Link between Impervious Cover and Base Flow in the White Clay Creek Wild and Scenic Watershed in Delaware

Field measurements indicate a correlation between increased impervious cover and decreased stream base flow in and near the White Clay Creek Wild and Scenic River watershed near Newark, Del. A stream base flow monitoring network was established in 19 watersheds near the University of Delaware campus. The watersheds have land uses varying from heavily forested to highly urbanized with impervious cover ranging from 3 to 44%. Using geographic information system land use mapping, watershed impervious cover was estimated based on the ratio of pavement and roof area for each land cover condition. Stream base flows were calculated using the continuity equation (Q=nu A) from velocity and channel cross-section area measurements recorded on 5 days during 2006 and 2007. Results from all five events indicate increased watershed impervious cover correlates with decreased stream base flows. For the five events, the coefficients of determination (R-2) based on linear regression of impervious cover and base flow data are 0.33, 0.35, 0.32, 0.46, and 0.58; evidence of fair to reasonably good correlation. Increased watershed imperviousness can result in dwindling drinking water and aquatic resource flows especially during drought periods. Water resource protection area ordinances, recharge augmentation, and pavement reduction techniques are available to reduce the impacts of impervious cover on watershed hydrology.