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. Itrepresents the amount of precipitation available in the crop root zone to meet the needs of evapotranspiration. Thecurrent 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’sprecipitation saved between the previous and current root-zone development is added to the Pe of the current day. Tothis end, we used three groups of data (meteorological, phenological, and soil characteristics data) related to 21 agrometeorologicalstations representing arid, semi-arid, semi-humid, and humid regions of the country. The results ofthis study indicated that, in spite of data limitations, the new procedure performed appropriately in estimating thatpart of the wheat yield which could be explained by Pe only. Coefficients of determination (R2) between annualprecipitation and Pe ranged from 0.50 in the humid climatic zone to 0.82 in the arid climatic zone. Ultimately, usingannual precipitation data collected from 181 Iranian synoptic stations and its correlation with Pe, the first annual Pemap of Iran was produced.