Rain Counts: What's the Value of the Green Water Footprint?

2014's biggest US water-related story is California's deepening drought, so withering that the three-year span is both the driest and hottest in over 100 years. The California drought is big news because of its negative effect on the country's most populous state, its largest economy and its leading food producer (by revenue). When the one state that produces nearly 50 percent of US-grown vegetables is hit with this sort of water crisis, that's big news.

The ongoing California drought has hit farmers hardest, with drastic cuts to irrigation water allocations, fallowed fields, reservoir levels down to only 36 percent of capacity and aquifers depleted from overpumping--but it could be worse.

Imagine how much more devastated California agriculture would be if they depended solely on rainfall with no access to irrigation water, as is the case with 70 percent of global cropland. Suddenly, agriculture's rainwater consumption - or its lack - becomes even more critical to survival. When rain doesn't fall, crops don't grow.

Despite the woeful hydrological circumstances that California now faces, the state is a hydraulic wonderland of well-developed freshwater infrastructure and delivery with capacity to draw on groundwater reserves to make up for shortfalls, even if those resources are strained. But as a dire situation sparks conversations about the water bond and Bay Delta Conservation Plan that pipes water hundreds of miles from north to south, people also need to be talking about how to stretch water resources.

To withstand climatic and precipitation variability, agriculture must get more "crop per drop." In other words, it must use less water more productively to get more crops. To better understand this, a deeper dive into the water footprint concept and its component parts, namely the blue, green and grey water footprints, is useful.

  • The blue water footprint is irrigation from surface water and groundwater that is consumed (i.e. evaporated or incorporated into the crop) during cultivation.
  • The green water footprint is the rainwater and soil moisture consumed by crops in their cultivation.
  • The grey water footprint is the freshwater required to dilute pollution and bring it up to safe water quality standards.

Why does getting more out of rainfall matter not just for agriculture in California, but also for the United States and the world? For all climates, but especially for California and the other 40 percent of semi-arid or arid lands worldwide, precipitation patterns under climate change are ever more significant because the timing and amount will likely vary fluctuate as compared to the past. As a shift towards a more variable "drought and deluge" cycle occurs, risk grows for planning, planting and harvests.

One major reason that a boost in green water footprint productivity is necessary is because it eases competition over surface and groundwater resources that are only going to become more precious and contested over time. Agriculture's water use, which stands at 75 percent of the world's total and represents 92 percent of humanity's water footprint, will encounter greater demands from urbanization, manufacturing and energy production. These demands will come from a growing global population and a rise in overall consumption and bigger appetites for processed foods and meat. Meat production is an especially water-intensive process because livestock and poultry eat a tremendous amount of grain, fodder and forage which, all together, have an enormous water footprint. To raise animals for meat means that water is being "eaten" through feed and other processes. (Read this analysis of the water footprint of beef for details.) For future food supplies to be met, it's crucial that agriculture shrinks its total water footprint, which in turn requires improved productivity of rain-fed agriculture to shrink the green water footprint. (Nuts and avocados also have large water footprints.)

Just because a given crop in a certain instance depends more on irrigation and less on rainwater doesn't mean it can survive without the rainwater. All water consumed by the crop (or process) counts equally.

Another reason to focus on green water productivity is to have a way to account for sustainable water use in crop production. (Read the Water Footprint Assessment Manual for more.) This is not an abstract exercise, but is instead a way to keep track of water use and consumption so that both our needs and the needs of the environment are met. A true accounting requires measuring all three components of the water footprint. Just because a given crop in a certain instance depends more on irrigation and less on rainwater doesn't mean it can survive without the rainwater. All water consumed by the crop (or process) counts equally.  This is also true for water polluted by agriculture (or other processes) that take water to clean and treat.

To explain further, let's take a look at a thirsty almond crop (notably, California produces all of the US supply of almonds). In a typical year rainfall provides more water to almond orchards than does irrigation water. Because of the drought, farmers have to pump more groundwater than normal for irrigation in order to save their trees and maintain almond yields. In this case, so much water is being drawn from the aquifer that it could be depleted. This means the blue water footprint is unsustainable making the entire water footprint unsustainable even though the green water footprint - the rain feeding the almond trees - is sustainable. Given the importance of all three components to create a sustainable water footprint, none of them can be excluded.

There are many agricultural methods to maximize water use by crops, whether it's rainwater or irrigation water. There are also many successful examples of increased rainwater productivity in drylands around the world. For example, integrated water harvesting which involves hardy, native perennial food-trees 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. Updated rainwater harvesting systems collect water for future irrigation but store it in aquifers to protect it from evaporation loss. Some the best ways to lower water footprints and help 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.

Farmers know better than anyone how important our water resources are. Fortunes are bound to change, particularly when taking climate change into account. Now is the time to think broadly and plan for how to maximize agricultural water use because whether more or less rain is falling, it's always a good time to save up for a (less) rainy day.


Image "Evening walk - the rain will clear" by where paths meet on Flickr used under a Creative Commons Attribution-NoDerivs 2.0 Generic license.