25 October 2004
Figure 4-2. Hydrologic Cycle of a Developed Environment
Source: McCuen, 1998.
While conventional approaches to stormwater management design typically
include only the hydrologic components of precipitation, runoff conveyance and storage
capacity within their scopes, LID design recognizes the significance of other
components of the hydrologic cycle as well. How these other components are actually
taken into account will depend on the information available and purpose of the design.
One LID design objective, for example, may be to maintain a natural groundwater
recharge rate for a given site. Determining the appropriate number, size, and location
of infiltration devices can require an extensive atmospheric data set (temperature and
precipitation) to calculate evapotranspiration rates, along with measures of soil hydraulic
The following section describes how LID design can make use of
precipitation, storage, infiltration, evaporation, and transpiration data. The discussion
includes a brief description of each of these types of data, and compares the use of
these data from LID and conventional stormwater management perspectives.
PRECIPITATION DATA. Precipitation data is often analyzed in terms of the
location. Stormwater management designs may take into account the total annual
depths or the volume generated by a storm of a specific frequency and duration (e.g. 2-
year 24-hour storm event). Hydrologic models may use precipitation data to develop a
synthetic design storm that reflects the pattern and intensity of precipitation for the
project location region or use actual gage data from a given storm event.
The level of detail and accuracy of data used is dependent on the requirements of the
hydrologic model. For example, to develop a simple water balance for on-site irrigation
only a few years of annual rainfall totals may be required. Some advanced urban