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INFLUENCE OF HUMAN ACTIVITIES ON THE HYDROLOGY OF THE INSIZA
RIVER, LIMPOPO BASIN, ZIMBABWE - IMPLICATIONS FOR CATCHMENT
PLANNING
Jean-Marie Kileshye Onema1,2*, Johan Rockström3,4, Dominic Mazvimavi5,6,
David Love3,7,8, Marloes L. Mul2,9, André van Rooyen7 and Steve Twomlow7
1
Department of Civil Engineering, University of Lubumbashi, PO Box 1825, Kasapa
Campus, Lubumbashi, Democratic Republic of the Congo. Tel.:+243-810-653896. Email.:
[email protected], [email protected]
2
Department of Civil Engineering, University of Zimbabwe, PO Box MP167, Mt.
Pleasant, Harare, Zimbabwe. Tel.: +263-4-336725. Email.: [email protected]
3
WaterNet, PO Box MP600, Mt. Pleasant, Harare, Zimbabwe. Tel.: +263-4-336725.
Email.: [email protected], [email protected]
4
Stockholm Environment Institute, Box 2142, SE-103 14 Stockholm, Sweden. Tel.: + 46
80412 1403. Email.: [email protected]
5
Department of Geography and Environmental Science, University of Zimbabwe, PO
Box MP167, Mt. Pleasant, Harare, Zimbabwe. Tel.:+263-4-303211. Email.:
[email protected]
6
Harry Oppenheimer Institute for Okavango Research, Maun, Botswana. Tel.:+ 267 686
1833. Email.: [email protected]
7
ICRISAT-Bulawayo, Matopos Research Station, PO Box 776 Bulawayo, Zimbabwe.
Tel.: +263-838-311/2/3/4. Email. [email protected], [email protected],
[email protected]
*
Corresponding author
1
8
Department of Geology, University of Zimbabwe, PO Box MP167, Mt. Pleasant,
Harare, Zimbabwe. Tel.: +263-4-303211. Email.: [email protected]
9
UNESCO-IHE, PO Box 3015, 2601 DA Delft, The Netherlands. Tel.: +31-15-2151715.
Email.: [email protected]
ABSTRACT
A study was undertaken to assess changes in flow characteristics of a medium-sized river over
time and interactions with human activity in a semi-arid sub-catchment. The study area,
Insiza sub-catchment, covers an area of 3400 km2 within the Zimbabwean portion of the
Limpopo Basin. The analysis covered 35 years of discharge data from four gauging stations
(flow duration, number of days with zero flows, runoff coefficients). Human influences over
water resources were assessed through the effect of hydrological infrastructure mainly the
construction of large dams and changes in land use. Results indicate a decrease in flows at
the sub-catchment scale (2,260 km2) from the 1960s to the 1990s. However, at meso-scale
(400 km2) the runoff coefficient increased from 2% in the 1960s to over 6% in the 1990s. This
can be related to increasing population and consequent intensification of agriculture,
conversion of mixed impacted land into crop fields as observed during the 1990s. These
changes could be the cause of the increase in runoff generation at meso-scale. Dam
management affected the flow regime of the river by reducing days with zero flows, the
hydrological regime downstream of the dams are dominated by releases, especially during
the dry season.
2
The results of this study suggest that population increase and consequent land use change
influence runoff generation at meso-scale, but have a much more limited effect on water
resource availability at sub-catchment scale. The latter, in an ephemeral river such as the
Insiza, seems to be mainly controlled by releases from major dams. This supports the
necessity of a community-level scale of decision-making for local catchments, since the
community-level activities have their effect at this level, coupled with catchment level
decision-making on major dams - which effect more distant communities.
Keywords: hydrological regime, dam development, catchment planning
INTRODUCTION
Human activities and hydrology
Both human activity and climate have a very significant effect on the hydrological cycle.
These effects are increasing as a consequence of the rapid increase in population, agricultural
production, and industrialization (Refsgaard et al.1989). Previous studies in Zimbabwe have
focused on the effect of land use changes on the hydrological response (Lorup et al, 1998).
The study by Lorup et al (1998) found that there were no significant changes in areas covered
by different land cover types on catchments located in regions where the sub-division of the
country into communal and commercial farming areas had been made over 30 years ago.
Flow characteristics did not change over time. Andrews and Bullock (1994) assessed the
effects of afforestation on hydrological responses on basins within the Eastern Highlands of
Zimbabwe. The initial clearing of indigenous vegetation in preparation for planting pines
resulted in increased water yield. Water yield decreased as the pines matured. Zimbabwe like
3
most of the southern African countries receives rainfall during a four month rainy season
(mid-November to mid-March) with the rest of the year being dry. Rainfall has high interannual variability. The country is underlain mostly by crystalline basement rocks with poor
potential for groundwater occurrence (Interconsult A/S, 1985). Due to the combination of
highly seasonal rainfall, and poor potential for groundwater occurrence which could sustain
dry season flows, most of the rivers only flow during the rainy season. Rainfall and therefore
runoff have also high inter-annual variability. Thus water shortages are experienced in some
years. The development of dams to store water during the rainy season for the dry season, and
also storing water during years with above average rainfall for supply during years with below
average rainfall has been a major strategy (Mazvimavi, 1998). The development of
commercial agriculture, mining, and urban areas has always been linked to dam development
in Zimbabwe. There are over 10,000 dams in Zimbabwe. The impact of dam development on
flow characteristics has not been adequately examined in Zimbabwe. Mazvimavi (1998)
compared the proportion of total storage in hydrological sub-zones of Zimbabwe to the mean
annual runoff of the respective sub-zones. However, no analysis was made of how specific
flow characteristics are modified by dams. This paper assesses the effect of dam development
on flow characteristic of one river basin in Zimbabwe.
Study area
The study has been undertaken in Insiza River basin (3401 km 2) located within the semi-arid
southern part of Zimbabwe (Fig 1). Mean annual rainfall varies from 480 mm/yr in the
southern part to 690 mm/yr in the northern part of Insiza River basin, and occurs over a
limited period of time between November and March, with often a large portion of the annual
precipitation falling in a small number of events (Butterworth et al., 1999). Rainfall in the
study area appears to be have been declining over the last 30 years (Moyo, 2005).
4
The Insiza River is a tributary of Mzingwane River, which drains into the Limpopo River,
contributing around 9 % of the unit runoff of the latter river (Görgens and Boroto, 1997).
The catchment is underlain by gneissic and granitic rocks, giving rise to moderately shallow,
coarse grained kaolinitic sands, and moderately shallow clays and loams (Ashton et al., 2001).
Land use is mainly a mixture of croplands, pastureland and woodland (Hearn et al., 2001).
The north part with moderately high rainfall has low open woodland of Combretum-acarciaterminalia associated with granitic or gneissic derived sandy soils. Towards the south sparse
low mopane woodland is gradually replaced by Terminalia sericea open woodland
(Timberlake 1989). Cropping includes commercial farming (largely resettled) in the north,
often under irrigation, and smallholder farming (mostly rainfed) in the south. Irrigation in the
south includes irrigation schemes, managed by farmer committees, and household level
irrigation – the latter mostly for vegetable gardens and often using drip kits (Maisiri, 2004;
Chigerwe et al., 2004).
Kileshye Onema (2004) observed land use changes in the Insiza sub-catchment, between
1991 and 2000. A combination of housing, grazing and crop fields (mixed impacted lands)
was turned into crop fields, this conversion was observed on 14% of the total area of the
Insiza sub-catchment. Further such changes can be expected due to the decrease in farm size
under the recent land reform programme (Moyo, 2004), especially in the Upper Insiza, where
the commercial farms were located.
5
Fig. 1. Location of Insiza subcatchment within Zimbabwe’s agro-ecological zones.
METHODOLOGY
There are four flow measuring stations that have were established by the then Department of
Water Development between 1966 and 1968 within the Insiza River basin (Table 1 and Fig.
2). These stations are equipped with automatic flow measuring instruments. Daily flow data
for these stations for the 1966-2001 periods were obtained from the Zimbabwe National
Water Authority
Table 1: Insiza River Gauging stations details. Source: Data and Research Division ZINWA,
2004
6
Station
River
Catchment Area
Opening
(km)2
Date
401
30/10/1968
570
5/11/1965
2260
5/10/1966
3025
22/09/1966
Location
Number
Insiza Upstream of Upper Insiza
B75
Dam
Insiza Downstream of Upper Insiza
B57
Dam
Insiza Upstream of Silalabuhwa
B69
Dam
Insiza Downstream of Silalabuhwa
B65
Dam
The flow measuring stations presented in Table 1 are all located along the same river, Insiza
River. Flow characteristics of these stations upstream and downstream of the dams are
compared to determine the effect of dams along the river. Secondly flow characteristic are
compared on trends between decades. The following methods were used to analyse the flow
characteristics.
Flow Duration Curve
Flow duration curves show graphically the relationship between any given discharge and the
percentage of time that the discharge is exceeded. The curve can be drawn for daily or
monthly flow data or for any consecutive D-day or month period. For this study, flow
duration curves were generated using daily flows for a period of ten years covering all records
available for each gauging station.
7
Number of days with zero flows
The number of days with zero flows within the period of assessment have been also looked at
to find out if there is any increase or decrease in the number of days with zero flows over time
and also to locate changes spatially and temporally for each gauging station in the subcatchment. The time step considered for this assessment was yearly.
Runoff coefficient
The runoff coefficients in the area were looked at last. The trends and changes over time were
analysed in order to assess if there is any increase and decrease in the runoff generation and
also to link those results with the land use changes and activities. From the mean annual
rainfall, the flows and the area of each gauging station runoff coefficients were determined for
each one of those gauging stations.
Water resources in Insiza sub-catchment
The Insiza sub-catchment is located in two hydrological zones, Upper Insiza located in the
highveld with moderate precipitations with mean annual rainfall ranging from 594 to
694mm/yr. For the Lower Insiza mean annual rainfall varies between 483 to 682mm/yr. In the
same way mean annual runoff declines from the Upper Insiza to the Lower Insiza, see table 2.
8
Table 2. Insiza sub-catchment surface water summary, after
Ministry of Water Resources and Development (1983).
Upper Insiza
Dam
B75
Catchment
Are
Mean
Coefficient Mean
a
annual
of
annual
Km2
runoff
variation
runoff
mm/yr
%
103m3/y
B57
Insiza Dam
r
Upper Insiza
1,57
50
130
Silalabuhwa
Dam
79,000
B69
2
Lower Insiza
1,82
B65
38
125
70,000
9
Fig. 2. Insiza subcatchment showing
major features and gauging stations.
With the coefficients of variation of run-off ranging from 125 to 130, there is a great potential
for carry-over of water stored in good years in order to balance out the low run-off in poor
years. This has been exploited in the development by government of three large dams on the
Insiza River and ten small dams on the tributaries (Table 3 and Fig. 3).
Table 2. Large dam development in the Insiza sub-catchment. Source: Data and Research
Division ZINWA, 2004
Name of Dam
Capacity
Year
(103 m3)
constructed
Insiza River Mainstem
Upper Insiza (formerly
8,829
1967
Fort Rixon)
9
Insiza (formerly Mayfair
173,491
1973
23,454
1966
or Cunningham)
Silalabuhwa (also known
as Silalajani)
Subtotal
205,774
Tributaries
Siwaze
2,325
Masholomoshe
1,068
Sukwe
1,700
1971
Makoshi
926
1998
Matiboni
228
1996
Nedzhiwa
19
1996
130
1998
Doro
262
1997
Thake
435
1999
Zhulube
440
Subtotal
7,533
Mleja (also known as
1987
Manzamhlope)
Total for Insiza
213,307
Subcatchment
10
250000
3
3
Cumulate capacity (10 m )
200000
150000
100000
50000
0
1960
1965
1970
1975
1980
1985
1990
1995
2000
2005
Fig. 3. Cumulative capacity of large dam development, Insiza sub-catchment.
As fig 3. shows the largest increase in storage was realised in the 1960s and early 1970s,
whereas the smaller dams located in the tributaries do not contribute to the total storage
capacity in the Insiza sub-catchment.
RESULTS AND DISCUSSION
Hydrology
Rainfall statistics
11
Figure 4:Rainfall- and flows for B75
The annual rainfall characteristics in the Insiza sub-catchment are represented by the rainfall
from Filabusi station (for location see fig 2.). As shown in fig 4, mean annual rainfall amounts
to 542 mm/yr. It also shows that the rainfall during the 1970s was substantially higher that
during the 1980s and 1990s, with 649, 489 and 495 mm/yr respectively.
Flow Duration Curves
Gauging stations B75 and B69 are located respectively upstream of Upper Insiza Dam and
Silalabuhwa Dam. Discharges occurrences and values are decreasing during the period, with
the lowest values located in the 1990’s for both B75 and B69. As compared to the 1970s, with
45% of time exceeding 0.1 m3/s, in the 1990s this is reduced to 20%. This trend is visible for
all flows, decreasing from the 1970s to the 1980s and decreasing even further during the
1990s. Although the trend from the 1970s to the 1980s is logical since the rainfall statistics
12
indicate that the 1970s were very wet years. Still during the last decade the flows has
decreased (figs. 5 and 6).
Figure 5.Flow Duration Curves of B75, Upstream Insiza Dam
Flow Duration curves B69
1000
100
Flow m^3/s
10
70s
1
80s
90s
0.1
0.01
0.001
0
20
40
60
80
% of time
Figure 6.Flow Duration Curves of B69, upstream Silalabuhwa Dam
13
100
120
Flow Duration curve B57
1000
100
flow m^3/s
10
70s
1
80s
90s
0.1
0.01
0.001
0
20
40
60
80
100
120
% of time
Figure 7.Flow Duration Curves of B57, Downstream Insiza Dam
Flow Duration curve B65
1000
100
Flow m^3/s
10
70s
80s
90s
1
0.1
0.01
0.001
0
20
40
60
80
100
120
% of time
Figure 8.Flow Duration Curves of B65, downstream Silalabuhwa
Gauging stations B57 and B65 are located downstream of Upper Insiza Dam and Silalabuhwa
Dam respectively. High values in discharges and occurrence are again found in the 1970s, the
wettest period of the study. For the station B57 the discharges in the 1980s and 1990s are
similar, however for station B65 the difference can clearly be seen in the medium flows
14
(1m3/s), which are more dominant during the 1980s. With low flows being dominant during
the 1990s.
Comparing the upstream flow duration curve with the downstream for Silalabuhwa Dam, B69
and B65, it is clear that the dam increases the low flows during the 1990s, with flows
exceeding 0.1 m3/s increases from 15-25 % and during the 1970s increasing from 40-90 %.
15
Number of days with zero flows
This flow characteristic is showing that the Insiza River is generally an ephemeral river with
an average of 200 days/yr with zero flows (Fig. 9).
Number of days without flows B65
350
300
250
Days
200
Series1
150
100
50
0
66
71
76
81
86
91
96
Years
Fig. 9. Numbers of days per year without flows recorded for the Insiza River at (from top to
bottom and left to right) B75, B57, B69 and B65.
During the 1970s the days without flows decreased due to the high rainfall amounts,
sometimes even reducing to zero. Comparing upstream to downstream gauging stations, it is
Number of days without flows, B69
Number of days without flows B57
400
350
350
300
300
250
250
200
Series1
Days
Days
400
200
Series1
150
150
100
100
50
50
0
0
66
66
71
76
81
86
91
96
71
76
81
86
91
96
Years
years
shown that in 1960s upstream of Upper Insiza Dam there are a substantial amount of days
without flow, where downstream there are flows almost throughout the year. Similar
behaviour is seen for Silalabuhwa Dam. This shows that dam management reduces days with
16
zero flows and turns an ephemeral river into a perennial river. However while doing the same
analysis in the 1990s, even downstream of the dams there are substantial amount of days with
zero flows comparable to upstream.
Runoff coefficients
For the runoff coefficient analysis only the gauging stations upstream of the dams were
considered since downstream is too much affected by dam management (Fig. 10).
Runoff coefficients B75
12
10
%
8
6
4
2
years
99
/0
0
97
/9
8
94
/9
5
92
/9
3
90
/9
1
88
/8
9
86
/8
7
84
/8
5
82
/8
3
80
/8
1
78
/7
9
76
/7
7
74
/7
5
72
/7
3
70
/7
1
68
/6
9
0
Runoff coefficients B75
Fig. 10. Runoff coefficients recorded for the Insiza River at (from top to bottom) B75,
andB69
For B75, located upstream of the upper Insiza Dam and with the smallest catchment area (400
km2), shows an increase in runoff generation from a maximum of 2% in the 1960s to a
17
maximum of 6% in the 1980s and 1990s, during the wet 1970s runoff coefficient even
increased to 11%.
Station B69, located the furthest downstream and with the largest catchment area (2,260 km2),
showed runoff coefficients from the 1960s compared to the ones in the 1980s and 1990s are
presenting similar values between 2-3 %. During the 1970s the runoff coefficient increased up
to 18%.
Comparing the runoff coefficient during wet periods (1970s) the runoff coefficient increases
with increasing catchment area, however during normal and dry years runoff coefficient
decreases with increasing catchment area.
CONCLUSIONS
It is clear that the dam development in the Insiza sub-catchment do have impacts on the
hydrology, where the major dams were built in the 1970s, with the gauging stations upstream
and downstream of the dam having different hydrological characteristics, decreasing the low
flows (<0.1 m3/s) and increasing the medium flows (1m3/s) from upstream to downstream.
Smaller dams in the tributaries of the Insiza sub-catchment were built at the end of the 1990s,
it is envisaged that these will alter the flows upstream of the Insiza and Silalabuhwa Dams.
However downstream the operation of these two major dams will still dominate the flow..
The results of this study suggest that runoff generation changes at meso-scale have a much
more limited effect on water resource availability at sub-catchment scale. The latter, in an
18
ephemeral river such as the Insiza, seems to be mainly controlled by releases from major
dams.
This supports the necessity of a community-level scale of decision-making for local
catchments, since the community-level activities have their effect at this level, coupled with
catchment level decision-making on major dams - which effect more distant communities.
ACKNOWLEDGEMENTS
The research presented in this paper was supported by a Belgian Technical Cooperation
scholarship and a grant from WaterNet to J.M. Kileshye Onema and in-kind contributions by
ICRISAT-Bulawayo. This paper is a contribution to WaterNet Challenge Program Project 17
“Integrated Water Resource Management for Improved Rural Livelihoods: Managing risk,
mitigating drought and improving water productivity in the water scarce Limpopo Basin”,
funded through the Consultative Group on International Agricultural Research’s Challenge
Program on Water and Food. The opinions and results presented in this paper are those of the
authors and do not necessarily represent the donors or participating institutions.
The cooperation of the office of the District Administrator (Insiza) and the Zimbabwe
National Water Authority (especially the Research and Data Branch and the office of the
Catchment Manager: Mzingwane) made this research possible.
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19
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21
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