A Comparative Study on Empirical Methods for Estimating Effective Rainfall for Rainfed Wheat Crop in Different Climates of Iran

Among factors affecting crop production, especially rainfed crops, the rainfall and its distribution during crop season have a significant role. In addition, all the precipitation that falls during the growing season does not have the same effect on crop yield, and is not used in agriculture. So, quantification of Effective Rainfall (ER), as a portion of the precipitation which is stored in plant root zone and meets the needs of evapotranspiration requirements in different climatic zones is an essential component of water resources in rainfed wheat areas. Effective rainfall used in this study is “That portion of the total precipitation on the cropped area, during a specific time period, which is available to meet the potential evapotranspiration requirements in the cropped area. A precise estimation of effective rainfall is still needed not only for planning and management of rainfed wheat production, but also for risk management strategies in farms. Since a precise estimation of effective rainfall is necessary for increasing agricultural production, major challenge is to design a soil-water balance model that provides more accurate calculation of effective rainfall. The main goal of this study was to compare different effective rainfall estimation methods for rainfed wheat. In this study, we adopted a two-layer soil–water balance (SWB) model. In the model,not only the portion of precipitation retained on root zone in current day is included, but a portion of the previous day’s precipitation saved between the previous and current root-zone development is also added to the effective rainfall of the current day. In the model, the soil reservoir is divided into two layers; 1) an active layer in which roots are presented at any given time, t, and from which both moisture extraction and drainage could occur; 2) immediately below the active layer, there is a passive layer of depth (maximum root depthroot depth attained any day after sowing) from which only drainage would occur. Because of a high cost associated with direct measurements, estimate of effective rainfall component is often based on empirical models. The aim of this study is to compare empirical methods of effective rainfall estimation with a proposed method based on soil-water balance equation. Following, six methods have been used to calculate effective rainfall for 21 agrometeorological stations of Iran:• Renfro Equation method • U.S. Bureau of Reclamation method • Potential Evapotranspiration/Precipitation Ratio method • USDA-SCS method • FAO method • TR21/SCS method For this purpose, four groups of data (including weather data, phenological data, soil characteristics, and wheat yield data) were used relevant to the 21 agro-meteorological stations representing arid, semi-arid, semi-humid, and humid regions of the country. Before using the weather data for estimating effective rainfall, data reconstruction was performed using Normalratio method (where required). The results of calculating the effective rainfall for rainfed wheat crop at the 21 agrometeorological stations, using selected methods and comparing the different methods of estimating effective rainfall, showed that: 1) in spite of data limitations, the new procedure had appropriate performance in estimation of that part of wheat yield which could only be explained by effective rainfall. Therefore, this method can be used as an efficient tool in computer based programs developed for agricultural risk management of rainfed area. 2) It has been observed that the higher the values of de Martonne Aridity Index, the lower is value of “effective rainfall/rainfall during the cropping season”. 3) The best result for arid and semi-arid climates was obtained by PET/P method (d-index= 0.8), and for semi-humid and humid climates by FAO method (respectively 0.9 and 0.8), and USDA-SCS method (respectively 0.8 and 0.7). A soil water balance model for estimating effective rainfall is applied for evaluating the accuracy of six established effective rainfall estimation methods. Renfro Equation provides an initial approximation based on aridity factor. The accuracy of this method is very low and it is exclusively empirical. USBR method considers only the runoff. The accuracy of this method is low and is not suitable for wide application. PET/Precipitation Ratio method takes the first approximation by runoff, soil and aridity factor. This method is suitable for preliminary plans, and is more effective than other methods, nearly in all regions. USDA- SCS method takes the first approximation of soil and crop beside aridity factor. This method is suitable for those areas that have low intensity of rainfall and high infiltration rate.