Big Uncertainties in the Global Water Budget

Results of Latest Models for Regional Precipitation and Temperature Distribution Deviate Strongly / Number of Measurement Stations Worldwide Is Decreasing Dramatically

No life without water. Catastrophes like droughts or strong rains reflect our dependence on the water cycle and climate system. Hence, it is important to understand details of the water cycle among the atmosphere, oceans, and land. A study in the Journal of Hydrometeorology now outlines significant differences of global models and measurement data sets. As the network of measurement stations worldwide is shrinking dramatically, uncertainties are increased (Doi: 10.1175/JHM-D-11-088.1).

“Climate change and the associated change of water availability are facts and will require partly significant adaptation,” emphasize Ha-rald Kunstmann and Christof Lorenz of Karlsruhe Institute of Technology, who are the authors of the said study. “This is the reason why we have to better understand interactions of evaporation, clouds, and precipitation also on the regional level.” To check the reliability of various global analyses, the hydrologists and climate researchers reevaluated three of the most modern global coupled atmosphere and ocean models with respect to the water budget and compared the results with measurement data of the years 1989 to 2006.

“We found very big uncertainties in the global water budget estimates,” says Kunstmann. The mean precipitations analyzed in some regions deviate by up to four liters per square meter and day. For comparison: In Germany, about two liters of rain fall per day and square meter on the average. Hence, these models do not allow for a reliable derivation as to when and where how much precipitation occurs. The models do not even provide simple relationships, such as that between the evaporation surplus above the oceans and precipitation above the continents. “Models will continue to tell us with very big uncertainties how much precipitation and, hence, permanently renewed freshwater is available on earth.”

KIT Climate and Environment Center

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