TY - JOUR ID - 35833 TI - A Comparative Study on Empirical Methods for Estimating Effective Rainfall for Rainfed Wheat Crop in Different Climates of Iran JO - Physical Geography Research JA - JPHGR LA - en SN - 2008-630X AU - Rahimi, Jaber AU - Bazrafshan, Javad AU - Khalili, Ali AD - Ph.D. Student of Agrometeorology, Meteorological Division, Dept. of Irrigation and Reclamation Engineering, College of Soil and Water Engineering, University of Tehran AD - Associate Prof., Meteorological Division, Dept. of Irrigation and Reclamation Engineering, College of Soil and Water Engineering, University of Tehran AD - Prof. in Meteorological Division, Dept. of Irrigation and Reclamation Engineering, College of Soil and Water Engineering, University of Tehran Y1 - 2013 PY - 2013 VL - 45 IS - 3 SP - 31 EP - 46 KW - Effective Rainfall KW - Empirical Methods KW - Iran KW - Rainfed Wheat Crop KW - Two-layer soil-water balance model DO - 10.22059/jphgr.2013.35833 N2 - Extended AbstractIntroductionAmong factors affecting crop production, especially rainfed crops, the rainfall and itsdistribution during crop season have a significant role. In addition, all the precipitation that fallsduring the growing season does not have the same effect on crop yield, and is not used inagriculture. So, quantification of Effective Rainfall (ER), as a portion of the precipitation whichis stored in plant root zone and meets the needs of evapotranspiration requirements in differentclimatic zones is an essential component of water resources in rainfed wheat areas. Effectiverainfall used in this study is “That portion of the total precipitation on the cropped area, during aspecific time period, which is available to meet the potential evapotranspiration requirements inthe cropped area. A precise estimation of effective rainfall is still needed not only for planningand management of rainfed wheat production, but also for risk management strategies in farms.Since a precise estimation of effective rainfall is necessary for increasing agriculturalproduction, major challenge is to design a soil-water balance model that provides more accuratecalculation of effective rainfall. The main goal of this study was to compare different effectiverainfall estimation methods for rainfed wheat. MethodologyIn this study, we adopted a two-layer soil–water balance (SWB) model. In the model,not onlythe portion of precipitation retained on root zone in current day is included, but a portion of theprevious day’s precipitation saved between the previous and current root-zone development isalso added to the effective rainfall of the current day. In the model, the soil reservoir is dividedinto two layers;1) an active layer in which roots are presented at any given time, t, and from which bothmoisture extraction and drainage could occur;2) immediately below the active layer, there is a passive layer of depth (maximum root depthrootdepth attained any day after sowing) from which only drainage would occur.Because of a high cost associated with direct measurements, estimate of effective rainfallcomponent is often based on empirical models. The aim of this study is to compare empiricalmethods of effective rainfall estimation with a proposed method based on soil-water balanceequation. Following, six methods have been used to calculate effective rainfall for 21 agrometeorologicalstations of Iran:• Renfro Equation method• U.S. Bureau of Reclamation method• Potential Evapotranspiration/Precipitation Ratio method• USDA-SCS method• FAO method• TR21/SCS methodFor this purpose, four groups of data (including weather data, phenological data, soilcharacteristics, and wheat yield data) were used relevant to the 21 agro-meteorological stationsrepresenting arid, semi-arid, semi-humid, and humid regions of the country. Before using theweather data for estimating effective rainfall, data reconstruction was performed using Normalratiomethod (where required).Results and DiscussionThe results of calculating the effective rainfall for rainfed wheat crop at the 21 agrometeorologicalstations, using selected methods and comparing the different methods ofestimating effective rainfall, showed that: 1) in spite of data limitations, the new procedure hadappropriate performance in estimation of that part of wheat yield which could only be explainedby effective rainfall. Therefore, this method can be used as an efficient tool in computer-basedprograms developed for agricultural risk management of rainfed area. 2) It has been observedthat the higher the values of de Martonne Aridity Index, the lower is value of “effectiverainfall/rainfall during the cropping season”. 3) The best result for arid and semi-arid climateswas obtained by PET/P method (d-index= 0.8), and for semi-humid and humid climates by FAOmethod (respectively 0.9 and 0.8), and USDA-SCS method (respectively 0.8 and 0.7).ConclusionA soil water balance model for estimating effective rainfall is applied for evaluating theaccuracy of six established effective rainfall estimation methods.  Renfro Equation provides an initial approximation based on aridity factor. The accuracy ofthis 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/PrecipitationRatio method takes the first approximation by runoff, soil and aridity factor. This method issuitable 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. Thismethod is suitable for those areas that have low intensity of rainfall and high infiltration rate. UR - https://jphgr.ut.ac.ir/article_35833.html L1 - https://jphgr.ut.ac.ir/article_35833_b7dc3c3b81e86772f9107222fdc416fd.pdf ER -