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Reaffirmation of large biases in a long-used method for projecting changes in Great Lakes water levels in climate change scenarios
  • Published Date:
Filetype[PDF - 959.79 KB]

  • Corporate Authors:
    Great Lakes Environmental Research Laboratory
  • Series:
    NOAA technical memorandum GLERL ; 167
    GLERL contribution ; no. 1779
  • Document Type:
  • Description:
    A method for projecting the water levels of the Laurentian Great Lakes under scenarios of human-caused climate change, used almost to the exclusion of other methods in the past, relies very heavily on the Large Basin Runoff Model (LBRM) as a component for determining the water budget for the lake system. This model uses near-surface air temperature as a primary predictor of evapotranspiration (ET); in addition to previous published work, we show here again that its very high sensitivity to temperature makes it overestimate ET in a way that is highly inconsistent with the fundamental principle of conservation of energy at the land surface. Under the traditional formulation, the quantity that has been called "energy available for evapotranspiration," which is proportional to what we call "potential evapotranspiration" (PET), is increased by large factors in future scenarios--by a factor of nearly 600 in the Lake Superior basin under forcing by one GCM case, but more typically by factors between 3 and 10. Because of the way that LBRM is formulated and calibrated, these factors can be thought of as corresponding to the factor of increase in solar radiation incident on the Earth, or, more vividly, as the number of Suns present in the sky of the virtual world simulated by LBRM. Therefore, we have created alternative formulations, which we regard as more reflective of what is being simulated by the driving GCMs, for the way that climate change is ingested into the modeling system that includes LBRM. In addition to the energy adjustment method in which PET is increased by an amount proportional to the change in net radiative energy at the surface, we add the Priestley-Taylor method, which augments the energy adjustment method by inclusion of a temperature-dependent factor with rigorous theoretical underpinnings that is a much weaker function of temperature than in the LBRM's basic formulation, as well as the Clausius-Clapeyron method, in which the PET is increased by an amount proportional to the increase in water vapor capacity of the atmosphere, again a weaker function of temperature than in the LBRM. We establish that all three of these alternative methods show, relative to the traditional method, often astoundingly less PET and less ET, more runoff from the land and net basin supply for the lake basins, and higher lake water levels in the future. The magnitude of these discrepancies is highly correlated with the air temperature change in the driving GCM (larger temperature changes lead to larger discrepancies). Using various methods of estimating the statistical significance, we find that, at minimum, these discrepancies in results are significant at the 99.998% level.

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