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Sustainability in the Yaqui Valley
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2) With water a constraining resource in this semi-desert environment, what are efficient uses of water in agriculture and in other sectors, and how vulnerable is the water resource to climate change, development in other regions of Mexico, and other external factors?

Water resources are used and influenced by all sectors of human activity. Experience in many regions of the world illustrate the importance of conflicts between the competing urban, industrial, and agricultural sectors as regions develop. In addition, ecosystem and recreation demands for water are also being added to the mix of demands. In the Yaqui Basin, the agriculture sector to date has had almost unrestricted access to water, with little competition from other sectors. Today, however, growing urban centers, industrial growth, and demands from outside the Basin suggest that new demands for water are just around the corner. We plan to explore questions about the vulnerability of water (and associated agricultural activities) to climate variation and changing demands in other sectors, and to develop a basis for decisions about water allocation and use in the Basin.

Water-Resource Planning.
While the reservoir system for the Valley buffers farmers and urban dwellers from variation in precipitation, long term draw-downs of the reservoirs during drought have at times had dramatic effects on decisions about cropping systems and management. Our work to date has indicated almost no long-term planning for the water system. Recent draw-downs have left little flexibility for future management decisions. We intend to assess the environmental (e.g., climate, soil) conditions for which the current allocation approaches are appropriate, and also how appropriate they are likely to be for future scenarios of rapid change or increased climatic variability?

Can more conservative management approaches be used to reduce vulnerability? What are the appropriate allocation decision rules to sustain the water resource and activities associated with it, and what is the political and scientific feasibility of establishing such rules? Can we develop a long-term drought plan? Using long-term records of precipitation, reservoir volume, and projections for future climate variation, we will develop decision rules for reservoir water draw-downs under varying precipitation regimes, and test the consequences of those models using allocation models developed in our current research.

Water-Soil, Drought-Salinity Interactions. Water availability drives many decisions in agriculture, including crop selection and fertilization and irrigation strategies. While the management focus has been on crop and soil performance under optimal water allocation, there has been little analysis of how crops perform and soils change under lower water supplies. We propose to link our water allocation models with agricultural ecosystem models to explore issues about the consequences of changes in water allocation and management on crop production and soil degradation (e.g., increasing salinity), and conversely, the consequences of crop changes (potentially driven by market forces or other external factors) for future water supply.

What are the most critical interactions among water use and soil degradation? How vulnerable is the valley to degradation and loss of productivity due to salinization? Our work has suggested that some flexibility in water allocation can be acquired through use of ground-water resources in combination with reservoir water (Addams and Gorelick). However, data on the variable salinity characteristics of the Valley's soils warn of a need for a decision model that accounts for the soil characteristics as well as water in deciding the proper mix of surface and subsurface water for irrigation. Such a decision model is likely to be spatially explicit. We will work towards an integrated model by combining GIS-based data on soil characteristics and ecosystem processes with the spatial water allocation models. For this work, we will utilize and build on TerraFlux, a spatially determined mathematical model of ecosystem production and biogeochemical processes with explicit treatment of soil processes and properties (Asner et al 2001), linked to the water allocation model now being developed by Steve Gorelick and Lee Addams.

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