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Capillary action (non-root) taken in consideration?

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    Rolando Soto

    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

     

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    Dom

    http://croptechnology.unl.edu/pages/informationmodule.php?idinformationmodule=1130447123&topicorder=3&maxto=13&minto=1

     

    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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    Rolando Soto

    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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    Andrei (Edited )

    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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    Rolando Soto

    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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