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Grapevines are thought to be a relatively drought-tolerant species. However, in commercial production, they are usually irrigated to increase yield and profitability. The crop water requirements for grapevine varies by region. Through years of research, grapevine crop water requirements for different climatic regions have been determined and are generally well understood. In the San Joaquin Valley, for example, grapevines require between 24 to 36 inches of water each season. Knowledge of how much water grapevines need at each growth stage during the season is key to efficient irrigation management. This is called irrigation scheduling in irrigation parlance. There are various approaches to irrigation scheduling. A common method is to track the soil water budget by accounting for precipitation, irrigation and evapotranspiration.
Soil moisture sensors offer a more direct way to estimate the amount of water in the soil at a given time, an attribute that makes them helpful for irrigation management. At the most basic, they are used to continuously track soil moisture status such that when the moisture level drops below a certain value (known as management allowable depletion), the irrigation system is turned on to replenish the soil profile.
Besides wide usage by growers in the United States, soil moisture sensors are also extensively used in irrigation research, and many studies have shown that they can help with water savings. For example, a recent study in southern Italy showed water savings between 10% to 17% by utilizing a soil moisture sensor-supported irrigation decision support system. Soil moisture sensors are also helpful for managing regulated deficit irrigation or partial rootzone drying, which are irrigation techniques that have been shown to improve grape quality and improve water use efficiency in grapevine production. In places like California’s Central Valley, where spring rain usually replenishes the soil with moisture, some water savings could be realized by delaying the onset of irrigation. However, to know how much water has been retained in the soil, one would need a device like a moisture sensor.
Soil Moisture Sensor Technology
This article discusses the main soil moisture sensing technologies available to growers. In the market, there are several brands of soil moisture sensors that are suitable. Disregarding brand, most of these moisture sensors are based on the measurement of either soil moisture tension or the dielectric constant of the soil.
Soil moisture tension is basically a measure of the force with which water is held within the soil. Tensiometers, which measure soil moisture tension and are well known in the irrigation community, are one of the oldest soil moisture sensing technologies. Another common moisture sensor that is based on measurement of soil tension is the resistance block (sometimes called gypsum blocks), which measures the electrical current through the soil, which is then translated into soil tension. Tensiometers and resistance-block devices typically report soil moisture status in units of centibars, or bars, or megapascals, which are not readily translatable to soil water amounts. However, vendors of such devices often provide further information to translate the tension readings to estimates of the amount of water in the soil. Compared to other common soil moisture sensors, basic tensiometers have a narrow measurement range (0 to 100 centibars) and therefore should be cautiously used, especially in light-textured soils like sands. At a soil water tension of about 85 centibars or above, there is a high risk of a tensiometer malfunctioning. This limitation has been overcome in more recently developed advanced tensiometers, which have wider measurement ranges.
There are two common types of devices based on the measurement of soil dielectric constants. These are TDRs (time domain reflectometry) and FDRs (frequency domain reflectometry). Basically, the dielectric constant is the measure of a nonconducting material to conduct an electromagnetic wave. The dielectric constant of a soil is dependent on the amount of water it contains. TDRs and FDRs typically report soil moisture in terms of volumetric water content (VWC) percentage, which, compared to tension devices, is a more direct measurement, considering that the goal in irrigation management is to know how much water is in the soil. VWC is basically the amount of water in a unit volume of undisturbed soil at any given time.
These days, many soil moisture sensor vendors offer cloud-data services as an optional addition to the hardware. Typically, these services involve processing and uploading the soil moisture data on user-friendly online platforms, where a client can access their soil moisture data through smartphones, tablets or computers, making real-time data available any time it is needed.
‘It is very important that moisture sensors are installed such that they make good contact with the surrounding soil.’
Soil Moisture Installation in Grapevines
Installing soil moisture sensors in grapevines is not markedly different from how it is done for other crops. It is important to install the soil moisture sensor close to the plant to get data that is representative of plant water use. For grapevines, it is reasonable to install the moisture sensor within 1 to 2 feet from the base of the trunk of the vine. This is because the highest root density is close to the base of the trunk of the plant.
Although mature grapevines are known to be able to extract moisture from depths of 5 feet or even more, as reported by a recent study conducted in Lodi, Calif., for irrigation management purposes, monitoring soil moisture to a depth of at least 4 feet is reasonable. For vineyards with cover crops, moisture sensors could also be installed between rows to monitor water use of the cover crop so that it can be accounted for in irrigation application amounts. This is especially important in vineyards that maintain cover crops during the period of grapevine peak water demand.
Generally, it is advisable to install more than one soil moisture sensor in a vineyard. A grower can use a soil map to guide their decision on where to install sensors. Field-scale soil maps are available for much of the continental United States. An additional factor to consider is to ensure sensors are installed at locations where there are healthy grapevines (i.e., grapevines expected to reflect the true water use across the orchard).
It is very important that moisture sensors are installed such that they make good contact with the surrounding soil. This is especially important for sensors that have an oblong architecture and thus must be installed into drilled holes. In circumstances where drilling a precise hole is challenging, an installer should make a slurry using soil drilled out of the hole and then pour the slurry back into the hole before (or while) installing the probe.
It is important to note that most commercially available soil moisture sensors estimate water in a small volume of soil that immediately surrounds the probe. Considering that soil texture naturally varies greatly in the field, both horizontally and vertically, the way water redistributes itself within the soil following irrigation is not homogeneous. It is therefore important to evaluate the reported soil moisture data after installation to ensure the data reported is related to wetting and drying periods and patterns.
Soil moisture sensors are an integral part of the suite of options that growers can draw from as they seek to improve their water use efficiency. Also, with advancements in digital agriculture, soil moisture sensors will remain key for gathering data that is used for ground-truthing and optimizing water use across various agricultural production systems.
Resources
Garofalo, S. Pietro, Intrigliolo, D. S., Camposeo, S., Alhajj Ali, S., Tedone, L., Lopriore, G., De Mastro, G., & Vivaldi, G. A. (2023). Agronomic Responses of Grapevines to an Irrigation Scheduling Approach Based on Continuous Monitoring of Soil Water Content. Agronomy, 13(11), 2821. https://doi.org/10.3390/agronomy13112821
McCarthy, M.G., Loveys, B.R., Dry, P.R. and Stoll, M. (2000). Regulated deficit irrigation and partial rootzone drying as irrigation management techniques for grapevines. Water Reports, 22. https://www.fao.org/4/y3655e/y3655e00.htm#TopOfPage
Peacock, B. (). Water Management for Grapevines. University of California, Tulare County. Pub. IG1-95. https://ucanr.edu/sites/default/files/2011-03/82035.pdf
Wilson, T. G., Kustas, W. P., Alfieri, J. G., Anderson, M. C., Gao, F., Prueger, J. H., McKee, L. G., Alsina, M. M., Sanchez, L. A., & Alstad, K. P. (2020). Relationships between soil water content, evapotranspiration, and irrigation measurements in a California drip-irrigated Pinot noir vineyard. Agricultural Water Management, 237, 106186. https://doi.org/10.1016/J.AGWAT.2020.106186
