Food and Water
There is great abundance of water at the Earth's surfance, but most of it is difficult for humans to use.
On a global level, there is sufficient water available that can be harvested sustainably.
However, water is distributed unevenly and is not generally traded internationally due to its low value-to-bulk ratio . Several countries, especially in the Middle East and North Africa, have water demands in excess of the renewable resource  and must make up the difference through trade or desalination.
Water usage is measured in three ways: withdrawal, consumption, and pollution. Withdrawn water may be returned to a river or lake in a usable form, while consumed water cannot because it is evaporated or polluted in a way that it cannot be used again.
Following are estimates of water consumption and pollution by major source.
As countries industrialize, the proportions shift toward industrial and municipal usage, and away from agriculture . The United States withdraws water as follows.
Major families of crops require water as follows.
Drip irrigation reduces a farmer's water needs by about 30-50%, but this effect may be negated by reduced percolation into the soil . Drip irrigation also helps reduce nitrogen requirements, soil erosion, and plant disease, but has a higher capital cost .
Highly intensive forms of farming, such as greenhouses and hydroponics, can also drastically cut water usage.
Power production consumes water as follows.
More energy-efficient power plants generally require less water as well, and the water intensity of thermal power generation has been generally decreasing . However, drilling unconventional oil and gas generally require more water than conventional oil and gas .
The method of cooling is a major determining factor in a thermal plant's water needs. Most commonly, there is a tradeoff between once-through plants, which withdraw large amounts of water, and closed loop systems, which withdraw less but consume more water through evaporation.
Dry-cooling and hybrid systems are not yet widely used. They reduce both water withdrawal and consumption, but generally with lower energy efficiency and higher capital cost .
Municipal water usage in particular trends to grow with urbanization and wealth, creating an infrastructure challenge for large cities in developing countries . However, water demand shows signs of declining when high levels of wealth allow for efficiency.
There is significant potential for water savings for municipal and non-energy industrial usage.
Water is often subsidized or otherwise not priced in a manner to reflect its true costs, thereby encouraging overuse . Most economists argue that water pricing is needed, and the efficient way to do this is set the price of water in a region equal to the marginal cost of provision . In practice, the complexities of water markets can make this challenging. Furthermore, water pricing is politically and socially difficult since this is often seen as hurting small farmers .
Water metering and submetering (such as meters for individual units in an apartment building) lead to more efficient water usage at the consumer level. Metering can help identify leaks, and for individual consumers, metering has been found to reduce water usage by 20-40% .
 Chini, C., Schreiber, K., Barker, Z., Stillwell, A. "Quantifying Energy and Water Savings in the U.S. Residential Sector". Environmental Science & Technology 50(17), pp. 9003-12. September 2016.
 Dieter, C., Maupin, M., Caldwell, R., Harris, M., Ivahnenko, T., Lovelace, J., Barber, N., Linsey, K. "Estimated use of water in the United States in 2015". United States Geological Survey Circular 1441, 65 p. 2018.
 Food and Agriculture Organization. "Nutritive Factors". Accessed January 7, 2020.
 Food and Agriculture Organization of the United Nations. "AQUASTAT". Accessed February 13, 2020.
 Hoekstra, A., Mekonnen, M. "The water footprint of humanity". Proceedings of the National Academy of Sciences of the United States of America 109(9), pp. 3232-3237. February 2012.
 International Energy Agency. "Water-Energy Nexus". Excerpted from the World Energy Outlook 2016. March 2017.
 Mekonnen, M., Hoekstra, A. "The green, blue and grey water footprint of crops and derived products". Value of Water Research Report Series No. 47, UNESCO-IHE, Delft, the Netherlands. December 2010.
 Mekonnen, M., Hoekstra, A. "The green, blue and grey water footprint of farm animals and animal products". Value of Water Research Report Series No. 48, UNESCO-IHE, Delft, the Netherlands. December 2010.
 Mohajan, H. "Sustainable Development Policy of Global Economy". American Journal of Environmental Protection 3(1), pp. 12-29. March 2015.
 Mohayidin, G., Attari, J., Sadeghi, A. and Hussein, M. "Review of water pricing theories and related models". African Journal of Agricultural Research Vol. 4 (13), pp. 1536-1544. December 2009.
 Monteiro, H. "Water Pricing Models: a Survey". August 2005.
 Pan, S., Snyder, S., Packman, A., Lin, Y., Chiang, P. "Cooling water use in thermoelectric power generation and its associated challenges for addressing water-energy nexus". Water-Energy Nexus 1(1), pp. 24-41. June 2018.
 Rao, P., McKane, A., de Fontaine, A. "Energy Savings from Industrial Water Reductions". Ernest Orlando Lawrence Berkeley National Laboratory, LBNL-190943. August 2015.
 Shock, C. "Drip Irrigation: An Introduction". Oregon State University Extension Service. March 2013.
 Spang, E., Moomaw, W., Gallagher, K., Kirshen, P., Marks, D. "The water consumption of energy production: an international comparison". Environmental Research Letters 9(10). October 2014.
 Tidwell, V., Moreland, B. "Mapping water consumption for energy production around the Pacific Rim". Environmental Research Letters 11(9). September 2016.
 Torcellini, P., Long, N., Judkoff, R. "Consumptive Water Use for U.S. Power Production". National Renewable Energy Laboratory. December 2003.
 U. S. Energy Information Administration. "2015 Residential Energy Consumption Survey". 2017.
 U.S. Department of Energy. "The Water-Energy Nexus: Challenges and Opportunities". June 2014.
 U.S. Geological Survey, U.S. Department of the Interior. "Estimated Use of Water in the United States in 2010". 2014.
 United Nations World Water Assessment Programme. "The United Nations World Water Development Report 2014: Water and Energy". Paris, UNESCO. 2014.
 United States Environmental Protection Agency. "How to Conserve Water and Use It Effectively". Accessed November 24, 2015.
 Ward, F. and Pulido-Velazquez, M. "Water conservation in irrigation can increase water use". Proceedings of the National Academy of Sciences 105(47), pp. 18215-18220. November 2008.
 WaterSense. "US Outdoor Water Use". U. S. Environmental Protection Agency. Accessed February 15, 2020.
 Wolfe, J. et al. "An Electric Power Industry Perspective on Water Use Efficiency". Universities Council on Water Resources, Journal of Contemporary Water Resaerch & Education, Issue 143, pp. 30-34. December 2009.