Water managers forecast future water demand for a variety of purposes, including planning future water purchases or building new infrastructure. There are several methods for estimating future water demand; the simplest and most traditional means is to estimate current per-capita water consumption and multiply this by expected future population.
In California, urban per capita water demand has declined dramatically over the past several decades, driven in part by greater uptake of water-efficient devices. These reductions have important implications for estimating future water demand. However, failure to account for the long-term trend of declining per capita water demand can lead to overestimation of future water demand, resulting in costly investments in unneeded infrastructure and new sources of supply.
At the California Irrigation Institute’s annual conference held in February of 2021, Heather Cooley, Director of Research for the Pacific Institute, shared some of the research the Institute has done evaluating urban and agricultural water use in California.
The data used for their analysis was generated by the Department of Water Resources and is included in the California Water Plan. The data are measures of applied water, for which there is an urban and an agricultural component.
For urban use, the estimates are based on annual water production measurements submitted by water suppliers in their annual reports. The data also includes estimates of use for energy production and self-supplied households and businesses. She noted that the data is a combination of measurements plus some estimates for sectors for which there isn’t very good data.
Ms. Cooley pointed out that in reality, there isn’t a good estimate of agricultural water use, which she attributed mainly to a lack of an estimate of groundwater use, a key water source for agriculture. To estimate agricultural water use, DWR uses models based on measurements of crop type, acreage, weather, and assumptions about crop coefficients and irrigation efficiencies.
“There’s a widespread perception in California, and frankly, around the world, that as our economy grows and as our population grows, we use more water, but in fact, California has experienced a dramatic decoupling of water use and growth, particularly since about 1980,” said Ms. Cooley.
Statewide water demand
She presented a figure showing growth and water demand. The green line is the gross state product for California, the yellow line is population, and the blue lines are water use.
“Prior to about 1980, we see water use going up at about the same rate as population, but then in 1980, we see a big break,” she said. “We see the economy and the population grow, but water use remains really pretty flat. And I think that is a testament to some pretty dramatic shifts in water usage.”
Agricultural water use
This figure shows agricultural water use from about 1960 through 2015. Agricultural water use is shown in the dots and the orange line at the top; precipitation is shown in blue, and the shaded areas depict the droughts.
“Agricultural use is highly variable from year to year, from a low of about 26 million acre-feet in the early 1990s to a high of 37 million acre-feet around 2007-2008,” Ms. Cooley said. “Weather is a big driver in agricultural water use and a driver of that variability, and in these wet years, we see agricultural use decline. But then in hot, dry years like during droughts, we tend to see usage go up.”
In some ways, Ms. Cooley pointed out that the higher demand shown in dry years is an artifact of the modeling. “The modeling is based upon weather as a driver of usage, so I don’t think it does a great job of capturing some of the things that farmers do during a drought, such as under irrigating. That’s one of the reasons we need more and better data.”
The key measure of agricultural water use is the economic productivity of water. Ms. Cooley noted that in California, the economic productivity of water has increased dramatically, more than doubling over the past 55 years, from about $550 per acre-foot of water applied to well over $1200 per acre-foot. She attributed the increase to farmers changing to higher value crops and an increase in water use efficiency.
“We were doing more with drip irrigation and with some of the new technologies around irrigation scheduling, and that’s enabled this trend,” she said.
Urban water use
Turning to urban water use, Ms. Cooley presented a figure showing urban use between 1960 and 2015. Total water usage is shown in the blue bars at the bottom, and per capita use is the green line at the top.
In 1960, California’s urban areas used about 3.1 million acre-feet of water or about 177 gallons per person per day. But in the subsequent 30 years, there was an increase in per capita usage and total usage, except for the period during the 1987-1992 drought when there was a rapid decline out of necessity. After the drought, there was an increase in water demand up until about 2000 or so.
Since 2000-2007, per capita water usage plateaued and then declined dramatically. “That reduction was sufficient really to drive a reduction in total usage,” she said. “Urban usage peaked in about 2006, and then it has declined dramatically. And in fact, in 2015, it was really to levels we hadn’t seen since the early 1990s. We have seen some increase since the drought ended. But it’s been far less than what many expected, and water usage was far less than it was before the drought.”
In some regions, water demand has been dropping even more. The previous graph showed a statewide estimate, suggesting the decline in per capita use started around the early 2000s. However, Ms. Cooley pointed out that when you look at individual suppliers, the reduction in per capita use, in fact, has been going on for far longer.
She presented a figure showing water demand for San Francisco, pointing out that the city is using less water today than in the mid-1960s, despite the economic and population growth during that time. She also noted that this is not unique to San Francisco; this is a trend in many communities across the state. The statewide estimates don’t necessarily reflect what is happening at the local level.
“That’s because, at the local level, we see those declines since 1980,” she said. “But we still see growth in those hotter, drier parts of California that tend to use more water. So I think that growth blunted this trend of reductions in per capita usage going back towards the 1980s.”
“In fact, if you look either at the statewide data or even at these local data, it’s driven by changes in our economy; we’re doing less manufacturing in California. It’s driven by greater uptake of efficiency improvements and greater uptake of climate-appropriate plants, In urban areas, there are denser developments, and so when you have people living closer together, there’s less outdoor space and less irrigation.”
Ms. Cooley then summarized the key findings:
California has experienced a dramatic decoupling of water use and growth that has enabled the state’s economy and population to grow without using more water.
Agricultural water use in California has remained flat since the 1980s, while the economic value of crop production has grown dramatically.
Urban use in California has declined rapidly, particularly since the mid-2000s, although many water suppliers have been seeing this trend for much longer.
In conclusion …
Ms. Cooley then gave her concluding thoughts.
“First is that the actions of local and state water managers, decision-makers, individual farmers, businesses, and households are such that California’s urban and agricultural regions are deriving much greater value from the state’s limited water resources, but despite these positive signs, we know that there are signs that water use in California remains high,” she said, noting that groundwater aquifers continue to decline and our rivers and streams are impacted by diversions. “There are many reasons for that, but one is the over-allocation or over-extraction of water, both surface and groundwater.”
“We also know that we have a number of challenges ahead. We have climate change which is increasing the variability of the supplies, but it’s also putting upward pressure on demand because of the higher temperatures. So because of these, we know that will continue to grow as well. So because of these, we will need to do more. A key element of doing more and evaluating how we’re doing is data; we need more and better data so that we understand where we’ve been, but also where we might go and evaluate our progress.”