Water scarcity is a threatening problem to the sustainability of agricultural production in many world regions. Wheat is a strategic and dominant crop and the most extensive irrigation water user in Iran. Hence, understanding wheat's "crop yield-water" relations across the country is essential for sustainable production and policy directions for future investments.
Using a calibrated hydrologic model, we modeled irrigated and rainfed wheat yield and consumptive water use (ET) with uncertainty analysis at a subbasin level in Iran. Simulated wheat yield and ET were used to calculate crop water productivity (CWP). The larger the CWP, the more suitable a region for investments.
The model was then used to analyze the impact of several stated policies to improve the agricultural system in Iran. These included increasing the quantity of cereal production through more efficient use of land and water resources, improving soil moisture conservation and retention activities, and optimizing fertilizer application.
Our analysis of the water use ratio to internal renewable water resources revealed that 23 out of 30 provinces were using more than 40% of their water resources for agriculture. Twelve provinces reached a ratio of 100% and even greater, indicating severe water scarcity and groundwater resource depletion. An analysis of the Yield-CWP relationship showed that one unit increase in rainfed wheat yield resulted in a lesser additional water requirement than irrigated wheat, leading to a larger improvement in CWP. The inference is that better water management in rainfed wheat, where yield is currently small, will lead to a larger marginal return in the consumed water.
An assessment of improved soil available water capacity (AWC) showed that 18 out of 30 provinces are more certain to save water while increasing AWC through proper soil management practices. Therefore, strategic planning in the national agricultural production and food trade to ensure sustainable water use is needed. This study laid the basis for a systematic analysis of the potential for improving regional and national water use efficiency. Furthermore, the methodology used in this research could be applied to other water-scarce countries for policy impact analysis and the adoption of a sustainable agricultural strategy.
A comparison of irrigated and rain-fed yield and crop water productivity in different regions of Iran based on the average annual (1990–2002) data. Provinces with large crop water productivity must be considered first for future investments in wheat production.