Part 2: Hydrological variability and change in the Zambezi River Basin

The hydrology of the Zambezi River Basin was first described in detail by Balek (1971). The most comprehensive treatment of Zambezi hydrological variability is provided by Beilfuss (2002), covering natural and dam-induced patterns of runoff, indicators of hydrological change, characteristics of extreme flooding events, and the reliability of the long-term hydrological data. Recent hydrological assessments, including Euroconsult and Mott Macdonald (2008), World Bank (2010) and SWRSD Zambezi Joint Venture (2010), have focused on hydropower investment potential and improved dam operations. Key findings of these hydrological studies are provided in this chapter.

The basin's climate is largely controlled by the movement of air masses associated with the Inter-Tropical Convergence Zone (ITCZ). Rainfall occurs predominantly during the summer (November to March), and the winter months (April to October) are usually dry. The average annual rainfall over the basin is about 960 mm, but varies from more than 1,500 mm per year in the northern highlands to less than 600 mm per year in the low-lying south/ southwestern portion of the basin. Rainfall is characterized by considerable variation across the basin and over time. Droughts of several years' duration have been recorded almost every decade. Large floods occur with similar frequency.

The natural flow regime of the Zambezi River reflects these rainfall patterns and is characterized by high seasonal and annual variability. Zambezi tributaries draining the steep gorges of the Central African Plateau peak rapidly with the rains, reaching their maximum discharge between January and March and decreasing to dry season minimal flows by October-November. In the Zambezi headwaters, Kafue River, and Shire River basins, large floodplain systems capture floodwaters and may delay peak discharges until late in the rainy season or early dry season. The average runoff efficiency1 across the entire basin is only 8.3% – on average only 80 mm runoff is generated annually from nearly 1000 mm annual rainfall. Most rainfall is stored in floodplains and other landscape depressions or intercepted by plants, where it is lost to evaporation (average annual potential evaporation2 is more than 1,560 mm) or infiltrates to groundwater to maintain Zambezi base flows during the dry season. The total volume of natural (unregulated) annual runoff is estimated to be 110,732 million m3 (Mm3), a flow rate of 3,511 m3/s.

For planning purposes, the Zambezi Basin is typically divided into three regions comprising 13 sub-basins (Figure 2). These include the Upper Zambezi, Kabompo, Lungwebungo, Luanginga, Barotse, and Cuando/Chobe sub-basins in Upper Zambezi region, the Kariba, Mupata, Kafue, and Luangwa sub-basins in the Middle Zambezi region, and the Tete, Lake Malawi/Shire, and Zambezi Delta sub-basins Lower Zambezi region. Table 1 gives the catchment area sizes, the mean annual rainfall, potential evapotranspiration, mean annual runoff, and runoff efficiency for each of these sub-basins. A detailed description of the hydrological characteristics of the Zambezi sub-basins is provided in the Appendix. In the following sections, we examine the unique patterns of hydrological variability that characterize the Zambezi River Basin.

1. Runoff efficiency (defined in terms of a dimensionless runoff coefficient) is the fraction of total rainfall that occurs as runoff.

2. Potential evaporation or potential evapotranspiration (PET) is defined as the amount of soil and water evaporation and plant transpiration that would occur if a sufficient water source were available.