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Hydrologic Abstractions

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.

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Hydrologic Abstractions

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  1. Hydrologic Abstractions

  2. Hydrologic Abstractions • Interception • Evaporation • Transpiration • Depression • Detention • Infiltration

  3. 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.

  4. 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.

  5. 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.

  6. 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

  7. 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

  8. 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.

  9. 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.

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

  11. 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.

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