How much water can a watershed store

Subsurface runoff dominates the hydrology of many steep humid regions, and yet the basic elements of water collection, storage, and discharge are still poorly understood at the watershed scale. Here, we use exceptionally dense rainfall and runoff records from two Northern California watersheds (~100 km2) with distinct wet and dry seasons to ask the simple question: how much water can a watershed store? Stream hydrographs from 17 sub-watersheds through the wet season are used to answer this question where we use a simple water balance analysis to estimate watershed storage changes during a rainy season (dV). Our findings suggest a pronounced storage limit and then ‘storage excess’ pattern; i.e. the watersheds store significant amounts of rainfall with little corresponding runoff in the beginning of the wet season and then release considerably more water to the streams after they reach and exceed their storage capacities. The amount of rainfall required to fill the storages at our study watersheds is the order of a few hundred millimeters (200–500 mm). For each sub-watershed, we calculated a variety of topographic indices and regressed these against maximum dV. Among various indices, median gradient showed the strongest control on dV where watershed median slope angle was positively related to the maximum volume of storage change. We explain this using a hydrologically active bedrock hypothesis whereby the amount of water a watershed can store is influenced by filling of unrequited storage in bedrock. The amount of water required to activate rapid rainfall–runoff response is larger for steeper watersheds where the more restricted expansion of seepage from bedrock to the soil limits the connectivity between stored water and stream runoff. Copyright © 2011 John Wiley & Sons, Ltd.