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)
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)