The hydrologic implications of ecologic succession: Depression storage and leaf litter

Background/Question/Methods Quantifying runoff changes from anthropogenic modifications to land cover are important for managing lakes and watersheds.  Traditionally this is accomplished by classifying aerial photography into the Anderson land uses classes (e.g. low density residential, agriculture etc.) and choosing appropriate parameters for the runoff model being used.  These parameters are typically based on soil type rather than the type of vegetation cover present despite previous studies noting the importance of vegetation in controlling soil infiltration.  We hypothesize that ecologic succession from mowed grassland to successional old field to forest will improve infiltration characteristics by increasing the connectivity of macropores, increasing the volume of available depression storage defined as the mm of rainfall that can be held within depressions, and increasing the volume of leaf litter which can retain water and reduce raindrop erosivity.  We test this idea by measuring surface roughness and leaf litter water retention rates in twelve sites containing a variety of slopes.  Surface roughness was then used to compute available depression storage.  Sites were also evaluated in which the slope and soil type were uniform but types of vegetation and management differed.         

Results/Conclusions Average ideal depression storage was 5.8, 11.1, 14.3, 18.7 mm for mowed grassland, old field, forest and wetland respectively.  Effective depression storage of unmanaged successional oldfield, shrubland and forest was significantly higher than mowed grassland at all slopes.  Mowed successional oldfield was not significantly different then mowed residential turf implying that management activities such as mowing retard the natural development of microtopography that appears to develop as the diversity and size of vegetation species increase.  For thicker old field, shrubland and forests the impact of small mammals on microtopography was noted.  Leaf litter water retention varied significantly between land covers but are much lower than depression storage change due to microtopography.    Maximum leaf litter water retention rates were only 1mm for forested wetland even after leaf fall, implying it is not a significant contributer to potential abstraction.  These results imply there are tight linkages between vegetation succession and runoff; and since managed grassland can evolve into successional old field after only a few seasons, changes in runoff from land use modifications are rapid.