The Lerma-Chapala river basin: a case study of water transfer in a closed basin

The water resources of the largest river basin in Mexico, the Lerma-Chapala River, are overcommitted by approximately 10%. A multi-state agreement provides for reallocation of water from agricultural use to environmental and municipal uses during periods of water stress as indicated by extreme low water levels in Lake Chapala. In 1999 the level of water in the lake was such that the ecology of the lake was severely threatened. Based on the agreement, 240 million cubic meters of surface water originally intended for irrigation was diverted to the lake. This represented approximately 70% of the surface supply available to the 700,000 ha of irrigation, and approximately 36% of the total supply. No compensation was paid to the irrigators who lost this resource. The over-use of both the surface and groundwater resources suggests that some permanent reallocation of water from agriculture is inevitable.