Abstract

Evapotranspiration (ET) was quantified for two rangeland vegetation types, aspen and sagebrush-grassland, over an 8-yr study period by comparing the following approaches for estimating ET: eddy covariance systems (EC, available for only 6 yr); soil water storage loss measured by time domain reflectometry (TDR) and neutron probe; and model simulation. The research site, the Upper Sheep Creek catchment in the Reynolds Creek Experimental Watershed, is part of a study on the effects of prescribed fire and vegetation removal. Estimates of seasonal ET for the aspen using EC with the turbulent fluxes adjusted to force energy balance closure, a 180-cm TDR soil profile, two 225-cm neutron access tubes, and the Simultaneous Heat and Water (SHAW) model agreed well with each other. If the two neutron probes were averaged, the RMSD of all approaches over the 6 yr was within 8% of the average. For the sagebrush-grassland, a 120-cm TDR profile underestimated seasonal ET for all years except the year immediately following the prescribed fire, when rooting depth likely had not recovered. A 195-cm neutron probe access tube located within 30 m of the stream underestimated ET for every year except when there was no streamflow, suggesting that lateral flow may have biased the results for this tube. A comparison of the other methods (EC flux adjusted to force energy balance closure, soil water loss measured from a 225-cm neutron access tube, and SHAW model simulation) agreed within 3% during the 6 yr with EC measurements at that site.

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