Soil moisture sensors are used for measuring the water content of soil. Multiple soil moisture sensors are combined to form a soil moisture probe. Frequency domain sensors such as a capacitance sensor is the most common sensor that is being widely used for commercial purposes.
Inexpensive sensors consisting of two electrodes and probes for measuring the soil resistance are often used for residential purposes. Time domain reflectometry (TDR) and time domain transmission (TDT) are also used for measuring the soil moisture content. A higher average dielectric constant for the soil is caused by a higher water concentration.
The soil moisture sensors measure the propagation speed in a buried transmission line to measure the average dielectric constant. These sensors provide real-time data and improve the irrigation efficiency. The sensors are easy to install and require very less maintenance.
Wu X et al (2011) from the University of Michigan studied the problem of optimal placement of sensors with respect to soil moisture sensing. They also suggested that by combining soil moisture data having unique features, more scalable algorithms can be developed for this technology.
As a result, a coarse-grained monotonic ordering of placements with respect to their soil moisture levels that changes over time can be accomplished. This in turn encourages a clustered sensor placement system, where the locations are grouped to form clusters corresponding to the ordering of the mean, with the number of sensors placed in each cluster determined based on the ordering of variances.
In addition, the team conducted several extensive experiments on a set of three-dimensional soil moisture data obtained from a novel soil moisture simulator. The results showed that the clustering approach performs efficiently, and is less sensitive to errors in data.
Paige G.B et al (2008) from the University of Wyoming studied the field performance of multiple soil moisture sensors in a semi-arid rangeland. The sensors that employ either the capacitance technique or time domain reflectometry for measuring the soil’s dielectric constant are arranged in shallow soil in a small semi-arid shrub covered subwatershed in Southeastern Arizona.
These sensors are tested under various conditions during three seasons with respect to the responses to natural drying and wetting by using water balance and infiltration simulation models. The experimental results showed that each of the sensors respond to most of the precipitation events. However, each of the sensors varied with respect to time and magnitude. Upon comparing the sensor responses or the infiltration model to individual precipitation events, no consistent or distinct trend was detected.
Traditional methods for measuring soil moisture have been limited to in situ measurements and hence remote sensing techniques are used most often. Hyperspectral sensors with narrow band widths and fine spatial resolution are best alternatives to traditional multispectral analysis of soil moisture in landscapes having high spatial heterogeneity.
Some of major applications of soil moisture sensor technology include the following:
- Wastewater reclamation
- Landfill management
- Irrigation management
- Crop yield forecasting
- Issuing early warning of droughts.
In the agricultural field, it is essential to determine the soil water content for assessing the profitability and viability of the business. It is also critical to restrict the usage of water with the increasing needs and cost of water. However, the soil moisture data corresponding to the surface of the soil can be acquired with the currently available novel technologies. There is a need for new technologies to collect data on sub-surface water measurements with the ever-increasing demand on water resources.
Moreover, energy fluxes present at the atmospheric interface or land surface and water exchange processes are highly impacted by surface soil moisture. Hence accurate measurement of temporal and spatial variations of soil moisture is required for several environmental studies. Recent advancements in satellite remote sensing technology have proved that various remote sensing techniques are suitable for measuring soil moisture. To support this development, there are now wireless irrigation control systems to help conserve water and help keep the turf in a health condition:
Microwave remote sensing is one of the most effective techniques for estimating soil moisture. In addition, soil moisture retrieval algorithms that integrate physically based model predictions, spaceborne measurements from multiple sensors along with in situ observations are also helpful in measuring soil moisture.