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1.9: Water Use

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    It is amazing what water is used for and how much is used on a daily basis. We tend to not think about all the daily activities we do which requires the use of water. We turn on the faucet in the morning without giving it much thought. We flush the toilet, make coffee, and shower without the consideration of where or how the water comes out of our tap. If we think about the simple and obvious water uses, we might list washing our face and hands, taking showers or baths, brushing our teeth, making coffee, boiling pasta, washing dishes and clothes, flushing toilets. However, the list of water use goes on and on. In addition to the obvious uses (i.e., drinking, bath, irrigation, etc.) millions of gallons are used in the manufacturing and processing of everything from semiconductors to beer and everything in between. Water is a vital resource for public health and safety and plays a significant role in our economy. One of the largest water uses in California is the agriculture industry. According to the Pacific Institute’s 2009 report titled Sustaining California Agriculture in an Uncertain Future, approximately 17.2 million acre-feet of water is used for agriculture annually. In addition, a large portion of the state’s water supply is undeveloped and used for environmental purposes.

    Water use can be broken down into several different categories: geographic, socioeconomic, availability, reliability, and quality. Geographically where water is plentiful very little is used for outdoor irrigation and sometimes use isn’t even tracked. In contrast, in areas of the world where precipitation is low and water is scarce, it is treated differently and very little is used for outdoor irrigation. In poorer areas of the globe, water is a vital resource for survival. Approximately one (1) billion people lack safe and adequate water. In these regions people are not concerned with keeping a lawn green with water. They are worried about surviving. Availability, reliability, and the quality of water also play an important role in usage patterns. Industrialized countries tend to have water readily available, with reliable infrastructure, and of high quality. However, as we have seen in recent years, in certain areas, such as Australia and California, long droughts can occur, jeopardizing water availability, reliability, and quality. The point of this introduction is not to memorize facts and figures; it is to illustrate the vast amount of water uses and variability of supplies throughout the world. The remainder of this chapter will focus on common water uses primarily in California. As mentioned above, water plays an important role in the manufacturing of items. The following table is an example of how much water is needed to prepare certain items.



    Pair of Jeans


    Cotton T-Shirt


    Barrel of Beer (32 gallons)


    Single Board of Lumber


    Gallon of Paint


    Individual Plastic Bottle of Water


    Units of Water

    Whenever we discuss water we talk in volumes. However, there are various units to measure volumes of water. Each unit has its own purpose and are commonly interchanged with each other. Below is a partial list of units and the common use for reporting.



    Meter Reads

    Cubic feet (cf)

    Hundred cubic feet (HCF / CCF)

    Groundwater Well Flow

    Gallons per minute (gpm)

    Daily Production

    Million gallons per day (MGD)

    Annual Production

    Acre-feet per year (AFY)

    Per Person

    Gallons per capita per day (GPCD)

    Understanding the terminology and units used in any industry is very important. The water industry uses a variety of different units and has a lot of terms not used in any other industry. As you start taking more classes in this field and begin your career in water you will become acquainted with the “language” of the industry.

    As shown in the above table, the common unit of measure to track water use on a per person basis is “gpcd”, which is gallons per capita (per person) per day. This is the amount of water used by one person per day. It is typically calculated by taking the total water demand (use) for the year and dividing it by the total population served. This will give you the amount of water each person uses for one year. Dividing it by 365 (the number of days in a year) will yield the gpcd, the gallons per person (per capita) per day. This number and unit were not used very often in the industry prior to 2009. In 2009, the governor of California passed legislation for conserving water and one of the parameters used to measure the required amount of conservation required by water suppliers is gpcd. Senate Bill X7-7 requires water suppliers to conserve at least 20% of their water demand by the year 2020. Hence, the bill is commonly referred to as 20x2020 (20 by 2020).

    In order to conserve water it is important to identify water use. Where is the water being used? This may seem like an obvious statement, but being able to identify where the most water is being used can help in targeting specific areas for conservation. For example, gpcd includes both indoor and outdoor water use from residential customers. As previously stated, it is simply the total amount of water used divided by the total population. However, it doesn’t distinguish between indoor or outdoor water uses. It also doesn’t discern between other uses such as business or industry (these will be discussed later in the text.) In the next section we will analyze and assume the entire population of the first few examples is a residential community. Therefore, the gpcd would be for all residential water use both inside and outside the home.


    Water usage varies from person to person, city to city, state to state, and more importantly climate to climate. Geography and socioeconomic status also plays an important role with water use. Little irrigation water is needed in areas with high amounts of precipitation. However, sometimes in areas with very little precipitation, little is used for outdoor irrigation too. For example, the gpcd for someone living in an area where there is a lot of annual precipitation will more than likely have a lower gpcd than compared to someone living in a warmer climate. Socioeconomic status will also have an effect on water use and gpcd. A single person living alone in a small apartment will probably have a lower gpcd than someone living in a Beverly Hills mansion with a large yard. Someone with high efficiency appliances, low flow toilets and faucets would tend to have a lower gpcd than someone with access to these types of things. Below is an example of the differences between geographic regions and the respective gpcd values.

    Average Daily Per Capita Water Use in Several Major U.S. Cities

    Metropolitan Area


    Phoenix, AZ


    New York, NY


    Seattle, WA


    Sacramento, CA


    San Diego, CA


    Before even looking at the actual gpcd values in the above table, you could probably guess which city would have the lower and which would have the higher average gpcd values. The only city which may have tricked a few people could possibly be Phoenix, Arizona. Phoenix is a very hot and arid city and one might expect a very high gpcd. However, the gpcd value is considerably lower to a similarly hot and arid Sacramento community. Why is this? If you have ever traveled to Phoenix and Sacramento, the difference between most of the homes in Sacramento compared to Phoenix is the type of landscaping at an average sized single family home. In Phoenix, many homes have “native” or natural landscaping such as rocks and cactus. In contrast, many Sacramento homes have grass (turf) for landscaping. There are other reasons why Sacramento has a higher gpcd than Phoenix, but this is one of the more obvious reasons. Water use is dependent on availability, reliability, and quality.

    Let’s now take a look at a hypothetical gallon per capita per day (gpcd) calculation to

    work with as an example to identify water use. Typically when a gpcd is calculated it is not done based on individual water usage. It is commonly calculated based on total annual consumption divided by the total population served. Assume a population of 5,000 people uses 1,120 acre-feet of water in one year. What would be the gpcd for this hypothetical community? Using the formula “total gallons divided by total population”, converts to a gpcd of approximately 200 (see below).

    1 AF = 325,829 gallons

    1,120 AFY x 325,829 gallons = 364,928,480 gallons per year

    364,928,480 gallons / 5,000 people = 72,986 gallons per person per year

    72,986 gallons per person per year / 365 days = 200 gpcd

    This type of calculation will be discussed in more detail in the Water 031 Advanced Waterworks Mathematics course. Therefore, you will not be required to perform this type of calculation in this course. It is used as an example and to help explain how water use is calculated and identified. Now that we have an example gpcd, let’s break down the actual usage.


    Among all the countries in the world, the United States has the highest average daily water use per person. In fact, on average, the U.S. uses approximately 410 billion gallons of water per day. The majority of this water use comes from surface supplies; about 80% and the remaining 20% comes from groundwater. Thermoelectric power is responsible for approximately half of all U.S. water demand. Irrigation (agriculture) and public water supply makes up just about all the remaining.

    California water use is strongly dependent on the Mediterranean climate throughout much of the state. However, California also has some very tall mountain ranges with subarctic conditions. The lower flatland areas of California consist of long dry summers, cool evenings, and mild rainy winters. This is why so many people flock to areas in Southern California. A summer day in Santa Monica or San Diego is almost the perfect climate condition; warm (not hot) days and cool evenings. Many of the mountain areas throughout the state experience a more traditional four-season year with snow lasting from November to April. However, because California is a coastal state, the climate is also dependent on the conditions of the Pacific Ocean. This means, there can be times of significant drought and also high amounts of rainfall during weather conditions such as El Nino.

    Because of California’s unique size and location, there are four (4) main climate regions; Central Coast, Mountain, Central Valley, and Imperial Valley. These four (4) regions have very diverse climate conditions. The Central Coast has more mild than hot summers and more mild than cold winters. Think of San Francisco as an example. The summers in San Francisco are not typically too hot and the winters are not extremely cold. The mountainous regions of California also have typically mild summers. However, in contrast the winters can be very cold with high amounts of rain and snow. The Central Valleys usually experience very hot summers and cool winters. The San Joaquin Valley for example can have summer temperatures, which exceed 100F and also see some snow in the winter. The Imperial Valley (e.g. Palm Springs) is very hot and dry in the summer and the winters are typically very mild with little to no rainfall.

    Most of the rain and snow occurs in the eastern and northern portions of California. While most of the people live along the coast and southern portions of the state. This presents a unique distribution challenge for water professionals. Bringing water from where it is to where most of the people are is something California does quite well. However, during years of drought the ability to distribute water throughout the state can be costly and difficult. Most of the time there are ample supplies of water in California for all of the various uses. During extremely wet years, there is sometimes too much water and not enough storage and a lot of the supply ends up in the Pacific Ocean.

    Indoor Water Use

    In this section we will analyze some potential indoor residential water use and compare it to the 200 gpcd we calculated in the previous section. If indoor and outdoor uses are not metered separately, certain assumptions will have to be made to distinguish between the two main areas of use. A lot of research and study has gone on and continues regarding water use. In January 2010, the California Homebuilding Foundation published a document titled “Water Use in the California Residential Home”. Many of the estimates used in this section were collected from this document. However, it is important to note this is a “fluid” topic (pun intended) and regulations and policies are constantly changing when it comes to water use and conservation.

    Now that we have established a 200 gpcd for this example community, we know how much water each person uses on an average per day. There are a number of variables affecting the actual use per individual, but this is the actual average gpcd for this example. Once the average gpcd is calculated you can break down the usage. First, let’s identify typical indoor uses. There are a number of different indoor water uses. For example, flushing toilets, brushing your teeth, bathing, washing your hands, cooking, cleaning dishes, and cleaning clothes. In addition to these common uses a certain amount of water can be lost through leaks in the plumbing system. There are obvious leaks such as a broken pipe or fixture. However, some leaks can go undetected for days or possibly longer. Toilets are probably some of the more common areas where leaks occur. The slightest little crease or crack in the rubber seal in the toilet tank can cause water to slowly drip into the bowl without being noticed. This is just one example of leaks which can account for a high percentage of water use both indoors and outdoors. The list of indoor water use can be larger, but this is a good start. Look at the table below for some estimated amounts and volumes of indoor water use.




    Volume per Use

    Time/Quantity of Use

    TOTAL based on 200 gpcd



    2.5 gpm

    8 min

    16 gal



    1.6 gpf


    6.4 gal

    Faucets (cooking/washing)


    2.2 gpm

    5.5 min

    12.1 gal

    Clothes Washer


    1 load per week

    25.5 per load

    3.6 gal



    16 gal

    Total Gallons per Day

    54.1 gpd

    Based on the information in the above table, the indoor usage would account for approximately 27% of a person’s usage with a 200 gpcd. This is to say 54.1 gallons of indoor water use divided by a total water use of 200 gpcd equates to about 27%. If 27% is identified as indoor water use, then the outdoor (or irrigation) usage would account for approximately 73% or 145.9 gallons per day.

    In the space below, make a list of your various indoor uses and estimate how much water you use per day in your home.

    Use Time or Frequency Volume

    Outdoor Water Use

    Sticking with the same 200 gpcd example, the average outdoor water use equates to approximately 73% or 146 gpcd. In California, the average outdoor residential water use ranges from approximately 60 - 75%. Most of this water is used to keep lawns green. Drive around any single family residential community in California and you’ll see acres and acres of turf. This green lush landscaping requires significant amounts of water to keep it green, especially in some of the dry semi-arid inland communities across the state. However, in 2015, the State Water Resources Control Board (SWRCB) adopted water conservation regulations mandating urban water suppliers to reduce water consumption on average of 25%. In addition, new home building standards are becoming stricter making it almost impossible to install grass as the normal focal point in front and backyards throughout California. As usage patterns change, there are still millions of homes with front and backyards covered in grass. So, how much water is used to irrigate a lawn. New efficient drip, micro-spray, and irrigation systems are coming out every day, but many outdoor irrigation systems use standard “sprinkler heads.” The amount of water sprayed out of a sprinkler head varies, but the average nozzle produces approximately four (4) gallons of water per minute. Let’s look at a hypothetical example.


    • 1 sprinkler head produces four (4) gallons per minute
    • 7 sprinkler heads per irrigation station
    • 5 irrigation stations
    • Each station operates for 10 minutes per day

    Based on the assumptions above, let’s calculate how much water this irrigation system produces in one day.

    7 sprinkler heads x 4 gallons per minute per head = 28 gallons per minute

    28 gallons per minute x 10 minutes = 280 gallons per station

    280 gallons per station x 5 stations = 1,400 gallons per day

    You can see by this example, the amount of water used for outdoor irrigation can add up in a hurry. In fact, based on the 200 gpcd example we used in the above example this would equate to a much higher gpcd. One other thing needs to be mentioned regarding the 200 gpcd. Remember, this is for one person. Someone with the irrigation system used in the above example may be from a family’s home. So, let’s assume the irrigation system is from a family of four (4) and they are not watering daily. Let’s assume they are watering three (3) days per week. How does this look in terms of a gpcd?

    First, take the 200 gpcd and multiply it by a family of four (4).

    200 gpcd x 4 = 800 gpcd for this home

    Next, take the 1,400 gallons per day, multiply it by three (3) days a week and then divide it by seven (7) days a week to get the per day amount.

    1,400 gpd x 3 days = 4,200 gallons per week

    4,200 gpw / 7 days in a week = 600 gallons per day

    The 600 gallons of outdoor water use is for the entire family of four. As a percentage, this equates to approximately 75%, which is consistent to the example given above.

    Now, divide this by four (4) for the family.

    600 gallons per day / 4 people = 150 gallons per person per day

    Therefore, in this example, one (1) person uses 200 gallons per day with 150 coming from outdoor uses and 50 from indoor uses. If you remember the indoor usage calculated in the earlier example it was 54.1 gallons, similar to the results in this exercise. Please note that this is just one example in a very complex and diverse world, but it should illustrate the point of how indoor and outdoor uses can be estimated.

    Other Water Uses

    This section will focus on other uses in California. The California Department of Water

    Resources (DWR) breaks down California’s total water use into three (3) main broad categories: Urban, Agricultural, and Environmental uses. Urban use includes domestic residential use as previously described. In addition, urban water use includes commercial, industrial, and institutional uses. These three (3) categories make up a component of water use in California and throughout the world. In California, these use categories are defined by the DWR.

    • Commercial: Water users that provide or distribute a service.
    • Industrial: Any water users that are primarily manufacturers or processors of materials as defined by the Standard Industrial Classifications (SIC) Code.
    • Institutional: Any water using establishments dedicated to public service. This includes schools, courts, churches, hospitals, and government facilities.

    As you can imagine, the use percent for these three (3) categories can vary widely based on the area. For example, the amount of commercial, industrial, and institutional water use in a small “bedroom” community would be quite low. However, in a city such as San Pedro, California, where there are a variety of industries this component of usage can be quite high. In California, these categories only account for a small percentage of all urban water use.

    Agricultural and Environmental Water Use

    In California, it is estimated that the agriculture industry accounts for approximately four (4) times as much water use as all urban water uses. This is not surprising since California’s unique geography and Mediterranean climate have allowed the State to become one of the most productive agricultural regions in the world. California produces over 250 different crops and leads the nation in production of 75 commodities. California agriculture irrigates approximately 9.6 million acres of land. Environmental uses are identified as coming from developed and undeveloped water supplies. A developed water supply is one controlled and operated by someone. For example, the State Water Project is a developed water supply controlled and operated by DWR. It comes from natural runoff from precipitation and snow melt runoff, but it has been developed into a supply for specific uses. In contrast, an undeveloped supply would be a “free” flowing stream or river which is not used for specific uses. Both of these supplies are used for and by the environment. Among other things, developed flows are needed to keep salinity levels lower in the Sacramento Bay-Delta area and to help keep cold, clean water needed for salmon migration and spawning. According to DWR, roughly 52 percent is used for agriculture, 14 percent for urban, and 33 percent for environmental uses.

    This page titled 1.9: Water Use is shared under a CC BY license and was authored, remixed, and/or curated by College of the Canyons (ZTC Textbooks) .

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