Environmental Engineering Reference
In-Depth Information
ground water, and will depend on slope, the transpirational activity of vegetation at differ-
ent places, the soil texture and horizons, the current water content, and the temperature of
the soil. With urban development, roads, pipes, and altered vegetation in the various
patches through which the water flows directly and indirectly alter the amount, timing,
and content of the water that arrives at the stream (
Figure 10.13
). Ground water and the
riparian fringe become much less important, while piped flow and overland flow become
more important. Urban riparian zones may, as a result, be much less effective in reducing
stream nutrient pollution than those in unmanaged watersheds (
Walsh et al. 2005;
Cadenasso et al. 2008
).
The interaction of atmospheric flow with a forest-field boundary exemplifies the control
of an above-ground mass flow. Experimental reduction of the density of vegetation on the
forest edge during the autumn peak for wind-dispersed field species demonstrated the
efficient filtering effect of the edge on wind-borne seeds and nitrogen deposition to the for-
est (
Cadenasso and Pickett 2001; Weathers et al. 2001
). Both of these materials have the
capacity to alter ecosystem function in the forest, the first by introducing species not cur-
rently present, and the second by increasing readily available soil nutrients.
Although we have spoken in terms of patchiness, which may imply discrete, homoge-
neous pattern in both landscapes and waterscapes, we intend the analysis to apply to all
kinds of heterogeneity. First, patches may be either homogeneous or internally heteroge-
neous. It is not necessary for each patch to be homogeneous, but rather only to be hetero-
geneous in a way that distinguishes it from other patches at the same spatial scale of
resolution. Furthermore, patches do not necessarily have to have hard and narrow bound-
aries. Boundaries may be zones across which gradients in the features that distinguish
patches are pronounced (
Cadenasso et al. 1997
). At one extreme, entire landscapes or
Canopy
interception
Evapo-
transpiration
Evapo-
transpiration
Surface
runoff
Surface
runoff
Interflow
Interflow
Baseflow
Baseflow
After construction
Before construction
FIGURE 10.13
Contrast in water movement between nonurban and urbanized watersheds. Flow paths and
magnitudes of flows across different flow paths are modified by development of human settlements. For example,
gutters and curbs impose new headwater structures, storm drains redirect flow below surface layers and effi-
ciently connect stream channels with rainfall, and alteration of vegetation type and cover changes infiltration and
evapotranspiration rates.
(Diagram from the Maryland Department of Environment and in the public domain.)
