Here is where you bring it all together, combining the factual information gleaned from the first seven chapters with your values and preferences, to generate tentative choices. The final step is to see what the consequences of those choices are, and how your choices interact with each other. To do that, we use water budget scenarios.

The table on the preceding page shows ten illustrative water supply and demand scenarios. The left side of the table shows municipal, industrial and agricultural demand expressed as percentages of current demand. The “supply” column describes who pumps groundwater and who uses CAP water under each scenario. (Note that use of effluent has little effect on the water budget bottom line, because nearly all effluent not re-used is directly or incidentally recharged.) These assumptions about water demand levels and supply allocations on the left side of the table are used to estimate resulting groundwater pumping by sector, CAP usage, and aquifer balance, as shown on the right side of the table.

The first three scenarios hold demand at current levels. The first scenario shows that if everyone uses pumped groundwater, we have a large deficit in the aquifer. The second scenario suggests that if all growth were halted and groundwater were reserved solely for municipal uses, the aquifer would be nearly in balance. The next three scenarios correspond to an eventual doubling of municipal demand, as population grows. Here, reserving groundwater for municipal uses does not bring the aquifer close to balance. By contrast, using CAP water for all municipal uses actually produces a surplus in the aquifer.

The third set of scenarios combines municipal growth with a halving of industrial and agricultural demand. Note that serving a 50-50 blend of groundwater and CAP water to all sectors nearly balances the aquifer. The final set of scenarios corresponds to a doubling of water demand in all sectors. This could occur, for example, if population growth continued unabated, mining expanded, and tribal water allocations were used to expand irrigated agriculture on reservation lands. In such a situation, there is not sufficient CAP water available

to bring the aquifer close to balance.

Those of you with Internet access are now invited to try your hand at water budgeting by making your own assumptions. An interactive version of this budget is on the Web at:

http://ag.arizona.edu/AZWATER/

Use it to construct a scenario that reflects your values, preferences and sense of fairness. Try out a number of options. For example, you may want to limit water use by controlling population, or by requiring more conservation; or you may want to lessen long-term salinity problems by using less CAP water.

See how close your preferred options come to balancing supply and demand. Remember that if you make some changes, other figures will be affected. For example, if you eliminate agriculture and replace it with naturally vegetated park land, the water savings will be greater than if you replace it with golf courses. If you use more effluent, you will have less incidental recharge. To get close to water sustainability, you will have to make hard choices. When push comes to shove, where are you willing to compromise?