The DRP outlines a plan for drought response that maximizes available water supplies and minimizes water usage.

This Drought Response Plan outlines procedures by which Denver Water could manage water supply and water use during drought. The procedures are designed to protect the community as a whole while considering individual customer priorities as much as possible. Because each drought is different, it is not practical to develop a set of hard and fast rules to apply to all droughts. These guidelines are intended to assist the five-member Denver Water Board in making decisions throughout the course of a particular drought. The Board may adjust or refine these procedures in response to actual drought conditions.

Recognizing the need for a plan to deal with short-term water shortages caused by drought, Denver Water created its first Drought Response Plan in 1991 with the help of a Citizen’s Advisory Committee task force. The staff revised that plan several times, most recently as part of updating its Integrated Resources Plan, which was completed in February 2002, just months before the current drought began.

The Drought Response Plan described in this document follows two years of drought restrictions in Denver Water’s service area during 2002–2003 and incorporates lessons learned. As the population in Denver Water’s service area continues to grow, the buffer in the reservoirs will diminish and droughts will have to be dealt with more quickly and more intensely. Accordingly, the Drought Response Plan must be updated regularly so that it is relevant to current conditions.

Before drafting the current Drought Response Plan, Denver Water's Planning Division thoroughly examined a range of drought scenarios, quantified the severity of Colorado's past droughts, revisited the history of water use restrictions in Denver, reviewed the lessons of past droughts, and projected options for obtaining additional water supplies in times of drought. These reports and analyses are published in a separate volume of technical appendices for interested readers.

The Drought Response Plan consists of four interrelated sections:

• Triggers – This section specifies the reservoir levels that activate four distinct stages of drought.
• Drought Response Program – This section provides guidelines for augmenting water supplies and reducing water use during times of drought.
• Public Outreach – This section outlines options for seeking customer suggestions and involvement in decisions, as well as methods to keep customers informed before, during and after a drought.
• Internal Communication – This section presents a process for keeping all Denver Water employees up-to-date on drought conditions and on the drought response measures adopted by the Board.

Drought is a normal, recurrent aspect of climate. Droughts occur in virtually all climatic zones, though their characteristics vary significantly from one region to another. When the amount of water flowing into streams and then into reservoirs is less than average, Denver Water will more closely monitor its water supply outlook. If continued low stream flows stress water supplies, Denver Water will work with customers to implement this Drought Response Plan.

Figure 1 depicts the variability of natural stream flows in Denver Water’s raw water collection system. Troughs in the plot indicate that droughts are a recurring natural phenomenon.

Note: Values are based on tree ring studies and streamflow measurements.
Figure 1. Natural stream flow in Denver Water’s collection system

Denver Water’s collection system is designed to dependably meet the needs of customers through hydrologic conditions similar to those of the past 50 years without supply shortages. Unfortunately, no one can predict how long drought conditions will continue once they begin. Timely response–neither too early nor too late–is Denver Water's goal.

Denver Water defines the term "firm yield" as the estimated amount of water available to meet annual average demand without the need to restrict water use (Figure 2). A key word in this definition is "average" because, in practice, customers use more water in warm, dry weather than in cool, wet conditions (Figure 3). Furthermore, shifts in weather patterns can be substantial from year to year and decade to decade, affecting both water supply and water use.

	Figure 2. The firm yield of Denver's water system–estimated at 375,000 acre-feet per year–compared with 2001 use

Firm yield is an estimate rather than an exact calculation. Of the many factors that affect firm yield estimates, weather is perhaps the most inexact. If the Denver area experienced a drought more severe than any ever recorded, this could stress the water supply system even more than is accounted for in the estimated firm yield. Still, in relation to reasonable predictions based on a century of measured stream flows, the firm yield estimate appears to be a relatively safe, prudent way to view the water supply for drought planning purposes. Although 2002 was the driest year on record for Denver Water’s system, a recurrence of the multiyear drought that occurred in the 1950s would be worse than a recurrence of a single dry year such as 2002.

Figures adjusted for population changes
Figure 3. Effect of weather on water use

On the whole, Denver Water’s customers are conserving water. Denver Water first launched an aggressive program to promote water conservation in the mid-1970s. By 1979, the Denver Water Board had adopted a formal Water Conservation Plan and set goals for reducing water use.

To track water use and water savings, Denver Water looks at customer water demand over the years and compares these numbers with population growth in its service area. Figure 4 shows 20 years of comparisons. The two biggest dips, in 1983 and 1995, occurred in very wet years. The good news is that, on average, customers appear to be reducing water waste. The bad news is that much more effort–by customers and water providers–may be required to achieve additional reductions during a severe drought.

	Figure 4. Customer water demand and population growth in Denver Water’s service area (1979–1999)

The impact of drought can generally be divided into two categories: effects on water supplies and effects on society.

Drought affects our water supply, our customers, and many other aspects of our communities and environment. Monitoring these effects and deciding how to respond are critical activities under drought conditions. The impact of drought can generally be divided into two categories: effects on water supplies and effects on society. Water supply effects can be measured or estimated by scientific means, but societal effects are less tangible. Individual water customers are impacted by these intangible societal effects. Societal effects also have ramifications in the broader context of economics, politics, relations with surrounding communities, and the environment.

How a drought affects individuals–plus public perceptions about how the drought should be addressed–will influence how customers respond to Denver Water’s calls to save water. The drought response activities of other water suppliers in the metro area, as well as news media and political responses, will also affect customer reaction. Although societal effects cannot be viewed in terms of simple formulas, they can be monitored and described for consideration in the Board’s decisions about drought response.

Response of Other Water Suppliers
Droughts affect the supplies of each metro area water providers in different ways. The northern suburbs with single watersheds may be better or worse off, depending on the localized effects of a drought. Other systems such as Aurora’s have less senior water rights and could be affected much sooner or more severely than Denver’s system. The Denver Water Board will weigh the importance of developing a unified metro-area response against the potential of restricting our customers’ water use either sooner or later than warranted by water supply conditions. As in 2002–2003, Denver Water will monitor the drought response decisions of other local water suppliers.

News Media Response
Much of the information customers receive about drought comes from the media. Members of the media can be very helpful in conveying factual information to customers, and they also play a key role in shaping public perception of drought. In 2002, for example, the media sometimes ignored or discounted the fact that the drought did not affect all metro-area water suppliers in the same way or to the same degree.

Political Response
Political response to the drought can take many forms and depends on the constituents affected. For example, the Board might be asked to save some of its water supply for use by providers with less reliable systems. Another example of a political response would be the reaction of other communities in Denver Water’s watershed to lowered water levels in nearby reservoirs.

Economic Impacts
Water restrictions imposed in response to drought affect businesses in different ways. In 2002–2003, some water-dependent industries, including landscape-related businesses and water bottling companies, felt that restriction programs unfairly affected them economically. As part of its public outreach efforts, Denver Water will continue to carefully coordinate restriction programs with these industries. Denver Water also encourages water-dependent businesses to develop their own Drought Response Plans to guide them through periods of drought and restrictions.

Environmental Effects
Reduced stream flows and lower reservoir levels caused by drought can affect the environment, recreation, fisheries, economic activity, and surrounding communities. As in 2002–2003, Denver Water will monitor stream and reservoir levels so that environmental effects are taken into account in drought response decisions.

Differences in Mountain and City Weather
City weather is considered in water supply forecasts, but weather in Denver Water’s mountain watersheds, which can differ considerably from that in the city, is the biggest single factor affecting our water supply. If conditions are dry in the city but wetter in the mountains, customer and media perception might be that drought is having a greater impact on supply than it actually is. The reverse could also occur, making it more difficult for customers to understand the severity of the drought. As during the 2002–2003 drought, Denver Water will monitor the difference between city weather and mountain watershed weather.

Long-Range Weather Forecasts
Denver Water’s drought response may differ depending on whether long-range weather forecasts indicate a prolonged drought or a short-term dry period.

Because most of the water supply during drought comes from storage reservoirs, reservoir levels are the best indicator of a drought’s impact on supplies. Reservoir levels are the “bottom line” result of most of the factors affecting supply–including weather, snowpack, soil moisture, runoff, water rights, bypass requirements (minimum stream flows required after diversion points are passed), collection system limitations, and water use. In the overall water storage picture, normal or generous stream flows (and thus higher reservoir levels) in one area of the water system can compensate for poor stream flows (and lower reservoir levels) in another. Customers benefit from a water system that is flexible enough to take advantage of circumstances such as these.

The water supply triggers that most heavily influence drought response are the storage levels forecast for July 1. Forecasts for July 1 are used because water storage in the Denver Water system usually reaches its maximum by this date each year; most of the snowpack has melted, and the high-water-use days of summer are ahead.

Denver Water will make its first official assessment of whether to declare a drought as soon as staff members can predict with reasonable certainty the total amount of water that will be stored in Denver’s reservoirs on July 1.

July 1 storage levels are forecast during the first weeks of February, March, April, May and June. Predictions are based on measurements of snowpack, stream flows, and the amount of water already in storage on the forecast date. Obviously, the ability to predict July 1 storage improves as that date approaches. Typically, forecasts of July 1 reservoir storage do not begin to be reliable until the April forecast. Thus, Denver Water would not declare a drought before April except under unusual circumstances.

Once a drought is declared, Denver Water will carefully watch day-to-day changes in stream flow, snowpack, precipitation, temperature, and water use until the drought is over.

If conditions change after a drought has been declared, the declaration can be lifted or the level of drought response can be adjusted. Denver Water’s drought response decisions must be adequate to deal with the lack of water without causing unnecessary hardships for its customers. The downside of starting restrictions in the spring is that wet spring weather can quickly extinguish the need for restrictions.

The following triggers are recommended as signals to declare a drought:

The threshold for declaring a drought–reservoir storage at 80 percent–was chosen for two reasons. The first reason: 80 percent is the amount of water that, with increasingly cautious use over time, is projected to see Denver Water's existing customers through a drought more severe than they experienced in the 1950s. The second reason is to avoid inconveniencing customers more often than necessary. As growth occurs or firm yield estimates change, this threshold may need to be changed.

Here is an example of how the process of drought assessment works. If in April of a given year, Denver Water predicts its reservoirs will be 90 percent full on July 1, declaring a drought would not be recommended.

April prediction of July 1 storage: 90% full
No drought declaration
Continue monitoring conditions

But suppose that May is unusually dry and the June prediction of July 1 storage drops to 78 percent full. In this case, Denver Water would likely recommend declaring a Stage 1 Drought.

June 1 prediction of July 1 storage: 78% full
Declare Stage 1 Drought
Take appropriate actions (see Stage 1 Drought Response)
Continue monitoring conditions: revise response level if conditions warrant

If the supply situation becomes significantly worse, the drought declaration would likely be changed to Stage 2. Generous rain, on the other hand, would likely cause the drought declaration to be lifted.

Denver Water staff spent a great deal of time analyzing and debating the advantages and disadvantages of taking early action in response to a possible drought versus delaying action until drought conditions are more clear. Tradeoffs are shown in Table 1.

A primary focus in the debate over early action versus delayed action was the potential hardships caused by reducing water use and drawing reservoir storage to lower levels. Reducing water use could affect metro-area businesses and damage water-intensive landscapes. Low reservoirs reduce or prevent recreation, affect the environment and create aesthetic problems. In the final analysis, Denver Water staff judged proposed storage levels and drought responses on the basis of three questions:

• How severe a drought could Denver Water withstand with a range of potential storage levels?
• With these storage levels, how often and to what degree would customers be asked to reduce water use?
• How low would storage levels get?

To answer these questions, the staff used the following assumptions:

• Current average annual demand is 285,000 acre-feet.
• Current annual firm yield is 375,000 acre-feet.
• Through close cooperation, Denver Water and its customers would cut annual unrestricted water use by 10 percent during a Stage 1 Drought, 30 percent during a Stage 2 Drought, 50 percent during a Stage 3 Drought, and 66 percent during a Stage 4 Drought (Table 2). Water use reductions were estimated on the basis of how customers use water during normal periods and how they have used it during past dry periods.

How severe a drought can the Denver system withstand?
The proposed trigger levels would enable Denver Water and its customers to endure a drought more severe than the 1950s drought. So far, the current drought has not stressed Denver Water’s supplies as much as a recurrence of the 1950s drought would. At the current level of water use, adopting the trigger levels proposed in Table 2 would enable Denver Water to withstand approximately four consecutive 2002-type drought years before depleting storage. If the Board chooses to maintain this level of system reliability as the population in Denver Water’s service area grows, the recommended water-savings goals will need to be increased.

How often and to what degree would customers be asked to cut water use?
At current water supply and water use levels, the triggers delineated in this plan would result in Denver Water’s customers having no water use restrictions about 90 percent of the time. During the other 10 percent of the time, Stage 1 or Stage 2 restrictions would be in effect. As water demand increases with population growth, Denver Water expects drought declarations to occur more frequently. System maintenance or repair activities that temporarily take a storage or conveyance system out of service could prevent Denver Water from storing all available water and thus make drought declarations more likely.

Sometimes a drought appears to be at hand, but Mother Nature has other ideas. Figure 5, for example, shows what happened in the South Platte River Basin in 1995. The South Platte basin is a major source of Denver Water’s supply. From February through mid-April of that year, water supplies from this source were below normal and drought conditions looked probable. The situation changed, however, as a result of higher-than-average precipitation during late April and most of May.

Figure 5. Snowpack in the South Platte River Basin–1995

Unfortunately, Denver Water is no better at predicting weather than anyone else. Forecasting a drought–and even knowing with certainty if one exists–can be difficult. When a dry year occurs, for example, we don’t know whether we are in the first year of a three-, five- or ten-year drought or merely in a dry year somewhere in a series of average-to-wet years. Even though droughts cannot always be predicted, customers will be kept advised of the latest information on Denver Water water supplies so they may take this into account in their own planning.