Application of remote sensing data in estimation of surface water in Bangladesh *
Md. Nazrul Islam , Taiichi Hayashi1, Toru Terao2 and Hiroshi Uyeda3 *
Department of Physics, Bangladesh University of Engineering & Technology, Dhaka, Bangladesh Disaster Prevention Research Institute, Kyoto University, Japan 2 Faculty of Informatics, Osaka Gakuin University, Japan 3 Hydrospheric Atmospheric Research Centre, Nagoya University, Japan 1
ABSTRACT: Proper estimation of precipitation is very essential for its various important features such as flood and drought which are very common natural disasters in Bangladesh (88.05°-92.74°E and 20.67°-26.63°N). It is well known that thunderstorms in Bangladesh cause death and huge damage including flash floods in each and every year. Precipitation is invaluable as a source of renewable fresh water and the importance is much more realistic in a region like Bangladesh which suffers from excessive rate of Arsenic in her ground water. In this situation knowledge of proper distribution and quantification of precipitation may be helpful for the disaster prevention and water management in that region. However, there are not so many detailed studies on the distribution of precipitation and absolutely no work using radar data over the country. At present, low-dense rain-gauge network over Bangladesh is only the tool to measure rainfall. The information from low-dense rain-gauge network sometimes is not sufficient for quantification of exact amount of precipitation and for monitoring flood situations. In order to gain the precise distribution of rainfall radar data available for 2000 from the Bangladesh Meteorological Department (BMD) are employed as the first research work in Bangladesh. The distribution pattern of rainfall retrieved from rain status of BMD radar PPI scan data represents the similarities with rain-gauge and TRMM 3B42 products determined distribution except the magnitudes are different. At present form of radar data, radar estimated rainfall amount multiplied by about four provides the ground-based rain-gauge amounts. 1
NTRODUCTION
The people in Bangladesh are repeatedly confronted by natural and human-made catastrophes such as flooding, surface and groundwater pollution, droughts, cyclones, riverbank erosion, air pollution, wetland loss, tornadoes, earthquakes, and coastal erosion. Some of these environmental degradations and calamities are not directly related to human activities and land-use practices (i. e., tornadoes, cyclones, heavy rainfall, storm surges and earthquakes). Others are related to human interactions with the nature. There are two types of floods which occur in Bangladesh, a) annual floods that inundate up to 20% of the land area and b) low frequency floods of high magnitude that inundate more than 35% of the area. The annual floods are essential and desirable for overall growth of the Bangladesh delta and the economy. The major floods such as those that occurred in 1954, 1955, 1974, 1984, 1987, 1988, 1993, 1998, 1999, 2000 and 2004 are destructive and cause serious threat to lives and economy. The water is polluted by floods that it affects immediately the health of, especially children in water-borne diseases like dysentery, diarrhea, may be the risk of cholera as well even polio. Acute respiratory infections and skin infections are also causes by flood water. During the flood the city area has gone under floodwaters polluted with sewage and muck, exposing people to serious health hazards. Rivers are receding, but as the waters go down, outbreaks of diarrhea, malaria, dengue, skin disease, typhoid, jaundice and other waterborne diseases are reaching an epidemic proportion, especially in remote areas where most tube-wells went under water. Thousands of people, mostly children, who are susceptible to the waterborne diseases, are falling sick everyday in absence of safe drinking water. Flooding is one water-related environmental problem. The magnitude and intensity of flooding are very much dependent on land-use practices in the watershed of each rivers or streams. Even though, natural weather event such Correspondence to: Md. Nazrul Islam, Department of Physics, Bangladesh University of Engineering & Technology, Dhaka-1000, Bangladesh.
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as heavy rainfall is one of the key parameter that contributes in flood-water, especially in flash flood. Rainfall in the upper catchment of the Ganges, Brahmaputra and Meghna (GBM) river basin is one of the major causes of floods in Bangladesh (Matsumoto, 1988; Matsumoto et al., 1996). Riverine floods occur when the amount of runoff originating in a watershed exceeds the carrying capacity of natural and constructed drainage system. Flooding can occur due to river overflow or surface runoff. We need to analyze possible underlying causes of recent unusual and frequent floods in Bangladesh in light of meteorological processes that operate in the excessive rainfall in this region. For the third consecutive year, Bangladesh is suffering from severe flooding and river erosion in 2000. The heavy monsoon rains in Bangladesh began six weeks early this year with no sign of letting up (IFRC-RCS report, 2000). The monsoon-related disasters have left some 4 million people homeless. In 2000 floods usually dry southwest parts of Bangladesh was affected and killed more than 100 people, damaged 625,000 acres (250000 hectares) of rice and other crops, and washed away nearly 675 miles (1080 km) of roads. By the 11 August 2000, approximately 1.26 million people in 41 sub-districts have been affected by the floods (ADRC Disaster Report, 2000). In such type of situation proper estimation and the precise distribution of precipitation is very essential for flood monitoring and mitigates the economical damage and safe lives. 2
CLIMATOLOGY
From the past fifty year statistical average, annual rainfall over Bangladesh is 2360 mm and ranges from 1650-2930 mm. At a particular place it is from 1200-5600 mm. The western part of the country is usually dry compared to the north-east and south-east parts (Fig. 1). Bangladesh is a riverine country with in mighty drainage systems (Ganges, Brahmaputra and Meghna (GBM)) along with its innumerable tributaries which originates in the Himalayas, and flow through Bangladesh to the Bay of Bengal. The Himalayan Mountains are largely responsible for the hydrometeorological cycles of the whole Indian subcontinent including Bangladesh. The Bay of Bengal is situated to the south of Bangladesh and plays role as a source of water vapor. Most of the areas of Bangladesh are plain. It is an active delta of the great rivers GBM and their tributaries and distributaries. The Ganges and Brahmaputra travel a long distance before entering to Bangladesh and ultimately falls to the Bay of Bengal. These (GBM) rivers and their tributaries carry huge rain-water from the upstream and contribute to the flood process. The rain-water inside the country contributes to severe floods and causes the flash floods. Still today the low dense rain-gauge network (triangle mark in Fig. 1) of BMD is the tool to estimate rain-water over the country. Sometimes this low dense raingauge network may not enough to catch the exact rainfall amount and to determine the spatial and temporal distribution of precipitation over the country. 26.08
Rain-gauge Rainfall (1950-1999)
11 10.5 10
25.08
9.5 9
LATITUDE (N)
8.5 24.08
8 7.5 7
23.08
6.5 6 5.5
22.08
5 4.5 4
21.08 87.44
mm/day 88.44
89.44
90.44
91.44
92.44
LONGITUDE (E)
Fig. 1. Rain-gauge estimated daily rainfall averaged for 1950-1999. The triangles represent the rain-gauge locations over the country. The circle represents the radar location.
The monsoon season in Bangladesh is divided into three viz; a) pre-monsoon, b) monsoon and c) post-monsoon (Das, 1995; Islam et al., 2004). Decadal average of rainfall averaged for all rain-gauge stations over the country is shown in Fig. 2. This Fig. 2 shows that rainfall over Bangladesh has increasing tendency in pre-monsoon and postmonsoon periods while almost constant during monsoon period. Usually, pre-monsoon and post-monsoon are the flash floods seasons in Bangladesh.
Fig. 2. Pre-monsoon(left), monsoon(middle) and post-monsoon(right) rainfall averaged for all rain-gauge stations from 1950-1999. The rainfall over the country reflects an increasing tendency (solid line in Fig. 3) from 1976. The closed circle in Fig. 3 indicates the major flood years 1954, 1955, 1974, 1984, 1987, 1988, 1993, 1998, 1999, 2000 and 2004 occurred in Bangladesh. It is seen that all flood years show surplus of rainfall except two years 1954 and 1955. These two years floods occurred by the rain-water from the upper catchments of GBM basin. The rainfall in other years contributes in the occurrence of major floods. There are also some years say 1956, 1959, 1971, 1983 and 1991 for those there are excessive rainfall but the country gets no major flood. Annual rainfall anomally (mm)
800 600 400 200 0 -200 -400 -600
2000
1998
1996
1994
1992
1990
1988
1986
1984
1982
1980
1978
1976
1974
1972
1970
1968
1966
1964
1962
1960
1958
1956
1954
1952
1950
-800
Year
Fig. 3. Rainfall anomaly calculated from rain-gauge network over the country. 3
RESULTS AND DISCUSSION
As previously mentioned, precise distribution of rainfall over the country is necessary for the proper estimation of rainfall amount. In this direction, radar data from the BMD are employed to estimate precise rain-water over the country. Details of the BMD radar specifications are described in Islam et al. (2005). The radar located at Dhaka, the centre of the country (circle in Fig. 1). It covers 600 km by 600 km data coverage and determines rain status instead of rain rate. There are six categories of rain status having different rain rate ranges. A procedure is employed to retrieve rain rate from rain status and depicted in Fig. 4. Figure 4 is the sample of a 5 × 5 pixels in a PPI scan of BMD radar data. Actually the radar data coverage is divided in 60 × 60 grid boxes, each box contains 16 pixels, each pixel having 2.5 km resolutions. So, one grid box is 10 km length along abscissa or ordinate that contains 4 × 4 = 16 pixels. Each pixel is the candidate of different status and knowing the status relative rain rate and pixel area the rain rate is retrieved (Islam et al., 2005) for each 10 km grid boxes.
S2
S2
S1
S3
S1
S2
S1
S4
S3
S2
S3
S6
S4
S3
S3
S5
S5
S4
S3
S3
S3
S4
S3
S1
Fig. 4. Schematic diagram for the radar analysis. Different pixels show different status identity. Each pixel is 2.5 km by 2.5 km in size.
The retrieved rainfall averaged for 16 April to 30 August 2000 is shown in Fig. 5. From the rainfall distribution it is seen that rainfall is high in the NE and east parts of the country. The low rainfall dominates in the western parts. The low rainfall in the extreme SE region may not the proper one because this portion is out of radar effective coverage (250 km from radar centre). Dhaka Radar AMJJA 2000
26.08
ut Brahmap
6
ra
6
INDIA
5
LATITUDE (N)
25.08
M eg 24.08
Ga n ges
hna
5 4 4
INDIA
3 3
23.08
2
INDIA
2 1
22.08
1 0
BAY OF BENGAL 21.08 87.44
0
88.44
89.44
90.44
91.44
92.44
mm/da
LONGITUDE (E)
Fig. 5. Radar estimated rainfall averaged for 16 April to 30 August 2000. The rainfall distribution estimated by BMD rain-gauge is shown in Fig. 6 for the same period of radar. The pattern is similar to that detected by radar except the amounts are high. It seems that radar underestimates (Fig. 5) in rainfall estimation compared to rain-gauge values. Reasons are described latter (Table 1).
BMD Rain-gauges AMJJA 2000
26.08
23 22
INDIA
21
25.08
LATITUDE (N)
24
20 19 18 17
24.08
16 15 14 13
23.08
12
INDIA
11 10
22.08
9 8
BAY OF BENGAL 21.08 87.44
88.44
89.44
90.44
91.44
7
92.44
6 mm/day
LONGITUDE (E)
Fig. 6. The same as in Fig. 5 except for estimation is from rain-gauge. The TRMM 3B42 daily products (combined instrument daily rainfall) are employed to verify the spatial distribution of rainfall over Bangladesh as shown in Fig. 7. In Fig. 7 daily rainfall is the average for the same period of radar data. The distribution pattern represents that rainfall is strong in the NE and coastal regions. This result is consistent with the radar and rain-gauge values except the magnitudes. It is mentioned that TRMM data are not yet validated for this region. Research is ongoing to do validation of TRMM data with ground-based data.
Fig. 7. TRMM 3B42 products daily rainfall averaged for same period of radar analysis (16 April to 30 August 2000).
The daily rainfall (mm) estimated by radar and rain-gauge averaged from 33 stations (as shown in Fig. 1) at different months in 2000 is tabulated in Table 1. The percentage (%) indicates how much rainfall radar can determine as it determined by rain-gauge. As is seen, radar can determine from 8.81% (Jul) to 45.58% (Apr) of rain-gauge rainfall. In average for all four months it is 24.41 %. This means radar estimates about one fourth of the rain-gage rainfall. One may think that radar under estimates in estimation of rainfall for the year 2000. The fact is that in 2000, most of the rain comes from SE coastal regions (Fig. 6). The radar effective radius is 250 km and it can not catch well the precipitation in these far distances from the radar centre. Another cause of underestimation is that the standard Z-R relationship (Z = 200 R 1.6) is used in radar data processing. The Z-R relationship is regional and cloud type dependent. It needs more year data to find a best fit Z-R relationship for this region which is the task of next job. The data type is also the candidate for the different results in radar and rain-gauge results. Rain-gauge provides point information while data is averaged in a 10 km grid for radar analysis. Table 1. Relation between daily rainfall estimated by radar and rain-gauge averaged from 33 stations (Fig. 1) at different months in 2000. Month Apr May Jun Jul Aug 4
Radar (mm/d) 3.83 4.14 2.70 1.22 2.20
Rain-gauge (mm/d) 8.41 14.30 13.70 13.89 11.59
% 45.58 28.95 19.70 8.81 19.01
Average % 24.41
CONCLUSIONS
Radar data from the Bangladesh Meteorological Department (BMD) are analyzed to retrieve rain rate from rain status. Radar PPI scans data for the year 2000 are successfully employed to estimate daily rainfall and compared with rain-gauge and TRMM 3B42 products estimated rainfalls. Radar underestimates rainfall due to Z-R relationship and limited data coverage. Historical data indicates that rainfall in Bangladesh increasing in premonsoon and post-monsoon seasons. Long term radar data are necessary to find a best fit Z-R relationship and to calibrate radar results with rain-gauge values. And validation of TRMM data for this region is also necessary which is not performed yet. Then radar data may employ for monitoring floods especially flash floods occurs during the pre-monsoon and post-monsoon periods which will help to assess the real damage and explore the real time situation during the disaster period. 5
ACKNOWLEDGEMENTS
The authors would like to thank the BMD for providing radar and rain-gauge data collected under JICA “Japan Bangladesh Joint Study Project- Phase II” under grant no 11691151. This study is partly supported by TRMM-RA4 of JAXA. 6
REFERENCES
ADRC Disaster Report (2000). Asian Disaster Reduction Centre Disaster Report FL-2000-0555-BGD, No 2000/09. Das, P. K., (1995). The Monsoons, Third Edition published by the Director, National Book Trust, New Delhi, India. Islam, M. N., Hayashi, T., Uyeda, H., Terao, T., and Kikuchi, K. (2004). Diurnal variations of cloud activity in Bangladesh and north of the Bay of Bengal in 2000. Remote Sensing of Environment, Vol. 93(3), pp. 378-388. Islam, M. N., Terao, T., Uyeda, H., Hayashi, T., and Kikuchi, K. (2005). Spatial and Temporal Variations of Precipitation in and around Bangladesh, J. Meteor. Soc. Japan, (in press). IFRC-RCS Situation Report (2000). International Federation of Red Cross and Red Crescent Societies, Situation report # 01, Aapeal # 20/2000. Matsumoto, J., (1988). Synoptic features of heavy monsoon rainfall in 1987 related to the severe flood in Bangladesh. Bulletin of the Dept. Geography, Univ. Tokyo, Japan, Vol. 20, pp. 43-56. Matsumoto, J., Rahman, M. R., Hayashi, T. and Monji, N. (1996). Rainfall distribution over the Indian Subcontinent during the 1987 and 1988 severe floods in Bangladesh. Bulletin of the Dept. Geography, Univ. Tokyo, Japan, Vol. 28, pp. 25-44.
