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Comparing Methods to Estimate Daily Evapotranspiration Over Chirgua River Basin, Venezuela, Using Landsat Satellite Images

DAILY EVAPOTRANSPIRATION,RIVER BASIN

The objective of this research was to compare methods to estimate daily evapotranspiration over the Chirgua River basin, Carabobo State, Venezuela, using satellite images. The processing of the data obtained was carried out through the application of the Surface Energy Balance Algorithm (SEBAL) and the use of measurements observed through meteorological monitoring stations. To carry out the research, it was necessary to have basic information collected from different public sources. Two satellite images of the Chirgua River basin were acquired from the EarthExplorer USGS website dated for January, 18^th, 2016 and April, 10^th, 2017. The information from meteorological stations was compiled from the databases of the National Institute of Meteorology and Hydrology (INAMEH). Data processing included the explanation of the procedure necessary for the application of temperature determination models for the estimation of daily evapotranspiration through the use of ArcGiS 10.0 Software. For the development of the research, the tools on model builder and satellite image processing contained in ArcGIS 10.0 were used. The estimation of evapotranspiration using the SEBAL method required the estimation of three main variables into the energy balance equation, that involved net radiation flux at the surface (Rn), soil heat flux (G), and sensible heat flux to the air (H). Mean values of these variables were found varying in the northern region (i.e. Rn (414 – 600 W/m²), G (35 – 194 W/m²) and H (0 – 56 W/m²)) while in the southern region –agricultural uses- (i.e. Rn (78 - 414 W/m²), G (88-116 W/m²) and H (356-1,278 W/m²)). Evapotranspiration resulted for the northern region varying between 0 and 6.3 mm/d while in the southern region between 0 and 3.24 mm/d. As second method, evapotranspiration was estimated based on the spatial interpolation through the Ordinary Krigging geostatistical model, using daily evapotranspiration observed values in 16 meteorological monitoring stations placed in sites around Chirgua basin. It was selected J-Bessel model from 11 available models. From the second method, evapotranspiration in the northern region resulted varying between 0 and 1.3 mm/d while in the southern region between 1.5 and 1.91 mm/d. A linear relationship was applied for estimating the adjustment between observations and estimates of evapotranspiration. The estimation of evapotranspiration through the SEBAL model provided an estimate that varied from one to three times the values observed in the field. It was directly affected by the proximity to the change of climatic season.