Location: Northwest Watershed Management Research
Title: Comparison of methods for estimating evapotranspiration in a small rangeland catchment Authors
Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 20, 2014
Publication Date: April 17, 2014
Citation: Flerchinger, G.N., Seyfried, M.S. 2014. Comparison of methods for estimating evapotranspiration in a small rangeland catchment. Vadose Zone Journal. 13:4. DOI: 10.2136/vzj2013.08.0152. Interpretive Summary: Reintroducing fire as part of the natural cycle to control invasive weeds and improve habitat on rangeland is becoming an accepted practice, however post-fire vegetation, water balance and streamflow responses are not well understood. This study compares several methods to quantify evapotranpiration before and after a prescribed fire conducted in the Upper Sheep Creek Watershed of the Reynolds Creek Experimental Watershed in the fall of 2007. The three methods used (eddy covariance, soil water measurements, and model simulations) all compared well with each other, demonstrating that we can accurately quantify evapotanspiration and plant water use within the waterhshed. This information will be used subsequently using model simulations to quantify the impact of prescribed fire on evapotranspiration and streamflow.
Technical Abstract: Evapotranspiration (ET) was quantified for two rangeland vegetation types, aspen and sagebrush/grassland, over an eight year study period by comparing several approaches for estimating ET: eddy covariance systems (EC, available for only six years); 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, RMSD of all approaches over the six years was within 7% 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. 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) compared within 3% during the six years with EC measurements at that site.