Estimating watershed level evapotranspiration using water budget method

Background/Question/Methods

Rapid encroachment of Juniperus species into semiarid grassland is thought to substantially affect the water cycle primarily through altering evapotranspiration, the largest component in water budget in these ecosystems. Establishment of evergreen woody junipers in grasslands alters both the structure and phenology of grassland canopy, leading to change in both rates and temporal dynamic of evapotranspiration.  Water storage change in soil is normally treated as a negligible error term when calculating annual evapotranspiration at watershed level. However, temporal dynamic of water storage in soil is an aggregated response of the watershed to rainfall input and runoff and evapotranspiration output. Precisely quantifying temporal dynamic of water storage throughout the soil profile in association with stochastic precipitation and runoff may assist understanding the temporal dynamic of evapotranspiration. We selected two paired watersheds on the Oklahoma State University Cross Timber Experimental Range. One watershed was a grassland with moderate grazing and the other was heavily encroached by Juniperus virginana. Runoff for each watershed was measured using an appropriate-sized flume. Water storage in soil was calculated at each watershed since May 2009 using soil water content stations (n =3). Each station included 1 rain gauge and 4 EH₂O EC-5 soil water content sensors at 5, 20, 45 and 80cm to represent the entire soil profile which is about 100 cm in these two watersheds. 

Results/Conclusions

Here we presented preliminary results based on water budget method. The preliminary results indicated that there was substantial difference in the temporal dynamic of evapotranspiration responding to the stochastic precipitation between the grassland and the J. virginana encroached grassland.