Have irrigated areas actually expanded around new irrigation dams?

Irrigation will play a vital role in meeting future food demand. Irrigation reservoirs currently account for approximately 40% of global surface water used for irrigation and play a critical role in agricultural production by providing water to overcome scarcity, buffer against climatic variability, and enable enhanced yields and multi-cropping. Yet while there has been ongoing establishment of dams for irrigation across the globe, there is little understanding of the extent to which irrigation has actually expanded within their potential command areas (PCA) (i.e., the maximum area that can be irrigated using water from a particular irrigation dam) and whether irrigation dams have realized the full extent of irrigated agriculture that they can potentially support. To assess this, we combine georeferenced information on irrigation dams completed since 2000 with elevation data to delineate the extent of each dam’s reservoir globally and estimate the extent of each dam’s potential command area (N = 422 across 14 countries) under gravity-fed and pump-extended allocation scenarios. We then use remotely sensed global cropland data to assess changes in cropland extent after dam establishment, finding an average increase from 39% to 46% of PCAs occupied by cropland. Finally, we apply thresholds of dry-season vegetation indices to estimate the share of cropland in PCAs that was likely irrigated, with an observed increase of 36% to 72% over the study period. These findings suggest that – while countries have realized substantial gains in irrigated areas surrounding dams – a large fraction of irrigation command areas may be under-utilized. Our study thus highlights that simply establishing an irrigation dam does not automatically translate into the expansion of irrigated agriculture. As such, our findings demonstrate that irrigation dam projects must be accompanied by deliberate planning to develop the necessary supporting infrastructure (e.g., canals) and ensure sustained access to enabling technologies (e.g., irrigation pumps and dry-season seed varieties) and services to realize the full productive use of new potential command areas.