SOUTH\AFRICA \
5.3.5 General
The field experiment was conducted over a period of two years (1 July 1998 until 30 June 2000). This period was divided into two 12-month periods: (i) 1 July 1998 to 30 June 1999 and (ii) 1 July 1999 to 30 June 2000, and are hereafter referred to as 1998/1999 and 1999/2000.
The components of the simplified soil water balance (Eq. 5.1) were estimated from in situmeasurements (Table 5.1, Figs 5.4 to 5.8). These soil water balances will illustrate the potential impact of grassland and E. viminalis trees on the total evaporation and soil water storage relationship of a site.
Table 5.1 A summary of the measurements made and methods and equipment used to estimate different parameters required during the field experiment
Parameter Method Measurements Equipment Manufacturer Total Bowen ratio Net irradiance Net radiometer Q*6, REBS,
evaporation energy balance Seattle, USA
(Fig. 5.4) Air Chromel- N/a
temperature contantan thermocouples
Water vapour Dew-lO N/a
pressure hygrometer
Soil Copper- N/a
temperature constantan
-- thermocouple
Soil heat flux Soil heat flux N/a plates
Output period Water content Campbell reflectometer Scientific
CS615 probe Transpiration Heat pulse Velocity of a Thermistors, Custom made
(Figs 5.5 and 5.6) velocity heat pules Heaters
Soil water Water content Output period Water content Campbell
content reflectometer reflectometer Scientific
(Figs 5.4 and 5.5) model CS615
Soil water Heat Change in Heat Campbell
potential dissipation temperature dissipation Scientific
(Figs 5.4 and 5.5) sensor model
229-L
Soil Relative Soil Wescor
psychrometry humidity psychrometer model PCT-55
Rainfall Automatic Rainfall Tipping bucket OSKOgawa
(Fig. 5.8) weather station Seiki Co. Ltd.
Other climatic Automatic Air Vaisala air Campbell
(Fig. 5.8) weather station temperature temperature Scientific
Relative and humidity humidity probe model
CS500
Solar Pyranometer Li-Cor
irradiance model LI-200
Wind speed Three cup RM Young
Wind direction anemometer model 03001
Barometric Barometric Campbell pressure pressure sensor Scientific
model CS105
Fig. 5.4 (a) Images ofthe E. viminalis experimental site and instrumentation used
Fig. 5.4 (b) Images of the E. viminalis experimental site and instrumentation used
Fig. 5.5 (a) Images of the grassland experimental site and instrumentation used
Fig. 5.5 (b) Images of the grassland experimental site and instrumentation used
5.3.6 Methods for estimating total evaporation and transpiration 5.3.6.1 Method for estimating total evaporation9
The Bowen ratio energy balance technique (Campbell Scientific, Inc., Logan, USA) was used to estimate the total evaporation of grassland (Fig. 5.4). The Bowen ratio energy balance method requires measurements of net irradiance, air temperature and water vapour pressure vertical profile differences, soil heat flux density, soil temperature and soil water content. A Campbell Scientific CR23X datalogger was used to record the measurements. Measurement intervals were 1 s for the air temperature and water vapour pressure profile differences, and 10 s for the net irradiance, soil heat flux density, soil temperature and soil water content. Measurements were averaged at 20 minute intervals.
The Bowen ratio sampling arms and net radiometer (Q*6, REBS, Seattle, USA) were mounted on a tripod and pole respectively. The sampling arms of the Bowen ratio energy balance system were orientated due north to avoid partial shading of the thermocouples on the arms, while the net radiometer was positioned north to prevent sensor shading. The air sensed by sensors mounted on these arms should be
representative of the surface studied (Campbell Scientific, Inc., 1998). The lower arm should be installed low enough for the bulk crop surface environment not to be sensed, whereas the upper arm should be installed low enough in order to not sense a different environment upwind. The measurement of the air temperature and water vapour profile differences should be within the resolution of the sensors. A separation distance
between the Bowen ratio sampling arms of 0.5 to 3 m is suggested in the Bowen ratio users' manual (Campbell Scientific Inc., 1998). With an increased distance between the arms, the water vapour pressure and air temperature differences are increased. A separation distance of at least 0.5 to 1 m between the sampling arms was therefore maintained throughout the experiment, with the height of the lower Bowen ratio arms at approximately 1 m above the vegetation (Fig. 5.5).
9Total evaporation (ET) can be defined as the total process of water movement into the atmosphere. In this experiment total evaporation refers to the sum of evaporation from the soil surface, transpiration by vegetation, and evaporation of water intercepted by vegetation (Rosenberget aI.,1983).
The soil heat flux density at the surface was estimated using measurements of soil heat flux density, soil temperature and soil water content for the upper 80 mm soil depth. As the grassland surface was uniform, only two heat flux plates and four averaging thermocouples were used. As the groundcover did not vary considerably, it was not considered necessary to include additional sensors. The soil heat flux plates were installed at 80 mm below the soil surface, the averaging thermocouples were installed at 20 mm and 60 mm below the surface, and the soil water content
reflectometer was installed at an angle over the upper 80 mm of the soil. Initially, the soil water content was estimated gravimetrically. Later, a water content reflectometer (Campbell Scientific CS6l5 probe) was used to estimate volumetric soil water content at 20 minute intervals.
5.3.6.2 Method for determining transpiration10
The heat pulse velocity technique (Huber and Schmidt, 1937; Swanson, 1974 cited by Dyeet al., 1992) was used to calculate the transpiration (sapflux) of six representative trees within anE. viminalistree stand. Four l2-channel heat pulse dataloggers (custom made) were used to measure the velocity at which a heat pulse moves through a tree stem at different depths below the cambium. Measurements were made at hourly intervals. Swanson (1983) found that radial differences in the sapflux occur, and suggested that sets of probes be implanted to different depths within the sapwood (e.g. dl to d4) (Fig. 5.6) (cited by Olbrich, 1994). Four sets of probes (a set consisting of a heater probe and two thermistor probes) were therefore installed at different depths (9,14,21,28 mm) below the cambium ofeachE. viminalis tree studied. This ensured that the variation in sapflux over the sapwood was covered and reflected in the tree transpiration estimated.
10Transpiration can be defined as evaporation of water that has passed through the plant. Transpiration therefore consists of vaporization of liquid water contained in the plant tissues and vapour removal to the atmosphere (Alien et al., 1998).
Cambium Hardwood
Sapwood
Fig. 5.6 Schematic of the implantation of sets of probes at different depths below the cambium (d1 to d4), into the stem of anE. viminalistree
As the accuracy of the sapflux measurements and heat pulse velocity depends on the distance between the probes, a drill jig with three aligned holes was used to install the probes accurately and parallel to each other. The heater probe was installed in the centre hole, and the two thermistor probes were installed at 5 mm below and 10 mm above the heater probe (Fig. 5.4).
5.3.7 Methods for determining soil water content11 and soil water