Materials and Methods 34

3 Energy and Water Balance Dynamics

Chapter 4 Materials and Methods 34

When using the net radiometer, getting too close to the canopy causes a shadow problem. Itis also possible for the net radiometer to be in a zone where there is a large temperature difference between the surface and the overlying air. This therefore calls for proper placement height for this sensor. Suppose that a net radiometer is placed a distancedfrom the banana canopy and "sees"

an area A.The view factor F for this area A for a distance d between the soil surface and the net radiometer is given by:

F= A/(A +n*d*d)

wheren is approximately 22/7 (Savageet al., 1997).

4.1

In order to avoid shading,the sensor was installed with its head facing north and the support arm facing south. It was horizontally mounted using a spirit level with the down dome facing downwards and the upper dome facing upward.The instrument was mounted at 1.5 m height above the canopy surface and approximately 3.5 m from the soil surface. This distance gives a view factor of 0.8(Eq.4.1). Itallows the sensor to sense the emitted long wave and the reflected solar irradiance from the crop surface.This distance is relatively sufficient to avoid the negative effect ofits own shadow.The net radiometer domes are cleaned every 21 to 30 days using distilled water and camel hair brush and they are replaced when need arises. The silica gel is replaced when its colour changes from blue to pink. If the radiometer domes have a "milky white"appearance, they should be replaced and,ideally, the net radiometer recalibrated at the same time.The sensor should be recalibrated at least twice a year (Savageet al., 1997). Net irradiance measurements,

R"el'

are fundamental in the Penman-Monteith equation calculation as will be illustrated in later

chapters.

4.5.1.2 Apogee infrared thermometer

Two self-powered high precision Apogee infrared, type K thermocouple sensors were used to monitor leaf surface temperatures.The IRTs have dimensions of60 mm long by 23 mm diameter with an accuracy of ±1 QC when the sensor body and target are at the same temperature. The relative energy received by the IRT detector depends on the sensor field ofview (FOY).The FOY is 50⁰for a full angle with 90 %of the target. The sensors were placed at 1.5 m above the

Chapter 4 Materials and Methods 35

canopy and this is referred to as a 1.5:1 FOV (Placed at 1.5 meters from the sensor the FOV is 1 m diameter circle) (Internet: http://\\rww.apogee-inst,2000).

A wide FO V integrates a larger target area than a narrow FO V.The distance between the IRT and the object dramatically affects the size of the area viewed. The radiation incident on a typical radiation sensor is determined by the cosine of the angle viewed, which therefore means that the incoming signal is primarily from the region directly in front of the sensor, but there is significant peripheral vision.The error in temperature measurement caused by the peripheral vision depends on the relative radiation emitted by the target and by the surroundings.

There are significant disadvantages to a narrow FOV. They see less ofthe target and more of their own temperature, which implies that they have increased sensitivity to sensor body temperature.

They also have a reduced signal to noise ratio and are more expensive than wider

FOV sensors. The best approach therefore is to use the sensor with the widest FOV possible and place it fairly close to the target (Internet:http://www.apoQcc-inst, 2000).

4.5.1.3 Three dimensional propeller anemometer

A three dimensional propeller anemometer (Model-08234, WeatherTronic, West Sacramento, CA, USA) was used to measure wind speed and wind direction. The propeller anemometer is a sensitive precision component wind speed instrument fitted with a structural foam polystyrene propeller moulded in the form of a true helicoid. The propellers have a very linear response for winds above 1 meter per second. Increased slippage occurs down to the threshold speed of 0.2 meters per second. The propeller drives a miniature direct current tachometer which produces an analogue output voltage proportional to wind speed.Allvoltages were measured every 30 s using a Campbell Scientific 21 X datalogger. The instrument measures both forward and reverse wind flow and the tachometer produces corresponding positive and negative currents.The propeller responds to only the component of wind in the axis of the instrument. The response follows closely to the cosine law. When the wind is at 90⁰ to the axis, the propeller will stop all together.

Chapter 4 Materials and Methods 36

Caution should however be exercised when handling and working around the instrument as the propellers are fragile and can break with impact (Savage, 1999a).

.All propellers are made from a single cavity mould to ensure uniformity and repeatable accuracy.

The smaller diameters are made from 230 mm mouldings by cutting with a hot wire.All propellers are then sprayed and balanced using a special acrylic paint. The effective pitch of the propeller is slightly different for each diameter. The 190 mm diameter propeller has a pitch of3 00 mm and is considered the nominal and the 150 mm propeller 2% less than normal. Itshould be noted that for all full scale calibrations to 6 m ^S-I or below, maximum accuracy is achieved by using a 0.13 m ^S-I zero offset and the 300 rev min-^I (or 250 rev min-^I) calibration point in reference to low speed calibration. For higher ranges use 1800 rev min-^I (or 1500 rev min-^I) calibration point without zero offset (Savage, 1999a).

4.5.2 Soil heat flux plates and soil thermocouples

Two soil heat flux plates (Middleton Instruments,Model CN3, Australia) were buried at a depth of 80 mm. Soil heat flux plates, if placed too close to the surface, disrupt energy and mass flow soil,hence burying them at that depth is recommended (Savageet al.,1997).Four thermocouples connected in parallel were used to measure the average temperature in the soil layer above the plates.

The soil heat flux density G was calculated using a spreadsheet (Savage, 1999b)as the sum of the measured soil heat flux using a heat flux plate (Gp) and that stored in the layer above the soil heat flux plate (Gslored) calculated from the thermocouples as:

4.2

The stored heat varies with changes in soil temperature (dTsoJduring a time interval, the soil bulk density (PbsoJ,the depth of the layer (Az), the specific heat capacity of the soil (Cp.) and water (Cp.}, and the soil (Pbso/PsoiJ and water (8va) fraction in the soil system (PsOil refers to the soil particle density,All the parameters were related by Hillel (1982) as follows: