Parts of Water
|Component is implemented in:|
|Related IMAGE components|
Key policy issues
- What is the combined effect of climate change and socio-economic development on water demand and availability, and on associated agricultural production?
- What is the potential of adaptation measures to reduce water stress and water-related crop production losses?
- How can water demand be reduced and still provide the adequate service levels to the sectors with the highest demand?
Water plays an important role in many natural and human processes. Its availability is essential for natural vegetation and agricultural production, for human settlements and industry. Around one third of the worlds’ population is living in countries already suffering from ‘medium’ to ‘high’ water stress (OECD, 2012). This number is expected to increase further, due to a growing population that will need more water and is living in a changing climate. Today, agriculture is responsible for 70% of the total global water withdrawals and is thus by far the biggest water user. Around one third of the total global crop production is harvested from irrigated areas, although they only occupy 17% of croplands (e.g. Portmann et al., 2010). This indicates that irrigation generally supports more productive agricultural practices. To meet a growing food demand (see Agriculture and land use), irrigation is expected to expand in the future (Fischer et al., 2005; Molden, 2007; FAO, 2011a) and, hence, will increase agricultural water demand. Moreover, the water demand in other sectors (domestic, electricity, manufacturing) is projected to increase strongly, over the coming decades (OECD, 2012). As a consequence, competition between different water users will increase and resulting water shortages may affect future food production. Although the total amount of fresh water on earth is more than enough to fulfil all human needs, it is the uneven distribution that makes water a scarce resource in some regions and watersheds. Climate change will lead to changes in precipitation patterns and, therefore, will also alter the future availability of water, adding to water stress in areas where precipitation levels are expected to decline.
To identify current and future areas of water stress, IMAGE now includes a hydrological model that calculates both water availability and demand. The hydrological module of LPJmL, coupled to the IMAGE model, is fully integrated with terrestrial carbon and land-use dynamics, and calculates agricultural water demand as well as water availability and withdrawals. Availability of renewable water is the net result of precipitation, interception and evapotranspiration by plants and soils. In the model, the surplus in each grid cell is flowing to neighbouring grid cells within a watersheds by means of a river routing scheme. River flows are modified by dams and reservoirs for irrigation, hydropower production or a mix of the two. The effects of water stress on crop production can be quantified by limiting the amount of available water for irrigation to the actually available amount of water in the LPJmL model. By including the feedback of water-limited crop production on land allocation, IMAGE is able to develop more realistic scenarios for cropland expansion and agricultural intensification in the future. The IMAGE model and the LPJmL model are fully and dynamically coupled (see Natural vegetation and carbon cycle), and IMAGE scenarios therefore include an integral assessment of the water cycle, and can be used to assess water availability and water demand at high spatial (0.5 x 0.5 degree grid cells) and daily resolutions.
Links to other parts of the model, input and output
The hydrological model of IMAGE is closely linked to the model on natural vegetation, crop and carbon cycles (see Natural vegetation and carbon cycle; Crop and grass), because all those submodels are represented in LPJmL, which is a global hydrology and vegetation model (Sitch et al., 2003; Bondeau et al., 2007). An overview of data exchanges with other IMAGE modules is given below. Data on annual land cover and land use are used as input into LPJmL, including information on the location of irrigated areas and types of crops. This influences the amounts of water that evaporate and run off, as well as the amount of water needed for those irrigated areas. Vice versa, information on water availability as calculated by LPJmL is taken into account by the land allocation model to find suitable locations for the expansion of irrigated areas. Climate is used as an input into LPJmL to determine reference evapotranspiration, and the precipitation input to the water balance (Gerten et al., 2004). The crop model, which is also part of LPJmL (Crop and grass), calculates irrigation water demand based on crop characteristics, soil moisture and climate. If the amount of water available for irrigation is limited, the crop model calculates the reduction in crop yield due to water stress.
Input Water component
|IMAGE model drivers and variables||Description||Source|
|Irrigation conveyance efficiency||Ratio of water supplied to the irrigated field to the quantity withdrawn from the water source, determining the quantity of water lost during transport. This parameter is defined at country level.||Drivers|
|Irrigation project efficiency||Ratio of quantity of irrigation water required by the crop (based on soil moisture deficits) to the quantity withdrawn from rivers, lakes, reservoirs or other sources. This parameter is given at country level.||Drivers|
|Crop irrigation water demand - grid||Water requirements for crop irrigation, calculated as daily moisture deficit during the growing season.||Crops and grass|
|Land cover, land use - grid||Multi-dimensional map describing all aspects of land cover and land use per grid cell, such as type of natural vegetation, crop and grass fraction, crop management, fertiliser and manure input, livestock density.||Land cover and land use|
|Precipitation - grid||Monthly total precipitation.||Atmospheric composition and climate|
|Temperature - grid||Monthly average temperature.||Atmospheric composition and climate|
|Digital water network - grid||Digital water network DDM30 describing drainage directions of surface water, with each cell only draining into one neighbouring cell, organising cells to river basins.|
|LOD (location of dams and reservoirs)||Location, building year, purpose and size of 7000 largest reservoirs.|
|Soil properties - grid||Soil properties that have an effect on vegetation growth and hydrology. These characteristics differ between soil types. Relevant characteristics are soil texture and depth and water holding capacity||HWSD database|
|Water demand other sectors - grid||Total annual water demand for non-agricultural sectors (households, industry and electricity production)|
Output Water component
|IMAGE model variables||Description||Use|
|Water withdrawal other sectors - grid||Total annual water withdrawal by non-agricultural sectors.|
|Irrigation water withdrawal - grid||Water withdrawn for irrigation, not necessarily equal to irrigation water demand, because of limited water availability in rivers, lakes, reservoirs and other sources.|
|River discharge - grid||Average flow of water through each grid cell.|
|Irrigation water supply - grid||Water supplied to irrigated fields; equal to irrigation water withdrawal minus water lost during transport, depending on the conveyance efficiency.|
|Water stress - grid||Water stress is a basin scale indicator of the mean annual water demand to availability ratio. This ratio gives an indication for the level of water stress experienced in the basin. Basins with a water demand to availability ratio above 0.2 are considered medium water stressed, basins with ratios above 0.4 are severely water stressed.||Final output|