The effects of suburban land-use change and climate on watershed hydrology in Oakland County, Michigan, 1975--2005

Oakland County, Michigan, is a rapidly growing area northwest of Detroit. Previous studies have determined that as much as 25 percent of the total area of several watersheds in the county has been converted to urban land uses during the period from 1980 to 2000, yet little overall change in streamflow characteristics has been observed. An impervious surface dataset was developed using automated classification of digital imagery, and compared to parcel-based land-use data. Impervious surface percentages were calculated for parcels of different sizes based on the year the parcel was developed. The results suggested substantial variation in impervious surface for residential parcels less than 1 acre (0.404 ha), ranging from less than 10 percent to more than 30 percent of parcel area, with an increasing trend through time in impervious surface even within parcels with similar size and use. However, because the number of small, highly impervious parcels has dropped as a fraction of total housing starts, the rate of impervious surface growth has slowed. Analysis of stream-flow records for six selected sites within the county showed little change in annual flow characteristics, and some decrease in spring season flows as a percentage of annual flow. The Soil Water Assessment Tool (SWAT) simulation model was used to test the effects of land-cover renderings, land-use change and climate on the Paint Creek Watershed. The results indicated that parcel and hand-digitized land-use data were not a good surrogate for land-cover data, and tended to overestimate runoff and underestimate recharge. The effects of land-use change on stream-flow were more than offset by the effects of climate, but both land-use and climate tended to reduce groundwater components of the hydrologic budget. These results were partially confirmed by calculating the total evapotranspiration for the six watersheds based on the difference between annual precipitation and annual stream-flow. Records from a nearby long-term monitoring well also suggest a long-term decline in ground-water levels in the region.