Residential Water Use: A Ten-Year Study

Denver Water wanted to know if the water consumption patterns of Denver Water’s single-family customers have truly changed over the past ten years. To find out, we turned to Aquacraft, a Boulder-based company that specializes in analysis of water use. During the summer and fall of 2005 Aquacraft updated water use analysis data gathered in 1994 and collected new data based on 2004 billings, survey information, and other research. Their report to Denver Water is available here in PDF* format.

The results were interesting, encouraging, and valuable. They document the impact of mandatory restrictions on water demands surrounding the recent drought. They show clearly how Denver's citizens met the challange of wise water use. And they point the way to future water conservation.

Important information came from flow trace data, which is gathered by a device that logs water use at the houses participating in the study. This data can be used to determine how much water specific appliances and fixtures are using. Comparison of current flow trace data with that from 1996 illustrates how much customers have chosen to adopt newer, efficient fixtures and appliances since 1996.

Household Use

In the one hundred sample homes studied, there was an average decrease of approximately 61,000 gal per year. From 166,000 gallons used annually in 1994, residential users cut back to 105,000 gallons. The biggest drop in water use occurred in 2003, when Denver Water was responding to the drought with restrictions, enforcement, and a publicity campaign.

Both indoor and outdoor water use dropped significantly between 1996 and 2005. Indoor use dropped from 173 gallons per day to 154, and roughly one-third of that reduction was due to increases in efficiency of fixtures and appliances. Highly efficient houses can reduce use to 100 gpd or less, so there is still significant room for improved efficiency. And habit changes can provide even more savings.

The information on showers illustrates something important about habit change. Word-of-mouth suggested that people were taking shorter showers to save on water. But there was no reduction in average shower length, although there was a slight decline in number of showers. In other words, people were taking showers slightly less frequently, but for all their good intentions, showering just as long as ever. Adjusting that behavior, reducing the average shower length from 8 minutes to 5 minutes, would reduce the amount of water used for showers by nearly one-third or roughly 10 gallons per day.

Outdoor use at single-family residences is the most significant value in water savings and waste. Outdoor use dropped between 1994 and 2004 by an average of 54,000 gallons. This is a 53% reduction in outdoor use. Not all of this reduction was due to restrictions or conservation. Looking at the weather data that determines the need for water (evapotranspiration), Aquacraft found that the climatic conditions in 2004 required less water than in 1994. In other words, people watered less because they didn't need to water more. However, even with weather factored in, reductions in outdoor use were still found. The report explains customer irrigation practices and the results at a high level of detail.

There is always some worry that outdoor usage may return to pre-drought levels, as people get out of the habit of conservation. But some of the savings of 2004 will be long-lasting. About 20% of the customers surveyed had made permanent landscape changes to save water.

The Full Report

The report provides details – charts and tables as well as photographs – to support its conclusions. It covers average water use, identifies where water efficiencies were, shows what percent of our customers have already invested in more efficient appliances (6% had ULF toilets in 1994, 20% by 2004) and other water saving devices.

The news is not all good. For example, leaks in the study homes increased significantly. Average leakage in the study houses rose roughly 50% between 1996 and 2005, from 16.1 to 24.5 gpd. The cause is not easy to determine. Leakage tends to be random and unpredictable, and the exact source is difficult to identify.

Summary and Conclusions

The following points are some of the key findings of this research project.

• Average per household use (indoor and outdoor combined) decreased by 37% from 166,000 gal to 105,000 gal between 1994 and 2004. Both indoor and outdoor use decreased during this time period.
• Indoor use decreased by an average of 7,000 gal (11%) per customer per year. Approximately 4% of this decrease was due to changes in demographics and 7% was due to efficient fixtures and appliances.
• Substantial indoor conservation potential remains, particularly through habit changes. Indoor use reductions can be achieved by Denver Water’s single-family customers in the coming years.
• Outdoor use decreased by an average of 53% between 1994 and 2004. The largest decrease was observed in 2003.
• Some of the reductions in outdoor use are the result of drought restrictions and pricing measures. However, the declining trend in outdoor water use appears to be related to weather patterns as well. Many of our customers understand the concept of using only what you need. It is not a hard lesson to learn.

Click here for the entire report in PDF* form.

* Note: These documents require Adobe Acrobat Reader to open.