Off-site effects of cassava cultivation and soil erosion on the environment

Off-site effects of agricultural activities - of which cassava production may be one, but seldom the only, component - are defined as the indirect effects of the activities on areas outside the immediate production area. Off-site effects may be in the neighbor's field, but usually refer to impact on the rest of the watershed, especially the effect of eroded soil on water quality and sedimentation. The latter may result in additional maintenance and dredging costs of irrigation systems, reservoirs and harbors.

Effect of cassava cultivation on quality of water resources

Reduced water quality as a result of agricultural activities is usually associated with the runoff of agricultural chemicals into nearby streams, causing either pollution of the water with toxic chemicals or with excessive amounts of nutrients from fertilizer, which may in turn lead to eutrophication of lakes and reservoirs. Cassava is grown mainly by poor farmers, who usually apply no or very low rates of fertilizers, no pesticides to control insects or diseases, and very few farmers are presently using herbicides. It is, therefore, very unlikely that cassava production will lead to water pollution; however, pollution from herbicides being washed into nearby streams could in the future, become an issue of concern.

Effect of eroded sediments on soil and water resources

Soil eroded from cassava fields may travel down-slope across other fields until either deposited in low spots elsewhere or washed into nearby streams, after which it may be deposited along river banks, in river deltas (after flooding), in reservoirs, irrigation systems, harbors or be carried out to sea. Only a fraction of eroded sediments will end up in streams and even a smaller fraction will end up at sea, as most sediments are deposited somewhere on land or along the water's course. Figure 12 shows the annual discharge of suspended sediments from various drainage basins of the world (Milliman and Meade, 1983).

It is clear that soil erosion and the associated discharge of sediments by the major river systems is most serious in Asia, followed by Latin America, and then Africa. Higher rates of soil erosion are observed in Asia due to high and poorly distributed annual rainfall, as well as extremely high population densities, which has resulted in intensive crop cultivation on steep slopes. The annual sediment yields of 100-1000 t/km², measured in much of southeast Asia, would correspond to "average" erosion rates of 1-10 t/ha for the whole drainage basin. These would be considered low rates of erosion in cassava fields, indicating that erosion levels for the whole watershed are usually only a fraction of that measured in individual plots or fields, as erosion tends to be much lower in undisturbed areas under native vegetation. In addition, much of the eroded sediments are deposited elsewhere in the landscape without ever reaching the streams and rivers below.

While erosion causes degradation of soils in the area of origin in the uplands, it may add fertility to lower spots on the slope, as well as to the lowlands and river deltas. These lower areas have greatly benefitted from erosion upstream and are the main areas for food production in many countries, such as along the river Nile in Egypt, the Chao Phraya river delta of Thailand, and the Red River and Mekong deltas of Vietnam.

Apart from these beneficial effects downstream, however, there are also negative effects, such as the deposition of eroded sediments in irrigation systems, reservoirs and harbors. This may require additional maintenance and dredging, while the siltation of reservoirs will also shorten the life of hydro-electric projects, and thus add to their cost. Deposits of soil sediments in rivers and reservoirs reduces their depths and water storage capacity, which may also increase the incidence and severity of flooding (El-Swaify and Dangler, 1982).

Figure 12. Annual discharge (millions of tuns) of suspended sediments from various drainage basins of the world. Width of arrow v correspond to relative discharge. Direction of arrows does not indicate direction of sediment movement.
Source: Milliman and Meade. 1983.

Cassava can definitely contribute to both the positive and negative off-site effects of soil erosion. It is impossible, however, to trace the sediments back to their place of origin, and establish a direct link with cassava production. Cassava is usually only one of many crops grown in the watershed, in addition to grasslands and forests, which all contribute in varying degrees to erosion and associated problems of sedimentation. Magrath and Arens (1989) tried to quantify the cost of erosion on Java island of Indonesia, both the on-site effects of present or future loss of income due to soil degradation as a result of erosion, and the off-site effects on the cost of maintenance and dredging of waterways, reservoirs and irrigation systems. Cassava is an important component of the upland cropping systems used in Java, where the crop is usually intercropped with maize, upland rice, peanuts, soybean and mungbean. Annual rates of dry soil loss of 25-100 t/ha have often been measured in erosion control experiments using these intensive intercropping systems (Wargiono et al., 1992; 1995; 1998). Magrath and Arens (1989) used values of 76 to 144 t/ha of annual soil loss for agricultural land in Java in their calculations, and estimated the resulting annual decline in yield of 3.8 to 4.7%. Table 16 summarizes their results and indicates that for the whole of Java the value of annual production losses due to erosion was about 315 million US dollars, while the annual off-site costs were valued at 26-91 million dollars.

Thus, it is clear that, while off-site effects are highly visible, and can cause serious damage and even loss of life during flooding, the "invisible" effect of loss of soil productivity seems to account for most of the actual costs associated with soil erosion (Magrath, 1990; World Bank, 1990).

Table 16. Annual on-site and off-site costs of erosion in Java, Indonesia.

¹⁾ Includes cost due to siltation of irrigation canals, reservoirs and harbors
Source: Magrath and Arens, 1989.