Numerical simulation of groundwater flow and advective transport in the Walnut Creek watershed
Date
2000
Authors
Vlachos, Sarah Rebecca
Major Professor
Advisor
Simpkins, William W.
Jones, LaDon C.
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Abstract
A three-dimensional, finite difference, numerical groundwater flow model was constructed and calibrated to simulate groundwater flow in a tile-drained agricultural watershed in central Iowa under steady-state and transient conditions. The model was constructed using hydraulic head measurements, hydraulic conductivity and storativity values derived from pumping tests, calculated recharge, and measured evapotranspiration. A no-flow boundary was located at the watershed boundary, and head-dependent boundaries were located at the stream and tile-drains. Flow was simulated through two homogenous layers of uniform thickness representing oxidized and unoxidized late Wisconsinan till of the Dows Formation. Recharge and evapotranspiration were applied uniformly over the model domain. The model was calibrated to 1996 and 1997 water tables at 16 piezometer nests located throughout the watershed. Particle-tracking simulations were performed on the steady-state model. Particles were inserted at the water table along the watershed boundary and subwatershed divides to trace groundwater flow paths and to determine groundwater travel times. Results of the steady-state solution show that tile drains in the western portion of the watershed capture most recharge to groundwater, influence groundwater flow paths, and reduce groundwater travel times. These results support the hypothesis given by Eidem et al. (1999) that tile drains remove a large portion of N03-N entering into the shallow groundwater system.
Groundwater captured by tile drains is discharged into Walnut Creek thereby providing a rapid pathway for agrichemicals from cropped fields to Walnut Creek. The calibrated steady-state solution was used as initial conditions for a transient simulation. Results of the transient solution suggest a non-uniform distribution of recharge and evapotranspiration due to variation in land use and topography across the watershed. The relationship between simulated and observed hydraulic head at each calibration site also suggest spatial distribution of storage in the oxidized till. A mass-balance of the transient model shows that most water removed from the shallow groundwater system occurs through tile drains and that creek discharge is supported primarily by tile drains.
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