2. Model Design


Water Futures and Solutions Initiatives (WFAS)

Water Futures and Solutions Initiatives is using a multi-model approach for global climatic, hydro-socioeconomic modeling in order to assess possible futures. We use three leading global hydrological models H08, WaterGAP and PCR-GLOBWB for estimating water demand and supply. This approach is used for a better understanding of the uncertainty and limitations of modeling. It provides a degree of confidence in the results an is in-line with the ISI-MIPS approach of multi-modeling


Figure 1: Potential population under severe water scarcity in 2050 - Middle of the Road Scenario - WFAS fast-track analysis

Nexus Integration - Water Energy Food Environment

In the framework of the Integrated Solution project
the Community Water Model (CWatM) will be coupled with the existing IIASA models MESSAGE and GLOBIOM
in order to do enhanced water assessments and an improved analysis feedback on water, energy, food and environmental aspects

Figure 2: IIASA model interactions

CWAT and the IIASA global hydro-economic model

The Community Water Model will help to develop a next-generation hydro-economic modeling tool that represents the economic trade-offs among water supply technologies and demands. The tool will track water use from all sectors and will identify the least-cost solutions for meeting future water demands under policy constraints. In addition, the tool will track the energy requirements associated with the water supply system (e.g., desalination and water conveyance) to facilitate the linkage with the energy-economic tool. The tool will also incorporate environmental flow requirements to ensure sufficient water for environmental needs. The new hydro-economic model will be linked to CWatM by GAMS output and input files (gdx-files).

Model design and processes


The Community Water Model (CWatM) will be designed for the purpose to assess water availability, water demand and environmental needs. It includes an accounting of how future water demands will evolve in response to socioeconomic change and how water availability will change in response to climate.


Figure 3: CWatM - Water related processes included in the model design


Calculation of potential Evaporation

Using Penman-Montheith equations based on FAO 56

Calculation of rain, snow, snowmelt

Using day-degree approach with up to 10 vertical layers Including snow- and glacier melt.

Land cover

using fraction of 6 different land cover types

  • Forest

  • Grassland

  • Irrigated land

  • Paddy irrigated land

  • Sealed areas (urban)

  • Water

Water demand

  • including water demand from industry and domestic land use via precalculated monhly spatial maps

  • including agricultural water use from calculation of plant water demand and livestock water demand

  • Return flows ((water withdrawn but not consumed and returned to the water circle)


Vegetation taken into account for calculating

  • Albedo

  • Transpiration (including rooting depth, crop phenology, and potential evapotranspiration)

  • Interception


Three soil layers for each land cover type including processes:

  • Frost interupting soil processes

  • Infiltration

  • Preferential flow

  • Capillary rise

  • Surface runoff

  • Interflow

  • Percolation into groundwater


Groundwater storage is simulated as linear groundwater reservoir

Lakes & Reservoirs

  • Lakes are simulated with weir function from Poleni for rectangular weir.

  • Reservoirs are simulated as outflow function between three storage limits (conservative, normal,flood) and three outflow functions (minimum, normal, non-damaging)


Routing is calculated using the kinematic wave approach

Features of the Model

Community Model



Programming Language

Python 3.x with some C++ for computational demanding processes e.g. river routing

Community driven

Open-source but lead by IIASA GitHub repository

Well documented

Documentation, automatic source code documentation GitHub Docu

Easy handling

Use of a setting file with all necessary information for the user Complete settings file and Output Meta NetCDF information


Python 3.x on Windows, Mac, Linux, Unix - to be used on different platforms (PC, clusters, super-computers)


Processes in subprograms, easy to adapt to the requirements of options/ solutions Modular structure

Water Model




different resolution, different processes for different needs, links to other models, across sectors and across scales


to be tailored to the needs at IIASA i.e. collaboration with other programs/models, including solutions and option as part of the model


including economics, environmental needs, social science perspectives


Sensitive to option / solution


Global to regional modeling – a mixture between conceptional and physical modeling – as complex as necessary but not more


Part of the ISI-MIP community

Demo of first results

Here are some first demonstration of the model run:

Demo of the model