Water System Planning: Estimating Water Needs

 

Find out how to estimate homeowner or farmer water needs and calculate how much water must be delivered from a private water supply to meet these needs.

 

Whether you are building a new house in a rural area or increasing the size of a dairy herd, adequate supply from a private water well or spring is critical to your plans. Planning should be done prior to having a well drilled or spring developed to ensure that an adequate supply of water is available.

 

 

In general, we use 50 to 100 gallons per person per day in our homes (200 to 400 gallons per day for a family of four). The household water use estimates given in Table 1 can be used to calculate more specific daily water use values for your home.

 

 

 

For the purposes of planning a water system, the total daily water use is less important than the peak daily water use or the peak demand. In reality, most of the water used in the home occurs over a very short time period, usually in the morning or evening. As a result, for planning purposes it is recommended that a water system be able to supply all of the days projected water use in a 2-hour peak demand period. If you estimate that your home water use will be 400 gallons per day, the water system should be sized to provide this much water in a 2-hour period.

 

So, how much water can be delivered from your well or spring in a given period of time? This is referred to as the well or spring yield. The yield from a spring can be easily measured by determining how many gallons of water flow from the outlet pipe every minute. This flow rate will likely vary considerably with weather conditions, but, for planning purposes, it would be best to measure flow during a dry time period. For a well, the yield is considered the maximum rate in gallons per minute (GPM) that a well can be pumped without lowering the water level in the borehole below the pump intake.

 

For most single-family homes, a minimum flow of 6 GPM is suggested from a well or spring. This flow would provide 360 gallons of water each hour, which would be sufficient to meet most home water peak demands. Higher flow rates may be necessary for larger homes with more fixtures, appliances, and residents that may all be using water at the same time. The values in the table below give the suggested minimum flow rates for various numbers of bedrooms and bathrooms in a home.

 

Ideally, the yield from the well or spring will exceed the recommended minimum flow rates in Table 2. If not, you may need to rely on water storage to meet peak demand periods. For a drilled well, the borehole can provide a significant amount of water storage. A typical 6-inch-diameter well will store about 1.5 gallons of water for every foot of standing water in the borehole and a 10-inch well stores about 4 gallons of water per foot. Therefore, a 6-inch-diameter well with about 100 feet of standing water in the borehole would contain about 150 gallons of stored water. In the case of a spring, a large spring box can be constructed where the spring emerges or a water storage tank can be added after the spring box to provide extra water storage to meet peak demand. The water stored in the borehole, spring box, or storage tank would be helpful when water use in the home exceeds the amount of water flowing from the well or spring. Well storage and spring flow can vary dramatically with the natural groundwater level, with the highest levels typically occurring in spring and the lowest levels in fall. These natural variations can be accentuated by drought conditions. So, while water storage can allow for the use of wells and springs with lower flow rates than shown in Table 2, it may not be reliable during severe droughts. An approximate estimate of the amount of water needed before a well or spring is developed can allow the professional contractor to utilize the combination of local knowledge, yield, and storage to meet water demand. For wells that yield extremely low amounts of water, an intermediate storage system can be added (see Water Facts #3--Using Low-Yielding Wells).

 

Table 2. Minimum flow rates (GPM) for homes based on number of bedrooms and bathrooms. (From Private Water Systems Handbook, 1992.)

 

 

 

Planning for water supply needs is generally much more important for farms because much larger amounts of water are often needed, especially for dairy operations or farms with large acreage in irrigation. Midwest Plan Service guidelines suggest that farms using 2,000 gallons per day (GPD) will need a water source flow rate of 16 GPM, those using 6,000 GPD will need 36 GPM, and those using 10,000 GPD will need 48 GPM. Planning for larger operations starts with an estimate of total daily water use from Table 3.

 

Table 3. Estimated daily water use in gallons for various farm animals, equipment, processes, and irrigation in Pennsylvania.

 

*The amount of water used for irrigation is seasonal and varies greatly depending on natural water availability from precipitation.ting Farm Water Use Needs

 

Using the estimates from Table 3, current and future daily water demands on the farm can be estimated. The farm water system would need to be designed to include sustained yield and storage from one or more wells or springs. Where large quantities of water are needed from a well, it may be worthwhile to hire a professional hydrogeologist to locate a high-yield well using fracture trace mapping. For more details on this technique, consult Water Facts #5--Water Well Location by Fracture Trace Mapping. It should also be noted that farms using more than 10,000 GPD must report their annual water use to the Pennsylvania Department of Environmental Protection as required by the Water Resources Planning Act.

 

The required water source flow rate does not necessarily need to equal the yield from the well or spring. If water availability is projected to be insufficient for the calculated peak water demand, additional sources must be developed or additional storage must be used (see Water Facts #3-- Using Low-Yielding Wells).