BASEFLOW RECESSION FROM UNSATURATED-SATURATED FLOW

Water flow in Unsaturated zones is an important process in subsurface flow cycles. A mathematical model for a coupled unsaturated-saturated flow in an unconfined aquifer is presented. Semi-analytical solutions for hydraulic heads and discharges are derived using Laplace transform and Cosine transform. The effects of the lateral discharge of the unsaturated zone and aquifer compressibility are specifically taken into consideration. The lateral discharge of the unsaturated zone is significant when dimensionless constitutive exponent κ_D of the unsaturated zone less than 1, and becomes negligible when κ_D larger than 100. The compressibility of the aquifer has a non-negligible impact on the discharge at early times. For late times, the power index b of the recession curve -dQ/dt~aQ^b , is 1 and independent of κ_D, where Q is the baseflow discharge, t is time, and a is a constant lumped aquifer parameter. For early times, b is approximately equal to 3 but it approaches infinity when t→0. The present solution is applied to synthetic data and field data. The present solution matches the synthetic data better than both the linearized Laplace equation and linearized Boussinesq equation solutions, with a minimum relative error of 16% for estimate of hydraulic conductivity. The present solution is well applied to the observed streamflow discharge in Iowa, and estimated values of the aquifer parameters are reasonable.