Negative Moisture levels?

Continuing the discussion from Why Doesn't 'Checkbook' Go Negative?:
I have a few questions about moisture level tracking from observing the moisture level graphs. I have been using fixed schedules to water so far this year, and have not tried to adjust moisture levels yet, so the graphs do not quite reflect the actual moisture level. The behaviour of moisture tracking does not quite match how I expected it should work.

I notice that when looking at a moisture graph for a zone that needs watering, the moisture level never falls below the allowed depletion level. In the case of a grass zone with an available water amount of 0.17 in, root depth of 6 inches. the total water available is 0.17 x 6, about 1.0 inches. If the allowed depletion is 50%, then the total moisture content should be between 1.0 inches and 0.5 inches. When at the allowed depletion, there is still about 0.5 inches of moisture in the soil. I get that the graph uses the allowed depletion level as a zero base point, but in reality the zone can still dry out below the allowed depletion level. Using the same example, I also notice that a flex zone watering time will always be the same, whatever is required to bring the moisture level from the allowed depletion level to field capacity (after factoring in the watering efficiency). I would think that the moisture level should be allowed to drop below the allowed depletion, and a flex watering should water the amount needed to bring the moisture level from the current level (not the allowed depletion) back to the field capacity, so the watering time on flex would vary a bit with each schedule.

I also notice that the moisture level is allowed to go above the field capacity. Isn’t the definition of field capacity the maximum amount of water the zone can retain without runoff? How can this moisture level be exceeded?

Are my expectations not realistic, or does the moisture tracking need some tweaking? I can see that once on a flex schedule, the end of day moisture level should normally be between field capacity and allowed depletion, so this might not be a big deal for most users.

Thanks for your attention.

We chose not to allow the graph to go below zero, but you are correct below available water capacity (AWC) is a permanent wilting point (PWP).

Interesting concept. We chose to go simple and once the zone is depleted, we fill it up. This keeps things a bit simpler from a tracking perspective along with min/max values.

Great overall questions, you definitely understand these concepts :wink:

This topic will help with some of our decisions.

If there is a domain expert out there that has a better way to implement, I’m listening.

I think there is room for improvement, but all in all the process seems to work for most customers that have chosen to use flex schedules.

Thanks for the great questions.


Hi franz, as always, thanks for the answers. In the topic 2 days before it rains the zone reaches MAD sometimes before 2 days, sometimes after 2 days. Adjusting watering durations might help stabilize the watering frequency in this situation. I also notice that the irrigation cycle might restore the moisture level to capacity, but by the end of the day, the ET has pulled the moisture level halfway back to depletion level. I live in BC, adjacent to the Pacific Northwest, so my zones will not dry out in 2 days. I am thinking that in the case of high ET, it might be beneficial to slightly overwater if any excess will dry out within the same day, to avoid watering 2 days in a row.

@jkb It sounds like you might just need a lower frequency.

This article explains the levers to adjust watering frequency.

Also note, I’m hoping by the end of the week to release code that will align the crop coefficient adjustment with flex, which will be by far the easiest way to affect frequency without affecting anything else (i.e. duration).