ENVIRONMENTAL FLOW REQUIREMENTS OF THE RUSAPE RIVER,
TRANSBOUNDARY SAVE BASIN, ZIMBABWE
F. Love a*, E. Madamombe b, B. Marshall c and E. Kaseke a
a Department of Civil Engineering, University of Zimbabwe, P.O.Box MP167, Mount
Pleasant, Harare, Zimbabwe, tel.: +263-11-749516
email.: fthmbi@yahoo.co.uk , ekaseke@eng.uz.ac.zw
b Data and Research Branch, Zimbabwe National Water Authority, Harare, Zimbabwe,
tel.: +263-4-703175, email.: hycos@mweb.co.zw
c Department of Biological Sciences, University of Zimbabwe, P.O.Box MP167, Mount
Pleasant, Harare, Zimbabwe, tel.: +263-4-303211, email.: bmarshall@science.uz.ac.zw
ABSTRACT
Environmental Flow Requirements for the Rusape River, a tributary of the Save River,
Zimbabwe, were determined using a rapid results approach. Thirty years of hydrological data
with daily time steps from gauging stations upstream and downstream of the Rusape Dam
were analysed using DRIFT Software. The dam appeared to have caused an increase in intraannual and inter-annual flood events downstream compared to upstream, including
significant dry season releases, while inter-annual floods were larger. Interviews with
members of the local community suggested that some native species could have declined
while one exotic had increased in the river after the dam was built. A vegetation transect was
integrated with the flood analyses and suggested that the altered flood patterns could be
related to a possibly increased moisture level in the banks On the other hand, water security
upstream of the dam, especially for Rusape Town, has improved. The water releases from the
dam differ from the natural flow in both volume and frequency, especially in the dry season
and may have had a negative impact on the local ecosystem and subsistence farmers.
Accordingly, a flow regime is recommended for the Rusape River that will reduce or reverse
these impacts, whilst ensuring sufficient water resources are released for economic needs.
Accordingly, a flow regime is recommended for the Rusape River that will reduce or reverse
these impacts, whilst ensuring sufficient water resources are released for economic needs.
Keywords: environmental flow requirements, managed releases, Rusape River
*
To whom all correspondence should be addressed.
1
INTRODUCTION
Environmental flow is the water left in a river, or released into it, for the specific purpose of
managing the condition of that ecosystem and as people are also part of the ecosystem their
needs have to be addressed as “population at risk” (Brown and King, 2003; King et al., 2003).
Environmental water requirements should not be understood as just “water for bugs” or
“water for fish”, but should be understood as water for the environment as a whole system
(Smakhtin, 2002). Thus most techniques used for determining environmental flow
requirements pay attention to multiple aspects of ecosystem functioning and services,
including maintenance of the geomorphology of the river channel and flood plain,
biodiversity, aquatic vegetation, water quality and recreational water use. All of these aspects
have social and economic implications. For example, flows at a certain level may be
associated with river fisheries, which provide food and livelihoods to millions of poor people
in developing countries (Smakhtin, 2002).
Dams change the flow regime of rivers often damaging the ecosystem; in Southern Africa, for
example, concerns have been expressed about the impacts of two large dams on the Zambezi
River (Attwell, 1970; Guy, 1981: Davies et al., 1975; Davies et al., 2000.) Disruption of the
natural flow of the river through changes in the fluvio-geomorphic processes can lead to
either siltation or erosion of riparian zones (World Commission on Dams, 2000). Many
countries are now considering the rivers health in water allocation and Environmental Flow
Requirements (EFR) are now being considered in some countries’ legislation (Ashton and
MacKay, 1996; Jaspers, 2003; Latham, 2001). The Zimbabwe Water Act of 1998 requires
that a certain amount of water must flow downstream for environmental purposes
Water managers therefore need to be in a position to determine how to maintain or restore
ecological health in rivers while meeting the demands of water users. This may lead to
controversial decisions, especially in areas of water scarcity where there might be competition
between people and the environment. The purpose of determining environmental flow
requirements is therefore to determine ways of accommodating the water needs of the
environment without depriving humans of water needed by them.
The only attempts to determine environmental flow requirements in Zimbabwean rivers has
been in the Odzi, Pungwe and Shashe Rivers (Mott Macdonald, 2004; Symphorian et al.,
2003), and the Nyatsime River near Harare (Tirivarombo, 2004). Much more remains to be
done before the EFR concept can become an integral part of the planning of water
development projects and this paper describes an attempt to determine the environmental flow
requirements of the Rusape River below the Rusape Dam in Zimbabwe. The Rusape is a
tributary of the Save, which drains an area of southeastern Zimbabwe before flowing into
Mozambique.
2
Fig. 1. The study area.
The river rises on Lesapi Source farm 45 km north-east of Rusape Dam, which has a
catchment area of 674 km2. Flow in the river is highly seasonal, reaching a peak of around 25
Mm3 per month in February falling to about 2.5 Mm3 in October (Fig. 2). The outflow from
the dam (gauged at station E131) was generally lower than the inflow (gauged at station
E136) during the rainy season from April to December but higher during the rest of the year.
The mean number of days with zero flow ranged from two above the dam to 12 below it
(Table 1).
Table 1. Annual flows at gauging stations in the study area, 1970-2002
Mean annual runoff Mean annual flows Mean annual
Number of days per
3
(Mm /a)
(mm/a)
maximum flow
year with zero flows
(m3/s)
E131
110.9
164.1
115.8
2
E136
111.7
175.9
111.5
12
3
3
Mean Monthly runoff (Mm)
30.00
E131, Downstream
E136, Upstream
25.00
20.00
15.00
10.00
5.00
0.00
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Fig. 2. Mean monthly flows at gauging stations in the study area, 1970-2002.
The only impoundment on the Rusape River is the Rusape Dam which was opened in 1972.
The Rusape Dam is an earth-rockfill dam, and the reservoir has a maximum depth of 36 m
and an effective capacity of 67 Mm3 with a yield of 55.36 Mm3.
Rusape Dam was built to supply water to large agricultural estates at Chisumbanje, as well as
to the town of Rusape. It now operates in conjunction with two other large dams in the Save
Catchment, Osborne, and Ruti, to supply water to Chisumbanje and most of this demand is
met by releases from Osborne. Thus water from Rusape Dam is currently mainly used for the
town of Rusape through upstream abstraction, and by local commercial farmers through
downstream abstraction. Rusape Dam therefore serves as a back-up or alternative water
supply for Chisumbanje, and can be drawn on for that purpose at any time.
METHODS
The use of a multidisciplinary team recommended for EFR data collection (King et al., 2003)
was not possible and the principle investigator, assisted by consultations with appropriate
specialists, undertook the entire programme. This rapid results approach (Figure 3) was used
to provide a quick estimation of EFR, which takes river ecology into account, because of a
shortage of time and a lack of funds.
4
Initial assessments
Hydrological
Analyses
Hydraulic
Assessments
Ecological
Assessments
Sociological
Assessments
Technical consultations
Assessment of
biophysical
impacts down
stream of
development
Assessment of
socio-economic
impacts
downstream of
development,
including
irrigation water
requirements
Outputs
Outputs can
be used for
management
scenarios
Environmental
flow
requirements
Stakeholder
Consultations
Fig. 3. Activities flow chart developed for this project, using some of the methods included in
the Downstream Response To Imposed Flow Transformations – DRIFT (King et al., 2003).
The principal steps in the rapid results approach to estimating ecological flow requirements.
A 30-year period (1970/1-2001/2) was studied. The period was selected because a time series
of more than 25 years is needed to give reasonably good results and the period of record
should include some of the wettest and driest periods, which in this case were the late 1970s
and late 1990s and early 1980s and 1990s, respectively (derived from rainfall records 19702003, Zimbabwe Meteorological Office). The hydrological data was obtained from gauging
stations on the Rusape River above (station E136) and below (station E131) the dam (supplied
by Zimbabwe National Water Authority, Research and Data Department). DRIFT software
(developed by Ninham Shand) was used for hydrological analyses: flow duration curves and
flood analyses. Mean daily flows were used for flood classifications, and mean monthly flows
for suggesting a possible flow regime.
For the purpose of analyses, floods are defined to include both natural high flows and
artificial high flows - due to dam releases. Flood events at the upstream station ranged from
2.30 m3/s upwards. Inter-annual floods (Table 3) are large floods which occur with a
frequency of less than one per year. Intra-annual floods (Table 4) are small to medium sized
floods, which are experienced within a year and they are grouped into the four different
classes.
Table 3. Inter-annual floods, calculated from upstream data using DRIFT software.
Return period Discharge (Q) range (m3/s)
(years)
1 : 20
140.0 and above
5
Return period Discharge (Q) range (m3/s)
(years)
1 : 10
95.0 – 139.9
1:5
73.3 - 94.9
1:2
58.1 - 73.2
Table 3. Intra-annual flood classes, calculated from upstream data using DRIFT software.
Flood Class
Discharge (Q) range (m3/s)
IV
42.6 - 58.0
III
29.2 - 42.5
II
15.7 - 29.1
I
2.3 - 15.6
Below Class I are the low flows.
The environmental flow requirements were then determined by integrating these results
obtained and using the building block method (King and Louw, 1998). This involves the
following steps:
1. The first building block is the minimum release, taken at 70 % of mean monthly
discharge, since a 30 % drop in flow is the generic minimal degradation level
recommended by Brown and King (2003) for rivers in southern Africa. This level can be
used as a starting point and can be altered if demand rises to balance the need for the
environment as well as meeting economic needs.
2. The second building block is the flushing floods, which maintain the channel by flushing
the bed and disposing of poor-quality water at the start of the rainy season (King et al.,
2000). All the Class I floods must be released in October, November and December.
These are determined as follows:
V ( I ) tot
V ( I ) m  n( I ) m 
n( I ) tot
Where V(I)m is the volume of flood events of Class 1 that should be released in month m
n(I)m is the number of Class 1 flood events recorded upstream in month m
V(I)tot is the total volume of class 1 floods for the full time series, as determined by DRIFT software
n(I)tot is the total number of class 1 floods for the full time series, as determined by DRIFT software
3. The third building block is habitat maintenance floods. Release of classes III and IV
floods in the middle of the rainy season maintains the physical habitat heterogeneity (King
et al., 2000).
4. The fourth building block is spawning floods: release of classes I and II floods triggers
spawning (Cambray et al., 1997; King et al., 2000). In Zimbabwe, spawning tends to take
place during November and December, i.e. at the beginning of the wet season. Because
Building Block 2 (flushing floods) requires release of class I floods in October to
December, Building Block 4 need only add the release of class II floods in October and
November.
5. The remaining (so far unallocated) upstream inflow received in any given month is
available for storage in the dam or for release and abstraction by downstream users.
RESULTS
The flow regime of the river has been altered by the dam, with a greater probability of higher
flows in the dry season May to October) occurring downstream because of water being
6
released from the dam while low flows are more likely to occur upstream throughout the year
(Fig. 4).
60
20
Wet season, upstream
Wet season, downstream
55
50
Dry season, upstream
Dry season, downstream
18
16
daily flow (m3/s)
daily flow (m3/s)
45
40
35
30
25
20
15
14
12
10
8
6
4
10
2
5
0
0
0
5
0
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
% Probability of Exceedance
% Probability of Exceedance
Fig. 4. Flow duration curves for the Rusape River, upstream and downstream and Rusape
Dam, 1970/1-2001/2, calculated using DRIFT software: (left) Wet season; (right) dry season.
(Figs. 5 and 6)
60
U/S
D/S
50
Duration ( days)
U/S
D/S
3
16
14
12
10
8
6
4
2
0
Volume Mm /s
No. of events
There are more of the inter-annual floods downstream than upstream of the dam, with more
chance of floods occurring with a smaller return period compared to a larger return period, for
both upstream and downstream floods (Fig. 5). There is more volume for downstream floods
than upstream for 1 in 2 and 1 in 20 years return periods and less volume for downstream than
upstream for the 1 in 5 and 1 in 10 years return periods.
40
30
20
10
0
2
5
10
Return Period
2
20
5
10
Return Period
18
16
14
12
10
8
6
4
2
0
U/S
D/S
2
20
5
10
20
Return Periods
Fig. 5. Characteristics of inter-annual floods, 1970/1-2001/2, as determined by DRIFT
software. U/S = upstream of the dam, D/S = downstream of the dam.
Downstream have more incidents of intra-annual floods than upstream and the number of
flood events decreases with increase in flood size (Fig. 6). There is more volume for upstream
floods than for downstream floods, except for Class IV. As is expected from the manner of
calculation, both upstream and downstream the volume increase with the increase in flood
classes, which is the opposite for the number of flood events. On average the duration in days
is higher for downstream than for upstream.
20
300
3
15
15
U/S
D/S
Duration (days)
U/S
D/S
Volume Mm /s
No. of events
400
200
10
5
U/S
D/S
10
5
100
0
0
I
0
I
II
III
IV
II
III
Classes
Classes
IV
I
II
III
IV
Classes
Fig. 6. Characteristics of intra-annual floods, 1970/1-2001/2, as determined by DRIFT
software. U/S = upstream of the dam, D/S = downstream of the dam.
7
Once the building blocks were assembled, on the basis of the hydrological analyses
performed, it was determined that for most months of the year (all months except November),
the mean monthly flows recorded for the thirty year time series are sufficient to meet the
environmental flow requirements (Fig. 8).
30.00
Available to store/abstract
BB4. Spawning floods
BB3. Habitat Maintenance Floods
BB2. Flushing Floods
BB1. Minimum release
25.00
BB3 class III & IV
20.00
BB4 class II
3
Release Volume (Mm / month)
SURPLUS
(should include class I,
but already in BB2)
15.00
BB2 class I
10.00
BB1
70 % of inflow
5.00
0.00
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Fig. 8. Building block model for environmental flow requirements of the Rusape River,
downstream of Rusape Dam. (BB = building block)
DISCUSSION
The differences in low flows, intra-annual floods and inter-annual floods for upstream and
downstream of the dam, can be attributed mainly to the dam releases creating an artificial
hydrological regime downstream: low flows are not being released as low flows, while
generally large numbers of small flood events are released. The EFR proposed should help
restore the flow to a less artificial regime. It should also provide sufficient volumes of water,
correctly timed to release floods that can assist in habitat maintenance and facilitate fish
spawning. The EFR proposed is practical: the recommended flow regime can be achieved
within mean monthly flows observed over the last 30 years. The slightly higher flows
required in November (where there is no surplus flow) can be met, so long as surplus water
from preceding months is retained in the dam for later release.
Storage of water in the wet season must allow for the retention of a surplus for downstream
users (e.g. Chisumbanje Estate, new irrigation schemes) for release in months where the EFR
estimated does not allow abstraction of any flows. The flow regime estimated in this study
should be presented by the responsible authority (ZINWA) to stakeholders in the Save
Catchment, as a first step leading to implementation of EFR.
A drawback of this study was that no detailed ecological investigation was undertaken, either
during this study or before the dam was constructed. An attempt was made to determine the
8
recollections of local people about how the river ecology had changed (Love, 2005).
However, the problem with the approach is that recollections of elderly people are not always
accurate on incidents during their youth: therefore, such reports can only be indicative.
Determining the EFR for the Save River as a whole is important, because the river is a
transboundary river shared by Zimbabwe upstream and Mozambique downstream. Sharing
this water, whilst ensuring availability of water for the environment, is a principle of the
SADC Protocol on Shared Watercourses (2000). This study has provided an estimate of EFR
for one of the major tributaries of the upper Save River. EFR for another major tributary, the
Odzi, is already known (e.g. Symphorian et al., 2003). If similar study to this one (or a more
detailed one) is carried out at Ruti Dam, then the results can be used in developing EFR for
the upper Save as a whole, which can be incorporated into the Save Catchment Outline Plan
(for Zimbabwe), and ultimately, a management plan for the shared river basin.
Water demand in any system does not remain constant. Further study should determine the
effects on the river, and on the model proposed, of increased demand for water from Rusape
Town, or irrigation schemes downstream, especially Chisumbanje.
ACKNOWLEDGEMENTS
This research was funded by a WaterNet M.Sc. fellowship to F. Love and incorporates
research results from work by F. Love for M.Sc. Integrated Water Resources Management
degree at the University of Zimbabwe. The support and cooperation of the Zimbabwe
National Water Authority (especially the Data and Research Branch and the office of the
Catchment Manager: Save), the Rusape Town Council, the Department of Agricultural
Research and Extension Services: National Herbarium, Chief Makoni and the residents of
Chiduku Communal Land made this study possible. Hodson Makurira and Pinimidzai Sithole
are thanked for helpful discussions.
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