Surface water storage influences streamflow signatures

Abstract. Extreme flow conditions in river discharge have far-reaching environmental and economic consequences. The retention of surface water in lakes, wetlands, and floodplains can potentially moderate these extreme flows by modifying the timing, duration, and magnitude of flow generation. However, efforts to characterize the impact of surface water storage on river discharge have been limited in geographic extent. In this analysis, a suite of hydrologic signatures, quantifying components of watershed flow regimes, was calculated from daily discharge at 72 gaged watersheds across the conterminous United States. Random forest models were developed to explain variability in six hydrologic signatures related to flashiness and high and low flow conditions. In addition to traditionally considered variables such as climate, land cover, topography, and geology, a novel remote sensing (Sentinel-1 & 2) approach was used to study the contribution of surface water storage dynamics to each signature's variability. While climate variables explained much of the variability in the hydrologic signatures, models for five of the six signatures showed an improvement in explanatory power when landscape characteristics were added. Automated variable selection is part of the modeling process and can be indicative of the relative importance of certain variables over others. When all variables were considered, four of the six signature models selected remotely sensed inundation variables. The amount of semi-permanent and permanent floodplain inundation, for example, was both negatively correlated with, and showed the greatest variable importance for wet season flashiness. Further, increases in seasonal floodplain inundation were positively correlated with increases in peak flows. This suggests that the storage of surface water on floodplains is relevant to both flashiness and high flow signatures. In addition, spatial variability in the amount of semi-permanent and permanent non-floodplain water helped explain variability in the baseflow index. These findings suggest that watershed surface water storage dynamics explain a portion of streamflow signature variability. The results underscore the need for protection and restoration of surface water storage systems, such as wetlands, across watersheds.