Annual groundwater evapotranspiration mapped from single satellite scenes

SummaryEvapotranspiration (ET) data measured using micrometeorological equipment were obtained from three separate studies conducted in arid and semi-arid shallow groundwater environments in California, New Mexico and Colorado. These locations have great diversity in (1) climate expression: monsoon versus Mediterranean annual precipitation patterns and comparatively long versus short growing seasons; and in (2) vegetation cover: alkali scrub, shallow groundwater meadows, and monocultures of saltcedar and cottonwood. Actual ET (ETa), measured from 24 site- and year-combinations was paired with NDVI∗, a derivation of normalized difference vegetation index. NDVI∗ was extracted from corresponding locations in nine mid-summer Landsat TM5 and TM7 scenes during 1999–2002, with single mid-summer scenes used for estimation of annual total ETa. NDVI∗ was a competent estimator of a derivation of ET, ET∗, calculated by subtracting annualized precipitation and normalizing by an annualized reference ET, (ET0). This relationship was used to estimate annual total ETa, derived solely from remote sensing and weather data. Residual error decreased as ETa increased with well-balanced data scatter indicating lack of systematic error. These results demonstrate a simple and robust method that can be used to map annual ET as a first-order approximation from a single mid-summer satellite scene for any arid or semi-arid shallow groundwater environment where sufficient weather data exist for calculating annual precipitation and reference ET.