DEPRESSION STORAGE IN LAND USES COMMON TO THE FINGER LAKES REGION
Paul L. Richards, Ryan Grimm and David Cannon
Dept. of Earth Science, SUNYCollege at Brockport
, (585) 395-5175
Depression storage can be an important factor governing runoff processes in the landscape. It can also impact erosion potential and overland runoff travel times. Although depression storage is an important concept in hydrologic models, it is typically evaluated by calibration and few studies have measured it in the field. Most work to date has involved theoretical investigations on the relationship between depression storage and physical variables like roughness, tortuosity and slope. The bulk of the depression storage data that exists, was collected for crop fields. In this study, we evaluated depression storage in natural and anthropogenic land uses common to the Finger Lakes region to determine its variability in the landscape. We also wanted to explore how terrestrial biologic processes, such as vegetation and bioturbation impact depression storage and depression storage – slope relationships, and to consider the implications on runoff prediction.
Depression storage was evaluated by digitizing the landsurface using a roughness clinometer. The device measures the depth to surface at 5 cm intervals relative to a plane parallel to the general slope of the surface. Specialized software was developed to compute the actual depression storage from measurements by adjusting the ideal depression storage to account for the free surface of water behind local topographic highs. The surface of the ground was cleared of debris (with the exception of active roots) prior to using the instrument. Sites were mapped using GPS. To date six different landuses have been evaluated (Table 1):
Land use / Level 3 landuse code / # obs / Mean effective storage (mm) / Std. dev. effective
storage (mm)
Wetland / 610 / 24 / 10.8 / 7.0
Mature deciduous forest
*includes tree fall depressions / 410 / 30 / 7.0 (9.9*) / 7.0 (13.0*)
Unused crop field / 250(300) / 32 / 9.4 / 5.0
Urban residential grassland / 113 / 20 / 0.5 / 0.4
Urban open (commercial) / 120 / 15 / 2.0 / 2.4
At one of our sites, we also had the opportunity to evaluate a region that had a relatively new land use on it (immature pine forest). Since the region had uniform landuse, soils and slope prior to the planting of pines 24 years ago, the effects of the tree growth on depression storage could be quantified.
Data collected so far suggests that natural land uses such as wetland, forest and herbaceous had considerably more depression storage than urban landcover at all slopes (Figure 1, top; Table 1). These land uses can store 5 or more times rainfall than pervious landcover in urban settings. Effective/ideal depression storage ratio is a strong function of slope, however with commercial and residential land use a sharp drop was noted at slopes 3% or more (Figure 1, bottom). The origin of the micro-topography varied with the land use. In the wetland, microtopography appeared to be caused by the base stems of cattails that collected sediment around them. Depression spacings appeared to be scaled with the distances between plants. In mature deciduous forests, microrelief is caused primarily by roots breaking the surface, as well as animal trails. Depressions caused by tree falls were also observed, however our instrument was not large enough to measure the storage for many of these features. The few measurements we took significantly skew the rest of the data. Depression storage for the unused crop field was also significant and caused by plow-related activities. The impact that small scale depression storage and the extent of larger scale internally drained topography have on watershed hydrologic connectivity are discussed.
Figure 1 Effective depression storage defined as the average volume
of water held (per unit area) by microrelief plotted against slope
(top) and effective/ideal depression storage ratio plotted against
slope (bottom). Notice the marked decrease in depression storage
at slopes between 2 and 3 percent for urban pervious landcover.