I have seen the statement that 90% of water used globally is used for agriculture, more and more frequently. This “new” statistic has replaced the often quoted World Health Organization statement that 80% of freshwater used is for irrigation. The WHO number is based on measurement of water withdrawals from rivers and groundwater for irrigation. The 90% number (it is actually 92%) is from a recent paper by Arjen Y. Hoekstra and Mesfin M. Mekonnen of the University of Twente in the Netherlands titled “The Water Footprint of Humanity.” They estimated the consumptive use of rainwater for agricultural production and add that amount of water to groundwater pumping and surface water diversion to examine the consumptive use of water for agriculture. Thus, the 90% of water used globally refers to an estimate of how much irrigation water and rainfall water is consumed by crops and agriculture in general.
The Water Footprint has no relationship to the rainwater available and does not relate in any way to the sustainability of surface and groundwater use patterns. In all countries the amount of water in agricultural products accounts for the largest proportion of water used. According to their methods agriculture accounts for 92% of the water footprint, industry 4.4% and domestic use (the water we drink and bath with) accounts for 3.6%. I’m not at all sure that utilization of rainwater in watering crops a meaningful measurement of water use. It leaves unaddressed sustainability of water use, the importance of (or lack of importance of) the rainfall on woodlands, recharging of groundwater that is not being depleted, stormwater runoff and other rainwater uses. It almost implies an ownership of rainwater that falls in various lands. The purpose of the work was to develop a global water management tool, but does not address the complicated aspects of water that is at times renewable and other times not.
In an attempt to look at water beyond the watershed these two Dutch scientists in a series of studies have attempted to trace the concept they called the Water Footprint, by including data on rainwater use and volumes of water used for human and animal waste assimilation to track waters movement in water-intensive commodities as they move across the globe. By importing food, a country externalizes their water footprint. The scientists identified the water content of various foods by estimates based on global precipitation, temperature, crop, and irrigation maps and the yield, production, consumption, trade and wastewater treatment statistics for nations. There are assumptions underlying this data on planting and harvesting dates per crop per region, feed composition per animal and country as well. In addition the scientists assumed that industrial water supply are spread according to population densities.
Arjen Y. Hoekstra and Mesfin M. Mekonnen then estimated the water content of all products and determined a trade balance with water content in a product as the measure. Using their methodology the major gross virtual water exporters are the United States, China, India, Brazil, Argentina, Canada, Australia, Indonesia, France and Germany. The scientists note that “all these countries are partially under water stress, which raises the question whether the …choice to consume the limited national (surface and groundwater) resources for export is sustainable and most efficient.” Good question. These scientists were trying to develop a way to look at the global dimension of freshwater resources to try to understand and ultimately solve the most pressing and urgent water problems, addressing the limits on the supply of and contamination of fresh water on the planet, and the ability of the planet to feed themselves. However, their operating framework ignores comparative advantage (French wine) and seems to suggest that water in agricultural products is not properly valued. However, they cannot actually determine what the country limit to agricultural production is because they have not addressed the limits of water supply, and unsustainable use of water.
Let’s look at this from another angle. All the water on earth is over 4 billion years old. “It's one of the more astonishing things about water — all the water on Earth was … here when Earth was formed, or shortly thereafter…in the first 100 million years or so. There is, in fact, no mechanism on Earth for creating or destroying large quantities of water.” The quote above is from Charles Fishman’s book, The Big Thirst: The Secret Life and Turbulent Future of Water. All the water that ever was or will be on earth is here right now. More than 97% of the Earth’s water is within the in oceans. The remaining 2.8% is the water within the land masses. The land masses contain all the fresh water on the planet. Of the land surface water, 77% is contained in icecaps and glaciers and for all practical purposes is inaccessible in the short run, and on a warmer planet will not be stored in ice. The remaining fresh water is stored primarily in the subsurface as ground water with a tiny fraction of a percent of water is stored as rivers and lakes which are renewed by rainfall.
Only a fraction of water falls as rain each year to make the rivers flow, recharge lakes and groundwater. The water on earth never rests, it is constantly moving within the hydrologic cycle along various complex pathways and over a wide variety of time scales. Water moves quickly through some pathways -rain falling in summer may return to the atmosphere in a matter of hours or days by evaporation. Water may travel through other pathways for years, decades, centuries, or more—the groundwater stored in the Wasia aquifer in Saudi Arabia fell from the atmosphere as rain thousands of years ago. In the Middle East, in California, in India and throughout the planet we are using groundwater faster than it is being recharged. We are using up our stored water reserves to grow food and the water reserves are shrinking. So, that determining the water footprint in the way that Arjen Hoekstra and Mesfin Mekonnen have attempted does not convey the limited time that mankind can continue to use water in the way that we are using water now.
As of 2010, 783 million people worldwide still relied on unimproved water sources (surface water from lakes, rivers, dams, or unprotected dug wells or springs) for their drinking, cooking, bathing and other domestic activities. In 2004 (the last year for which statistics were available), water, sanitation and hygiene was responsible for 1.9 million annual deaths from diarrhea. Most diarrhea deaths in the world (88%) are caused by unsafe water contaminated by human or animal waste, sanitation or hygiene. In addition, there are estimated to be as many as one billion hungry people in the world, some even in the United States.
The earth has a fixed amount of land and water. Water is complicated by the variability in weather and the variable length of different parts of the water cycle. Precipitation does not fall in the same amounts throughout the world, in a country, or even a region and varies from year to year. We are on a trajectory towards a world where ever increasing numbers of people will not have food security and will starve during drought years. Farmers in the United States feed 20% of the world’s population on just 10% of the earth’s surface that is how we ended up the largest virtual water exporter. The U.S. agricultural sector is the most successful in the world, but will not be able to meet the world’s projected food demand and we may not want to mine groundwater in California to export Almonds. California might want to drink some of that water. Even if all the world’s farmers adopted conservation-based agricultural production techniques (emphasizing soil health) there are limits to what the earth can reliably produce each year. During a “good” period of temperature and rainfall in the most agriculturally productive areas and the most marginal areas the world’s population and demand for food will grow to exceed the average production and the next drought or the exhaustion of a groundwater aquifer will bring catastrophic consequences. It has always been the nature of man (see the Mayan Empire).
According to the Dutch scientists, over a fifth of the nations are net water importers, they have an external water footprint. Many highly water scarce countries (that can afford it) are externally water dependent- Kuwait, Jordan, United Arab Emirates, Israel, Yemen, Malta, and Cyprus. Though, not all countries with a large external water footprint are water scarce. One of the interesting observations was that the Netherlands and United Kingdom are net importers of food and thus water. Arjen Y. Hoekstra and Mesfin M. Mekonnen state “For governments in water-scarce countries such as in North Africa and the Middle East, it is crucial to recognize the dependency on external water resources and to develop foreign and trade policies…” to ensure a sustainable and secure import of water intensive commodities (food). It is not viable to irrigate crops with desalinated water. According to the US Geological Survey it takes 20 gallons of water (on average) to grow one apple, 4,000 gallons of water to grow one bushel of corn, 11,000 gallons of water to grow one bushel of wheat, 15,000 gallons of water to raise a cow. The Dutch scientists finish by pointing out that China with a relatively internal water footprint is leasing lands in Africa to secure their food supply and water resources outside their country.