Hi folks,

I’m new here. I got my OSPi a few months ago and have been experimenting with the software. This thread has provided a lot of useful information. The approach I’m pursuing now is to calculate a daily ET value using the solar radiation reading from a Weather Reach station about 10 miles from my house. I use other data (rain, wind, humidity) from my neighborhood (via Weather Underground) to calculate a more localized ET. Calculations using historical data yield ET values very close to those at Weather Reach, so using their values directly would be a reasonable simplification.

I came across this very informative document which covers a lot of relevant information and is easy to read:

It recommends a soil moisture balance or “checkbook” method for calculating an irrigation schedule. This technique keeps a running daily balance of moisture required that is increased by the day’s ET (water lost) and decreased by the day’s rainfall. When the depletion exceeds a particular threshold, irrigation takes place and the balance is decreased by the amount of water applied. This approach waters the same amount each time, but skips days when no irrigation is necessary. The idea is that when irrigation takes place the amount needs to be enough to penetrate deeply into the soil. I ran a simulation of this approach using historical data from last year and the schedule seemed pretty reasonable. During the rainy season there was no irrigation and during the driest part of the year watering happened every two to three days. I’m thinking I might use a base schedule based on this approach augmented by shorter but more frequent watering in zones containing shallow-rooted plants. I’m curious to hear what people think of this approach.

One of my pet peeves is having my sprinklers come on when there’s significant rain imminent or, worse, when it’s actually raining. To prevent this I plan to use forecast data to reduce watering when rain is likely. Currently I’m using the Weather Underground QPF (Quantitative Precipitation Forecast) value to adjust my watering times. I look out a couple of days and scale today’s QPF value at 100% and later days at lower values. I’m wondering if there might be an alternative approach where future ET values are estimated based on forecast conditions and the result integrated into the irrigation scheduling calculation. What approaches are other people taking to adjusting watering based on forecasted rain?

At this point I’m just simulating watering. Here in the Seattle area it’s common knowledge that the rainy season ends on July 5 (or later), so I’ve still got plenty of time to experiment.

Dan in WA