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Explanation:

Calibration variables: all except fsol are usually taken from measurements on-site during calibration. P0 is arbitrary but is usually something typical for the site.
N: Raw counting rate averaged over the nominal calibration period.
P: Pressure over the nominal calibration period.
P0: Reference pressure; must be the same P0 used in calculating the soil moisture timeseries. Can be calculated from International Standard Atmosphere https://crnslab.org/util/atmo.php
T: Air temperature (for specific humidity correction).
RH: Relative humidity (for specific humidity correction).
fsol: Solar activity factor. This can be obrained from https://crnslab.org/util/solar.php

Soil constants: these are assumed to be constant soil properties of the site.
ρd: soil bulk density
SOC: Soil organic carbon.
LW: Lattice water.
BM: Biomass.

Site location (optional): For scaling N0 to high latitude/sea level.
Latitude: site latitude.
Longitude: site longitude.
Year: year calibration was performed.

Additional notes:
For simplicity, values of N, P, T and RH represent the mean value over the calibration period. This allows the user to input a single value for each variable rather than an array of hourly values. Such a simplification is justified when the variable, particularly P, does not change much over the averaging period. If the change in pressure is <10mb then the error in N0 will be less than 0.1% and can be safely ignored.