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

Abstract

Summary 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, ET 0 , 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 (ET g ; 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 ET g were compared to measurements generated by micrometeorologic methods, demonstrating that NDVI ∗ underestimated ET g due to insensitivity to the component of direct soil-surface evaporation of groundwater, E g . Logarithmic curves were fitted to the two data sets and the difference between these curves yielded an estimate of E g that approximated two published values. These results provide mathematical curves to estimate ET g based upon DTW that are suitable for dynamic groundwater modeling applications for alkali scrub.