Why do a Long-Term Hydrologic Impact Assessment
It is important to consider the effects land use changes have on surface runoff, streamflow, and groundwater recharge. Expansion of urban areas significantly impacts the environment in terms of ground water recharge, water pollution, and storm water drainage. Urbanization leads to creation of impervious surfaces which lead to an increase in surface runoff volume, this in turn contributes to downstream flooding and a net loss in groundwater recharge. Eventually loss of recharge affects residential and municipal water supplies. Minimizing the disturbance on an urbanizing watershed is one way of ensuring continued water supply. Since each land use has a different level of impact, careful physical planning can minimize these impacts. Although the impacts of urban sprawl on groundwater recharge and surface water quantity and quality are of considerable importance, many planners, city managers and water resource professionals lack the ability to provide estimates of the potential hydrologic impacts of land use change.
Assessment of the hydrologic impacts or urban land use change traditionally includes models that evaluate how land use change alters peak runoff rates, and these results are then used in the design of drainage systems. Such methods however do not address the long-term hydrologic impacts of urban land use change and often do not consider how pollutants that wash off from different land uses effect water quality. Techniques traditionally used to assess the impacts of land use change on runoff typically focus on individual short-term "design" storm events of specific recurrence intervals, and are used to calculate peak discharge rates and hydrographs. Single storm methods are suitable as engineering approaches in estimating flood intensities for stormwater facilities management, they do not address the long-term, cumulative hydrologic impacts of land use change.