Seth N. Goldstein (Institute at Brown for Environment and Society (IBES), USA); Laurence Smith (Department of Earth, Environmental, and Planetary Sciences, Brown University, United States)

A growing body of research describes water security using quantitative indices combining
physical and government-reported human factors. Few of these indices are updated regularly at
the global scale given a variety of data availability challenges. Here, we show that the correlation
between remotely sensed SWOT seasonal reservoir level changes and regional Precipitation -
Evaporation (P - ET) offers a global measure of water availability independent of a traditional
report-based water security indices. Anticorrelation between changes in reservoir water level and
P - ET (negative Reservoir Water Balance Correlation or RWBC; Ryan et al., 2020) reflects
water resource management with operational independence from climatic water input, while
positive correlation (positive RWBC) signifies operational dependence on climatic water inputs.
We find that countries with negative RWBC have high water availability as measured by the
traditional Global Water Security Index (GWSI; Gain et al., 2016). Countries with positive
RWBC can have high or low water availability as measured by the GWSI, with differences
largely explainable by two readily obtainable metrics (remotely sensed P - ET and national GDP
per capita). Our study affirms the potential of satellite remote sensing for tracking reservoir
operations, and for enhancing insight into the water security of nations with limited data
availability.