Vulnerable waters confer watershed resilience

Abstract:

Scientific evidence is urgently needed on the importance of vulnerable waters – e.g., wetlands – to hydrologic resilience in order to slow down their degradation and destruction. Hydrologic resilience is the ability of a watershed to resist disturbances and recover its natural hydrologic functional traits [e.g., storage, fill and spill, connectivity, transit times, and the (a)synchronicity between precipitation (P) and discharge (Q)]. Hydrologic resilience can be measured by the degree to which Q is coupled with P (i.e., the slope of the P-Q correlation) – watersheds that store water and release it gradually to the stream have a high resilience(low P-Q correlation), whereas watersheds that do not store water and release it quickly to the stream have low resilience (high P-Q correlation). The importance of wetlands to hydrologic resilience has been acknowledged, but there is no scientific evidence of how they influence hydrologic resilience at national (policy relevant) scales. We present a novel framework that quantifies and compares the hydrologic values of wetlands of different types (surface/subsurface), locations (far or near to stream), and connectivity (low versus high connectivity). We consider factors that influence the way a watershed collects, stores and releases water, and links P to Q, including: (1) “distal factors” [e.g., water availability as indicated by the theoretical energy and water limits on the watershed’s water balance, and whether the P and Q are in phase (synchronous) or out of phase (asynchronous)]; and (2) “proximal factors” [e.g., the potential volumetric storage capacity of a wetlands, the residence time of water stored within it, the location of a wetland to a nearby stream, and its hydrological connectivity to the stream]. We apply this framework to explore the relationships between wetlands and P-Q correlations to minimally disturbed watersheds throughout the conterminous United States to understand the importance of wetlands to the hydrologic resilience. Our study shows complex relationships exist between vulnerable waters and hydrologic resilience. Despite this complexity, vulnerable waters – covering a broad range of hydrological functional traits – are essential to conferring hydrologic resilience.