Background Model Intercomparison

Considerable effort in the last 20 years has been directed into the research and development of global hydrologic models (GHMs) and Ocean Atmosphere Global Circulation Models (OA-GCMs). To improve the simulation of hydrological cycle the OAGCMs, land surface hydrological models (LSHM) have been developed and improved.  GHMs have been used to assess the global-scale implications of climate change for water resources, their availability and water resources stresses. The development of LSHMs has mainly focused on improving the representation of the hydrological cycle in OAGCMs to improve studies on climate change.

The LSHMs tend to describe the vertical exchanges of heat, water, and sometimes carbon, very well but are traditionally weak in their representation of the lateral transfers of water. The GHMs are traditionally more focused on lateral transfer than are LSHMs, but have limited process representation and their application is restricted by the availability of global data.

‘First GWSP International Workshop on Computing the World Water Balance’
Currently there are very few global hydrological models exists which compute the basic dimensions of the global water system on a grid cell basis. While results of most of these models have appeared in the scientific literature, their estimates have never been systematically compiled or compared. These studies differ in their simulation of resource availability, through the use of different input data sets, and in their interpretation of the socio-economic implications of changes in resource availability, through the use of different indicators of impact and representations of demands for water.

To initiate the first intercomparison of estimates from global water models, the Global Water System Project (GWSP) organized a workshop in Kassel, Germany, in April 2007, at which representatives from six global modelling groups came together for the first time to exchange information and plan common activities. Such a comparison will provide important insights into the range of uncertainty of key dimensions of the global water system. The participating modelling groups are now using standardized inputs to compute various aspects of the world water balance including grid-cell runoff (gross water availability), river discharge, soil water content, evapotranspiration, average snow water equivalent, and volume of snow melt. These components are being computed for current climate conditions and for the 2080s under the A1b and B1 IPCC scenarios. Not only continental-totals are being estimated but also values for different climatic zones based on the Koeppen-classification as well as totals for selected major river basins on each continent.

Previous intercomparisons of Land Surface Hydrological models
Previously, there have been large scale comparisons between LSHMs through the “Project to intercompare Land-Surface parameterization Schemes” (PILPS) and the “Global Soil Wetness Project” (GSWP). PILPS is an element of GLASS under the auspices of GEWEX and the World Climate Research Programme. Since its establishment in 1992, PILPS has been responsible for a series of complementary experiments, with focuses on identifying parameterization strengths and inadequacies. About 30 land surface process modelling groups have been participating in PILPS. The Global Soil Wetness Project (GSWP) is an ongoing environmental modelling research activity of the Global Land-Atmosphere System Study (GLASS) and the International Satellite Land-Surface Climatology Project (ISLSCP), both contributing projects of the Global Energy and Water Cycle Experiment (GEWEX). Its goals are to 1) Produce state-of-the-art global data sets of land surface fluxes, state variables, and related hydrologic quantities, 2) Develop and test large-scale validation, calibration, and assimilation techniques over land, 3)  Provide a large-scale validation and quality check of the ISLSCP data sets, and 4) Compare LSHMs, and conduct sensitivity studies of specific parameterizations and forcings, which should aid future model and data set development. (Information taken from PILPS and GSWP webpages.)

The GWSP - Watch Water Balance Intercomparison Project
The ‘First GWSP International Workshop on Computing the World Water Balance’ held in April 2007 was meant to be the first in a series, which has engaged an international community of global modelling experts. As the efforts move forward, this community is now seeking the advice of a broader body of experts, who can provide insight in past and future water availability and water use. To open the forum will mean, to be able to include not only the Global Hydrological Models (GHMs) but also the Land Surface Hydrology Models (LSHMs) within the Global Climate Models for an enhanced analysis and improvement of modelling the global water balance.

Therefore it is intended to incorporate the integrated EU-Project WATCH in the next step of model intercomparison. By bringing together hydrologists, water cycle experts and climate modelers, WATCH will develop new methods to assess the risk of floods and droughts in the 21st century with assessments of human and ecological water demands. There are many uncertainties in our understanding of the current water cycle and how it will develop in the future. WATCH will help to clarify these overall vulnerabilities of global water resources related to the main societal and economic sectors. In the development of a new consolidated dataset, and an enhanced, consistent modelling framework for water resources, hydrology and climate studies, it can be possible to identify future hotspots of water scarcity. Within this modelling framework several global hydrology models and land-surface schemes are integrated, therefore the WATCH consortium is the ideal partner to have a joint GWSP – WATCH effort for a global water balance intercomparison study.

This new comparison is different from the previous ones because this intercomparison will also use forcing data for future scenarios and will look at a more integrated scale, e.g. looking at discharge of major river systems and eventually at the impacts of land use in terms of large scale irrigation. Besides this, we will also look at global consumptive water use in different sectors not only for irrigation but also for domestic, manufacturing and livestock farming purposes. In addition, focus will be on building interfaces between water resources, hydrological and climate models, attempting a maximum possible consistency in spatial and time scales involved, and in related process descriptions, which is one of the main innovative components of WATCH. Through integrated model intercomparisons and evaluations, participating models will improve the parameterization of physical processes and human interactions with the global water cycle. At least 11 models will be involved in the model intercomparison.