Introduction

Units

All inputs and results use the units chosen on the page Processes & units: a length unit (mm, cm, m), a time unit (s, min, h, d, yr) and a mass unit, which is only a label. As in HYDRUS-1D, the dimension of a quantity is given in the help and the tooltips with L (length), T (time) and M (mass), e.g. Ks [L/T], α [1/L], concentrations [M/L³].

Unit conversion

When the length or the time unit is changed with Convert all values when a unit changes switched on, every input is rescaled:

value_new = value_old · f_L^a · f_T^b

with f_L and f_T the ratios of the units and a, b the dimension of the quantity. Converted are heads, depths and lengths, conductivities and fluxes, α (1/L), times and print times, atmospheric records, root uptake parameters, heat parameters, transport parameters (dispersivities, diffusion coefficients, rate constants, Freundlich / Langmuir coefficients Ks and η with the exponent β), concentrations (M/L³), the initial profile and the tables of tabular soil properties.

Not converted are the mass unit (only a label), dimensionless quantities, temperatures, activation energies and the meteorological inputs that HYDRUS reads in fixed units (radiation in MJ/m²/d, wind speed in km/d, sunshine hours, crop height and measurement heights in cm, latitude, altitude in m, the interception constant in mm). The time and rooting-depth columns of the meteorological data are converted.

With the option switched off only the unit label changes and all numbers stay as they are – useful when the values were entered in the wrong unit label.

Changing a unit discards the results and the measured data of the project (they are in the old units).

Units of the heat parameters

The thermal parameters are given in the project's length and time units with the mass in kg, as in HYDRUS-1D: the coefficients b1, b2, b3 of the thermal conductivity in kg L T⁻³ K⁻¹ (W/m/K = kg m s⁻³ K⁻¹ in SI units) and the volumetric heat capacities Cn, Co, Cw in kg L⁻¹ T⁻² K⁻¹ (J/m³/K). For cm and days:

SI value	in cm and days
1 W/m/K	100 · 86400³ = 6.45 × 10¹⁶ kg cm d⁻³ K⁻¹
1 J/m³/K	86400² / 100 = 7.46 × 10⁷ kg cm⁻¹ d⁻² K⁻¹

The buttons on the page Heat transport enter typical values already converted to the project's units.

Vapor flow depends slightly on the length unit, as in HYDRUS: the capacity term of the equivalent vapor content contains the unit conversion factor in a way that is exact only for metres. Rusteau keeps the HYDRUS expression so that its results match HYDRUS 4.17 (vapor flow).