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Evapotranspiration, surface conductance and water-use efficiency of two young hybrid-poplar plantations in Canada's aspen parkland

University of British Columbia

Hybrid poplar (HP) plantations established on former agricultural land in the aspen parkland of Canada have the potential to provide fibre, bio-energy and ecosystem services. The low precipitation and large summertime vapor pressure deficits in the aspen parkland raise questions about HP plantation water use and its effects on regional water supplies. In 2010 and 2011, I began using the eddy-covariance (EC) technique to measure CO₂, water vapor and sensible heat fluxes above two young HP plantations planted in 2009 (HP09) and 2011 (HP11) on clay loam Chernozemic soil located near Edmonton, AB and Winnipeg, MB, respectively. Measurements showed that both HP09 and HP11 shifted from carbon (C) sources to C sinks in the 3rd year of growth. EC measured evapotranspiration (E) and climate data were used to calculate bulk surface conductance (Gs) using the inverted Penman-Monteith (PM) equation and were compared to Gs estimates derived from a biophysical model that permits the partitioning of E into canopy transpiration (Ec) and evaporation from the soil (Es). Es was estimated using the equilibrium evaporation rate modified to account for soil moisture effects on Es using a soil water content based multiplier (f), and Ec was estimated using a canopy conductance (Gc) sub-model and the PM equation. Modelled half-hourly values of Gs showed excellent diurnal and seasonal agreement with EC-calculated Gs. Measured and modelled E also had excellent agreement, and using the Gs model, I was able to show the relative contribution of Ec and Es to E as the plantation grew. For example, in the 5th year of growth at HP09, measured and modelled E was 400 and 428 mm, respectively, of which 138 and 290 mm occurred as Es and Ec, respectively. Values of water use efficiency calculated as gross primary productivity divided by E, increased every year of growth and were similar at both sites. Results show Es dominates E during the first 2 years of HP growth and as Ec becomes increasingly dominant in the following years, E can exceed P, suggesting HP planted on highly productive agricultural soils in Canada’s aspen parkland can become water limited.

Text

2014-08-19

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/50050

Soil Science

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/