Cotton and Water Use

Cotton and Water Use

(Adapted from a recent publication by ICAC and FAO: Measuring Sustainability in Cotton Farming Systems, Towards a Guidance Framework. http://www.fao.org/3/a-i4170e.pdf)

Resource use is a critical factor in all production systems. With the world population growing to more than 9 billion by 2050, combined with rising incomes leading to increased commodity consumption, there will be increased pressure on the use of agricultural land, water and energy. Cotton will be competing with other users, both agricultural and non-agricultural, for access to resources, and all farmers will be under pressure to raise yields per hectare, yields per liter and yields per calorie in order to remain viable in the economy of the future.

Water is important to increase yields and to improve cotton quality, and cotton production is limited by water supply in most countries, whether irrigated or rainfed. Cotton can grow with minimal amounts of water, or if water is available it can utilize significant amounts at certain times during its production cycle. Requirements vary widely depending on region, length of growing season, climate, cultivar, irrigation method and production goal, and range from 600 to 1200 mm. The relationship between yield and water use for cotton is linear: other things equal, more water means more cotton.

Among major producing countries, nearly 100% of cotton production is irrigated in Western China, Egypt, Northern India, Pakistan, Turkey and Uzbekistan; about 90% of production is irrigated in Australia and Eastern China; about three-fourths of production is irrigated in the United States, and about one-third of production in Central and Southern India is irrigated.

Water use can be measured and reported in a range of ways, often referred to as a measure of “water use efficiency” (WUE). Various metrics are used in describing WUE, including evapotranspiration (ET) of water from the field. Water crop productivity (WCP) is an index that measures the production associated with the water use. Irrigation water use index (IWUI) is defined as the quantity of output per volume of water applied through irrigation. The gross production water use index (GPWUI) is similar to the IWUI, but includes seasonal rainfall and stored soil moisture at the start of the season. Water use efficiency percentage (WUE) (%) is the ratio between the amount of water actually used (ET) and the amount of water withdrawn or diverted from its source (river, lake etc.). It is sometimes also referred to as “water supply efficiency” or “irrigation efficiency.”

Three significant sustainability issues associated with water management are relevant to cotton: 1) water depletion (irrigated cotton only), 2) soil salinization (generally associated with irrigated cotton) and 3) water pollution, including eutrophication (relevant to both irrigated and rainfed cotton production).

Water Depletion: Cotton cultivation accounts for an estimated 3% of the world’s irrigation water (Hoekstra and Chapagain, 2007), which is proportional to cotton’s share of global arable land. While water withdrawal for irrigation can be sustainable if the amount of water withdrawn is replenished by equal amounts in a timely manner, over-withdrawal can occur when removal for irrigation exceeds replenishment over a number of years.

Salinization of soil results from evaporation of water and leads to higher concentrations of mineral deposits in the root zone. With irrigated cotton, soil salinization is a consequence of limited drainage, combined with the application of saline or sodic water. One estimate is that in six leading cotton-producing countries, 12–36% of the irrigated area was damaged through salinization (Dinar, 1998).

Water Pollution: Pollutants (pesticides and fertilizers) can affect freshwater quality. The main pathways for pollutants into rivers and bodies of water are spray drift during application, water runoff from fields and soil erosion. The likelihood of spray drift occurring and reaching water is influenced by weather conditions at the time of application, the method of application, crop stage and the distance between the crop and the water. Pesticides and fertilizers can be transported when dissolved in water or attached to soil particles carried by runoff water. Therefore, minimizing runoff and erosion will reduce the risk of pesticides and fertilizers contaminating water bodies. Water eutrophication is a specific form of pollution in which nitrogen and phosphorous reach bodies of water and cause excessive growth of algae.

Why is cotton produced in arid and semiarid regions?

Cotton is often criticized as being a “thirsty” crop (World Wildlife Fund) or for consuming water that would otherwise go to “better” uses. A recent story in the Guardian (http://www.theguardian.com/sustainable-business/2015/mar/20/cost-cotton-water-challenged-india-world-water-day) implies that 100 million people in India do not have access to safe drinking water because of cotton exports.

While there are many metrics that can be used in describing water use efficiency, the measure most often reported in the general press and used by many NGOs is simply the kilograms of output per cubic meter of water use. As is always the case in agriculture, measured yields are highly variable, but there are averages that can be used to illuminate basic relationships.

Using average values of kilograms of output per cubic meter of water, cotton appears to be inefficient. As shown in the table below, more kilograms of wheat, rice, maize and soybeans can be produced per cubic meter of water than kilograms of cotton lint. By this measure, many NGOs criticize the cotton industry and imply that consumers should buy polyester clothing rather than products made of cotton.

But, you have to wonder, if cotton is inefficient in the use of a scarce resource like water, why do farmers grow it? Are they dumb?

As shown, farmers choose to grow cotton in regions of water scarcity because the resulting gross revenue is approximately double what they would receive growing wheat, maize or soybeans.
The reason farmers grow cotton in areas where water is scarce becomes apparent when you look at crop values. Yes, more kilograms of grains and oilseeds can be produced per unit of water, but a kilogram of wheat was only worth one-ninth as much as a kilogram of cotton during the decade of the 2000s; a kilogram of soybeans was only worth about one-fourth as much; and a kilogram of maize was only worth about one-twentieth as much as a kilogram of cotton. Only rice produces and economic return comparable to that of cotton, and rice is far more water intensive than cotton and can only be grown in areas of water surplus.

Cotton is a drought- and heat-tolerant crop, and thus generally well suited to climates with low rainfall, and it is grown in many regions where precipitation is low. Irrigation is typically applied in these areas to ensure crop maturity and to stabilize and maximize productivity. Irrigated cotton accounts for half of all the land devoted to cotton production and is responsible for 73% of global cotton production.

Farmers grow cotton in areas where water is scarce because cotton can be grown in those areas and because cotton provides the best economic return on the use of a scarce resource. Regions are not arid or semiarid because cotton is grown in them. It is not correct to say that cotton requires more water than other crops; rather scarce water is applied to cotton because cotton makes better use of it than other crops would.

Furthermore, even if cotton exports from India were reduced to zero, there would still be 100 million people without access to safe drinking water. People do not have access to safe water because infrastructure has not been built or family incomes are so low that they cannot afford water, not because cotton is grown in India.

Nevertheless, while it is sensible to grow cotton in regions that experience water scarcity, the cotton industry must continue to improve in the use of water to ensure that supplies are not depleted and that soil and water are not polluted.

Nevertheless, while it is sensible to grow cotton in regions that experience water scarcity, the cotton industry must continue to improve in the use of water to ensure that supplies are not depleted and that soil and water are not polluted.