This Issue

H2O in 2100


Sustainable landscaping with agave plants.


By Matthew Heberger

What will California’s water resources look like at the end of the century?


Agreat deal of research on climate change over the last decade has focused on changes to the hydrologic cycle—the continuous process by which water is circulated throughout the earth and the atmosphere—which naturally impacts water supply. These changes—earlier spring snowmelt, increased evaporation from higher temperatures, and more frequent and intense droughts—are obvious. And the changes are alarming. Water suppliers and large water users simply cannot afford to ignore climate change as they plan for the future.

Many water suppliers have begun to consider how climate will affect their water supplies, whether it is from a lake or river, stored behind a dam, or drawn from underground aquifers. Not as much attention has been paid to the other side of the equation: What will climate change do to water consumption?

Most people familiar with the state of climate change expect that a warmer climate will drive up water demand for landscapes and the inevitable evaporative cooling. Yet there has been little research on this subject and even less practical guidance for water planners and managers.

Two recent research projects have contributed to this emerging field of study, to better predict how climate change will affect future water demand. In 2012, my colleagues and I at the Pacific Institute developed a planning tool focused on the state of California that forecasts water use out to the year 2100. A year later, in 2013, the Water Research Foundation published a nationwide study, “Changes in Water Use under Regional Climate Change Scenarios.”

Both of these groundbreaking studies demonstrated how climate change can help predict future urban water use. We worked closely with climate scientists to translate the output from their models into information on water demand for use by water managers. Our work focused mostly on how temperatures are causing an increase in evaporation and water lost to the atmosphere by plants. In California, as in much of the West, more than half of publicly supplied water is used outdoors. Some of this is used for washing cars or sidewalks, or for filling pools and spas, but most is for landscape irrigation.

The two maps here show irrigation requirements. The first reflects recent conditions; the other reflects a warmer climate at the end of the century. Grid cells represent how much water is required to grow turf grass—the depth of irrigation water required in meters per year. The darker green color indicates higher water needs. The gray dots represent population clusters now and in the future, considering one scenario of population growth.

Our model of future water use took into account the effect of higher irrigation water demand due to warming. Our conclusion: With all other factors holding constant, climate change could contribute to a 15 percent increase in future urban water demand in California by the year 2100.

The green building trade is on the forefront of the effort to use water wisely and more sustainably. Every time someone designs a water-efficient building or landscape, they are leading the charge.

It’s clear that where growth occurs has a big impact on future water use. This is especially true in California, where the future population growth is expected to occur mainly in the hot, dry Central Valley. New greenfield development can lead to big increases in water use, even when they are planted with low water-use landscapes. A better approach may be to promote water-neutral development and minimize the creation of new landscapes by encouraging urban infill or brownfield development.

Strong evidence suggests that the effects of climate change are already driving changes in water consumption; regional models show that temperature, evaporation, and crop water use are slightly higher today than they have been over past decades.

It’s not inevitable, though, that water consumption will continue to rise. Forecasters often made predictions in the past of skyrocketing water use that never came true. Water use in many areas of the country has held steady or even declined, despite a growing population, because we adopted more efficient appliances and fixtures for homes and businesses. For example, all toilets sold countrywide since 1994 must use just 1.6 gallons per flush or less, a big improvement over the old 3.5 gallon models. Today’s ENERGYSTAR washing machines use only 15 gallons of water per load, a major savings over standard machines and even those produced ten years ago.

Despite these gains, a great deal of wasteful, inefficient water use continues. The U.S. Geological Survey estimates that domestic water use in the country is 170 gallons per person per day. There certainly is significant room for improvement: Australians use an average of 54 gallons per person per day (for both indoor and outdoor uses); and residents of the Australian state of Victoria use only 40 gallons each. Australians have not always been water misers, but they have lowered their consumption dramatically over the past decade, after the unprecedented Millennium Drought. The Australians’ solution was simple: adopting new water-efficient technology and water-saving habits. For instance, dual-flush toilets are now in nine out of ten Australian homes.

Climate is only one factor influencing future water demand. Landscape water use is driven mostly by plant types and the efficiency of irrigation systems. Programs have cropped up across the West to encourage the planting of native plant species that require minimal irrigation. Besides having colorful blooms that attract birds and pollinators, these plants have other benefits, such as easier maintenance and less need for fertilizers and pesticides.

Promoting water conservation and efficiency is one of the most important things we can do to climate-proof our cities and create more resilient water systems. When we use less water to meet human needs, we can better withstand future droughts, in addition to saving energy and reducing greenhouse gas emissions.

The green building trade is on the forefront of the effort to use water wisely and more sustainably. Every time someone designs a water-efficient building or landscape, they are leading the charge.