Crop Water Allocator (CWA). The CWA is a seasonal planning tool to find the optimum net return from all of the combinations of crops, irrigation amounts, and land allocations that the program user wants to examine."
Even without being motivated by reduced water capabilities, this tool is a very interesting exercise at looking into various combinations of crops and acreages that may surprise you with their profitability - or lack thereof. There are a lot of operation-specific values, rates and numbers you must input as you set up your fields, but the "what-if" capabilities are well worth it. While the current version is a downloadable item, KSU is working on a web-based version that should be even better. If you're a crop producer, and especially if you have limited water, you may want to give this free program a look-see. All the background data (rainfall, ET reference values, etc.) are geared for Kansas, but these data from any region in the world can be manually input as well.
KSU also has a related program called Crop Yield Predictor which is also handy, but for a different aspect of a producer's operation. This program helps predict crop yields with limited irrigation. This program can also be used to determine some of the inputs into the crop water allocator program.