UBC Theses and Dissertations
Soil water regimes on a forested watershed Giles, Donald George
The quantitative evaluation of summer water stress over a range of sites on a forested watershed was accomplished by determining the soil water deficits occurring during the growing season. The relationship of soil water deficit to forest productivity was studied using site index, total stemwood volume and annual incremental stemwood volume per unit area to quantify forest productivity at each site. Two procedures were used for evaluating growing season soil water deficits. In the first procedure, deficits were calculated at each site during the growing seasons of 1980 and 1981 by summation of the weekly shortfalls of the actual transpiration rate (limited by available soil water storage) below the maximum transpiration rate (limited by net radiation). This required the determination of coefficients in the relationships of transpiration rate to available soil water storage, to net radiation, and to the evaporation rate of intercepted rainfall. These coefficients were calculated by weekly water balancing of evapotranspiration against measured precipitation plus measured soil water withdrawal from storage during periods when drainage, run-off and capillary rise were negligible. The second procedure for calculating growing season soil water deficits was by monthly water budgeting over the growing season of each year from 1964- to 1981, for which years the required climatological data was available. Maximum evapotranspiration calculated from the average daily net radiation was balanced against available soil water storage plus precipitation on a monthly basis, with carry over of unused soil water storage to the next month. Growing season soil water deficit variations between sites for 1980 and 1981 were found to be well correlated with forest productivity as quantified by site index and by total stemwood volume. Relative differences in soil water deficits between sites for the years 1980 and 1981 were thus concluded to be representative of the average relative site to site differences over the life of the stand. These conclusions were confirmed by comparing the average growing season soil water deficits over the years 1964 to 1981 with the average annual incremental stemwood occurring at a site over this period as determined by tree ring width measurements. Yearly incremental stemwood volume for 1964-81 was also found to be well correlated with growing season soil water deficit, although less variation of growth with deficit was apparent between years at a given site, than when comparing variation of growth with deficit between sites.
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