Analysis of Spatial and Temporal Variations in Crop Water Productivity of the Rainfed Wheat for a Regional Scale Analysis

Water productivity is a suitable indicator in water potential analysis at a location. Hence in this study, changes in water use and productivity are studied in spatial and temporal scale simultaneously. Temporal changes in water productivity in Hamedan region (Iran), was determined with Standard Precipitation Index (SPI). To estimate regional water productivity, GIS and Kassam method were coupled to estimate the potential yield of rainfed wheat. Thus, layers of radiation and temperature were created using geo-statistical subprogram in GIS environment and subsequently, potential yield for the entire area were determined. To determine actual evapotranspiration, the region was discrete into homogeneous areas using Thieson method in Arcinfo software, and potential evapotranspiration of the crops were calculated by Cropwat software. Using the method recommended by FAO, these values were converted to actual evapotranspiration. The map of spatial coverage of the water productivity was prepared for the whole region by overlapping the two layers: the potential yield and the actual evapotranspiration. Results of this study indicate that the combined analysis of spatial and temporal characteristics is advantageous. This can help in carrying an effective analysis and results in a broader knowledge of the indicators under consideration. Amount of water productivity in drought condition in comparison with the other two conditions was higher and from geographical point of view, the southern parts of the region have higher potential production when compared to other locations of the province. The analysis shows that the variation in amount of active radiation received by the earth surface is causing these differences.