3 Energy and Water Balance Dynamics
Chapter 3 Energy and Water Balance Dynamics 26
(1982) as well as by Ham et al.(1991).Precaution must be taken regarding the maximum number of days of use of the same sample, minimum length of the microlysimeter and the thermal differences between the sample and the adjacent soil as affected by wall and end cap materials (Savage et a!., 1997).
Depending on the prevailing climatic conditions and method of measurement, it is crucial to estimate annual evapotranspiration (ET) of banana in order to evaluate its water use patterns and recharge the profile if found necessary. Estimates of annual evapotranspiration (ET) of banana plantations range from 1200 to 2690 mm (Robinson and Alberts, 1989). The high evaporative demand in semiarid environments combined with a large transpiring surface area and sometimes a fairly shallow rooting system make the crop susceptible to water deficits and lodging.Consequently, banana plants may require irrigation during the dry periods to prevent reductions in yield and plant quality (Normanetal., 1984). This aspect however is not applicable to some equatorial countries like Uganda.Evaporation is a major component of the soil water balance, and can be determined from measurements made on soil,crop and the microclimate.Methods ofobtaining estimates ofET range from direct measurement techniques using microlysimeters to energy balance measurements based on the Bowen ratio, flux profile and eddy correlation techniques. Eddy correlation and the surface temperature techniques can provide estimates of evaporation using the energy balance equation and the estimated sensible heat for crops such as grass (Savageet al., 1997).
Evaporation measurements provide valuable information about temporal changes in water use by a given cropping system. However, lysimeter methods can only measure one cropping system at a given location. The use of micrometeorological techniques is preferred because of easy automation and sound theoretical basis. Micrometeorological techniques provide a way of comparing ET rates among different cropping systems and can be more easily located in multiple fields to estimate evapotranspiration from a number of different cropping systems than lysimeters can.However,Jackson and Wallace (1999) reported a new microlysimeter method for estimating evapotranspiration which can overcome this shortcoming, details ofwhich are reported later.
Chapter 4
4.1 Site description
Materials and Methods
CHAPTER 4
4 Materials and methods
27:
The research was conducted on Inselele farm, located at latitude 30°19' S;longitude 29°28'E, Mtwalume, in KwaZulu-Natal, South Africa.The site has an approximate slope of 1.5 %and is at an elevation of about 95 m above sea level. The KwaZulu-Natal south coast is characterised by steeply undulating terrain. The valleys are subject to large diurnal and seasonal air temperature fluctuations (Gevers, 1987).The minimum and maximum air temperatures cited for the coldest month, July, were 7.44 °C and 28.5 °C respectively and a mean monthly temperature of 15.64
c c.
The hottest month was January with minimum and maximum temperatures of 19.63°C and 31.24°C respectively and a mean temperature of24.32 °C (Table 6.1).The main rainy season is February to April,with little or no rainfall during May to September/October. Bananas require an optimum growth temperature of 27°C preferably in humid lowland tropical regions where the annual rainfall varies between 1000 mm to 2500 mm (Gevers, 1987). In Mtwalume,with specific reference to Inselele,average rainfall is below 100 mm/month for approximately six months ofthe year which include January and May to September (Gevers, 1987).However, four ofthese months are during winter when water requirement is lower (South Africa Sugar Association, 1999).Itis therefore evident that the rainfall in the region is relatively low.However, it has a fairly high level of relative humidity,which alleviates the effects of high temperature by reducing the evaporative demand since the vapour pressure deficit would be minimal. This region is also prone to regular off-shore winds (Gevers, 1987).The plantation was irrigated with a 4-5 day cycle with the drag line method during the dry season. This resulted in a fairly low soil water tension,and ensured that water stress was not a limiting factor. The experimental field (Fig. 4.3) was bordered on the east side by sugarcane, south by sugarcane and shrubs, while the north and west sides were occupied by banana.
Chapter 4 Materials and Methods 28
The baseline data for the individualbananavariates such as height (m),pseudostem circumference (m), number ofleaves and leaf emergency rates (LER) were measured at regular intervals of30 days between day of year (DOY) 215 and DOY 365,1999,resulting in six baseline data sets.The plants sampled were ratoon suckers in order to determine the impact of three shade levels and a control (solar irradiance with no artificial shade) on morphology, growth and productivity of bananas during their production cycle. The radiation screens (Fig.4.1), Knitex type were black and had mesh sizes of30 % knitting,60% knitting and 80% knitting yielding 70 %,40 % and 20 %irradiances respectively.These were erected on DOY 28,2000 and sampling continuedtill DOY 304, 2000. This helped compare plant morphological features before and after the treatments were imposed.
4.2 Experimental design and plot layout
Each treatment plot had 16 banana trees (4 rows by 4 trees in each row) that were sampled from it. Hence there were 48 data plants sampled from each of the three blocks.The banana spacing was at three meters between the tree rows and two meterswithin the tree rows creating a planting density of 1666 plants ha". The plot sizes were 12.5 m by 6 m (75 nr') occupying a portion equivalent to 0.09 hectares ofthe total area of 0.655hectares designated to bananas depicted by the shaded section in Fig.4.3.
During plant data sampling, the key growth stages were closely monitored, namely, the mid vegetative stage, pre-flowering stage,flowering emergence and harvest marked by the beginning of ripening (physiological maturity). Muchowet al.(1994) proposed removing of dead leaves at weekly intervals from areas where tube solarimeters and the LAI-2000 are used.The LAI was determined from both the pre-flowering and flowering stages. At least 50 % green leaf material was the yardstick for measurement with a leafarea meter (Sinclair and Muchow, 1999).
Measured growth parameters included plant height, pseudostem circumference (girth), number ofleaves and leaf emergence rates. The pseudostem height was measured from the exposed mid corm level to the neck ofthe bunch stalk at flowering and stem circumference was measured 1000