A water resources planning response to climate change in the Senegal River Basin

The Senegal River Basin (SRB), located in the Sahel region of West Africa, is simultaneously undergoing fundamental environmental, hydrologic and socioeconomic transitions. The tri-nation (Senegal, Mauritania and Mali) river basin development authority, the Organisation pour la Mise en Valeur du Fleuve Senegal (OMVS), is attempting to execute a shift to irrigated rice production for domestic consumption in the river basin in order to ease the severe foreign exchange shortfalls these riparian nations face. With the recent completion of the Manantali and Diama dams, year-round irrigated agriculture is now possible in the SRB. The full agricultural development potential of the SRB is constrained, however, by the basin's limited water resources. Significantly, a time series analysis of Senegal River hydrology has provided powerful evidence that the prolonged Sahelian drought may be permanent. The basic hydrologic constraint on development is revealed in a time series decomposition of Senegal River annual flow volumes, which strongly suggests that water resources availability has been substantially curtailed since 1960. Two alternative time series mechanisms are hypothesized to account for the decreased flow volumes in recent decades. The first time series model suggests the presence of a long-term periodicity, while the second model hypothesizes an ARMA(1,1) process. The second hypothesis provides a superior model fit. The stationary ARMA(1,1) model can be fit successfully, however, only after explicitly removing a non-stationary component by linearly detrending after 1960. The implication of non-stationarity in Senegal River hydrology provides analytic evidence that the landscape degradation and desertification processes observed in Sahelian Africa can be in part attributed to climate change effects. The negative impact of the state-imposed rice production policy compounds the severe effects of the drought on the river basin ecology. Rice production in the arid river valley has been a financial and social failure. Irrigated rice projects suffer a high rate of abandonment and have intensified the desertification process in the river valley. As an alternative use of the basin's scarce water resources, an agricultural development policy based on village-scale irrigation projects and intensive, irrigated agro-forestry projects has been proposed. Village-scale irrigation is dedicated to low-water-consumption cereal grain crops and is managed by traditional socio-political structures. The agroforestry production system analysed has the dual objectives of using irrigation to re-establish a protective bio-mass cover in the desertifying river valley and of reversing the drought-induced migration from rural to urban areas. A comparative river system simulation study was conducted to analyse the effects of both the rice production development policy (policy RP) and the natural resources management policy (policy NRM) on the SRB's full agricultural development potential. The simulation study compared three alternative hydrologic scenarios using, (1) the pre-drought era, (2) the 1970s level drought and (3) the 1980s level drought. Principles of dynamic programming were applied to Manantali reservoir management to optimize water allocation in each scenario. All the hydrologic scenarios generated showed that the lower overall water-demand pattern exerted by policy NRM allows a higher full agricultural development potential than does policy RP. (C) 1997 Academic Press Limited