Analysis of Surface Water Trends for the Conterminous United States Using MODIS Satellite Data, 2003–2019.

Satellite imagery is commonly used to map surface water extents over time, but many approaches yield discontinuous records resulting from cloud obstruction or image archive gaps. We applied the Dynamic Surface Water Extent (DSWE) model to downscaled (250‐m) daily Moderate Resolution Imaging Spectroradiometer (MODIS) data in Google Earth Engine to generate monthly surface water maps for the conterminous United States (US) from 2003 through 2019. The aggregation of daily observations to monthly maps of maximum water extent produced records with diminished cloud and cloud shadow effects across most of the country. We used the continuous monthly record to analyze spatiotemporal surface water trends stratified within Environmental Protection Agency Ecoregions. Although not all ecoregion trends were significant (p < 0.05), results indicate that much of the western and eastern US underwent a decline in surface water over the 17‐year period, while many ecoregions in the Great Plains had positive trends. Trends were also generated from monthly streamgage discharge records and compared to surface water trends from the same ecoregion. These approaches agreed on the directionality of trend detected for 54 of 85 ecoregions, particularly across the Great Plains and portions of the western US, whereas trends were not congruent in select western deserts, the Great Lakes region, and the southeastern US. By describing the geographic distribution of surface water over time and comparing these records to instrumented discharge data across the conterminous US, our findings demonstrate the efficacy of using satellite imagery to monitor surface water dynamics and supplement traditional instrumented monitoring. Plain Language Summary: Daily satellite images can be used to create cloud‐free maps of surface water for most of the United States (US). An analysis of these maps over multiple decades reveals surface water change in large‐scale water features such as lakes, reservoirs, and major rivers. Results show that increasing surface water trends are more common across the central US while decreasing trends are present across much of the western and eastern regions over the past 17 years. More specifically, we observed certain seasonal trends including increasing spring flooding along the Mississippi River. Comparing these data to trends in streamgage discharge provides additional insights into certain regions where surface water changes are more directly related to dynamic surface water flows. Results from this study identify national and regional surface water variability over space and time that can contribute to more effective water resource management. Key Points: Monthly Moderate Resolution Imaging Spectroradiometer maps can be used to calculate and compare surface water area trends across US ecoregionsThe central United States (US) underwent an increase in surface water while most of the western/eastern US underwent a decline from 2003 through 2019Monthly trends in surface water were congruent with monthly trends in discharge for the majority of Level III Ecoregions in the US [ABSTRACT FROM AUTHOR]