Satellite-Based Potential and Reference Evapotranspiration

A potential evapotranspiration (PET) and reference evapotranspiration database was created at a 2 km grid scale and a daily time scale from 1995 to 2004 for the entire state of Florida. While some PET methods require only temperature data, these methods are not suitable for the humid and cloudy conditions typically found in the southeastern United States. In Florida, the determination of PET requires direct measurements of net radiation or net radiation derived from measured incoming solar radiation. However, as the ground-based network of radiation instrumentation is extremely sparse or non-existent from 1991 to present, alternate methods to determine spatially distributed estimates of solar radiation must be used. The primary component used in estimating incoming solar insolation is 30-min visible-channel data from Geostationary Operational Environmental Satellites (GOES)-8 and 12, along with radiative transfer modeling, provided by a parallel research initiative.

This presentation details the framework in which the GOES solar radiation product was applied. The Priestley-Taylor method was selected as the most appropriate methods for Florida following a comparison of measured and modeled data at 18 eddy flux and Bowen ratio validation sites. The sites were distributed throughout the Florida and represent a variety of land cover types: open water, marshland, grassland/pasture, citrus and forest. A four component method was shown to provide best estimates of net radiation. While a GOES albedo product was identified as a promising approach, it was determined not to be viable for operational purposes. The method is applied using two albedo values determined from the measured values for land (0.149) and for water (0.062) is recommended for use in Florida net radiation calculations. Net radiation approaches were compared using a database from eleven sites in central Florida having two years of 15 minute data measured by Kipp & Zonen CNR1 four channel radiometers.

The final reference ET and potential ET dataset available for the water resource managers in State of Florida Water Management Districts are presented here in a summary format and compared to prior state ET data resources.