Patterns of River Flow Regulation Across the World's Climate Regions

ABSTRACT Dams profoundly alter river systems, but broad‐scale comparisons of water storage relative to natural flow across different climate settings remain limited. The Impoundment Ratio (IR), defined here as reservoir storage capacity divided by mean annual river runoff, is a simple, first‐order metric that allows such comparisons. We calculated IR for 16 060 dams spanning the major Köppen climate regions using global datasets of reservoir storage and natural river discharge. Spatial and temporal patterns were analysed using linear mixed effects models and generalised additive models. Approximately one‐third of the dams had IR ≥ 1 year, indicating storage capacity equivalent to at least 1 year of mean annual inflow. IR differed significantly across climate regions, with highest values in arid and semi‐arid settings, where reservoirs often regulate flows in non‐perennial rivers. Variation between countries was also evident, although climate region explained more of the observed variation in IR than country. Median annual IR of dams commissioned each year between 1800 and 2020 oscillated between 0.5 and 1. As a first‐order, non‐process‐based metric, IR does not capture reservoir purpose, operating rules, or the temporal structure of inflow, and therefore cannot be used directly to indicate impacts or guide management decisions. Nevertheless, by integrating storage and natural inflow, IR highlights patterns in the magnitude of river impoundment that are not evident from other metrics. It can be used as an early warning tool to identify cases where more detailed, processes‐based assessments are needed.