Capillary action (non-root) taken in consideration?
Hi,
It seems that field capacity is a function of root depth and soil type. But it also seems that during excessive rain events, the filed capacity is 'clipped' to a certain value and ofter times, after big rains, the ground seems to still be wet below the surface yet the RainMachine decides that the field capacity has been exhausted and thus needs replenishment.
But I believe that water below root level will still percolate upwards though a phenomenon called non-fibrose capillary action where water slowly migrates upwards (against gravity force!) though capillary effect, to be more precise through non-fibrose (non-root) capillary effect. This is a slow process, but nevertheless it is an important process, In some cases this is how the parts from water table makes it's way to plants. But this is not about water table....
I want to know if there is a way to account for *SOME* of the capillary 'replenishment' effect from rain water, to be more precise from long rain episodes where the RainMachine decides to water after 2-3 days when in fact maybe 3-4 days will be more appropriate.
Can your formula take this in consideration? Even an empirical capillary effect is better than no capillary effect.
Thank you for your consideration.
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Official comment
Hi Dom
Field capacity in our latest math is heavily dependent from soil type - on the same root depth the field cap of clay is around double of sand!
This is partially because of the capillarity You are talking about - as You probably know the particle size of clay (0.002mm) is much smaller than sand (0.05-2mm) and the capillarity effect is much stronger in smaller gaps between particles.
This is also visible in the soak time (or intake rate) for different type of soils - much higher for sand than for clay.
regards,
Brown
Comment actions -
I found this interesting article. I understand that different soils have different field capacities and value is multiplied with root depth so each foot of soil represents a certain amount. But giving a situation where a plant has a root depth of only 1 foot, I believe that the soil below the 1st foot can 'communicate' with the top layer of soil via capillary action?
From what you tell me, it looks like RainMachine is using only 1ft of available soil and discards everything below that? (if the root depth is 1 foot, for example).
In other words, if the 1st foot of soil is 'drained' of water, will any water from below migrate upwards and replenish some of the soil capacity via capillary effect?
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Capillarity is not a source of endless water rise otherwise we wouldn't have to irrigate. It has his limits and it is responsible more for retention than for replenishment.
Lets take the figure from your article: http://croptechnology.unl.edu/Image/NolanDiane1129928529/figure3-4.jpg
You can see that capillarity rises the water FROM the current water level. That water level slowly drops in time thus the overall height which gets moisture via capillarity drops too.But let's forget "science" - how many backyards You have seen which got water back on the surface when was not irrigated or was no rain? :) There are lot's of experiments (empirical ones) measuring field capacity and all of them have one result - if the soil does not get water from rain or irrigation it will drain out sooner or later due to gravity (and evapotranspiration) to a percent from which plants cant extract water anymore.
There are two special cases when this won't happen:
1. underwater pressure together with capillarity is big enough to form a spring or fountain
2. the underwater level is close to the root level (due to low sea level or close underwater reservoir)I think for case 1. we shouldn't worry and case 2 is special case - can happen after a serious inundation or in marshlands. And I did not seen mathematical models which can deal with inundations to be honest because there are so many parameters from sea level to local drainage that makes impossible to have a generic model which works on every place of the earth well.
So I would recommend to the users in case they see their irrigation system starts to irrigate too soon after a serious rain to either change the soil type to one which has a better water retention (even use a custom value) or increase the root depth a bit.
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I think Dom was referring to large rain events where more water is stored in the ground. This is very helpful for greater than 1inch rain events. Rainmachine is simply starting watering sooner than it should be, IMHO. For small rains it works well, but for big rains RainMachine starts watering a bit too soon.
I see that the beta release has control over soil capacity that offers over 100% increase -- this way I can accept more water from big rains.
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Beside the 100% increase, the base value is heavily influenced by the soil type.
For example Lawn with Sand has 0.22inch capacity, with clay has 0.54 inch and with loam has 0.74inch. If not even that is enough custom soil type can be selected and "soil field capacity" [%] permits setting any custom value (setting 0.5 there will increase the overall field capacity to 1.5inch).
Same is valid for plant type - any root depth increase will increase the field capacity too.
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