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irrigation is essential to meet the crop’s water demands. Under current climate scenarios, the demand for irrigation will rise. Irrigation may cancel temperatureinduced drought effects on pesticide degradation (Gonczi 2016).
3.4 Pesticide Transport
The pesticides currently used in sugarcane production in Malawi utilize water as a
solvent. High temperatures result in increased volatilization of highly and semivolatile pesticides through evapotranspiration of the pesticides and their metabolites
to the atmosphere (Bloomfield et al. 2006). Most of the pesticides in use in the
sugarcane industry in Malawi are, however, less volatile (Kasambala Donga and
Eklo 2018). Water-based pesticides such as monosodium methylarsenate (MSMA)
and its metabolites show some persistence in soil and sediments because they tend
to move slower than water and remain concentrated in shallow soil depths (Mahoney
et al. 2015; Bloomfield et al. 2006), increasing the possibility of pesticide contamination of the environment after initial application. A study in Australian forests
found residues of atrazine and its metabolite desethylatrazine in 1.8-m-deep groundwater (Kookana et al. 2010).
Rainfall is a key factor influencing the transport of pesticides in the environment.
The onset of the rainy season is around October to November in most parts of
Malawi, with the highest rainfall occurring around February to March or early April,
especially in the north. The rains tail off in late April and May when winter begins.
The amount and timing of rainfall in relation to pesticide application is a much more
important factor than average annual rainfall and temperature (Wang et al. 2018).
For Malawi, the observed and predicted increases in the proportion of rainfall that
falls in heavy events during the wetter months of January and February affect the
following pesticide pathways: leaching to surface and groundwater, runoff and erosion. There is a high probability of pesticide movement to surface and groundwater
at higher rainfall intensities since wetter soils have higher hydraulic conductivities
(Bloomfield et al. 2006). Hydraulic conductivity varies with soil type and the water
content of the soil. The soils in the main sugarcane-growing areas of Malawi are
chiefly alluvial in the Nkhotakota District and alluvials and vertisols in the Chikwawa
District. The water holding capacity of vertisols is high compared to alluvials. This
implies that there will be a higher likelihood of pesticide-rich water percolating to
groundwater in areas with vertisols in situations of higher rainfall intensities. On the
other hand, a higher soil water content will result in increased degradation rate of
pesticides (Jebellie et al. 1996) and thus lower the pesticide risk estimate.
Increased rainfall intensities may also result in flooding and runoff. Runoff will
directly influence the fate of pesticides through the increased erosion of soil particles and transport of sorbed pesticides (Bloomfield et al. 2006). Increased precipitation may enhance runoff contamination by pesticides (Silburn et al. 2013; Probst
et al. 2005). Rainwater and floodwater runoff account for the transport of a quarter
of the diuron applied yearly to sugarcane in Australia (Camenzuli et al. 2012).
Impact of Climate Variability on the Use and Exposure of Pesticides in Sugarcane…
irrigation is essential to meet the crop’s water demands. Under current climate scenarios, the demand for irrigation will rise. Irrigation may cancel temperatureinduced drought effects on pesticide degradation (Gonczi 2016).
3.4 Pesticide Transport
The pesticides currently used in sugarcane production in Malawi utilize water as a
solvent. High temperatures result in increased volatilization of highly and semivolatile pesticides through evapotranspiration of the pesticides and their metabolites
to the atmosphere (Bloomfield et al. 2006). Most of the pesticides in use in the
sugarcane industry in Malawi are, however, less volatile (Kasambala Donga and
Eklo 2018). Water-based pesticides such as monosodium methylarsenate (MSMA)
and its metabolites show some persistence in soil and sediments because they tend
to move slower than water and remain concentrated in shallow soil depths (Mahoney
et al. 2015; Bloomfield et al. 2006), increasing the possibility of pesticide contamination of the environment after initial application. A study in Australian forests
found residues of atrazine and its metabolite desethylatrazine in 1.8-m-deep groundwater (Kookana et al. 2010).
Rainfall is a key factor influencing the transport of pesticides in the environment.
The onset of the rainy season is around October to November in most parts of
Malawi, with the highest rainfall occurring around February to March or early April,
especially in the north. The rains tail off in late April and May when winter begins.
The amount and timing of rainfall in relation to pesticide application is a much more
important factor than average annual rainfall and temperature (Wang et al. 2018).
For Malawi, the observed and predicted increases in the proportion of rainfall that
falls in heavy events during the wetter months of January and February affect the
following pesticide pathways: leaching to surface and groundwater, runoff and erosion. There is a high probability of pesticide movement to surface and groundwater
at higher rainfall intensities since wetter soils have higher hydraulic conductivities
(Bloomfield et al. 2006). Hydraulic conductivity varies with soil type and the water
content of the soil. The soils in the main sugarcane-growing areas of Malawi are
chiefly alluvial in the Nkhotakota District and alluvials and vertisols in the Chikwawa
District. The water holding capacity of vertisols is high compared to alluvials. This
implies that there will be a higher likelihood of pesticide-rich water percolating to
groundwater in areas with vertisols in situations of higher rainfall intensities. On the
other hand, a higher soil water content will result in increased degradation rate of
pesticides (Jebellie et al. 1996) and thus lower the pesticide risk estimate.
Increased rainfall intensities may also result in flooding and runoff. Runoff will
directly influence the fate of pesticides through the increased erosion of soil particles and transport of sorbed pesticides (Bloomfield et al. 2006). Increased precipitation may enhance runoff contamination by pesticides (Silburn et al. 2013; Probst
et al. 2005). Rainwater and floodwater runoff account for the transport of a quarter
of the diuron applied yearly to sugarcane in Australia (Camenzuli et al. 2012).
Impact of Climate Variability on the Use and Exposure of Pesticides in Sugarcane…