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Evapotranspiration from a dry Douglas fir forest Curtis, John Reeves 1975

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EVAPOTRANSPIRATION FROM A DRY DOUGLAS FIR FOREST by JOHN REEVES CURTIS B . S c , M c G i l l U n i v e r s i t y , 1972 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n t h e Department o f S o i l S c i e n c e We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA A p r i l 1975 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r a n a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l m a k e i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e H e a d o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 D a t e Aon/ 1^7^ - i i -ABSTRACT E v a p o t r a n s p i r a t i o n measurements u s i n g the Bowen r a t i o / e n e r g y b a l a n c e t e c h n i q u e were, made o v e r a dry Douglas f i r f o r e s t . S o i l w ater m a t r i c p o t e n t i a l over the p e r i o d o f measurement v a r i e d from 0 t o -10.5 b a r s . With t h e premise t h a t t h e e q u i l i b r i u m e v a p o r a t i o n r a t e a s s o c i a t e d w i t h adequate water s u p p l y e s s e n t i a l l y e x p r e s s e s e v a p o t r a n s p i r a t i o n as a f r a c t i o n o f d a i l y net r a d i a t i o n , the r a t i o o f l a t e n t h eat f l u x t o net r a d i a t i o n f o r 24-hour p e r i o d s i s examined as a f u n c t i o n o f s o i l w a ter m a t r i c p o t e n t i a l . T h i s r a t i o , i s reduced t o a p p r o x i m a t e l y h a l f i t s maximum v a l u e as the m a t r i c p o t e n t i a l approaches -10 b a r s f o r t h e e x p e r i m e n t a l s i t e . The r e d u c t i o n i s not as g r e a t - a s t h a t i m p l i e d by changes i n s o i l w a ter s t o r a g e , i n d i c a t i n g t h a t evapo-t r a n s p i r a t i o n o f wa t e r not o r i g i n a t i n g i n t h e r o o t zone i s c a r r i e d on d u r i n g p e r i o d s o f c o n s i d e r a b l e w a t e r s t r e s s . W h i l e some water may be f l o w i n g upward from below the r o o t zone, i t i s f e l t t h a t o v er h a l f o f the d a i l y e v a p o t r a n s p i r e d water i s r e l e a s e d from s t o r a g e i n t r e e stems d u r i n g t h e most severe p e r i o d . - i i i -TABLE OF CONTENTS Page ABSTRACT i i LIST OF FIGURES i v ' ACKNOWLEDGEMENTS v i LIST OF SYMBOLS v i i INTRODUCTION 1 EXPERIMENTAL PROCEDURE 4 A. Experimental S i t e 4 B. Experimental Methods 6 RESULTS AND DISCUSSION 10 A. Soil Water Content and Matric Potential 10 B. Daytime Course of Net Radiation and Latent Heat Flux 13 C. The Relationship between LE/R and i> 2 4 n rm D. Evapotranspiration Estimates Using R and i> data 32 ° n rm E. Water Balance of the. Forest 34 CONCLUSIONS 40 REFERENCES ' 43 APPENDIX I 45 APPENDIX I I 4 7 - i v -LIST OF FIGURES F i g u r e 1. Water r e t e n t i o n c u r v e f o r Dashwood s e r i e s . g r a v e l l y sandy loam. Each p o i n t i s t h e average water c o n t e n t 0 o f the 15, 30., 45 and 60 cm depth samples a t a s i n g l e . v a l u e o f s o i l w a ter m a t r i c p o t e n t i a l Bars• i n d i c a t e range o f water .contents. F i g u r e 2. S o i l w ater m a t r i c p o t e n t i a l ^ f o r June;.17-August 19, 19 74. Each p o i n t i s t h e average v a l u e o f t h e m a t r i c p o t e n t i a l s a t t h e 15, 30 and 4 5 cm d e p t h s . F i g u r e 3. Average v o l u m e t r i c water c o n t e n t f o r June 20-August 20 o f t h e 0-60 cm depth o f s o i l from g r a v i m e t r i c s a m p l i n g . F i g u r e 4. D i u r n a l t r e n d i n wet- and d r y - b u l b t e mperature d i f f e r e n c e s AT W and AT r e s p e c t i v e l y o v er 1. m h e i g h t above the canopy f o r June-18 , 1974. Note t h e r e v e r s i n g o f t h e g r a d i e n t s around 0800 PST. F i g u r e 5. Daytime c o u r s e o f s u r f a c e r e s i s t a n c e r (from energy b a l a n c e measurements)and C s t o m a t a l r e s i s t a n c e r s ( p r o j e c t e d l e a f a r e a b a s i s ) f o r June 18 and J u l y 25, 1974. The 'disagreement o f t h e two v a l u e s i s l a r g e on a p e r c e n t a g e b a s i s i n t h e morning. F i g u r e 6a. D i u r n a l t r e n d i n R n and LE f o r June 18, 1974. S o l i d d o t s are energy b a l a n c e measurements o f LE, w h i l e c i r c l e s show LE e s t i m a t e d from s t o m a t a l r e s i s t a n c e measurements. The s o l i d l i n e f o r LE was drawn by eye. F i g u r e 6b. D i u r n a l t r e n d i n R n and LE f o r J u l y 25, 1974. S o l i d d o t s are energy b a l a n c e measurements o f LE, w h i l e c i r c l e s show LE e s t i m a t e d from s t o m a t a l r e s i s t a n c e measurements. The s o l i d l i n e f o r LE was drawn by eye. F i g u r e 7. F i g u r e s 6a and 6b t o g e t h e r t o i l l u s t r a t e t h e e f f e c t s o f d e c r e a s e d s o i l water m a t r i c p o t e n t i a l i j j m on LE. Note t h a t R n i s h i g h e r on June 18. F i g u r e 8. R n and LE on J u l y 19, t h e . ' f i r s t c l e a r day a f t e r the r a i n y p e r i o d i n m i d - J u l y . The m i d - a f t e r n o o n peak i n LE i s p r e s e n t , but not pronounced. - V -F i g u r e 9. R and LE on J u l y 8, t h e l a s t day o f the f i r s t d r y i n g p e r i o d , and t h e most se v e r e o b s e r v e d day from t h e a s p e c t o f wa t e r s t r e s s . LE/R n was o n l y 0.26. F i g u r e 10. LE/R n as a f u n c t i o n o f ' s o i l w a t e r m a t r i c p o t e n t i a l ^ m f o r t e n s e l e c t e d f i n e days. The number a d j a c e n t t o each p o i n t i s t h e dat e . F i g u r e 11. LE/LE eq- as a f u n c t i o n o f i j j , m where L E e q r e p r e s e n t s t h e d a i l y e q u i l i b r i u m e v a p o r a t i o n r a t e . D i f f e r e n c e s i n t h i s c u r v e and F i g u r e 10 are due o n l y t o d i f f e r e n c e s i n d a i l y mean t e m p e r a t u r e s . F i g u r e 12. L E / L E m a as a f u n c t i o n o f > m ( l o g a r i t h m i c s c a l e ) "for v a r i o u s a u t h o r s ( t a k e n from van B a v e l , 1967). Note t h a t t h e maximum e v a p o . t r a n s p i r a t i o n r a t e L E m a x i s det e r m i n e d d i f f e r e n t l y f o r d i f f e r e n t a u t h o r s , and i n t h i s s t u d y i s t a k e n as t h e e q u i l i b r i u m r a t e . F i g u r e 13. LE/R n as a f u n c t i o n o f v o l u m e t r i c water c o n t e n t . T h i s and F i g u r e 10 a r e r e l a t e d t h r o u g h t h e water r e t e n t i o n c u r v e f o r t h e s o i l , and a l l o w e v a p o t r a n s p i r a t i o n e s t i m a t e s from water c o n t e n t measurements.to be made. F i g u r e 14. D a i l y t o t a l s o f R R and LE f o r June 17-August 14, 1974 . P o i n t s used t o . - o b t a i n F i g u r e 10 are shown as t r i a n g l e s . F i g u r e 15. Water b a l a n c e o f t h e 0-60 cm depth o f s o i l f o r June 17-August 14, 1974. V a l u e s a re ' d a i l y averages i n mm o f water. The r e s i d u a l t e r m i s d i s c u s s e d i n the t e x t . F i g u r e S o i l water m a t r i c p o t e n t i a l a t "four depths f o r June 17-August 19, 1974, i n d i c a t i n g d i r e c t i o n o f g r a d i e n t s i n t h e v e r t i c a l a x i s . - v i -ACKNOWLEDGEMENTS I cannot hope t o f u l l y e x p r e s s my a p p r e c i a t i o n o f the guidance,:. i n s i g h t , and e n t h u s i a s m o f my s u p e r v i s o r , Dr. T.A. B l a c k , i n a l l phases o f my M a s t e r ' s s t u d i e s . I s i n c e r e l y f e e l t h a t t h e a c c o m p l i s h -ments r e p r e s e n t e d i n t h i s work are every b i t as much h i s as mine. A p a r t i c u l a r debt o f g r a t i t u d e i s owed t o Mr. Joe Nnyamah, whose m e t i c u l o u s and r e l i a b l e work i n the ' f i e l d o f s o i l m o i s t u r e measurements formed an i n d i s p e n s a b l e c o r n e r s t o n e o f t h i s s t u d y . I w i s h t o thank Mr. P a u l Tang and Mr...Ron Toth f o r t h e i r key r o l e s i n t h e s u c c e s s o f t h e i n s t r u m e n t a t i o n and d a t a l o g g i n g . I would e s p e c i a l l y l i k e t o e x p r e s s thanks t o Mr. C.S. Tan and M i s s Ruth Hardy f o r t h e i r s u p p o r t i n t h e f i e l d and f o r t h e i r f r i e n d s h i p , d i s c u s s i o n , and c r i t i c i s m i n t h e o f f i c e . I am a l s o i n d e b t e d t o t h o s e who s e r v e d on my committee; Drs. C.A. Rowles, T.M. B a l l a r d , T.R. Oke and R.A. F r e e z e . Thanks a r e due t o Mrs. Retha Gerstmar, who took t h e t i m e t o t y p e t h i s t h e s i s . Funds f o r t h i s r e s e a r c h were made a v a i l a b l e t h r o u g h g r a n t s from Canada Department o f t h e Environment (CFS and NCWRR), and from t h e N a t i o n a l Research C o u n c i l o f Canada. - v i i -LIST OF SYMBOLS D Drain a g e E E v a p o t r a n s p i r a t i o n G S o i l h eat f l u x d e n s i t y L L a t e n t heat o f v a p o u r i z a t i o n LE L a t e n t heat f l u x d e n s i t y LE E q u i l i b r i u m l a t e n t heat f l u x d e n s i t y eq ^ J M S t o r a g e o f s e n s i b l e and l a t e n t heat w i t h i n the canopy R n Net r a d i a n t energy f l u x d e n s i t y v.p.d. Vapour p r e s s u r e d e f i c i t AT D r y - b u l b temperature d i f f e r e n c e o v e r h e i g h t AT w Wet-bulb te m p e r a t u r e d i f f e r e n c e o v e r h e i g h t A9 Change i n s o i l w a ter s t o r a g e g Bowen r a t i o 9 V o l u m e t r i c s o i l w a ter c o n t e n t S o i l w ater m a t r i c p o t e n t i a l m r INTRODUCTION E v a p o t r a n s p i r a t i o n i s an i m p o r t a n t t e r m i n b o t h the water b a l a n c e and t h e energy b a l a n c e o f a f o r e s t . However, t h e c o m p l e x i t y o f t h e s u r f a c e f e a t u r e s and p h y s i c a l .geometry, as w e l l as the sheer s i z e o f a f o r e s t canopy, makes t h e a p p l i c a t i o n , o f t h e o r e t i c a l e x p r e s s i o n s f o r e v a p o t r a n s p i r a t i o n a d i f f i c u l t t a s k f o r t h e f o r e s t h y d r o m e t e o r o l o g i s t . N o n e t h e l e s s , r e s e a r c h e r s have been a c t i v e l y s e e k i n g a means o f making u s e f u l e s t i m a t e s o f e v a p o t r a n s p i r a t i o n t a k i n g i n t o a c c o u n t t h e b a s i c m e t e o r o l o g i c a l and h y d r o l o g i c a l parameters o f a v a i l a b l e energy and a v a i l a b l e w a t e r i n a p r a c t i c a l way. I f p o t e n t i a l e v a p o t r a n s p i r a t i o n were t a k e n t o be the e v a p o t r a n s p i r a t i o n when f e t c h and s o i l w a ter a r e adequate, t h e n t h i s p a rameter i s o f n e c e s s i t y a m e t e o r o l o g i c a l one. I t has been shown t h a t t h i s i s a p p r o x i m a t e l y e q u a l t o t h e " e q u i l i b r i u m " r a t e (McNaughton, 1974) f o r a w e s t - c o a s t f o r e s t environment. When used as an e s t i m a t e o f e v a p o t r a n s p i r a t i o n under s u i t a b l e c o n d i t i o n s , the e q u i l i b r i u m model i s p h y s i c a l l y j u s t i f i e d and e x t r e m e l y s i m p l e t o a p p l y , s i n c e i t r e p r e s e n t s e s s e n t i a l l y a s l i g h t l y temperature-dependent f r a c t i o n o f the d a i l y net r a d i a t i o n . - 2 -The s i m p l i c i t y o f t h e e q u i l i b r i u m model i s due i n p a r t t o t h e f a c t t h a t i t a p p l i e s o n l y t o a w e l l - w a t e r e d system, and i t a v o i d s t h e q u e s t i o n o f an e v a p o t r a n s p i r a t i o n r a t e w i t h s o i l m o i s t u r e and p h y s i o l o g i c a l parameters t a k e n i n t o a c c o u n t . A t h e o r e t i c a l m o d i f i c a t i o n o f t h e e q u i l i b r i u m model a p p l y i n g t o a l l c o n d i t i o n s o f s o i l m o i s t u r e and p l a n t water s t r e s s would r e n d e r i t u n w i e l d y from a p r a c t i c a l p o i n t o f view. However, t h e f r a c t i o n o f d a i l y net r a d i a t i o n r e p r e s e n t e d i n t h e e q u i l i b r i u m r a t e p r o v i d e s a u s e f u l v e h i c l e f o r a s t r a i g h t f o r w a r d study o f t h e r e d u c t i o n i n e v a p o t r a n s p i r a t i o n under c o n d i t i o n s o f water s t r e s s . Some e a r l i e r s t u d i e s o f t h e r e d u c t i o n o f e v a p o r a t i o n a s s o c i a t e d w i t h water s t r e s s i n a g r i c u l t u r a l p l a n t s were r e p o r t e d by Denmead and Shaw (19 62) and Gardner and E h l i g (1963). F u r t h e r s t u d i e s on c r o p s c a r r i e d - o u t : . i n - . " f i e l d p l o t s were made by.a number o f o t h e r workers (van B a v e l , 19 67; Denmead and M c l l r o y , 1970; Davie s and A l l e n , 197 3; R i t c h i e , 1973). Only a few have l o o k e d a t t h e problem r e l a t e d t o f o r e s t v e g e t a t i o n , and t h e s e have been p r i m a r i l y on s e e d l i n g s i n t h e l a b o r a t o r y ( Z a t i k o v s k y and F e r r e l , 1968; L o p u s h i n s k y and K l o c k , 1974), owing t o t h e d i f f i c u l t y i n making.in situ measurements o f evapo-t r a n s p i r a t i o n o f a f o r e s t s t a n d . - 3 -The f a c t t h a t such a study o f a f o r e s t s t a n d must u l t i m a t e l y be made i n t h e f i e l d , t o g e t h e r w i t h t h e c o n f i d e n c e g a i n e d i n t h e t e c h n i q u e and a p p a r a t u s used i n t he Bowen r a t i o approach ( B l a c k and McNaughton, 1971), p r o v i d e s t h e b a s i s f o r t h e o b j e c t i v e s o f t h i s s t u d y . These o b j e c t i v e s can be summarized as f o l l o w s : 1. To determine whether t h e e v a p o t r a n s p i r a t i o n r a t e a t t h e e x p e r i m e n t a l s i t e when water i s not l i m i t i n g i s e q u a l t o t h e e q u i l i b r i u m r a t e , t h e r e b y p r o v i d i n g a f u r t h e r check on the r e s u l t s p r e v i o u s l y o b t a i n e d i n a west-c o a s t Douglas f i r s t a n d (McNaughton and B l a c k , 1973). 2. To measure e v a p o t r a n s p i r a t i o n from a dry Douglas f i r s t a n d , and observe t h e e f f e c t o f d e c r e a s i n g s o i l m o i s t u r e on t h e f r a c t i o n o f d a i l y net r a d i a t i o n used i n e v a p o r a t i n g w a t e r . D i u r n a l t r e n d s i n t h e energy b a l a n c e p r o v i d e some i n s i g h t i n t o the p r o c e s s e s c o n t r o l l i n g e v a p o t r a n s p i r a t i o n , and t h e r e f o r e w i l l be touched upon. N e v e r t h e l e s s , t h e emphasis o f t h i s s t u d y w i l l be upon d a i l y changes i n t h e e v a p o t r a n s p i r a t i o n component o f the water b a l a n c e , and i t s r e l a t i o n t o d a i l y changes i n s o i l w a ter s t a t u s and net r a d i a t i o n . - 4 -EXPERIMENTAL PROCEDURE A. Experimental Site S e l e c t i o n o f a s i t e f o r t h i s s t u d y was c o n t i n g e n t upon the n e c e s s i t y f o r an a r e a which c o u l d be c o n s i d e r e d s i m p l e from a m i c r o m e t e o r o l o g i c a l p o i n t o f vi e w , and y e t one which r e p r e s e n t s a n a t u r a l environment from which p r a c t i c a l c o n c l u s i o n s c o u l d be drawn. C o n s i d e r a b l e e f f o r t was t h e n put i n t o f i n d i n g an e x i s t i n g f o r e s t s t a n d which met the f o l l o w i n g c r i t e r i a : 1. The s i t e must e x p e r i e n c e water s t r e s s d u r i n g the summer. 2. The s i t e must be f r e e o f a d v e c t i v e e f f e c t s , r e q u i r i n g an e x t e n s i v e f o r e s t e d a r e a o f r e a s o n a b l y u n i f o r m v e g e t a t i o n , f l a t t o p o g raphy, and good f e t c h . 3. The st a n d s h o u l d be p r i m a r i l y Douglas f i r . 4. The s i t e s h o u l d be a c c e s s i b l e by r o a d . The s i t e chosen was on Crown Z e l l e r b a c h Company p r o p e r t y a p p r o x i m a t e l y 17 m i l e s n o r t h w e s t o f Co u r t e n a y , B. C., on t h e e a s t e r n c o a s t o f Vancouver I s l a n d . The s i t e met most o f t h e r e q u i r e m e n t s f o r t h e study from both a m i c r o m e t e o r o l o g i c a l and an o p e r a t i o n a l p o i n t o f view. The measurements were made i n an u n t h i n n e d - 5 -sta n d o f Douglas f i r p l a n t e d i n 1953, l o c a t e d on the r e l a t i v e l y e x t e n s i v e c o a s t a l p l a i n between Courtenay and Campbell R i v e r . The wet, m i l d w i n t e r s and d r y , warm summers e x p e r i e n c e d by the a r e a p r o v i d e d t h e range o f s o i l m o i s t u r e needed f o r the s t u d y . The topography i s g e n e r a l l y f l a t , a l t h o u g h t h e r e a r e s e v e r a l r i d g e s o f a p p r o x i m a t e l y 20-30 m r e l i e f , and some d e p r e s s i o n a l a r e a s o f a swampy n a t u r e which d r y out d u r i n g the summer. The s i t e was l o c a t e d a t an e l e v a t i o n o f 150 m. At t h e time o f t h e r e s e a r c h , t h e t r e e s ranged i n h e i g h t from 7 t o 10 m, and averaged a p p r o x i m a t e l y 15 cm i n d i a m e t e r . There were a p p r o x i m a t e l y 17 3 0 stems -1 2 -1 ha , and the b a s a l a r e a was 2 7.5 m ha , w i t h a c o m p e t i t i v e s t r e s s i n d e x o f 34 0. The s o i l , b e l o n g i n g t o t he Dashwood s e r i e s , was a w e l l - d r a i n e d g r a v e l l y sandy loam o f 45-6 0 cm depth o v e r l y i n g a deep l a y e r o f compacted b a s a l t i l l . Measurements o f r o o t d e n s i t y i n d i c a t e d t h a t t h e b u l k o f the r o o t s were i n t h e upper 45 cm o f the s o i l . O p e r a t i o n a l l y , the s i t e p r o v i d e d good a c c e s s w i t h main l o g g i n g r o a d s l e a d i n g t o w i t h i n 2 m i l e s o f the study a r e a from the I s l a n d Highway. S m a l l e r r o a d s l e d r i g h t t o t h e s i t e , a l o n g s i d e which were p l a c e d t h e two t r a i l e r s and the g e n e r a t o r . The 12.2 m h i g h i n s t r u m e n t a t i o n tower was p l a c e d 30 m from the t r a i l e r s . - 6 -I t i s f e l t t h a t t h e a r e a i s an e x c e l l e n t one f o r a m i c r o m e t e o r o l o g i c a l s t u d y . A d v e c t i v e e f f e c t s were f e l t not t o be p r e s e n t , w i t h f i v e m i l e s o f f o r e s t e d l a n d s e p a r a t i n g the s i t e from the S t r a i g h t o f G e o r g i a , and f o r e s t e x t e n d i n g upwards o f 20 m i l e s i n . e i t h e r d i r e c t i o n a l o n g the c o a s t . A l t h o u g h t h e w e s t - t o - s o u t h e a s t s e c t o r i n c l u d e d i r r e g u l a r t e r r a i n and d e f o r e s t e d a r e a s on the s l o p e s o f mountains r i s i n g i n t o t h e F o r b i d d e n P l a t e a u a r e a w i t h i n 2 m i l e s o f t h e s i t e , winds were v i r t u a l l y never from t h a t d i r e c t i o n . P r e v a i l i n g winds were from t h e n o r t h t o n o r t h e a s t . B. Experimental Methods Continuous h a l f - h o u r l y measurements o f e v a p o t r a n s -p i r a t i o n were made from June 14 u n t i l August 15, 1974 u s i n g t h e Bowen r a t i o / e n e r g y b a l a n c e t e c h n i q u e . The Bowen r a t i o g was measured a t the 8.5 m l e v e l u s i n g t h e p s y c h r o m e t r i c a p p a r a t u s d e s c r i b e d i n B l a c k and McNaughton (1971). Optimum placement o f t h e s e n s o r over the canopy was sought a f t e r some e x p e r i m e n t a t i o n w i t h v a r i o u s h e i g h t s and p o s i t i o n s r e l a t i v e t o t h e t o p o f the tower. The s e n s o r remained i n t h e same l o c a t i o n from June 17 u n t i l August 1. On August 1, a second Bowen r a t i o a p p a r a t u s i d e n t i c a l t o t h e f i r s t was p l a c e d above t h e f i r s t one, and the p o s i t i o n s o f the two machines were p e r i o d i c a l l y s w i t c h e d i n o r d e r t o check on the o p e r a t i o n o f each, and t o t e s t f o r any s e n s o r placement problems. Placement o f t h e s e n s o r s i s d i s c u s s e d b r i e f l y i n t h e next s e c t i o n . Net r a d i a t i o n R was measured a t t h e top o f the n ^ canopy w i t h a S w i s s t e c o S - l net r a d i o m e t e r , c o n t i n u o u s l y v e n t i l a t e d w i t h d r i e d a i r from an aquarium-type pump. S o i l heat f l u x G was measured a t t h e 5 cm l e v e l w i t h two heat f l u x p l a t e s , and c o r r e c t e d f o r s t o r a g e i n t h e upper 5 cm u s i n g an i n t e g r a t e d t e m p e r a t u r e measured w i t h a diod e i n t e g r a t i n g thermometer (Tang et a l . , 1974). S t o r a g e o f s e n s i b l e and l a t e n t heat w i t h i n the canopy M was e s t i m a t e d from wet and d r y b u l b t e m p e r a t u r e s t a k e n e v e r y 15 minutes a t t h e 3, 5 and 7 m l e v e l s , and from e s t i m a t e s o f the heat c a p a c i t y o f the canopy based on the work o f St e w a r t and Thorn (1973). E v a p o t r a n s p i r a t i o n E was t h e n c a l c u l a t e d u s i n g the e q u a t i o n E = (R - G - M ) / [ L ( 1 + 6)] where L i s the l a t e n t heat o f v a p o u r i z a t i o n . S u p p o r t i n g c l i m a t o l o g i c a l measurements were made of i n c i d e n t s o l a r r a d i a t i o n R g a t t h e top o f the tower, and a t v a r i o u s l e v e l s t h r o u g h t h e canopy a t s e l e c t e d t i m e s . Wind speed and d i r e c t i o n were m o n i t o r e d u s i n g a C a s s e l a s e n s i t i v e anemometer and C l i m e t wind vane - 8 -mounted a t the top o f the tower. P r e c i p i t a t i o n was r e c o r d e d each day w i t h a 10.2 cm (4") d i a m e t e r r a i n gauge a t the t r a i l e r . Data s i g n a l s were c a r r i e d back t o the d a t a l o g g i n g t r a i l e r by 7 5 m s h i e l d e d c a b l e s where they were r e c o r d e d w i t h a H e w l e t t - P a c k a r d 2707 A d a t a l o g g e r , and R n, R , and Bowen r a t i o d a t a s i g n a l s were i n t e g r a t e d u s i n g v o l t a g e i n t e g r a t o r s ( B l a c k et al., 1974). Bowen r a t i o d a t a was f u r t h e r m o n i t o r e d on a Honeywell s t r i p -c h a r t r e c o r d e r . The system was powered by a 6.5 kW K o h l e r d i e s e l g e n e r a t o r . S o i l w ater c o n t e n t was measured b o t h g r a v i m e t r i c a l l y and by use o f the n e u t r o n m o i s t u r e meter. Neutron m o i s t u r e measurements were made eve r y 2 t o 3 days i n s i x a c c e s s t u b e s . The b a s a l t i l l l i m i t e d the depth of the tubes t o 45-60 cm. G r a v i m e t r i c s a m p l i n g o f the r o o t zone-was t a k e n - e v e r y 5 t o 16 da y s , depending on the s t a t e o f the d r y i n g p e r i o d . S o i l water p o t e n t i a l between 0 and -1 bar was measured by a t e n s i o m e t e r - t r a n s d u c e r system. Four t e n s i o m e t e r s were used and were l o c a t e d a t depths o f 15, 30, 45 and 60 cm. S o i l water p o t e n t i a l l e s s t h a n -1 b a r was measured u s i n g a Campbell S c i e n t i f i c HR-33T dew p o i n t m i c r o v o l t m e t e r and PT-10 hygrometers. S i x - 9 -hygrometers, two each a t the 15 and 3 0 cm d e p t h , and one each a t t h e 45 and 6 0 cm d e p t h , were used. S o i l w a ter p o t e n t i a l by the t e n s i o m e t e r - t r a n s d u c e r system was r e c o r d e d a t 15 minute i n t e r v a l s , w h i l e s o i l w a ter p o t e n t i a l s by the hygrometers was measured t h r e e t i m e s each day. I n t h i s s t u d y , o n l y d a i l y v a l u e s o f s o i l w ater p o t e n t i a l a r e used i n the a n a l y s i s . S o i l w ater r e t e n t i o n c u r v e s were de t e r m i n e d i n the l a b o r a t o r y by ( i ) hanging column method from s a t u r a t i o n t o a water p o t e n t i a l o f -0.12 bar and ( i i ) p r e s s u r e p l a t e e x t r a c t i o n from -0.33 bar t o -15 b a r s . In a d d i t i o n , t e x t u r e , b u l k d e n s i t y , and r o o t d i s t r i b u t i o n measurements were made. .Stomatal r e s i s t a n c e r a t t h e 10 m h e i g h t was s & measured on a r o u t i n e b a s i s t h r e e t i m e s a day on the m a j o r i t y o f days d u r i n g the p e r i o d June 14 t o August 18 and measurements were made e v e r y two hours d u r i n g the daytime on 16 s e l e c t e d days. The measurement was made u s i n g t h e v e n t i l a t e d porometer d e s c r i b e d i n B l a c k et al. (1974). A t l e a s t two samples o f f o u r n e e d l e s each were used t o o b t a i n a v a l u e o f r . - 10 -RESULTS AND DISCUSSION A. Soil water content and matric potential The average water r e t e n t i o n c u r v e f o r the 0-60 cm depths i s shown i n F i g u r e 1. The c u r v e r e p r e s e n t s the average o f l a b o r a t o r y a n a l y s e s f o r f o u r d e p t h s , w i t h the b a r s i n d i c a t i n g t h e range o f d e v i a t i o n s . F i e l d measure-ments o f s o i l water c o n t e n t and m a t r i c p o t e n t i a l c o n f i r m t h i s c u r v e . The a v a i l a b l e water c a p a c i t y i s a p p r o x i m a t e l y 3 - 3 0.14 cm cm f o r t h e m a t r i c p o t e n t i a l range -1/3 t o -15 b a r . The r e s u l t s o f the m e c h a n i c a l a n a l y s i s o f s o i l p a r t i c l e s w i t h a d i a m e t e r l e s s than 2 mm was 63.5% sand, 21.9% s i l t , and 14.6% c l a y by w e i g h t . The volume f r a c t i o n o f s t o n e s ( p a r t i c l e s g r e a t e r than 2 mm) was 20%. F i g u r e 2 shows the average s o i l water m a t r i c p o t e n t i a l it. f o r the p e r i o d June 17 t o August 19 , 1974 f o r t h e r o o t m c zone (0-45 cm). D a i l y v a l u e s o f p r e c i p i t a t i o n i n mm a r e i n c l u d e d f o r r e f e r e n c e . I t can be seen t h a t t h e summer can be d i v i d e d i n t o t h r e e p e r i o d s : ( i ) a d r y i n g p e r i o d i n the l a t e s p r i n g , which was a l r e a d y i n p r o g r e s s when the equipment was s e t up i n June, ( i i ) a w e t t i n g p e r i o d i n m i d - J u l y , when the b u l k o f the summer's p r e c i p i t a t i o n o c c u r r e d ( p r e c i p i t a t i o n f o r J u l y , 1974 was a b n o r m a l l y h i g h ) , - 11 -i 1 1 1 1 1 1 r 1 I I I 1 r F i g u r e 1. Water r e t e n t i o n c urve f o r Dashwood s e r i e s g r a v e l l y sandy loam. Each p o i n t i s the average water c o n t e n t 6- o f t h e 15, 30, 45 and 60 cm depth samples a t a s i n g l e v a l u e ,of s o i l w ater m a t r i c p o t e n t i a l tym. Bars i n d i c a t e range o f water c o n t e n t s . - .12 -1 i I i i i i i r o - 5 ( 0 " 4 5 c m depth, bar) I -10 J I ft-fl i n 20 30 June JL J i t_ Precip. 20 (mm) 0 20 30 July 1974 10 20 August F i g u r e 2... S o i l w ater m a t r i c p o t e n t i a l f o r June ' 17-August 19, 1974. Each p o i n t i s . t h e average v a l u e o f the m a t r i c p o t e n t i a l s a t t h e 15, 30 and 4-5 cm depths. - 13 -r e s t o r i n g the s o i l t o z e r o m a t r i c p o t e n t i a l , and ( i i i ) a second d r y i n g p e r i o d w i t h no p r e c i p i t a t i o n o c c u r r i n g t h r o u g h o u t u n t i l the end o f the p e r i o d . T h i s weather a f f o r d e d two s e p a r a t e p e r i o d s a t the d r y end o f t h e range o f water c o n t e n t s i n t h e s o i l . Average s o i l w a ter c o n t e n t , shown i n F i g u r e 3, ranged from 9-23 p e r c e n t on a v o l u m e t r i c b a s i s which i n c l u d e s s t o n e s . The 0 t o 60 cm l a y e r was used i n t h i s average because i t i s f e l t t h a t water c o n t e n t changes down t o 60 cm may be s i g n i f i c a n t i n the f o r e s t water b a l a n c e . T h i s m a t t e r w i l l be d i s c u s s e d i n more d e t a i l l a t e r . P r e c i p i t a t i o n i s a g a i n i n c l u d e d i n t h e f i g u r e f o r r e f e r e n c e . B. Daytime course of net radiation and latent heat flux D e t a i l e d c o m p u t a t i o n o f h a l f - h o u r l y energy b a l a n c e s was r e s t r i c t e d p r i m a r i l y t o f i n e days when r e s o l u t i o n was good i n d i u r n a l f l u c t u a t i o n s o f energy f l u x e s . I t was found t h a t t h e s m a l l g r a d i e n t s o f wet and d r y b u l b t e m p e r a t u r e s over t h e d r y f o r e s t produced more s c a t t e r i n the evapo-t r a n s p i r a t i o n measurement than, had been e x p e c t e d . The s i z e o f t h e s e g r a d i e n t s i s e v i d e n t when F i g u r e 4, showing AT and AT t h r o u g h the c o u r s e o f the day on June 18, i s compared w i t h t h e g r a d i e n t s shown i n F i g u r e 6 i n B l a c k and McNaughton (1971). - 14 -T 1 1 i i i i i I r F i g u r e 3. Average v o l u m e t r i c water c o n t e n t f o r June 2 0-August 20 o f the- 0-60 cm depth o f s o i l from g r a v i m e t r i c s a m p l i n g . - 15 -T ~~ i i : — i 1 1 — i 1 r J -i 1 1 I J I i i L L 0 4 8 12 16 20 Hour PST F i g u r e 4. - 16 -N e g a t i v e v a l u e s o f AT, c a u s i n g o v e r e s t i m a t e s o f LE f r e q u e n t l y e x c e e d i n g R , were c o n s i s t e n t l y seen i n the p e r i o d from s u n r i s e u n t i l a p p r o x i m a t e l y 1000 PST. I t was f e l t t h a t t h e problem was caused by the s e n s o r b e i n g p l a c e d too low and c o n s e q u e n t l y measuring g r a d i e n t s w i t h i n the canopy r a t h e r t h a n over i t a t c e r t a i n t i m e s . A s e t t l i n g o f the maximum temperature o f the p r o f i l e i n t o the canopy a f t e r 1000 PST c o u l d e x p l a i n morning i n v e r s i o n s and s u c c e s s f u l a f t e r n o o n Bowen r a t i o measurements. A l s o , s i n c e t h e t e r r a i n s l o p e d g e n t l y t o t h e n o r t h e a s t , the apparent h e i g h t o f t h e s e n s o r was g r e a t e r when winds were from the n o r t h e a s t , which was the predominant wind d i r e c t i o n i n the a f t e r n o o n . S i n c e a d v e c t i v e e f f e c t s were c o n s i d e r e d a b s e n t , i t was f e l t t h a t t h e t r u e v a l u e o f LE s h o u l d be somewhat l o w e r t h a n R d u r i n g t h i s t ime p e r i o d . R e s u l t s from the f o r e s t n 6 ^ a t Haney (McNaughton and B l a c k , 1973) would suggest t h a t a v a l u e c l o s e r t o o n e - h a l f o f R would be a b e t t e r e s t i m a t e . n Independent e s t i m a t e s o f LE u s i n g t h e M o n t e i t h e q u a t i o n and observed v a l u e s o f s t o m a t a l r e s i s t a n c e t o c a l c u l a t e s u r f a c e r e s i s t a n c e shew good agreement w i t h energy b a l a n c e measure-ments i n the a f t e r n o o n , but tend t o s u p p o r t t h e o n e - h a l f R R r e l a t i o n s h i p i n the morning. F i g u r e 5 compares s t o m a t a l r e s i s t a n c e r _ w i t h s u r f a c e r e s i s t a n c e r _ c a l c u l a t e d from - 17 -6 9 12 15 18 Hour PST F i g u r e 5. Daytime c o u r s e ' o f s u r f a c e r e s i s t a n c e r c (from energy b a l a n c e measurements) and s t o m a t a l r e s i s t a n c e r s ( p r o j e c t e d l e a f a r e a b a s i s ) f o r June 18 and J u l y 25, 1974. The disagreement o f the two v a l u e s i s l a r g e on a p e r c e n t a g e b a s i s i n t h e morning.. the energy b a l a n c e e s t i m a t e s o f LE and t h e M o n t e i t h e q u a t i o n . Note t h a t i n t h e morning r g and r c o f t e n d i f f e r by o v er a f a c t o r o f two, whereas i n the a f t e r n o o n t h e y show good agreement on a p e r c e n t a g e b a s i s . At t i m e s when the second Bowen r a t i o a p p a r a t u s was p l a c e d above the f i r s t one, i t s measurements a l s o suggest t h a t t h e o n e - h a l f R n r e l a t i o n s h i p i s a good. e s t i m a t e . The s u c c e s s o f the second machine i n t h i s p o s i t i o n tends t o s u p p o r t the c o n c l u s i o n t h a t w i t h such s m a l l g r a d i e n t s , placement o f t h e s e n s o r s can be c r i t i c a l : t oo low a s e n s o r may be s u b j e c t t o i n v e r s i o n e f f e c t s i n the morning, and too h i g h a s e n s o r may not be a b l e t o s u f f i c i e n t l y r e s o l v e the s m a l l e r g r a d i e n t s . I t may w e l l be t h a t the most v i a b l e a l t e r n a t i v e t o v a r i a b l e placement of the s e n s o r w i t h h e i g h t o v e r th e day i s an i n c r e a s e d s e p a r a t i o n o f the s e n s o r s f o r d r y f o r e s t measurements. F i g u r e 6 shows the d i u r n a l c o u r s e o f R and LE f o r ° n (a) June 18, 1974, and (b) J u l y 25, 1974. The s o l i d d o t s show the energy b a l a n c e v a l u e s o f LE, w h i l e the c i r c l e s r e p r e s e n t LE e s t i m a t e s from s t o m a t a l r e s i s t a n c e measurements. D a i l y t o t a l s o f e v a p o t r a n s p i r a t i o n were o b t a i n e d from th e s o l i d l i n e , drawn by eye. Note the a p parent o v e r e s t i m a t i o n o f measured LE over what i s f e l t t o be the b e t t e r approxim-a t i o n i n the d a y l i g h t p e r i o d b e f o r e 1000 h r s . T h i s - 19 -T I I r r 1 1 1 1 r _i _ L j t i i I 1 1 i i i I 0 4 8 12 16 20 24 Hour PST F i g u r e 6a. D i u r n a l t r e n d i n R n and LE f o r June 18, 1974. S o l i d dots a re energy b a l a n c e measurements o f LE, w h i l e c i r c l e s show LE e s t i m a t e d from s t o m a t a l r e s i s t a n c e measurements.. The s o l i d l i n e f o r LE was drawn by eye. - 20 -6 0 0 Wm 4 0 0 r -2 0 0 Hour PST F i g u r e 6b. D i u r n a l t r e n d i n R n and LE f o r J u l y 25, 1974. S o l i d d o t s are energy b a l a n c e measurements o f LE, w h i l e c i r c l e s show LE e s t i m a t e d from s t o m a t a l r e s i s t a n c e measure- • ments.. The s o l i d l i n e f o r LE was drawn by eye. - 21 -o v e r e s t i m a t i o n u s u a l l y amounted t o a p p r o x i m a t e l y t e n p e r c e n t o f the d a i l y t o t a l e v a p o t r a n s p i r a t i o n . F i g u r e s 6 (a) and (b) a r e combined i n F i g u r e 7 t o c o n t r a s t t h e i r c u r v e s o f LE and R . J u l y 25 was a f i n e n J day s u f f i c i e n t l y l o n g a f t e r the r a i n y p e r i o d t h a t t h e r e was no f r e e water on t h e t r e e s , y e t a day when s o i l water c o u l d be c o n s i d e r e d adequate a t a m a t r i c p o t e n t i a l o f -0.7 b a r . The l a t e n t heat f l u x on J u l y 25 does not d i s p l a y the m i d - a f t e r n o o n peak obse r v e d a t Haney by McNaughton and B l a c k (1973). In f a c t , a l t h o u g h i t was p r e s e n t , i t was not pronounced i n the l a t e n t heat f l u x p l o t f o r J u l y 19 (shown i n F i g u r e 8 ) , the f i r s t c l e a r day a f t e r t h e w e t t i n g p e r i o d . T h i s s u g g e s t s more r e s t r i c t i o n t o t r a n s p i r a t i o n t h a n was p r e s e n t a t Haney i n the p e r i o d o f peak a f t e r n o o n vapour p r e s s u r e d e f i c i t . The 24-hour v a l u e s o f LE/R o f 0.54 and 0.58 f o r J u l y 19 n J and 2 5 r e s p e c t i v e l y a r e s l i g h t l y l o wer t h a n th e r e s p e c t i v e v a l u e s of 0.61 and 0.63 p r e d i c t e d by the e q u i l i b r i u m e v a p o r a t i o n r a t e . The r e s t r i c t i o n may w e l l be a f u n c t i o n o f the low h y d r a u l i c c o n d u c t i v i t y a s s o c i a t e d w i t h a v e r y c o a r s e s o i l . June 18, a day when the m a t r i c p o t e n t i a l was -6.5 b a r s f o l l o w i n g s e v e r a l weeks o f g e n e r a l l y c l e a r weather, p r e s e n t s a much d i f f e r e n t p i c t u r e . Note, f i r s t , t h a t R - 22 -6 0 0 Wm'2 4 0 0 2 0 0 25 July, 1974 Y m = -0 .7 bar — 18 June, 1974 Y m = - 6.5 bars 0 0 8 12 Hour 16 PST 20 24 F i g u r e 7. F i g u r e s 6a and 6b t o g e t h e r t o i l l u s t r a t e t h e e f f e c t s o f • d e c r e a s e d s o i l water m a t r i c p o t e n t i a l i | ^ m on L E . Note t h a t R i s h i g h e r on June 18. n to - 23 -l_ I 1 I : i I I 1 L I I I I I 0 4 8 12 16 20 24 Hour PST F i g u r e 8. •' R n and LE on J u l y 19, the f i r s t c l e a r day a f t e r t h e r a i n y p e r i o d i n m i d - J u l y . The m i d - a f t e r n o o n peak i n LE i s p r e s e n t , b u t . n o t pronounced. - 24 -i s h i g h e r on June 18, b e i n g o n l y 3 days from the summer s o l s t i c e . The c u t b a c k i n LE was apparent as e a r l y as 10 00 PST, a t whi c h time the e v a p o t r a n s p i r a t i o n r a t e reached i t s d a i l y maximum. The l a t e n t heat f l u x d e c r e a s e d d u r i n g t h e l a t e morning and e a r l y a f t e r n o o n t o a minimum o f 80 Wm~2 a t 1430 PST when t h e v.p.d. and s t o m a t a l r e s i s t a n c e b o t h r e a c h e d maximum d a i l y v a l u e s ( B l a c k et al., 1975). R e s i s t a n c e t o t r a n s p i r a t i o n i s o b v i o u s i n t h i s case w i t h the 24-hour v a l u e o f LE/R n b e i n g o n l y 0.38. J u l y 8, whose d i u r n a l c o u r s e s o f LE and R n a r e shown i n F i g u r e 9, can be c o n s i d e r e d t h e most severe o b s e r v e d day o f the summer, w i t h an average ^ m o f -10.5 b a r s , R n . • o f 6.75 mm " e q u i v a l e n t , and LE/R n o f o n l y 0.26. G r a d i e n t s i n the wet and d r y b u l b t e m p e r a t u r e s were so s m a l l t h a t the observ e d vapour p r e s s u r e g r a d i e n t was n e g l i g i b l e a t 1600 PST. C. The relationship between LE/Rn and i>m As mentioned i n the i n t r o d u c t i o n , the e q u i l i b r i u m e v a p o r a t i o n r a t e p r e d i c t s an a p p r o x i m a t e l y c o n s t a n t v a l u e of t h e r a t i o LE/R n f o r adequate s o i l w a t e r . S i n c e the e v a p o t r a n s p i r a t i o n r a t e depends on e v a p o r a t i v e demand as w e l l as water s u p p l y , we w i l l examine the e f f e c t o f d e c r e a s i n g s o i l w a ter m a t r i c p o t e n t i a l on LE/R n, F i g u r e 10 shows t h e 24-hour v a l u e o f t h e r a t i o LE/R n as a f u n c t i o n o f f o r t e n s e l e c t e d f i n e days i n c l u d i n g - 2 5 -T i : i 1 r r- 1 1 — — \ 1 T J i i i i i i i i i i i I 0 4 8 12 16 20 24 Hour PST F i g u r e 9. R n and LE on J u l y 8, the l a s t day o f the f i r s t d r y i n g p e r i o d , and the most severe o b s e r v e d day from t h e a s p e c t o f water s t r e s s . LE/R was o n l y 0.26. - 2 6 -0-8, 0-6 LE 0-4 0-2 0 1 - 2 5 / 7 i i i i i i i x e .19/7 \ ^ . 2 / 8 -19/6 - 3 / 8 ^ ~ ^ 17/6 -- l 8 / 6 ^ ~ ~ — — -2 5 / 6 -i i i • i I I 8 / 7 -0 - 2 -4 - 6 . - 8 ^ m ( 0 - 4 5 c m depth, bar.)' -10 F i g u r e 10. LE/R as a f u n c t i o n o f s o i l water m a t r i c n . p o t e n t i a l ^ m f o r t e n s e l e c t e d f i n e days. The number a d j a c e n t t o each p o i n t i s t h e date . - 27 -the f o u r days p r e v i o u s l y d i s c u s s e d . ^ m r e p r e s e n t s t h e average v a l u e f o r the r o o t zone, as i n F i g u r e 2, A c u r v e o f b e s t f i t was drawn by eye. The c u r v e i n d i c a t e s t h a t LE/R^ d e c r e a s e d c o n t i n u o u s l y t o a p p r o x i m a t e l y h a l f i t s maximum v a l u e as IJJ^ approached -10 b a r s . I t i s p o s s i b l e t h a t l e s s s c a t t e r would be o b t a i n e d by t a k i n g i n t o a ccount such f a c t o r s as s t o m a t a l r e s i s t a n c e , v.p.d,, and t h e magnitude o f R , but the c o m p l e x i t i e s i n v o l v e d i n d o i n g t h i s d e f e a t the purpose o f the s i m p l e approach used h e r e . The h i g h v a l u e o f LE/R n f o r June 19 (0.41) may be due t o the somewhat h i g h e r v.p.d. on t h a t day, but f u r t h e r s p e c u l a t i o n would not be j u s t i f i e d w i t h o u t a g r e a t d e a l more d a t a . Workers i n a g r i c u l t u r e have o f t e n used v a r i a t i o n s on the Penman p o t e n t i a l e v a p o r a t i o n e q u a t i o n r a t h e r t h a n R R t o n o r m a l i z e LE t o e v a p o r a t i v e demand. The d i f f i c u l t i e s i n c a l c u l a t i n g '-and i n t e r p r e t i n g the Penman p o t e n t i a l e v a p o r a t i o n r a t e have been d i s c u s s e d by McNaughton and B l a c k (1973). F i g u r e 11 shows the r a t i o LE/LE as a eq f u n c t i o n o f , where LE i s the e q u i l i b r i u m e v a p o r a t i o n m eq ^ ^ r a t e d e t e r m i n e d from d a i l y a i r t e m p e r a t u r e s and net r a d i a t i o n . The s i m i l a r i t y between t h i s graph and F i g u r e 10 e s s e n t i a l l y u n d e r l i n e s the degree t o which th e e q u i l i b r i u m r a t e can be e x p r e s s e d as a c o n s t a n t f r a c t i o n o f R . Because - 28 -0.4 r-I I • I I I I I 1—. 1 1 1 0 - 2 - 4 - 6 - 8 -10 (bar) F i g u r e 11. L E / L E e q as a f u n c t i o n o f tym where L E e q r e p r e s e n t s t h e d a i l y e q u i l i b r i u m e v a p o r a t i o n r a t e . D i f f e r e n c e s i n t h i s c u rve and F i g u r e 10 are due o n l y t o d i f f e r e n c e s i n d a i l y , mean t e m p e r a t u r e s . - 29 -o f the f r e q u e n t l y s t r o n g dependence o f water c o n t e n t on h i g h v a l u e s o f ^ m f o r most a g r i c u l t u r a l s o i l s , c u r v e s such as t h a t i n F i g u r e 11 a r e o f t e n shown w i t h il> on a m l o g a r i t h m i c s c a l e t o expand t h e h i g h ^ m range. Such a c u r v e , t a k e n from F i g u r e 11, i s shown i n F i g u r e 12 a l o n g w i t h v a r i o u s o t h e r c u r v e s t a k e n from van B a v e l (19 67) f o r comparison. I t s h o u l d be noted t h a t d e t e r m i n a t i o n o f maximum r a t e s o f LE i s not the same f o r a l l a u t h o r s . A l s o i n the c o a r s e s o i l o f t h i s s t u d y , h y d r a u l i c c o n d u c t i v i t y tends t o drop o f f f a s t e r w i t h d e c r e a s i n g m a t r i c p o t e n t i a l than i n most a g r i c u l t u r a l s o i l s . W h i l e the energy-based parameter I|J i s used e x t e n s i v e l y by e n v i r o n m e n t a l p h y s i c i s t s , h y d r o l o g i s t s and a g r i c u l t u r a l i s t s o f t e n f i n d i t more c o n v e n i e n t t o use v o l u m e t r i c water c o n t e n t 6 as a measurement o f s o i l w a t e r . I t s h o u l d be p o s s i b l e , t h e n , t o "draw a r e l a t i o n s h i p .between LE/R n and 8 by use. o f F i g u r e 10 and a w a t e r ; r e t e n t i o n c u r v e f o r the s o i l under s t u d y . Such a r e l a t i o n s h i p , d e r i v e d from F i g u r e s 1 and 10, i s shown i n F i g u r e 13. The use o f such a c u r v e would a l l o w t h e use o f a d a i l y g r a v i m e t r i c o r n e u t r o n meter measurement o f 8 a l o n g w i t h d a i l y R n d a t a t o e s t i m a t e d a i l y v a l u e s o f e v a p o t r a n s p i r a t i o n . - 30 -r~ i i 1 r a u t h o r s ( t a k e n from van B a v e l , 1967). Note t h a t the maximum e v a p o t r a n s p i r a t i o n r a t e L E m a x i s d e t e r m i n e d d i f f e r e n t l y f o r d i f f e r e n t a u t h o r s , and i n t h i s s t u d y i s t a k e n as the e q u i l i b r i u m r a t e . - 31 -.25 .20 .6 .10 .05 9 (cm3 cm"3) F i g u r e 13. LE/R n as. a f u n c t i o n o f v o l u m e t r i c water c o n t e n t . T h i s and F i g u r e 10 are r e l a t e d t h r o u g h the water r e t e n t i o n c u r v e f o r the s o i l , and a l l o w e v a p o t r a n s p i r a t i o n e s t i m a t e s from water c o n t e n t measurements t o be made. - 32 -D. Evapotranspiration estimates usinq R and i> data c r u n m A r e l a t i o n s h i p such as t h a t i n F i g u r e 10 can be used t o e s t i m a t e e v a p o t r a n s p i r a t i o n u s i n g measured v a l u e s o f d a i l y and i j ^ . T h i s approach r e q u i r e s o n l y one more measurement, t h a t o f t h a n th e e q u i l i b r i u m a p p r o a c h , and y e t i t a p p l i e s t o a wide range o f s o i l m o i s t u r e . F i g u r e 14 shows the s e a s o n a l t r e n d i n e v a p o t r a n s p i r a t i o n i n mm e s t i m a t e d from from F i g u r e 2 and d a i l y v a l u e s m • • J o f R n ( e x p r e s s e d i n mm), u s i n g t h e . r e l a t i o n s h i p b e t w e e n ,LE/R n and if; drawn i n Figure,. 10. V a l u e s o f - - : e v a p o t r a n s p i r a t i o n f o r days used t o o b t a i n F i g u r e 10 were used i n F i g u r e 14 f o r t h o s e days, and a r e shown as t r i a n g l e s t o d i s t i n g u i s h them from v a l u e s o f LE d e r i v e d from th e r e l a t i o n s h i p between LE/R and i> . W h i l e n m i t may be argued t h a t F i g u r e 10 s h o u l d a p p l y o n l y t o f i n e days of h i g h R n, i t can be seen i n F i g u r e 14 t h a t e v a p o t r a n s p i r a t i o n on days when R n i s low i s o n l y a s m a l l f r a c t i o n o f the summer t o t a l , and s m a l l . e r r o r s i n e s t i m a t i n g e v a p o t r a n s p i r a t i o n on days o f low demand are not c r i t i c a l f o r l o n g term p r e d i c t i v e e s t i m a t e s . V a l u e s o f R n and LE shown i n F i g u r e 14 a r e g i v e n i n t a b u l a t e d form i n Appendix I . - 3 3 -8 r and L E (mm) 01 -T 1 1 1 r IV, Rn 2 0 3 0 June i r j i_ 10 2 0 3 0 July 1974 10 2 0 August F i g u r e 14. D a i l y t o t a l o f R n and LE f o r June 17-August 14, 19 74. P o i n t s used t o o b t a i n F i g u r e 10 are shown as t r i a n g l e s . .R ' and LE are shown as mm wa t e r e q u i v a l e n t , - 34 -E. Water balance of the forest W h i l e t h e changes i n s o i l w a ter c o n t e n t shown i n F i g u r e 3 c o u l d be used t o check on t h e e s t i m a t e d v a l u e s o f e v a p o t r a n s p i r a t i o n , the s i g n i f i c a n c e o f such a comparison i s d o u b t f u l s i n c e no independent measurements o f d r a i n a g e were made. On the o t h e r hand, a water b a l a n c e c o u l d be o b t a i n e d u s i n g e v a p o t r a n s p i r a t i o n and s o i l water c o n t e n t v a l u e s , w i t h d r a i n a g e as the r e s i d u a l u s i n g t h e water b a l a n c e e q u a t i o n P = E + -A9 + D where P i s t h e p r e c i p i t a t i o n , A9 i s t h e change i n water c o n t e n t i n t h e 0-60 cm l a y e r o f s o i l , and D i s t h e ' d r a i n a g e . S u r f a c e r u n o f f was not o b s e r v e d a t t h e s i t e . Average d a i l y v a l u e s o f A 9 were o b t a i n e d from the c u r v e i n F i g u r e 3 f o r the e i g h t p e r i o d between d a t a p o i n t s . D a i l y v a l u e s o f P and E f o r the c o r r e s p o n d i n g p e r i o d s were averaged from F i g u r e s 3 and 14 r e s p e c t i v e l y , and average d a i l y v a l u e s o f d r a i n a g e were th e n c a l c u l a t e d . The water b a l a n c e components thus o b t a i n e d a r e p r e s e n t e d i n F i g u r e 15, and g i v e n i n t a b u l a r form i n Appendix I I . Note t h a t d r a i n a g e appears as an i n p u t as w e l l as an o u t p u t t o t h e r o o t zone. Whil e t h e n e g a t i v e d r a i n a g e - • 35 -~T i i 1 — — i ( 1 1 1 1 r Depth (mm) 16 • | 12 8 4 i 0 - 4 - 8 I2 | 8 [ Outputs Depth (mm) 0 - 4 Storage change - f t I 1 1 : I I U I T Precipitation ["^] Evapotranspiration-| | Residual J L I l_ 2 0 3 0 10 20 3 0 10 June July 1974. August F i g u r e 15- Water b a l a n c e 1 o f the 0-60 cm depth o f s o i l f o r June 17-August 14, 19 74. V a l u e s are d a i l y averages i n mm o f w a t e r . The r e s i d u a l term i s - d i s c u s s e d i n t h e t e x t . - 36 -c a l c u l a t e d f o r the p e r i o d J u l y 14-17 c o u l d e a s i l y be caused by s m a l l e r r o r s i n measurements o f p r e c i p i t a t i o n and s o i l water s t o r a g e f o r t h i s s h o r t p e r i o d , t h e r e i s an o b v i o u s t r e n d i n t h e r e s i d u a l o f t h e wa t e r b a l a n c e f o r d r i e r p e r i o d s t h a t i n d i c a t e s t h a t e v a p o t r a n s p i r a t i o n c o n t i n u e d a t p r o g r e s s i v e l y h i g h e r r a t e s t h a n would be p r e d i c t e d from s o i l m o i s t u r e d e p l e t i o n . S i n c e t h e r e i s l i t t l e e v i d e n c e t o i n d i c a t e t h a t r o o t s extended below the 4 5 cm l e v e l , t h e r e i s an apparent s o u r c e o f water which i s not t a k e n i n t o account i n a c o n v e n t i o n a l w ater b a l a n c e o f t h e r o o t zone. There i s some e v i d e n c e t h a t t h i s s o u r c e i s , i n p a r t , upward movement o f water i n t o t h e 0-4 5 cm l a y e r . F o r t h i s r e a s o n t h e water c o n t e n t a t t h e 6 0 cm l e v e l was i n c l u d e d i n c a l c u l a t i n g A6. I n s p e c t i o n o f t h e p r o f i l e o f s o i l m a t r i c p o t e n t i a l ( F i g u r e 16) i n d i c a t e s t h a t i n d e e d g r a d i e n t s e x i s t e d d u r i n g t h e d r i e r p e r i o d s t h a t would t e n d t o in d u c e f l o w upwards from below t h e 6 0 cm l e v e l . A p p l i c a t i o n o f . C a r e y ' s / law t o t h e s e g r a d i e n t s and t h e magnitude o f t h e n e g a t i v e d r a i n a g e v a l u e s g i v e s v a l u e s o f u n s a t u r a t e d h y d r a u l i c c o n d u c t i v i t i e s t h a t a r e r e a s o n a b l e f o r a c o a r s e s o i l . However, t h i s c o n d u c t i v i t y s h o u l d become p r o g r e s s i v e l y l ower as t h e apparent n e g a t i v e d r a i n a g e i n c r e a s e s . I t i s a l s o d i f f i c u l t t o - 37 -T 1 r~ i 1 1 1 ~T i~ r 20 30 10 2 0 3 0 10 2 0 June July 1974 August F i g u r e 16. S o i l w ater m a t r i c p o t e n t i a l a t f o u r depths f o r June 17-August 19, 1974, i n d i c a t i n g d i r e c t i o n o f g r a d i e n t s i n t h e v e r t i c a l a x i s . \f - 38 -b e l i e v e t h a t o v er h a l f o f t h e wa t e r t r a n s p i r e d d u r i n g t h e d r y p e r i o d i n . mid-August, as i n d i c a t e d by t h e water b a l a n c e , o r i g i n a t e d from the: n e a r l y impermeable l a y e r o f b a s a l t i l l below 60 cm. I t has been r e c e n t l y suggested t h a t t h e amount o f water c a p a b l e o f b e i n g s t o r e d i n t h e wood o f c o n i f e r s can be i m p o r t a n t i n the t r a n s p o r t o f water t h r o u g h t h e t r e e s (Running et al. , 1975). S i g n i f i c a n t amounts o f water may be r e l e a s e d by t h e wood when s u p p l y a t t h e r o o t s becomes i n a d e q u a t e , i n o r d e r tormeet e v a p o r a t i v e demand. T h i s i s i n a c c o r d w i t h some r e c e n t .experimental f i n d i n g s f o r Douglas- f i r (Lasso'ie, 1973 ) and White Oak ( H i n c k l e y and B r u c k e r h o f f , 1975). The f a c t t h a t water p o t e n t i a l s i n t h e t w i g s and r o o t s o f t h e t r e e s under s t u d y d e c r e a s e t h r o u g h o u t t h e d r y i n g c y c l e ( B l a c k et al. , 1975) i n d i c a t e s t h a t t h e t r e e s a r e l o s i n g water which has not e x p l i c i t l y been t a k e n i n t o account i n t h e water b a l a n c e . A l t h o u g h measurements o f t r e e water c o n t e n t were not made, i t i s r e a s o n a b l e t o s p e c u l a t e t h a t p r o g r e s s i v e l y more o f t h e " n e g a t i v e d r a i n a g e " term i n t h e water b a l a n c e i s due t o changes i n t r e e water s t o r a g e t h a n i n s o i l w a ter s t o r a g e . T h i s amounts t o over h a l f t h e d a i l y average e v a p o t r a n s p i r a t i o n i n t h e l a s t p e r i o d . I t i s e x p e c t e d t h a t t h e n e g a t i v e d r a i n a g e term would - 39 -b e g i n t o decrease under more extreme d r y i n g c o n d i t i o n s , c a u s i n g a more marked d e c r e a s e i n t r a n s p i r a t i o n . The water b a l a n c e a n a l y s i s s e r v e s t o i l l u s t r a t e t h e u s e f u l n e s s o f m i c r o m e t e o r o l o g i c a l measurements i n u n d e r s t a n d i n g t h e water regime o f a f o r e s t , and i n d e e d i t has p r o v i d e d some i n s i g h t i n t o t h e p r o c e s s o f water t r a n s p o r t t h r o u g h t r e e s i n a w a t e r - s t r e s s e d f o r e s t . I t i s q u i t e c o n c e i v a b l e t h a t w i t h a b e t t e r u n d e r s t a n d i n g o f t h e r e l a t i o n s h i p between s o i l m o i s t u r e and t h e p r o p o r t i o n o f t r a n s p i r e d water r e l e a s e d from s t o r a g e i n t h e wood, a r e a s o n a b l y a c c u r a t e water b a l a n c e c o u l d be p r e d i c t e d from e s t i m a t e s o f d a i l y , net r a d i a t i o n and p r e c i p i t a t i o n a l o n e . B e g i n n i n g w i t h a known c o n d i t i o n o f s o i l m o i s t u r e i n t h e e a r l y p a r t o f t h e season, e v a p o t r a n s p i r a t i o n f o r each day can be used, a l o n g w i t h an e s t i m a t e o f t h e p r o p o r t i o n o f t h e day's t r a n s p i r e d w ater t a k e n from t r e e stem s t o r a g e , t o c a l c u l a t e a v a l u e o f IJ f o r the next day. T h i s w i l l i n t u r n be used m J t o d e termine LE/R^ f o r t h e next day. I n t h i s way good h y d r o l o g i c a l p r e d i c t i o n s c o u l d be made from the s t r a i g h t -f o r w a r d measurements o f p r e c i p i t a t i o n and net r a d i a t i o n . CONCLUSIONS E v a p o t r a n s p i r a t i o n from a water s t r e s s e d Douglas f i r f o r e s t has been measured th r o u g h o u t t h e range o f s o i l m a t r i c p o t e n t i a l s from 0 t o -10.5 b a r s , and d a i l y v a l u e s o f LE/R have been c a l c u l a t e d f o r s e l e c t e d days. n J These v a l u e s have been used t o e x p r e s s a r e l a t i o n s h i p between e v a p o t r a n s p i r a t i o n , net r a d i a t i o n , and s o i l m a t r i c p o t e n t i a l t o be used t o p r e d i c t e v a p o t r a n s p i r a t i o n from r a d i a t i o n and s o i l w a ter d a t a . The f o l l o w i n g c o n c l u s i o n s can be drawn: 1. E v a p o t r a n s p i r a t i o n a t t h e e x p e r i m e n t a l s i t e i s n e a r l y e q u a l t o t h e e q u i l i b r i u m r a t e when s o i l water . i s f e l t not t o be l i m i t i n g . C a l c u l a t e d d a i l y v a l u e s o f LE/R n were s l i g h t l y l o w e r t h a n t h o s e a s s o c i a t e d w i t h e q u i l i b r i u m r a t e s even f o r f i n e days f o l l o w i n g r a i n , and i t i s seen from the d i u r n a l t r e n d i n LE t h a t t h e r e appears t o be a r e s t r i c t i o n t o t r a n s p i r a t i o n i n t h e a f t e r n o o n p e r i o d . T h i s r e d u c t i o n may be t h e r e s u l t o f low h y d r a u l i c c o n d u c t i v i t y near the r o o t s p r e v e n t i n g t h e m i d - a f t e r n o o n peak, i n LE obser v e d a t Haney. While i t appears t h a t t h e e q u i l i b r i u m r a t e would be a good e s t i m a t e o f e v a p o t r a n s p i r a t i o n when th e s o i l i s c o m p l e t e l y s a t u r a t e d , t h e s e r e s u l t s - 41 -suggest t h a t i t may s l i g h t l y o v e r e s t i m a t e t h e t r u e v a l u e even a t m a t r i c p o t e n t i a l s as h i g h as -0.5 bar i f the s o i l d i s p l a y s poor water r e t e n t i o n c h a r a c t e r i s t i c s . 2. The r a t i o LE/R n d e c r e a s e s t o a p p r o x i m a t e l y h a l f i t s maximum v a l u e as \\>. d e c r e a s e s t o -10 b a r s m at t h i s s i t e . The r e l a t i o n s h i p between LE/R n p r o v i d e s a s i m p l e approach t o e s t i m a t i n g e v a p o t r a n s p i r a t i o n from a water s t r e s s e d f o r e s t u s i n g o n l y d a i l y v a l u e s o f R and . The n m dependence o f LE/R on R i t s e l f i s f e l t t o be v n n o f l e s s e r i mportance when e s t i m a t i n g e v a p o t r a n s - : p i r a t i o n o v er t h e l o n g term. The r e l a t i o n s h i p of LE/R n t o such f a c t o r s as s p e c i e s , s o i l t y p e , p h y s i o l o g y and m a c r o - c l i m a t e has y e t t o be de t e r m i n e d . 3. The d e c r e a s e i n LE/R f o r t h e e x p e r i m e n t a l s i t e n r appears t o be immediate f o l l o w i n g a drop i n <Pm from s a t u r a t i o n . E v a p o t r a n s p i r a t i o n appears t o be more s e n s i t i v e t o a g i v e n change i n m a t r i c p o t e n t i a l a t the wet end o f t h e range t h a n a t t h e dry end. While t h e s o i l i s a t h i g h ip , changes i n s o i l w a ter s t o r a g e account f o r t h e d a i l y - 42 -average e v a p o t r a n s p i r a t i o n r a t e s r e a s o n a b l y w e l l . However, a t low i|> e v a p o t r a n s p i r a t i o n c o n t i n u e s a t a r a t e h i g h e r t h a n t h a t r e f l e c t e d i n the s o i l w a t er s t o r a g e change, i n d i c a t i n g a sou r c e o f water t h a t i s r e l e a s e d o n l y when s o i l w a ter i s not adequate. W h i l e some upward f l o w o f water from below the r o o t zone i s i n e v i d e n c e from m a t r i c p o t e n t i a l g r a d i e n t s , i t i s b e l i e v e d t h a t : r e l e a s e o f water s t o r e d w i t h i n t h e t r e e stems w i l l p r i m a r i l y account f o r t h e c o n t i n u i n g e v a p o t r a n s -p i r a t i o n under f a i r l y extreme d r y i n g c o n d i t i o n s . - 43 -REFERENCES B l a c k , T.A. and K..G. McNaughton. 1971. P s y c h r o m e t r i c apparatus, f o r Bowen r a t i o d e t e r m i n a t i o n over f o r e s t s . Boundary-Layer M e t e o r o l . 2: 246-254. B l a c k , T.A., P.A. Tang, C.S. Tan, J.R. C u r t i s and K.G. McNaughton. 19 74. Measurement t e c h n i q u e s used i n f o r e s t h y d r o m e t e o r o l o g y . F i n a l Report f o r t h e D i r e c t o r , P a c i f i c F o r e s t R e s e a r c h C e n t r e , Department o f t h e Environment, V i c t o r i a , B.C. B l a c k , T.A., C.S. Tan, J.R. C u r t i s and J.U. Nnyamah. 1975. F a c t o r s a f f e c t i n g e v a p o t r a n s p i r a t i o n o f an. u n t h i n n e d D o u g l a s - f i r f o r e s t . F i n a l R e p o r t t o t h e D i r e c t o r , P a c i f i c F o r e s t Research C e n t r e , Department o f t h e Environment, V i c t o r i a , B.C. D a v i e s , J.A. and C D . A l l e n . 1973. E q u i l i b r i u m , p o t e n t i a l , and a c t u a l e v a p o r a t i o n from cropped s u r f a c e s i n Southe r n O n t a r i o . J . A p p l . M e t e o r o l . 12: 649-657. Denmead, O.T. and I.C. M c l l r o y . 1970. Measurements o f n o n - p o t e n t i a l e v a p o r a t i o n from wheat. A g r i c .  M e t e o r o l . 7: 285-302. Denmead, O.T. and R.H. Shaw. 1962. A v a i l a b i l i t y o f s o i l w a ter t o p l a n t s as a f f e c t e d by s o i l m o i s t u r e c o n t e n t and m e t e o r o l o g i c a l c o n d i t i o n s . Agronomy J . 54: 385-390. Gardner, W.R. and C F . E h l i g . 1963. The i n f l u e n c e o f s o i l w a ter on t r a n s p i r a t i o n by p l a n t s . J . Geophys.  Res. 68: 5719-5724. H i n c k l e y , T.M. and D.N. B r u c k e r h o f f . 1975. The e f f e c t s o f drought on water r e l a t i o n s and stem s h r i n k a g e o f Quercus alba. Can. J . Bot. 53: 62-72. L a s s o i e , J.P. 1973. D i u r n a l d i m e n s i o n a l f l u c t u a t i o n i n a D o u g l a s - f i r stem i n response t o t r e e water s t a t u s . F o r e s t S c i e n c e 1 9 ( 4 ) : 251-255. L o p u s h i n s k y , W. and G.O. K l o c k . 1974. T r a n s p i r a t i o n o f c o n i f e r s e e d l i n g s i n r e l a t i o n t o s o i l w a ter p o t e n t i a l . F o r e s t S c i e n c e 20: 181-186. - 44 -McNaughton, K.G. 1974. A s t u d y o f t h e energy b a l a n c e o f a D o u g l a s - f i r f o r e s t . Ph.D. T h e s i s , Department o f S o i l S c i e n c e , U n i v e r s i t y o f B r i t i s h C olumbia, Vancouver, B.C. McNaughton, K.G. and T.A. B l a c k . 1973. A s t u d y o f e v a p o t r a n s p i r a t i o n from a D o u g l a s - f i r f o r e s t u s i n g the energy b a l a n c e approach. Water Resources Res. 9: 1579-1590. R i t c h i e , J.T. 1973. I n f l u e n c e o f s o i l water s t a t u s and m e t e o r o l o g i c a l c o n d i t i o n s on e v a p o r a t i o n from a c o r n canopy. Agron. J . 65: 893-897. Running, S.W., R.H. Waring and R.A. R y d e l l . 19 7^. P h y s i o l o g i c a l c o n t r o l o f water f l u x i n c o n i f e r s . O e c o l o g i a • 1 8 : 1-16. S t e w a r t , J.B. and A.S. Thorn. 197 3. Energy budgets i n a p i n e f o r e s t . Quart. J . R. Meteorol-. Soc. 99 :! 154-170. Tang, P.A., K.G. McNaughton and T.A. B l a c k . 1974. I n e x p e n s i v e diode thermometry u s i n g i n t e g r a t e d c i r c u i t components. Can. J . F o r e s t Res . 4 :.. 2 5 0-2 54 . van B a v e l , C.H.M. 1967. Changes i n canopy r e s i s t a n c e t o water l o s s from a l f a l f a induced.by s o i l water d e p l e t i o n . Agr. M e t e o r o l . 4: 165-176. Z a v i t k o v s k i , J . and W.K. F e r r e l l . 1968. E f f e c t o f drought upon r a t e s o f p h o t o s y n t h e s i s , r e s p i r a t i o n , and . t r a n s p i r a t i o n o f s e e d l i n g s o f two e c o t y p e s o f D o u g l a s - f i r . Bot. Gaz. '129: 346-350. - 45 -APPENDIX I Date 1974 R (mm) LE (mm) LE/R ^„ ( b a i —n n —m June 17 7.03 2 .69 . 38 -5 . 8 18 7 .27 2 .62 . 36 -6 . 5 19 7.29 2.99 .41 -7 . 3 20 7 . 41 2.45 .33 -7 .4 21 4.69 1.50 . 32 -7 . 6 22 6 .57 2 . 04 . 31 -8.1 23 5 . 44 1.69 . 31 -8 . 3 24 A at* _f_ a-* ft A A A rft 4a 4\ ,»? j». -8.4 25 6 .49 1.78 .27 -8 . 7 26 0.97 0 .27 .28 -9.4 27 4.66 1.33 . 28 -9 . 3 28 2 .84 0 .80 .28 -9 .-4 29 1.92 0 .55 .28 -9 . 3 30 3 .43 0 .98 .28 -9 . 3 J u l y 1 3.73 1.05 .28 -9 . 5 2 5 .58 1.56 .28 -9.6 3 4 .28 1.16 . 27 -9.9 4 4.73 1.28 .27 -10 .1 5 7 .40 2 .00 .27 -10.1 6 7 . 38 1.96 .26 -10. 3 7 6 .87 1. 79 .26 -10 . 4 8 6 .75 1.74 .26 -10 . 5 9 1.23 0 .32 .26 -10 . 6 10 1.76 ' 0 .45 .25 -10 . 8 11 4.60 1.38 .30 -8 . 8 12 3.40 1.09 .32 -7.6 13 6.61 2 .38 .26 -6 .1 14 5.62 2 .19 . 39 -4.9 15 3.03 1.27 .42 -3.9 16 0 . 84 0 . 36 .43 -3.4 17 3 .97 2 .38 .60 -0.1 18 3.72 2 . 22 . 59 -0.2 19 6 . 54 3 .55 .54 -0 . 2 20 6 .55 3.83 .58 -0 . 3 21 3 .32 1.94 .58 -0 . 3 2 2 3.69 2 .14 . 58 -0.3 23 4.99 2 . 89 .58 -0.4 24 5 .65 3 .14 .55 -0.4 25 5 .66 3 . 31 .58 -0.7 - 46 -Date 1974 J u l y 2 6 27 28 29 30 31 Aug. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 R (mm) LE (mm) 5 . 69 3 . 07 5 .77 3 .12 5 .67 3 .00 5.96 3 .16 5 . 39 2.80 5 . 82 2 .73 6 . 29 2.92 6 .51 2.98 6 .49 2 . 75 6.44 2 .84 5 .99 2 .55 6 .00 2 . 46 6.27 2 . 51 6 . 26 2 . 31 6 .11 2 . 29 5.68 2 . 04 5 .52 1.88 6.03 1.99 6.23 2.03 2.08 0.66 LE/R tjj ( b a r s ) -n ' Tin . 54 -0.9 . 54 -0 . 9 .53 T - l . 0 .53 -1.1 .52 -1.3 .47 -2 . 3 .46 -2.4 .46 -2 . 7 . 42 -3.0 .44 -3.2 .42 -3 . 6 .41 -4 . 2 . 40 -4 . 6 . 37 -5 . 0 . 37 -5.4 . 36 -6 .1 . 34 -6 . 7 .33 -7.1 .32 -7 . 5 . 31 . -7.9 - 47 -APPENDIX I I Water Balance Data D a i l y averages g i v e n i n mm P e r i o d P E A 0 June 20-June 2 6 June 2 6-J u l y 5 J u l y 5-J u l y 14 0.00 1.50 -0.90 1.19 1.19 0.00 5.50 1.48 3.27 JTn } y J"!} 12.33 1.34 14.80 J u l y 17 T U3" y i o 0-80 2 . 74 -7 . 32 J u l y 2 2 J T U ^ Y ll~ 0-00 3.09 -2.60 J u l y 2 8 ^Uly I8' 0.00 2.88 -2.14 Aug. 4 Aug. -4-Aug. 14 0 .00 2 .07 -0 .90 

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