Novel Sensing System for in-Situ Monitoring Soil Water Content

Water shortage is a world-wide issue and currently 70% of global freshwater consumption is agriculture. Unfortunately, the water use in agriculture is often inefficient with only a fraction of the water diverted for agriculture effectively used in plant growth, with the rest lost via soil drainage, evaporation, or transpiration. Water efficiency in agriculture has been extensively researched. Conversion from traditional furrow irrigation to micro-irrigation methods has been shown to reduce irrigation needs for about 50% while improving yields. Universally applied micro-irrigation solutions many not realize improved yields due to different crop requirements and the high specificity of agriculture practices. Improving the efficient use of water still demands the development of new technologies that cost less and save energy by accurately assessing crop status for a given location, climate and soil condition. To reach this, it is critical to monitor the soil water content (SWC) or soil moisture in a real-time manner. 

Many technologies have been developed and are currently widely used to determine the SWC. Most of these technologies are stationary. That is, the soil samples have to be collected in the field and it is, then, immediately stored in a special bag to keep the moisture and delivered to a laboratory for the test. This is time consuming process and adds additional cost to the characterization of SWC. Technologies, such as time domain reflectometry, have the capability to determine the in-situ SWC in a real-time manner, but it is only for a localized spot. Therefore, it is very critical to find a right spot to perform the characterization/analysis, which is somehow arbitrary and requires trained personnel. For a farm land, many spots have to be analyzed to determine the average SWC for the farm land.

This work is designed to develop a rationally new technology that can monitor the average SWC over a farm land. The technology is for in-situ analysis and has the potential to be developed as a wireless network. A sensing system is designed and built to determine the average SWC over a farm land, as illustrated in Figure 1. A few spots (i.e., the corners and center of a farm land) are selected as the nodes, as shown in Figure 1 (a). At each node, an electrical pole (i.e., probe) will be installed as illustrated in Figure 1 (b). Each probe contains a few electrodes which are located at different depths underground. The part of the electrical pole outside of soil is either an electronic deliver or electronic receiver. Two probes form a measuring sensor/system, such as contacting with a lock-in amplifier, which determines the average SWC over the area, as shown in Figure 2 (c and d). The system is tested in real farm land to determine the maximum area can be analyzed. Other commercial sensors are used to validate the sensor system for SWC determination.  

Figure 1 (a) Schematic of sensor networks for measuring SWC, (b) electrical pole, and two probes to form a measuring sensor: (c) top view and (d) side view.