Soil Type / Field Capacity vs. Available Water

Good Morning Rachio

I would like you to help me shed some light on the soil specific values for ‘available water’.

Available Water (AW) is AFAIK defined as Field Capacity (FC) - Wilting Point (WP) in units [in/in] or [%]. It’s the volumetric fraction of water extractable by plants, i.e. volume water per volume soil.

There are many tables out there and they do not all agree but I found this to be a representative summary:

Now in order to avoid water stress we do not want to deplete the water reservoir all the way to WT.
Literature recommends to keep moisture levels between 100% and 60% of FC, so readily available water is RAW = 0,4 x FC.

Below I put together a table summarizing these values for 4 soil types:
The last two columns are Rachio’s numbers.

SOIL          FC %   WP %    AW %   RAW %      Rachio AW %  AW (@50% depl)
sand          9.1    3.3     5.8    3.6        5.0          2.5 
loamy sand   12.5    5.5     7.0    5.0        7.0          3.5  
sandy loam   20.7    9.5    11.2    8.3       12.0          6.0 
loam         37.0   11.0    26.0   14.8       17.0          7.4

First question: Do y’all agree on the above?
Seconds question: Should we not deplete further than 50%, say 70-80% to consume all of RAW before irrigating?

The motivation behind all of this is to irrigate deeper thus promoting deep-rooting grass over types like poa trivialis. It has little effect on the total amount of water.

Cheers, Sebastian

I’d agree with your analysis, but soil type is not an absolute thing. Here in Texas, we have access (via agriculture extension service or university web sites) to the actual AWC (Available Water Capacity) from topographical studies and maps. Loam in Michigan is 'way different than loam in Texas!

As I understand it (perhaps paraphrasing you), AWC is the amount of water that can be retained in the “average” arable surface profile of your area. It’s measured in inches of water per inch of the profile. Your table labels it “Field Capacity”. Texas A&M says 'TX073 Sacul fine sandy loam to underlying clay/clay loam @10" ’ with AWC values with local variation from 0.16 - 0.20 where I live. (Yes, this would be “arid” to a lot of Rachio users!)

AWS (Available Water Supply) - what Rachio calls “Available Water” is the available reservoir of water within the root zone, where it matters. Generally, it’s determined by multiplying AWC by the depth of your root zone. For home irrigation it’s measured in inches of water - the irrigation requirement to refill it. “Allowed Depletion” is a percentage of the FULL capacity of the root zone that can be removed by all factors before water MUST be added to recharge the root profile. To confuse things a bit, Rachio’s “Soil Moisture” shown for zones is the percentage remaining of that portion of the zone profile allowed to be depleted (100% is full, 0% is at or below Allowed Depletion). Tap “Soil Moisture” to see this. The higher you set Allowed Depletion, the less frequently Rachio needs to water and the tougher its flora needs to be. 50% is Rachio’s default - seems a pretty good number. It turns out, as the wilting point and residual columns of your chart illustrate, plants wilt - and then die - as you drop into the bottom half of the soil profile’s capacity.

I’d say, in answer to your second question, no - there’s little benefit from dropping too deep into that bottom half of the profile’s capacity - Rachio will not water less overall to do so, it will just delay filling it. Your “wilting point” data says that all soil types get risky close to 50%.