Evaluating physical controls on conduit flow contribution to spring discharge

Abstract

The physical controls on the contribution of conduit to spring discharge, quantified as the ratio of conduit flow to spring discharge (Qc/Q), are evaluated through numerical simulations using MODFLOW-Conduit Flow Process. The controlling factors include the ratio of conduit surface roughness to conduit diameter (ε/D), the hydraulic conductivity ratio between the conduit wall and the porous matrix (Kc/K), the ratio of groundwater recharge to the hydraulic conductivity of the porous matrix, and conduit density (Cd) defined as the ratio of total conduit length to springshed area. For a single conduit, Qc/Q increases with Kc/K when flow in the conduit is laminar. When flow is turbulent, Qc/Q is stable and then decreases with ε/D, increases with Kc/K, and decreases with R/K due to the limited exchange capacity between matrix and conduit (i.e., small Kc/K). The results of hypothetical conduit networks show that Qc/Q increases with Cd but is not sensitive to the two sets of network structure evaluated. The sensitivity analysis for the Silver Springs in Florida shows that the spatial heterogeneity of conduit diameter causes higher Qc/Q, and Qc/Q is most sensitive to the conduit density, followed by R/K, and then Kc/K.