J. Sylhet 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, Bangladesh 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 Meghna river tributaries. This region is characterized by the presence of numerous large and deeply flooded depressions between the rivers known as haors. 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 beels (FAP-6, 1993). It is situated just below the hilly regions of the states of Assam, Meghalaya and Tripura of India, the haor 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] Islam et al. (2017) 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 Bengal 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 Barak River, 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 Sylhet district, 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 Sunamganj 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 Surma River in Sylhet, SW-158 at Khowai River in Habiganj, 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. 316 Dynamics of river water level 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 (Khowai River) is almost same in every year except in 2011 and 2012.