Locations:
Sixteen districts of the northwest region of Bangladesh, Bogura, Chapainawabganj, Joypurhat, Naogaon, Natore, Pabna, Rajshahi and Sirajganj in Rajshahi division, and Thakurgaon, Rangpur, Panchagar, Nilphamari, Lalmonirhat, Kurigram, Gaibandha and Dinajpur in Rangpur division were selected for the study.
Data collection:
Daily climatic data (precipitation, temperature, wind speed, sunshine hours, solar radiation and relative humidity) for 31 years (1985-2015) were collected from the Bangladesh Meteorological Department (recorded at the meteorological stations located in Bogura, Rajshahi, Ishurdi, Dinajpur, Syedpur, and Rangpur) with the help of an on-going collaborative project (SDIP II: Sustaining groundwater irrigation for food security in the North-West region of Bangladesh) with CSIRO, Australia. Data on crops such as length of growth stage, crop co-efficient, root depths, depletion factors, yield reduction factors, maximum ponding depth, minimum ponding depth, refill ponding depth, planting date and planting duration were collected from sub-district (called upazila) and district Agricultural Offices through SDIP II project.
After expert consultation and literature review, a total of eight most commonly practiced crops and cropping patterns were selected for this study. The selected cropping patterns were: Aman-Boro−Fallow, Aman–Wheat−Fallow, Aman–Maize−Fallow, Aman–Potato−Fallow, Aman−Fallow-Aus, Aman–Wheat-Aus, and Aman–Wheat–Boro. We grouped the minor crops as Other Rabi (for dry season crops) and Other Kharif (for wet season crops) and put them into Other Rabi–Other Kharif pattern. The Other Rabi includes pulses, spices, oilseeds and vegetables of the Rabi season, and Other Kharif includes mungbean and vegetables of the Kharif season. For simplicity, we presented ‘T. Aman’ as ‘Aman’ and dropped out ‘Fallow’ from the cropping patterns. In the dominating cropping patterns, Boro, Aman, Potato, Wheat and Maize were the major crops. Aus was also found in many occasions in other cropping patterns. So, we selected the major crops, Other Rabi and Other Kharif.
CROPWAT model:
Actual crop evapotranspiration (ET), total effective rainfall (TER), crop-usable effective rainfall (ER) and irrigation requirement (IR) were estimated by using a daily Soil-Water Balance Model, which is exactly similar to CROPWAT (SWBcropwat) model, which is a macro in MS excel sheets. TER is the amount of rainfall after deducting all losses and available for usage by crops, while ER is the portion of TER that crops could utilize. Details on SWB model were reported in Mainuddin et al. (2014).
ET, TER, ER and IR were estimated for eight major crops and cropping patterns of all 16 districts in the NW region for 31 years (1985–2015). Also estimated are the volumetric quantities of ET, ER and IR for each crop by multiplying ET, ER and IR by crop acreage. These were summed up district-wise for all crops, crop-wise for all districts and also for all crops for the whole NW region (16 districts).
Trend analysis:
The trend of seasonal ET, ER and IR, and their volumetric quantities of the eight major crops and cropping patterns were determined by using Mann-Kendall-Sens (MAKESENS) trend statistics (Salmi, 2002). The used statistical methods are the non-parametric Mann-Kendall test for analyzing the presence of monotonic increasing or decreasing trend and the non-parametric Sen's method for estimating the slope of a linear trend (Salmi, 2002). MAKESENS utilizes two statistics, called Z-statistics and S-statistics to estimate trend. Based on data type, especially sensitivity of change, the MAKESENS model determines statistical significance of the trend at p ≤ 0.10, 0.05, 0.01 and 0.001. The Mann-Kendall test requires at least four values, and calculation of the confidence intervals for the Sen's slope estimate requires at least ten values in a time series.