A. The NCA-LDAS dataset currently provide soil moisture values in units of [m^3 m-3] for volumetric soil moisture. To convert to units of [kg m-2] for the total soil moisture amount in each layer, all you do is multiply the [m^3 m-3] value by the thickness of the layer in [mm]. The assumption is made that the density of the water in the soil is 1000 [kg m-3].
NCA-LDAS SM volumetric ----> [converting to] ----> NCA-LDAS SM amount
1000 kg 1 m layer in mm kg
volumetric soil moisture X ------- X ------- X ----------- = --
m^3 1000 mm 1 m^2
As an example, for the 0-10cm layer, just multiply by 100mm to convert the data from volumetric soil moisture to get units of [kg m-2]. For the 10-40cm layer, multiply by 300mm, and so on for the other soil moisture layers. The volumetric soil moisture values range between wilting point and porosity (e.g., ~0.01 to ~0.5). If we divide the volumetric soil moisture [m^3 m-3] by porosity [m^3 m-3], we will get the soil moisture fraction which ranges from 0 to 1.