Environmental Engineering Reference
In-Depth Information
subsurface pathways and deeper stores becomes
important. More knowledge about the subsurface
routes below the trees is therefore desirable. We
assume here that connection to field drainage
systems exists.
We also have incomplete understanding of the
relative functioning of unimproved grassland and
wetland areas within Pontbren. Hence these areas
are currently modelled and conditioned using
catchment-scale data: data show that these areas
are less flashy than improved grassland; however,
the extent to which the unimproved grassland
dampens response is an open and important ques-
tion currently being addressed.
Figure 3.7 illustrates the simulated response
for a representative hillslope (100
100m) using
the detailedmodel for a range of landmanagement
types, including grazed and ungrazed drained
grassland, grassland with tree shelter belts (80m
length, 15mwidth) in different locations, and full
tree cover. The envelopes of response represent the
range of parameter uncertainty.
.
Drain flow remains important in winter flood
conditions, and may be the dominant flow mech-
anism in most summer floods.
There is certainly significant small-scale hetero-
geneity and strong evidence for significant non--
stationarity of small-scale responses. There is very
little understanding of compaction effects in these
soils, to allow us to distinguish the response of
grazed and ungrazed grassland.
Data fromtree-planted areas inPontbren suggest:
.
increased infiltration within tree shelter belts;
.
increased capacity to store water underneath trees;
.
large interception losses;
.
significant tree shelter-belt edge effects.
Despite the extensive field programme, some
uncertainties in the perceptual model remain. A
major knowledge gap regarding the tree-planted
areas is the fate of subsurface water. For extreme
events, the interception and localized near-surface
storage that is known to be associated with the
trees is not necessarily adequate to significantly
reduce flood generation; and the activation of slow
graze
d
drained grassl
a
nd
grazed drai
n
ed grassland, l
o
w trees
grazed drain
e
d grassland, h
i
gh trees
1.5
1.5
1.5
Maximum flow
Minimum flow
Maximum flow
Minimum flow
Maximum flow
Minimum flow
1
1
1
0.5
0.5
0.5
0
0
0
Jan07
Feb07
Mar07
Apr07
Jan07
Feb07
Mar07
Apr07
Jan07
Feb07
Mar07
Apr07
g
razed drained gr
a
ssland, perpe
n
dicular trees
woodland
ungraz
e
d drained gras
s
land
1.5
1.5
1.5
Maximum flow
Minimum flow
Maximum flow
Minimum flow
Maximum flow
Minimum flow
1
1
1
0.5
0.5
0.5
0
0
0
Jan07
Feb07
Mar07
Apr07
Jan07
Feb07
Mar07
Apr07
Jan07
Feb07
Mar07
Apr07
Fig. 3.7
Field-scale runoff (drain flow
รพ
overland flow) for different land use types, with uncertainty bounds.
