LYSIMETERS. INSTRUMENTS TO KNOW THE SOIL WATER BALANCE
We all know the importance of water in plants. It is a fact that has been known for millennia. Where there is water there is life and some of the first species to appear are plants. At Gardenprue we have already talked about irrigation, water in the plant and in the soil, but today we are going to delve slightly into a somewhat more unknown concept in the measurement of soil water. Lysymmetry.
THE WATER ON THE GROUND AND ON THE PLANT, A SMALL TOILET.
Before talking about how to measure the water contained in the soil, we should know in what forms this water appears. We have already told this in previous articles so if you want to consult it then we leave you the link about the water in the ground and its behavior. We can count on the fingers of one hand the forms in which the water is on the ground. Free water, capillary and hygroscopic. The one that interests us for crops is the capillary, which is easily available for plants.
The same goes for the water in the plant. We also know how it behaves and how the plant kingdom makes use of it. In another article that our collaborator Eduardo Casasnovas wrote, he told us how water interacted in the plant and the importance of watering well, in an optimized way so as not to incur in water imbalances both by default and by excess.
MOISTURE MEASUREMENT VS WATER BALANCE
In the first link about water in the soil that we have given you here, it talks about the humidity of the soil. This concept has nothing to do with lysymmetry. Let’s differentiate them.
El contenido en humedad de un suelo medido de forma correcta debería ser una vez se haya drenado el sobrante de forma natural. Con esto conseguimos saber el contenido en agua que queda retenida, que al fin y al cabo, parte de ese agua es la que las plantas van a utilizar en sus procesos. Este contenido en humedad se puede medir fácilmente por un tensiómetro, o sonda de humedad que nos puede dar mediciones puntuales en el tiempo con la frecuencia que se estime.
However, the lysimeter is much more complex in the data it offers. The water in a soil undergoes changes and physical processes of all kinds, as we have seen in previous articles. The lysimeter, thanks to its operating principle, can record the water outlets on the ground. As inputs mainly in an agricultural plot we can have 2 which are the rainfall and irrigation that we can give it. We easily measure this with rain gauges, flow meters, etc.
And as outputs we can have drainage, evapotranspiration (if there is plant cover) and the water that remains on the ground. With a tensiometer we get to know the humidity of the soil at a given moment that in theory should be when the drainage water has already passed. What is not easy is to determine when this happens on the ground. The lysimeter manages to measure the amount that is lost by evapotranspiration and drainage, therefore, we can also know the volume of water that remains in the soil.
Evapotranspiration is a widely used measure since it allows us to know the water consumption of crops and therefore gives us exactly the water needs to provide in the form of irrigation. It can be calculated using well-established formulas such as Penman-Montheith that estimate it indirectly. However, the lysimeter can calculate it directly after calibration and validation.
We are not going to go into the types of lysimeter today because we would spend hours and each one is a world but basically we can tell you that it depends on what you want, we can have lysimeters that calculate the soil drainage water and the water balance and others in which we can recover the soil solution to determine the composition of the soil solution and see possible contaminants, excess or lack of nutrients for the crop, etc. They are called suction lysimeters.
ALTERATION OF SOIL SAMPLING BY LYSIMETER
One of the most pressing battles in the measurement of lysimeters is the separation of the soil volume being evaluated. Being separated from the real floor, there may be variations in the measurements although today, there are already materials and designs that compensate and eliminate the error or “edge effect” that can cause that volume of floor necessary for the instrument.
Another day we will talk about different types of lysimeters.