Page 98 - Climate Change and Food Systems

climate change and food systems: global assessments and implications for food security and trade
conditions or forcing functions (such as via shifts in yield–response functions) (Bell et al., 2014). For example Hurd et al., (2004) applies the Watershed Allocation and Impact Model (Water-AIM), a partial equilibrium model, which links the investment decisions of water resource planning authorities, the water allocation decisions of water managers, and the water consumption decisions of water users together in a spatially and temporally differentiated framework that is consistent with the geophysical features of individual basins.
The model produces basin level “lower bound” estimates of the potential economic impacts of climate change on water resources.
The Water-AIM model structure depicts
key physical characteristics of the natural and man-made water supply system, including tributaries, inflows and return flows, diversion points, reservoirs, and basin imports and exports. Seasonal runoff into each basin is based on historical records, and the models solve simultaneously for water allocations and implicit water prices for both consumptive and non-consumptive uses, reservoir storage and releases, and in stream flows over a planning period spanning a number of years and seasons. The model objective function is the expected
net economic returns of water users (sum of consumer and producer surplus) as a function of both consumptive and non-consumptive uses of water over time and space subject to a system of constraints (seasonal runoff; surface water diversions; inter-temporal balances in reservoirs between runoff into the reservoir; water storage; water losses; and storage releases).
The scenarios were used to determine runoff under climate change and to condition irrigation demands. To convert the climate scenarios into hydrologic impacts (runoff), runoff projections were obtained through a module which translates changes in monthly average precipitation and temperature into changes in monthly runoff and aggregated at the seasonal level and reported so as to coincide with the inflow points for each of the basin models (Hurd et al., 2004). Water-AIM was then used to estimate the implicit price or
marginal value of water for every time period and location.
Water resources at the river basin level can
be incorporated into an economic model in
one of two ways: through “reservoir” storage (tanks) or through explicit routing of flows along watercourses within a basin. The former is applied in the IMPACT-WATER model (Rosegrant et al., 2008) – an optimisation model that minimizes water shortages within the river basin. The model calculates net irrigation water demand, taking
into account crop evapotranspiration, effective rainfall and basin efficiency. Effective rainfall for crop growth can be increased through rainfall harvesting technology. The basin efficiency measures the ratio of beneficial water depletion (crop evapotranspiration and salt leaching) to the total irrigation water depletion at the river basin scale. Future years’ basin efficiency is assumed to increase at a prescribed rate in a basin, depending on water infrastructure investment and water management improvement in the basin. The
model also determines off-stream water supply for household consumption and for industry, livestock and irrigation sectors. To determine the total amount of water available for various off-stream uses in a basin, hydrologic processes, such as precipitation, evapotranspiration, and runoff are taken into account to assess total renewable water. Once the model solves for total water that could
be depleted in each month for various off-stream uses, the model determines the water supply available for different sectors. Assuming domestic water demand is satisfied first, priority is then given to industrial and livestock water demand, whereas irrigation water supply is the residual claimant
and is allocated based on profitability of the crop, sensitivity to water stress, and irrigation water demand (total demand minus effective rainfall) of the crop. Using effective irrigation water supply in each basin by crop and by period over a 30-year time horizon, the results are then incorporated in simulating food production, demand and trade.
The second approach to integrating water resources is used by the Water – Global Assessment and Prognosis (WaterGAP) model
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