Evaluation indices of drought of winter wheat in North China

As a natural disaster, drought is characterized by short of precipitation and reduction of crop yield. In this paper, the sequence of historic yield of winter wheat is processed with cubic polynomial and thus the yield can be divided into two parts—— tendency yield and meteorological yield which is mainly affected by meteorological factors. Then the correlation between meteorological yield and precipitation anomaly was analyzed. It was found that there are close relationship between meteorological yield and precipitation anomaly in whole growing season and in jointing stage, especially in the first period. The regression equations between them were established. Considering the drought is classified in light, moderate, heavy and severe grades in agriculture, two sets of drought indices were determined. The negative anomaly of precipitation of less than 15%, 15%～35%, 35%～55% and more than 55% in whole growing season of winter wheat correspond to light, moderate, heavy and severe drought respectively and the yield was reduced by less than 10%, 10%～20%, 20%～30% and more than 30%. But in period of jointing stage, the drought indices are less than 30%, 30%～65% and 65%～100% of negative anomaly of precipitation, which corresponds to light, moderate and heavy drought. The yield was reduced by less than 10%, 10%～20% and 20%～30%. Therefore the affection of short of precipitation in jointing stage on yield is not as much as that in whole growing season. The drought indices in whole growing season should be in priority compared with that in jointing stage when evaluating drought status. 