DEPRESSION STORAGE IN LAND USES COMMON TO THE FINGER LAKES REGION
Paul L. Richards, Ryan Grimm and David Cannon
Dept. of Earth Science, SUNY College at Brockport
prichard@brockport.edu, (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
Wetland
Mature deciduous forest
*includes tree fall depressions
Unused crop field
Urban residential grassland
Urban open (commercial)
Level 3 land
use code
610
410
# obs
24
30
Mean effective
storage (mm)
10.8
7.0 (9.9*)
Std. dev. effective
storage (mm)
7.0
7.0 (13.0*)
250(300)
113
120
32
20
15
9.4
0.5
2.0
5.0
0.4
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.
Effective depression storage (mm)
60
Wetland Landcover
50
Low Density Residential
40
Mature Deciduous Forest
30
Inactive Cropland
Urban Grassland
(Commercial)
20
10
0
0
1
2
3
4
5
6
7
8
9
10
% Slope
Effective/ideal storage (mm/mm)
1.20
1.00
0.80
0.60
0.40
0.20
0.00
0
1
2
3
4
5
6
7
8
9
10
% slope
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.