EV222 – Quantitative Methods in Environmental Science
Block 2 – 2006
Stream Flow Model
1. Description:
The watershed is a square plot with 800 km2 of land. The main channel is 20 km
long with a cross section of 5 m2. The soil thickness in the watershed is 2 m and
the aquifer underneath is 20 m thick. Both the soil and aquifer porosity is 10%.
The base flow of the stream is 7200 m3 per hr.
When it rains, surface water either runs off the watershed to the stream or
infiltrates the soil. From the soil, it either flows into the stream or it percolates
down to the ground water in the aquifer. The ground water gradually flows to the
stream.
2. Stocks:
Surface Water – initially set to 0.
Stream – initially set to the volume of the stream channel (in cubic meters).
Soil Water – initially set to half the volume of the soil layer times the porosity.
Ground Water – initially set to the volume of the aquifer times the porosity.
3. Converters:
Base Flow – 7200 (m3 per hr)
Basin Area – 800 km2 (convert to m2 )
Soil Porosity – 0.1 (10%)
Soil Capacity – Basin Area * Soil Thickness * Soil Porosity
Soil Thickness – 2 (m)
Aquifer Porosity – 0.1 (10%)
Aquifer Thickness – 20 (m)
Runoff Constant – 0.4 (accounts for vegetation, slope, etc.)
4. Graphical Converters:
Infiltration Factor – function of Soil Water / Soil Capacity. At 0 it is 5.00 and at
1.0 it is 0 (concave down). The units here are cm/hr.
Percolation Factor – function of Soil Water / Soil Capacity. At 0 is it 0 and at 1 it
is 0.05 (S-shape curve). Units of cm/hr.
Rain – Bell-shaped curve beginning at 10 reaching a height of 2.6 at 15 and
dropping back to 0 at 20. (units are cm/hr )
5. Flows:
Precipitation –basin area times the rainfall. (Convert this to m3/hr.)
Runoff – Surface Water times the runoff constant.
Stream Flow – The amount in the Stream as a percentage of the initial amount in
the Stream times the base flow.
Infiltration – Basin area times Infiltration Factor. Convert so units are m3.
Percolation – Basin area times Percolation Factor. Convert to m3.
Soil Discharge –Base flow times 0.05 times Soil Water as a percent of initial Soil
Water. (5% of the base stream flow comes from the soil).
Ground Discharge – Base flow times 0.95 times Ground Water as a percent of
initial Ground Water.
(Hydrograph on next page)
6. Actual Hydrograph of a Stream: