Management of favorable conditions for plant growth within partially wet soil volumes under drip irrigation presents a measurement challenge. Typically, soil water status is determined from a single point measurement or several point measurements within the wet volume requiring various assumptions for quantitative interpretation of total water available for irrigation decisions. We propose a new coaxial time domain reflectometry (TDR) probe capable of measuring the total soil water storage within well-defined soil volumes (referred to as the variable volume coaxial probe, VVCP). The measurement volume is contained between an array of stainless steel conductors embedded in the soil and arranged in a coaxial configuration with variable spacing. The average water content in the spacing between the inner and outer circular arrays of conductors is determined from travel time analysis of VVCP waveforms. Laboratory and field tests of the VVCP established that waveforms and travel time analyses resulted in dielectric permittivities and water contents similar to those obtained with standard three-rod TDR probes. For uniform distribution of soil water VVCP measurements were independent of conductor spacing or soil volume. Variable volume coaxial probe-determined water balance within a partially wet soil volume was in excellent agreement with the amount of water applied from a point source indicating accurate and integrative volumetric measurement.