Fluvial systems: catchments and rivers - Fundamentals of the Physical Environment - page 321

(a)

Figure 14.8
The general character of storm
hydrographs (a) and the annual
hydrograph (b) of the river
Thames at Teddington.
Source: Compiled from data from the
DoE Water Data Unit (1983)

Precipitation

The discharge hydrograph

Peak

Lag
time

Rising limb
Time of rise

Falling limb

Direct
run-off

Depletion
curve

Base time

BASE FLOW

Time (hours)

(b)

1000.00
500.00

100.00
50.00

10.00
5.00

1.00
0.50

0.10

J FMAMJ J ASOND

precipitation, riparian vegetation drip and throughflow.
Vegetation and soil humus largely preclude Horton flow
in humid climates but soil throughflow may emerge as
saturated overland flow towards valley floors.
The impact of these rapid transfers is dramatic but
short-lived, leading to a sharp rise in the constant or
slowly declining baseflow of the hydrograph. This rising
limb has four important parameters ( Figure 14.9b ) . The
peak discharge value (PDV) represents the maximum
event discharge, prior to its recession down the falling
limb , and is separated by lag time from the precipitation
peak. Time of rise is determined by the antecedent storage
capacity and connectivity of catchment components to
the channel. This includes the upstream channel network
for trunk river hydrographs ( Figure 14.10 ). Therefore, the
greater the antecedent stores and connectivity of the
system, the shorter the lag time and time of rise. The PDV
is also higher, since less time has elapsed in which other

losses may occur. Delayed flow , or slower throughflow and
baseflow, continues to rise after the PDV has passed.
Driven by higher hydrostatic pressures in rising soil and
groundwater tables, it meets the falling limb at a higher
discharge than during commencement. The depletion
curve returns to long-term baseflow once the quickflow,
flood peak and delayed flow have passed. Where this is
insufficient to sustain perennial river flow, channels may
still be maintained by seasonal or intermittent flow. This
is typical of small tributaries, channel segments over
highly porous strata and ephemeral streams in hot and
cold (permafrost) deserts.

STREAM FLOW IN CHANNELS

Gravity-induced overland flow is both unstable and
inefficient, since land surfaces are neither homogeneous