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(particularly precipitation estimates) will be greater in
these remote parts of the basin than elsewhere.
This study concludes that an uncertainty approach to
PUB can be a useful tool and that the methods suggested
by Kapangaziwiri et al. (2009) are generally appropriate for
quantifying parameter uncertainty in South Africa. How-
ever, it is not always straightforward to apply the parameter
uncertainty estimation approach when the required physical
basin property data are not readily available, as is the case in
the Lesotho parts of the Caledon River basin.
precipitation varies between 1000 and 1250 mm/yr and is
evenly distributed throughout the year. Snowfall is
common in the months of December, January and Febru-
ary, with more snow falling away from the coastal areas
during these months. A record drought occurred over a
several-year period during the mid to late 1960s. Under-
standing the sustainability of water allocation through a
period of drought of such severity is of great interest to
stakeholders in the study area. Therefore, it was important
that estimated runoff
included the period of
record
covering the 1960s drought.
The geology and hydrology of the study region were
heavily affected by the growth and retreat of glaciers
during the last ice age, which formed the present-day
stream network and drainage patterns. The retreat of the
glaciers filled the river valleys with outwash sands and
gravel as well as fine- to coarse-grained lake deposits,
and these sand and gravel deposits have been found to be
important controls on the magnitude and timing of base-
flows in the study region.
We selected those stream gauges for which the contrib-
uting catchment areas were considered to be least affected
by regulation (
Figure 11.24
). Of the 66 stream gauges in
the study region that fit this criterion, 48 stream gauges
had at least 20 years of continuous daily runoff records
that extended through the drought-of-record (black tri-
angles in
Figure 11.24
). The distributions of selected
catchment characteristics for the 66 stream gauges are
shown in
Figure 11.25
. In general, the study stream
gauges represent the likely values encountered at most
ungauged basins in the region. For example, the range
of mean annual precipitation observed at the study stream
gauges (1100
11.7 SETTING ENVIRONMENTAL
FLOW TARGETS IN NORTH-EAST
USA
s. a. archfield
The issue from societal and hydrological perspectives
This case study focuses on the north-eastern USA, where
water managers are under increasing legal pressure to
ensure that water for public supply is allocated with con-
sideration for flows needed to support ecological services
and not in excess of the total surface water available in a
basin. Lack of runoff information at ungauged locations
resulted in conflict between the state
s largest stakeholder
groups: the environmental advocacy groups and the water
suppliers. With no information available to justify water
allocation permits within ungauged catchments, environ-
mental groups asserted that water managers were being too
liberal in their allocations, resulting in lowered runoff that
could not support environmental flows. Conversely, water
suppliers claimed state water managers were being too
conservative in their issuing of water allocation permits,
thus limiting the water suppliers
'
1435 mm/yr) is representative of the range
of mean annual precipitation values reported over the
entire study region. More information on the stream
gauges and catchment characteristics can be found in
Archfield et al.(
2010
).
-
abilities to meet customer
demand. To provide a common framework for negotiation
between these stakeholder groups and to inform the water-
allocation decision-making process, water managers
required a technically defensible decision-support tool to
estimate the total surface water available in ungauged
catchments across the region. The tool enables users to
compare these runoff estimates to time-varying ecological
flow targets and to compute water availability for
ungauged basins in the north-eastern USA.
'
Method
Existing tools to estimate runoff at ungauged locations were
determined to be inadequate for water allocation decisions.
Low flow statistics were available at ungauged basins in
Massachusetts through regional regression equations
developed by Ries and Friesz (
2000
) that related flow statis-
tics such as the median monthly August runoff to measure-
able physical and climatological characteristics of the basin.
At the time these regional regression relations were
developed, ecological flow targets were typically defined as
a constant, minimum runoff that would remain in the river
throughout a year to support ecological services and func-
tions. However, in recent years, the work of Poff et al.(
1997
)
challenged this convention. They declared that the ecological
Description of the study area
The study area is located in the southern portion of the
north-eastern USA (
Figure 11.24
) and covers an area of
approximately 30 000 km
2
. The region is characterised by
a
temperate
climate with distinct
seasons. Annual
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