Estimation of effective precipitation for winter wheat in different regions of Iran using an Extended Soil-Water Balance Model

Estimated Effective Precipitation (Pe) in dryland areas is an essential element of water resource management. It represents the amount of precipitation available in the crop root zone to meet the needs of evapotranspiration. The current study compared different approaches for estimating Pe in different climatic zones of Iran. A two-layer soil– water balance (SWB) model was adopted based on the proposed approach in which a portion of the previous day’s precipitation saved between the previous and current root-zone development is added to the Pe of the current day. To this end, we used three groups of data (meteorological, phenological, and soil characteristics data) related to 21 agrometeorological stations representing arid, semi-arid, semi-humid, and humid regions of the country. The results of this study indicated that, in spite of data limitations, the new procedure performed appropriately in estimating that part of the wheat yield which could be explained by Pe only. Coefficients of determination (R2) between annual precipitation and Pe ranged from 0.50 in the humid climatic zone to 0.82 in the arid climatic zone. Ultimately, using annual precipitation data collected from 181 Iranian synoptic stations and its correlation with Pe, the first annual Pe map of Iran was produced.