Performance Evaluation of Different Methods for Estimation of Evapotranspiration in Pakistan’s Climate

Pakistan experiences a variety of climates ranging from a small humid patch in the north to extremely arid southern plains. Evapotranspiration is not a directly measurable element among agrometeorological parameters influencing crop growth and development. Many empirical and semi-empirical methods for estimation of crop evapotranspiration exist and are being used by individual scientists and researchers in Pakistan as per their claimed validations in other parts of the world. In this paper a comparison is made in reference with pan evaporation to test the performance of four widely used methods in the world under various climatic conditions of Pakistan. Although lysimeteric data are believed to be the best judge to assess the performance of any method yet the date are not available in continuous series at all location. The second best of choice is made on evaporation well documented in PMD archives from four Regional AgroMetrological Centers representing four major agro-climatic zones of Pakistan. FAO PenmanMonteith equation worked well in all the major climatic zones of Pakistan and may be adapted to draw closer and more realistic estimates of evapotranspiration for crop water requirements, various operational agricultural practices and water balance studies. Introduction Evapotranspiration process is the combination of two separate processes commonly known as Evaporation and Transpiration. In this process water is lost on the one hand from the top soil or water surface by evaporation and on the other hand from the crop plant tissues through transpiration by stomatal dynamics. Evaporation and transpiration occur simultaneously therefore there is no easy way of distinguishing between the two processes. Instead of water quantity in the topsoil, the evaporation from a cropped soil is mainly determined by the fraction of the solar radiation reaching the soil surface. When the crop is small, water is predominately lost by evaporation from the soil surface, but once the crop is well developed and completely covers the soil, transpiration becomes the main process (Allen et. al., 1996). Estimates of evapotranspiration provide an outlook of soil water balance in association with the amount of precipitation. Such estimates are of immense importance for calculation of water demand of the field crops and irrigation scheduling (Rasul, 1992). It also determines the nature of agro-climate a region has, agro-climatic potential of that region and suitability of crops or varieties, which can be grown successfully with the best economic returns (Rasul et al., 1993). Evapotranspiration is not easy to measure. Specific devices and accurate measurements of various physical parameters or the soil water balance in lysimeters are required to determine evapotranspiration. The methods are often expensive, complex, demanding in terms of accuracy of measurement and can only be fully exploited by well-trained research personnel. Many scientists developed mathematical equations to estimate * Pakistan Meteorological Department 2 evapotranspiration in different parts of the world but no one can be universally recommended and adopted. Sometimes error upto the unacceptable limits appear when applied in climates different than where they were originally developed. Pakistan is inherited by a variety of climates ranging from extremely arid to humid having similarities with desert to temperate climates. The local scientists generally apply the well-known methods believed to be giving good results in other parts of the world despite the fact that their accuracy is highly sensitive to climate. An effort has been made in this paper to compare the accuracy of various widely used methods under Pakistan’s climatic conditions and to identify the best suited method giving the more accurate approximation of this most important agro-climatic and agro-hydrological parameter. Materials and Methods The relationships developed in different parts of the world were often subject to rigorous local calibrations and proved to have limited validity. Testing the accuracy of the methods under a new set of conditions is laborious, time-consuming and costly. It has been stated earlier that Iysimeters data is the reliable tool to test the accuracy of a method but it should cover a period of years to incorporate possible climatic fluctuations on one hand and its installation, maintenance and accuracy of measurements are also another highly demanding aspect. All that was not economically and practically viable. Hence the evaporation from a standard pan (WMO class-A) exposed to the natural environment has been taken as reference to carry out this comparison. These pans are installed in the same enclosure where other meteorological parameters are recorded for estimation of evapotranspiration with the help of methods under comparison. Data The data on meteorological parameters like maximum and minimum temperatures (oC), relative humidity (%), wind speed (m/s) and sunshine duration on daily basis for the period of last seventeen years (1989-2006) were obtained from the records maintained by Meteorological Services of Pakistan. The evapotranspiration was calculated by each method for all the selected stations on daily as well as monthly basis. Evaporation from Class-A pan recorded at all the four stations was also considered for comparison. Evaporation from Class-A was taken as reference because same instrument was used at all the four places and uniform method for measurement was adopted (WMO1983). Evapotranspiration calculated by four methods under review was then compared with daily and monthly class-A pan evaporation values. The normal values based on 30 years average (except Tandojam for which 15 years average is taken) of rainfall (mm), relative humidity (%) as well as maximum and minimum temperatures (oC) for selected climatic zones are presented in the table. Except Rawalpindi, all the other three locations are deficient of rainfall. On the other hand, the temperatures are extremely high in summer which keeps the water balance mostly in negative.