mass_fraction_of_water_in_surface_soil_layer 1 [XXXX] "Mass fraction" is used in the construction "mass_fraction_of_X_in_Y", where X is a material constituent of Y. It means the ratio of the mass of X to the mass of Y (including X). A chemical species or biological group denoted by X may be described by a single term such as "nitrogen" or a phrase such as "nox_expressed_as_nitrogen". "Layer" means any layer with upper and lower boundaries that have constant values in some vertical coordinate. Quantities defined for a soil layer must have a vertical coordinate variable with boundaries indicating the extent of the layer(s). "Water" means water in all phases.