Calculating crop production within the command areas of irrigation reservoirs at a global scale to support sustainable water management under climate change

Water reservoirs play a crucial role in water resources management, providing a range of benefits that contribute to societal, economic, and environmental well-being. One of the primary functions of reservoirs is to provide water for agriculture, which is essential for irrigating crops and sustaining agricultural productivity. About 70% of global freshwater withdrawals are used for irrigation, creating substantial demand and alterations for regional hydrology. A study conducted by the World Commission on Dams revealed that irrigation dams frequently fail to deliver the projected water supply for their command areas (i.e., the irrigated area extent and location associated with each individual irrigation reservoir), underscoring inefficiencies in reservoir management.

Several additional challenges are expected to affect reservoir management in the future. For example, climate change can alter the volume of stored water in the reservoirs, whereas land cover change can increase sediment delivery, reducing reservoir storage capacity. Pollution may trigger harmful algal blooms, and conflicting objectives such as balancing domestic water supply, hydropower generation, and irrigation demands can make reservoir operation even more complex. Therefore, reservoir management is inherently a multi-objective task that must account for future storage conditions while balancing various water demands to ensure their sustainable supply.

Many researchers have tried to map irrigated areas and the crops produced therein, but no current global dataset provides command areas and crop production information at the resolution of individual irrigation reservoirs at a global scale. As a result, there is a clear global need for continuous, high-resolution information on command areas and crop production linked to individual water reservoirs.

This study proposes a new framework to cope with the complexity of irrigation reservoir management through two different steps, including (i) the identification of command areas and (ii) the determination of crop production and water demand for each command area for all large irrigation reservoirs worldwide, totaling more than 21,000.  Command areas are estimated based on different criteria such as proximity to the reservoir, gravity-based water transfer (i.e., only downhill movement is allowed), irrigation and crop maps, exclusion of water bodies and urban areas, and slope considerations. Then, using crop production maps alongside the estimated command areas, geospatial analyses are applied to extract the total crop production within each command area. For each command area, the first step is to identify the different crop types harvested during the target year. Once the crop types are identified, the water requirements for all crops within the command area can be calculated. The annual water requirement for each command area then represents the sum of the water needed for all crop types grown within the command area during the target year.

Integrating crop production data with command area mapping allows for improved optimization of reservoir operations to balance irrigation needs with other competing uses such as hydropower generation, flood control, and environmental flows, as well as a better management of downstream irrigation production.