Causes of Drought Occurrence

Drought occurs due to restricted irrigation, lack and uneven distribution of precipitation in any season, late onset of rainy season, and occurrence of dry spells. Agricultural water use is the largest component (45%) of total direct water use in Korea (MOCT, 2001). Sometimes agricultural water usage is curtailed to meet the urban and industrial water demands, and as a result drought may occur.

The pressure distribution of the marine tropical air mass, polar air mass, or continental air mass determines the amount of precipitation in summer season. When one of the air masses is excessively strong or weak such that the high pressure system is centered over the Sea of Ohkotsk and not over the northern Pacific Ocean, a drought begins (Park and Schubert, 1997).

Table 30.1 The main drought years during I960s-90s

Year

Rainfall (mm) (May-July)

Rainy days No. Frequency

Rate of reservoir storage

% Frequency

No. of dried reservoirs

Drought area (ha)

Amount of damage (million $)

Budget for drought mitigation (million $)

1967

307.4

56

7

5

25

402,547

522

5

1968

122.2

72

50

4

30

470,422

584

5

1981

658.2

50

5

46

2

5,306

145,547

181

43

1982

300.8

54

7

27

7

13,593

231,244

287

40

1994

231.3

68

30

15

15

6,728

113,300

Low Precipitation during the Spring Season The period from April 1 to May 15 is the spring rainy season (Tian and Yasunari, 1998; Byun and Lee, 2002). The warm and humid southerly airflow in the layer below the 700 hPa level combines with the upper-level, cold northerly current (that is still dominant in the layer over 500 hPa) and causes rains with intensified hydrostatic instability. However, sometimes, for reasons not adequately understood at present, the precipitation is low and evaporation is high in spite of the gradual rise in temperature and humidity. Recently Byun and Lee (2003) found that the low precipitation is caused partly due to the decrease in the meridional gradient of the adjacent sea-surface temperature. In March, the groundwater level rises due to the melting of snow and ice. However, this has only a small effect on the replenishment of water resources. As a result, drought occurs and hampers the process of germination and sprouting of seeds, thus damaging the crops.

Late Onset of the Summer Rainy Season Influenced by the Asian monsoon, the summer rainy season in Korea begins in the second half of June and continues until the end of July. The precipitation during this period makes up more than 50% of the annual precipitation. Therefore, if the summer rainy season begins late, nonirrigated land is affected first, but occasionally irrigated farming also suffers. The main reason for a late onset of the summer rainy season is that the marine tropical air mass (mT; Oliver, 1987) called the Northern Pacific High becomes weak and develops late due to low heating of the Asian continent. Hence, the warm southerly flow does not reach Korea. For example, when snow is widespread on the Tibetan Plateau, it reflects more solar rays and retards the heating and melting of snow. Further, if there is a significant amount of ice in the Okhotsk Sea, a marine polar air mass (mP; Oliver, 1987) develops due to snow thawing in May and June. When the mP is strong, the frontal system that forms the rainy season is obstructed from proceeding northward, and there is a delay in onset of the rainy season (Byun et al., 1992a, 1992b).

Low Precipitation in Summer Rainy Season Low precipitation during summer seasons is attributed not only to the absence or infrequence of low-pressure systems, but also to lower temperature, a lower moisture gradient, and the persistence of upper level high-pressure systems (Byun and Han, 1994; Byun, 1996).

Low Precipitation during Winter Kaul-Changma (the early fall rainy season in Korea) begins about the middle of August and ends in the middle of September. This phenomenon is dominant in the southern half of the Korean Peninsula, and the precipitation during this period is stored in reservoirs or dams for irrigating crop in the following year. In this case, the precipitation is supplied mainly by the lows that pass near the Korean Peninsula. At times, the weakened tropical cyclones pass the peninsula and cause heavy rains. Also, an early-winter downpour occurs between late

October and mid-December and replenishes reservoirs. But such rains occur only once in 2.5 years (Jeong, 2002). Without heavy rains, shortage of water resources occurs in the following spring.

0 0

Post a comment