Role of hydrological forecasts and river flow modelling in rational agricultural water management in the perspective of a climate change — A case study of the Rivers Upper Nile and Niger
Abstract An international scientific conference under the auspices of WMO, UNEP and ICSU on the role of CO 2 and other greenhouse gases in climate variation made a statement that, if the concentration of CO 2 in the atmosphere doubles in the near future as present trends indicate, the global mean surface temperature would increase by 1.5–4.5°C. The paper discusses the impact such a climate change may have on water resources and on their management in agriculture. It stresses the importance of the possible increase in variance and frequency of extreme climatic events and subsequent water yield. It further outlines the ways of exploring the consequences of such development by using hydrological and water management models. It also proposes that real-time hydrological forecasting systems, able to produce forecasts with long, medium and short lead times, are one of the most effective measures to offset possible negative effects of the climate change, by increasing the effectiveness of storage management. These propositions are documented by means of case studies from developing countries using models on the River Nile, in particular in the basin of Lake Victoria, and of a hydrological forecasting system on the River Niger. It is proposed that it is important to develop a simulation model of the whole River Nile, together with a real-time forecasting system on the river in order to operate efficiently existing and future storages. Modelling of climate change impact on the water yields of River Niger is also discussed.
Sensitivity of water resource systems to climate variation
ABSTRACT The problem of the response of water resource systems to climate variations is stated. The importance of the sensitivity of the systems to such variations is demonstrated with respect to the stationarity of the mean and the extremes. A definition of moderate climate variation is adopted. Results of deterministic modelling of the influence of such a variation on streamflow are reported, in extreme conditions of arid and humid basins. The results are used in modelling the influence of climate variations on reservoir storage systems. The model indicates a considerable amplification of the effect of climate change on runoff and storage and emphasizes the importance of studies of sensitivity of water resource systems to climate variation.
Lake Muzahi, Rwanda: limnological features and phytoplankton production
Lake Muhazi, a small lake of Rwanda (East Africa) was studied from 1986 to 1990. A dramatic decrease of the catch of Oreochromis niloticus (350 T y−1 in the fifties vs 30 T y−1 in 1982) suggested a loss of productivity or overfishing. In the same period, other ecological changes occurred: the submerged macrophytes regressed and there was a decrease in Secchi depth (0.65 m in 1987 vs 1.5 m in the fifties). Compared to other lakes of the same area, the plankton production seemed low. The results of the present study characterize lake Muhazi as a shallow lake with a rather unstable diurnal stratification and with slight differences in mixing regime between its eastern, deepest part and its western, shallowest part. Secchi disk depth does not vary seasonally to a large extent. The water has a rather high mineral content (conductivity of about 500 µS cm−1 at 25 °C) and low concentrations of dissolved N and P, except in the hypolimnion, where NHinf4sup+-N can be high.Two species, Microcystis aeruginosa and Ceratium hirundinella, account for most of the phytoplankton biomass, which is about 50–80 mg chlorophyll a m−2 in the euphotic zone, usually with little seasonal variation. Daily gross production estimates amount to about 6 to 9.5 g O2 m−2 d−1 with a significant difference between the two parts of the lake. Data on C:N and C:P ratio in the phytoplankton suggest that some N deficiency might occur in the eastern part. Moreover, the Zm:Zc ratio could also lead to rather low net production rates (0.21–0.25 d−1 for a mixed layer of 4 m)In conclusion, the primary production of lake Muhazi is medium for African lakes and the hypothesis that decreased planktonic production could account for a reduced fish production should be discarded. Whereas the present yield of the fishery is only 20 kg ha−1 y−1, the yield estimated from primary production ranges between 46 and 64 kg ha−1 y−1. This could be reached through proper management. Finally, some hypotheses are given to explain the ecological changes which occurred in the lake.