The heavy drought of 2002 and the large flood of 2003 in NLB
Kong Xiangguang1 Qian Mingkai2(1Yi Shu Si River Water Resources Bureau, Huaihe River Commission
No.186,Jiefang Road, Xuzhou, 221009, Jiangsu, China. E-mail:kongxg@yss.org.cn2
The Bureau of Hydrology, Huaihe River Commission, E-mail:mkqian@hrc.gov.cn )Abstract: The paper gives an introduction of the heavy drought of 2002 and the large flood of 2003 in theNasihu Lake Basin (NLB), including its geographical location as well as hydrometeorological features. It
also includes Nasihu Lake river system, natural river system and man-made channels. The precipitation,drought, flood and hydrological situation in 2002 and 2003 are analyzed. In 2002, the NLB experienced aheavy drought. The disaster in northwest area was especially serious. In 2003, the second largest of flood
since 1950 happened. It caused flood disaster over large area. The lasting time of flooding was long and thewater volume was great. All this led to the highest water level for some hydrological stations in the recent
30 years. The paper also gives analysis on the climatic cause of the floods and drought disasters.
1. General introduction to the basin
The Yishusi River Basin(YSSRB) is an independent watersheds. It is a part of Huaihe River Basin(HRB). The basin bounded by the old Yellow River and Huaihe water system in the south. The Nasi lake is
6th fresh water lake in China. It is composed of the Nanyang lake, Dushan lake, Zhaoyang lake and Weishanlake. NLB located in the westward of the YSSRS. There are 53 main tributaries flowing into the lake with a2catchment area of 31400km 。In 1960 a gate (called Erjie gate ) was built in the lake and the lake is divided
two parts: the upper and down lakes. NLB is located in the transition belt of climate between the north andthe south of China. The climate change is violent. The change of rain between years and months is unevenand the flood and drought occur frequently. The average annual rainfall is 695.2mm. The rainfall in theflood season from June to September is about 70% of annual rainfall. The average potential evaporation
amount in the lake per year is 1000mm and the average evaporation amount for land is 500～650mm.Theriver networks in the west of the lake are richer, and their bed slopes are less so that the flow velocities are
small and the flood amount are big.
Because the slope of the main stream is bigger in east of lake, the flood lasts a short period of time andthe magnitude is large with rising up and falling suddenly. Although having been harnessed for many times,
the criterion of the present flood control is still lower and it is to protect for 20-years occurrence flood. The
typical flood events occurred in 1730, 1974, 1957 and2003. The typical drought events occurred in 1966，
1988 and 2002. As an whole river basin flood happened in the YSSRS in 1957, the occurrence of flood of
NSL was near to a hundred years.

2.Rainfall and water information of flood season in 2002
2.1 Rainfall
The average monthly precipitation during flood season from June to September in YSSRB are: 101mm, 236 mm, 157 mm and 83 mm, and monthly precipitation from June to September in 2001 were 87 mm,97 mm, 59 mm, 33 mm. It is clear that rainfalls in 2002 were less 13.9%, 58.9%, 62.4%, 60.2% than thenormal year. The rainfall during flood season in 2002 was 276mm, and was less 52.2% than that of the1average annual precipitation. It was a drought year of 2002. The rainfall amount of 2002 was the minimum
since the year 1953 and the heavy drought happened in the northwest of basin. In regional distributing therainfall decreases from the southeast to northwest gradually. Fig. 1 shows Isohyets for flood season (June toSeptember, 2002) in the YSSRB.
The average precipitation during flood season from June to September was 231 mm in NLB, and wasless 52.5% than that of the average annual precipitation. The rainfalls in the northwest of upper lake of NLB
were small, which was between 100 to 200mm. The rainfall in other parts of the basin was between 200 to300mm. In regional distributing the rainfall decreased from the southeast to northwest gradually.
2.2 Water Information
The Nanyang station located in the upper lake of NLB, on the 1st of January, the water level of thestation was 33.43m, just 0.43m higher than its minimum pool elevation, measured that the lake dried up at
the 16th July, which was the 6th since the recordation of hydrological data. The upper lake was just sere in
the end of flood season. The flood volume of the lake is 0, the flood volume has decreased 77 million m3
during the flood season.
The Weishan station located in the lower lake of NLB, on the 1st of January, the water level of thestation was 31.48m, just 0.02m less than its minimum pool elevation. On August 25, the water stage in thelower lake was 29.85 m, it was lowest which means the lake dried up. The water level of upper lake wasjust 30.36m in the end of flood season. The flood volume was 27 million m3, the flood volume hasdecreased 143 million m3 during the flood season.
During the flood season of 2002 there are no obvious floods. The peak discharge of the Si river, thebiggest tributary of the lake was only 2.5 m3/s. The flow from other tributaries was also much less. The
amount during the flood season of 2002 was only 8.2% compared with the averaged amount of same season.
The gates in the upper and down lake were closed in this year.
The groundwater was over used because of the shortage of the water resources this year The
groundwater table was 4.15m deeper than that of the normal year. The area of cone of depression 12289.9
km2 and the depth was from 13 to 49m。
2.3 Draught Information
The rainfall in NLB in 2002 was badly small, which is much less than that of normal year. It led theheavy drought. The occurrence of the drought is 50 year according to calculation, the drought was serious
especially in the west of the basin.
After the 16th July 2002, the upper and lower lakes dried up one after the other, till 16th June 2003.
Only a few water in the lake such as deep-routes and river channels in the lake. The large big continuous
water surface was not visible. The whole bottom of the lake looked like regions of drainage system. Theinter-crossed channels were visible. There were farmlands and pounds on each side and grass flourished in
the wasteland. Cars could travel freely. Every tributary dried up so that the lake dried up. There were no any
water in the lake and river. There were some cracks in the bottom of lake and river with 10 cm wide.
The economic development, people living and social stabilization in NLB had been impacted seriously
by draught. The economic loss reached 1.33 billion RMB. The loss of farming, fishing, aquiculture, ship
and tourist was great. The natural environment had been destroyed greatly.
2.4 Formation of weather
The average temperature in YSSRB persisted high from Jan. to April in 2002 and as a result,evaporation augmented. Subtropical high was a little stronger than normal year in April and May. However,2the main rain belt was located in the north region of HRB. The precipitation was small in the NLB.
In June, the subtropical high was a little weak and the rainfall was small. In normal years, Meiyu
period is between the 22nd June and the 10th July. The stabilization time of subtropical high was short inthis year and Meiyu period of the basin was just nine days from 19 to 27 June. During the Meiyu period,
there was only one day, when the area rainfall exceeded 5mm in NLB.
In July and Aug. of the normal year, the rainfall is rich in main flood season of NLB. From last days ofJune to 20th July 2002, the subtropical high was a little weak There was a course of heavy rain, but the timewas short and the rainfall was small in NLB. In September this year, the subtropical high was a little weakcontinuously and the precipitation was small.
3. Rainfall and water information of flood season in 2003
3.1 Rainfall
The rainfall was 835mm from June to September of 2003, which exceeded the average annual
precipitation by 50.5%. Monthly rainfall from Jun. to Sep. was 140mm, 315mm, 272 mm, 108mm
respectively, and exceeded the average annual precipitation by 40.0%, 42.5%, 68.9% and 45.9%,respectively. There was much precipitation in the continuous four months and it was the abundant water
year. Total rainfall in the flood season of 2003 was 839mm, and ranked in the 2nd since 1953. Fig. 2 shows
isohyets for flood season (Jun. to Sep., 2003) in the YSSRB.
The rainfall was 800mm~1000mm in the east of YSSRB, and exceeded the average annual precipitation
for this area by 30%~50%. The rainfall was 600mm~800mm in the west of YSSRB. It exceeded the averageannual precipitation by 20%~40%. It was smaller than 600mm in the northwest of the basin. In distribution,the rainfall decreased from the southeast to northwest.
3.2 Water Information
From last ten days of June in 2003, the hydrological condition in NLB was changed so much thatdrought quickly turned into flood. The water stage of the upper lake on 22 July reached the 33.00m(minimum pool elevation), and then reached the 34.20m, the flood prevention level on August 31, it reachedthe 35.28m on 22 July, the highest stage since 1973. The water stage in the Weishan station of lower lake
reached the 31.00 m (minimum pool surface)) on 12 July, it reached 32.50m, the flood prevention level on
August 27, and it reached 33.33m, the highest since 1974 on October 14,. The water gate in Erjie dam（it isway out of the upper lake）, Hanzhuang water sluice（it is way out of the lower lake） were opening fully inSeptember. The flood amount flowing from the two gates are 1.404 and 1.636 billion m3, respectively. They
were the largest ones since 1985 and 1974, respectively.
After the flood season of 2003, the groundwater table rose 4.32m and the groundwater amountincreased 3.57 billon m3.
The occurrence of moving maximum flood amount of 15 days is 10-years. The flood caused theinundation of the farmland around the lake for long time.
At the end of flood season, the storage volume compared with those of the last ten days of June inupper lake and lower lake increased 1.251 and 1.223 billion m3, respectively. During the flood season, theinflow volume was 4.11 billion m3 in the NSL.
During the flood season, the reservoirs stored the flood volume as much as possible according to theflood management and regulation rules to guarantee the safety of rivers and lakes.
3.3 Main water information in other parts of YSSRB
3The runoff amount from upper Luoma lake basin was 7.09 billion m3. The biggest flood discharge was3290 m3/s and the accumulative discharged water amount was 5.90 billion m3 and the sixth biggest one
since 1974.
During the flood reason of this year, HRB suffered the heavy flood, is the second largest (The flood in1954 is the largest.) since 1950. The Huaiyin water gate was opened for discharging flood from Huaihebasin to Yishusi Watershed on July 6.The maximum discharge was 1720 m3/s. The accumulative wateramount at Huaiyin water gate is 264,000,000 m3.
Shuyang Station of the New Yi River witnessed the highest water level since 1974. The water level
reached the highest level of 10.71 m. It surpassed the Alert Water Level by 1.71m and ranked second in thehistory. The flood peak at Shuyang Station was 4860 m3/s and ranked third in the history.
3.4 The Storm mechanism
In the summer of 2003, the precipitation amount was more than usual in the YSSRB. The precipitation
concentrated on the period from June 20 to September 8.The precipitation in this period could be divided
into two stages.
The first stage was Meiyu precipitation from June 20 to July 28. The second stage was from August 14
to September 8. The Subtropical high during this stage was evidently stronger and extending to west thanthat’s in the first stage. The cold air moved towards deflection north and also the typhoon was very activity
during the period. The precipitation of YSSRB was stronger.
4.Conclusion
2002’s drought and 2003’s flood situations expressed the circumstance of two kinds of typical models
of drought and flood in YSSRB. It reflected the basic characteristic of YSSRB:(1) The drought is very easy to happen due to average annual precipitation is small and have no stablewater source to supply in YSSRB with a short sources of river and complicated river systems;(2) The criterion of flood control is low to hydraulic engineering in YSSRB, so flood is probably totake place.
Now, constructing South-to-North Water Transfers project and the second period of towards
east-to-towards south flood control project which can advance the flood control standard of YSSRB to be
50-year, we will completely change this fact that flood and drought is very easy to happen.

Fig.1 Isohyets from Jun. to Sep., 2002Fig.2 Isohyets in Jun. to Sep., 2003

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