Geoscience Reference
In-Depth Information
Case study
A LYSIMETER USED TO MEASURE EVAPORATION FROM TUSSOCK
A narrow-leafed tussock grass (
Chionochloa rigida
,
commonly called 'snow' or 'tall tussock') covers
large areas of the South Island of New Zealand.
A field study of a catchment dominated by snow
tussock (Pearce
et al
., 1984) showed high levels
of baseflow (i.e. high levels of streamflow between
storm events). Mark
et al
. (1980) used a perco-
lation gauge under a single tussock plant and
estimating evaporation, showed that the water
balance can show a surplus. They suggested that
this may be due to the tussock intercepting fog
droplets that are not recorded as rainfall in a
standard rain gauge (see Plate 3). The nature of a
tussock leaf (long and narrow with a sharp point),
would seem to be conducive to fog interception
in the same manner as conifers intercepting fog.
Another interpretation of the Mark
et al
. (1980)
study is that the estimation of evaporation was
incorrect. An understanding of the mechanisms
leading to high baseflow levels is important for
a greater understanding of hydrological processes
leading to streamflow.
In order to investigate this further a large
lysimeter was set up in two different locations.
The lysimeter was 2 m in diameter and contained
nine mature snow tussock plants in an undis-
turbed monolith, weighing approximately 8,000
kg. Percolating runoff was measured with a
tipping-bucket mechanism and the whole lysi-
meter was on a beam balance giving a sensitivity
of 0.054 mm (Figure 3.5). The rainfall was
measured immediately adjacent to the lysimeter.
Campbell and Murray (1990) show that although
there were times when fog interception appeared
to occur (i.e. the catch in the lysimeter was greater
than that in the nearby rain gauge) this only
accounted for 1 per cent of the total precipitation.
The detailed micro-meteorological measurements
showed that the tussock stomatal or canopy
resistance term was very high and that the plants
had an ability to stop transpiring when the water
stress became too high (see earlier discussion on
plant physiological response to evaporative stress).
The conclusion from the study was that snow
tussocks are conservative in their use of water,
which would appear to account for the high
baseflow levels from tussock-covered catchments
(Davie
et al
., 2006).
Figure 3.5
Large weighing lysimeter at Glendhu
being installed. The weighing mechanism can be seen
underneath.
(Photograph courtesy of Barry Fahey)
The Thornthwaite technique is extremely use-
ful as potential evaporation can be derived from
knowledge of average temperature (often readily
available from nearby weather stations) and latitude.
There are drawbacks to its usage however; most
notably that it only provides estimates of monthly
evaporation. For anything at a smaller time-scale
it is necessary to use another technique such as
