Challenges in Large-Scale Water Availability Changes.

Abstract:

The WCRP Grand Challenge on “Changes in water availability” has a key question “How can we better understand and predict precipitation variability and changes, and how do changes in land surface and hydrology influence past and future changes in water availability and security? This is a central focus of GEWEX over the next 10 years in addressing a very important yet tractable problem. The anticipated ways forward include the exploitation of improved data sets of precipitation (especially from the Global Precipitation Measurement mission), soil moisture, evapotranspiration, and related variables such as water storage and sea surface salinity expected in the coming five years. Closure of the surface energy budget globally remains a major issue, with the main tradeoffs being between radiation and evapotranspiration latent heating. The broad 22% spread in CMIP5 models in the latter from 76 to 92 W m^-2 versus a new observed estimate of 79 W m^-2 undermines confidence in projections. Continental scale water budgets that are forced to reconcile all fields provide examples of ways forward. The advanced datasets and analyses should improve our ability to close the water budget over land and thus provide improved information for products related to water availability and quality for decision makers and for initializing climate predictions from seasons to years ahead.