Historic Water Retention Capacity of Wetlands in a Tile Drained Agricultural Landscape.

Non-point source pollution (NSP) is currently one of the largest sources of water quality impairment in the United States.  NSP is frequently linked to drainage from agricultural landscapes requiring efforts to manage drainage water, including the use of constructed and restored wetlands. However, the water retention capacity of drained wetlands in agricultural landscapes of the Upper Midwestern United States is not known.  Using airborne light detection and ranging (lidar), we calculated the baseline water holding capacity of existing and drained wetlands across the Greater Blue Earth River Basin (GBERB), an area dominated by agriculture that is largely tile drained.  We also analyzed the degree to which wetland water retention capacity had changed in the GBERB after the initial drainage period (i.e. prior to 1940).  We estimate that wetlands in the GBERB had a historic retention capacity of 152 mm.  For the 11 HUC 12 watersheds within the GBERB, the retention depth ranged from 23-915 mm, indicating large spatial disparities in retention capacity among watersheds.  Large depressions (>10 ha) accounted from 86% of the historic retention capacity and 0.9% of the observed wetlands, suggesting that the majority of wetland drainage had little impact on the water retention capacity of the area.  The analysis of historic aerial photographs revealed that at least 54% of retention capacity had already occurred been lost by 1938.  This suggests that if drainage has impacted the hydrology of the GBERB, the impact occurred prior to 1940.