20th Conference on Hydrology


Estimation of evaporative fraction and evapotranspiration from remotely sensed data using complementary relationship

Virginia Venturini, Universidad Nacional del Litoral, Santa Fe, Argentina; and S. Islam, G. Bisht, and L. Rodriguez

The knowledge of evapotranspiration (ET) or evaporative faction (EF), defined as the ratio between actual evaporation to available energy, is a key component in studies related to water and energy balances on Earth's surface, as well as many water and agricultural managements applications. In the last two decades many models have been developed to estimate ET for a wide range of spatial and temporal scales, and surface conditions. Most of these models are variations of Penman's (1948) and Priestley-Taylor's (1972) equations, while some of them have been taken advantage of complementary relationships like those proposed by Bouchet (1963) and Grager and Gray (1989). Ease of availability of land surface variables (e.g., temperature, vegetation index, etc.) at large spatial scales from remote sensing has led to development contextual approaches that exploit the relationship among heat fluxes, vegetation indexes and temperature. In many current approaches to estimate ET, air temperature is only available from ancillary sources. One of the most relevant advances introduced by Earth Observing System (EOS) satellites is the atmospheric profiles product derived from MODIS sensors onboard EOS-Terra and EOS-Aqua. MODIS's atmospheric profile product (MOD07/MYD07) provides several atmospheric parameters, for instance air and dew point temperature profiles. This newly available remote source of atmospheric data as well as the already existing and widely used surface temperature maps obtained from different satellites, open a new opportunity to revise the complementary relationship concepts that relate the actual rate of evapotranspiration (ET) and potential rate of evapotranspiration (Epot). We develop a new formulation to derive EF and ET maps from remotely sensed data without auxiliary relationships such as those relating a vegetation index and land surface temperature or site-specific relationships. The new equation is based on Grager and Gray's complementary relationship and Priestley-Taylor's equation. We propose to demonstrate the validity and robustness of spatially distributed maps of EF and ET obtained from a simple and scalable equation that is applicable to different surface wetness and vegetation conditions during clear sky days. A key advantage of working with the proposed model that includes a relative ET/Epot parameter, allows us to eliminate the use of wind functions and associated resistance parameterizations commonly used for ET calculation. By combining this, Grager and Gray complementary relationship and Priestley-Taylor equation we obtain a simple equation to estimate ET. This proposed ET and EF formulation will be applied and validated over the Southern Great Plains (SGP) region for several clear sky days.

extended abstract  Extended Abstract (468K)

Session 3, Hydrologic applications of satellite data, including GRACE, AMSR-E, TRMM and MODIS
Wednesday, 1 February 2006, 1:30 PM-4:30 PM, A403

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