A satellite-based Daily Actual Evapotranspiration estimation algorithm over South Florida

Water resources and agricultural applications require the knowledge of evapotranspiration (ET) over a range of spatial and temporal scales. Due to paucity of surface based hydro-meteorological stations, the spatial resolution of ET estimates is fairly coarse and is not particularly suitable or reliable for hydrologic modeling, water resources planning and decision making. An ET estimation algorithm has been developed by combining data from satellite and ground observations. The method extends the applicability of a commonly used energy balance formulation of ET and utilizes the contextual relationship between remotely sensed surface temperature and vegetation index. The required parameters are derived from the Advanced Very High Resolution Radiometer (AVHRR) aboard NOAA-14 satellite. First, the Evaporative Fraction (EF) is estimated by utilizing the relationship between a vegetation index and radiometric surface temperature observed from AVHRR for each day. Then spatio-temporal interpolation and filtering techniques are applied to obtain daily EF values for cloudy pixels to produce the EF map for the entire region. Daily Actual ET (DAET) maps are derived from these EF maps and net radiation maps obtained from ground-based observations. The comparisons between satellite derived DAET and ground measurements showed overall low bias and root-mean-square-error for both clear and cloudy days at South Florida in 1998 and 1999. The proposed satellite-based DAET (SatDAET) algorithm has its EF component primarily estimated from satellite data and the resulting DAET has been validated using multi-year ground observations over the South Florida region. The SatDAET algorithm appears to be robust and has the potential to provide near real-time land surface evapotranspiration monitoring over large heterogeneous areas at a very fine spatial and temporal resolution.