L-THIA Impervious Input

When to use the 'Percent Impervious Surface Input' Screen

The ‘Percent Impervious Surface Input Screen’ of L-THIA has been developed mainly for assessment of long-term hydrologic impacts in urban land use scenarios. Percent impervious surface of a development is a number easily available to most land use planners and developers. Also the percent of a watershed that is covered by an impervious surface is a good indicator for amount of land development and its effects of the hydrology in urban watershed (Schueler, 1994). Urbanized watersheds and the process of urbanization turns land uses such as forests, grasslands, agricultural lands to impervious surfaces such as rooftops, roads, parking spaces, runways etc. This change from pervious to impervious surfaces has various long and short-term consequences.

In urbanized watersheds land is either covered by an impervious surface (rooftops, roads, driveways, etc) or is green space (lawns, parks, golf courses). In non-urbanized watersheds the land uses include forest, agriculture land, pasture, lawn and some impervious surfaces. However runoff is not generated exclusively by impervious surfaces. L-THIA uses the equation provided by the Natural Resource Conservation Service provides to convert percent impervious surfaces to curve numbers (USDA, 1986).

For the ‘Percent Impervious Input Screen’ the equation assumes the pervious surface to be grass in fair condition. This description is valid for most pervious areas in urban areas. Another assumption made is that the impervious areas are connected to each other. For years the goal was to remove water from the land as quickly as possible. This resulted in the curbs, gutters, pipes and culverts that now line streets and buildings in urban areas connecting most impervious areas. TR-55 states that if the percent impervious area is over 30% the ability of the remaining pervious areas to infiltrate will be nominal. Thus, the assumption that all impervious areas are connected is made.

This input screen should only be used for urbanized areas. The assumption that the pervious areas are grassland is not valid for most rural areas. In many areas, especially the mid-west, the agricultural land functions as the pervious area within a watershed in terms of the runoff generated. The runoff from this land use is different from grass in terms of both volume and non-point source pollution.

The errors that can occur by using this input screen for a rural watershed are shown in the following example: One hundred acre plot of row crops and farmstead is being converted into 1/8 acre residential lots. The row crops and farmstead have 3% impervious surface (roads and roof tops), while the condominiums will have 65% impervious areas. L-THIA converts the 3% impervious to a curve number of 70 while row crops should have a curve number of 78. While this many not seem significant, it will have a considerable change in the amount of runoff.