How Much Water Does My Landscape Need

This is a commonly asked question for the Gardenworks Inc. team especially during this era of water conservation and water use efficiency.  The current drought has created a lot of tension and concern over outdoor irrigation needs.  The recently updated California Water Plan indicates that upwards of 34% of urban water use in this state is for landscapes. This is significant and amounts to maybe as much as 10 million acre feet of water per year.
What you might not know is that there is a process to determine the right amount of water a landscape should be using per year, month, week or day no matter where it resides in California.  This process is recognized by the State in its recent updating of its Model Water Efficient Landscape Ordinance (MWELO) which went into effect in January of 2010.
This is a water budget and every landscape has one or should.  As you probably know a budget is a plan to meet a target and is usually thought of in terms of money.  Generally is a list of all planned expenses and revenues. It is a plan for saving and spending.

  •  A water budget is a plan for managing water use and saving.  It tracks planned water use versus actual water use. A water budget helps manage costs and improves site maintenance and efficiency

To calculate an overall water budget for a landscape the following formula is used:
Water Budget:  Et x LF x LA  x .62 = gallons


Et = evapotranspiration rate (the amount of water lost through a combination of plant transpiration and soil evaporation) and is measured per day, week, month and year.  In the Santa Rosa area it is about 44 inches per year
LF = the Landscape factor – which is made up of the overall water needs of the plants as a whole.  In California the plant factor default is .70 but in Sonoma County it is .60 ( more restrictive than the state by a factor of 10%)
LA = Landscaped area measured in square feet
.62 = conversion to gallons

Et = 44”/year
LF = .60
LA = 2000 square feet

Becomes:  (Et)44 x (LF).6 x (LA)2000 x .62 which equals 32,736 gallons needed per year

The evaporation rate mentioned above is different though out areas of the state and times of year and is directly the opposite of  our rainfall cycle.  The highest month for Et is July and the lowest is January. In the Santa Rosa area Et is around 6” for the month of July which is also the highest month for landscape water use.  So using the formula above:

6 x .6 x 2000 x .62 = 4,464 gallons of water should be applied.

The next logical question then is “how is this water tracked?”  The simple answer is with a water meter.  Unfortunately, most water meters for residences are not dedicated to the irrigation system and read both indoor and outdoor usage.  So it becomes important to know how much water is used indoors so that water can be subtracted from the meter reading to get the amount used outdoors.  One way to get that is by looking at your water consumption/bill during the parts of the year when the irrigation system is turned off.  A better way is to install an irrigation sub meter that will only read water used for the landscape.  A third and the most inaccurate way is to use a default of approx. 70gallons per person per day.  Whichever route chosen, the water meter should be read at least weekly to determine how much is being used and then forecast out to the not to exceed total for the month.  In our scenario above if the meter indicates the weekly landscape usage is 1000 gallons then at the end of the month the total will be 4,000 gallons.  Our budget above is set at 4,464 gallons.  Congratulations you are under your water budget!

It is commonly accepted that Californians in general over water their landscapes by 30% – 300% of what their landscapes actually need.  Using landscape watering budgeting avoids this entirely and results in great looking water efficient gardens.

Please feel free to contact us at Gardenworks Inc. if you would like more information about the scientific approach to healthier and better looking landscapes.  And by the way, over two thirds of our clients are on a water budget and get an annual report of how their watering year went.