by Marc Airhart
May 2, 2008
Climate scientists have long warned that climate change will disrupt the water supply we depend on for drinking, growing food, and generating energy by altering rainfall patterns, intensifying droughts and floods, and reducing snow-fed water supplies in many regions.
According to Bridget Scanlon, a researcher at The University of Texas at Austin’s Bureau of Economic Geology, such predictions are cause for concern but with regards to water, society should not overlook an equally important factor: land use change.
“Land use change is on a par with climate change when it comes to impact on water resources,” she says.
Scanlon has made some surprising discoveries. For example, under certain conditions, croplands can actually recharge an aquifer faster than undisturbed native vegetation. And irrigation, which has been blamed for shrinking aquifers and degrading soil and groundwater in some areas, can in some cases improve water resources.
Her work is also revealing tradeoffs between water quantity and water quality. Land use practices that improve one might harm the other. And what works in one region might not work in another.
Scanlon says the impact of climate change on water resources has been difficult to estimate, and hence difficult to address Rather than feeling hopeless, she prefers to focus on the things that we can do that have a large impact.
“I see this as a great opportunity,” she says. “If we can understand how this all works, we can manage our water resources. Agriculture can actually be part of the solution.”
Over one in six people in the world (over 1 billion) live without access to safe drinking water.
Now, some scientists warn of an impending global water crisis brought on by a perfect storm of increasing water demand from population growth, rising energy production, dietary changes, and industrialization, as well as dwindling supplies from pollution, climate change, and poor management.
“By 2025, over half the world’s population will live in water stressed or water scarce countries,” according to a 2005 report by the Center for Strategic and International Studies, a public policy research institution in Washington, D.C.
Against this backdrop Scanlon, who came to the Bureau in 1987, strives to find ways to make our use of water more sustainable. She points out that agriculture must be part of the solution because it accounts for more freshwater consumption than any other human activity—90 percent of all freshwater consumed globally and 80 percent in the U.S in the last century.
|One of Scanlon’s favorite examples of how land use drives groundwater supply comes from the north central African country of Niger. When farmers there converted land from natural vegetation to crops such as millet over the last half century, a curious thing happened: groundwater increased and the water table, or the top of the aquifer, rose. Here they were growing
crops using rainwater, removing the produce and yet somehow more water was recharging the aquifer than ever before. Even more startling, this recharge took place during a period that included several major droughts.
It turns out that native vegetation tends to be deep rooted and grow year round, whereas crops are more shallow rooted and cover the ground for only a few months of the year. As a result, during their fallow periods, agricultural fields allow more water to seep down to the aquifer. Scanlon’s work shows that recharge has also increased in the Southern High Plains when natural grasslands and shrublands have been replaced with cropland.
The same effects of conversion of grasslands and shrublands to rain-fed agriculture have been documented in Southern Australia where natural eucalyptus forests were replaced with crops in the late 1800s and early 1900s. Forests generally transpire—or release water vapor to the air—more than crops. So the conversion from forests to crops means less water is lost to the atmosphere and more is stored underground.
So clearly rain-fed agriculture (we’ll get to irrigated agriculture in a bit) can be a tool for boosting water resources.
Agriculture, however, is a double-edged sword. While it has boosted groundwater quantities in (semi)arid regions of Africa, Australia and the U.S., agriculture also degraded the quality of the water in those regions to varying degrees. That’s because the increased flow of water through the shallow subsurface tends to mobilize natural salts, carrying them down into the aquifer. The salts are a natural byproduct of Earth’s soils slowly drying out since the end of the Pleistocene in the southern High Plains (10,000 to 15,000 years ago) and up to 30,000 years ago in Australia.
In Southern Australia, the problem is particularly acute because rainwater there contains more salts than in other parts of the world (ten times the chloride concentration of rain in the southwestern U.S.). Over thousands of years, those salts have accumulated in shallow soils. Also, increasing recharge has raised the water table to the land surface in some places where it can easily evaporate and leave behind crusts of salt. This is already happening in some areas of Australia.
The use of fertilizers in farming can also upset the nitrogen cycle, with negative impacts for aquatic animals and people. In the second half of the last century, nitrogen fertilizer use in the Mississippi Delta Basin increased 500 percent. Excess nitrogen washed down the Mississippi River to the Gulf of Mexico and boosted nitrate levels in water near the coast by 200 percent. This excess nutrient loading reduced oxygen levels in deep water and has been linked to huge dead zones in the Gulf.
All of the above is true for rain-fed agriculture. But what about irrigated agriculture—pumping surface water from streams, lakes and reservoirs or from the ground to irrigate crops?
Irrigated farming only represents 18 percent of all cropland, yet it accounted for 90 percent of global freshwater consumption in the last century. And it’s expected to expand by 20 percent globally by the year 2030. Irrigation increases crop yield by about a factor of 2.5 globally. Half the world’s irrigated land lies in just three countries: China, India and the U.S
How will the world handle increased croplands? Global cropland for food production is projected to expand this decade. Scanlon’s research shows that if the new cropland is primarily irrigated, it will stress water supplies and water quality, whereas rain-fed croplands could enhance recharge.
Map prepared by Boston University (Earth Observing System Data Gateway) based on MODIS satellite data. “Cropland (2%)” refers to cropland/natural vegetation mosaic.
It turns out that irrigated agriculture can cause all of the same problems with salinization and excess nutrient loading that can occur with rain-fed agriculture. And for the most part, it doesn’t have the saving grace of increasing available water supplies. The story becomes a little more complicated at this point because the impacts depend on where the irrigation water comes from.
In the big three irrigating countries, an increase in groundwater-fed irrigation in recent decades has lowered some water tables by as much as a meter a year and reduced stream flow. Scanlon’s work shows that since 1945, groundwater-fed irrigation in the Southern High Plains has caused groundwater to decline by an average of 40 meters over an area of 10,000 square kilometers.
Where surface water from streams, lakes or reservoirs was used for irrigation rather than groundwater in the big three countries, stream flows were greatly reduced, yet water tables rose. But the gains in groundwater were ephemeral. It merely shifted water from the surface to the ground, so there was no net gain of water. Overall, according to Bridget Scanlon, irrigation is a lose-lose proposition. You lose more ground and surface water per pound of crop than with rain-fed farming and the water that’s left is worse off than what you started with.
“Irrigated agriculture is basically unsustainable,” says Scanlon.
To lessen the negative impacts of agriculture and make water use more sustainable, Scanlon has several recommendations.
1. Decrease irrigated agriculture.
First, decrease use of irrigated agriculture by adding rainwater harvesting to the mix, rotating irrigated with non-irrigated agriculture or converting completely to rain-fed agriculture in some places.
Scanlon points out that not all irrigated agriculture is bad. In some places, it could actually help improve water quality. In Australia and Niger, where the water table has risen to the land surface and is directly evaporating, artificial salinization is occurring and there’s more than enough groundwater, the careful use of groundwater-fed irrigation to lower the water table would improve water quality. There is no one-size-fits-all solution. Good land and water management will mean different things in different places.
Likewise, rain-fed farming may not be appropriate everywhere. Because it can mobilize natural salts and nitrates in shallow soils, before it expands into new land, scientists need to communicate to farmers which areas have the least chance of degrading water quality.
2. Get “more crop per drop.”
In other words, use water more efficiently by reducing evaporation, runoff and drainage. Also decrease fallow periods by growing winter cover crops. In Australia and Niger, shorter fallow periods would have the collateral benefit of reducing salinization by slowing aquifer recharge. More efficient use of water also means that less land has to be converted to cropland to feed growing populations.
3. Breed crops that grow well with less water and more salt.
So far, geneticists and traditional plant breeders haven’t been very successful on this front. But a breakthrough here could have a big impact in the developing world.
4. Grow crops suited to the local climate.
One of the biggest crops in northern China is winter wheat, a crop that grows in the winter and requires a lot of water. Yet, the monsoons that provide 70 percent of annual rainfall occur in the summer. As a result intensive irrigation (mostly from groundwater) is the only way to grow winter wheat in that area. This has caused streams to dry up and groundwater declines of up to 0.7 meters per year. It would be much more sustainable to grow more corn or other summer crops that can take advantage of the monsoons.
Keep in mind that some efforts to improve water quality, such as planting winter cover crops or increasing forest cover, can lead to less groundwater recharge.
“I hope this work can help people in developing countries produce the food they need while also protecting their water resources,” says Scanlon.
Reporters sometimes ask her what individuals can personally do, given that most aren’t actually farmers. She responds that the biggest impact any one of us can make would be to switch to a vegetarian diet. Peer-reviewed studies indicate that it takes about half as much water to grow the food that a vegetarian eats as it does for the food that a non-vegetarian eats. Specifically, it takes about 2,600 liters per day to feed a vegetarian, compared to 5,400 liters to feed a non-vegetarian.
Scanlon is now looking at impacts on water resources of other farming practices such as no-till farming, in which seeds are planted without plowing the soil. She and graduate student Gil Strassberg are also working with NASA to calibrate gravity data from the GRACE satellite that can be used to monitor seasonal changes in groundwater storage in the High Plains.