Impacts Of Land Use And Watershed Characteristics On Stream Flow In Chesapeake Bay Watershed

Watershed characteristics can directly affect stream flow. We selected 150 subbasins from Chesapeake Bay drainage and developed proportions of land use types, impervious surface coefficients and soil physical properties for those subbasins. Based on the daily observed stream flows from 1984 to 2004 collected from 150 USGS sites and the hourly precipitation records from 186 meteorological sites, 33 flow metrics were calculated by IHA and 17 of the most informative ones were chosen. We used statistical methods to quantify the effects of land use types, impervious surface, and soil physical properties on 17 flow metrics in three aspects.Firstly, we examined the correlations between land use, impervious surface, soil physical properties and flow metrics. The result showed higher forest proportions reduced flooding, prolonged pulses of higher flow, and reduced flow variability. Higher proportions of agriculture reduced flow variability and prolonged pulses of higher flow in all physiographic provinces and reduced flooding in the highland physiographic provinces. Higher grassland proportions reduced flooding and flow variability and prolonged high flow pulses in all provinces. Higher proportions of developed land and impervious surface reduced infiltration, increased flooding and flow during high rainfall periods, intensified flow variability, and shortened high flow pulses; there are different hydrologic effects in different urbanized region. Higher proportions of sand in soil increased flow in flooding periods. Deeper bedrock decreased flooding, intensified flow variability and increased base flow index. Base flow index and flow variability decreased with the higher group of hydrologic group. Higher soil infiltration rate intensify base flow index and intensify flow variability.Secondly, we analyzed the effects of urbanization and agricultural activities by the method of the relative departures of urban and agriculture watersheds to natural watersheds. The result demonstrated that the urbanization had different effects on flow regime with different degrees and directions in coastal plain, piedmont and highland and increased mean annual flow, flooding, high flow and flow variability. However, agricultural activities did not have obvious effects on stream flow.Thirdly, we used stepwise regression to quantify the effects of land use types, impervious surface, and soil physical properties on 17 flow metrics measured for watersheds in three physiographic provinces of the Chesapeake Bay drainage. The result revealed that the factors that most affected stream flow were the proportions of grassland, developed land, forest, and impervious surface and soil hydrologic group. Land use types had more significant effects on stream flow than soil physical properties. There were different responses of flow metrics to land use and watershed characteristics in different physiographic provinces.