An integrated biosphere model (IBIS) and st hydrological routing algorithm
(HYDRA) are used in conjunction with long time-series climate data to inves
tigate the response of the Lake Chad drainage basin of northern Africa to c
limate variability and water use practices over the last 43 years. The simu
lated discharge, lake level, and lake area of the drainage basin for the pe
riod 1953-1979 are in good agreement with the observations. For example, th
e correlation coefficient (r(2)) between the simulated and the observed lev
el of Lake Chad for the 288 months of available observations is 0.93. Altho
ugh irrigation is only a. modest portion of the hydrology in the period 195
3-1979; representing only 5 of the 30% decrease in simulated lake area for
the decade 1966-1975, the simulated lake level and area are in better agree
ment with the observations when irrigation is included. For the period 1983
-1994 the observed water use for irrigation increased fourfold compared to
1953-1979. A comparison of the simulated surface water area, with and witho
ut irrigation, suggests that climate variability still controls the interan
nual fluctuations of the water inflow but that human water use accounts for
roughly 50% of the observed decrease in lake area since the 1960s and 1970
s.