Abstract:
The Water Simulator Tool is an integrated hydrologic and economic model designed to better understand the key linkages between water, food security, and environment. The tools helps develop scenarios for exploring key questions for food water, food, and environmental security, at the global, national and basin scales. Broadly speaking the model consists of two integrated modules: the ‘food demand and supply’ module, which is adapted from IMPACT model (Rosegrant, Cai and Cline 2002); and the ‘water supply and demand’ module which uses a water balance based on the Water Accounting framework (Molden 1997, Fraiture 2007) underlying PODIUM combined with elements from the IMPACT-WATER model (Cai and Rosegrant 2003). The schematic model structure is given in figure 1.
Model Structure:
Broadly speaking the model consists of two integrated modules: the ‘food demand and supply’ module, which is adapted from IMPACT model (Rosegrant, Cai and Cline 2002); and the ‘water supply and demand’ module which uses a water balance based on the Water Accounting framework (Molden 1997, Fraiture 2007) underlying PODIUM combined with elements from the IMPACT-WATER model (Cai and Rosegrant 2003). The schematic model structure is given in figure 1.
The model estimates food demand as a function of population, income and food prices. Crop production depends on economic variables such as crop prices, inputs and subsidies on one hand and climate, crop technology, production mode (rain fed versus irrigated) and water availability on the other. Irrigation water demand is a function of the food production requirement and management practices, but constrained by the amount of available water.
Water demand for irrigation, domestic purposes, industrial sectors, livestock and the environment are estimated at basin scale. Water supply for each basin is expressed as a function of climate, hydrology and infrastructure. At basin level, hydrologic components (water supply, usage and outflow) must balance. At the global level food demand and supply are leveled out by international trade and changes in commodity stocks. The model iterates between basin, region and globe until the conditions of economic equilibrium and hydrologic water balance are met.
Model Spatial Disaggregation:
In order to adequately model hydrology, it makes most sense to use river basin as basic spatial unit. When it comes to food policy analysis, administrative boundaries should be used (trade and policy making happens at national level, not at river basins scale). Therefore, WATERSIM takes a hybrid approach to its spatial unit of modeling. Firstly the world is divided into 125 major river basins of various sizes with the goal of achieving accuracy with regard to the basins most important to irrigated agriculture. Next the world is divided into 115 economic regions including mostly single nations with a few regional groupings of nations. Finally the river basins were intersected with the economic regions to produce 282 Food Producing Units or FPU’s (figure 2). The hydrological processes are modeled at basin scale by summing up relevant parameters and variables over the FPU’s that belong to one basin. Similarly economic processes are modeled at regional scale by summing up the variables over the FPU’s belonging to one region.
Economic processes are modeled at an annual time step, while hydrological and climate variables are modeled at a monthly time-step. Crop related variables are either determined by month (crop ET) or by season (yield, area). The food supply and demand module runs at region level on a yearly time-step. Water supply and demand runs at FPU level at a monthly time-step. For the area and yield computations the relevant parameters and variables are summed over the months of the growing season.
The model, written in GAMS, is developed by IWMI with input from IFPRI and the University of Illinois.
Model Applications:
- Scenario analysis for the Comprehensive Assessment of Water Management in Agriculture (CA)
- Scenario contribution to the International Assessment of Agricultural Science and Technology (IAASTD)
- Water implication of biofuels
- Impact of trade and agricultural policies on water use
- Sub-Saharan Africa investment study
- Scenarios at basin level for the benchmark basins in the Challenge Program on Water and Food
[Summary provided by the International Water Management Institute.]
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