Fundamental resources/Water/Water-efficient agriculture

"We need a 'Blue Revolution' in agriculture that focuses on increasing productivity per unit of water — more 'crop per drop'.

— Kofi Annan

70% of world water use goes to irrigation for agriculture. The single most important thing we can do if we want to reduce water demand is to increase the water-efficiency of agriculture. Along with protecting the environment and securing food supplies, this is a third reason to move to sustainable agriculture, which consistently effects large reductions in water used ^[1]. The various types of food-production suggested in the Food article are all very economical with water. There are several very simple ways to reduce agricultural water use.

A combination of these techniques — applied intelligently and with a sensitivity to local needs and resources — can enable even rather dry climates to secure their own food supply without the need to pump in water from elsewhere. Rainfed agriculture currently provides about 60% of the world's food ^[2]. This could be expanded to nearly 100% using the methods mentioned here, while we can also make irrigation at least twice as efficient. This would massively unburden the world's water needs and can be done very cheaply, as most of the methods are very simple, low-tech interventions.

The recent tragic famines in Niger, Sudan, Chad and the Horn of Africa have been blamed on droughts, but they have more complex causes than a simple lack of rain. While drought was the immediate cause of the crop failure, the drought would not have done the same damage to an agricultural system that used water more effectively. By building soil and using groundcover, earthworks and trees, we can build agricultural systems that can maintain their productivity through times of drought and put an end to catastrophic crop failures.

Building soil

Sustainable farming practices emphasize building the structure of the soil, not disturbing it with tilling or digging. This builds soil infiltration (the ability of water to enter soil) and water retention (the amount of water a soil can hold, which is mostly determined by the amount of humus in the soil).

Several practises typical of sustainable agriculture have the effect of dramatically increasing soil infiltration and retention capacity, thus dramatically decreasing the need for water to be applied —

• No-till farming , which is gaining popularity around the world, increases infiltration and retention capacity while reducing labor and increasing yields.
• Permaculture designs generally avoid leaving earth bare. The ground is covered either with cover crops (such as clover and grass), or with mulch such as straw or woodchips. When ground is covered, water does not easily evaporate from it, and the layer of groundcover acts as a sponge, holding extra water after rainfall. This method alone reduces water requirements 25-50%.
• Compost, the basic fertilizer of sustainable farming, which is essentially manmade humus, can increase soil infiltration 125%^[3]
• Mob-grazing (a method of raising livestock that emulates natural grazing patterns), has been shown to increase infiltration as much as 775%^[4]
• Burying charcoal in the soil improves its structure and thus increase water retention capacity. This method is gaining attention recently.

Using trees

Sustainable agriculture emphasizes polyculture; rather than growing a single crop, many different species grow together and interact with one another. Trees have a role to play in nearly all agroecosystems. Planting trees between crops reduces the need for water in at least six ways —

• Tree roots secrete humic acid, which creates humus in the soil. This humus has a spongy texture and helps the soil hold more water.
• Trees have much deeper roots than food crops, so they can draw up deep groundwater and bring it into the field. This moisture then enters the water cycle on the farm.
• The shade provided by the trees reduces evaporation by two-thirds ^[5]
• Trees may increase orographic rainfall  by deflecting cool winds upwards. This would mean that planting forests can actually increase the amount of rainfall an area receives.
• Air rising off trees contains pseudomonas syringae bacteria, which create ice crystals in clouds, leading to rain^[6].
• Trees condense moisture from the air and drip it down to the soil below. This is known as occult precipitation. Trees are natural atmospheric water generators.

Earthworks

Swales are a simple and effective rainwater harvesting method that can be used anywhere. They are long depressions in the earth, dug along contour lines, with an earthen wall on the downhill side. Without swales, rainfall runs off the hillside (picture on left). When a series of swales are dug (picture on right), the rainfall cannot run off. It pools in the swales and gradually soaks into the soil. A single rainfall will then keep the soil wet for months or even years.

Very simple earthworks can turn arid areas into productive farmland. This is exemplified in Syria and Jordan, where there are productive rainfed farms in areas with as little as 120mm of rainfall per year. The simple structures include stone walls built on contour to stop water draining downhill, hollows dug into the ground to accumulate rainwater, swales, which are a combination of these two methods, and mini-dams to redirect water towards productive areas. A combination of agroforestry and earthworks can make any area productive, as has recently been demonstrated in Burkina Faso, Niger and Kenya.

Alternative food production methods

• Aeroponics uses a tiny fraction of the water of drip irrigation. It is by far the most water-efficient means of growing food yet devised.
• Seawater agriculture can grow food in coastal regions, requiring no fresh water at all.
• Aquaponics recycles the same water over and over, so that a given amount of plants can be grown in only 10% of the water a garden would need.