Two land degradation problems that can result from irrigation include salinization and waterlogging . We'll discuss these in a bit, but first we need some background about irrigation in agriculture.
Increased irrigation has been very important in increasing cultivated acreage, and increasing productivity on cultivated lands. Today, almost 40% of the global harvest comes from the ~20% of the world's cropland that is irrigated. [Vital Signs 2010]
Humans now appropriate about 54 % of the accessible freshwater for all uses combined, and agriculture is a big user of water: Farming accounts for about 70% of the world's water consumption (streams + groundwater combined) (Science 11 Apr 08). (Water "consumption" refers to uses of water that do not return the water directly to the source from which it was withdrawn in a form that is readily re-useable. For example, much of the water that is used in irrigation returns to the atmosphere as water vapor via either evaopration or transpiration. You don't "consume" water when you brush your teeth if you live in a town such as Corvallis -- the water goes back to to the river via the sewage system -- or percolates back to ground water, in the case of septic systems.)
Food demand is projected to increase greatly, so the volume of water for food production will need to increase as well, given that irrigated lands produce so much of our crops. Currently, ~ 2 billion people live in water-stressed areas [Science Aug 25 2005]. "Water stressed conditions" are defined as not having enough fresh water to meet the inductrial, municipal, and food production needs of the people. By 2025, about six times that many people are expected to live under such conditions, with concentrations in Africa, S. Asia, and the Middle East. Many people in these nations are poor and malnourished now, and are unlikely to be able to afford the import of foods that would be necessary to make up for their water deficiencies. Efficiency of water use MUST increase! Water (and land) shortages are thought to have led to some major conflicts already -- including those in Sudan's Darfur region....
About 62 % of the water used in agriculture, globally, comes from surface sources (e.g., rivers) while about 38 % comes from ground water (underground aquifers). However more of Earth's freshwater is in aquifers than in surface sources -- in fact, about 99 % of all liquid freshwater is in groundwater. (Issues in Ecol (9) 2001). Much (> 75 %) of this groundwater is "fossil water;" -- water that is not being recharged but is relic from wetter ancient climate conditions and from melting ice after the Pleistocene ice ages. Once we use it, it is "gone" for all practical purposes.
Between 1900 and 1950 irrigated land area world wide nearly doubled, and the increase since 1950 was even greater, as you can see in the figure below (Updated figure is available in Course Documents on Blackboard).
(Figure modified from Vital Signs 1997, World Watch Institute, WW Norton Inc.)
Between 1900 and 1950, irrigated acreage world wide nearly doubled, and the increase since 1950 was even greater. There has been a slowing of expansion in irrigated acreage since the 1970's however, largely because the best irrigation areas had already been developed; remaining options for expansion are, by and large, very expensive; and the social and environmental liabilities of some new irrigation projects have made them politically unfeasible (e.g., people being flooded out of their home lands by the creation of new reservoirs, or the consequences of low river flows for fish). Nevertheless, increases in irrigated acreage have been a very important cause of increased global harvests -- between 1961 and 2004, total yields increased by 2.4 times, while harvested area increased by only 10% -- irrigated acreage more than doubled during this time (Science 25Aug06) (and we've already seen what happened to use of inorganic fertilizers...)
Slowing expansion of irrigated acreage is also a consequence of low commodity prices (which made water and pumping it too expensive), high energy costs, and shortages of available land and water. In some areas, such as Mexico and portions of the US (e.g., areas watered from the Ogalalla Aquifer), irrigated land area has actually decreased in recent years. (See more information on water shortages and competition for water resources in the notes on problems with maintaining the Green Revolution's ability to keep increasing agricultural production.)
The potential to increase substantially the gross irrigated area of the world is limited. 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. For example, the water demands of Phoenix and Tucson have resulted in the abandonment of formerly irrigated farm lands (as discussed in your assigned reading about "Feeding Nine Billion.")
Also, there is increasing concern about environmental damages associated with dam projects (which, of course, involve creation of reservoirs whose water can be used for irrigation). In the US, the Commissioner of the Bureau of Land Reclamation said in 1994 that, "Federally funded irrigation water supply projects will not be initiated in the future." The Commissioner cited limited money and the need to, "Focus on increasing efficiency and remediating adverse impacts of existing projects." (which would include problems with salmon, inundation of valuable areas by flooding, and so forth).
Finally, there is also another kind of competition from nonfarm water uses; restoration of wetlands and fisheries. For example, a 1992 California law requires that 800,000 acre feet of water be kept in rivers to restore them! We had a local example of this kind of competition during the drought summer of 2001 in Oregon. Farmers in the Klamath basin were nearly cut off from irrigation water provided by local lakes and rivers, as the Federal listing of several fish species (two species of sucker fish and also coho salmon downstream) apparently made it necessary to retain water in the streams. I say "apparently" because the US Interior Department called on the National Academy of Sciences to review scientific and technical information related to the listing of the species and the implications for users of water in the area. In the summer of 2002, of course, the farmers were allowed to use more water, and massive kills of salmon resulted during the fall runs. As of 2012, a consortium of interest groups (fishermen, tribes, environmentalists, farmers, and power companies) reached an agreement to remove all four dams on the Klamath River -- the hope is that this will restore the formerly large salmon runs, retain water for other species of endangered fish, and protect farmers from uncertainty about their water supplies for each growing season. The dam removals are anticipated to begin in 2020.
Over recent decades, as you might expect, per capita irrigated area, has basically declined, as illustrated in the figure below (note that the figure illustrates irrigated acreage per 1000 people). Rapid increase in population have overtaken the smaller increases in irrigated area.
(Figure modified from VitalSigns 1997, WorldWatch Institute, WW Norton Inc.)
The future of water supplies is very difficult to project, in part because good data on current water availability and use are spotty and uneven in quality. However, most analyses suggest that humans are currently using over half of the accessible fresh water runoff. Climate change will be important. Areas experiencing more water stress in the near future are anticipated to increase far more (by twice as much) than areas experiencing less water stress. Less rainfall is anticipated in already arid areas, such as the Mediterranean Basin, the western US, S. Africa, and NE Brazil. Further, glaciers and snowpack may be greatly diminished in the near future, both of which have historically fed summer river flows and recharged aquifers [Vital Signs 2010]. Stress on natural aquatic ecosystems is likely to increase. Globally, 20% of freshwater fish species are threatened or extinct, and, in 2001, freshwater species comprised 47% of all animals that are federally listed as endangered in the US. Most analysts suggest that the only viable solution for meeting human and natural ecosystem water needs for the future will be increases in efficiency of water use -- in agriculture as in all aspects of human endeavor.
Many irrigation systems are very inefficient. In fact, it is common for less than half the water diverted for irrigation to actually benefit crops. Inefficiencies result as water:
seeps out of unlined canals as it is transported to fields
evaporates from canals and soil
evaporates as its being applied (you have probably all seen this on hot summer days)
is delivered to plants on a fixed schedule that doesn't match the plants' needs
evaporates from between plant rows
Farmers in the US, since water is relatively cheap to those who have water rights, tend to apply enough water to make sure that no area of the field is under water stress, which often means that many areas are overwatered instead.
In addition water rights in portions of the US tend to work on a "use it or lose it" basis so that if farmers don't use their full allocation in one year, the amount allotted to them may decrease the next year. 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. The adverse effects on soils that we'll talk about are caused in part by this excessive application of water
Excessive quantities of water are also often required because people grow water hungry crops (such as cotton or corn) in semi arid regions.
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. In fact, in most years, the Colorado River doesn't make it to the ocean. This has consequences for the river and its riparian ecosystems, as well as for the delta and estuary system at its mouth, which no longer receives the recharge of fresh water and nutrients that it normally did. The same is true for the Yellow River in China. The San Joaquin River in California is so permanently dewatered that trees are growing in its bed and developers have suggested building housing there.
In the last 33 years, the Aral Sea in central Asia, which was the world's 4th largest fresh water lake, lost 50% of its surface area and 75% of its volume, with a concomitant tripling in its salinity, owing largely to diversion of water from its feeding rivers for irrigating cotton and rice fields. Shorelines receded by as much as 120 km, and fisheries that formerly supplied jobs to thousands and yielded thousands of tons of fish have shut down completely. You can find photographs of docks and fishing boats stranded in what appears to be virtual desert. (A new bright spot, though -- Science 14 April 2006 -- as the Aral Sea shrank, it became divided into the N and S Aral Seas, The World Bank and Kazakhstan build a dam across the strait separating the two seas, and made changes to the Syr Darya River, one of the primary "feeders" of the N sea that allowed more water to flow along its course. While it was imagined that it would take years for the N Aral to begin filling significantly, within 8 MONTHS it had filled to the point where it now spills water into one portion of the S Aral! There is great hope that the associated decrease in the N Aral's salinity will allow the return of indigenous fish from the rivers that feed it....
Huge water withdrawls and changes in riparian systems (related in part to overgrazing as well) and degradation of streams by silt (also related in part to overgrazing) are considered responsible for the extinction, Federal listing, or candidacy for endangered status for 122 out of 150 native fish west of the continental divide.
Groundwater is being mined, essentially (as we discussed earlier briefly when we talked about problems with Green Rev style agriculture.) In the US, about 20% of our irrigated agricultural area is watered by pumping in excess of recharge.
Texas has lost 14% of its irrigated acreage since 1980 as a result of aquifer depletion. In India's Punjab (its breadbasket) pumping exceeds recharge by 1/3, causing water tables to drop by 1 m/yr or more! Problems with overpumping from aquifers are occuring in many places in the world -- in Gaza, groundwater depletion in coastal areas has allowed salty ocean water to intrude into aquifers, rather than the aquifers draining into the sea....Can't irrigate with salty water!
Other environmental price tags associated with this irrigation are the problems of waterlogging and salinization. To review notes on soil salinization resulting from irrigation, click >> at the bottom of this page; for general reminders on how to navigate within and among these pages, click "Navigate ", or click "waterlogging" to jump to those notes.
Page maintained by Patricia Muir at Oregon State University. Last updated Nov. 5, 2012.