Maintaining soil water content within the readily available range is optimum concerning crop production and water use. In this regard, the continuous monitoring of soil water content is a crucial element for identifying the key parameters for sustainable agricultural water management. In this study, volumetric soil water content (θv) was monitored and analyzed in a bare soil agricultural field (Sakaecho experimental field of Tokyo University of Agriculture and Technology). The field consisting of volcanic ash soils was monitored from June 2016 to September 2017 using capacitance sensor (CS). The optimal range of readily available water for plant uptake was defined as the range between field capacity and depletion level (θDep). These values were determined from the soil water potentials and θv values measured using pressure plate and soil cores, respectively. The 16-month period monitoring result revealed substantial temporal variability in θv in response to rainfall, evaporation and deep percolation. The monitored values of θv were above the θDep (0.307 cm3 cm−3 measured at suction (pF) value of 3.0) throughout the monitoring period for the 10–20 and 20–30 cm soil layers. In contrast, for the surface soil (0–10 cm), the θv fell below θDep for 27% of the monitoring period despite the high rainfall during those periods owing to high evaporation and deep percolation. The below θDep results for the surface soil suggests the need to conduct continuous θv monitoring, to support decision for planning efficient irrigation water management to avoid yield loss of shallow-rooted crops and deep-rooted crops at their earliest growth stages as well as quality reductions due to moisture stresses.