Causative Factors of Drought

A significant decrease in rainfall, especially during the dry season, causes serious damage to crops. There are many factors affecting rainfall variability in Indonesia.

Types of Rainfall

There are three types of rainfall within Indonesian region (Boerema, 1938). The first type is monsoon rainfall, which peaks in December. The second is equatorial rainfall, characterized by two monthly rainfall peaks in March and October. The third is a "local" type with a monthly rainfall peak in July-August (figure 26.1).

According to Boerema (1938), monsoon rainfall can be categorized into two groups (types A and B) that are prevalent in southern Indonesia. Types A and B indicate a clear distinction between a dry and a wet season throughout the year. Type A shows a longer dry period (eastern part of Indonesia, Nusa Tenggara islands) and is drier overall compared to type B (Java, South Sumatra, and South Sulawesi). The region with type A rainfall experiences severe drought more frequently. In general, rainfall variation is larger in the dry season (April-September) compared to the wet season (October-March).

Type C (local type), which differs from the types A and B, is located mainly in the eastern equatorial part of Indonesia (e.g., Maluku and So-rong). Wet seasons of this type are between April and September, while dry seasons are between October and March. However, the dry periods of this type are not as dry as those of type A or even type B. As a result, the total rainfall in a year is relatively high.

Equatorial-type rainfall can be divided into two groups, types D and E. Type D covers the west coast of North Sumatra with no pronounced dry season, and rainfall increases slightly around March and October. Type E covers the west coast of South Sumatra with uniform distribution of rainfall throughout the year.

The length of the wet season varies from as long as 280-300 days to as short as 10-110 days. The seasonal rainfall during the wet season varies from as high as 4115 mm to as low as 640 mm. The early onset and the

Figure 26.1 Climatology of seasonal rainfall (updated, based on Boerema, 1938, and ASEAN Secretariat, 1982).

Figure 26.1 Climatology of seasonal rainfall (updated, based on Boerema, 1938, and ASEAN Secretariat, 1982).

late withdrawal of the monsoon result in a lengthy wet season, while the late onset and the early withdrawal entail a relatively short rainy season and a longer dry season.

The dry season ranges between 5 and 35 dekads (one dekad equals 10 days). Most of the area in the eastern part of Indonesia has relatively longer dry seasons. The longest and driest periods are found in central and eastern Lombok (30-35 dekads). The second longest dry season (26 dekads) occurs in Lombok North, and the third one (25 dekads) occurs in eastern Sumba, Flores, Pasuruan-Probolinggo, East Java and Subang, Indramayu, and West Java. The length of the dry season in most of East Java is between 15 and 25 dekads. The shortest periods are found in western and central Java and along the eastern coast of Sumatra. Rainfall during the dry season ranges from 250 to 500 mm in most areas of the archipelago. However, in some parts around the northern coast of West Java, the southern coast of East Java, central and northern Lampung, Sumbawa, West Lombok, the northern part of East Timor, and the northern and southern coasts of Irian Jaya, the rainfall amounts are between 500 and 750 mm.

Impact of El Niño/Southern Oscillation on Rainfall Variability

Indonesia is a tropical country influenced by the Indian and Pacific oceans. Rainfall variability is affected by at least five factors (Tjasyono, 1997): meridional circulation (Hadley), zonal circulation (Walker), monsoon activity, local effects (topography), and tropical cyclones. These five factors work simultaneously throughout the year, but in certain conditions, one factor might become more dominant than others. Table 26.1 shows that all but six drought events were associated with El Niño during the 18441998 period. Many studies imply that droughts in Indonesia were mostly associated with El Niño events (Braak, 1919; Berlage, 1927; Nicholls,

Table 26.1 El Niño and drought events in Indonesia, I844-I998a

1844-

-97

1902

98

Drought

El Niño

Drought

El Niño

1844-45

1844

1902-03

1902

1845-46

1845

1905-06

1905

1850-51

1850

1913-14

1914

1853-54

None

1918-19

1918

1855-56

1855

1923-24

1923

1857-58

1857

1925-26

1925

1864-65

1864

1929-30

1929

1873-74

1873

1932-33

1932

1875-76

1875

1835-36

None

1877-78

1877

1940-41

1939

1881-82

1880

1941-42

1941

1883-84

None

1944-45

1943

1884-85

1884

1945-46

1946

1888-89

1887

1953-54

1953

1891-92

1891

1861-62

None

1896-97

1896

1963-64

1963

1965-66

1965

1967-68

None

1969-70

1969

1972-73

1972

1976-77

1976

1980-81

None

1982-83

1982

1986-87

1986

1991-92

1991

1994-95

1994

1997-98

1997

Source: Updated from Quinn et al. (1978) and ADB and BAPPENAS (1999).

Source: Updated from Quinn et al. (1978) and ADB and BAPPENAS (1999).

aIn drought years, about 60-90% of the Indonesian regions have rainfall below normal.

1981, 1983; Hackert and Hastenrath, 1986; Hastenrath, 1987; Malin-greau, 1987; Harger, 1995; Yamanaka, 1998; chapter 3).

El Niño-related droughts occur when the Hadley cells are weak and the massive subsiding air over Indonesia feeds the increasingly westerly flow of surface air over the Pacific Ocean (Tjasyono, 1997; Kirono and Partridge, 2002). This results in the weakening or failure of the Walker circulation (Tjasyono, 1997). When the Hadley circulation is strong and the favorable local climate conditions prevail, subsidence can be localized, even during an El Niño event.

The effect of El Niño on rainfall varies between regions. It is strong in regions that are strongly influenced by monsoon systems, weak in regions that have equatorial systems, and unclear in regions that have local systems

(Tjasyono, 1997). The impact of the El Niño/Southern Oscillation (ENSO) on rainfall is more pronounced during the dry seasons than the wet season (USDA, 1984; Las et al., 1999). Various studies on the influences of ENSO on interannual rainfall variability in Indonesia reveal some seasonal patterns (USDA, 1984; ADPC, 2000; Yoshino et al., 2000; Kirono and Partridge, 2002): (1) The end of the dry season occurs later than normal during El Niño and earlier during La Niña years, (2) the onset of the wet season is delayed during El Niño and advanced during La Niña years, (3) a significant reduction of dry season rainfalls could be expected during El Niño and significant increase could be expected during La Niña years, (4) long dry spells occur during the monsoon period, particularly in eastern Indonesia. Observations at five locations during 1982-83 showed these patterns (table 26.2).

An analysis of time-series rainfall data (1951-98) for Pandeglang and Lebak (West Java) revealed that the normal onset of the dry season in this zone could be around the first week of June (ADPC, 2000). However, during extreme La Niña years, it could be delayed to the first week of August. During extreme El Niño years, the onset of the dry season could be advanced as early as mid-April. Against the normal rainfall of 379 mm during April-September, the rainfall could be as high as 979 mm during the La Niña years and as low as 15 mm during El Niño years (ADPC, 2000).

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