2.1 Location The study area, Lohagara Municipality, is located in Lohagara Upazila (sub-district) under Narail district of Bangladesh. The area is about 16.16 sq. km for the present case study. It is bounded by the Ganges River on the North, the River Padma on the East and the Western border upazillas on the west. The southern boundary of the study area is defined as the boundaries of the Upazila's of Sharsha, Jhikargacha, Jessore Sadar, Narail Sadar, Lohagara, Kashiani and Muksudpur.
2.2 Climate The study area experiences a typical tropical monsoon climate, with hot wet summers from May to September and cool dry winters. The rainy season occurs approximately from May to October and almost 90% of the total annual rainfall occurs during this period. Both temperature and relative humidity remain high in this season. Mean daily temperature is fairly constant between the months of April and September and show little variation across the region, being of the order of 280 C. From October, the temperature begins to decline, and the mean daily temperature reaches to a minimum of about 19 to 19.50 C in January. In April, the maximum daily temperature in the region can often exceed 350 C, while in January minimum daily values can be below 100 C. The rainfall distribution in the study area is not uniform. The lowest mean annual rainfall is about 1800 mm, which is observed in northwest stretched strip and increases towards eastward and reaches about 2100 mm and 2400 mm in Narail and Bhanga, respectively.
The study area has an almost flat topography and is characterized by a fairly plane land except for the water bodies such as rivers, ponds, depressions and beels etc. Peat occurs extensively in the Gopalganj-Khulna Beels and locally in some Haors of the Sylhet basin. The soils contain organic matter at the surface or buried under a mineral soil layer below at a depth of up to 40 cm. The organic material that forms the Histic horizon varies from dark brown, fibrous peat to semi-liquid black muck. They have been included as Histosols. Soils of this area are result of the deposition of Gange's alluvium. Gange's alluvium is calcareous when deposited, but most basin clays and some older ridge soils have been decalcified and acidified in their upper layers; lime is found only in the subsoil or substratum of such soils. Clay soils predominate in basins and on the middle parts of most ridges, with loamy soils (and occasionally sands) occurring mainly on ridge crests. The cut-off parts of the Meghna floodplain have a smooth relief and predominantly silty soils, which are deeply flooded by rainwater in the monsoon season. The unit covers most of the districts of Rajshahi, Natore, Pabna, all districts of the Khulna Division, and parts of Manikganj, Narayanganj, Munshiganj, Shariatpur, Madaripur, Barisal, Gopalganj. This physiographic unit is almost triangular in shape and bounded by the Ganges tidal floodplain on the south. On its southern end it traps the Gopalganj-Khulna Beels. Values of pH in soil range from 7.0 to 8.5.
2.4 Geomorphological and Hydrogeological Setting Geomorphologically, Lohagara Upazila belongs to the Ganges River floodplain which comprises the active floodplain of the Ganges and the adjoining meander floodplain. The latter mainly comprises a smooth landscape of ridges, basins and old channels. The relief is locally irregular alongside the present and former river courses, especially in the west, comprising a rapidly alternating series of linear low ridges and depressions. The Ganges channel is constantly shifting within its active floodplain, eroding and depositing large areas of new char land each flood season, but it is less braided than that of the Brahmaputra-Jamuna. Seasonal flooding is mainly shallow in the west and north, with the highest ridge crests remaining above normal flood levels, but flood depths increase towards the east and the south. Flooding is mainly by accumulated rainwater and the rising GW table except on the active Ganges floodplain and close to distributary channels that cross the meander floodplain. Hydrogeological investigations had been undertaken to assess aquifer information of sedimentary formation. Past studies (BWDB, 1993) give an idea about the depth and thickness of the aquifer layers in the study area. The hydrogeological parameters of this area are governed by the litho-stratigraphic and prevailing tectonic activities, which is part of the regional hydrogeological setting and tectonic features. The Quaternary sequence provides good aquifers, which have been extensively exploited in Bangladesh. The aquifers are generally thick multilayered with high transmissivity and storage coefficient. In addition, the aquifer systems can broadly be distinguished in the study area is recent sand forming both confined and semi-confined aquifers.
2.5 Existing Landuse and Agricultural Practices The major part of the study area is agricultural land. It has a homestead, pond, and depressions (locally named as beels) also. The crop calendar reveals that T. Aman, HYV-Aman, HYV-Boro, and Potato are the main crops in the study area. Land use and vegetation are used in the model to calculate actual evapotranspiration depending on the actual crops grown in the project area. Under the present study, the spatial distribution of crops has been determined from a comprehensive field campaign. However, for the model input, these cropping types and cropping patterns have further been simplified considering the major crops that require irrigation water. A crop database for each crop, which defines leaf area index, root depth, and other properties of each crop is developed based on Food and Agricultural Organization (FAO) of United Nations (UN) publications (FAO, 1979) and used in the model. Paddy is grown almost everywhere through the year in each of the Karif-I (March-June), Karif-II (July-October), and Rabi (November-February). Pulse, oilseeds, vegetables, jute, wheat, sugar cane, potato, Mango, etc, are major non-paddy crops. With all availability of irrigation water, growers of the study area shifted towards growing more productive Boro-Fallow-HYV T. Aman. The spatial distribution of land use and cropping patterns is presented in Figure 2.