Historical Perspective on the Accounting for Evaporation and System Losses in the Lower Colorado River Basin

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.

My Science Be Dammed co-author Eric Kuhn and I have just published a dive into the issues:

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.



  1. Several years ago I calculated average evaporation rates for Lake Mead using data from the study

    “Evaporation from Lake Mead, Nevada and Arizona, March 2010 through February 2012”
    By Michael T. Moreo and Amy Swancar

    Using the area-capacity data (Lake volume as a function of its surface area) given by


    I found the average evaporation rate for the two periods studied in the paper (March 2010-Feb 2011 and March 2011 to Feb 2012) to be

    M10-F11: 2.07 m/yr
    M11-F12: 1.87 m/yr

    which is almost identical to the values given in the paper. These come to 5.7 and 5.1 mm/day, respectively.

  2. Nice piece. You laid it out quite well. I agree with most of your talking points but want to make a point or two myself.

    We’re all pretty much are on the same page saying that the resources were over allocated.

    Also, I think that most of us understand that politics was the driving action that brought us in this direction. Politics was and now is the driving factor in where we’re going in the basin. Science, environmental and social justice also factor in. Pure politics is and will be the main factor.

    Water losses in the system:

    Understand this. There is no precise way to measure total losses in the system. We can only make an estimate based on the available data that is available. The focus of your post here was evaporation. You briefly mentioned about losses due to seepage and vegetation growth adjacent to the river. The other (ghost) loss is the accounting aspect. All of these factor into the total equation. Again, I want to stress that the losses inherent in the Lower Colorado and other river basins are based on estimates and not cold hard data.

    Your paper also stresses that the Federal Government and the lower basin States should be held accountable for not looking into the evaporation on the reservoirs and the river on the Lower Colorado. I see from the perspective from the Upper Colorado that this should be factored in. I do know from my personal experience that evap was being measured in Boulder City by Joe Donnelly in the mid 90’s. In the 2010 timeframe a more detailed study was conducted by the USGS on Lake Mead. Dr Paul Miller instigated that study. I know of no other studies done at the other reservoirs or river elsewhere. As for the Lower Basin States, can’t vouch for them.

    It’s all about politics. That’s the bottom line.


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