Geoscience Reference
In-Depth Information
5
RUNOFF
LEARNING OBJECTIVES
When you have finished reading this chapter you should have:
An understanding of the process of runoff leading to
channel flow
.
A knowledge of the techniques for measuring streamflow and runoff directly.
A knowledge of techniques used to estimate streamflow.
The amount of water within a river or stream is of
great interest to hydrologists. It represents the end-
product of all the other processes in the hydrological
cycle and is where the largest amount of effort has
gone into analysis of historical records. The methods
of analysis are covered in Chapter 6; this chapter
deals with the mechanisms that lead to water
entering the stream: the runoff mechanisms.
Runoff
is a loose term that covers the movement of water
to a channelised stream, after it has reached the
ground as precipitation. The movement can occur
either on or below the surface and at differing
velocities. Once the water reaches a stream it
moves towards the oceans in a channelised form,
the process referred to as
streamflow
or
riverflow
.
Streamflow is expressed as
discharge
: the volume
of water over a defined time period. The SI units for
discharge are m
3
/s (
cumecs
). A continuous record of
streamflow is called a
hydrograph
(see Figure 5.1).
Although we think of this as continuous measure-
ment it is normally either an averaged flow over
a time period or a series of samples (e.g. hourly
records).
In Figure 5.1 there are a series of peaks between
periods of steady, much lower flows. The hydrograph
peaks are referred to as
peakflow
,
stormflow
or
even
quickflow
. They are the water in the stream
during and immediately after a significant rainfall
event. The steady periods between peaks are referred
to as
baseflow
or sometimes slowflow (NB this is
different from
low flow
; see Chapter 6). The shape
of a hydrograph, and in particular the shape of
the stormflow peak, is influenced by the storm
characteristics (e.g. rainfall intensity and duration)
