The world's population is growing dramatically, and with this growth will come increased demand for food. The majority of this food will come from agricultural land that must be irrigated.
It is clear that water will be in great demand in the future. In anticipation of future water scarcity, there has been a large push to increase the efficiency of water use in agriculture.
The assumption here is that this increase in efficiency will leave more water available for further agriculture, cities, and other uses. Although this is a reasonable assumption, the reality is far more complex, and it has not been tested.
Frank Ward (New Mexico State University, Las Cruces) and Manuel Pulido-Velazquez (Universidad Politécnica de Valencia, Spain) have shown this efficiency assumption to be false, at least in the Rio Grande River basin. Increasing water efficiency in agriculture, especially through subsidies, can actually increase the total use of water.
Carrying out the study.
The scientists focused their efforts on the Elephant Butte Irrigation District, an 89,000 acre region in southern New Mexico. This is a component of the Rio Grande River basin.
Thirteen crops were studied, including wheat, pecans, and alfalfa. Each one of these crops has a positive linear relationship between crop water use and crop yield, regardless of the technology used to irrigate the crops.
Additionally, detailed data is available for each crop regarding (1) the amount of water applied per technology, (2) crop water use per technology, (3) crop price per unit, (4) crop yield per technology, and (5) crop production cost per technology, with or without production cost subsidies. This enabled the scientists to critically evaluate the agricultural, hydrological, and economic effects resulting from water conservation in agriculture.
This experimental data also enabled the scientists to extrapolate the data into computer models, to predict water use under a given set of conditions. Both experimental data and modeling were combined into their analysis.
The scientists also take into consideration treaties and environmental laws that dictate water use. They constrain their model to not allow water levels to fall below the legally permissable limit.
Effects of public subsidies.
Using their model, the scientists discovered that as subsidies increase, net farm income increases from $34.1 million (no subsidy) to $45.5 million (100% subsidy). At maximum subsidy levels, water-efficient irrigation is used for 53% of the available land.
Increasing subsidies for water conservation in agriculture does reduce the amount of water applied to farmlands. However, total water use by the crops increases.
Increasing water use through conservation.
How can water conservation increase total water use?
Water depletion in agriculture is water from a hydrologic basin that is used by the crops. Water that is simply diverted to the crops, but not used by the crops, is often returned to the basin through reabsorption into the ground, or as runoff.
Drip irrigation is widely assumed to conserve water. By slowly supplying water to the crops' roots, less water is lost through ground reabsorption or runoff.
However, this means that (1) the crops will use all of the water that was provided for them, and (2) the crops will grow more, both of which will increase water use even further. While this increased crop growth increases revenue for farmers, more water overall has been used.
This means that providing subsidies for water conservation will lead to increased crop yields, increased crop acreage, and water depletion.
Limitations of the model.
This model is best applied to regions similar to the Elephant Butte Irrigation District. Geographic areas in which water that is diverted to agriculture, but not used by the crops, and returns to a nonusable source such as the ocean, can actually decrease total water use.
How can we decrease water use?
The scientists note several steps that can be taken to decrease water use in agriculture.
Clearly, the common assumption that switching to water-efficient irrigation saves water, and therefore leaving more water available for other uses, is not necessarily true. The reality is far more complex.
Conflicting effects of water depletion and water application, when considered over the level of an entire watershed basin, can have unexpected consequences. Conserving water in agriculture can actually increase the total use of water, leaving less water available for other purposes.
for more information:
Ward, F. A.; Pulido-Velazquez, M.
Water conservation in irrigation can increase water use.
Proc. Natl. Acad. Sci. USA 2008, 105, 18215-18220.