What is AWC?
There are several different ways to represent soil moisture content; however Available Water Content (AWC) and Volumetric Water Content (VWC) are the most common. In order to understand AWC, it is helpful to first understand VWC.
VWC Presents soil moisture as either a depth measurement such as inches, or a percentage. It represents the total amount of water in a column of soil. For example, if you had a column of soil 12” deep, and were able to squish it until the soil is one solid mass, the water would also squeeze out. In this case, you may find that of the original 12” of soil, the actual mineral and organic components of the soil squished down to 6”. Whereas, the water that squeezed out was is 3” deep. In this case, VWC would show you that 3” is the amount of water in the soil. In percentage form it would give you 25%, in that 25% of the column of soil was actually water.
VWC is can be quite accurate in terms of how much water is in the soil, what it doesn’t do is tell you how much water that is compared to how much the soil can actually hold. Similarly, it doesn’t do a good job of telling you if it is enough water for the crops that are being grown. As soils vary greatly, each place a probe is placed will have a different “full” value, and it is very difficult to track all of those differences from location to location.
AWC is a normalized version of VWC. In order to change VWC to AWC the “full point”, also called “Field Capacity” (FC) has to be determined. There are several different ways to do this including looking for key events in the data trend that indicates FC. FC represents the point at which the soil is holding as much water as it can hold onto. This value is presented as 100%. All other values are then presented on the scale that creates. This normalization aids in interpreting the data because regardless of soil differences, 100% is always the point at which the soil is full.
How is Field Capacity determined?
Field Capacity can be determined several ways including lab tests, however it can also be determined by watching for key moisture trends that occur after sufficient irrigation events. Semios uses AI to watch for these trends and to determine FC based on multiple of these events. On top of this, because is is using AI it is continually scanning the data for improved data that will help to refine and adjust the “calibration”.
Why is my soil moisture showing more than 100%.
FC represents that amount of water a soil can hold onto without allowing drainage. Just because that is the amount of water it can hold onto, it doesn’t mean that it isn’t possible to fit more water into the soil. This extra water doesn’t stay there though. If your soil is over 100% it is likely because it is currently, or was recently irrigating. The extra water will pass through the upper layers of the soil and move into deeper, drier soil. If your soil stays at more than 100% for more than 24 hours, contact your Semios representative for extra support.
How do I know when to irrigate?
The AWC chart is split into four colored sections.These colored sections are a guide to when to irrigate. The default settings that the system come set with have the green band set between 100% and 70% AWC. In general, if you keep the soil moisture in the green band, the crop should experience little to no stress.
The chart also includes a forecast of what the AWC will do over the next several days. You can use this forecast to estimate when would be a good time to irrigate. Ideally, irrigation will start before AWC drops into the yellow band.
Are the default irrigation threshold settings best in my situation?
Default threshold settings on the AWC chart include the size and location of the colored bands. The default settings are a great place to start. Most do not change them. However, if you have extra insight into your soil or crop, you may want to adjust them. Or, you may want to adjust them during different periods when you may want to increase the time between irrigations to allow for regulated deficit irrigation.
In general, if you were to change them, the green zone should be smaller for heavier soils like loam and clay-loam, and it can be larger for sandy soils.
Thresholds can be adjusted in the settings menu next to the AWC chart.
What depths should I monitor?
There are multiple different thoughts on this matter. I will present two.
The active rootzone. The active rootzone does not represent the entire area where roots are present. Instead, it is the area where most of the water and nutrition are taken up. In full-sized trees this is usually from about 6” to 24”. Dwarfed tree rootzones are slightly smaller, from 2” to 18”. Annual crops vary, but generally 2” to 12” captures the active rootzone.
Leaching Zone. Some growers like to know when they have over-irrigated and so will watch deeper soil depths to see if water is moving beyond the rootzone. They will monitor as deep as 48”. Some also believe it is possible to “bank” water to be used later at deeper depths. Although banked water is not usually accessible until the plant is under significant stress, the water will eventually be used when enough stress is induced.