| South-East Asia's Environmental Future: The Search for Sustainability (1993) |
|Part II - Climatic change and variability|
|8. Climatic change and agriculture: Problems for the Asian tropics|
Problems in climate-agriculture relationships: Rice yields in three areas
The case of paddy (rice) production in Sri Lanka is of relevance, because it demonstrates the complexities of climate-agriculture relationships in an area that has several elements in common with South-East Asia. It is discussed with reference to results obtained by Yoshino and Suppiah (1984).
There are two cropping seasons in Sri Lanka corresponding with the northeast monsoon, or Maha season, and the south-west monsoon, or Yala season. Sowing dates in the former season extend over several months, but over only a short period in the latter (Yoshino, 1984b: 95). Most of the dry-zone districts (roughly, the north-eastem part of Sri Lanka) show a significant relationship between harvested area and rainfall in the Maha season, while in the wet-zone districts (roughly, the south-westem part of Sri Lanka), the significant relationship is with rainfall in the Yala season. But there are some districts which show strong relationship in both seasons, as they are affected by both monsoons. Figure 8.3, calculated from data over 20 years (1960 80), relates deviations in harvested or sown area to seasonal rainfall deviations for three places.
In the dry zone, Maha season rainfall which is less than one standard deviation below the mean has caused severe crop losses, extending into the subsequent Yala season and, in some cases, into the following Maha season. The failure of the south-west monsoon (Yale) results in water-deficit conditions in the wet-zone highland paddy lands. Excessive rainfall in either or both seasons causes floods and waterlogged conditions in the lowlands of the country. In the driest part of Sri Lanka, there is a unique relationship in the range of anomalous negative departure of rainfall below -10 centimetres: the relationship is positive; but above -10 centimetres, the rainfall has no effect on the harvested area. The reasons for this relationship are not clear.
In Figure 8.4, the secular changes of total paddy area harvested and sown for the whole of Sri Lanka over 20 years, and the average seasonal rainfall of all of Sri Lanka, are presented, using data drawn from Yoshino et al. (1983). From this figure, it can be seen that paddy production is influenced more by rainfall in the Maha season than in the Yala season. In fact, the area sown minus area harvested has the closest relationship to the rainfall. A total product departure from the calculated value, which is an experimentally calculated value in view of the secular increase in area, is significantly correlated for the Maha season, but not for the Yala season. However, absolute values of sown area, harvested area and total product have no significant correlation with rainfall. This is due to the sharp increase in these values during the 20 years under consideration, not to an increase in the rainfall parameters.
On the basis of individual years, a number of other interesting effects can be seen; for instance, a low Yala rainfall coincided with low harvested and sown areas in 1976, but not in 1966. Table 8.1 presents the years of minimum sown and harvested area: 1965, 1969, 1972, 1973, 1975, 1976 and 1979. These are compared with minimum points on the rainfall curves, and the years marked according to perfect fit, relatively good fit and non-fit between rainfall and agriculture. Years for minimum sown and harvested area in the Yala seasons of 1965, 1969, 1972 and 1975 correspond to the rainfall minima in the preceding Maha seasons, in each of which, rainfall was below about 800 millimetres. Table 8.1 also compares conditions of drought in the 1964-5 and 1974-5 Maha seasons. In the 1974 5 Maha season, the drought was most serious, rainfall being less than two standard deviations below the mean. The sown and harvested areas, however, were smaller in 1964-5 than in 1974 5, not because of rainfall but because of development of cultivation techniques and irrigation systems.
TABLE 8.1 Fitness of Minimums of Yala Sown Extent (Y.S.) and Yala Harvested Acreage Y.H.) to the Minimums of Yala Rainfall (Y.R.) and Maha Rainfall (M.R.) in Sri Lanka, 1961-1980
|Y.S. Minimum||Y.H.. Minimum||M.R. Minimum||M.R. (mm)|
|Average (1961 -80)||977|
Source: Yoshino et al. (1983).
Note: · = perfect fit: o = relatively good fit; x = non-fit.
The timing of cultivation is a serious problem for water management in Sri Lanka. It depends upon water being available at the proper time. The phenomena described above show something of the real nature of this problem. Domroes (1978) wrote that the relationship between crop yields and drought conditions is complicated, and that rainfall conditions may not immediately affect crop production. It is therefore of interest that in the worst cases noted here, when deficient Maha rainfall carried its effect over into the succeeding Yala season, Maha season rainfall was less than a standard deviation below the mean. Perhaps this represents a critical climatic value in this complex relationship.
Indonesia: The Drought of 1982 in Java
Indonesia covers a wider climatic range than Sri Lanka. In the months between November and February-March, the northwesterlies bring humid air, in a season generally termed the north-west monsoon. There are some local differences in rainfall distribution in accordance with topography and position in relation to the prevailing wind direction. During the period from April to October-November, Indonesia is influenced by the relatively dry southeasterlies from across Australia.
Of the total area of Indonesia, around 14.2 million hectares are classified as arable land. This arable land can be divided into several types, among which rice-field (sawah) land and upland are most important. Sawah land, in which rice is grown under flooded conditions, can be subdivided into irrigated, rainfed and swamp land. Upland agricultural areas have greater variety (Oldeman, 1984). Three-fifths of sawah land and more than one-third of the upland areas are located in Java.
Since 1967, the average yield of rice has increased markedly, from a fairly constant 2 000 kilograms of rough rice per hectare, to reach 3 170 kilograms per hectare in 1978. Yields in the dryland areas showed a much smaller improvement, from 1 100 kilograms per hectare in 1968 to 1 300 kilograms per hectare in 1978. Because of the large year-toyear variation of rainfall, and the differing water supply conditions, there is great variety, even in Java, in the seasonal patterns of planting and harvesting. Table 8.2 shows the contrast between different parts of the island. The main harvest peaks are between April and June, which coincides with the end of the wet season, but there are different crop calendars in some areas. In Tuban, East Java, there is a sharp concentration in the harvest period, soon after the end of the short December-to-April rainy season; elsewhere, multiple cropping gives a wider annual spread of production peaks.
Malingreau (1987) has analysed the effects of the severe drought of 1982 in Java. In the main producing areas, the 1982 dry-season crop was planted in April-May, more or less on schedule, but started to suffer from the water shortage as the season progressed. In all parts of Java, the harvested area was reduced more than 10 per cent in 1982, compared to previous dry seasons. Then, because of the late arrival of the monsoon at the end of 1982, wet-season planting was delayed, so that the rice was in the ground only by the end of December instead of early November, as normally. The resulting compression of the 1983 wet-season harvest in April put heavy pressure on harvest labour, and on the milling and storage systems. Moreover, many farmers planted corn rather than rice, because the rainfall was still insufficient at the end of 1982. This led to a further reduction in rice production in 1983, especially in East Java.
TABLE 8.2 Share of Monthly Harvested Area of Wetland Rice in Total Annual Harvested Area in Selected Regions in Java (per cent)
Source: Oldeman (1984).
Supplementing production data with innovative use of remote sensing, Malingreau (1987) concluded that the most important effect of the drought was a drastic reduction in the growth rate. A satellite-derived vegetation index for an area in West Java showed a peak value in June 1982 only 60 70 per cent of that during other dry seasons in a threeyear period; there was then no recovery of the vegetation index in the second half of the year. The drought affected both the dry-season crop of 1982 and delayed the planting of the 1982-3 wet-season crop. This is a similar pattern to that found in Sri Lanka, discussed above.
Hainan Island, South China: The Tropical Margin
A different sort of climate-agriculture relationship is exhibited on Hainan Island, South China. The northern boundary of the true Tropics in South-East Asia might be placed at the southern limit of cold-wave invasion in winter; these waves extend into continental South-East Asia. The frequency and magnitude of cold waves differ according to largescale synoptic events and regional-scale topography (Yoshino, 1988). In the eastern part of South China, cold waves occur quite frequently under the influence of anticyclones over the region. In the most striking cases, the temperature fall at ground level may be more than 20 °C. The so-called 'cold-dew winds' (hanlufeng) which blow during the flowering period in the autumn are also important (Yoshino, 1984a, 1986).
Rice production in Hainan has increased substantially since the introduction of International Rice Research Institute (IRRI) varieties in the mid-1970s. With fastermaturing cultivars, there has been a substantial increase in the early rice crop, to now about four-fifths the size of the late rice crop and, under good conditions, giving even better yields. However, the full potential productivity is not realized. The early rice cultivars present the major problem. They are sown from December to May and harvested in 160-170 days. Solar radiation in the ripening months is more than adequate, but the greater part of the growing period of early rice is the dry season. Irrigation is necessary, and winter sweet potatoes are also grown as a dry crop. But the actual crop calendar depends strongly on the incidence of cold waves, winter monsoons and also typhoons in late summer. Combinations of crops would provide better insurance against these hazards.