The irrigated area per person reversed from the previous increasing trend since 1978, and decreased 6% by 1991. According to the FAO, this tendency in the irrigated area is a serious problem, for more than a half of the increase in the global food production resulted from the increase in irrigated area from the mid-sixties to the mid-eighties. This slowdown in the growth of the global irrigated area is governed by economic, managerial and environmental reasons. 10) Economic factors are firstly, the reduction in the real international grain price after the second world war, and secondly, the increase in the cost for building large scale surface irrigation systems within the past decades. 11) The cost efficiency of small scale irrigation systems such as Danbos in Sahel and tube-wells in India are important to note. Managerial factors are such that more than half the irrigation facilities in the developing countries currently need to be repaired and also these facilities are operated far below their designed capacity, so much water is wasted, and waterlogging and soil salinization are not appropriately managed. The environmental factors are soil salinization, which is said to occur in 10% of global irrigated areas, human diseases related to irrigation water, environmental damage caused by dams, and the external values derived from aquatic ecological environment.
As agriculture uses two thirds of the world's fresh water supply, there are strong restrictions on agricultural water resource. This water resource is being diverted to industrial and household uses as the economy grows in all the countries of the world. Exhausting underground water resource by over-utilization for agricultural purposes has been occurring in America, Northern China and India. 12) Irrigation investment has stagnated as previously mentioned. During my survey in America in the late eighties, I have personally witnessed the water shortage in California due to the difficulties in building irrigation dams because of the environment protection movements, and destructive reduction of underground water levels due to excessive pumping-out for rice crops in Texas.
The needed future increase in the world grain supply must rely on yield increase under the restrictions on cultivated land, irrigated areas and water.
