To investigate the correlations between annual yields with climate variables during each growth period across the stations for each zone, the yields were firstly linearly de-trended to get the de-trended yield series that were mainly affected by seasonal climate variability. Then the climate variables were also linearly de-trended, and the partial correlation analysis was applied to investigate the correlation between the de-trended yields series and the de-trended climate variables series during 1981–2009. Statistical significance was tested using the two-tailed t-test.