|DE Walle D R, - PENN STATE UNIVERSITY|
|Pionke H B,|
Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 22, 1994
Publication Date: N/A
Interpretive Summary: In order to control chemical losses from land to groundwater or streamflow it is critical to know from which land most storm runoff and soil drainage originates. Without this information, there is no practical or efficient way of focusing on the critical flow and chemical contributing areas within a landscape or watershed. This work addresses chemical and isotopic tracer use for delineating those critical areas that generate stormflow. Presently, researchers are developing combinations of methods to trace timing or position in the watershed. One particularly attractive combination has been to exploit the differences between different natural occurring components including stable isotopes. Specifically, our work points out that chloride, nitrate, silica and sulfate may also be useful tracers of mixing, dilution, agricultural land use, and springflow contribution to streams. The addition of these tracers to those hydrologic and isotopic tools we already have provide us with a systematic way of analyzing chemical losses and control at the watershed scale. These are the kind of tools extension, SCS and consultant engineers will use to better decide land use management and position on watersheds.
Technical Abstract: In order to increase understanding of hydrologic flowpaths, two- and three- component mixing models were used with conservative tracers to determine streamflow sources on a small agricultural catchment in central Pa. Three storms were studied during the 1989 fall recharge period with the first and largest storm having a 5-10 year return period rainfall. Continuous precipitation and streamflow records were augmented by observations and sampling of spring and seepage discharge as well as shallow wells and soil water. During the largest storm event, a two-tracer, three-component model separation using 0-18 and silica indicated that 50% of streamflow was shallow subsurface stormflow, 18% overland flow and/or channel precipitation, and 32% deep groundwater flow. Subsurface stormflow in this event was a mixture of infiltrated event water (26%) and pre-event subsurface water (74%) based upon analysis of fluctuations of 0-18, silica, ,Cl and NO3-N in soil water, shallow well and springflow samples during the event. Thus, two major pathways for transferring event water to streamflow existed for this event: overland flow (18% of streamflow) and shallow subsurface stormflow (13% of streamflow). In other storms analyzed, rainfall amounts were either insufficient to generate subsurface stormflow or subsurface stormflow water could not be isotopically or chemically distinguished from deeper groundwater. Early autumn storms occurring before subsurface water becomes well mixed appear to offer the greatest opportunity to differentiate shallow and deep subsurface pathways for streamflow generation. Event water contributed an average of 40% of streamflow, with peak contributions up to 58%, for the highest rainfall intensity event.