We offer the option to display soil moisture measurements in mm instead of volume percent. We have this option because for many growers it says more about the amount of available soil moisture and possible replenishment capacity. The precipitation in a weather forecast is also displayed in mm, making the two graphs more comparable.
How it works
To get from volume percentages (volumetric water content or VWC) to mm moisture, a short calculation is needed. Below we will explain this calculation:
In this example we assume a clay soil with a field capacity of 53% VWC and a wilting point of 28% VWC. The sensor measures the following values in three layers of 10 cm:
0-10cm = 35% VWC
10-20cm = 38% VWC
20-30cm = 41% VWC
The available soil moisture in mm is then:
([measuring value VWC] - [wilting point]) * [height per layer in mm] = [mm available soil moisture per 100mm]
In other words:
0-10cm: 35% - 28% = 7% of 100mm = 7mm per 100mm
10-20cm: 38% - 28% = 10% of 100mm = 10mm per 100mm
20-30cm: 41% - 28% = 13% of 100mm = 13mm per 100mm
Together the total available moisture is 30mm over a depth of 300mm. (To check: average 10% available moisture at a depth of 300mm; 10% of 300mm = 30mm available moisture).
What you can do with it
As a grower, you then want to know how much water the soil can hold at maximum. This is where the given field capacity comes in handy.
To bring the zones to field capacity, the following amount of water must be added:
0-10cm: 53 - 35 = 18%. 18% VWC is 18mm on 100mm ground.
10-20cm: 53 - 38 = 15%. 15% VWC is 15mm on 100mm ground.
20-30cm: 53 - 41 = 12%. 12% VWC is 12mm on 100mm ground.
All in all, this is 45mm water (because an average of 15% over 300mm is 45mm water).
So, we will first work with the available soil moisture, later we will show what the replenishment capacity will be.
To ensure that the calculation is correct it is necessary to have the correct data of the soil, field capacity and wilting point play an important role in these calculations. We currently do this by linking standard data per type of soil. These are selected automatically based on the indicated soil type. One sandy soil is not the other, and even within a plot large differences are possible. At a later stage, we will also add the possibility to give custom values. This gives the possibility to specify certain values for both wilting point and field capacity for each (sensor) location and layer depth.