Cumulative Effects of Small Reservoirs on Streamflow in Northern Coastal California Catchments

With small reservoirs increasingly employed to meet human water needs, tools that consider cumulative effects of multiple small reservoirs through space and time are essential for understanding impacts of these spatially distributed stresses on catchment hydrology and related ecological processes. We used a GIS-based hydrologic model to predict streamflow impairment caused by 438 small reservoirs in a 743 km(2) study area in Sonoma County, California, USA. The GIS model was designed to consider the varying effects that these reservoirs have through the project area drainage network, as well as the varying effects they have over time (as reservoirs fill and no longer impair flow downstream). Results indicate that, at the onset of the water year (when reservoirs are assumed to be empty), more than 25 % of the drainage network below reservoirs is impaired by over 50 %. Nine weeks into a normal-type water year, approximately 25 % of the drainage network below reservoirs is impaired by at least 25 %; and at 15 weeks, five percent of the drainage below reservoirs is impaired at least 25 %. Impairment is more persistent in a dry-type year. Nine weeks into a dry year, almost 40 % of the drainage network below reservoirs is impaired by at least 25 %; and at 15 weeks, 25 % of the drainage is impaired by at least 25 %. Results illustrate that impairment caused by reservoirs varies appreciably over space, but as reservoirs fill over time, impairment is lower through most of the drainage network.