Fresh water is a critical resource for life on earth. As competition for it increases, fresh water’s scarcity now, and in the future, drives our need to measure its use. Water footprint is similar to other ecological footprint methods (i.e. carbon footprint) in that it is a measurement of human impact. However, unlike carbon footprint—which only needs to track emissions into the atmosphere and their potential to affect global warming—water footprint has to account for the changing availability and demand of water at specific times and specific places on the planet.

Consumers and consumer groups are increasingly applying market pressure to improve sustainability in the products they purchase. Additionally, more attention is paid to the ecosystem services of urban forests and landscapes. City planners and community organizations looking for horticultural products to meet these demands represent a market segment that is very aware of water scarcity and sustainability, and they have strict production standards for the products they purchase. Water footprint analysis in nursery crop production is a critical piece of meeting those needs as we move into the future.

Table 1. Water footprint comparison of common items.
Table 2. Selected Water Scarcity Indices in 2011. Higher numbers, tinted red, represent higher scarcity. Lower numbers, tinted blue, represent higher abundance.

Water footprint measures “consumptive use” of fresh water, as defined by impact. This is based on a simple statement of fact: water used for one purpose cannot be used for another purpose at the same time. For example, captured rainfall runoff that has evaporated behind a dam cannot also flow downstream and provide drinking water. Water is a vital resource with many direct uses in human activity, including irrigation, drinking water and sanitation, but it also provides habitat support of wildlife, including aquatic animals and vegetation that support indirect human desires, such as recreation. Given the numerous competitive uses for water, it should not be a surprise that the word for river and rival are so similar.

The Water Footprint Network, a global organization started in 2008, addresses the world’s water crises by advancing fair and smart water use. Today, the Water Footprint Network has more than 200 partners from large companies to small suppliers, regulatory bodies, non-profit organizations, and academics. In 2011, they developed a set of methods for assessing water footprint. This international standard for water footprint assessment has broad application, allowing for meaningful and science-based comparison of processes, products, and commodities (Table 1), but also communities and organizations. Furthermore, since it connects consumers to very distant supply chains, it is important to recognize water as a global resource. Put plainly, industrial and agricultural products in the United States are exported to distant markets, making North America an exporter of “net virtual water.”

A water footprint analysis is not meant to be a regulatory stick to beat on consumers or producers, but is rather a method to analyze consumptive water use and make crucial improvements with momentous impacts.

How It works

A water footprint is the sum of four other water footprints (WF): Embodied WF, Green WF, Blue WF, and Grey WF. Embodied WF refers to the water footprint of input materials, including plastics, fertilizers, pesticides, fuels, etc. Green WF refers to the volume of irrigation avoided due to rainfall events. Blue WF refers to rainfall runoff captured or added from surface or groundwater sources (e.g. when one saves rainwater in a reservoir). Grey WF refers to the required volume of water necessary to dilute to public standards any water containing nutrients or pesticides released into a stream, lake, or other public waterway. Blue and Grey WF can then be weighted according to water availability at the location of use or discharge.

Embodied water footprint. The synthesis and fabrication of plastics have a relatively high water footprint. A 1-pint plastic bottle uses 1.4 gallons of water to create. Globally, the grey water footprint required to produce new plastic packaging was 330 billion cubic meters (over 87 trillion gallons) of fresh water in 2016. This is roughly the same amount of water India uses for domestic purposes during a four-year period. The water footprint for each fabricated, consumable component of a marketable nursery plant, including containers, trays, fertilizers, insecticides, and herbicides, are all totaled and represent the embodied water footprint. Unfortunately for producers, the water footprint of these individual components is only referenced in complex databases that are not easily accessible. For reference, preliminary data shows the Embodied WF for a single, marketable #3 container shrub is around 4 gallons.

Green water footprint. Green water is the volume of water from avoided irrigations due to rainfall events (Figure 1). Water captured from rainfall or melted snow does not count towards green water. One would expect Green WF for a production system on the east coast of the United States, a wetter climate, to be much higher than a production system in southern California, a drier climate.

Figure 1. Green water is water provided directly during atmospheric events that avoid having to irrigate.

Blue water footprint. Blue water refers to the volume of all surface and ground water used, including captured rainfall runoff (Figure 2). Blue water can be thought of as water that is extracted or otherwise prevented from flowing downstream for other uses. An example would be building a reservoir to capture rainfall. All rainfall runoff that is prevented from flowing off property is blue water. Even if a grower allowed all captured rainfall to evaporate from the reservoir, that evaporation is consumptive use, because the water has been prevented from being available downstream. Assuming total volume as constant, deep reservoirs lose less water per year than wide shallow reservoirs.

Unlike green water, blue water is a weighted water volume. The weighting process reflects the scarcity of water specific to the time of year, geographic location, and the source of water.

Grey water footprint. Grey water refers to the volume of water required to dilute pollutants down to acceptable levels (Figure 2). It is an attempt to quantify the amount of water a pollutant is “using” from the body of water, be it a stream, river, bay, or other public waterway. Acceptable levels of pollution standards are generally taken from local water boards or state or federal regulatory agencies, where available. Locations with high demand for limited water resources tend to have stricter standards for water discharges. However, discharges of sufficiently toxic or damaging pollutants without a safe or acceptable level can result in an infinite grey water footprint.

Weighting by scarcity

Although the methods of calculating water scarcity are still being discussed by researchers to date, water scarcity is reflected numerically as a ratio or fraction between 0.001 and 100 (Table 2). Lower numbers represent high water abundance while higher numbers represent increased scarcity. The ratio is meant to reflect volume of use or demand divided by availability, and obviously this value changes from year to year based on weather patterns and human activity. Therefore, a complete water footprint analysis will include the volume of water used as well as the volume of water weighted by scarcity. Both numbers allow a grower with operations in multiple regions to make meaningful comparisons of efficient water use, despite significant differences in availability. Operations in southern California may have a much higher weighted WF than a location in the southeastern U.S., while the unweighted WF would likely reveal the southern California system to be thriftier with water use.

Figure 2. Blue water and Grey water concepts, illustrated.

The process of weighting consumptive water use is calculated on a per month basis, as withholding or extracting water from a stream has a greater impact during a dry season than a wet one. It is important to note only blue water and grey water are weighted.

Example water footprint analysis

In Figure 3, we see a detailed, weighted water footprint analysis for a single, marketable #3 container holly grown on the East Coast of the U.S. It shows the water footprint of each of the four components (embodied water, green water, blue water, and grey water). It further breaks down the blue water portion to show how the water is consumed by the system, with about 11 gallons being used during irrigation events while almost 8 gallons were lost to evaporation while sitting in the reservoir. Figure 4 and 5 characterize a water footprint of young greenhouse plants between an older overhead irrigation and newer ebb and flood irrigation.

Ongoing research

Water footprint research and outreach discussed in this article is being performed in collaboration with Dr. Dewayne Ingram at the University of Kentucky and Dr. Charlie Hall at Texas A&M. It is part of a larger five-year, multi-state National Institute of Food and Agriculture, USDA Specialty Crop Research initiative called Clean Water3 (www.cleanwater3.org) to evaluate production practices and technologies related to water use reduction, recycling, and remediation for specialty crops. An end-product goal is to develop a freely available decision-making support tool for growers to evaluate production practices and technologies using science-based data.

Joshua Knight is an Extension Associate, Nursery Crop Production, at the University of Kentucky, Department of Horticulture; joshua.knight@uky.edu

FIGURE 3. Preliminary Water Footprint of a single, marketable #3 container holly grown on the East Coast, U.S.
FIGURE 4. Preliminary Water Footprint of a marketable greenhouse grown, 72-count tray of young plants in the Southeastern, U.S. using overhead irrigation in an older greenhouse.
FIGURE 5. Preliminary Water Footprint of a marketable greenhouse grown, 72-count tray of young plants in the Southeastern, U.S. using ebb-and-flood irrigation in a newer greenhouse.