The Altiplano is an endoreic catchment (196 000 km2) of the Central Andes. At present, the wet season corresponds to the southward displacement of the intertropical convergence zone (I.T.C.Z.). The annual rainfall decreases from the north (about 800 mm.yr-1) to the south (about 200 mm.yr-1). The studied sub-catchment is the northern one of Titicaca lake (56 275 km2). Over the last decades, the water level of this lake has fluctuated of few meters around 3810 m.a.s.l. and the mean value of the surface outflow was 50 m3/s. During the last 11 000 years, the lake level has dropped down to 3710 m. An hydrological annual balance model allows to simulate the time evolution of the water level according to different rainfall scenarios. This lumped model takes into account the modifications of the ratio between the lake area and the watershed area. Using present potential evaporation, this model shows that a 18% decrease of the rainfall induces a drop of 100 meters in 380 years. On the base of this annual model, we developed a second one with a three-month time step. It takes into account the reservoir effect of the soil. This model includes two parameters whose values were calibrated with the hydrological data available on the last 15 years. For another 10- year period, the computed lake levels are in good agreement with the observations. The computed altitude of the water level decreases, like for the first model, of 100 m when the annual rain amount is 18% less than the present amount. However, the duration to achieve this value is lower than with the annual mass balance model. It is 340 years when 80% of the rainfall concentrates on three months and it is 310 years when the rainfall amount is well distributed all over the year.
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