Effective Rainfall is What is Important to Plants

Q. With all the beautiful rain that we had recently, what should we do regarding future watering? Has it been enough water to just stop watering until after the first of the year?

National Weather Service Wettest Day of 2013 

A. Rains come in all forms. This one was rather unique in that it came down over a nice, long period of time giving it a chance to soak into our landscapes instead of running off the surface and into the streets. 

            Landscapes are supposed to be contoured to force rain off landscapes and into the streets. Once water enters the streets they act as “storm sewers” and help remove water from properties. If landscapes are not contoured correctly it is feared that water might accumulate on the landscape and flood homes causing damage. 

Storm Drain System in the Las Vegas Valley 

            For gardeners who are interested in “harvesting” water on their property, this rule may seem to be counterproductive but it is meant for the “general good”. There are ways to store water and improve the effectiveness of rainfall such as dry stream beds and pools but you must be careful when doing this. I will post some ideas on my blog as examples.

            As a general rule we consider about 40% of our rainfall to be what we call “effective”. This means that 40% of the rain, four tenths to every inch, actually gets into the soil where it is stored rather than running into the streets. This rain event however was more “effective” than most. 

What is Effective Rainfall? 

            Another problem was that the rain was not evenly spread throughout the valley. This makes it hard to make a general statement about how long to turn off irrigation systems throughout the valley. For shallow rooted plants like flowers and turfgrass they will still need a couple of irrigations the rest of this year. Deeper rooted plants, such as trees and large shrubs, can probably get by.

            Unless you know your rainfall amount, I would count this rainfall as a single irrigation event, skip one irrigation and then continue irrigations unless we get more rain. The savings will still be significant.