Prediction of low flows in ungauged basins - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 166

movement and resulting in low flow. These are often called
winter low flows ( Figure 8.2b ). As air temperatures rise
with the onset of spring, ice and snowmelt and the subsur-
face pathways become reactivated. The melt water is added
to the system, restoring flows that may persist into the
summer, even if there is low summer precipitation.
In mid- and high-latitude climates, there is often one
particular season of low flow occurrence: either summer or
winter. In low-latitude climates, there may be more than
one dry season and consequently more than one distinct
low flow period. In arid and semi-arid climates, the com-
bination of low precipitation and high evaporation results
in sparse river networks and ephemeral flows. These
regime types reflect the major impact of climate on low
flows and climate maps provide a valuable source of infor-
mation for assessing the climatic drivers and the expected
seasonality of low flows. Seasonality of low flows, in turn,
is an indicator of the regime type and may help identify
characteristic processes as a basis for regionalisation
(Laaha and Blöschl, 2006a ; WMO, 2008 ; van Loon and
van Lanen, 2011 ). The seasonal variability may be com-
pounded by strong inter-annual variability, as experienced
in countries such as Ethiopia, South Africa, Australia and
India ( Chapter 6 ).

aquifer all govern the overall storage and release properties
of the aquifer and, in consequence, groundwater discharge
into the stream. During dry periods, groundwater discharge
will continue as storage is slowly depleted. In hilly or
mountainous regions, discharge from shallow aquifers of
weathered hard rock often provides the most important
source during dry periods. In lowland areas (for example,
deltas, coastal plains), deep aquifers typically exist beneath
shallow aquifers, and aquifer layers are often connected. In
lowland areas or large valleys, aquifers thus act as large
storage systems and are able to feed rivers during pro-
longed dry periods.
Depending on the storage characteristics of the aquifers,
the low flow characteristics may vary immensely, and this
is the reason for the different low flow behaviours seen in
the two catchments in Figure 8.1 . The less flashy Kennet
overlies a highly fractured chalk aquifer that allows rapid
infiltration of precipitation (Maurice, 2009 ). It closely
interacts with the highly pervious chalk aquifer system
leading to sustained low flows. In contrast, the more flashy
South Tyne catchment is underlain by carboniferous lime-
stone that does not allow such rapid infiltration, leading to
more overland runoff.
Storage can also occur in lakes that maintain low flows
during dry periods, in particular in humid climates. In arid
climates, however, lake evaporation may in fact decrease
low flows downstream of the lake.
Low flows are a runoff signature that may be very
strongly affected by anthropogenic activities, including
abstractions from and discharges into rivers and reservoir
storage. Groundwater abstractions close to a river can also
have a major impact on the low flow regime (e.g., Clausen
et al., 1994 ; van Lanen and van de Weerd, 1994 ; van
Lanen et al., 2004b ). Discharges from treatment plants
can also have a great effect on the low flow regime and
in some instances the artificial discharges may be larger
than the natural runoff (Gustard and Demuth, 2009 ). Res-
ervoirs may have a major effect on low flows, governed
primarily by the mode of their operation. In the case of
reservoirs used for hydropower, peak power production
will result in a redistribution of runoff over time and cause
fluctuations that are particularly apparent during low flow
periods. This is illustrated in Figure 8.3 which shows
monthly runoff (left) and the daily average fluctuations in
the hourly average (right) (see Holko et al., 2011 , for
method). Before 1990 the fluctuations are small during
the winter months (October to March), which can be attrib-
uted to the river

Catchment processes
Catchment processes, especially those factors that control
storage within a catchment, are very important for low
flow conditions, as they affect the rate of depletion of
runoff (WMO, 2008 ; van Lanen et al., 2004b ). Topo-
graphic slope, soil depth and texture, geology and land
cover (e.g., lakes, bogs) all determine the storage and
drainage properties (i.e., whether they are fast-responding
or slow-responding catchments) and geology is usually the
most important factor of these catchment processes. Fast-
responding catchments may respond to precipitation events
with a larger number of short-lived low flow events. On the
other hand, in slow-responding catchments the number of
low flow events may be smaller, but they may last longer.
Surface processes (interception, surface and snowpack
storage) determine how much of the incoming precipitation
infiltrates into the soil. The permeability of the topsoil, in
combination with catchment topography, determines the
rate at which water infiltrates and how fast soil moisture is
replenished. Soil water is depleted by evaporation and tran-
spiration, governed by the climate, vegetation and soil prop-
erties. Water-saturated soils (e.g., swamps and bogs) are
subject to the highest evaporation losses, generally con-
sidered equivalent to evaporation from open water surfaces.
The drainage capacity of the soil determines how quickly the
infiltrated water may recharge the groundwater system.
The groundwater potential gradients along with the stor-
age characteristics and hydraulic conductivity of

s winter low flow regime, meaning that
runoff is consistently low throughout the winter. In April
1990, however, the situation changes and the winter fluc-
tuations increase dramatically. This is because the Wald
power plant was put into operation and the stream gauge
was then moved below the power plant in April 1990.

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