Storms release old water from hillslopes

Posted on June 4, 2011

Although it’s known that subsurface water movement from hillsides can make significant contributions to surface stream flows, aquatic connections between slopes and streams have been difficult to quantify because the presence of groundwater in the intervening riparian zone tends to obscure flows arriving from the hillslope.   Oregonscientists dealt with this problem by working in an area where the riparian zone had been removed by debris flows.  They used a variety of techniques to measure the time scales of background water flows and those associated with 18 storm events.  They found that although there was a rapid runoff response from the catchment during storms, the subsurface water arriving in the valley stream was dominated by “old” pre-event water.  Most of this was deep baseflow water with an average transit time of 1-2 years.  At times when the slope was saturated from previous rain events, baseflow water was supplemented by younger water that had passed vertically through the soil over the previous 10-25 days.  A third source of water, from storm events, had a transit time of 10-30 hours, and tracer experiments showed that during storms the subsurface contribution to the stream included water that had travelled a long way down the hillslope.  A minimum threshold rainfall was needed to start subsurface runoff from the hillside but this threshold varied depending on the wetness of the catchment and on the properties of the soil and bedrock. 

Reference:  McGuire, K. J., and J. J. McDonnell, J.J.  2010.  Hydrological connectivity of hillslopes and streams: characteristic time scales and nonlinearities.  Water Resources Research 46, W10543, doi:10.1029/2010WR009341.