ROLE OF UNSATURATED-ZONE THICKNESS ON SPATIAL AND TEMPORAL VARIABILITY OF GROUND-WATER RECHARGE

Water generally percolates through an unsaturated zone between land surface and the water table in areas of diffuse infiltration prior to becoming ground-water recharge. The effect of unsaturated zones on ground-water recharge typically is neglected in humid areas where the thickness is less than a few tens of meters. Thin unsaturated zones (less than 1 m) in permeable soils (K's > 1 m/d) do not delay or attenuate a recharge pulse from a single infiltration event unless evapotranspiration from plants is actively removing water. However, thick unsaturated zones (greater than 15 m) in permeable soils delay and attenuate the recharge pulse such that multiple infiltration events may merge to form a much longer period of ground-water recharge that is not correlated to any single infiltration event. The delay and attenuation of ground-water recharge is increased as the permeability of the soils decreases; either as a function of soil texture or moisture content. The effect of moisture content is more pronounced in areas with distinctly wet and dry seasons.

Incorporating spatial and temporal variations in unsaturated-zone thickness alters the distribution of ground-water recharge. These variations affect model calibration and results. In ground-water models that neglect unsaturated zones, recharge rates generally are estimated independently of the model and applied regardless of unsaturated-zone thickness. Neglecting thin unsaturated zones where the water table seasonally intersects land surface and discharges ground water causes hydraulic conductivity to be overestimated. Model error and parameter uncertainty are increased where storage in and attenuation of recharge rates from unsaturated zones are not simulated. Examples will be used to illustrate effects of unsaturated-zone thickness that include (1) varying thickness as a result of a changing water table and (2) rejecting recharge in areas of intermittent discharge to land surface.