IMAGEIntegrated Model to Assess the Global Environment.

Water

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Water module of LPJmL, in IMAGE 3.0
Flowchart Water. See also the Input/Output Table on the introduction page.

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?

Introduction

Water availability is essential for natural vegetation and agricultural production, human settlements and industry. Around one third of the world’s population lives in countries suffering from medium to high water stress (OECD, 2012). This number is expected to increase as the water demand will increase due to the population growth, and as water availability may decrease due to global warming.

Today, agriculture accounts for 70% of the total global water withdrawals. Around one third of the total global crop production is irrigated although only occupying 17% of croplands (e.g. Portmann et al., 2010). Irrigated agriculture is expected to increase further to meet the growing demand for food (Fischer et al., 2005; Molden, 2007; FAO, 2011a). Moreover, water demand in other sectors (domestic, electricity, manufacturing) is projected to increase substantially in the coming decades (OECD, 2012). As a result, competition between water uses will increase and the resulting water shortages may affect future food production.

Although the global total quantity of freshwater is more than sufficient to meet all human needs, uneven distribution makes water a scarce resource in some regions and watersheds. Furthermore, climate change will lead to changes in precipitation patterns, thus altering future water availability and adding to water stress in areas where precipitation levels are expected to decline.

To identify current and future areas of water stress, IMAGE includes a hydrology model that calculates water availability and demand. The hydrological module of LPJmL is fully integrated with the terrestrial carbon and land-use dynamics of LPJmL and the rest of IMAGE and dynamically calculates agricultural water demand as well as water availability and withdrawals. Availability of renewable water is the net result of precipitation, interception loss and evapotranspiration by plants and soils. In the model, the surplus in each grid cell flows to neighbouring grid cells in a watershed by means of a river routing scheme. However, river flows are modified by dams and reservoirs used for irrigation, and hydropower production or both.

The effects of water stress on crop production can be quantified, and by including the feedback of water-limited crop production on land allocation, IMAGE can produce more realistic scenarios for cropland expansion and agricultural intensification. IMAGE and LPJmL are fully and dynamically linked (see Carbon, vegetation, agriculture and water), and thus IMAGE scenarios include an integrated assessment of the water cycle, and can be used to assess water availability and demand at high spatial (0.5x0.5 degree grid cells) and daily resolutions.

Input/Output Table

Input Water component

IMAGE model drivers and variablesDescriptionSource
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
External datasetsDescriptionSource
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 variablesDescriptionUse
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 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.
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