Have a look at the graphs below with the live measurements of the soil moisture station at the RMA office in Dwingeloo in the Netherlands.
Soil, where plants grow, can contain a certain amount of water. When soil is completely filled up with water it reaches the point that is called Field Capacity. At this soil moisture level the soil is filled up to the maximum water holding capacity. Additional water added to the soil will percolate away immediately.
Plants take water from the soil for growing. This process is called evapotranspiration. Plants can take the easily available water from the soil as long as the water is not being held by the soil too tight. Beyond this point crops are starting to suffer from drought stress. We named this point the irrigation trigger. The moment that irrigation should be started to avoid yield losses.
Any soil has a certain amount of water readily available to the crop. One can compare this to the fueltank of a car or tractor.
Our sensors measure the amount of water in the soil at every 10cm depth. The curves go up because of rain or irrigation. The curves go down because of the crop using the water.
As the major uptake by plants from the roots is happening in the top 30cm of the soil, we create a separate management graph to guide the irrigations. The readings at 10, 20 and30cm are being summed. This graph runs from Field Capacity to the Irrigation Trigger.
The gauge is actually the fuel tank of the soil. At blue the soil is full of water and at red all the readily available water has been used up. The colours match the management graph for the rootzone. The values indicate the Soil Moisture Deficit in mm.
The nearby weather stations is used to calculate the reference evapotranspiration (ETo). The readings are shown together with the readings of the raingauge.
Our sensors also measure the soil temperature at two depths.