Are we dammed or damned?

As I mentioned in the previous post, the assessment of water resource may influence the direction of the water resource management. A prime example of this is the transition from large scale development such as dam construction towards a more integrated water resource management that recognises the social and economic needs of people as well as the natural environment. The latter no longer focuses solely on the volume/storage of water, it also pays attention to the demands and distribution of water. Furthermore, the environmental flow regime of natural habitats is also recognised under such a framework. This could be greatly attributed to the work many scientists have done in exploring the often not-so-obvious effects of dam constructions. Today in this post, I would like to focus on the hydrological and ecological impacts of dams in Africa through three case study: Itezhi-tezhiDam on Kafue River, Tiga and Kafin Zaki Dam and Challawa Gorge in theHadejia-Nguru river basin and lastly, the small scaled dams in South Africa.

The impacts of the dams on the hydrological regime and flow have been proven in all three locations.

For the Kafue River, the unregulated historic peak and low flow would occur between April and May and between October and November respectively. However, after the installation of the Itezhi-tezhi Dam, the natural hydrological regime has been compromised. The once gradual change in the volume of discharge had turned to episodic increase of flash flow released by the dams. As soon as the dam stop releasing water, the flow runs much lower than historic value. The number of month in which inundation takes place also increases. According to Howard and Williams(1982), discharge greater than 170 cumecs will overcome the embankments and lead to inundation of the floodplain. As can be seen in Figure1, the flow regime curve shows that the area downstream of the Itezhi-tezhi dam now have average flow higher than the threshold capacity of the channel between June and October, a period historically associated with low flow.

Figure1.

Monthly mean discharge of the Kafue River before (solid triangle) and (empty circle) after dam construction. 

For the Hadejia-Nguru river basin, similar situation is also observed due to the construction of dams and a series of irrigation schemes. According to the analysis, if the construction of Kafin Zaki Dam is completed and operated with the aim of providing irrigation to 84000 ha of agricultural land, the discharge downstream would decrease by 80%. This would effectively lead to similar level of drought experience in the 1980s. This would consequently lead to a rapid diminishing level of groundwater storage. Furthermore, the goal of sustaining the proposed 84000 ha of irrigation would not be viable during the 1970s and 80s according to the model simulation. As opposed to the large scale irrigation scheme mentioned in the two previous example, it is also shown that small scales dams with high spatial densities could have significant impact on hydrological regime and reduction on riverflow as well.

One common theme that runs across all example is the shift away from the natural hydrological regime. This has great ecological impacts on the environment and their biota. For example, due to changes in flow disturbance and extended period of inundation in the Kafue river and floodplain, alien invasive woody species such as the Mimosa pigra outcompetes the native aquatic species such as Echinochloa stagnina and Oryza longistaminata. Within a short 7-year period since the commissioning of the Itezhi-tezhi Dam in 1978, the infestation of Mimosa had increased by 50-folds from 2 ha to 100ha. Its extent spread from the head stream to the downstream floodplain in the process. As a keystone species with great potential to alter the nutrient energy flow within the ecosystem through trophic cascade, this is certainly a red flag for the water management practice in the region. Similarly, due to changes in the volume of flow and the time at which it happens, there is imminent threat to the Hadejia-Nguru wetland as well. However, in this case, it is due to the reduced wet season inundation that lead to the degradation and diminishing extent of habitat. Furthermore, the reduced quality of aquatic environment in South Africa caused by the high densities of small dams was shown to have benefitted the opportunistic taxa of macroinvertebrates that are better adapted to slower currents and higher disturbance and pollution. Overall, the construction of dams and its operation have led to multi-faceted on the regional ecology.

Another commonality among the three case studies is the need to strengthen hydrological monitoring. According to Mumba and Thompson (2005), more data is needed to distinguish the impact of climate change and operation of Itezhi-tezhi dam. As for the Hadejia-Nguru river basin, the overall quality of hydrological data has deteriorated significantly due to the lack of maintenance of hydrometric stations (Thompson and Hollis, 1995). Sukhmani et al (2010) also call for consistent assessment of environmental water requirement and this requires integration of reliable and representative data.

To sum up, the call for a better water resource management needs to be met by reliable water resource assessment, which in turn relies on the data. It has been used successfully by scientists to explore the negative impacts of dams.