3. MAPPING SUITABLE PRODUCTION AREAS
3.1 Factors Affecting Plant Growth and Distribution
3.1.1 Rainfall
Water usually accounts for 80 % of the weight of the herbaceous plant and it is a principal component of the plant cell. It is biochemically important as the principal component of physiological processes that occur within the plant (Manske, 2001). According to Manske (2001), water is essential for the maintenance of the rigidity of plant tissues. When water is limited, biological processes such as temperature control, nutrients and metabolite transport can be affected and these can impact plant growth and development (Manske, 2001). Seasonal crop water use, rainfall concentration index and the crop coefficient concept are discussed in the next three sub-sections.
3.1.1.1 Seasonal crop water use
The seasonal crop water use indicates in which months the majority of seasonal rainfall falls.
This is important from a crop growth point of view, as most crops are sensitive to water stress during particular growth stages. According to Seckler (2003), most crops are sensitive to water stress at both the vegetative growth and fruit/grain development stages. For example, maize is most sensitive to water stress during pollination and requires more water during the flowering stage (Figure 3.1).
(a) (b)
Figure 3.1: (a) Water use by maize under well-watered conditions; (b) Sensitivity of maize to soil water stress (Pannar, 2003)
Figure 3.1b shows that maize is particularly sensitive to water stress at 35 days after planting (during the vegetative growth stage) and again at 70 days after planting (during the grain filling stage). A lack of rainfall during the crop’s peak water use period (i.e. approximately 70 days after planting) will result in a yield loss of up to 10 % (Figure 3.1b). This coincides with the peak water use at day 70 after planting as shown in Figure 3.1a. According to FAO (2013), the maize crop requires most water during mid-season stage (day 75 to day 120) as shown in Table 3.1. This approach is further explained in cf. Section 3.1.1.3, using soybean as an example feedstock. Soybean was chosen as the example feedstock because preliminary results were presented for soybean at the Soybean World Conference in February 2013. A number of indices have been developed which highlights the importance of rainfall distribution across the growing season and these are described next in more detail.
Table 3.1: Single crop coefficients (Kc) for each maize growth stage as suggested by FAO (FAO, 2013)
Growth stage
Length of growth stage
(days)
Kc
Initial 15 – 30 0.30 – 0.50
Development 30 – 45 0.70 – 0.85
Mid-season 30 – 45 1.05 – 1.20
Late-season 10 – 30 0.80 – 0.90
At harvest 0.55 – 0.50
3.1.1.2 The rainfall concentration index
The rainfall concentration index, determined using Markham’s (1970) methodology, calculates a value ranging from 0 to 100 %. An index of 100 % implies that all rainfall falls in a concentrated time period (e.g. one month). On the other hand, a concentration index of 0 % implies a similar rainfall amount in each month. Hence, lower values have been used to identify the all-year rainfall season along the southern Cape coastal areas (Schulze and Maharaj, 2007e). Hence, rainfall concentration describes the duration of the rainy season and varies spatially across southern Africa.
Schulze and Maharaj (2007d) calculated rainfall concentrations for each quaternary catchment, whereas Schulze and Kunz (2010) repeated the exercise at the quinary catchment scale. Both studies highlighted that the highest and lowest rainfall concentrations are found in the Limpopo and Western Cape provinces, respectively. Schulze and Maharaj (2007d) highlighted that plant growth is affected by the duration of the rainy season, i.e. whether the rainfall is concentrated over a short period of the year or spread over a longer period. The importance of seasonal rainfall distribution is highlighted by the concept of rainfall concentration.
3.1.1.3 The crop coefficient concept
In hydrology, the crop coefficient is used to estimate crop water use which varies with the crop’s growth. The single crop coefficient (Kc) approach combines soil water evaporation
and crop transpiration, whereas the basal crop coefficient (Kcb) describes plant transpiration only. The crop coefficient is calculated by dividing crop water use (transpiration and soil water evaporation) by the reference crop evaporation. Reference crop evaporation is calculated using solar radiation, air temperature, relative humidity and wind speed data using the Penman-Monteith equation given by Allen et al. (1998). Typical values for Kcb and Kc for soybean are provided in Table 3.2 and 3.3 respectively. The Kcb values indicate that transpiration peaks during the development and mid-season growth stages. However, Kc
values show that total crop water use (i.e. evapotranspiration peaks during the mid-season growth stage).
Table 3.2: Basal crop coefficients (Kcb) derived from the SAPWAT3 database for each soybean growth stage (van Heerden, 2013)
Growth stage
Length of growth stage
(days)
Kcb
Initial 30 0.10
Development 30 1.15
Mid-season 60 1.15
End-season 01 0.90
Table 3.3: Single crop coefficients (Kc) for each soybean growth stage as suggested by FAO (FAO, 2013)
Growth stage
Length of growth stage
(days)
Kc
Initial 20 – 25 0.3 – 0.4
Development 25 – 35 0.7 – 0.8
Mid-season 45 – 65 1.0 – 1.2
Late-season 20 – 30 0.7 – 0.8
At harvest 0.4 – 0.5
The length of each growth stage in relation to soybean’s crop coefficient curve is shown in Figure 3.2. The rate at which the crop develops and the time to reach full canopy cover are affected by weather conditions, in particular mean daily air temperature. Therefore, the length of time between planting and full canopy cover varies with climate, latitude, elevation,
planting date as well as cultivar (crop variety). Thereafter, the rate of further physiological development (flowering, seed development and ripening) is more dependent on plant genotype and less dependent on weather. Stress caused by high temperatures or lack of soil water can shorten the mid- and end-season growing periods (Allen et al., 1998).
Figure 3.2: Generalised crop coefficient curve based on the single crop coefficient approach (Allen et al., 1998)
According to Allen et al. (1998), the initial crop coefficient value (Kc ini) varies with the frequency of wetting events during the initial growth period, i.e. Kc ini is large when the soil is wet from frequent rainfall or irrigation events and is low when the soil is dry.