Transcript Applied Hydrology Rainfall Analysis - RSLAB-NTU

Applied Hydrology
Hydrological Forecasting
Prof. Ke-Sheng Cheng
Department of Bioenvironmental Systems Engineering
National Taiwan University
Introduction
• Floods and droughts are two most severe
natural disasters hydrologists frequently
encounter.
• Droughts are disastrous events with creeping
effects.
• In contrast, floods often are expected to occur,
although the exact time of occurrence and their
magnitudes are difficult to forecast.
• Flood forecasting for large and long rivers in
continental countries depends heavily on
channel routing.
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• Floods in Taiwan and other similar islandcountries are typically flash floods.
– Flash floods are dangerously fast moving floods
caused by a large amount of heavy rain in a
localized area.
– Flash floods occur for a variety of reasons including
concentrated rainfall during a slow moving
thunderstorm, cyclones, and tropical storms.
– Flash flood forecasting is more difficult than normal
flood forecasting since it depends on successful
realtime rainfall forecast of high temporal resolution
(for example, hourly rainfalls).
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Definition of droughts
• Drought is a period of dryness especially when
prolonged and causes extensive damages to
crops.
• Types of drought
– Meteorological drought (氣象乾旱，降雨)
– Agricultural drought (農業乾旱，土壤水分)
– Hydrological drought (水文乾旱，河川流量與水庫
蓄水量)
– Socioeconomic drought (社經乾旱，民生資源)
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Progressive occurrence of droughts
不同類型旱災發生順序
氣象乾旱
旱災之前兆
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農業旱災
水文旱災
社經旱災
旱災之發生
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Drought Indices
• Meteorological drought index
– Standardized Precipitation Index, SPI
• Agricultural drought index
– Palmer Drought Severity Index, PDSI
– Surface Water Supply Index, SWSI
• Hydrological drought index
– Shortage Index, SI
– Deficit Rate, DR
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Standardized Precipitation Index
• Usually calculated from daily rainfalls
• Simplicity in calculation
• Can be associated with different drought
severity levels
SPI range
-1 < SPI  0
-1.5 < SPI  -1
-2 < SPI  -1.5
SPI  -2
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Drought severity level
Close to normal or mild
Moderate
Severe
Extreme
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Calculation of SPI
• Two time scales
– Operation scale (ts)
The time period for operation usage of SPI. For
example, paddy irrigation water supply is scheduled
on a ten-day-period (TDP) basis. As a result, SPI
must be calculated and assessed on TDP basis.
– temporal resolution (tr) of the index
The time span for cumulative rainfall calculation.
Cumulative rainfall of a specified time span forms
the basis for SPI calculation.
• Spatial scale
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Cumulative rainfalls for SPI
calculation
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Transforming from cumulative
rainfall to SPI (TDP-specific)
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Finding distribution parameters
• TDP-specific cumulative rainfalls are considered
having a gamma distribution.
1 x
f X ( x ; ,  ) 
 
( )   
 1
e ( x /  ) , 0  x  
• Calculate maximum likelihood estimates of the
distribution parameters.
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Return period
(years ?)
2
6.30
14.97
43.96
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SPI乾旱指標與旱災等級
氣象乾旱
旱災之前兆
輕度旱災
中度旱災
嚴重旱災
農業旱災
水文旱災
社經旱災
旱災之發生
訂定各旱災等級之SPI範圍
啟動供水分析模式
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嘉南灌區之乾旱分析
-使用標準化降雨指標
雨量站
#
名稱
測站號碼
資料長度
1
小公田(2)
01L360
41
2
中坑(3)
01L910
46
3
里佳
H1M220
38
4
嘉義
00L090
46
5
六溪
01O080
50
6
曾文
02N730
45
7
曾文新村
H0O660
39
8
虎頭埤
01O710
28
9
關山
01O760
24
10
王爺宮
01O750
25
11
大埔
01L430
26
12
塭港
00M300
23
16
研究區代表站六溪雨量站歷史年雨量紀錄之
一致性檢驗(使用日降雨紀錄)
1959
水利署網頁資訊
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本研究分析結果
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嘉南地區歷史乾旱紀錄
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六溪雨量站之歷史SPI(累積9旬雨量)序列
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歷史乾旱事件及其SPI特性
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2009嘉南降雨與SPI指標
WRA Raingauges
#
站號
站名
縣市
TM2X
TM2Y
記錄年份
統計年數
1
01L360
小公田(2)
嘉義縣
212557
2591545
1967~2008
42
2
01L390
大湖山
嘉義縣
210373
2597161
1953~2008
56
3
01L480
樟腦寮(2)
嘉義縣
208592
2603575
1960~2008
49
4
01L490
沙坑
嘉義縣
200217
2603298
1965~2008
44
5
01L910
中坑(3)
嘉義縣
200270
2607743
1962~2008
47
6
01M010
溪口(3)
嘉義縣
187931
2609686
1957~2008
52
7
01M310
新高口
嘉義縣
233701
2596708
1983~2008
26
8
01N840
西阿里關
台南縣
206500
2558138
1972~2008
37
9
01N850
南化(2)
台南縣
195814
2549518
1972~1992,2000~2008
30
10
01N860
崎頂
台南縣
183525
2540753
1951~2008
58
11
01O070
關子嶺(2)
台南縣
198753
2581219
1957~2008
52
12
01O080
六溪
台南縣
193866
2578818
1958~2008
51
13
01O190
東原
台南縣
192476
2573984
1958~2008
51
14
01O200
北寮
台南縣
197653
2576737
1957~2008
52
15
01O710
虎頭埤
台南縣
181234
2547489
1980~2008
29
16
01O750
王爺宮
台南縣
187808
2569223
1983~2008
26
17
01O760
關山
台南縣
207721
2563745
1984~2008
25
WRA Raingauges
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2009 Rainfall
莫拉克颱風(88水災)百年降水
2009 SPI
*黑色區塊應為白色,為轉檔之錯誤
*黑色區塊應為白色,為轉檔之錯誤
*黑色區塊應為白色,為轉檔之錯誤
莫拉克颱風(88水災)百年降水
*黑色區塊應為白色,為轉檔之錯誤
*黑色區塊應為白色,為轉檔之錯誤
*黑色區塊應為白色,為轉檔之錯誤
• 莫拉克颱風於八月上旬有百年降雨
• 除了上述降雨(八月上旬)之外, 2009年第15旬
至30旬(五月到十月) 由SPI指出為明顯氣象乾
旱
Drought monitoring
• Drought duration (dry spell)
• Intensity (can be characterized using average
SPI or cumulative anomaly)
• Bivariate distribution of dry spell and drought
intensity
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Drought monitoring and
forecasting
• Reference
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Drought Monitoring System
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This is typical of storms in Taiwan.
A storm event of less than 24-hr
duration may completely eliminate
all drought effects.
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Drought forecasting
•
•
•
•
•
Drought index forecasting
Autoregressive model
ANN
Markov-chain
Ensemble forecasting (probabilistic forecasting)
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