11 Chapter 2 Biophysical characteristics INTRODUCTION The Limpopo River Basin is situated in the east of because of its orographic effect. There is also a southern Africa between about 20 and 26 °S and north–south rainfall gradient towards the Limpopo 25 and 35 °E. It covers an area of 412 938 km2. River. Figure 4 shows the basin in relation to major Rainfall varies from a low of 200 mm in the hot physical features of the subcontinent. The basin dry areas to 1 500 mm in the high rainfall areas. straddles four countries: Botswana, Mozambique, The majority of the catchment receives less than South Africa and Zimbabwe. Figure 5 shows the 500 mm of rainfall per year (Figure 7). The hot dry main overland transport routes, urban centres, areas receiving about 200–400 mm of annual rainfall rivers and nature conservation areas in the basin. are located mostly within the main Limpopo River Valley itself. CLIMATE Rainfall is highly seasonal with 95 percent Classification of the climate of the Limpopo occurring between October and April, often with River Basin a mid-season dry spell during critical periods of Climate conditions vary considerably in southern crop growth. It occurs on a few isolated rain days Africa, as the subcontinent lies at the transition of and isolated locations, seldom exceeding 50 rain major climate zones. The climate in the Limpopo days per year. Rainfall varies significantly between River Basin is influenced by air masses of different years, with maximum monthly rainfall being as origins: the equatorial convergence zone, the high as 340 mm compared with mean monthly subtropical eastern continental moist maritime rainfalls of 50–100 mm for January, February and (with regular occurrence of cyclones), and the dry March. continental tropical and marine west Mediterranean (winter rains) (Bhalotra, 1987b; GOB–MMRWA, 1991; Schulze, 1997; Unganai, 1998). FIGURE 4 According to the Köppen Classification The Limpopo River Basin in relation to country (Köppen, 1918; Rosenberg, 1999), the basin is boundaries and physical features of the subcontinent predominantly semi-arid, dry and hot (BSh in Figure 6). The central river valley is arid, dry and hot (BWh). Here, the average rainfall is less than 400 mm with likely crop failure in 75–90 percent Zimbabwe of years (Reddy, 1985; 1986). The South African highveldt part of the basin is temperate with Mozambique summer rainfall and cool to hot summers (Cwc Botswana Limpopo and Cwa). The Mozambique coastal plain is River Basin mainly warm-temperate with no dry season and hot summers. Rainfall The Limpopo River Basin is a region of summer South Africa rainfall, generally with low precipitation. The overall feature of the mean annual precipitation is that it decreases fairly uniformly westwards from the northern reaches of the Drakensberg Escarpment across the interior plateau. However, rainfall is highest on the Drakensberg Escarpment 12 Drought impact mitigation and prevention in the Limpopo River Basin FIGURE 5 The Limpopo River Basin in relation to transport routes and urban centres Legend Towns International and basin boundaries Parks Water bodies Main roads National roads Rivers Railways The Limpopo River Basin generally experiences FIGURE 6 Köppen climate classes short rainfall seasons, except for some of the outer limits of the basin that have higher rainfall and longer seasons. The rainfall concentration index Gwanda is 60 percent and above, and this limits crop Francistown production because most of the annual rainfall is Selebi Pakwe Bokonang received in a short period of time. Serowe Musina Pafuri A rainfall concentration index of 100 percent Thohoyandou Chigubo implies that a location receives all its rainfall in a single month. The rainfall season usually begins in Polokwane Massingir early summer (late November to early December) Gaborone for the southernmost parts of the basin and in Chokwe mid-summer (mid-December to January) for the Xai-Xai central parts of the basin around the Limpopo Pretoria Johannesburg Legend River itself. The rainfall season lasts an average of Af BSk Cwc four months. BSh BWh Cfa Cwa Rainfall in Botswana is caused mainly by convection thunderstorms, which typically occur Chapter 2 – Biophysical characteristics 13 FIGURE 7 Average seasonal total rainfall in the Limpopo River Basin 27 °E 30 ° 33 ° BOTSWANA N 21 °S ZIMBABWE Mean annual rainfall (mm) Tuti Umzingwani MOZAMBIQUE 300 Shashe Bubye 400 500 Motloutse Mwanazi 600 700 Limpopo 800 Lorsane 900 1 000 Sand Levuwbu Mogalakwena 1 250 Changane Shingwedzi Middle Laphalala Lelaba Mokolo Great Lelaba 24 ° Ngotwane Selati Olifants Blyde Crocodile Steelpoort Marico Wilge Pienaars MOZAMBIQUE Elands Little SOUTH Olifants AFRICA Rolt Indian Ocean 050100 150 200 km Source: UNCTAD (2003). as localized events with a high spatial and temporal 40 percent in the Limpopo River Basin. Along variability. The annual rainfall in the Botswana part the coastal strip, the mean annual rainfall is 800– of the Limpopo River Basin varies from 350 mm in 1 000 mm, declining to less than 400 mm in the dry the northeast to about 550 mm in the southeast. interior bordering Zimbabwe. Zimbabwe experiences a single annual rainy Rainfall generally has to exceed a minimum season of five months (November–March), threshold of 20–30 mm before any runoff occurs, associated with the summer movement of the Inter- owing to high temperatures, low humidity and Tropical Convergence Zone over southern Africa. flat terrain. Many rainstorms are less than this and Within the Zimbabwe part of Limpopo River Basin, hence the flow regimes of rivers vary considerably. the mean annual rainfall varies from slightly more This results in high storage requirements for dams than 600 mm in the southern highveldt (Bulawayo) in order to deliver the yields that are required. to less than 400 mm in the southeastern lowveldt Increased storage is costly and causes increased (Tuli and Beitbridge). The annual variability is evaporation losses. considerable, with a coefficient of variation (CV) of about 40 percent. The probability of receiving more than 500 mm of rainfall in any year is less Evaporation than 60 percent in the southern highveldt and less Evaporation within the Limpopo River Basin varies than 30 percent in the southeastern lowveldt (with from 1 600 mm/year to more than 2 600 mm/year. less than 10 percent in Beitbridge). The highest evaporation occurs in the hot Limpopo In Mozambique, the generalized rainfall pattern River Valley. High levels of evaporation mean that shows a sea-to-land gradient with a CV of about the soil dries up quickly and this reduces the 14 Drought impact mitigation and prevention in the Limpopo River Basin reduces effective rainfall, runoff, soil infiltration FIGURE 8 Estimated length of growing season derived from and groundwater recharge. Evaporation loss from satellite imagery dams is significant owing to the high storage–yield relationship and flat dam basins. LGP Zero Dryland subsistence farming is generally not ZIMBABWE 1 – 60 days 61 – 70 days viable given the variable rainfall, high evaporation 71 – 80 days 81 – 90 days 91 – 100 days and high evapotranspiration. Figure 8 shows the 101 – 110 days 111 – 120 days estimated length of growing period (LGP) derived > 120 days Excluded from satellite imagery. Figure 9 shows the average BOTSWANA MOZAMBIQUE crop water availability for selected months. SOUTH AFRICA Limpopo basin boundary International boundary Temperature Summers in the Limpopo River Basin are generally warm, and winters are mild. In summer, daily amount of water available for plant uptake. This temperatures may exceed 40 °C, while in winter results in crops being more prone to drought. temperatures may fall to below 0 °C. The general Evaporation from open water in Botswana varies figures for air temperature are related closely to from about 1 900 mm/year to 2 200 mm/year. altitude, and also to proximity to the ocean. The Slightly lower annual figures of about 1 500 mm mean maximum daily temperature in most of are derived from evapotranspiration calculations the Limpopo River Basin, notably South Africa, based on the Penman method. Daily figures range Botswana and Zimbabwe, varies from about 30– from about 2 mm to 5 mm. Evaporation is highest 34 °C in the summer to 22–26 °C in winter. The during the rainfall season, and it significantly mean minimum daily temperature in most areas FIGURE 9 Average crop water availability expressed as the relation between monthly rainfall, R, of selected months and the reference evapotranspiration, ETo September November January March May July R > ETo R = 0.5 – 1.0 ETo R < 0.5 ETo Source: E. Mellaart, personal communication (2003). Chapter 2 – Biophysical characteristics 15 lies between 18–22 °C in summer and 5–10 °C in prerequisite to effective agriculture is a description winter. of the rainfall regime in response to questions The eastern and northern parts of the Limpopo (Dennett, 1987) such as: River Basin are frost-free while the southern and • When is rain likely to occur? western areas experience winter frosts. Frost • What is the probability of a dry spell greater does not occur in Mozambique and it occurs than a certain length? only occasionally in the southern highveldt of • What is the probability of receiving a daily Zimbabwe, associated with an influx of cold dry air rainfall greater than a particular amount? from the southeast. Frost-free areas also exist in the Probabilistic rainfall models to address these lowveldt of South Africa and along the Limpopo questions have been developed by Stern, Dennett River in the Messina area. and Dale (1982), and Stern and Coe (1982). These Most of the higher-lying areas in South Africa models have been applied to rainfall data from and Botswana within the Limpopo River Basin Gaborone and Tshane, two Botswana stations, experience frost, occurring most severely in the 350 km apart.