Influences of Changes in Rainfall and Solar Radiation on Performance of Kharif Sorghum

Kharif sorghum is an important crop of the northern transition zone (NTZ) of Karnataka. Historically this zone was characterized by the assured and uniform distribution of rainfall during the southwest monsoon. The last decade has witnessed increased erraticity in the onset, progress and distribution of rainfall, but days without rain also remain cloudy for weeks during Kharif season, thus lower the crop canopy, which affects the yield, interrupts solar radiation. Solar radiation, rainfall are the two important climatic factors affecting crop performance, but it is logistically difficult, and resource demanding to artificially create study-growing environment under field conditions. Alternately, Crop Simulation Models can be effectively used for such studies by creating customized weather scenarios within the model. Four rainfall scenarios (±10 and ±20 % over observed) and four solar radiation scenarios (±10 and ±20 % over observed) were created by using 32 years’ observed weather data (1985-2016) within the calibrated and validated DSSAT-CERES-Sorghum model [1].

Simulations were run across all the above scenarios for 32 years' seasonal analysis with the best four kharif sorghum cultivars sown across three dates of sowing under the standard package of practices followed for NTZ. Model simulated annual outputs for grain yield over 32 years were averaged and presented. The model simulated results revealed that for NTZ changes in solar radiation was found to have more effect on yield than rainfall. Any reduction in solar radiation over observed drastically reduces the yield. Across cultivars and dates of sowing under observed weather (1985-2016), on average, 1720 kg ha-1 yield was simulated. When solar radiation was reduced by 10 % across rainfall scenarios the average yield was reduced to 1424 kg ha-1 which further reduced to 670 kg ha-1 (61% reduction) when solar radiation was reduced by 20 %. In contrast, when solar radiation was increased by 10 % and 20 %, the model simulated 2967 kg ha-1 and 3181 kg ha-1 yield, respectively which is 42 and 46 % more over the yield of observed weather. This study showed that for NTZ of Karnataka during the Kharif season increased cloudy period will have a more adverse effect on yield than changes to rainfall.