Estimation of evapotranspiration at different scales using traditional and remote sensing techniques

Water and its use or loss is critically important in the Southwestern United States where population growth is rapidly approaching the limit of available drinking water. It is therefore important to gain an understanding of water use by native and non-native species to ensure that sufficient water remains to maintain native ecosystems. This study examines water loss by the non-native tree species Tamarix ramosissima (saltcedar) at the leaf, branch and whole stand level using traditional methodologies as well as remote sensing. Transpiration measurements were estimated for open and closed stands of Tamarix ramosissima (saltcedar) at two sites within a desert riparian corridor on the lower Virgin River floodplain, southern Nevada. One site (open and closed stands) was within 10 meters of the river channel (River site), and the other site (open and closed stands) was more than 50 meters from the river channel near a Bowen ratio tower (Bowen site). At the leaf level, mid-morning stomatal conductances in trees from the River site were nearly three times higher than the Bowen site for all dates during the summer growing season. At the branch level, the results from sap flow measurements were not as clear-cut. While mean daily, accumulated sap flows were higher for the River site in comparison to the Bowen site, these differences were only significant for one date for each stand density. A comparison of the April 1994 and April 1996 remotely sensed data demonstrate the marked negative impact of a flood-induced channel diversion on downstream transpiration. Additionally, it was quite evident from the ET maps that even within apparently homogeneous closed stands there is a high degree of variability in transpiration.