PUB in practice: case studies - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 289

Figure 11.20. The Caledon River
basin showing modelled sub-basins
and runoff gauging stations (P1 to P3
refer to the Google Earth images in
Figure 11.21 ).

ungauged regions such as the Canadian Prairies. Many
streams in the region are ephemeral, and are not only
ungauged, but lie in poorly defined internal drainage
basins. Providing estimates of runoff variation in droughts
and pluvials adds new information for land and water
managers. The methodology was demonstrated by model-
ling the extent of hydrological droughts on the Canadian
Prairies during the period 1999

transfer scheme abstracting water from Welbedacht dam
for the city of Bloemfontein located in the Modder River
basin to the north-west. As a consequence there exist
many small farm dams as well as a number of much larger
dams. Midgley et al.( 1994 ) list a total of 53 impound-
ments with a combined storage capacity of approximately
202 × 10 6 m 3 , compared with their estimate of the mean
annual runoff of 1244 × 10 6 m 3 . There are, however, many
more small storage dams with unknown capacities that are
not included in the list provided by Midgley et al.( 1994 ).
While there are six runoff gauging stations ( Figure 11.20 )
within the study area, their data records are short and cover
different periods, rarely measure the full range of high
flows, and are impacted by poorly quantified upstream
abstractions. The combined uncertainties in the observed
data make the basin effectively an ungauged one. The
observed data may be useful for constraining some aspects
of simulated flow data, but are not useful for conventional
model calibration.

- 2005 with reference to the

normal period of 1961 90. Because suitable meteoro-

logical forcing data are only available at points separated
by many hundreds of kilometres, only very large-scale
spatial trends are discernible in runoff. Nevertheless, plots
of interpolated exceedance probabilities were able to dem-
onstrate the substantial spatial and temporal variation of
the hydrological drought in regions where it would other-
wise be unquantifiable.

-

11.6 SEASONAL FLOW PREDICTION
WITH UNCERTAINTY IN SOUTH
AFRICA AND LESOTHO
d. a. hughes

Description of the study area
The Caledon River forms the north-western boundary
between Lesotho and the Free State Province of South
Africa ( Figure 11.20 ) and is one of the major tributaries
of the upper Orange River. The total area of the Caledon
River basin at its junction with the Orange River is 21 884
km 2 , while this study focuses on the area (15 270 km 2 )
upstream of Welbedacht dam (D23J). The headwater sub-
basins rising within Lesotho are characterised by steep

The issue from societal and hydrological perspectives
The water resources of the Caledon River basin are import-
ant locally to sustain water supplies for many small towns
as well as the Lesotho capital city of Maseru, and for
irrigated agriculture in the South African parts of the basin.
They are also important regionally through an inter-basin