2.1. Site selection Soil salinity is more hazardous in the southwestern part than in any other part of the coastal region. Most rice lands of this part fall under the categories of moderately saline (ECe4±8 dS mÿ1 ) and saline (ECe8±16 dS mÿ1 ) soils. We selected a site in Barodanga village and another in Mirzapur village, both in Dumuria Thana of Khulna district, to respectively represent the saline and moderately saline soil categories. The two sites are located at an altitude of 4 m above mean sea level between 22.478 and 22.498N latitude, and 89.248 and 89.348E longitude (FAO, 1988).
2.2. Conceptual framework In the coastal region, groundwater level remains close to the ground surface during July to November. Generally, salt build-up in the soil starts in January or February and continues up to April or May, when evaporation exceeds rainfall. The leaching of salts from the soil profile usually starts in May or June, at the onset of the monsoon season. The salt dynamics is mainly governed by the evaporation process and rainfall. 2.2. Conceptual framework In the coastal region, groundwater level remains close to the ground surface during July to November. Generally, salt build-up in the soil starts in January or February and continues up to April or May, when evaporation exceeds rainfall. The leaching of salts from the soil profile usually starts in May or June, at the onset of the monsoon season. The salt dynamics is mainly governed by the evaporation process and rainfall.
2.3. Groundwater level and salinity Groundwater levels were measured at one-week intervals from March 1996 to April 1997 at both the research sites by using a water-level recorder. Two observation wells were installed at two corners along the diagonal of the 28 m45 m experimental field of each site to monitor the fluctuation of groundwater. The wells were 12 m deep at Barodanga and 25 m deep at Mirzapur. The average of these readings over a week was taken as the depth to groundwater for that week. A hand-pumped tube well was also installed at each location, in the vicinity of the experimental field, to collect groundwater samples for determining its electrical conductivity (EC) and sodium adsorption ratio (SAR) values. The EC of groundwater was measured at 15-day intervals from March 1996 to April 1997. About 500 ml of water samples was collected from the hand tube well. During sampling, the well was pumped continuously for about 5 min before taking a sample so that the effect of standing water within the tube was removed. The samples were collected in clean glassware and its EC was determined immediately by using a portable EC meter. The SAR of groundwater was measured only once during the study period, as EC values of groundwater did not vary considerably. For SAR, water samples were collected the same way as for EC and these were analyzed at the Soil Resources Development Institute's (SRDI) laboratory at Khulna City for Na, Ca, and Mg concentrations. SAR was then determined by using the formula (USSL STAFF, 1954):
2.5. Rainfall and evaporation A Taylor rain gage (305 mm Clear-VU Rain and Sprinkler gage with a rainfall capacity of 127 mm) and a USWB Class A evaporation pan were installed in the vicinity of the experimental field at both sites for measuring daily rainfall and evaporation rates, respectively. These devices were read every day at 9:00 AM. Rainfall and evaporation data for the study period were also collected from the Khulna meteorological station located about 10 and 15 km away from Barodanga and Mirzapur, respectively.
2.6. Crop production In order to develop relationships of soil salinity and crop culture, experiments were conducted at both sites during the 1996 and 1997 dry seasons. Two non-rice crops were grown under irrigated and rainfed conditions at each location. The treatments were:
C1=fallow (farmers' practice) C2=cowpea/mungbean (rainfed) C3=sesame (rainfed) C4=cowpea/mungbean (irrigated) C5=sesame (irrigated)
Cowpea and sesame were grown at Barodanga, and mungbean and sesame at Mirzapur in the 1996 and 1997 dry seasons. Cowpea seeds were sown in line by dibbling method when soil moisture content was close to saturation. Both sesame and mungbean were sown in line after plowing the land by a power tiller when soil moisture was at ®eld capacity. Row-to-row and plant-to-plant spacing were 3020 cm for cowpea and 3015 cm for mungbean and sesame (Bangladesh Agricultural Research Institute, 1995). The experiment was laid out in plots of 8 m5 m size, using randomized complete block design (RCBD) with 5 replications (Gomez and Gomez, 1984).
Groundwater was used to irrigate non-rice crops. A hand sprinkler was used for uniform application of irrigation water. Irrigation water was applied to replenish half of the evaporative demand to match with the crop coefficient factor. The amount of irrigation water applied was determined by the following formula:
Dir = 0:50*CPE- CRF
where Dir is the depth of water applied per irrigation (mm), CPE the cumulative pan evaporation preceding irrigation (mm), and CRF the cumulative rainfall preceding irrigation (mm).