Evaluation of techniques for estimating reference evapotranspiration for Jawa-Indonesia

Abstract

Due to the shortage of measured consumptive use of water for crops (i.e., evapotranspiration, ETc) in Indonesia, estimation of reference evapotranspiration (ETo) using climatological data is necessary in order to determine crop water requirements for irrigation planning and operation. -- The objectives of this study are to compare 24 methods of estimating ETo, and to adjust selected methods to the standard method recommended by the Food and Agricultural Organization (FAO) of the United Nations and to derive new empirical equations using regression analysis of published climatological data. This study uses 4156 monthly data values from 50 stations on Jawa. -- In this study, the methods are classified into 4 categories according to the number of climatic variables (i.e., from one to four) used in the equation. From each category, the most promising method is selected and the possibility of adjusting the method to give results closer to that of the standard method is investigated. In addition, simpler empirical equations for each category are derived using regression analysis. -- The methods which use a greater number of variables and where solar radiation is the dominant factor in estimating reference evapotranspiration gave the best results. All the non-standard methods required adjusting to give results that are, on average, equal to the standard method. The new empirical equations derived by regression analysis gave excellent results and are easy to use when compared to both the standard and nonstandard methods.