Oregon State University

4.1 Firm yield

Water demand and firm yield overview

A key parameter in designing reservoirs for water supply is the safe (or firm) yield, the maximum quantity of water that can be guaranteed with some specified degree of confidence during a critical period. For example, the guaranteed volume of water available to residential users during low summer flows is considered firm yield. Determining the firm yield necessary to meet water supply objectives at a certain level of confidence depends upon the volume and timing of supply and demand. Residential water use tends to remain relatively constant throughout the year, though demands in the Western US are generally higher in the summer. On the other hand, irrigation demand is highly seasonal, with peak demand in the summer and virtually negligent demand in the wet winter months of the Western US mediterranean climate. To simplify the following exercises, it is assumed that Folsom dam operates in isolation to only supply water within Sacramento County, rather than operating as part of the multipurpose Central Valley Project. 

Estimating and projecting residential water demand

Residential demand - Residential demand for water is primarily a factor of population size and per capita water use, which are themselves influenced by a variety of factors such as:  household income, lot size, water pricing, climatology, etc. The following graphs display historical population and per capita water use in Sacramento County and California, respectively, prior to the construction of Folsom Dam. In 1940, approximately 170 thousand people lived in Sacramento County and urban residents in California consumed about 150 gallons of water on average every day.

170,000 ppl * 150 gal = 25,000,000 gallons

conversion factor:   325,851 gal/ 1 acre-ft

25,000,000 gallons / (325,851 gal/ 1 acre-ft) ~ 78 AF/ day of residential demand in 1940


Imagine you are planning Folsom dam in 1940, what trends do you observe in the population and water use data? Based on the data available during the planning stages of Folsom dam, how do you think the graphs' shape might look in the future? Click on the trend buttons to fit different forms of simple regression equations to the pre-Folsom data.  

Initially just show the graphs with pre-Folsom data points. Display buttons for: "linear", "exponential", "power"? When a user clicks on a button, draw the trendline.Do not show post-Folsom points until the next set of exercises. NEED TO GET DATA for per-capita graph (SF from Water Plan shown)

Figure 4.1a. Population trends in Sacramento Co.

Figure 4.1b Per capita water use in California

If you were planning Folsom operations over a 50 year planning period, for what size population in 2000 would you plan?

Approximately, what value per capita water use would you expect in 2000?

Based on your estimates, residential water demand in 2000 =

Population * per capita use = Gal/day = AF/day = AF/ yr

Fill in these numbers and calculate based on user input. Display a message:  The 2000 Census reported the Sacramento County population of 4.1 million residents  (check year 2010?) and PPIC reported per capita water consumption of 249 gal/ day in the Sacramento River Valley in 2005 (Sac. County is higher - need to get number).

Your population estimate is too high/low/close and your per capita water use estimate is too high/low/close. Over all, your projection for water demand in 2000 is ____ AF/day and ___% lower/higher than the actual residential demand.   


Storage-yield relationship

Suppose that you wanted to plan for a firm yield of ___ AF/day during the driest time of the year








Other water sources

Physical constraints which need to be considered in storage-yield studies

Meeting current water demands

increased efficiency (decrease in residential per capita use)


Planning for changing water demands

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