Models for the water balance are faulty

Model results are very different from measured values

Rain showers in a bay © Mila Zinkova / GFDL
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Our knowledge about the global water balance is obviously worse than expected. Average precipitation models deviate from each other and from readings in some regions by up to four liters per square meter per day. That's twice as much as Germany's average in one day. The models can therefore not be reliably deduced from where, how much rain falls, the researchers warn in the journal "Journal of Hydrometeorology". Here must urgently be improved.

There's no life without water. Disasters such as drought or heavy rain prove our dependence on the water cycle and the climate system. Accordingly, it is important to understand the details of the water cycle between the atmosphere, the oceans and the mainland. "Climate change and the changing availability of water is a fact and will in some cases require major adjustments, " clarify Harald Kunstmann and Christof Lorenz from the Karlsruhe Institute of Technology, who have written the current study. "That's precisely why we need to better understand the interplay between evaporation, clouds and precipitation on a regional level."

Three current models compared with measurement data

To test how reliable the various global analyzes are, hydrologists and climatologists have re-evaluated three of the most modern global coupled atmospheric and ocean models for the water balance and compared them with measurements from 1989 to 2006. "We have found very great uncertainties in global water budget estimates, " explains Kunstmann. For example, average precipitation analyzes differ by as much as four liters per square meter per day in some regions. For comparison, in Germany fall on average about two liters of rain per day and square meters.

From these models can not be reliably deduced when and where really how much precipitation falls. Even simple relationships, such as the excess of evaporation over the oceans and precipitation over the continents, are not consistent in the models. "From the models, we continue to know only with great uncertainty, how much rainfall and thus constantly renewing fresh water on the earth really are actually available."

Data shortage tends to increase

"On the other hand, there are insufficient data from many regions of the world, " explains Kunstmann. "And the situation is getting worse and worse." In South America, for example, the number of measuring stations has fallen by more than 85 percent from around 4350 to 550. But even in Europe, a significant decline can be seen. Between January 1989 and December 2006, the number of precipitation monitoring stations has almost halved from around 10, 000 to 5800, with about half of all European stations alone in Germany. "And without a solid database, the water balance models can not be decisively improved, " Kunstmann describes the problem. QuantA quantification of the trends of rain and drought is thus made considerably more difficult. Display

Therefore, it is urgently necessary to invest again in precipitation monitoring stations and to strengthen the meteorological services even in remote regions. "If we understand the hydrological change and want to effectively adjust to it in the future, we absolutely must create and maintain the necessary infrastructure, " says Kunstmann. (J. Hydrometeor., 2012; doi: 10.1175 / JHM-D-11-088.1)

(Karlsruhe Institute of Technology (KIT), 25.06.2012 - NPO)