Characteristics of the watershed area directly affect the hydrologic analysis. Basic features of the watershed basin include size, shape, slope, land use, soil type, storage, and orientation.
The size of the watershed basin is the most important characteristic affecting the determination of the total runoff. It is generally measured in acres, square miles, or square kilometers and is defined by the limits of the topographic divide. A topographic divide is a line that separates water flow between basins, thus causing the rainfall that falls on one or the other side to flow into a particular watershed. The location of this divide, and thus the perimeter of the basin, may be determined from aerial photographs, topographic maps available from the U. S. Geological Survey (USGS), and field surveys.
The shape of the watershed primarily affects the rate of water flow to the main channel. Because the rainfall in narrow watersheds reaches the main stream relatively quickly, a narrow basin generally has a low peak discharge compared with a fan — or pear-shaped basin of otherwise similar characteristics.
The main effect the slope has on water flow is on the time of concentration, or the time it takes the rainfall to flow from the farthest point in the watershed to the point under consideration. Everything else being equal, steeper slopes cause a shorter time of concentration, and thus a higher peak discharge, than do flatter slopes.
The use of the land and the type of surface the precipitation falls upon have an obvious impact on the flow of water. Developed areas covered by asphalt or concrete will allow a much greater percentage of the rainfall to flow to the point under deliberation than will an undeveloped vegetated area.
Peak flows may be reduced by the effective storage of drainage water. Of the three main types of storage—interception, depression, and detention—detention storage has the major impact in determining runoff. Interception refers to storage on aboveground fixtures such as plants, and depression refers to storage in depressions in the ground surface. Interception storage will eventually evaporate, and depression storage will either evaporate or infiltrate into the ground. Detention storage is runoff that is either in transit to the main channel or in storage in a pond, swamp, basin, or constructed detention chamber prior to transmission.
The final characteristic of the watershed basin is orientation. Taking into account the slope of the basin, if it is north — or south-facing, the runoff may be affected. If the basin accumulates snow and faces north, the snow may not melt until the late spring. If the snow melt is caused by a spring rain, the total runoff will be increased. On the other hand, if the basin faces south, the snow melt may come much earlier in the year, and with evaporation and infiltration, it may not contribute as greatly to the runoff. Basin orientation for small, steep basins also affects the peak rates of runoff. Where these basin types are in line with prevailing storm movements, the watershed responds with higher, shorter peak discharges.
