The effect of gradual decrease in hydraulic conductivity with depth on groundwater flow in small drainage basins

Distribution of hydraulic conductivity is critical for modeling of regional groundwater flow, which is critical for understanding a variety of geologic processes. Unfortunately, few studies on regional groundwater flow have accounted for the gradual decrease in hydraulic conductivity with depth. Following Freeze and Witherspoon’s (1967) study, the distribution of hydraulic head and resulted flow systems in a synthetic basin under a series of different rates of hydraulic conductivity decrease with depth are obtained using MODFLOW and PMPATH. It is found out that when all conditions except for the decay exponent of hydraulic conductivity are fixed, the development of local versus regional flow systems varies greatly. With an increase in decay exponent of hydraulic conductivity, regional flow systems are becoming more difficult to form, which results in an increase in penetration depth of local flow systems. Furthermore, the decrease in hydraulic conductivity with depth would also have effect on the total recharge amount as well as discharge amount in different discharge zones. With an increase in decay exponent of hydraulic conductivity, the amount of total recharge decreases significantly, less water recharged in the whole basin can reach the regionally lowest discharge zone, and more water discharged in the regionally lowest zone is recharged from the nearest recharge zone. Therefore, to analyze hydrologic problems related to regional groundwater flow, the gradual decrease in hydraulic conductivity with depth is should not be neglected. Equipotential lines (dashed lines) and random pathlines (red lines) under different decay exponents (A) of hydraulic conductivity. (a) A=0 1/m; (b) A=0.003 1/m; (c) A=0.01 1/m.