McDonald Group International, Inc

Water, Wastewater, Environmental Engineering 

 

 

Estimating Irrigation Demand of Residential Subdivisions

A common problem in the design of a public water supply and distribution systems is how to estimate the effect on peak hour demand  in a predominantly residential system when a large number of   the connections begin simultaneous  irrigation. Irrigation demand for single residential connections often far exceed normal household use.  It is impractical to design a public water supply system to simultaneously serve 100% of such connections, yet to not to design a system for a reasonable expected number of  connections simuitaneously irrigating may result in low pressure problems.   This technical topic provides one theoretical approach to the problem

The following example calculation indicates one method irrigation demand for residential subdivisions with in ground lawn sprinkler systems could  be estimated.   There are many assumptions that have to be made after the size of the lot and dwelling are established.  In this example,  it is assumed that the grassed area of the lot is irrigated at 1 inch per week.  This results in average day demand of 669 gallons per day per lot.  While high, this is not altogether unusual.
   

Site of lot 0.25 ac
Size of Dwelling and Driveway 3375 sf
Average Irrigation Rate 1 in/wk
average gallons per day 669 gpd
Average Sprinkler Head 2 gpm/each
Coverage' 680 sf/each
Number of Heads 11
Zones 2
Domestic Demand 1
Demand Per Home 13 gpm
Irrigation Time Per day 51 minutes
Probability One On, any time 0.053

 

The instantaneous demand per connection has to be estimated.  There are several ways to do this, but considering  a reasonable number of zones and gallons per minute per emitter or sprinkler head,  it is  likely that 13 gpm or more will be the instantaneous irrigation demand from one lot.   Thus, the typical home would satisfy their irrigation requirement of 1 inch per week with just over 50 minutes of operation per day.

If we then assume then that for all intents and purposes that  people will only   irrigate their lawns  between 6 in the morning and 10 at night, then the probability that one  home would  be irrigating at any one time is about 5%.

With a hundred homes, the question then becomes how likely is it that 5, 10, 15 etc, homes could be simultaneously irrigating.

Use of the so-called binomial formula suggests the following : 
    

From this chart, we can see that  of 100 homes,  it is most likely that 5 would be irrigating at any one time.  However, we are not concerned with what is most likely to occur on a typical day,  but what could occur given enough time that would cause us supply and pressure problems.   With this view,   we see that it is not until we anticipate more than 14 or 15 homes out of 100 simultaneously irrigating that the probability of such occurring drops to less than 1 percent.

Thus, in this case,  our suggested instantaneous irrigation demand for the 100 homes would be :

100 x 15% x 13 gpm per home =  195 gpm.  To that we need to add the ordinary peak demand for the rest of the homes which are not irrigating,    which is probably 100 gpm.  Total demand is therefore practically 300 gpm.

If the distribution system piping is designed to transmit fireflows of at least 500 gpm,  then it is likely that it will handle irrigation demand as well.   However, there may be cases where this is not so.

A final note:  no mathematical exercise can entirely substitute for real data.  If possible, surveys of neighborhoods where high irrigation demands are thought to occur should be checked.

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