The Class A pan coefficient to estimate the reference evapotranspiration for the Tumbaco valley
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Abstract
The precise determination of the Class A pan coefficients allows to estimate the reference evapotranspiration and real values of crop water requirements. Daily and monthly climatological data was used to determine the pan coefficient (Kp) and the reference evapotranspiration (ETo). The models evaluated to determine Kp were: Allen – Pruit, Cuenca, Pereira, Snyder and the computed coefficient. The following indices were used to evaluate the cross analysis between the models to compute ETo: absolute mean error, concordance coefficient and the coefficient of determination. The results show that: i) for daily data, the Allen - Pruitt and Snyder contrast presented the best indices; ii) for monthly data, the Snyder and the computed coefficient contrast presented indices with higher accuracy; iii) the maximum average ETo was registered in August with 139.32 mm and the minimum in April with about 76.54 mm. The main conclusions are: i) for monthly average climatological data, the Snyder and the computed method can be used to determine Kp and the Snyder method for daily data and ii) the standard Penman Monteith FAO 56 method presented poor inferential indices for daily data and better indices for monthly data, considering the environmental conditions of the Tumbaco valley.
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