J. Agril. Univ. 4(2):315-324, 2017 ISSN: 2308-1597

DYNAMICS OF RIVER WATER LEVEL IN SYLHET BASIN M Islam* 1 and S Ahmed 2 1Department of Irrigation and Water Management, Sylhet Agricultural University, Sylhet-3100, 2MS Student, Department of Irrigation and Water Management, Sylhet Agricultural University, Sylhet-3100, Bangladesh

(Available online at: www.jsau.com.bd) Abstract Rivers carry water and nutrients to areas all around the earth and provide excellent food and habitat for different microorganisms and also important for livelihood resides in the bank of the river. With an ever- growing population demand for water and increasing climate uncertainty, there is pressing need for improved understanding of the underlying patterns of natural variability of water resources and consideration of their implications for water resource management and conservation. In this regard various climatic parameters such as rainfall, temperature, humidity etc. shown significant trends in river water level. Rainfall, which was regarded as a main influencing factor found to same in the changes occurred during the study period, except the month of April to July. The change of the maximum and minimum temperature over the year was also found almost constant. As the temperature increases, the rate of evaporation also increases which tends to decline the river stage. But, the highest rainfall has put a significant influence to maintain the evaporation loss. In respect of humidity, the higher relative humidity occurred during the months from May to July in every year. In the winter season, evaporation occurred at a higher rate as the relative humidity decreases. As a result, river water level declines but during monsoon the river stages regain its original phase. Climatic parameters played an important role on river water level. Studies on geological settings reveal that the sediment deposition during the Holocene passing through the Sylhet basin occurred at a higher subsiding rate. It may be concluded from the study that an optimum utilization of land and water resources by understanding the morphological process and subsequent prediction for any intervention in Sylhet basin for the development of lives and livelihoods in the area.

Keywords: River, climatic parameters, livelihood and Sylhet basin. Introduction River water level is the key factor on the inundated extent and water level in floodplains, while the infilling and draining of floodplains inversely affect the discharge in river channels by attenuating water level variations by flood waves (Yamazaki et al., 2011). Along with water exchanged between river channels and floodplains, large amounts of carbons and nutrients are drained from floodplains and wetlands to river channels (Richey et al., 2002). Thus, quantifying the water level dynamics is a key step to understand the detailed hydrological effects on river basins. Furthermore, water level is critical information for flood management as well as river discharge, and is used for urban hydraulic engineering (e.g., deciding embankment height and urban zoning). In this contrast the study is undertaken in the greater part of the northeast region is occupied by a number of wetland basins, which comprises the floodplains of the tributaries. This region is characterized by the presence of numerous large and deeply flooded depressions between the rivers known as . Sylhet-Mymensingh basin is the largest natural depression (62,106 ha) in the country occupying approximately 15.6 % of the inland open water area and 58.28 % of the total (FAP-6, 1993). It is situated just below the hilly regions of the states of , and of , the area has some of the most severe hydrological conditions like extreme rainfall and subsequent flooding. Since rainfall on the adjacent Indian side largely affects flooding in the haor area, the rainfall pattern of the upstream catchment has great influence in these areas. Any change of the hydro-climatic pattern in this region will significantly affect the balance among natural features and also other parts of the country (Hasan et al., 2012). In Sylhet basin flash flood due to excess rainfall in the upstream and subsequent runoff is the main disaster here which devastates the primary production sector (i.e., agriculture) and thus threatens the livelihood. This needs to explore the potentiality of surface water of the study area. The objectives of this study were to evaluate the dynamics of river water level and to observe the influence of climatological parameters on the river water level.

*Corresponding author: M Islam, Department of Irrigation and Water Management, Sylhet Agricultural University, Sylhet-3100, E-mail: [email protected]

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Materials and Methods Description of the study area The Surma Basin is a sub-basin of the country situated in the north-eastern part of Bangladesh. The basin is bounded on the north by the Shillong plateau, east and south-east by the Chattogram-Tripura fold belt of the Indo-Myanmar ranges, and west by The Indian Shield platform. To the south and south-west, it is open to the main part of the Basin (Johnson and Alam, 1991). The published Bouger anomaly map shows gradual higher values (negative) towards the center of the basin. The Aeromagnetic interpretation map by Hunting (1980) indicates a gradual deepening of basement towards the center of the basin and also reveals subsurface synclinal features and faults within the basin. Its topography is predominantly flat with some north-south trending ridges of twenty to several hundred meters elevation present in the north-eastern border. It is actively subsiding (Johnson and Alam, 1991).

Geological setting of the north-east region of Bangladesh The Sylhet trough is a sub-basin of the Bengal Basin and consists of 13-20 km thick alluvial and deltaic sediments underlain by much older genesis and granitic rock. Goodbred and Kuehl (2000) estimated the sediment thickness during the Holocene based on the analysis of borehole data and carbon dating information. The long profile of sediment deposition during the Holocene passing through the Sylhet basin up to the sea indicates that higher deposition occurred at the northern boundary of the basin (Fig. 1), which suggests a higher subsiding rate.

Fig. 1. Geological Setting in the North-East region of Bangladesh (Goodbred and Kuehl, 2000)

River system The water resources in the area originated from the „Surma‟ River which is hydraulically connected to the underlying alluvial aquifer system. The Surma originates in the hills of Shillong and Meghalaya of India. The main source is , which has a considerable catchment in the ridge and valley terrain of Naga-Manipur hills bordering Myanmar. Barak-Meghna has a length of 950 km of which 340 km lies within Bangladesh. On reaching the border with Bangladesh at Amalshid in , Barak bifurcates to form the steep and highly flashy rivers Surma and kushiyara (http://en.banglapedia.org). Surma flows west and then southwest to Sylhet town. From there it flows northwest and west to town. Then it maintains a course southwest and then south to Markuli to meet Kushiyara. The joint course flows up to Bhairab Bazar as the Kalni. Flowing north of the Sylhet basin, Surma receives tributaries from Khasi and Jaintia Hills of shillong plateau. All these rivers and its tributaries are silt-carrying, flashy and eroding (Fig. 2).

Selection of gauge station and data analysis The study was conducted using secondary data. The selected gauge stations were SW-167 at in Sylhet, SW-158 at River in , SW-201 at Monu River in Moulvibazar, SW-269 at Surma River in Sunamganj. The daily and monthly rainfall data, relative humidity, the daily temperature of Sylhet rain gauge station was collected from the Bangladesh Meteorological Department (BMD) for the period of 2000-2015. The river water level data were collected from Bangladesh Water Development Board (BWDB) for the period of 2000-2015.The rainfall data were taken as monthly total, the temperature, and relative humidity manipulated as monthly average. The collected data were processed graphically to make them error free and to fill out the missing data where possible. The processed water level, rainfall, temperature and relative humidity data were analyzed to evaluate the variation with time and to observe the pattern of variations.

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Fig. 2. River System Map (Sarker and Akter, 2011)

Results and Discussion

This study aims to present and evaluate the behavior of river water level fluctuations by statistical properties and the major influencing factors on river water level. The present paper focuses on the overall status of water level fluctuation in a micro watershed at North East part of Bangladesh. For this purpose, the analysis has been carried out in two stages. Firstly the periodical behavior of river water level, which gives the overall status of water level in the study area. Secondly, the relationships between meteorological variables and river water level are investigated using statistical analyses.

Monthly trend of river water level The variation of water level at different river in the study area shows in Fig. 3. The mean monthly river water level in Sylhet (Surma River) is almost same in every year. It is visible that the maximum water level occurred between April to July in each year. In those months, the fluctuations were about 2 to 11.7 m. This higher water level in those months is due to monsoon season where the highest rainfall occurs. The highest river water level occurred in the month of July (11.7 m) 2004. On the other hand, the lowest water level lies in the month of December (2 m) 2004 and 2007. The mean monthly river water level in Habiganj () is almost same in every year except in 2011 and 2012. Maximum water level occurred between April to July and the fluctuations were observed about 12.2 m to 16.6 m. This higher water level in those months is due to monsoon season where the highest rainfall occurs. The highest river water level occurred in the month of July (16.6 m) 2012. On the other hand, the lowest water level lies in the month of January (12.2 m) 2015. The mean monthly river water level in Moulvibazar () is almost same in every year. Maximum water level occurred between April to July in each year and the fluctuations were observed about 16.4 m to 19.2 m. The highest river water level occurred in the month of July (19.2 m) 2004. On the other hand, the lowest water level lies in the month of February (16.4 m) 2015. The mean monthly river water level in Sunamganj (Surma River) is almost same in every year. Maximum water level occurred between April to July in each year and the fluctuations were observed about 1.8 m to 8.9 m. The highest river water level occurred in the month of July (8.9 m) 2004. On the other hand, the lowest water level lies in the month of February (1.8 m) 2007.

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Fig. 3. Spatio-temporal variation of river water level

Fig. 4. Monthly variation of total rainfall

Monthly trend of rainfall From the Fig. 4, it is seen that there is no significant changes occurred during the study period. It is found that the maximum rainfall is approximately 1400 mm occurred in the month of June 2004. The months between April to July receive the maximum rainfall (about 800 to 1400 mm) over the year. From this Fig., it is found that there is very little amount of rainfall (48 to 148 mm) between the months of November to January during the study period.

Seasonal variation of rainfall In general rainfall starts in summer or pre-monsoon, monsoon and post monsoon respectively and there is very little amount of rainfall in winter compared to the other season. In the monsoon season due to this heavy rainfall water level rises to from lower level to higher level i.e., almost near to the earth surface, whereas in the case of non-monsoon water level rises slowly declines after receding the rainfall (Fig. 5). Although few rainfall events occur in winter season but the river water level does not increases. This is due to heavy demand of water as well as there is no alternative source of water in this area.

Monthly variation of average maximum and minimum temperature The variation of maximum and minimum temperature over the study period from 2000 to 2015 has been analyzed. The maximum temperature lies between 24oC to 35.8oC and the minimum temperature lies between 12oC to 26oC (Fig. 6). It also seems that the highest average temperature is 35.8 0C that occurred in the month of May 2014 and the lowest average temperature is 12 oC that occurred in the month of January 2013. From Fig. 7 it is found that the warmest and the coldest period over the year in the study period from April to July and December to February, respectively. The temperature of every month gradually increases in every year under the study period. This increasing temperature has adverse impact on river water level.

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Dynamics of river water level

Fig. 5. Seasonal rainfall pattern

Fig. 6. Monthly variation of average maximum and minimum temperature

Variation of relative humidity The change of relative humidity with time in the study period was analysis and depicted in Fig. 7. The relative humidity varies from 53 to 93% over the study period. The maximum relative humidity (93%) occurred in the month of June 2002. On the other hand, the minimum relative humidity (53%) was found in the month of March 2007 and 2009. It is also found that the higher relative humidity occurred between the months from May to July in every year through the study period. The lower relative humidity was found between the months from January to March.

Effect of temperature on river water level Temperature is one of main factor which has direct influence on water level in any catchment. The highest temperature has a great impact on river water lever as the rate of evaporation increases. The variation of river water level Surma River at Sylhet (SW-167), Khowai River at Habiganj (SW-158), Monu River at Moulvibazar (SW-201) and Surma River at Sunamganj (SW-269) with temperature has been discussed. The average maximum and minimum temperature are observed as 35.8oC and 12oC on May 2014 and January 2013, respectively. It is found that the river water level increases with the increase of temperature (Figs. 8 - 11). Normally higher temperatures have led to decrease of river water level but in this case river water level is increasing with temperature. This is mainly due to high rainfall starts at the month of June which recover the evaporation loss during summer.

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Fig. 7. Variation of relative humidity

Fig. 8. Relation between maximum temperature and river water level in Surma river at Sylhet

Fig. 9. Relation between maximum temperature and river water level in Khowai river at Habiganj

Fig. 10. Relation between maximum temperature and river water level in Monu river at Moulvibazar

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Fig. 11. Relation between maximum temperature and river water level in Surma river at Sunamgonj

Fig. 12. Relation between rainfall and river water level at Sylhet

Fig. 13. Relation between rainfall and river water level in Khowai river at Habiganj

Fig. 14. Relation between rainfall and river water level in Monu river at Moulvibazar

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Fig. 15. Relation between rainfall and river water level in Surma river at Sunamganj

Fig. 16. Relation between relative humidity and river water level in Surma river at Sylhet

Fig. 17. Relation between relative humidity and river water level in Khowai river at Habiganj

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Fig. 18. Relation between relative humidity and river water level in Monu river at Moulvibazar

Fig. 19. Relation between relative humidity and river water level in Surma river at Sunamganj

Effect of rainfall on river water level Here focused on the variation of river water level at four selected gauge station are analyzed with rainfall and is depicted in Figs. 12 - 15. From these Figs. it is found that the river water level increases with the increase of rainfall and vice-versa. For the maximum rainfall between April to July the river water level rises although the loss of water by high temperature and evaporation.

Effect of relative humidity on river water level The variation of river water level with relative humidity at four selected gauge station is depicted in Figs. 16-19. It is found that the river water level rises with the increase of relative humidity and vice-versa. The higher relative humidity occurred between the months from May to July in every year under the study period. At this time the river water level rises since the evaporation rate decreases for higher relative humidity. The lower relative humidity is found between the months from January to March. At this time the river water level fall since the evaporation rate increases for lower relative humidity.

Water level fluctuation is very common in the rivers, as they depend on seasonal precipitation and the inflow from upland sources. The rising rainfall tends to increase the river water level. It receives maximum rainfall in the month of April to July which also augmenting the river stage. As the temperature increases, the rate of evaporation also increases which tends to decline the river stage. But, the highest rainfall has put a significant influence to maintain the evaporation loss. In winter season, the rate of evaporation is high as the relative humidity decreases which results in declining river water level. In monsoon season, the river stage rises as the relative humidity increases.

References

FAP-6 (Flood Action Plan). 1993. Bangladesh Water Development Board, Flood Plan Coordination Organization, December 1993, Manu River Improvement Project, FAP 6, Northeast Regional Water Management Project (Draft Final Report) SNC, LAVALIN, Northwest Consultants in Asssociation with EPC Ltd, BETI, IDEA and NCM.

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Goodbred Jr S L and Kuehl S A. 1999. Holocene and modern sediment budgets for the - system: Evidence for highstand dispersal to flood-plain, shelf, and deep-sea depocenters. Geology. 27(6): 559- 562. Goodbred Jr S L and Kuehl S A. 2000. Enormous Ganges- Brahmaputra Sediment load During Strengthened Early Holocene Monsoon, Geology. 28. 1083-1086. Http://en.banglapedia.org/index.php?title=Surma- Meghna_River_System&oldid=19871. Hunting Geology and Geophysics Ltd. 1980. “Aeromagnetic Survey: Bangladesh”, Unpublished Report, Petrobangla. Hasan G M, Alam R, Islam Q and Hossain S. 2012. Frequency Structure of Major Rainfall Events in the North-Eastern Part of Bangladesh, JESTEC, School of Engineering, Taylor‟s University; 7(6): 690 - 700. Johnson S Y and Alam A M N. 1991. Sedimentation and tectonics of the Sylhet trough, Bangladesh. Geol. Soc. Ann. Bull. 103: 1513–1527. Richey J E, Melack J M, Aufdenkampe A K, Ballester V M and Hess L L. 2002. Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2. Nature; 416:617–620, doi:10.1038/416617a. Sarker M H and Akter J. 2011. Evolution of Rivers in Subsiding Sylhet Basin: Northeast of Bangladesh, Poster for the Workshop on Advances in River Science: accounting for Sediment, Habitat and morphology in Integrated River Management, 18-21 April, Swansea, United Kingdom. Yamazaki D S, Kanae H Kim and Oki T. 2011. A physically based description of floodplain inundation dynamics in a global river routing model, Water Resour. Res.Vol. 47, W04501, doi:10.1029/2010WR009726

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