Hydrologic Abstractions

Hydrologic Abstractions Interception Evaporation Transpiration Depression Detention Infiltration

INTERCEPTION A portion of the rainfall is intercepted by plant foliage, buildings, and other objects. This water is not available for runoff. Interception typically removes about 0.5 mm during a single storm event. Values as high as 1.5 mm have been reported.

EVAPORATION Evaporation is the change of the state of water from liquid to vapor as a result of heat addition. Evaporation from a body of water occurs only if the surrounding air is not completely saturated with water vapor, that is, if the relative humidity is less than 100 percent.

TRANSPIRATION Transpiration is the transfer of soil moisture from the soil to the atmosphere by the action of vegetation. Plants transpire water vapor through their foliage. Transpiration has minimal effect on individual storms and is usually only taken into account in long-term hydrologic budgets. Evaporation and transpiration are commonly lumped in one variable called evapotranspiration.

DEPRESSION STORAGE Depression storage accounts for the water that becomes ponded in land surface irregularities. Depression storage depends on the land use of the watershed and typically amounts to 0.5 to 8 mm during a single rain event. It is inversely proportional to the watershed’s slope

DETENTION STORAGE Detention storage is the volume of water that moves as overland flow (sheet flow). This volume is temporarily detained in transit to the stream. Sheet flow storage depends on watershed parameters including land use, vegetation, slope, and rainfall intensity. Typical values range from 2.5 to 10 mm

INFILTRATION Infiltration takes place as part of the rain percolates through the soil. The rate of infiltration depends on the soil type, slope, vegetation, soil moisture content, temperature, and the precipitation intensity. Infiltration usually is the largest abstraction and therefore has the most significant effect on runoff.

INFILTRATION Infiltration rates generally decrease with time as the rainfall proceeds and the soil becomes saturated. Many infiltration models have been developed from very simple to very complex.

Infiltration Rainfall Excess Rainfall Intensity in/hr fi -Index Time in hours Rainfall Intensity in/hr -Index Rainfall Excess fi

Total Hydrologic Abstractions The M-Index method lumps all of the abstractions into an average infiltration rate. The Soil Conservation Service (SCS) Curve-Number method allows for an initial abstraction and then infiltration as a function of soil type and land use. The Green-Ampt Infiltration equation allows for the infiltration to become a function of time.