Journal of River Basin Management
Vol 5 N° 2
Influence of soil-moisture and land use change on
precipitation in the Volta Basin of West Africa
By HARALD KUNSTMANN and GERLINDE JUNG
An ongoing intensification of agriculture in West Africa
has lead to changes in surface and subsurface
characteristics that may directly affect evaporation rates
and, in turn, regional rainfall patterns. To investigate
the effects of those changes, the Penn State
University/National Center for Atmospheric Research (NCAR)
mesoscale meteorological model (MM5) was applied for the
identification of feedback mechanisms between land surface
(soil and vegetation) and atmosphere. MM5 was applied in
three different resolutions (81 x 81, 27 x 27, and 9 x 9
km2) with 2-way interactive grids. The objective of this
work was to learn to what extent regional (intra-domain)
evaporation determines rainfall within the domains. The
effect of decreased and increased initial soil moisture on
total rainfall and on precipitation recycling indicators
was investigated. Scale dependent, positive (increased
precipitation at increased initial soil moisture) as well
as negative (decreased precipitation at increased initial
soil moisture) feedback mechanisms were found. Detailed
atmospheric water budget analysis showed that negative
feedback mechanisms on the small domain appeared to have
been caused by an increased inflow of atmospherc moisture.
Sensitivity of precipitation with respect to soil moisture
was very variable over space. Both negative and positive
sensitivities were observed. To investigate the effect of
land use change, the total rainfall distribution was
computed under the assumption of the observed land use
substitution from cropland woodland mosaic into shrub land
and finally into grassland. Sensitivity of precipitation
with respect to land use change was very heterogeneous.
Total precipitation change was of the same order of
magnitude as in case of the initial soil moisture
perturbations.
Climate trends of temperature, precipitation and river
discharge in the Volta Basin of West Africa
By REBEKKA NEUMANN, GERLINDE JUNG, PATRICK LAUX and HARALD
KUNSTMANN
The impact of climate change on precipitation and water
availability is of major concern for policy makers in the
Volta Basin of West Africa, whose economy mainly depends
on rainfed agriculture and hydropower generation. It is
therefore essential to know if, and to which extent
climate trends in the Volta Basin exist that impact water
availability. In this study, the present trends in
precipitation, temperature, and river discharge for the
Volta Basin were analysed. Linear trend and corresponding
levels of significance were calculated for time series of
annual and monthly maxima and corresponding means
respectively. Trends of total annual precipitation and
standard deviations for all considered variables were
analysed. In addition, the stability of linear trends was
considered via reverse arrangement test. Clear positive
trends with high levels of significance were found for
temperature time series. Precipitation time series showed
both positive and negative trends, whereas most
significant trends were negative. However, due to the
small number of significant cases, only weak trends
towards a decrease in precipitation can be concluded. Most
of the significant trends of the standard deviation in
precipitation were negative. Due to this observation a
trend towards a decrease in the variability of
precipitation is concluded. In case of discharge time
series, a small amount of (predominantly positive)
significant trends for the wet season was observed. The
majority of the significant trends for the dry season were
negative. For discharge no clear trend could be evaluated
though, as the anthropogenic influences (e.g. building of
dams, intensified irrigation) could not be quantified.
Both, standard deviation of temperature and of river
discharge show positive and negative significant trends.
Thus one can not draw the conclusion of a change in
temperature and river discharge variability. It is
additionally shown that monthly precipitation trends can
be weakly linked to climate indices. This was achieved by
linear correlation analysis between monthly precipitation
amounts and the climate indices NAO, SOI, TNA, TSA
Demand and supply of improved water in the Ghanaian Volta
Basin
By STEFANIE ENGEL, MARIA ISKANDARANI and MARIA DEL PILAR
USECHE
Growing efforts are undertaken at the local, national, and
international level to improve water supply in developing
countries and thereby help people to satisfy their basic
water needs. This study aims at an evaluation of
households' actual access to and use of improved water
sources in rural communities in the Ghanaian Volta basin.
A statistical analysis of a household survey conducted in
the Volta Basin helps to demonstrate that access to
improved water does not automatically translate into use
of it for all households. Our analysis shows that 43 per
cent of those households that had access to an improved
source still use unsafe water as their main domestic water
source. Relative distance to improved and unimproved
sources, the pricing system established locally, and
quality perceptions all are likely to influence household
water source choices.
Water and electricity sector reforms in Ghana: Back on
track?
By SUSANNAWOLF, VERONIKA FUEST and FELIX ASANTE
The lack of continuous and reliable supply of energy and
water is a major hindrance to industrial and agricultural
productivity in Ghana. Similar to the public utility
management problems found in most developing countries,
the electrical and water utility companies of Ghana have
suffered from a number of institutional and operational
shortcomings, including low collection rates, high
wastage, operational losses, and inadequate staffs.
Reforms of the energy and water sectors have been under
way since the beginning of the nineties. Prices have
increased significantly, but there has been little or no
improvement in services. This analysis will focus on ongoing reforms, their likely effects on different user
groups and the obstacles for their implementation. Those
who provided water and electricity were plagued with
institutional weaknesses, and regulating agencies did not
perform according to their mandates. The regulating
agencies were lacking financial and human resources for
monitoring, and their functioning suffered from principal
agent problems. The government policy of private sector
participation in utility provision has been only partially
implemented for various reasons: the providers of water
and electricity and the government agents that manage the
reform process have a greater effect on manufacturers than
they do on agriculture because the former have the greater
need for regular supplies of water and electricity. The
potentials and interest of the clients, regulating
agencies and providers as well as the relationships
between them have to become a key issue in the reform
process. There is a need for more transparency and
accountability on the part of politicians and providing
agencies - whether public or private, local or national.
Hydrological parameterization through remote sensing in
Volta Basin, West Africa
By MOHSIN HAFEEZ, MARC ANDREINI, JENS LIEBE, JAN FRIESEN,
ANDREAS MARX and NICK VAN DE GIESEN
Ground-based hydrological data collection tends to be
difficult and costly, especially in developing countries
such as Ghana and Burkina Faso where the infrastructure
for scientific monitoring is limited. Remote sensing has
the potential to fill the gaps in observation networks.
The GLOWA Volta Project (GVP) seeks to maximize the
information to be gained from satellite imagery by
combining remotely sensed data with strategically chosen
ground observations. However, there is very limited
information about the coupling of remotely sensed data
with ground based data over the mixed savanna terrain of
West Africa.
This paper provides an overview of innovative techniques
to measure hydrological parameters as actual
evapotranspiration, rainfall, and surface runoff over
mixed savanna terrain in a semi-arid region in West
Africa, and their potential use.
Evapotranspiration - The Surface Energy Balance Algorithm
for Land (SEBAL) was used to calculate sensible heat flux
and evapotranspiration through the energy balance. The
SEBAL parameterization is an iterative and feedback-based
numerical procedure that deduces the radiation, heat and
evaporation fluxes. Along a 1,000 km gradient in the Volta
Basin, three scintillometers were installed to measure
sensible heat flux over distances comparable to NOAA-AVHRR
pixels, approximately two kilometers. The comparison of
sensible heat flux measured from remotely sensed data and
scintillometers provide accurate results. This will help
to increase the reliability of SEBAL parameterization.
Rainfall - Depending on the region within the Volta Basin,
up to 90% of the precipitation in originates from squalllines. The Tropical Rainfall Measuring Mission (TRMM)
imagery provides a valuable tool to monitor such squall
lines. However, the TRMM signal should be validated for
squall line rainfall. To increase the reliability of
space-based rainfall measurements, TRMM based rainfall
rate estimates were calibrated with rainfall measurements
from a dense network of rain gauges. Surface Runoff Remote sensing has limited value in estimating surface
runoff. The savanna of West Africa, however, is dotted
with a large number of small reservoirs used to supply
water for households, cattle, and small scale irrigation.
Bathymetry of sixty reservoirs in Ghana's Upper-East
Region produced a very regular correlation between surface
area, as observable by satellites, and volumes. By using
all-weather RADAR imagery and the measured surface/volume
curves, surface runoff volumes can be monitored throughout
the year. These indirect runoff measurements will help
researchers to develop surface-runoff models for the Volta
Basin.
A hybrid metric-conceptual (HMC) model for monthly
riverflow prediction in the semi-arid Volta Basin of West
Africa
By B.A. AMISIGO and NICK VAN DE GIESEN
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Data-based mechanistic modelling techniques have been
applied to catchment monthly runoff, potential evaporation
and rainfall time series to model monthly catchment runoff
at selected gauging sites in the Volta Basin of West
Africa. The aim of the study was to obtain a modelling
framework that not only accounts for the rainfall-runoff
non-linearity in the basin and provides acceptable
predictions of the monthly catchment runoff for the subbasins studied, but also reveals the necessary insights
for a plausible interpretation of the rainfall-runoff
mechanism in the basin. The rainfall-runoff process was
considered in two stages - a nonlinear transformation from
rainfall to effective rainfall and then a linear
transformation from effective rainfall to runoff. First a
linear time varying, state dependent parameter (LTV-SDP)
transfer function model was applied to the monthly
rainfall-runoff calibration series of each sub-basin to
determine the form of the rainfall-effective rainfall nonlinear transformation. The observed series (runoff or
rainfall) that was significantly related to effective
rainfall was then identified. Next, the functional form of
this relationship was established and used to fit linear
time invariant (LTI) transfer function models relating
monthly runoff to monthly effective rainfall and potential
evaporation. The best estimate of the monthly effective
rainfall from the nonlinear modelling was obtained from
the product of monthly rainfall and a fractional power of
monthly runoff - the runoff acting as surrogate for
catchment wetness. Since this form of effective rainfall
cannot be used for simulation in validation mode (as the
runoff series is unknown then), a non-linear rainfall
filter used in the Identification of unit Hydrographs And
Component flows from Rainfall, Evaporation and Streamflow
data (IHACRES) application was used to model effective
rainfall in this mode. The study showed that monthly
catchment runoff in the basin can be decomposed into two
parallel flows, an instantaneous (within a month) and a
slower (with one month delay) flow components. These
results suggest that baseflow contribution to streamflow
is insignificant in the Volta Basin.