To calculate soil moisture the NRCS uses the Soil Moisture Equation developed in cooperation with the Agricultural Research Service (ARS). The equation tracks the actual soil moisture by volume. This work has been peer reviewed and published by Seyfried, M.S., L.E. Grant, E.Du, and K Humes. 2005. Dielectric loss and calibration of the hydra probe soil water sensor. Vadose Zone J. 4:1070-1079.