Soil moisture sensors for irrigation: tension or water content, and where to put the probe
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A water balance calculated from the weather tells an irrigator how much water the crop has probably used. A soil moisture sensor tells him what is actually left in the ground, at one spot. Most precision irrigation systems, FIGARO's included, combine the two. But the word “sensor” covers instruments that measure different physical quantities, in different units, over very different volumes of soil. Choosing and placing one well matters more than the brand.
Two questions, two kinds of measurement
Soil sensors answer one of two questions.
- How much water is in the soil? This is the volumetric water content, the volume of water per volume of soil, written as m³/m³ or as a percentage. It fits naturally into a water balance: a change of 0.01 m³/m³ over a 50 cm root zone is 5 mm of water.
- How hard does the plant have to pull to get it? This is the soil water tension (or matric potential), usually given in kilopascals. A wet soil holds its water loosely, near 0 kPa; as it dries, the remaining water is held more tightly and the tension rises.
The two are linked by the soil's water retention curve, which depends on texture and structure. The same 20% water content can be comfortable in a clay and close to stress in a sand. This is why a water content reading means little until it is set against the soil's own reference points, while a tension reading can be compared from one field to another more readily. Plants respond to tension; water budgets are written in content.
The main types of sensor
| Sensor | Measures | Strengths | Limits |
|---|---|---|---|
| Tensiometer | Tension, directly | Simple physical principle, no soil calibration, reads the wet range accurately | Stops working when the soil dries beyond its range; needs refilling and servicing |
| Granular matrix (resistance) block | Tension, through electrical resistance | Cheap, little maintenance, works in drier soil than a tensiometer | Slow to respond, less precise near saturation and in coarse sands; needs temperature correction |
| Capacitance / FDR probe | Water content, through the soil's dielectric permittivity | Continuous logging, multi-depth profile probes, moderate cost | Small zone of influence; sensitive to air gaps, salinity and temperature; needs calibration for accurate values |
| TDR probe | Water content, through the travel time of an electrical pulse | Accurate, less affected by soil type than capacitance sensors | More expensive electronics; used more in research and high-value crops |
| Neutron probe | Water content, through neutron moderation by hydrogen | Large measured volume, long a reference method | Radioactive source: licensing, training and transport rules make it rare on farms |
A tensiometer is a sealed, water-filled tube with a porous ceramic cup at the bottom and a vacuum gauge or pressure transducer at the top. As the soil dries, it draws water out through the cup and the gauge reads the resulting suction. Once the tension approaches atmospheric pressure, air enters the cup and the instrument must be refilled, which in practice limits tensiometers to roughly the first 80 kPa. That is enough for most drip-irrigated vegetables and orchards on light soils, which are kept fairly wet, and is the reason tensiometers remain common there.
Dielectric sensors, the most widespread in commercial systems, rely on a physical contrast: liquid water has a relative permittivity of about 80, against roughly 1 for air and a few units for the mineral grains of the soil. The bulk permittivity of the soil therefore rises steeply with its water content. In 1980, Topp, Davis and Annan published an empirical equation linking the two for a range of mineral soils, measured by time domain reflectometry; it is still the default conversion in many instruments. Capacitance and FDR (frequency domain reflectometry) probes estimate the same permittivity more cheaply, from the frequency of an oscillating circuit, at the cost of a greater sensitivity to salinity, temperature and imperfect contact with the soil.
From reading to decision
A sensor reading becomes an irrigation decision only through thresholds. With tension sensors, the usual approach is a refill point: irrigate when the tension in the main root zone reaches a value chosen for the crop, soil and stage, taken from local extension guidance or trials. With water content sensors, the reading is placed between two reference levels for that soil: an upper level reached a day or two after a thorough wetting (field capacity, in the field) and a lower level that should not be crossed. The difference between them is the water the crop can use comfortably, the same quantity that the FAO-56 water balance calls readily available water.
The simplest way to find those levels is to watch the sensor. After a heavy irrigation the readings jump, then fall quickly while the excess drains, and the decline slows as drainage stops: the level at the bend is a practical field capacity. The daily slope after that is the crop's water use. Many growers use the trend lines this way, without trusting the absolute value, which is sound practice when the sensor has not been calibrated for that soil.
Sensors and calculation are best used together. The weather-based balance gives a forecast for the coming days and a quantity to apply; the sensors check whether the soil really behaves as calculated, and reveal drift, such as a crop coefficient that is too high or a block that receives less water than planned. This was the logic of the FIGARO platform, which fed probe data into crop models and scheduling software (see the sensors behind the platform). At the Picassent citrus site near Valencia, readings from FDR probes at four depths were relayed automatically, along with data from the regional weather stations (the field data retrieval service).
Where to put the probe
A capacitance probe reads a few centimetres of soil around it; a tensiometer cup, a few cubic centimetres. Whatever is true of that small volume is assumed true of a whole irrigation block. Placement decides whether that assumption holds.
- Choose representative ground. Avoid field edges, wheel tracks, low spots where water collects and unusually vigorous or weak plants. If a block has two distinct soil types, it needs two stations, or two blocks.
- Measure at more than one depth. One sensor in the main root zone drives the decision; one below it shows whether water is draining past the roots. Rising readings at depth after each irrigation mean the doses are too large.
- Respect the wetting pattern. Under drip irrigation, water spreads in a bulb beneath each emitter. A probe too close to the dripper reads permanently wet; one midway between emitters may never see the water. Install it at a consistent distance within the wetted zone, near emitters whose flow is close to the block average: our article on drip irrigation uniformity explains why.
- Get good contact. Air gaps around the sensor are the commonest cause of nonsense readings. Follow the installer's method, usually a hole of the exact diameter or a slurry of the native soil, and give the soil a few irrigation cycles to settle before trusting the data.
- Keep a record. Note the depth, the distance to the emitter and the soil at each station. When readings change, the first question is whether the soil changed or the sensor did.
What sensors cannot do
Even a perfectly placed sensor is a point measurement. It cannot tell an irrigator that the far end of a lateral receives a fifth less water, and it cannot see the variability of a field it does not sample. Strategies that deliberately keep the crop slightly short of water, such as regulated deficit irrigation, depend on readings being reliable within a narrow band, so they need the most care in calibration and placement. Plant-based measurements, like stem water potential in orchards, are sometimes added for that reason. Used with these limits in mind, a few well-placed sensors remain the cheapest way to check that a schedule calculated on paper works in the field.
Sources
- Topp G. C., Davis J. L., Annan A. P. (1980), “Electromagnetic determination of soil water content: measurements in coaxial transmission lines”, Water Resources Research 16(3): 574–582, doi:10.1029/WR016i003p00574.
- Evett S. R., Heng L. K., Moutonnet P., Nguyen M. L., eds. (2008), Field estimation of soil water content: a practical guide to methods, instrumentation and sensor technology, IAEA Training Course Series 30, International Atomic Energy Agency, Vienna.
- Allen R. G., Pereira L. S., Raes D., Smith M. (1998), Crop evapotranspiration: guidelines for computing crop water requirements, FAO Irrigation and Drainage Paper 56, Rome.