Estimating crop water deficit using the relation between surface-air temperature and spectral vegetation index

The crop water stress index (CWSI), developed at the USDA-ARS U.S. Water Conservation Laboratory, Phoenix, Arizona, is a commonly used index for detection of plant stress based on the difference between foliage and air temperature. Application of CWSI at local and regional scales has been hampered by the difficulty of measuring foliage temperature of partially vegetated fields. Most hand-held, airborne, and satellite-based infrared sensors measure a composite of both the soil and plant temperatures. The concept proposed here, termed the vegetation index/temperature (VIT) trapezoid, is an attempt to combine spectral vegetation indices with composite surface temperature measurements to allow application of the CWSI theory to partially-vegetated fields without knowledge of foliage temperature. Based on this approach, a new index [water deficit index (WDI)] was introduced for evaluating evapotranspiration rates of both full-cover and partially vegetated sites. By definition, WDI is related to the ratio of actual and potential evapotranspiration; in practice, WDI can be computed using remotely sensed measurements of surface temperature and reflectance (red and near-infrared spectrum) with limited on-site meteorological data (net radiation, vapor pressure deficit, wind speed, and air temperature). Both the VIT trapezoid and WDI concepts were evaluated using 1) a simulation of a two-component (soil and vegetation) energy balance model and 2) existing data from an experiment in an alfalfa field in Phoenix, Arizona. Results from both studies showed that the WDI provided accurate estimates of field evapotranspiration rates and relative field water deficit for both full-cover and partially vegetated sites.