The Value of Rainwater: Why Green Water Footprints Matter

In concepts & definitions, footprints


Counting on Rain

For the United States, the most memorable water story in recent history was California’s epic five-year drought that spanned 2012 to 2016. This withering drought was one of the state’s worst on record and ranked as one of its driest and hottest in over 100 years. It was big news because it had such a negative effect on the country’s most populous state, its largest economy and its leading food producing region (by revenue). Considering the state that produces nearly 50 percent of US-grown vegetables was struck with a major water crisis, simply put, California’s drought was America’s drought.

Water-dependent California farmers were hit hard as low precipitation and stream flows meant fallowed fields, drastic cuts to allocated irrigation water, reservoir levels down to only 36 percent of capacity and aquifers depleted from overpumping. And yet, it could have been worse.

Imagine how much more devastated California agriculture could have been if it had no access to irrigation water, as is the case with 70 percent of global farmland. Agriculture’s rainwater consumption – or the lack thereof – could have become even more critical to survival.

When rain doesn’t fall, crops might not grow.

Greening the Water Cycle

Water footprints are important for all climates, but they are especially important for California and the 41 percent of arid and semi-arid lands that cover the world’s land surface. Shifting precipitation patterns under climate change have changed the timing and amount of precipitation compared to the past. As this shift towards less predictable “drought and deluge” cycles occur, risks increase for farmers in terms of planning, planting and eventual harvests. Just because a given crop in a certain place or time depends more on irrigation and less on rainwater doesn’t mean it can survive without the rainwater.

Certainly California is not unique in the US, because drought can strike any part of the country and force people to stretch and better manage their water resources. To withstand the variability in precipitation and available water resources in California and across the country, agriculture must become more efficient and productive to get “more crop per drop.” In other words, farmers must use less water more productively to grow more crops. There are many methods and technologies available to achieve this, including high-efficiency drip-irrigation and other innovations as widely deployed in Israel, or groundwater banking and “on-farm recharge” of aquifers as practiced in California.

Improving the productivity of rainfall and shrinking the green water footprint requires measuring and tracking crop water use. A true accounting requires measuring all three components of the water footprint. This is not an abstract exercise; keeping track of water use and consumption ensures that the needs of humans and society as well as the needs of the environment are met.

This is important, because agriculture, which accounts for 75 percent of the world’s total water use and represents 92 percent of humanity’s water footprint, is often in competition with urban use and development, manufacturing and energy production. Globally, the competition comes from a growing global population, an overall rise in the consumption goods and services and bigger appetites for meat, dairy and processed foods. A boost in productivity of rainfall eases competition over surface and groundwater resources that become more valuable, strained and contested in times of shortage.

For instance, if a number of corn farmers increase their potential to grow with rainwater and decrease the amount of irrigation water withdrawn from a river or aquifer, more water is available within the shared watershed for users in other sectors. The decrease in water withdrawals can be especially important in water-scarce regions or when a locale is in drought.

Meat production is such a water-intensive process because livestock and poultry eat a tremendous amount of grain, fodder and forage which, together, have an enormous water footprint. These food-producing animals “eat” large amounts of water through their feed and other processes. (Read this analysis of the water footprint of beef for greater explanation.)

The Water Footprint of Almonds: A Case Study

The best way to understand how farmers can achieve more productivity is to dive deeper into the water footprint concept and its component parts – blue, green and grey water footprints. (Read the Water Footprint Assessment Manual for more detailed information.)

  • Green Water Footprint refers to the rainwater and soil moisture consumed by plants and crops in their cultivation.
  • Blue Water Footprint refers to irrigation from surface water and groundwater that is consumed (i.e. evaporated or incorporated into a crop) during cultivation.
  • Grey Water Footprint refers to the freshwater required to dilute pollution (e.g., farm runoff) and bring the water resource up to safe water quality standards.

Take a look at the thirsty California almond crop. The Golden State produces almost all US almonds and about 80 percent of the world’s almonds. They are not only an economically significant crop for the state, but also a very water-intensive one. Almond growers annually receive state water allocations equal to three times the volume of Los Angeles’ allocation.

A recent study commissioned by the Almond Board of California found that the average total water footprint of California almonds is 1,230 gallons per pound of almond kernels (the nut), which equates to about 3 gallons of water per almond. In a typical year, almond orchards consume significantly more irrigation water than rainfall. The water footprint of almonds breaks out as follows:

  • Blue Water Footprint: 635 gallons per pound (2,404 L)
  • Green Water Footprint: 68 gallons per pound (257 L)
  • Grey Water Footprint: 526 gallons per pound (1,991 L) – due to pollution caused by fertilizers and pesticides.

According to the authors, during the study’s 2004 to 2015 timeframe, an increase in the water footprint “appears to be driven by increased blue water requirements, possibly associated with drought and higher temperature conditions.” Because drought reduces the amount of rain that feeds the almond orchards, that rainwater deficit must be made up through greater use of irrigation. Regardless of the water source, whether by irrigation ditch or a steady downpour, the almond crop must receive and consume approximately the same volume of water in order to grow and produce fruit.

For almond trees that suffered under California’s most recent drought, farmers had to pump more groundwater than in wetter times to keep trees alive and maintain yields, thereby depleting the aquifer, reducing stored groundwater, lowering water quality and causing the land to sink (called subsidence). These negative consequences mean that the blue water footprint is unsustainable, which, in turn, makes the entire water footprint unsustainable.

Shrinking the Water Footprint of Crops

There are agricultural methods and practices that can maximize water use by crops and food production, whether from rainwater or irrigation water. Many options are available to help food producers reduce their impact on water resources. These options take into account factors such as the type of crop or animals grown, soil type and soil health, location and climate.

There are many successful examples of increased rainwater productivity in drylands around the world:

  • Integrated rainwater harvesting can involve harvesting and storing rainwater for future use in pits, pools and ponds. It can also rely on hardy, native perennial trees (sometimes fruit-bearing) planted as screens on western crop edges to offer shade, reduce drying winds, limit soil erosion and enhance the soil’s ability to retain rainwater and use it more productively.
  • Updated rainwater harvesting systems collect water for future irrigation and store it in aquifers to protect it from evaporation loss – a system called water banking. In one example of a water banking system, a series of University of California-designed pilot projects where some almond orchards have been flooded in the wet winter months have successfully restored groundwater in depleted aquifers.
  • Dry farming, or growing during the dry season where the crops rely on residual soil moisture, is regularly practiced for crops like wine grapes and olives, as well as other crops like tomatoes and quinoa.

Some of the best ways to lower water footprints and facilitate water uptake is by taking care of the soil by using low or no-till practices, using crop rotations and planting cover crops to replenish soil naturally. Agroecology is a promising approach that can help build healthy soils on farmland to store more carbon and water while requiring less polluting fertilizer and pesticides.

Farmers are very familiar with importance of water resources and the tradeoffs inherent with shifting scenarios and the risks involved. The effects of climate change will create new challenges including less predictable precipitation and compel farmers and growers to think broadly and plan for how to maximize agricultural water use regardless of how much rain is falling.

Image: Evening walk – the rain will clear. Credit: Andrew Hill (Creative Commons).