Even within one growing season, irrigation systems need continual maintenance. The long-term hazards of irrigation are even greater. Investment in irrigation projects pays best in dry areas where evaporation is high. Water is never pure, but has mineral salts dissolved in it. Evaporation will therefore make it saltier still.
Rivers flowing through dry or desert areas lose water by evaporation, and become salty.
For example, the water in the lower Colorado contains over a ton of salt per acre-foot of water. Many areas in dry climates have natural brackish or salty lakes, or even dry salt pans. Promising irrigation areas thus may have natural salts in rocks or soils that will easily be transferred into fields as soon as irrigation water is applied, and even that water may come from rivers that have become saltier from evaporation along their courses. As water is used on crops, it spreads out as a thin sheet, exposed to the surface. Much of it may evaporate, making it more saline.