Evaluating Problems with Long-Established Methods for Calculating Evapotranspiration Under Climate Change in the Great Lakes Basin: Take II

Abstract:

A 2011 paper by a group from the NOAA Great Lakes Environmental Research Laboratory established that a long- and widely-used method for projecting evapotranspiration (ET) and runoff from the land portions of the Great Lakes basin exhibited severe deficiencies in terms of conservation of energy at the land surface, and consequent errors in projected runoff and lake levels. The key component of this older method is known as the Large Basin Runoff Model (LBRM). A simple alternative method was developed to better account for energy conservation, and this was run for two different general circulation model (GCM) datasets, in order to demonstrate the corresponding discrepancies in terms of ET, runoff, and lake water level. In the Third National Climate Assessment, the regional chapter on the Midwest acknowledged these results, while Appendix 3 (Climate Science Supplement) expressed less credence, with the lead authors of that appendix maintaining that the models needed to be run with more GCMs as input. We will report on the results of runs using more than 40 GCM realizations from the Climate Model Intercomparison Project 5 class. In addition to the previously-used method of adjusting future potential evapotranspiration (PET) according to changes in net radiative energy available at the surface, we introduce one that additionally estimates the air temperature dependence term of the Penman-Monteith formulation, and one in which PET varies in proportion to the Clausius-Clapeyron relation (i.e. PET increases by about 7% per degree C, in contrast to LBRM, in which PET typically increases by 30-50% per degree C).