comparison of effective rainfall estimation methods in agriculture

the part of precipitation that directly meets the water requirements of a plant, is termed the effective rainfall. effective rainfall estimation methods consist of: direct measurement, empirical methods, and soil-water balance modeling. due to the high costs of measurements and storing devices such as lysimeter, empirical and balance methods are most commonly used to estimate the effective rainfall. in this paper, as well as introducing the main methods of effective rainfall estimation, the benefits, drawbacks, and applications of each method are discussed. based on the reported literature, the balance method, in spite of its need for exact data regarding the physical elements, after the direct measurement methods, has the most accuracy. the potential evapotranspiration ratio method is the most effective approach for basic projects. the renfrew equation because it is very empirical, is therefore not a suitable method. the usda method is also not appropriate for areas where the quantity of rainfall is more than the potential evapotranspiration rate. the highest effective rainfall amount is given by the usda method and the lowest level is given by the reliable method. in addition, the results of the empirical approaches, as well as the us soil conservation service, us department of agriculture and percentage method give similar results. in arid and semi-arid areas, the potential evapotranspiration to precipitation method and the soil conservation department of the united states provide the best results. in wet areas, the reliable method is chosen as the most appropriate approach to planning because it provides more realistic values.