Parameter

Case 1

Case 2

Average transmissivity, T (m ²/day)

20

20

Specific yield, S _y

0.1

NA

Storage coefficient

NA

2 x 10 ^-4

Porosity, n

0.3

0.3

Starting head - aquifer (m)

5

5

Mean head - river (m)

5

5

Amplitude, s_r (m)

5

5

Time period (days)

1

1

Size of model (m)

500 x 10

500 x 10

Grid

251 x 6

251 x 6

Cell size, Δx (m)

2 x 2

2 x 2

Time step, Δt(days)

0.1

0.1

The model parameters and inputs are explained below:

• The transmissivity values are 20 m ² /day for both cases; specific yield is 0.1 for case 1 and storage coefficient is 2 x 10 ^-4 for case 2.
• The sinusoidal oscillation in the river is characterized by the following parameters: amplitude of 5 m and a time period of 1 day.
• The time step, Δt , is selected such that the temporal variability of head in the river is resolved. The head in the river varies sinusoidally over a time period of 1 day, and hence a time step of 0.1 days is selected.

Download model - 1(To download, right click and select "Save Link As" )

Download model - 2

Problem Statement

This video compares the effect of a sinusoidally oscillating boundary condition on the hydraulic head in an unconfined aquifer with that in a confined aquifer. The two aquifers differ only in the magnitude of their storage properties (specific yield and storage coefficient). The modeling domain consists of a time-variable head boundary on the left extreme, and a no-flow boundary on the right. Details are provided in Table 2.5.

 Key Observations

The following observations can be made from the video:

• In comparison to a confined aquifer, a similar unconfined aquifer has a smaller response zone.
• Unconfined aquifers release water by draining water-filled pores, while confined aquifers release water by elastic expansion of the aquifer matrix and decompression of water. Therefore, unconfined aquifers are able to drain more water from a unit area of aquifer material per unit decline in head; hence, the smaller response zones.

 Additional Observations

Case 1: The average transmissivity of the unconfined aquifer is 20 m ² /day and the specific yield is 0.1. The sinusoidal oscillation of head in the river is characterized by an amplitude of 5 m and a time period of 1 day. The response zone is approximately 50 m in length.

Case 2: The transmissivity of the confined aquifer is 20 m ² /day and the storage coefficient is 2 x 10 ^-4 . The sinusoidal oscillation of head in the river is characterized by an amplitude of 5 m and a time period of 1 day. The response zone is larger than in case 1, since the confined aquifer has a much lower storage coefficient compared to the unconfined aquifer.

Since storage properties of confined aquifers are smaller than those of unconfined aquifers by several orders of magnitude (typically 2 or more), the response zones are proportionally larger.