The study area comprises 82 upazillas (sub-districts) of northwest Bangladesh. Geographically, the area extends from 23047′45′′ N to 25046′34′′ N latitude and from 88000′37′′ E to 89049′11′′ E longitude, and covers approximately 23,295 km2. Total population of the area is 20.06 million, of which more than 80% people live in rural area and directly or indirectly depend on agriculture. The density of population is approximately 860 persons per square kilometer.
Three distinct seasons can be recognized in area from climatic point of view: (1) the dry winter season from December to February, (2) the pre-monsoon hot summer season from March to May, and (3) the rainy monsoon season which lasts from June to October (Rashid 1991). Climatically, the study area belongs to dry humid zone with annual average rainfall vary between 1,400 and 1,900 mm. The seasonal distribution of rainfall shows that almost 92.7% rainfall occurs during May to October. Less than 6% rainfall occurs during the irrigation period of Boro rice (January to April).
The rainfall also varies widely from year to year. Annual variability of non-monsoon rainfall in the area is more than 50% (Shahid 2008). Temperature in the region ranges from 250C to 400C in the hottest season and 80C to 250C in the coolest season.
The economy of the area is completely agriculture based. About 74.8% land of the study area is used for agriculture among which about 86% is used for dry season crop cultivation. High Yield Varity (HYV) Boro rice is the dominating crop which shares almost 81.2% of total cultivated crop in the dry season. It can be seen from the figure that the study area is one of the most densely Boro cultivated area of Bangladesh. About 3.4 million metric ton of Boro rice is produced from the study area which shares 12.3% of total food production in Bangladesh (Bangladesh Country Almanac 2004).
Irrigation method commonly used in Boro rice field is Basin method in which water is supplied from one side of the plot and the whole plot is flooded with 5–7 cm standing water. Level land is required for successful irrigation by this method (Banglapedia 2003). The recurrence of irrigation is necessary to assist in the production of crops.
Groundwater is the main source of irrigation in Boro rice field. Recent declination of groundwater level during dry season in northwest Bangladesh has posed a major threat in irrigated agriculture system. Drought is a common phenomenon in the region (Shahid 2008; Shahid and Behrawan 2008). In last 40 years the area suffered eight droughts of major magnitude (Paul 1998). Recurrent droughts and availability of irrigation equipments have made the people depended on groundwater as a source of irrigation. During the period 1980–2000, groundwater irrigation coverage rose from 6% to 75% in Bangladesh (Bangladesh Agricultural Development Corporation 2002). Ever increasing ground water extraction for irrigation and no increase in rainfall have caused the groundwater level falls to the extent of not getting fully replenished in the recharge season causing overdraft in northwest Bangladesh (Government of Bangladesh 2002). The problem is becoming progressively more acute with the growth of population and extension of irrigated agriculture.
4 Estimation of water required for land preparation The soil moisture deficiency is calculated by using the soil water balance method developed by Thornthwaite (Thornthwaite 1948; Mather 1978). The soil moisture-holding capacity (MHC) of different soil groups in the study area, precipitation and potential evapotranspiration data are used for this purpose. The deficiency of soil moisture in the study area is computed in the end of December considering that land is prepared in this time for the cultivation of Boro rice in the beginning of January.
5 Percolation and seepage losses through the paddy field Total water losses of water through percolation and seepage from the rice field are calculated from soil data. Following Brouwer et al. (1986), it is considered that percolation and seepage losses for a sandy soil are 8 mm/day, for clay are 4 mm/day, and for loom are 6 mm/day. Percolation and seepage losses through a particular soil class are calculated according to the percentage of sand, clay and loom exists in that soil class. For the better yield of crop, 50 to 70 mm standing water in the plot is expected during the growing stages of rice. In the present study 50 mm of water is considered to establish water layer in the rice field.