Edition 5, July, 2007

NOTE: Sections (8-A) and (8-C) defines terminology and units used below.Four factors contributed to a doubling of global food output during 1950-75 (78B1):

The global land area covered by irrigation systems is about 2.5 million km2 - 16% of all croplands, although it is not clear that this figure adequately accounts for system abandonment. The global land devoted to irrigation grew by nearly 3%/ year from 1950 to the mid-70s. Between 1970-82 global irrigated area grew 2.%/ year (99P1). Between 1982-94 global irrigated area grew 1.3%/ year (99P1). Per-capita irrigated area has declined 5% since 1978 (96P1). About 40% of global cropland output (weight-basis) comes from irrigated croplands (96P1). Thus irrigated cropland is about 3.6 times more productive per unit-area than non-irrigated cropland. The dollar-value of production of a given area of irrigated cropland is about 6.6 times that of non-irrigated cropland and about 36 times that of rangelands (97C1). The ratio of the productivity of irrigated land to that of non-irrigated cropland is thus larger in dollar-terms (6.6) than in weight-terms (3.6), reflecting the fact that irrigated croplands must be used to grow higher-value crops to justify the high capital costs involved.

Irrigation systems, globally, use about 3500 km3 of water/ year (74% is consumptive use), as compared to a total human global water use of 4400 km3/ year, a reliable global water supply of 14,000 km3/ year and a global runoff from ice-free land of 40,000 km3/ year (96P2). These figures may seem to contradict the fact that many irrigation systems are being abandoned due to the reallocation of water supplies to urban use. But a large fraction (2/3-3/4?) of the 14,000 km3/ year of reliable supply is in regions far removed from the semi-arid regions (22.2 million km2), arid regions (22.1 million km2), and hyper-arid regions (5.9 million km2) that typically need irrigation. Global run-off from semi-arid, arid, and hyper-arid lands is only about 860 km3/ year, so clearly the bulk of irrigation water must come from run-off from surrounding (usually upstream) moist areas and from groundwater. Large-scale inter-basin transfers of water are rarely economically feasible without large government subsidies.

As more dams are built, more of the 40,000 km3/ year of global run-off is added to the 14,000 km3/ year of "reliable supply". But water accessible to irrigation on arid and semi-arid regions can hardly be expected to reach, even theoretically, several times the current irrigation-related water use. Overdrafts of groundwater for irrigation have hit their limits in some areas, but may last for some decades or centuries (at current extraction rates) in other regions. The global total of overdrafts of groundwater has been estimated at about 200 km3/ year (99P1) and massive increases in water scarcity are forecast for the coming decades. Yet the FAO estimates that global irrigated area could theoretically be increased 50% (96P1) - down from an earlier forecast (80S1) of a 400% expansion.

From the 1950s to the mid-1970s, about 1000 large dams came on line annually. By the early 1990s, about 260 large dams were being completed annually (99P1). This represents a 0.65%/ year growth in the number of large dams (40,000) which are filling with erosion sediments at about 1%/ year. So the world seems to now be starting to lose the battle to add more reservoir storage capacity - one reason why irrigation systems are increasingly obtaining water from groundwater supplies.

Thus numerous reasons indicate a slowing of net irrigation growth:

5-8% of global irrigated area depends to some degree on over-pumped groundwater (96G2) and this fraction is growing rapidly. In the US this fraction is 20-25%. It seems from all this that the future will not see many more years of irrigation system growth much over the current rate - 1.3%/ year. Increasing numbers of analysts are suggesting that if accounting were done more carefully, current net irrigated-cropland growth could easily be negative. One recent study projects an irrigated-area growth of 0.3%/ year over the next five decades (96G2) and another suggests 0.6%/ year or less growth over the next 2.5 decades (99P1). Clearly the world is rapidly losing one of its major engines that fueled global food-supply growth in recent decades.

The rate of abandonment of irrigation systems due to salination, alkalization and waterlogging is 15,000 to 25,000 km2/ year (96G2) (0.6-1.0%/ year). Nearly 60% of the world's irrigation systems are less that 50 years old (99P1), and several decades are typically required for salinity and waterlogging effects to appear. So abandonment rates must grow significantly in decades to come. The rapidity of the increase of the abandonment rate can also be seen by comparing the estimated global area of abandoned irrigation systems (200,000-250,000 km2) accumulated over all of human history to the current abandonment rate (15,000-25,000 km2/ year). Salination and waterlogging are usually prevented by underground drainage tiles. This adds significantly to the cost of irrigation systems-costs governments are increasingly unable to subsidize.

While no global data exist on loss of irrigation systems due to reallocation of water supplies, US data suggests a rate of about 0.5%/ year. The global rate could be similar, because US population growth is lower than the global average, and the US has a disproportionately large fraction of the world's dams.

The total rate of irrigation system loss from the above four degradation effects (1.1-1.5%/ year) must therefore be at least comparable to the anticipated global rate of expansion of irrigated lands (0.6-1.3%/ year).

Would higher food prices help? Probably not, because the system is so fraught with positive feed-backs (instabilities) that negate the stabilizing effects of a free market. For example, higher pressures on irrigated land to produce tend to translate into trying to get more out of a given water supply. This results in greater salination that counteracts increases in irrigated area that higher food prices produce. Would elimination of irrigation-water subsidies help? Almost certainly yes, by forcing people to use water more efficiently (e.g. drip-irrigation). But the cost of this is the pricing of many Third-World folk out of the market for water-something they aren't likely to accept passively.

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(1TC) Top of this Chapter - Overview
(8) Top of this Review's Appendices (units, conversions, definitions)
(9) Top of this Review's Reference List
(IR) Irrigated Land Degradation: A Global Perspective (Table of Contents)
(T) Title Page of this entire web site (visit early!)

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