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Beginners Guide: How Much Water Does My Landscape Need

Water Cycle Graphic 1

How much water does my landscape need continues to be a common question from landscapers and homeowners. The garden and landscape industry contributes to the mystery in a variety of ways. We talk in terms of ET, and make watering recommendations – like, you need an inch of water a week – then manufacture drip irrigation products that apply water in terms of gallons per hour or gallons per minute. This is super confusing to many homeowners and gardeners. Fortunately there is lots of science behind how much water to give plants and below you will learn a couple of the basic steps to optimizing your watering schedule. This article will solve the mystery of converting  gallons per hour to inches per week and how much time to run your water to satisfy the water needed.

How much water does my landscape need – Evapotranspiration is key

Contractors and homeowners should use evapotranspiration (ET) to determine how much to water. ET is reported in inches –For example the rate of ET yesterday in San Diego was .17 inches. The easiest way to understand evapotranspiration is to think the opposite of rain. ET is the amount of water that evaporates from the soil and plant surface plus the amount of transpiration through the plant. Temperature, solar radiation, humidity and wind velocity all affect ET on a daily basis. Knowing when plants need water and knowing when they are full is the first step to a healthy thriving landscape. Under watering and over watering are both detrimental to your plants. The majority of plant issues seen today are a result of too much water. When you observe a plant that is not doing well the first impulse is to give it water. Often this is an incorrect step and this article helps you understand when you are overwatering plants. ET helps guide us to the best time to water. Imagine if you had a way to determine the moment you were getting hungry and then were able to satisfy that hunger without overeating. You would be happy and productive. By monitoring ET we can do exactly that and have happy thriving landscapes as a result. If we measure the rate of ET over time (days, weeks, months) we know exactly how many inches of water the soil needs.

Gallons per hour to inches per hour

To determine how many inches per hour your drip irrigation system waters you need to collect a little information. This includes emitter flow rate in gallons per hour, the emitter spacing, and the space between rows. This information should be available on the product labels.

For emitterline use the formula below:

Inches per hour = 231.0 x q
                                 s x l

q = emitter flow rate in gallons per hour
s = emitter spacing (inches)
l = lateral spacing (inches)

For example if you are using ¼” emitterline to water a raised bed and the emitters are .5 gallons per hour, 6 inch spacing, and the rows are 12 inches apart:

Inches per hour = 231 x .5 6 x 12

A total of 1.6 inches per hour

Run time calculation

Using the information above a run time of one hour will result in 1.6 inches of water applied, or .026 inches per minute. If the reported ET for the week was 1 inch and we need to add 1 inch of water to the soil we need to run our controller for 38 minutes.

1 inch/by .026 (the inches per min) = 38 minutes to get 1 inch of water.

Remember this is the amount of water per week. We can divide the 38 minutes by the number of days we want to water. If we want to water 5 times a week we divide 38 by 5 to get 7.6 minutes. If we want to water two times a week it would be 38 divided by 2 for 19 minutes per watering cycle.

There are many additional variables needed to apply the appropriate amount of water to your garden or landscape. This covers just one basic concept. Other factors like soil, sun intensity, root depth, plant type, sprinkler efficiency, slopes and micro-climates (just to name a few key factors) all contribute as well. In future articles we will be discussing all those factors as well as how best to calculate run times for sparse plantings using point source emitters. Bewaterwise.com has an excellent calculator here for southern California. If you enjoyed this article please consider subscribing to the blog or following me on twitter @H2oTrends.

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