Water Consumption: Drier Summers Are Increasing Demand

By Brian Kennedy & Dr. Patrick Jones

Spokane County sits upon one of the greatest resources needed for human survival, water. The Spokane Valley-Rathdrum Prairie Aquifer contains roughly 10 trillion gallons, covering about 370 square miles across Washington and Idaho. Despite its abundance, there are risks: it is the sole source of water for most people residing in Spokane County. With a growing number of residents tapping into the aquifer how has consumption changed, and how can we protect our means of growing but not abusing this resource? Indicator 4.4.1 tracks the water a resident consumes each day to help monitor usage across the county.

This county estimate is based on the thirteen largest water districts, as tallied by residential connected. The average water consumption per person within the county was roughly 274 gallons per day in 2017. This has actually seen about a 3% decline since the peak, as well as the start of the trend in 2009, where residents were using about 282 gallons per day but still about 10 gallons per day more than the series average of 264 gallons. The last three years have shown abnormally high water consumption for these 13 districts, reaching a max in 2015 of over 290 gallons per day. Since 2015 consumption has slowly started to stabilize back down, but what could be causing these fluctuations?

According to Hilary Nickerson, City of Spokane Water Department, and Doug Greenlund, an Environmental Analyst of the City of Spokane, “single family residential water use is the largest water user sector for most water providers in the area. A home can use four times or more water per month in the summer than the rest of the year.”

In this indicator, we peg water consumption to the total inches of rainfall during the summer months (May through September). Peak consumption is nearly always linked to dry summers. In fact, the two phenomena show a strong, negative correlation of -70%, as summer rainfall declines, local water consumption climbs. Other measures one might expect having a similar relationship, such as snowpack on Mt Spokane, showed no significant correlation. Observing consumption and rainfall in 2015 and 2016 show this relationship the clearest. With these years being the direst summer of the trend having just 1.81” and 1.93” of rainfall respectively, it is clear why water consumption peaked over this time period and began falling again in 2017 when rainfall picked up.

The National Weather Service’s Climate Prediction Center creates probability outlooks for both temperature and precipitation looking at the 2019 summer months. For the Spokane region, they estimate that there is about a 40% to 50% chance that temperatures will be above the average and that precipitation has about a 33% to 40% chance of being near normal. Given that the latest 5-year average of rainfall is about 3.05” and the inverse relationship stated earlier, it would not be unexpected to see water consumption per person to be somewhere in the 260 gallons a day.

However, within the County, there is quite a lot of variation potentially affecting the estimate, of which maps of the district layouts can be found here. Take the City of Spokane, which makes up the largest water district of the thirteen used to create the County estimate. In 2017, its use sat at about 240 gallons used per person per day. Following the county trend, the city peaked in 2015 at roughly 270 gallons and began to fall to where it sits at about 10 gallons above its trend average.

This differs quite a bit from that of Spokane Valley, largely made up from Vera Water and Power, Modern Electric Water, Model Irrigation District, and Spokane County Water District 19. Residents consumed 382 gallons of water per day in 2017. Again, following a similar trend of peaking in 2015 (at 430 gallons) and starting to fall back down towards the trend average of 370.

The difference between the City of Spokane and Spokane Valley residential water usage can in part be attributed to lot size and population density. The City of Spokane has roughly 600 more people per square mile than the Valley, shown on indicator 4.3.1. Given that there are more people packed in a square mile would lead to reason that there is less room for lawns, the largest contributor to water consumption.

So where do we go from here to curb water usage throughout the County? Dan Kegley, director of Wastewater Management for the City of Spokane, states “our next step to continue or lesson current per capita use is smart irrigation and landscape design in conjunction with adaptive planning. Although we currently sit on top of one of the most prolific aquifers in the world we cannot deny that our future would change dramatically in the event of water shortage or scarcity. We need to take steps now to insure we do not walk down that same path.” And the City of Spokane has begun doing just that through the SpokaneScape program.

Its focus is on the replacement of lawns with low volume irrigation and drought tolerant plants. Residents can earn up to a $500 credit towards their water bill, claiming $0.50 per square foot of lawn transitioned to less water intensive landscaping. While the program is still young - 2018 being the first year, 22 residential and one commercial customers have completed the program. Longer term, it has the potential for significant water reductions. Estimates from Nickerson and Greenlund show that “by using drip irrigation coupled with low water demand plants and mulch there could be as much as an 80% reduction in water used, or a 1,000 square foot project could save 11,000 gallons a year.”

So with new programs starting to take hold within the region, water consumption in the region will be an interesting trend to follow. With most residents sharing the same aquifer, our water supply, while currently quite large, is relatively fixed. As population grows, continuing to monitor and reduce usage per person will help ensure its supply for years to come.