Inserting man’s irrigation and drainage wisdom into soil water flow models and bringing it back out: how far have we progressed?

Half of the world food production originates from irrigated and drained soils. However, the future management of these systems must accept the paradigm shift away from managing abundant water supplies (with focus on conveyance and distribution) and toward the beneficial use of scarce water resources with the emphasis on deficit irrigation, sustainable groundwater exploitation and optimized crop water productivity. Mechanistic computer models – together with economic and social decision rules – have a key role to play in supporting water allocation decisions. Unfortunately, computer models for prediction and better understanding of unsaturated soil water flow processes have low operational focus, especially in many irrigation countries where they are most needed. Advanced models have the potential to contribute to the solution of relatively complex problems, provided that field data are available to calibrate and run them. Calibration techniques, especially with the help of GIS and remote sensing, have progressed rapidly, but the required level of expertise tends to make the application of sophisticated tools highly dependent on modeling experts. The challenge, now, is to reduce the gap between the abundant supply of advanced models and the low demand by the irrigation and drainage community. The likelihood of adoption by a broader user community will increase as models become more user- and data-friendly (or -tolerant) and heterogeneity-aware. The time to formulate and market the unsaturated-zone model as a necessary ingredient to the solution of crop water production problems and the time to equip users around the globe is no