Buzi, Pungwe and Save River Basins

Buzi, Pungwe and Save River Basins

The Buzi, Pungwe and Save (BUPUSA) Basins have a total area of 161 446 sq km shared by Mozambique (MOZ) and Zimbabwe (ZWE) (Table 1, Figure 1). The largest parts of the Pungwe and Buzi, 95.3% and 87.1% respectively, occur in Mozambique with the remainder being in Zimbabwe. In contrast, the Save Basin has 83% of the area in Zimbabwe and covers 21.5% of this country. The Save Basin covers 9% of the national area of Mozambique. The total area of the three basins is almost shared equally by the two countries with 44.7% being in Mozambique and 55.3% in Zimbabwe (Table 1).

Table 1: Basin areas of the BUPUSA Basins, and the proportions of the basin areas falling in Mozambique and Zimbabwe respectively.

PungweBuziSaveTotal – BUPUSA
Basin Area (km2)31 15128 870101 425161 446
Area of Basin in MOZ (km2)29 69025 13717 27772 104
Area of Basin in ZWE (km2)1 4613 73384 14989 343
% of the Basin in MOZ95.387.117.044.7
% of Basin in ZWE4.712.983.055.3
% of MOZ National Area3.73.12.29.0
% of ZWE National Area0.41.021.522.9

The three basins have watersheds or catchment divides with relatively high altitude, 800 – 2500 m above sea level, occurring along the Mozambique – Zimbabwe border (Figure 1). These high altitude areas comprise the Vumba (peak height 1911 m), Chimanimani (2430 m) and Nyanga Mountains (2592 m) which have a north-south orientation stretching for about 300 km. The Save Basin has on the western and northern part a 1400 – 1700 m high watershed with the Zambezi River. The Save Basin also shares a watershed with the Limpopo River on the southern part. The areas with relatively high altitude, greater than 1400 m, have significant effects on climate such as having a) relatively low average annual temperatures than would be expected in a tropical location, and b) high rainfall due to the orographic effects. Although these areas have limited spatial extent, they contribute significantly to river flows in each basin.

Figure 1: BUPUSA River Basins which are exclusively shared by Mozambique and Zimbabwe

Surface Water Resources

The Save Basin has been subdivided into 12 sub-basins (9 in Zimbabwe, 3 in Mozambique) for water resources assessment and management at the transboundary level (Figure 5). The Pungwe Basin has 12 sub-basins (2 Zimbabwe, 10 Mozambique), while these are 11 for the Buzi Basin (3 Zimbabwe, 8 Mozambique).

Figure 5: Sub-basins of the Save, Pungwe and Buzi Basins used for water resources assessment and management.

Information contained in the Water Sharing agreements and reports of previous investigations was used to determine the available water resources;

Figure 6: Mean annual runoff with most parts of the Save Basin generating less than 100 mm/year, and  200 – 1200 mm/yr for the mountainous border regions.. 

The Mean Annual Runoff (MAR) ranges from 80 mm/yr (8157 Mm3/yr) for the Save Basin, 143 mm/yr (4450 Mm3/yr) for the Pungwe Basin, and 197 mm/yr (5701 Mm3/yr) for the Buzi Basin.(Table 1, 2, 3) (Hydroconsult, 2017; Consultec, 2013, 2015). The most productive regions in terms of runoff production are the mountainous areas with altitudes above 800 m and occurring mainly along the Mozambique-Zimbabwe border (Figure 1, 2 and 5). Due to the significant topographic gradient dominating the mountainous areas, the proportion of rainfall forming runoff or runoff coefficient varies from 30% to 60%. Globally, mountain areas like the ones in the BUPUSA Basins that produce significant amounts of surface flows and sustain base flows during dry season are considered as strategic water source areas or water towers.

 

Buzi Basin

About 60% of the Buzi Basin has relatively high runoff with MAR ranging from 200 to 380 mm/yr (Table 2). As expected the mountainous areas are the most productive with MAR of 260 – 380 mm/year.  The Lucite Zimbabwe sub-basin covers 5% of the Buzi Basin and generates 10% of the 5701 Mm3/yr basin flow (Table 1) (Consultec, 2015).

Table 1: Mean annual runoff of the sub-basins of the Buzi Basin

Sub-basinBasin Area (km2)% of Basin AreaMAR (Mm3/yr)MAR (mm/yr)% of Basin Runoff
Revue Zimbabwe5101.81733393.0
Lucite Zimbabwe1 5405.358237810.2
Buzi Zimbabwe1 6905.94462647.8
Sub-total ZWE3 740131 20132121.0
Upper Buzi4 48015.589820015.8
Middle Buzi4 33015411957.2
Lower Buzi3 26011.3189583.3
Upper Lucite3 25011.3112434519.7
Lower Lucite1 8806.52671414.7
Upper Revue2 3308.14992138.8
Middle Revue2 4608.54511827.9
Lower Revue3 14010.966121011.6
Sub-total MOZ25 130874 50017979.0
Total28 8701005701197100.0

The runoff coefficient for the basin is as high as 27% in the mountainous upper parts of the basin and 18% for the whole basin.

Pungwe Basin

Table 1 gives an area of the Honde Sub-basin in Zimbabwe as 776 km2 which differs from that given in the Pungwe Water Sharing Agreement. The agreement gives this area as 1245 km2 which is incorrect as this includes the part of the Honde Basin within Mozambique up to the Pungwe River confluence. The Pungwe Monograph in Annex I Sector Study on the Surface Water Resources of the Pungwe River Basin states on Page 13 that 1463 km2 of the whole Pungwe Basin is in Zimbabwe (SWECO, 2004). This is made up of 687 km2 of the Pungwe Zimbabwe, 528 km2 Honde River up to the border, and 248 km2 of the Nyamkwarara River. The combined area of the Nyamkwarara River and Honde River up to the border is 776 km2 referred to in this report as Honde Zimbabwe. Thus Honde Mozambique covers 469 km2. Table 1 provides the mean annual runoff (MAR) of all the sub-basins of the Pungwe Basin.

The Pungwe Basin in Zimbabwe covers about 5% of the whole basin. This has an MAR ranging from 426 – 1124 mm/yr, and contributes 25% of the average annual flow of the whole basin (Table 1 and Figure 6) (Consultec, 2013). The Nyazonia Sub-basin of the Pungwe Basin is also highly productive and has an MAR of 325 mm/yr.  The Urema and Lower Pungwe Sub-basins have altitude in the 10 – 200 m, low gradient and hence have relatively low runoff, 82 and 71 mm/yr respectively.

Table 2: Mean annual runoff of sub-basins within the Pungwe Basin (Consultec, 2013)

Sub-BasinArea (km2)% Basin AreaMAR (Mm3/yr)MAR (mm/yr)% Basin MAR
Pungwe Zimbabwe6872.27721 12417.3
Honde Zimbabwe7762.53304267.4
SUB-TOTAL ZWE1 4634.71 10275324.8
Honde Mozambique4691.52004264.5
Upper Pungwe2 8469.13281157.4
Nyazonia1 3604.44423259.9
Middle Pungwe5 39017.357410712.9
Vunduzi3 43911.049914511.2
Muda1 3364.31601203.6
Lower Pungwe6 44520.74587110.3
Urema8 40227.06878215.4
SUB-TOTAL MOZ29 68795.33 34811375.2
TOTAL31 1501004 450143100

The proportion of mean annual rainfall (MAP) that constitute mean annual runoff or runoff coefficient is 60% on the mountainous part in Zimbabwe, 5% – 6% in the lowlands, i.e. Urema and Lower Pungwe sub-basins, and 11% for the whole basin.

 Save Basin

The MAR of the Save Basin is 8157 Mm3/yr or 80 mm/yr (Hydroconsult, 2017). The Mozambican part of the basin contributes 16% while this is 84% for Zimbabwe. The southern and eastern parts of the Save Basin are semi-arid receiving 500 – 700 mm/yr of rainfall. These regions generate in comparison to all other BUPUSA sub-basins relatively low runoff, 30 – 70 mm/yr (Table 3, Figure 6). The Upper Save and Odzi sub-basins are the most productive with MAR of 115 mm/yr and 159 mm/yr respectively.

Table 3: Mean annual runoff for sub-basins of the Save Basin.

Sub-BasinArea (km2)% of Basin AreaMAR (Mm3/yr)MAR  (mm/yr)% of MAR
Upper Runde15 80215.69005711.0
Lower Runde5 8025.7179312.2
Tokwe7 9477.8543686.7
Mutirikwe7 9417.8477605.8
Chiredzi 35643.5166462.0
Devure-Nyazwidzi8 1538.0768949.4
Upper Save17 79117.52 04111525.0
Odzi7 2397.11 15415914.2
Lower Save9 9109.8595607.3
Sub-total ZWE84 14983.06 8228183.6
 
Massagena4 5104.4198442.4
Conare6 5016.4523816.4
Save-Estuary6 2666.2613987.5
Sub-total MOZ17 27717.01 3357716.4
 
Total101 426183.08 15780100.0

The Save Basin has a low runoff coefficient of 11% with this being 15% – 16% in the Odzi and Upper Save sub-basins, and 6% in the low lying Lower Save and Massangena sub-basin.