Statewide estimation of potential and reference evapotranspiration in Florida

A strategy is presented for estimating of potential and reference evapotranspiration (PET and RET, respectively) throughout the State of Florida at 2-km spatial scale and daily time scale from 1995 to the present. PET and RET are critical variables required for water management, both in hydrologic flow simulations and water allocation. However, existing information on PET and RET is often based on a small number of point measurements at micrometerological stations. Additionally, variations exist among water management entities in the assumed methodology for computation of PET and RET. The product resulting from this work will provide seamless and consistent estimates of PET and RET over all five State of Florida Water Management Districts. Data input for PET and RET equations will be derived from satellite- and land-based measurements, along with land cover mapping. A GOES satellite-based technique will be used to estimate spatial and temporal variations in solar insolation, a critical variable in RET calculations. Solar insolation also is the largest determinant of net radiation, a critical input to PET calculations. Land-based insolation, net radiation, and four-component radiation measurements in Florida will provide a cross-check on the algorithms used to process the satellite-derived radiation data. Additional meteorological data (air temperature, relative humidity, and wind speed) required for PET and RET calculations will be derived from the many State and Federal weather stations in Florida.

Albedo, surface resistance, and canopy resistance will be derived from the existing literature values, refined for Florida conditions based on historical and on-going field experiments. RET will be computed based on the American Society of Civil Engineers – 2000 method for a grass reference at daily resolution. PET will be computed by three methods (simple radiation-based approach, Priestley-Taylor, and Penman-Monteith) and the results will be compared to eddy correlation and Bowen ratio measurements of evapotranspiration under “well-watered” conditions to determine the “best” PET estimator.

This presentation will highlight the development of the GOES-based solar insolation processing, and the project goals.