GIS-based estimates of former and current depressional wetlands in an agricultural landscape

Before European settlement, 23% of Illinois (3.2 million of 14 million ha) was covered by wetlands. It is estimated that 90% of those wetlands Were lost during conversion of the landscape to agriculture and urban, use. Champaign County was one of the most extensively drained counties in Illinois, with 39-60% of original county area estimated to have been drained. Current and future efforts to conserve and restore wetlands would benefit from information on the number and distribution of former wetlands. We used GIS to estimate the spatial extent, density, pattern, and sizes of former and extant depressional wetlands in Champaign County. We derived several models of former wetlands; all models used hydric soils but varied by using Digital Raster Graphics (DRG), 30-m Digital Elevation Models (DEM), or Digital Orthophotography Quarter Quadrangles (DOQ). We also combined the DRG and DEM models, and we conducted visual field surveys for saturated or ponded conditions to test the models. The DRG model was conservative: it identified fewer and larger wetlands than the DEM model (the DOQ model was judged inadequate). Depending on the model selected, we estimated that 1077-4090 depressional wetlands formerly existed in the county, and that 78.6-91.6% were drained, accounting for 1108-2777 ha of lost wetland habitat in Champaign County alone. Thus, depressional wetlands accounted for the vast majority of historical wetland loss and should be a priority for wetland restoration efforts. Spatial pattern among wetlands also changed: an organism adapted to the former landscape had >50% probability of reaching another wetland within 260 m: today that same species faces a 7.8% probability at that distance. The modern landscape of Champaign County (and others like it) poses potential risk for remaining wetland metapopulations, and GIS models of precise former wetlands locations can be a valuable initial tool for wetland conservation and restoration efforts.