How Much Water is Being Applied at One Time, or ‘Why am I watering the street when I don’t own it?”
It was surprising to me, at first, to find out that the total water applied by irrigation per week does not vary with root depth and soil type, as explained previously. Well, where do those come in? The main answer is: The amount of water applied at one time, or rather on one day, in each zone.
First, why do we care? If we give the lawn the 1.4” or whatever water per week it requires, does it really matter how much water is applied at any one time? Yes, it does, for the health of your lawn, in particular its ability to withstand hot weather. If we apply that 1.4” by watering just a bit per day, in this example 0.2”, we keep moisture in the top layer of the lawn, but never really water deep enough for the roots to grow, thus protecting the lawn and making it healthier. Watering every day is just bad for your lawn. Basically, every time you water, you should water enough to soak into the lower roots of the lawn, or at least where you’d like them to grow to.
Watering too much at a time is bad as well: Water will no doubt be wasted, either by running off, or sitting in puddles which can actually kill the grass. So how much is right? Rachio uses the Field Capacity model for watering, explaining with their Bank and Gas Tank analogies. I’ll use a bucket. You want to fill the bucket to the top, which will water to the bottom of the bucket (roots). The water then evaporates daily from the bucket until it reaches about the halfway point (an Allowed Depletion of 50%). If you let the water level get too low, it will stress your lawn, not providing enough water to the roots. If it’s too high, you overflow the bucket.
In our case, the bucket isn’t really empty; it’s filled with your soil, and different soils can hold different amounts of water. That’s the soil’s Available Water, which generally varies from 0.05 to 0.21, in my case 0.17. How much water, in inches, will your soil hold? That’s easy. For 8” deep roots it’s:
Inches of water held = AW x Root Depth = 0.17 x 8 = 1.36” of water
While the soil holds that much water, we don’t want to apply it all at once, because 1) we’d probably over-water, as it couldn’t accept it all at once, and 2) we don’t want the water in the bucket to ever be “empty”, or your grass will be on the point of dying. Rachio suggests a AD of 50%, which means we’ll wait until half the water is gone. That amount of water is:
Inches of water held = AWC x Root Depth x MAD = 0.17 x 8 x .50 = 0.68” of water
That value of represents the amount of water that Rachio will apply every time it waters, unless it reduces it due to rainfall, and answers the question we started with. It’s trying to top up the bucket every time it waters, then waits until the bucket is half full before watering again.
Once again, there are surprisingly few variables that affect the water applied at one time: the Available Water (determined by your soil type), the Root Depth (determined by your grass or crop) and the Allowed Depletion (you get to choose that one, but it should probably stay not far from 50%). If you want to water less often, you can decrease the Allowed Depletion some; if you want to water more often, you can increase it some, but watch that you don’t overflow your bucket and waste water in the street.
Note that changing these variables will change the amount of water applied in one day, and also the total amount that Rachio thinks your soil will hold, but it will NOT change the total water, on average, that your lawn receives. That is done only by the Crop Coefficient, as explained in the first post above.
Also note that while the amount calculated here will be applied each day Rachio waters that zone, it may not be all applied at once. Letting Smart Soak do its job will let the system break the 0.68” up into 2 or more separate applications with soak times between, totaling 0.68”, but allowing your yard time to soak it up. This is often necessary for soils such as clay, which hold a lot of water but absorb it very slowly, and for sloped yards, which let the water run off if too heavy.