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Exfiltration from the buffer strip into the underlying soil can be modelled by defining the exfiltration rate (mm/hr). Representative exfiltration rates for different soil types are provided in the table over the page. The water that seeps from the buffer strip is lost from the catchment, and cannot re-enter the system downstream. Contaminants in the water that is lost to exfiltration are removed from the buffer strip, along with the exfiltrated water and are also lost from the catchment. Representative exfiltration rates for different soil types are shown in the following table.

 

the statistics#nodewaterbalance>Node Balance item to production+and+interpretation+>Production

Soil Type

Median Particle Size (mm)

Saturated Hydraulic Conductivity

(mm/hr)

(m/s)

Gravel

2

36000

1 x 10-2

Coarse sand

1

3600

1 x 10-3

Sand

0.7

360

1 x 10-4

Sandy loam

0.45

180

5 x 10-5

Sandy clay

0.01

36

1 x 10-5

 

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This simple example may help to explain the specifications for buffer strips. If a Source Node with an impervious area of 100 ha has buffer strips applied to 20 ha, and the buffer strips occupy 5 ha of land:

Percentage of Upstream Impervious Area Buffered = 20%

Buffer Area = 5%

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It is possible to record flux data for the buffer, including inflows, outflows, water quality, etc. Data are recorded for each timestep. These data can be exported to a text file, for importing and analysis in other software packages:

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Note that for small timesteps (e.g. 6 minute), the resulting flux file can be very large.

 

NOTE: The user also has the option to view a simple summary of the water balance (e.g. evapotranspiration, infiltration, overflow, etc) within a node by using

the Node Water

Balance item under the Statistics sub-menu; refer

to Production and Interpretation of music Output.  In many cases this will provide the extra detailed water balance information needed, without the need to handle the large files generated by the flux file option.

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The Notes button allows you to record any important details or assumptions for the buffer (for example, you may provide an explanation of how the area was calculated). It is good practice to provide notes of any important assumptions, for future reference by others using the model.