WMS:OC Losses

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Watershed outflow in Orange County is a function of losses. Examples of losses include depression storage, vegetation interception and transpiration, minor amounts of evaporation, and infiltration. Many, if not all, of these losses are affected by the land cover or soil type.

Fm, or the maximum loss rate, is a measurement of the effect of these losses on the peak flow rate. This value corresponds to the soil group, cover complex, and imperviousness of the drainage subarea. Fm is calculated by the following equation: Fm = ap * Fp where,

  • ap – Pervious area fraction.
  • Fp – Pervious loss rate. This is a function of the soil group where different values are shown in the following table:
Fp .40 .30 .25 .20
Group A: Soils with low runoff potential, high infiltration rates.
Group B: Soils with moderate infiltration rates.
Group C: Soils with slow infiltration rates.
Group D: Soils with high runoff potential, very low infiltration rates.

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