The soil holds water. We call this mud. There is a limit to how much water the soil can hold, which we measure with a parameter called the soil water (or pervious surface) capacity (also sometimes "field capacity" of the soil). We estimate the soil water holding capacity on an area-weighted basis for the pervious surfaces, based on the soil types associated with the ecosystem types. "Pervious surfaces" are areas where the soil is open to the atmosphere (i.e. not covered by impervious surfaces like cement and asphalt). We also assume that at the beginning of the precipitation event that the soil water is filled according to the "proportion of pervious water storage filled- initial conditions" parameter. The difference between the soil water store capacity and the current amount of water held is called the soil water deficit. During a precipitation event, the amount of surface inputs (from precipitation and outdoor water use) is compared to the soil water deficit. If the deficit is larger, all the surface water hitting the pervious surface is captured and there is no surface runoff from the pervious surfaces. If the surface inputs are larger than the deficit, then the soil fills with water to its capacity, and any excess water runs off becoming stormwater.
Soil water storage capacity = area of pervious surface * average soil water capacity
Soil water deficit = soil water storage capacity * (1 - proportion of soil water filled- initial conditions)
Depends upon:
—Parameters: pervious (soil) storage capacity, proportion of impervious surface and proportion of pervious water storage filled - initial conditions
—Other metrics: ecosystem area