How Much Water Do I Need to Irrigate?

Calculating 1" of Water

Deciding how much water any plant needs can be difficult.  There are so many factors to consider.  The first seems obvious, what type of plant? Grass, a tomato plant, an apple tree?  Within each plant may be several varieties that may need more water than others. You also need to consider factors such as soil type and evaporation due to wind or sun.  Don’t forget about your limitations on how much water you can access and the irrigation infrastructure (pumps, pipe, filters etc.) in place to capitalize on this opportunity of water.

In today’s age there are all kinds of “smart tools” that can help like soil moisture sensors, weather stations, and remote irrigation controls.  These types of tools come with a price tag… as you could expect for such powerful tools.  Some of these can also seem daunting in order to use the tools to its full capacity.

At the end of the day, the key to designing an irrigation system is ensuring that it will apply the correct amount of water over the intended space, efficiently and uniformly.  Whether it be dripline in a flower bed, sprinklers on a golf green, or a center pivot on a potato field they all have the same goal, a uniform distribution of water.  How much water though? Soil moisture sensors can help you tell exactly.  Sticking your hand in the dirt can give you a gut feel. But is there a general rule of thumb you can always come back to?

 1” of water per week. 

“In a vacuum” it isn’t far off, no matter what you are growing.  It has been commonly used for generations because it works.  So, the question is, how can I guarantee these plants get 1” of water a week?

Well, you can’t control Mother Nature.  Unless you are indoors, overwatering is always a threat.  Under watering on the other hand is completely within your control.  Supplement the rain by installing an irrigation system that you can feel confident in to deliver 1” of water to the plants in a weeks’ time.  If you can apply 1” of water in a week to your plants, you are covered; the plants will not skip a beat through a drought.   If the drought continues… you’ll have to do it all over again next week.

Finally, how much water do I need from my water source to supply 1” of water?  Once you know that, you can use the constant to calculate how much water you are going to need to irrigate.

1” of water on an acre = 27,200 gallons

1” of water on 1,000 square feet = 620 gallons

1” of water on a square foot = 0.62 gallons

For example: If you are looking to irrigate 1 acre of grass during a drought, you should prepare to deliver 27,200 gallons of water per week until the drought is finished.

So, how much water? Soil moisture sensors can help, and sticking your hand in the dirt can give you a gut feel, but the general rule of thumb you can always come back to is 1” of water per week!