MODFLOW-CFP is an efficient tool that helps us to model karstic conduits and learn about their interaction with groundwater flow.

A practical case where these elements have been conceptualized using ModelMuse will be explained in this post. The purpose is showing the capacities of MODFLOW-CFP for karst modeling.

To start, is necessary to have the CFP package. CFP version 2 can be downloaded from the following link from Dresden University.

Conceptualization

First of all, a general model of the area of study was generated. The following information was entered: topography, geology, recharge, evapotranspiration and a discharge zone of groundwater flow conceptualized as a drain.

The model is composed of 4 aquifers with 3 sub layers each. The temporal discretization is static. The solver used is PCG Preconditioned Conjugate Gradient.

Karstic conduit

For this model, a cavern at an elevation of 3950 masl. has been considered. This element discharges to a spring.

The karstic conduit can represented by a Line object, where the CFP condition was assigned. This package requires certain data to properly represent the element. The required parameters and the values considered for this model and the following:

Diameter: 0.5 m
Tortuosity: 1 m
Roughness height: 0.01 m
Lower critical Reynolds number (turbulent to laminar): 2000
Higher critical Reynolds number (laminar to turbulent): 4000
Conduit wall conductance: 0.01 m2/s

To represent the spring, the package CFP has the capacity to use a Fixed Head, which was located in the final node of the conduit with a head equal to the ground level.

Results

The water balance of the karstic conduit indicates that the spring releases a flow equal to 26.56 l/s. This flow is given by the matrix along the length of the conduit.

The matrix water balance indicates that the recharge contributes with 41.4 l/s to the system, the river has a flow of 12.1 l/s and the evapotranspiration has a rate of 2.73 l/s.