Land Use, Natural Storage, Vegetative Cover, and Soil Property Information
Data that describe the watershed properties are needed for the conceptual models, and to a limited extent, by certain empirical models.
A conceptual model of watershed runoff, with components as illustrated in
, represents processes of infiltration and overland flow. To do so, the model must be configured and calibrated with knowledge of the properties of the watershed that will affect infiltration and overland flow. Those include:
- Land use in the watershed. Especially important in this is gathering information about the distribution of impervious and pervious cover in the watershed. Rain that falls on impervious surfaces, such as parking lots and rooftops, will run off as overland flow. Rain that falls on a pervious surface may infiltrate, entering the soil layers, and not running off immediately or at all. The rate of this infiltration is related with land use, as well.
- Natural storage in the watershed. Water that ponds in natural depressions, lakes, and similar features in a watershed will not run off or may runoff with some delay and with reduced rates. The location of, capacities of, and behavior of storage must be identified if this is to be represented in computations of design flows.
- Vegetative cover and soil property information. Rates of infiltration depend on properties of soils in the watershed and upon the presence of vegetation. For example, water ponded on sandy soils may infiltrate at four or five times the rate of water ponded on clay soils. And crops planted on clay soils will increase the rate of infiltration there. Thus, the designer must gather information on the cover and soils. That information should define the spatial variations across the watershed.
These data are needed with conceptual models that do not seek to represent in great detail the physical processes. For example, with the
, a runoff coefficient relates runoff rate and rainfall rate. That coefficient is related to land use within the watershed. And knowledge of land use, particularly knowledge of presence or absence of impervious area, is critical for assessing the applicability of
.