Geoscience Reference
In-Depth Information
Case study
WATYIELD - MODELLING CHANGES IN WATER YIELD FROM ALTERING
LAND COVER CHANGE
The degree of vegetation cover in a catchment will
affect the amount of water flowing down a stream.
The physical processes that cause this effect have
been described in Chapter 3 and the impacts of
this change are discussed in Chapter 8. A water
balance model has been developed to quantify
the impact of land use changes on the stream
discharge. The model is simple to use and can be
downloaded for free from the World Wide Web
(look for WATYIELD at http://icm.landcare
research.co.nz). Also available at this site are a
series of reports that help parameterise the model
and a full user's guide.
The WATYIELD model was developed for New
Zealand conditions and works best in a humid
temperate environment. It has been designed for
catchments up to around 50km
2
in size. In the
modelling terminology outlined earlier in this
chapter, WATYIELD could be described as a
lumped, conceptual model. However, there is
detailed process representation of rainfall inter-
ception and soil moisture storage within the model
so it moves slightly towards being physically
based. The spatial representation is at the catch-
ment scale; although it is possible to split a
catchment into sub-sections with different vege-
tation covers and rainfall distributions. However,
these sub-sections have no spatial differentiation
within the catchment, i.e. the model doesn't know
where they are within a catchment, just that there
are subsections. For catchments larger than 50km
2
the underlying assumptions of spatial uniformity
start to breakdown and it is necessary to start
introducing elements such as flow routing down
a stream (presently ignored at the daily time step
of WATYIELD).
WATYIELD works by adding daily rainfall to
two storage terms which release water to a river
based on hydrograph recession coefficients (a full
description of the model can be found in Fahey
et al
., 2004). The storage terms represent soil
moisture and a deeper groundwater store. Daily
rainfall is processed by the model so that any
interception loss from a vegetation canopy is
removed and all the resultant rainfall infiltrates
into the soil moisture store. In order to operate the
model a daily rainfall record, potential evapotrans-
piration, soil parameters and knowledge about
flow characteristics from a nearby stream are
required. Much of this type of data is readily
available from the scientific literature and resource
management databases.
WATYIELD has been applied to a 23 km
2
catchment (Rocky Gully) in the South Island of
New Zealand to investigate two possible land
use change scenarios. The current vegetation cover
for the catchment is a mixture of tall tussock
grassland, pasture grasses and a small amount of
scrubland forest. The catchment has an altitude
range from 580 m to 1,350 m with an increasing
rainfall with altitude.
In testing the model against daily streamflow
from 1989-2001, WATYIELD was able to predict
mean annual flow within 2 per cent accuracy and
mean annual seven-day low flow within 3 per cent.
The two scenarios simulated were:
•
40 per cent of the catchment was converted to
plantation forestry (
Pinus radiata
). All the
planting occurred in the lower half of the
catchment;
•
50 per cent of the catchment was converted
from tussock to pasture grassland. All the
tussock grassland in the upper half of the
catchment was replaced with pasture species.
