Limpopo River: steps towards sustainable and integrated water resources management

Transcription

1 Regional Management of Water Resources (Proceedings of a symposium held during die Sixth IAHS Scientific Assembly at Maastricht, The Netherlands, July 2001). IAHS Publ. no. 268, Limpopo River: steps towards sustainable and integrated water resources management R. A. JEAN BOROTO Department of Water Affairs and Forestry, Private Bag X313, Pretoria 0001, South Africa borotoj@dwaf.pwv.gov.za Abstract This paper discusses the pattern of past developments in the Limpopo River basin which is shared by Botswana, Mozambique, South Africa and Zimbabwe and the prospects of future development potential. Water use is mainly for irrigation, urban supply and mining. In the past, each country exploited its resources at will, whereas at present, collaboration with other co-basin states is a requirement. This cooperation has resulted in joint studies; the scope and findings of two such studies are discussed in this paper. From these studies, the need for the improvement of the hydrological monitoring network has emerged as a priority. A stronger institutional cooperation and the implementation of new water legislation in each country are expected to favour a sustainable and integrated development of the Limpopo River basin. Key words international water sharing; southern Africa; Limpopo River; semiarid; transmission losses; Pitman monthly hydrological model; flow balance INTRODUCTION The water resources of the Limpopo River (catchment area: km 2 ) are shared by Botswana, Mozambique, South Africa and Zimbabwe. The basin's climate varies spatially from arid in the west through semiarid and temperate in the central zones to semiarid in the east, but with a few sub-humid pockets near the centre. The Limpopo main-stem environment is semiarid throughout its length and its flow is not perennial. The mean annual rainfall ranges between 300 and 1500 mm (with an average of 450 mm) and the mean annual potential evaporation varies between 1400 and 2200 mm. The river is some 1750 km long, serving as the border between South Africa on one side and Botswana and Zimbabwe on the other. It then flows through Mozambique before discharging into the Indian Ocean. Given its international character as well as the relatively high degree of water utilization, the Limpopo River is a prime candidate for integrated water resource management at a regional scale. However, institutional collaboration between the four countries was only initiated in the mid 1980s and two subsequent joint studies took place afterwards as further discussed. Table 1 which has been derived from Pallet (1997) gives details of selected shared rivers by countries of the Southern Africa Development Community (SADC). It illustrates the semiarid climate prevailing in the Limpopo River basin with a unit runoff of 13 mm compared to 330 mm for the Congo River (which is located in tropical and equatorial climates). The unit runoff is defined as the ratio between the Mean Annual Runoff (MAR) at the mouth and the total surface area of the river basin.

2 34 R. A. Jean Boroto Table 1 Geographic details of selected SADC international river basins. River basin Basin area (km 2 ) River length (km) Mean Annual Runoff at mouth (10 6 m 3 year" 1 ) Congo Cunene Limpopo Okavango Orange Save Zambezi Unit runoff (mm) PATTERNS OF PAST DEVELOPMENTS In the past, each country developed its water resources unilaterally, without consultation with the other co-basin states. Developments were dictated by the respective needs of each country. Impoundment of surface water per country as derived from Pallet (1997) - Botswana has some four dams (including the newly constructed Letsibogo Dam) in the Limpopo catchment, the biggest is the Gaborone Dam with a capacity of 144 x 10 6 m 3. The combined storage capacity of these dams is less than 350 x 10 6 m 3. Most of this water is used for urban consumption in Gaborone and the expanding southeastern part of Botswana. The Letsibogo dam on the Motloutse tributary will augment water supply to Gaborone via the North-South Carrier. - Mozambique has one major dam, Massingir Dam on the Olifants River, with a storage capacity of 2840 x 10 6 m 3 which is mainly for irrigation. The Chokwe irrigation scheme ( ha) along the banks of the Limpopo River is supposedly its largest consumptive water user (an estimated 517 x 10 6 m 3 year" 1 ) in Mozambique; however, most of the water flows back to the Limpopo River as return flow because large parts of the scheme are not operational at present. - South Africa with more tributaries has many dams (more than 20 with the largest dam being Loskop Dam on the Olifants River with a capacity of 348 x 10 6 m 3 ) in the Limpopo catchment with a combined storage capacity of the order of 1900 x 10 6 m 3. The main water use is for agriculture and the urban sector with some degree of support to the mining sector. The portion of the Limpopo River catchment in South Africa is highly developed with the economic heart of South Africa, Gauteng, mainly located within the catchment. This situation makes it necessary to import water from adjacent catchments such as the Incomati and Orange/Vaal rivers. - Zimbabwe has more than 10 dams in the Limpopo River catchment with the largest being Manyuchi Dam (319 x 10 6 m 3 ) and a total storage capacity of nearly 1000 x 10 6 m 3. Most of the water is used for irrigation, urban supply to the town of Bulawayo (the second largest town in Zimbabwe) and for mining.

3 Limpopo River: steps towards sustainable and integrated water resources management 35 Development in major tributaries The extent of total development (large and small farm dams combined) in tributaries is illustrated by Gôrgens & Boroto (1997) in Table 2 which gives, for the major tributaries and where available, an estimate of the natural and present day Mean Annual Runoff (MAR) as well as the derived percentage reduction in MAR which is a direct measure of the extent of development in each tributary. This table has been derived from different studies on the tributaries and it is to be noted that these studies covered different longterm periods and denaturalization horizons and used different hydrological estimation methods. Along the Limpopo River main-stem, mainly between Botswana and South Africa, some 74 weirs were erected for storage of water when the river is flowing. In addition, there are important alluvial aquifer deposits along the Limpopo River and water is pumped from these aquifers. Irrigation is mainly by means of centre pivots or by normal sprinkler systems. The losses that are incurred by these systems are surmised to be high because of their inefficiency and because of the high evaporation rates. Table 2 Limpopo tributaries ranked per estimated "current-day" MAR. Tributary Catchment area (km 2 ) Naturalized MAR (10 6 m 3 ) "Current-day" MAR Reduction in (10 6 m 3 ) MAR (%) Olifants > Luvuvhu Shashe ? 462? Umzingwani ? 350? Mwenezi ? 256? Crocodile Mokolo Motloutse * 0* Lephalala Nzhelele Mogalakwena Lotsane Bubi 8 140? 53? Marico Sand River Notwane Matlabas Bonwapitse Mahalapswe * Will change when utilization of Letsibogo Dam increases. INITIATIVES RESULTING FROM INTERNATIONAL COLLABORATION Chenje & Johnson (1996) report that the first steps towards international collaboration amongst the co-basin states of the Limpopo River saw the creation of the Limpopo River Basin Permanent Technical Committee (LBPTC) by the four co-basin states in Two studies subsequently were commissioned, the first between Botswana and

4 36 R. A. Jean Boroto South Africa and the second involving the four countries. The two studies are summarized below, with the second being more extensively discussed given that it is the most recent and that the author was the main researcher. Other developments resulting from the international collaboration between the four co-basin states are also discussed. The Joint Upper Limpopo Basin Study Botswana and South Africa undertook the Joint Upper Limpopo Basin Study (JULBS) which was commissioned in the late 1980s and completed in This study by Gôrgens et al. (1991) provided a first understanding of the hydrological dynamics of the Limpopo River and showed that transmission losses were an important component of the water balance along the main-stem and its major tributaries. This study also showed that there were serious hydrological monitoring shortcomings; these were subsequently addressed by the building of two cableway stations in the Upper Limpopo at Buffelsdrift and Seleka Farm respectively. The latter cableway was particularly useful in the hydrological study that ensued as explained below. The hydrological modelling of the main-stem of the Limpopo River The activities of the LBPTC continued and intensified in the mid 1990s. The utilization of the main-stem flows had become a point of interest for water planners in the four co-basin states, as unabated development had taken place in most tributaries. This resulted in the first study undertaken on the main-stem of the Limpopo River. This study, by Gôrgens & Boroto (1997, 1999) focused on the hydrology of the main stem of the Limpopo River, defined by a fictitious line joining the most downstream flow gauging station on each tributary. It proceeded as follows: - A hydrometric review focused on streamgauging, rainfall and evaporation, alluvial geohydrology as well as water and land use. Greater attention was however given to streamgauging as it provided a direct measure of the available surface water resources. - The evaluation of streamgauging revealed that the actions taken as a result of the JULBS study have yielded fruits in that flow balance could be reconciled satisfactorily in the upper reach; this was attributed to more reliable flow information gathered with the erection of the Buffelsdrift and Seleka Farm cableways. The flow record at the Seleka Farm cableway was particularly useful in this regard in that it provided the means to establish that, of the two streamgauging stations at Oxenham Ranch (A5H003) and Martinsdrift (A5H006), 3.6 km apart and consisting each of a weir, the latter was more reliable. This provided an anchoring point for the hydrological model that was set up for the Limpopo River main-stem. - South Africa and Zimbabwe had each a streamgauging station (A7H004/A7H008 and B35 respectively) on either side of the Limpopo River at Beit Bridge. Upon analysis of their respective records, discrepancies of the order of as much as 60% were revealed for selected periods. This resulted in a need to thoroughly compare the

5 Limpopo River: steps towards sustainable and integrated water resources management 37 two records in order to reconcile them. The reconciliation necessitated that South Africa and Zimbabwe agree to exchange primary information from their respective stations, in the form of recorder charts. These had to be compared, week for week, for a period between 1959 and This analysis took two years to complete and resulted in a reconciled record that is now providing a second anchoring point for the hydrological model. - The configuration of a monthly hydrological model for the main-stem of the Limpopo River was done on the SHELL modelling system developed by Berg et al. (1991) which is based on the monthly Pitman model which is widely used in southern Africa. Model parameters for South Africa by Midgley et al. (1994) were used as a starting point in the model calibration. Input data had to be prepared, consisting of rainfall, evaporation, upstream and tributary inflows, land- and water-use details relating to dams, irrigation, afforestation, point abstractions and sources, channel and riparian zone dimensions, alluvial depth and extent, and porosity. More than 90 modelling cells were required to cover the entire study area. - The hydrological model made it possible to derive the flow balance given in Table 3 for the period from 1971 to It showed that transmission losses in the form of alluvial channel, riparian and flood plain recharge, evaporation and consumptive use are significant components of the water balance and have in the recent past been markedly greater than irrigation and other water use. Table 3 Flow balance details for primary sub-reaches for period (10 6 m 3 year" 1 ). Component Upstream from Sterkloop Sterkloop to Shashe Shase to Beit Bridge Pafuri to Beit Brid] je to Pafuri Chokwe Inflow: upstream 0 497(obs.) Inflow: tributaries Study area incremental runoff Total entering the reach Irrigation abstraction/other use: * main-stem and lower sections of tributaries in study area Alluvial channel and riparian vegetation losses Simulated at sub-reach exit Recorded at flowgauging station % Difference +8.8% * Diverted to Chokwe Irrigation Scheme, but a significant proportion is returned, unused, to the Limpopo downstream of the flow gauging station. Note: a follow up study using the improved record from Beit Bridge will provide an updated flow balance. This new balance was not available at the time of writing this paper. Strengthening of the institutional framework of collaboration The management of the water resources of the Limpopo River is effectively taking place under a stronger institutional cooperation. The four countries are members of the Southern African Development Community (SADC) and have ratified the UN Convention for Non Navigational Use of Shared Water Courses as well as the SADC

6 38 R. A. Jean Boroto Protocol on Shared Water Resources. The latter, which is specific to SADC is inspired by the former, its importance stems from the fact that 15 river basins are fully or partially shared by SADC countries. In the spirit of this collaboration, any development intended by any of the countries has to involve the others by at least sharing the relevant information. Botswana, for instance, is considering constructing the lower Shashe Dam and has to consult the other co-basin states. Likewise Zimbabwe and South Africa have to consult the others for the construction of a weir across the Limpopo River upstream of Beit Bridge for the abstraction of flood water by farmers on either side of the river. Table 4 shows the possible impact that the proposed weir would have on the MAR of the Limpopo River at Beit Bridge, assuming that a certain flow (0.8 mv, 1 mv 1 and 2 m 3 s" 1 in this illustration) is abstracted at a given rate only when the flow recorded at Beit Bridge exceeds a certain rate. The calculation has been done using the reconciled flow daily record at Beit Bridge, based on data from 1971 to The table shows that if 0.8 m 3 s" 1 is abstracted at the proposed weir, when the flow recorded at Beit Bridge exceeds 10 m 3 s" 1, then the MAR at Beit Bridge will be reduced by 0.47%, assuming that there is sufficient capacity to store all the water abstracted. Such information is shared with all the co-basin states so as to appreciate the impact of the proposed development and to enable them to authorize it or impose their conditions. Table 4 Reduction in MAR at Beit Bridge if a certain flow (0.8 to 2 m 3 s~ l ) is abstracted then the flow recorded at Beit Bridge exceeds a certain level (10 to 25 m 3 s" ). Abstraction at weir If flow at Beit If flow at Beit If flow at Beit IfflowatBeit Bridge exceeds Bridge exceeds Bridge exceeds Bridge exceeds lonrs 1 15 mv 20 rr? s" 1 25 rn 3 s" mv 0.47% 0.42% 0.39% 0.36% 1.0 mv 0.59% 0.53% 0.48% 0.45% 2.0 mv 1.19% 1.06% 0.97% 0.90% THE WAY FORWARD In a context of greater socio-economic development in the SADC countries, shared water resources will have to be efficiently used in order to meet future competing demands. The potential for conflict is great if countries do not put in place a strong structure with mechanisms of consultation and collaboration for intended water resource developments. The Limpopo Basin Commission, which will soon be established, will be a statutory body that will be able to commission future studies and to advise co-basin states on future developments of the water resources of the Limpopo River. The forthcoming Basin Study will be the first comprehensive study that will assess the resources of the Limpopo River basin. Its stated objective, as defined by the Limpopo River Permanent Technical Committee (2000), is to quantify the present and future water balance in the Limpopo River basin in each of the four co-basin states, and to plan future water resource development or management options so as to meet the future water demands as optimally, sustainably and equitably as possible. Each country has to respond to growing needs. The political changes that have taken place in South Africa for example have made it necessary to develop an appropriate legal

7 Limpopo River: steps towards sustainable and integrated water resources management 39 framework for the management of water resources. The National Water Act (Act 36 of 1998) makes provision, among others, for international water sharing obligations and the protection of the environment. A reform in the ownership of water resources has brought the concept of a water use license as opposed to a water right. As a corollary, a new water pricing strategy is being devised which will inevitably force irrigators to implement more efficient methods to limit their water consumption. Finally, the impact of future market forces in SADC on water utilization in the Limpopo River basin will have to be watched with interest. It is possible that agriculture might develop in the wetter countries and importing food from these countries might be a cheaper alternative. This might make more water available for other sectors. REFERENCES Berg, R. R., Beuster, H. & Gorgens A. H. M. (1991) Innovative modelling strategies for flow generation in developed winter rainfall catchments. In: Proceedings of the Fifth SA National Hydrological Symposium (Stellenbosch), to SANCIAHS, Stellenbosch, South Africa. Chenje, M. & Johnson, P. (eds) (1996) Water in Southern Africa. SADC/IUCN/SARDC, Maseru/Harare, Zimbabwe. Gôrgens, A. H. M. & Boroto, R. A. J. (1997) Limpopo River: flow balance anomalies, surprises and implications for integrated water resources management. In: Proceedings of the Eighth SA National Hydrological Symposium (Pretoria). SANCIAHS, Pretoria, South Africa. Gôrgens, A. H. M. & Boroto, R. A. J. (1999) Limpopo River: hydrological investigations to prepare for integrated water resources planning. In: Proceeding of the Ninth SA National Hydrological Symposium (Cape Town). SANCIAHS, Cape Town, South Africa. Gorgens, A. H. M., Beuster, H. & Hallifax, P. (1991) The Upper Limpopo: flow generation in a very large semi-arid river basin. In: Proceedings of the Fifth SA National Hydrological Symposium (Stellenbosch), to SANCIAHS, Stellenbosch, South Africa. Limpopo River Permanent Technical Committee (2000) Draft terms of reference of the Limpopo River basin study (June 2000). Midgley, D. C, Pitman, W. V. & Middleton, B. (1994) Surface Water Resources of South Africa Water Research Commission, Pretoria, South Africa. Pallet, J. (ed.) (1997) Sharing Water in Southern Africa. Desert Research Foundation of Namibia, Windhoek, Namibia.