| Subsurface runoff dominates the hydrology of many steep humid regions、and yet the basic elements of water storage、collection and discharge are still poorly understood at the watershed scale. Here we use rainfall and runoff records from two Northern California watersheds (~100 km2) with distinct wet and dry seasons to develop a new conceptual framework for storage-discharge. Stream hydrographs from 17 sub-watersheds through the wet season suggest a pronounced “storage excess” pattern; i.e. where 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 capacity. To estimate how much of water a watershed can store、we conducted a water balance analysis to estimate total storage changes (dV) during a rainy season. We analyzed also the streamflow recession characteristics、from which we estimated dynamic storage changes (dS). The estimated dV and dS from the 17 sub-watersheds in the study areas showed diagnostic patterns whereby dV and dS were themselves negatively correlated and that the sub-watersheds with steeper hillslopes tended to have a larger dV and smaller dS. We explain these patterns with a simple storage-discharge relationship that reflects storage excess dynamics、and offer this as a new framework for assessing runoff dynamics in upland humid watersheds where subsurface flow predominates. |
