Simplifying Daily Evapotranspiration Estimates over Short Full-Canopy Crops
2000
Ibá n ˜ ez, Manuel | Castellv ı ´, Francesc
A knowledge of daily evapotranspiration (λE) from surface radiative temperature at a regional scale is of interest in agronomy. The aerodynamic formulation and the radiative Bowen ratio–energy balance method provide good estimates of instantaneous λE based on remote-sensed canopy temperatures. Instantaneous latent heat flux estimates for the daytime period can be integrated to obtain a value for daily λE when detailed crop knowledge and continuous meteorological measurements are available. We propose a different method for estimating daily evapotranspiration for short, unstressed crops with a leaf area index (LAI) > 3. The method is based on the radiative Bowen ratio–energy balance method and on the similarity between the diurnal course of λE and solar irradiance. This regression-based approach uses continuous measurements of air vapor pressure, air temperature, surface radiative temperature, and solar irradiance during daylight hours. Moreover, detailed crop knowledge is not required. The method was tested in a Mediterranean area with semicontinental climatic characteristics on grass (Festuca arundinacea Schreb.), wheat (Triticum aestivum L.), and alfalfa (Medicago sativa L.). Daily λE was estimated with an error <15% compared with estimates of λE made using a Bowen ratio–energy balance equipment. This new approach is promising for remote sensing applications at a regional scale, although it requires further verification for other climatological conditions.
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