P. C. D. Milly, USGS Workshop on

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Presentation transcript:

Runoff in a Changing Climate: Climate-Model Choice Matters, and So Does PET Choice P. C. D. Milly, USGS Workshop on “The Development of Climate Projections for Use in Chesapeake Bay Program Assessments” Scientific and Technical Advisory Committee, Chesapeake Bay Program Annapolis, MD 7 March 2016

Based on Table 7 of Najjar et al. (2009)

Observed Runoff Multi-Model Median

The global pattern of historical (20th century) runoff trends was reproduced skillfully by a multi-climate-model ensemble, more skillfully than by the most skillful single climate model (Milly et al., 2005). OBSERVED TREND MODELED TREND

Model-Projected Changes in Annual Runoff, 2041-2060 Percentage change relative to 1900-1970 baseline. Any color indicates that >66% of models agree on sign of change; diagonal hatching indicates >90% agreement. -40 -20 -10 -5 -2 2 5 10 20 40 (After Milly, P.C.D., K.A. Dunne, A.V. Vecchia, Global pattern of trends in streamflow and water availability in a changing climate, Nature, 438, 347-350, 2005.)

Model-Projected Changes in Annual Runoff, 2041-2060 Percentage change relative to 1900-1970 baseline. Any color indicates that >66% of models agree on sign of change; diagonal hatching indicates >90% agreement. -40 -20 -10 -5 -2 2 5 10 20 40 (After Milly, P.C.D., K.A. Dunne, A.V. Vecchia, Global pattern of trends in streamflow and water availability in a changing climate, Nature, 438, 347-350, 2005.)

Based on Table 7 of Najjar et al. (2009)

“Offline” Hydrologic Modeling projected hydrology climate model projected climate change downscaling and bias correction downscaled climate change hydrologic model projected hydrology

Downscaling and Bias Correction projected hydrology climate model projected climate change downscaling and bias correction P downscaled climate change PET model projected PET change hydrologic model projected hydrology

Some PET Methods Used in Offline Climate-Change Studies Name Equation Penman-Monteith Priestley-Taylor “Energy-Only” Samani-Hargreaves Oudin Hamon Thornthwaite

Typical Temperature Dependence of PET, Radiation Being Held Constant Relative Change in PET, Percent

% Change in Annual PET, RCP8.5, 2081-2100

Skill for Change in PET GFDL-ESM2G Energy-Only PET

Exaggeration of Change in PET by Standard Methods Average over all non-water-stressed gridcell-months

Change of Runoff: Climate Model and Budyko/PET Runoff (%)

Based on Table 7 of Najjar et al. (2009)

Summary To estimate historical Susquehanna R. basin (SRB) runoff, the use of the median across many climate models is more accurate than the use of most individual models or small collections thereof. Similarly, a many-model ensemble was more skillful than any single model in reproducing global pattern of 20th century streamflow trends. A large number of climate models is needed to obtain a stable estimate of future SRB runoff change. Variation in past estimates of SRB runoff change is significantly affected by at least two factors: Use of different climate models. Use of different hydrologic models, especially PET formulations. Offline estimates of runoff change based on empirical PET estimates are generally biased low relative to runoff changes in climate models themselves. Use of a more process-based approach to PET in “offline” hydrologic modeling of climate change requires surface radiation. Climate models produce their own runoff, and this is a useful source of climate-change information.