India is the world’s second largest producer of wheat (FAOSTAT, 2017), which is one of the important cereal crops maintaining the food security of the country. Burgeoning population along with climate change situation is going to threaten the food security of India. Changes in temperature and precipitation in future may impact on water resource availability, crop water requirement and thus, affect the crop and water productivity to a greater extent (Abeysingha et al., 2016). According to IPCC 5th assessment report (2014), global mean surface temperature and concentration of carbon dioxide (CO2) will increase by 4.8°C and 539 ppm respectively, at the end of 21st century under RCP 8.5 scenario. Indian Institute of Tropical Meteorology (IITM) ran various global and regional climate models with different emission scenarios, and indicated an increase of 3-5oC temperature and 5–10% summer monsoon rainfall for Indian sub-continent (NATCOM, 2004).
Climatic parameters govern different biological and chemical processes within plant system and thus regulate crop growth and development (Porter and Gawith, 1999). Temperature, rainfall, solar radiation (SRAD) and CO2 solely and in combination influence the crop growth and productivity to a greater extent. Both high and low temperatures reduce dry matter production and, at extremes, can drastically reduce wheat productivity (Grace, 1998). Agricultural productivity is sensitive to climate change due to direct effect of changes in temperature, precipitation, SRAD and CO2 concentration and also due to indirect effect through changes in soil moisture, incidence of pest and diseases (Mendelsohn, 2014). Effect of climate change on crop performance varied with the climate change scenario used, cropping system, management practices and also from location to location (Islam et al., 2012; Hillel and Rosenzweig, 2011). Climate change without any adaptation measure, will negatively impact wheat production in tropical and temperate regions under 2oC or more local temperature increase during late 20th century levels, although individual location may benefit (Porter et al., 2014). The overall relationship between weather and crop yield is often region specific, depending on difference in baseline climatic condition, agronomic managements and soil conditions (Porter et al., 2014). Sommer et al., (2013) predicted that climate change will benefit wheat production in Central Asia through the positive effects of temperature increase. In India, the potential yield of wheat will be reduced by 11% and 6%, under 2oC increase in maximum (Tmax) and minimum (Tmin) temperature (Subhas and Mohan, 2012). Xio et al., (2008) predicted that climate change related warming might lead to an increase of 3% in wheat at low latitude and 4% at high latitude in China by 2030. Warmer temperatures could modify the rates of photosynthesis and respiration, thus affecting crop growth rates (Long, 1991). Increase in temperature will shorten the length of growing period, and reduce biomass accumulation and yield (Butterfield and Morison, 1992). The increase in temperature under climate change scenario is expected to increase the crop evapotranspiration (ET) rate leading to crop water stress and inhibiting yields. Therefore, understanding the impact of climate change on crop production and water resource utilisation is of prime importance for developing suitable adaptation strategies (Abeysingha et al., 2016). Elevated CO2 (eCO2) alone could be beneficial to most of the crop by enhancing photosynthetic rates and water use efficiency (WUE) value (Conroy et al., 1994). The effect of eCO2 tends to be higher in C3 plants (wheat, rice) than C4 plants, due to high photosynthesis rate in C3 under eCO2 (Leakey, 2009). Since pre-industrial times, increase in atmospheric CO2 by more than 100 ppm, has certainly increased WUE and yield of C3 crops such as wheat and rice (McGrath and Lobell, 2011). Doubling of CO2 concentration from 330 ppm to 660 ppm increased wheat yield by 28 percent. However, increase in ambient temperature by 1oC and 2oC offsets yield increase to 25 and 16 % respectively and hastened maturity by 5 and 10 days (Lal et al., 1998). Periods of abundant radiation are favourable for agricultural production (Long, 2012). Decrease in SRAD reduced the potential wheat yield by 3.1% (Subhas et al., 2012) and 9.7% (Pathak et al., 2003) in Indian condition.
The mean annual wheat yield will increase by 13.9 to15.4, 23.6 to 25.6 and 25.2 to 27.9 % during 2020s, 2050s and 2080s in Gomti river basin of India (Abeysingha et al., 2016). Mishra et al., (2013) reported a change in wheat yield − 17% to + 5.4% in lower Indo-Gangetic Plain under REMO and HadRM3 projected scenarios respectively. In contrast, according to Naresh Kumar et al., (2014), 6 to 23 and 15 to 25 % reduction in wheat yield respectively during 2050s and 2080s is expected under projected climate change scenarios in India. Thus, climate change impact will vary region to region. There is need for comprehensive? assessment of the vulnerability in agricultural productivity under future climate change at different agro-climatic regions, delineated with different soil, climate and management practices.
Crop simulation models perform well across different environmental and management conditions to understand the effect of various climatic factors on crop growth and yield by taking into account their interactions with biotic and abiotic factors. Thus, crop models are increasingly being applied in agricultural and environmental studies as complementary tools to field experiment (Matthews et al., 2013). Several studies were carried out to develop an integrated evaluation of climate variability as well as climate change on regional and global food production through dynamic modeling (Alexandrov and Hoogenboom, 2000). The Decision Support System for Agro-technology Transfer (DSSAT) was widely used for analysing yield gap, decision making and planning, framing strategic and tactical management policies and climate change studies worldwide (Arya et al., 2017; Attia et al., 2016; Kassie et al., 2016; Aggarwal et al., 2006a; Aggarwal and Mall, 2002; Pathak et al., 2003; Li et al., 2015; Timsina et al., 2008). In India, climate change impact assessments were studied to establish the necessity of crop simulation models (Das et al., 2012; Behera and Panda, 2009; Saseendran et al., 2000; Aggarwal 2003).
Increased atmospheric temperature reduces the total duration of crop by inducing early flowering and shortening the grain filling period and thus reduces yield per unit area. Positive effect of CO2 increase will be nullified by negative response of temperature increment. Reduced transpiration, due to decrease in stomata closure under eCO2, increases water productivity. On the other hand increased temperature will enhance the atmospheric evaporative demand. Thus the question comes, how do different crop and weather interactions will effect on wheat yield and water productivity in future at different agro-climatic region of West Bengal? To address the above mentioned facts the study was undertaken i) to find the changing pattern of climatic variables over six selected locations of WB, India during 2021-95 and ii) to assess the impact of climate change on crop and water productivity of wheat.