Geoscience Reference
In-Depth Information
Each of these is a realistic land use change scenario
for the region; conversion of pasture land to
forestry is common practice, as is 'improving'
grassland by over-sowing with rye grass species.
The land use change scenarios were simulated in
the model using the 1989-2001 rainfall data (i.e.
repeating the earlier simulation but with a
different land cover).
The results from the modelling are shown in
Table 6.5. An initial look at the results suggests a
surprising result: the amount of interception loss
from a 40 per cent increase in forestry does not
transfer through into much of a change in mean
annual streamflow or low flows. There is a larger
change in flow regime from the replacement of
tussock grassland in the upper catchment; despite
this land use change resulting in a lowering of
interception loss (tussock grassland has higher
interception losses than pasture grass). The reason
for these results is that it is the upper part of the
catchment, with a higher rainfall, that produces
most of the streamflow, particularly the low flows.
Hence a change in land use in the lower section
makes a relatively small change in the flow
regime. However, a change in land use in the
upper region of the catchment has a larger effect
because this is where the effective rainfall is
occurring. A change from tussock to pasture
grassland increases the transpiration loss which
more than offsets the decrease in canopy
interception.
In this case WATYIELD was able to tease out
the difference between canopy interception and
canopy transpiration. The difference between the
two is what made the most difference in the
simulations. The final scenario modelled was to
place the forestry in the upper reaches of the
catchment; this reduced flows by around 25 per
cent. However this is a highly unlikely land use
change scenario as commercial forestry at this
latitude does not normally extend beyond 850 m
above sea level.
Table 6.5
Results from WATYIELD modelling of land use change
Flow measure
Scenario 1 (forestry in lower
Scenario 2 (replacement of
half replaces pasture)
tussock grassland with pasture)
Mean annual flow
Reduced by 6%
Reduced by 7%
Mean annual 7-day low flow
Reduced by 3%
Reduced by 7%
hydraulic parameters (e.g. wetted perimeter, stream
depth, etc.) and stream health; and the habitat
method uses actual measurements of stream health
with changes in flow regime to predict the impacts
of flow changes. The way these methods treat the
relationship between increased streamflow and the
biological response is shown in Figure 6.19. The
historic method assumes that there is a linear
relationship so that more flow results in a greater
biological response. The hydraulic method recog-
nises that stream beds are non-linear in form and
therefore a small change in flow may result in large
increases in biological productivity but that this
decreases as the flow increases. The habitat method
recognises that there is a maxima in the biological
productivity and high flows may lead to decreasing
biological response.
The most well known of the
historic flow methods
is 'Montana method' proposed by Tennant (1976),
also called the Tennant method. Tennant (1976)
used hydraulic data from eleven streams in the
USA and knowledge about depths and velocities
required to sustain aquatic life to suggest that 10
per cent of average flow is the lower limit for aquatic
life. Tennant (1976) also recommended that 30 per
cent of average flow provides a satisfactory stream
