Geography Reference
In-Depth Information
11.15 REMOTELY SENSED LAKE
LEVELS TO ASSIST RUNOFF
MODELLING IN GHANA
j. liebe, n. van de giesen, m. t. walter and
t. s. steenhuis
irrigated agriculture. Population growth places pressure
on scarce land and water resources. The scarcity of usable
water resources is mainly due to the climate, especially the
mode of rainfall. The Upper East
s semi-arid climate is
characterised by a three-month, mono-modal rainy season.
Ninety per cent of the region
'
s total rainfall (986 mm/yr)
occurs as thunderstorms, originating from squall
'
lines.
Rainfall intensities often exceed the soil
s infiltration rates
causing surface runoff, without significantly replenishing
soil moisture and groundwater. Small reservoirs capture all
runoff until they are filled, after which any incoming water
flows over the spillway. In the Upper East Region, 154
reservoirs with a surface area between 1 ha and 100 ha
were identified on the basis of remote sensing. In addition,
two larger reservoirs (Tono, 1894 ha; Vea, 435 ha) are
located in the area.
The landscape is slightly undulating with typical max-
imal height differences within the studied catchments of
less than 100 m.
Figure 11.61b
shows a representative
small
irrigation scheme adjacent to a small reservoir and
gives a general impression of the landscape during the dry
season. The landscape can be described as a park landscape
of fields with scattered individual trees.
Figure 11.61c
shows a very small reservoir at the end of the dry season,
with the dam wall in the background.
'
The issue from societal and hydrological perspectives
In semi-arid areas of the developing world, rural water
supply is increasingly insufficient. Supplying the rural
population in semi-arid developing countries with water
requires spatially distributed sources of different qualities
and quantities of water. Access to clean drinking water is
being improved with borehole programmes, but the
equally important large volume demand for non-drinking
purposes is currently not addressed sufficiently. In many
regions, small reservoirs act as multi-purpose water
sources in support of irrigated agriculture and gardening,
livestock watering and fishing, as well as personal hygiene,
domestic uses and building. They are as important for rural
development, health improvement and poverty reduction
as for access to safe drinking water.
One of the key advantages of small reservoirs is their
existence in large numbers, greatly improving the water
availability at village level. They are often the only
adequate and economically feasible source of large volume
water supply for non-drinking purposes and important for
economic development and the reduction of poverty. Their
small size, existence in large numbers and widespread
distribution leads to many desirable socio-economic
effects. From a PUB perspective, they offer a unique
possibility to monitor the occurrence and, to a lesser extent,
quantity of runoff over large areas (see
Chapter 3
). In order
to do this, a monitoring and modelling framework was
developed based on radar (ASAR) imagery and a simple
conceptual model. The case study concerns the Upper East
Region of Ghana, a region with a relatively high density of
small reservoirs. The method has previously been pre-
sented in Liebe et al.(
2009a
,
b
) and draws on several
earlier publications regarding remote sensing of small res-
ervoirs. The work is part of the Small Reservoirs Project
Method
To calculate the catchment discharge, we combine time
series of remotely sensed reservoir surface areas with
known relationships between reservoir volume and surface
area to calculate runoff volumes that are then used to
parameterise the
Thornthwaite
Mather (1955)
water bal-
ance model. The reservoir surface areas were extracted
with a spatial resolution of 30 m from 12 ENVISAT ASAR
images as described in Liebe et al.(
2009a
). An example of
one of the images and the reservoir outline is shown in
Figure 11.62
.
The reservoir
-
'
s storage volumes were determined with a
generalised area
volume equation developed by Liebe
et al.(
2005
). Bimonthly inflow into the reservoir is esti-
mated as the change in reservoir volume, adjusted for
evaporation from and rain falling on its surface. Catchment
runoff, on days with precipitation that fall within the
bimonthly period, is interpolated. Stream network and the
catchment area were extracted from SRTM V3 elevation
data (
Jarvis et al., 2008
) after performing a pit-removal
procedure and choosing the dam wall as a seed point. Daily
rainfall data with a horizontal resolution of 10 km were
provided by the Famine Early Warning Systems Network
that is based on Meteosat infrared data, rain gauge reports
and microwave satellite observations (Xie and Arkin,
1997
).
-
Description of the study area
The Upper East Region of Ghana is situated in the centre
of the Volta Basin. The Upper East is inhabited by approxi-
mately one million people and has a population density of
about 100 inhabitants/km
2
. With a poverty incidence of
88% in 1998/9, the Upper East has the largest proportion
of poor people of Ghana
'
incomes are generated from mostly rain-fed and some
'
s ten regions. The residents
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