vadose zone A zone beneath the land surface and above the level of permanent groundwater or phreatic zone in which water has a hydraulic head less than atmospheric pressure and is retained by a combination of adhesion and capillary action. ENVO ENVO:00000328 Wikipedia: If the vadose zone envelops soil, the water contained therein is termed soil moisture. In fine grained soils, capillary action can cause the pores of the soil to be fully saturated above the water table at a pressure less than atmospheric. In such soils, therefore, the unsaturated zone is the upper section of the vadose zone and not identical to it. unsaturated zone environmental zone