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Introduction and structure of thesis
The genus Citrus (family Rutaceae) is a range of fruits which includes oranges, mandarins, tangerines, tangelos, clementines, satsumas, lemons, limes, and grapefruits (Syvertsen and Lloyd, 1994; Carr, 2012). Citrus are perennial evergreen trees believed to have originated from the humid tropics of the south-eastern parts of China and in southern India (Kriedemann and Barrs, 1981; Sippel, 2006). The species have, however, become widely adapted to the semi-arid regions of the world (Carr, 2012). They are cultivated primarily for fresh fruit and juice, although there are other by-products, such as food additives, pectin, marmalades, cattle feeds (from the peel), cosmetics, essential oils, chemicals, and medicines. Citrus is a one of the most important fruit crops grown across the world, as well as in South Africa. South Africa has an average production of 2.16 million tons per year, which accounts for 4.2 % of world production and as a result the country is ranked thirteenth in the world in terms of citrus production (FAO, 2012). In South Africa, Valencia (including mid-seasons) and Navel orange orchards occupy the bulk of the 64 510 ha planted with citrus at the present moment, accounting for 40 and 24.3 % of the total area planted to citrus, respectively (Citrus Growers Association of Southern Africa, 2015a) (Figure 1). Figure 2 shows total orange production and the quantity of fruit that was used for local consumption, processed and exported in South Africa in recent years (Citrus Growers Association of Southern Africa, 2015b).
While the quantity of fruit used for local consumption and processing has remained fairly constant over the years, there has been a gradual increase in the fruit sold on the export market. This is probably due to the ever increasing price of fresh fruit on the export markets (Figure 3) where South Africa is ranked third. Citrus is grown in all provinces of South Africa with rare to frost-free conditions (this excludes Gauteng and Free State) (Bijzet, 2006). The major producing provinces are the Eastern Cape, Mpumalanga, Limpopo and Western Cape (Figure 1). A large portion of these areas receive seasonal rainfall, less than 500 mm per annum, making them semi-arid and therefore irrigation is required in order to meet crop water requirements and enable the successful production of citrus in these areas. Currently, it is estimated that irrigated agriculture uses approximately 70 % of the available fresh water resources on a global scale (FAO-AQUASTAT, 2003) and in South Africa it is estimated at 60 % (DWAF, 2013). It has been predicted that as the world population expands and economies grow, competition for water will increase, while water resources become more polluted. These changes are expected to be exacerbated as a result of climate change, which is believed to have already increased the frequency and severity of droughts in southern Africa in recent years (IPCC, 2007). Consequently, the demand for water will exceed supply, and less water will be available for agriculture. These population and climate dynamics will have catastrophic effects on 90 % of the fruit production industry in South Africa which, according to Annandale et al. (2005), is entirely dependent on irrigation water.
Water-saving agricultural practices and sound water management strategies are therefore urgently required to ensure the long-term sustainability of the industry. The advent of precision irrigation systems, such as drip, provides an opportunity to match crop water requirements and irrigation amounts, but needs to be coupled with appropriate water management techniques (Jones, 2004). Insufficient knowledge of fruit tree water use under these relatively new systems has been identified as a major factor hindering the development of sound water management tools in South Africa (Pavel et al., 2003; Volschenk et al., 2003). Available literature on citrus water use measurements is in the form of Green and Moreshet (1979) and Du Plessis (1985), which emphasizes that very little has been done in terms of research on citrus water use in South Africa in recent years. There is need to update such information to cater for changes in production practices.
Accurate and reliable methods of measuring water use in fruit tree orchards are needed to gather information to fill this knowledge gap. Crop evapotranspiration (ETc) can be measured using a number of systems which include lysimeters, micrometeorological methods, soil water balance, sap flow, scintillometry and satellitebased remote sensing (Allen et al., 2011). Quantification of the transpiration component that forms the larger portion of orchard water use (Reinders et al., 2010) and is directly related to productivity (Villalobos et al., 2013), is critical in microirrigated fruit tree orchards. Reviews by Swanson (1994), Smith and Allen (1996), Wullschleger et al. (1998) and Allen et al. (2011) have reported sap flow measurements as accurate and reliable methods for estimating long-term transpiration of woody species such as citrus trees, provided the method is calibrated and that due care is taken during installation of the equipment.
Measurements of crop water use are considered to be expensive and time consuming, requiring specialised expertise, which is not always available. Physicallybased models can be used for the extrapolation of measured crop water use to different environments for the improvement of irrigation water management (Boote et al., 1996). The term irrigation water management used here encompasses the activities of irrigation system planning, irrigation scheduling and issuing of water rights/permits. According to Allen et al. (2011), ETc is typically modelled on a physical basis using weather data and algorithms that describe the surface energy and aerodynamic characteristics of the crop. Simultaneous measurements of ETc and weather data are required to improve the current models; as well as adapt the models to account for new production practices e.g. use micro-irrigation. The crop coefficient approach first published by Doorenbos and Pruitt (1977), and later refined by Allen et al. (1998), has been used extensively and is currently considered as the standard model of ETc. Although site specific crop coefficients are probably best, the crop coefficients for a generic citrus tree orchard are often used for water management, due to a lack of reliable information on tree water use. The basal crop coefficient (Kcb), which is directly linked to transpiration, is very important in micro-irrigated orchards, as it represents the major component of orchard water use and is directly linked to productivity (Villalobos et al., 2013). Villalobos et al.
(2013) highlighted that the Kcb factors published by Allen et al. (1998) contain some residual diffusive evaporation component supplied by soil water below the dry surface and by soil water from beneath dense vegetation; such that the term transpiration coefficients (Kt) should be used when basal crop coefficients are determined using transpiration determined by sap flow methods.
Sap flow measurements of crop water use
A number of methods have been used to directly estimate transpiration of trees in general, including fruit trees (Wullschleger et al., 1998). These include weighing lysimeters, large tree potometers, ventilated chambers, radioisotopes, stable isotopes and a range of heat balance/heat dissipation methods. The most accurate and accepted method is the weighing lysimeter and as such it has been used in a number of studies in orchards (Moreshet and Green, 1984; du Plessis, 1985; Castel, 1996; Girona et al., 2002; Ayars et al., 2003; Girona et al., 2003; Yang et al., 2003; Williams and Ayars, 2005). However, they are expensive to install in existing orchards, as well as to maintain.
Amongst the several methods that have been used to measure tree transpiration, the use of thermometric sap flow measurements has shown considerable promise (Smith and Allen, 1996; Wullschleger et al., 1998; Allen et al., 2011). Sap flow measurements at stem level can be used to estimate tree transpiration if performed over sufficiently long periods to negate changes in stem storage and neglecting the 2-5 % of the water used for photosynthesis (Salisbury and Ross, 1978; Waring and Roberts, 1978). Some of the advantages of these methods include their ability to estimate transpiration, which has a direct relationship with productivity; easy interface to data loggers for direct automated transpiration measurements; their applicability is not inhibited by complex terrain and heterogeneity within fruit tree orchards; and they exhibit relatively good accuracy when compared to conventional methods of measuring orchard water use, e.g. the eddy covariance method (Rana et al., 2005).
Thermometric techniques that have been used for transpiration measurements in woody species can be broadly categorized into heat pulse, heat balance and heat dissipation methods (Smith and Allen, 1996). Heat pulse techniques, particularly the compensation heat pulse method (CHPM), have been widely used, mainly because they are relatively easy to use and have low power requirements (Green et al., 2003; Edwards et al., 1997; Smith and Allen, 1996). However, there are limitations to the CHPM, of which the major shortcoming is the inability to measure low and reverse sap flow rates, which usually occur under water stress conditions or at night (Becker, 1998). Burgess et al. (2001) developed the heat ratio method (HRM) as an improvement to the CHPM, in order to cater for low and reverse sap flow rates in woody species. The HRM has been used to study transpiration dynamics in Jatropha curcas L. (Gush, 2008), olive (Williams et al., 2004; Er-Raki et al., 2010), Eucalyptus marginata (Bleby et al., 2004) and Prosopis (Dzikiti et al., 2013).
Chapter 1: Literature Review
1.1 Measurement of citrus water use
1.2 Modelling citrus water use
1.3 Scope of the study
Chapter 2: Calibration of the heat ratio method (HRM) sap flow technique for longterm transpiration measurement in citrus orchards
2.1 Introduction
2.2 Materials and methods
2.3 Results
2.4 Discussion
2.5 Conclusions
Chapter 3: Measurement of long-term transpiration and transpiration coefficients in citrus orchards
3.1 Introduction
3.2 Materials and methods
3.3 Results and discussion
3.4 Conclusions
Chapter 4: Estimation of transpiration crop coefficients from ground cover and height in citrus orchards
4.1 Introduction
4.2 Materials and Methods
4.3 Results and discussion
4.4 Conclusions
Chapter 5: Predicting transpiration of citrus orchards using the Penman-Monteith equation coupled with a Jarvis-type multiplication canopy conductance model
5.1 Introduction
5.2 Materials and methods
5.3 Results
5.4 Discussion
5.5 Conclusion
Chapter 6: General conclusions and recommendations
6.1 Overview of study
6.2 Calibration of the heat ratio method (HRM) sap flow technique for long-term transpiration measurement in citrus orchards
6.3 Measurement of long-term transpiration and transpiration coefficients in citrus orchards
6.4 Estimation of transpiration coefficients from plant height and canopy cover
6.5 Predicting transpiration of citrus orchards using the Penman-Monteith equation coupled with a Jarvis-type canopy conductance model
6.6 Recommendations for future research
References
