Water resource policy evaluation using a combined hydrologic-economic-agronomic modeling framework: Yaqui Valley, Sonora, Mexico

An integrated hydrological-economic-agronomic modeling framework is developed to address water policy in the Yaqui Valley, a semi-arid agricultural region in northwest Mexico. The modeling framework includes physically-based hydrological and agronomic processes and hierarchical decision-making by water managers and farmers. The hydrologic component consists of (a) a physically-based canal routing model, and (b) a complete, stand-alone three-layer groundwater flow model. The agronomic component represents the yield response of major Yaqui Valley crops to total seasonal irrigation depth and salinity. The economic component is a set of mathematical formulations representing the decision-making processes of (a) groups of farmers (modules), and (b) water managers in the Yaqui Valley. Each Module-Level Decision Model determines crop, irrigation, and pumping levels to maximize single-year profits, subject to land, water, and scheduling constraints. The set of module-level decision models successfully reproduced 1996--2002 Valley-wide crop production, in part due to the underlying spatial variability in yield potential and irrigation efficiency.; The complete set of process and decision-making models (collectively, the "District Distribution Model") was used to explore improvements in the Irrigation District's water-allocation system. In the District Distribution Model, short-term canal operations are controlled through the embedded canal routing model. Long-term effects of groundwater pumping and canal operations are measured through the interannual groundwater model. Farmer response to water allocations and prices are measured through the Module-Level Decision Models.; The Irrigation District "Historical Policy" (set of distribution objectives) was successfully replicated, including the effects of an existing "Water Bank" Policy. A more efficient "Historical-Improved" Policy maximized the water allocation to the modules, resulting in a more efficient operation and an approximately 10% increase in water deliveries, though only a <1% increase in profit. Two "Custodial" policies were also proposed, both of which allowed greater groundwater use than historically pumped. Compared to the "Historical Policy", higher allocations were achieved with the "Salinity Limits" Custodial Policy, but the consequent increase in water cost (from increased pumping) resulted in no added economic benefit (using historical Yaqui Valley crop prices). The "Equivalent Yield" Custodial Policy represented a second hypothetical policy that allocated water to each module based on equivalent crop-growing potential.