Monitoring Variably-Saturated Water Flow in Volcanic Ash Soils Using the Matric Potential Sensor, MPS-1.

In this study, we used the new matric potential sensor (MPS-1) from Decagon Devices to monitor variably-saturated water flow in soils. The sensor measures the dielectric constant of a porous material disk that is in contact and at equilibrium with surrounding soils. The observed dielectric constant is then used to find the water content of the porous material. Because of a known unique water retention characteristics of the porous material, from the water content, we can obtain the water potential through the retention curve. This relatively new sensor is known to be low cost, low maintenance, and longevity compared to traditional and standard water potential measurement devices. However, the application is still limited. The objective of this study was to monitor variably saturated water flow in Japanese volcanic ash soil, known as Kuroboku, using the MPS-1. In a small cylindrical lysimeter, two MPS-1 sensors were installed at 5 and 15 cm depths with tensiometers and Decagon soil moisture sensors, 5TE, installed at the same depths. After a given amount of water was applied three times with certain intervals at the surface, changes in the soil water potential and the water content were monitored by those sensors. Although there were no significant time lags between the outputs of the sensor and the tensiometers, large discrepancies in readings between them were observed when the soil was relatively wet. This result suggests that individual calibration is necessary for those sensors to improve the performance of the sensor for wet soils. In general, infiltration processes were well monitored using the MPS-1 sensor. This sensor possesses a lot of potential to be used in many applications where soil water potentials need to be monitored.