Gains from new capacity are expected to be largely offset by losses from the problems we will discuss, including waterlogging and salinization, as well as retirement of areas being irrigated by pumping water in excess of rates of recharge. In fact, most new water capacity is predicted to come from increasingly efficient use of existing supplies rather than harnessing of new supplies. In addition, agriculture faces increasing competition from nonagricultural water users. Residential and industrial demands for water are increasing, and cities often can pay a higher price for water than can farmers. Rapid increase in population have overtaken the smaller increases in irrigated area.
Many irrigation systems are very inefficient.
In fact, it is common for less than half the water diverted for irrigation to actually benefit crops. Such a system obviously encourages wasteful use. This inefficiency often means that more water is diverted for irrigation and is applied to fields than needs to be. These heavy water withdrawls have effects on river and groundwater systems, as you can imagine. For example, the Colorado river often contains essentially no water by the time it crosses the border into Mexico, owing to both urban and agricultural withdrawls.