# What is Darcy flow analysis?

The groundwater tools allow you to perform rudimentary advectionâ€“dispersion modeling of constituents in groundwater. Darcy flow analysis models two-dimensional, vertically mixed, horizontal, and steady state flow, where head is independent of depth.

## Darcy flow analysis

The purpose of Darcy flow analysis is twofold. First, it is used to check the consistency of groundwater datasets and to generate rasters of groundwater flow vectors. The standard output raster is the groundwater volume balance residual raster, which measures the difference between the flow of water into and out of each cell.

Since the flow calculations are performed through each of the four cell walls independently (flow is governed by the differences between adjacent cells), it is possible that more (or less) water may flow into a cell than out of it, resulting in a positive (or negative) volume balance residual.

Smooth, consistent input rasters with no sources or sinks should produce small residuals near zero. Large residual values indicate that the head raster is not reasonable with respect to the transmissivity, porosity, and thickness rasters. A head raster contains the values of the groundwater head elevation for each cell in the raster. The head is typically an elevation above some data, such as mean sea level. When large residuals are output, the input data would be inconsistent and produce meaningless results.

The second purpose of Darcy flow analysis is to calculate the flow field using Darcy's Law, as discussed below. The flow field is a vector field of groundwater seepage flow velocities and is expressed as two rasters, one for magnitude and one for direction.

The first step in groundwater flow modeling is to determine the flow velocity and direction at each point in the flow field. Darcy Flow does this and calculates the volume balance within each cell, which should be small in the absence of sources or sinks, such as wells, infiltration, or leakage. A zero volume balance residual indicates a balance between flow in and flow out of the cell. The flow field is assumed to be steady (constant in time). A thorough discussion of groundwater hydraulics can be found in several sources including Bear (1979), Freeze and Cherry (1979), and Marsily (1986).

### Darcy Flow

The standard output of Darcy Flow is the groundwater volume balance residual raster, which measures the difference between the flow of water into and out of each cell. The residual is used to check the consistency of groundwater datasets.

Since the flow calculations are performed through each of the four cell walls independently (flow is governed by differences between adjacent cells), it is possible that more (or less) water may flow into a cell than out of it, resulting in a positive (or negative) volume balance residual.

Smooth, consistent input rasters with no sources or sinks, such as wells, infiltration, or leakage, should produce small residuals near zero. Large residual values indicate that the head raster is not reasonable with respect to the transmissivity, porosity, and thickness rasters. In such a case, the input data would be described as inconsistent and produce meaningless results.

Darcy Flow can optionally create the Darcy Velocity outputs, described in the following section.

### Darcy Velocity

Darcy Velocity uses Darcy's Law to calculate the flow field. A flow field is a vector field of groundwater seepage flow velocities.

The flow velocities are expressed as two rasters, one being the magnitude and the other the direction.

Darcy Velocity is useful when the volume balance residual of Darcy Flow is not needed.

## References

Bear, J. Hydraulics of Groundwater. McGraw-Hill. 1979.

Freeze, R. A., and J. A. Cherry. Groundwater. Prentice-Hall. 1979.

Marsily, G. de. Quantitative Hydrogeology. Academic Press. 1986.