Evaluating Historical Impacts of Surface Reservoir Storage on Catchment Memory Across the US

Today there are tens of thousands of storage structures in the US ranging from Hoover Dam, with a capacity more than 34 million cubic meters, to small structures less than 2 m tall.  While there exists a myriad of water management tools that capture storage operations for local to regional systems, national and global scale hydrologic models struggle to incorporate this storage. Large scale earth system simulations generally exclude management operations or rely on generic operating policies due to lack of data.  Reservoir storage capacity is much more easily obtained and can tell us about the potential for regulation of a system, but without evaluating actual operations we can’t capture the actual influence of human storage on catchment dynamics.  Here we use the National Inventory of Dams to evaluate the evolution of total storage capacity across the US over the last century. Consistent with previous work we show spatial trends in storage volume relative to streamflow and storage purpose (i.e. flood control as opposed to water supply). To quantify the actual impact of operations on hydrologic regimes though, reservoir capacity is not sufficient. Therefore, we also assemble a dataset of reservoir inflows, outflows and changes in storage focusing on large reservoirs in the western US.  Using these timeseries we can isolate the historical regulation imposed by reservoirs and their impact catchment memory. Furthermore, we compare our historical observations to generic operating policies to evaluate how well storage dynamics are captured by existing models and the potential for these tools to over or underestimate reservoir impacts.