Do Human Activities Influence Reservoir based Hydrological Droughts?

Droughts pose enormous challenges to food security and freshwater availability across the globe. Reservoirs serve as a lifeline for drought mitigation, and as a prime source of irrigation water during extreme dry conditions. Despite the critical role reservoirs play during droughts, reservoir-based hydrological droughts have only been explored in a limited manner. Specifically, droughts based on reservoir storage levels and evaporative losses have not been accounted for at a global scale. Here, we use NASA’s new MODIS global water reservoir product, GRACE TWS, and GLDAS meteorological data to evaluate global reservoir-based hydrological droughts for 164 reservoirs located in different climatic zones and geographic settings. We introduce an Integrated Reservoir Drought Index (IRDI) that was developed using the concept of the copula, which incorporates the effects of reservoir storage and evaporation rates to effectively monitor reservoir-based hydrological droughts. From the observed climate data (2002-2020), we find that the frequency of reservoir based droughts is increasing in the western part of North America (NA), the eastern part of South America (SA), south-central Asia, and the majority of Africa, New Zealand, and Europe. Based on this information, we have reconstructed the drought characteristics (mean intensity, max intensity, max duration) using the IRDI for all of the 164 reservoirs during 2002-2020. We find that reservoirs in western Australia, southern Europe, southeast Asia, and the northern part of North America have gone through more prolonged droughts (of about 80-90 months, with a maximum intensity of about -2.6 to -3.2). We find contrasting trends of TWS anomalies and IRDI in south Asia, south Africa, eastern North America, and other places—which highlights the need for reservoir-based drought information (as TWS tends to be governed by groundwater changes, and does not necessarily give insights on reservoir droughts). Despite having high precipitation anomalies (wet conditions), reservoirs were mostly found to be under drought conditions, which points to the significant influence of human activities on reservoir-based droughts. Major factors contributing to this are inefficient water management policies and practices—especially during drought conditions—in different countries. Overall, our study highlights how human activities alter reservoir-based hydrological droughts, which has significant implications on sustainable and resilient water resources planning and management across the globe.