Contents

...

The geometrical conceptualisation and the unsaturated and saturated zone models are described in more detail below.

Figure 1. Conceptual illustration of the GN1D model structure, showing one bank of the river reach only.

Geometrical Conceptualisation

...

1. The position of the left hand and right hand riverbanks. The groundwater model uses the positions of the left and right banks to determine when overbank flow events occur.
2. Manning's N and the link slope. Manning's N and the slope parameters can be used, if desired, to generate a streamflow rating curve from the cross section. Alternatively, the user can calculate and input their own rating curve.

Figure 2. Cross-section conceptualisation.

Groundwater Model Geometry

...

• The subsections start either side of the riverbanks, as the groundwater model does not extend underneath the river.
• The left and right extents of the subsections are implemented as no flow boundaries.
• The depth of the subsections (z-coordinate) is defined by the user and is implemented as a no-flow boundary.
• The initial groundwater table height is constant across the subsections and is set by the user.

Figure 3. Example illustrating the representation of a cross-section within the groundwater model using 4 subsections per bank (grey shading).

Subsection height calculation

...

Diffuse infiltration takes as input a time series of rainfall (or other diffuse source) intensities. The Mein and Larson equations model diffuse infiltration in two stages as illustrated in Figure 4 (Mein and Larson, 1973). The first stage predicts the volume of water that will infiltrate before the soil surface becomes saturated. From this point, surface runoff begins and the infiltration capacity is predicted by the Green-Ampt equation.

Figure 4. Infiltration rate as a function of time for steady rainfall.

 
Additional assumptions:

• The depth of ponded water at the surface is negligible
• The rainfall is of constant intensity until run-off begins.
• The soil is a homogeneous medium.

...

Equation 1 is derived from Darcy's law using conservation of mass and assuming that transmissivity is homogeneous (constant) and isotropic (equal in all directions).

Figure 5. Illustration of the finite difference scheme.

 
The GN1D model solves equation (1) for groundwater head using an explicit finite difference scheme (illustrated in Figure 5):

...

Equation (2) is valid for lateral flow in a confined aquifer and can also be applied to an unconfined aquifer providing that the spatial variation in head is small compared to the aquifer thickness. In the context of estimating stream – aquifer interactions, this requirement is generally met if the stream is not deeply incised compared to the unconfined aquifer thickness. The phreatic surface (i.e. the groundwater table) can be highly variable close to things such as extraction bores. However, at the spatial scale at which the model is designed to operate, the localised impacts of groundwater extractions are unlikely likely to be less significant.

Equation (2) becomes unstable if the following criteria is not met:

...

The GN1D model settings and parameters are described in Table 1. Typical values of the parameters for the Mein-Larson infiltration equations are given in Table 2.
 

Table 1. GN1D model settings and parameters.

Table 2. Average values of Green-Ampt parameters by USDA soil texture class (adapted from Rawls et al., 1983).

...

Each time step, the GN1D model requires the river stage, which is calculated by the link using a rating curve. Other inputs are configured by the user and are summarised in Table 3.
 

Table 3. GN1D model data requirements, including typical units.

...

Mein, R.G., C.L. Larson (1973) Modeling infiltration during a steady rain, Water Resources Research, 9(2), 384-394.

Rawls, W. J., Brakensiek, D. L., & Miller, N. (1983). Green-Ampt infiltration parameters from soils data. Journal of hydraulic engineering, 109(1), 62-70. doi:10.1061/(ASCE)0733-9429(1983)109:1(62).