An export coefficient approach to hydrological and non-point source (NPS) pollution modeling enables quick and simple assessment of long-term impacts for planning purposes. An export coefficient and geographic information system based L-THIA (Long-Term Hydrologic Impact Assessment) model was applied to the Richland Creek basin (Illinois, USA) to assess the impacts of future urban growth on direct runoff, NPS total nitrogen (TN), total suspended particles (TSP), and total phosphorous (TP) loads.

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... The model predicted that mean annual direct runoff and TSP loading would increase by around 7% and 4% respectively by 2030 with moderate and rapid urban growth simulated by a land-use change model, while TN and TP loads would change little. Such changes are due to the projected land-use change patterns, mainly from agriculture to commercial/industrial or low-intensity residential, and to the different contributions of land-uses to runoff and NPS pollutant loads. At a subbasin scale, the most developed subbasin is projected to experience the greatest increase in commercial/industrial land at the expense of agricultural land and thus notable increases in runoff and TSP load. The changes in runoff and TSP load in other subbasins and the changes in TN and TP loads in all the subbasins show little spatial variability even though the range of per cent increases in low-intensity residential is extremely wide. This study reveals the effect of different 'urban' land-use types on water quality and suggests that proper simulation or planning of different urban land-use types must be carried out for impact assessments.