Trends in climatic variables and their combined effects on irrigation water requirement and yield of wheat

Abstract

Wheat is the second most important staple food crop in Bangladesh. Within a period of 40 years, wheat has been firmly established as a secure crop. At present climate change is thought to be putting an extra stress on wheat production and hence the food security of Bangladesh. Climatic factors such as temperature, rainfall, atmospheric carbon dioxide, solar radiation, etc., are closely linked to wheat production. This study was conducted to assess the vulnerability of wheat production in Chapai Nawabganj district of north-west Bangladesh. Different aspects of climate change and its effects on wheat cultivation have been assessed in this study. Historical data of different climatic parameters such as temperature, rainfall, humidity, sunshine and wind speed, were collected for Rajshahi meteorological station to assess the past and present climatic conditions at wheat growing period (November-March). Then the impacts of climate change on irrigation water requirement and yield of wheat were assessed for the years of 2025, 2055 and 2085 compared to the base year of 1975 which is the average condition of 1961-1990. The data for future projections were collected from the PRECIS outputs available with the Climate Change Cell of BUET. To assess the vulnerability of wheat yield, a simulation study was conducted for the Kanchan variety of wheat using a crop growth model called Decision Support System for Agrotechnology Transfer (DSSAT). For the model simulation climate data, soil data and crop management data were required. Four soil samples were collected following the composite method prior to ploughing of land and then analyzed in the laboratories. The crop management data were collected through FGDs, KIIs and informal interviews with the local farmers during the four field visits in the year of 2010-11. The analysis of historical and predicted climate of wheat growing period reveals that the observed and predicted maximum temperatures have increasing trend of 0.10C and 5.40C per century, respectively. The observed minimum temperature has decreasing trend of 0.50C per century but the predicted minimum temperature shows increasing trend of 6.20C per century. The observed rainfall has decreasing trend of 3.67% per decade, whereas the predicted rainfall has increasing trend of 10.15% per decade. The sunshine hour has decreasing trend of 4.22% per decade for the observed data and the predicted data has an increasing trend of 0.46% per decade. The observed relative humidity has increasing trend of 3.18% per decade whereas the predicted humidity has decreasing trend of 0.64% per decade. From the analysis of climatic data and local agricultural practices, it was found that net irrigation water requirement (NIR) may increase by 11.1%, 13.9% and 20.3% by the years of 2025, 2055 and 2085, respectively. The simulation study revealed that the wheat yield may decrease by 7.57%, 12.16% and 15.16% by the years of 2025, 2055 and 2085, respectively at fixed CO2 concentration (334 ppm). At variable CO2 concentrations wheat yield may increase by 3.76%, 6.07% and 7.42% by the years of 2025 (460 ppm), 2055 (535 ppm) and 2085 (620 ppm), respectively, compared to the base year of 1975 (334 ppm). The increasing CO2 concentrations are likely to offset to some extent of the adverse effects of other climatic variables. Increase in maximum and minimum temperatures are found to be primarily responsible for yield reduction through sensitivity analysis. With the increase of incoming solar radiation wheat yield may increase. Any change (increase or decrease) in base year soil pH may reduce wheat yield. With the addition of soil OC by 0.25% yield may increase by 1.88% compared to the base year. Application of three irrigations was found to result in the highest yield and 15 November was found to be the optimum time of seeding from the crop model simulation.