Managing wetlands to solve the water crisis in the Katuma River ecosystem, Tanzania

The formerly perennial Katuma River in western Tanzania starts in a protected forest, it then flows through irrigated rice farms before reaching Lake Katavi, a floodplain wetland whose outflow regulates the river flow through the Katavi National Park (KNP) down to its outlet at Lake Rukwa, which has no outlet.  In recent years, due to overexploitation of water for irrigation, the Katuma River dried out for up to four months per year and this greatly degraded the KNP ecosystem, the siltation of river lead to flooding of the adjacent areas during heavy rains, and the water level of Lake Rukwa has decreased by 4 m since 1992 while its fishery yield and water quality also deteriorated.  In 2016, a total of 46 illegal weirs were removed from the Katuma River upstream of KNP. Following that, the river zero-flow periods were reduced by two months and Lake Rukwa water level rose by 1 m. We suggest that the construction of a low V-notch weir at the outlet of the Lake Katavi wetlands would further reduce the Katuma River zero-flow periods by an additional month, thus returning the river nearly to its former perennial status. The enforcement of regulations governing the construction of irrigation weirs is essential. These ecohydrology solutions do not eliminate the threats, but they amplify the opportunities for sustainable development at the basin scale. This example of active governance of water resources at the basin scale can be applied throughout Tanzania and in semi-arid East Africa in general.

Introduction

The African human population is projected to increase by about 1.3 billion from 2017 to 2050, and almost half of this increase will occur in sub-Sahara Africa (UN-DESA, 2017). Increasing human population and associated freshwater demands pose a major threat to freshwater resources and biodiversity in sub-Saharan Africa (UNEP 2006; Jury and Vaux, 2007; UN-DESA, 2017). In sub-Saharan Africa surface water abstractions are often affected by dams, weirs, and diversions which range from small to large scale, and these are geared towards meeting the high food and energy needs of a fast growing human population (UNEP 2010; Lehner et al., 2011). As human needs increase, so does abstraction of water whose practice is often unsustainable and consequently leads to freshwater problems and associated conflicts between people in upstream and downstream areas. This is already the case in much of semi-arid East Africa (Crisman et al., 2003). There are numerous such examples, some resulting in upstream ‘winners’ and downstream ‘losers’, a case where the upstream users enjoy extracting all/most of the water while leaving nothing/little to the downstream users. Such situation also causes human-wildlife conflicts. For example, Gichuki (2002) documented illegal abstraction of water for irrigation farming in the upper Ewaso Ng'iro River in Kenya that extracted up to 80% of the available water in the upstream areas during the dry season, resulting in the lowlands in a marked water scarcity, affecting both people and wildlife and leading to human-wildlife conflicts that resulted in the killing of wild animals. As another example the overharvesting of freshwater for rice irrigation in the historically perennial, upper Great Ruaha River in Tanzania has resulted in drying up the lower reaches of the river for typically three months a year, with severe human and ecological impacts (Kihwele et al., 2012, Kihwele et al., 2018, Mtahiko et al., 2006). Tanzania is severely affected by the developing water crisis due to the lack of or weak/non-existent governance of water resources (Elisa et al., 2010, Kabote and John, 2017, Kihwele et al., 2012, Kiwango et al., 2015). In spite of the regulations relating to water resource use and conservation, improved governance countrywide at the river catchment scale is needed (Seeteram et al., 2019).

This paper focuses on the Katuma River, in the Lake Rukwa drainage basin (Figure 1a). This river is the key source of water for wildlife in Katavi National Park (KNP) in the dry season, and this wildlife is abundant with thousands of buffalo, elephant, impala, zebra, eland, giraffe, sable and hundreds of hippo (WCS 2019). The Katuma River is the main river draining into KNP and Lake Rukwa, which has no outlet and supports an important artisanal fisheries and communities along its shore. The human population growth rate in the river basin is ~ 3.8% per annum, which is well above the 2.9% per annum national average (URT 2014), and this is largely attributable to a high influx of people from other regions of western Tanzania. Their main socio-economic activity is farming and livestock keeping (Silangwa, 2016; Salerno et al., 2017). A severe water crisis is developing in the river basin because of the excessive use of Katuma River for rice irrigation upstream of KNP (Figure 1). In 2014 irrigation accounted for about 80% of all water consumed in the entire river basin (URT 2014) and this lead to the former perennial Katuma River drying up for up to four months a year. The irrigation area generates ‘winners’ and ‘losers’. The winners are the irrigators in the upstream areas. The losers are KNP and its wildlife, as well as the fishermen and cattlemen living downstream of KNP and around Lake Rukwa (Elisa et al., 2010; LRBWB, 2016). Lake Rukwa faces a crisis due to a rapidly growing human population, overfishing, polluted agricultural and mining runoff, siltation from erosion upland from deforestation, overgrazing and poor agricultural practices, decreasing river inflows, and decreasing water levels; indeed it has been predicted that the lake could further shrink and even disappear altogether in the next few decades (Down To Earth, 2017; IUCN-Netherlands, 2017; Lakepedia, 2017; IPP-Media, 2018; NASA Earth Observatory, 2019).

This paper describes how solutions to this water crisis are being sought following the ecohydrology principles described by Zalewski (2002, 2011). First, monitoring and research was carried out to demonstrate the link between hydrological and biological processes at the catchment scale, e.g. the key role of water in sustaining wildlife in KNP and in maintaining Lake Rukwa, and the human impact in degrading that system by withdrawing water from the natural system. Second, an effort was made to restore the basin ecosystem absorbing capacity by removing 46 illegal weirs and restoring the river channel in the irrigated areas upstream of KNP (KNP, 2016). By late 2019 about 20 illegal weirs were already re-built for irrigation farming upstream of KNP. These weirs were later removed in the same year. Third, initiatives are underway to use the ecosystem properties as a management tool, e.g. by designing a low-level V-notch weir to extend the river outflow from the upstream wetland (Lake Katavi) in order to diminish the period of no flow in the river. Those initiatives do not eliminate the threats posed by the water crisis, but they amplify the opportunities for sustainable development at the basin scale.

This paper does this by describing the results of a water budget evaluation focusing on Katuma River flow and its regulation by Lake Katavi, which is misnamed as it is a floodplain wetland, and on Lake Rukwa (Figure 1a, b, c). We used a combination of data obtained from satellite altimetry, field hydrological study and modelling to obtain a time series of the flows in the Katuma River and the water level in Lake Katavi from 2013 to 2019. This enabled us to quantify both their seasonal and inter-annual water variability, the key role of Lake Katavi in regulating the flow in the river, and the relative importance of interannual fluctuations in rainfall and the abstraction of water for paddy irrigation.