Environmental Engineering Reference
In-Depth Information
Hillslope monitoring
biological properties have been observed. For both
treatments, i.e. no grazing and planting of trees, a
significant (t-test significant at the 0.05 level for
both treatments) reduction in soil bulk density for
the top 3-5 cm of the soil has been measured.
There is also a significant increase in the number
of earthworms observed within the top 15-cm
depth of soil under both treatments (ANOVA
returned an F-value of 0.001 for treatments). The
presence of earthworms can have a significant
impact on the movement of water through soil
(Li and Ghodrati 1995), especially in heavy-tex-
tured soils such as those found at Pontbren.
Changes in hydrological response have been
observed post-treatment. Relative increases in soil
infiltration rate and a reduction in overland flow
occurring in treatment plots compared to the graz-
ing control are observed at three out of the four
plots. Figure 3.2 illustrates an example of this from
manipulation plot M
2
. For the one site where
no apparent difference between treatment and
control has yet been detected, conditions are par-
ticularly wet, due to the heavy-textured soil and
the slope location. This research is ongoing, in-
cluding quantitative analysis of soil properties,
soil states and runoff processes.
In considering the catchment-scale response it is
important to understand the nature of the runoff
processes at hillslope scale; hence, detailed mon-
itoring of climate, soil water states and runoff
processes was established for an instrumented
hillslope (
0.4 ha) under improved grassland pro-
duction. The results illustrate the importance of
drain and overland flow (OLF) in characterizing
the runoff response at this scale of observation.
Figure 3.3 indicates that the field drainage sys-
tems, which are extensively installed throughout
the areas of improved grassland, appear to domi-
nate the runoff response in terms of volume at the
hillslope scale. However, there are times when
overland flow rate exceeds that of the drain flow
and is a significant contributor to total runoff peak
flow rates. Pore water pressure readings indicate
that when overland flow occurs it does so as a
result of saturation excess and not infiltration
excess. This is illustrated in Figure 3.4, where
there is relatively more overland flow occurring
during the first event (a) compared to the second
event (b) due to the saturated soil conditions
indicated by the pore water pressure data in corre-
sponding plots below. A relatively impermeable
Fig. 3.2
Difference in overland flow runoff event volume (treatment - control) (mm) from manipulation plot M
2
.
