Sedimentation in Saudi Arabia's 574 reservoirs: Nationwide assessment using remote sensing and erosion modeling

Reservoir sedimentation poses a critical threat to water security by reducing storage capacity and impairing functionality. Despite its importance in water resource management, sedimentation remains poorly quantified at large scales. Here, we present the first nationwide assessment across 574 reservoirs in Saudi Arabia, with a total design capacity of approximately 2.58 billion cubic meters (BCM). We combine long-term water extent trends (1986-2024) from Landsat imagery with sediment yield and reservoir infill time estimates from the Revised Universal Soil Loss Equation (RUSLE). Analysis of 488 reservoirs (>0.1 million m³) with reliable records revealed a median annual water extent decline of -1.5% year^-1, with 63% (307 reservoirs) showing decreasing trends. Statistically significant reductions (p<0.05) were found in 30% (147 reservoirs), while only 3% (12 reservoirs) showed significant increasing trends. Furthermore, 26% (126 reservoirs) experienced ≥5 consecutive years of negligible water presence, suggesting severe sediment accumulation. Climate variability was not the dominant driver, as precipitation and evaporation trends were minor and poorly spatially correlated with water extent trends. Reservoir infill times from RUSLE (median 43 years) aligned reasonably with satellite-based estimates (median 66 years), with shorter infill times corresponding to stronger declines. Our estimates indicate that sedimentation may have reduced the total usable storage capacity of Saudi Arabia's reservoirs by approximately 32% to 1.77 BCM, although this does not account for potential sediment management interventions. Given government plans to construct 1,000 additional dams, effective watershed management, sediment removal, and sustainable design are vital for Saudi Arabia's water future.