VARIABLE SOURCE AREA HYDROLOGY APPLICATIONS TO WATER QUALITY PROTECTION AND RISK ASSESSMENT: IDEAS FOR BRIDGING SCIENCE AND PRACTICE

New ideas are needed to bridge the gap between hydrological science and applied water quality protection, especially with respect to variable source area (VSA) hydrology. Most hydrological models predict the outflow of a watershed accurately but for water quality issues the locations of runoff generation also need to be predicted accurately. Very few models accurately predict the locations of VSAs, i.e., areas in the landscape that are prone to saturate, and those models that do predict VSAs are not applicable to practical water quality management because they either require information or expertise that are not readily available. We are currently investigating two approaches for predicting these runoff areas that can be practically applied by watershed managers. The first approach uses topographic indices, based on watershed areas and local land slopes, to assess the location and timing of runoff risks. We have found that topographic indices correlate well to runoff generating areas over periods of months or seasons. Another, even simpler approach, assumes that runoff risk can be linked to “distance from stream channel.” The latter concept is attractive because of its simplicity and ready applicability to the field, but it may not consistently capture VSAs as accurately as the former. In both cases we look at runoff generation monthly to account for seasonal changes in landscape water content. The adoption of simple methods like these is encouraged with the understanding that this is a rapidly evolving aspect of water quality protection that will improve the skills of consultants to identify areas that generate runoff and therefore have a high potential of carrying pollutants to the streams.