Geoscience Reference
In-Depth Information
In reality this is very rarely observed, a factor of the
complicated nature of storm runoff processes. The
recession limb
of the hydrograph is after the peak
and is characterised by a long, slow decrease in
streamflow until the baseflow is reached again. The
recession limb is attenuated by two factors: storm
water arriving at the mouth of a catchment from the
furthest parts, and the arrival of water that has
moved as underground flow at a slower rate than the
streamflow.
Exactly how water moves from precipitation
reaching the ground surface to channelised stream-
flow is one of the most intriguing hydrological
questions, and one that cannot be answered easily.
Much research effort in the past hundred years has
gone into understanding runoff mechanisms; con-
siderable advances have been made, but there are
still many unanswered questions. The following
section describes how it is believed runoff occurs,
but there are many different scales at which these
mechanisms are evident and they do not occur
everywhere.
5
4
3
2
1
0
1 1
21
31
41
51
61
71
81
91
101
Time (days)
Figure 5.1
A typical hydrograph, taken from the river
Wye, Wales for a 100-day period during the autumn of
1995. The values plotted against time are mean daily
flow in cumecs.
and many physical characteristics of the upstream
catchment. In terms of catchment characteristics the
largest influence is exerted by catchment size, but
other factors include slope angles, shape of catch-
ment, soil type, vegetation type and percentage
cover, degree of urbanisation and the antecedent soil
moisture.
Figure 5.2 shows the shape of a storm hydrograph
in detail. There are several important hydrological
terms that can be seen in this diagram. The
rising
limb
of the hydrograph is the initial steep part
leading up to the highest or peakflow value. The
water contributing to this part of the hydrograph is
from
channel precipitation
(i.e. rain that falls directly
onto the channel) and rapid runoff mechanisms.
Some texts claim that channel precipitation shows
up as a preliminary blip before the main rising limb.
RUNOFF MECHANISMS
Figure 5.3 is an attempt to represent the different
runoff processes that can be observed at the hillslope
scale.
Overland flow
(
Q
o
) is the water which runs
across the surface of the land before reaching the
stream. In the subsurface, throughflow (
Q
t
) (some
authors refer to this as
lateral flow
) occurs in the
shallow subsurface, predominantly, although not
always, in the unsaturated zone. Groundwater flow
(
Q
G
) is in the deeper saturated zone. All of these
are runoff mechanisms that contribute to stream-
flow. The relative importance of each is dependent
on the catchment under study and the rainfall
characteristics during a storm.
Overland flow
Some of the earliest research work on how overland
flow occurs was undertaken by Robert Horton
(1875-1945). In a classic paper from 1933, Horton
Time
Figure 5.2
Demonstration storm hydrograph.
