Site selection- The study area was in the western region of Bangladesh (24.1-23.2 °N and 89.1-89.2 °E, altitude 7 to 14 m above mean sea level). The site includes Pabna, Kushtia, Chuadanga, Jessore, Jhenaidah and Meherpur districts, which belong to the agro-ecological zone (AEZ)-11 (High Ganges river floodplain) and AEZ-12 (Low Ganges river floodplain) (FAO, 1988). AEZ-11 covers 43% silt loam, 32% loamy and 12% clayey soil and it has an area of 1321062 ha. AEZ-12 has 797139 ha with 31% clayey soil, 29% silty clay and 13% silt loam (FAO, 1988). Silt loam soil was considered in this study since it covers the maximum area in Bangladesh. ETo was calculated according to Penman- Monteith method using CROPWAT 8.0 model. The Penman–Monteith equation (FAO, 1998) integrated in the CROPWAT program is expressed by equation (1) as follows; ETo was calculated according to Penman- Monteith method using CROPWAT 8.0 model. The Penman–Monteith equation (FAO, 1998) integrated in the CROPWAT program is expressed by equation (1) as follows;
where, ETo is reference crop evapotranspiration (mm d-1); Rn is net radiation at the crop surface(MJ m-2d-1);G is soil heat flux (MJ m-2d- 1); T is average air temperature(°C); U2 is wind speed measured at 2 m height (m s-1); (ea−ed) is vapor pressure deficit (kPa); Δ is slope of the vapor pressure curve (kPa°C-1); γ is psychrometric constant (kPa °C-1) and 900 is a conversion factor. The FAO -CROPWAT 8.0 model (FAO, 2009) incorporates procedures for estimating ETo and crop water requirements and allows simulation of crop water use under various climate, crop and soil conditions. Average maximum and minimum temperatures (°C), relative humidity (%), wind speed (m s-1) and sunshine hours (h) were collected from data from Bangladesh Meteorological Department (BMD) from 1970 to 2013 for Ishwardi and Jessore and from 1989 to 2013 for Chuadanga. The latitude, longitude and altitude of the stations were also collected. The ETo was calculated for every 10 days (defined as “decade” by FAO) and then accumulated to monthly data. CROPWAT model considers four methods as fixed percentage, FAO/AGLW formula, empirical formula and USDA soil conservation service method to estimate effective rainfall. Crop data- Rice was grown in T. Aman and Boro seasons. Rice growing period was divided as: nursery/land preparation (seedling stage, 30 days in T. Aman and 40 days in Boro); initial stage (transplanting to seedling establishment, usually 10 days); crop development stage (tillering to panicle initiation); mid stage (panicle initiation to 100% flowering, 30 days) and late stage (flowering to maturity, 30 days). In this study, irrigation scheduling was done for popular T. Aman variety, BRRI dhan49 (135 days duration) and Boro variety, BRRI dhan28 (140 days duration).Three transplanting dates (15 July, 1 and 15 August) were considered for BRRI dhan49 depending on farmer’s choice. In Boro season, farmers usually transplant Boro varieties from the last week of December to middle of February and grow the crop under fully irrigated condition. For simplification, 15 January was considered as a suitable transplanting date since most of the farmers’ follow this practice in the study region. Maximum rooting depth of rice was considered 40 cm. Soil data- Bulk density of soil was1.5 g cm-3 with maximum infiltration rate 40 mm day-1. Moisture content at field capacity and wilting point were 26.8% and 12.9%, respectively. Drainable porosity was approximately 18% (BRRI, 2014). Default value of maximum percolation rate in the model was 3.4 mm day-1 for silt loam soil. Estimation of irrigation water requirement and time of application- Crop irrigation water requirement (CWR) refers to the amount of water that needs to be supplied, while crop evapotranspiration (ETc) refers to the amount of water that is lost through evapotranspiration (Allen et al., 1998). CWR (mm) was determined according to FAO (2005) as; Default value of Kc for rice by FAO in the model was used in calculation of actual evapotranspiration. Model considered Kc dry value was 0.7, 1.05 and 0.7 in nursery, development and late stage respectively. Where Kc wet value was 1.2, 1.2 and 1.05 for nursery, development and late stage respectively. Since rice is a water loving crop, suitable time of irrigation was considered when moisture level went down to field capacity. In case of silt loam soil, 18% desaturation indicates the field capacity and it needs 3 days to reach the field capacity from saturation (BRRI, 2015). So, three days after disappearing of standing water was considered as suitable time of irrigation. Irrigation efficiency was assumed 70%. Irrigation was not applied in the last 10 days of its growing period because water in this stage would delay ripening of crop.