Over the past year, the question of how to account for evaporation and system losses in the Lower Colorado River Basin has become a hot political and policy topic. With the recent Lower Basin water use reduction scheme, we seem to have set the question aside for now. But it’s not going away.
Water management of the Lower Colorado River has long sidestepped the questions of how to account for and assess the impact of reservoir evaporation and system losses. To date, the preferred strategy has been to ignore those losses. The hydrologic gap left by this approach, which leaves an imbalance between the water flowing into Lake Mead and the amount released for downstream users, has been covered by simply releasing water stored in Lake Mead from the wet decade of the 1990s ensuring that no user bears the brunt of a legal interpretation that might reduce their supply. This disconnect between the river’s allocation framework and hydrologic reality is the result of longstanding governance failures by the U.S. and the Lower Basin states – Arizona, California, and Nevada – including failure of the U.S. to factor in reservoir and system losses in the 1944 Treaty with Mexico and failure of the states to negotiate a Lower Basin compact to apportion their share of the river.