Remotely-sensed groundwater evapotranspiration from alkali scrub affected by declining water table

NDVI∗ is a remotely-sensed atmospherically-resistant form of the normalized difference vegetation index (NDVI), stretched from zero to one using satellite scene statistics. This green vegetation index, derived from mid-summer Landsat TM satellite data, was evaluated for its ability to predict the annual total evapotranspiration (ET) of alkali scrub vegetation in San Luis Valley, Colorado, when used as a scalar with an expression of the atmospheric driving force, ET0, known as grass reference evapotranspiration. The study site experienced a 2.5 m decline in depth to groundwater (DTW) during a 17-year period, with known decline of vegetation cover and ET. Estimates of the ET component from groundwater (ETg; estimated by subtracting annual total precipitation from annual total estimates of ET) were developed from NDVI∗ using single TM scenes for each year. The resulting estimates of ETg were compared to measurements generated by micrometeorologic methods, demonstrating that NDVI∗ underestimated ETg due to insensitivity to the component of direct soil-surface evaporation of groundwater, Eg. Logarithmic curves were fitted to the two data sets and the difference between these curves yielded an estimate of Eg that approximated two published values. These results provide mathematical curves to estimate ETg based upon DTW that are suitable for dynamic groundwater modeling applications for alkali scrub.