Spatiotemporal variation of drought frequency of winter wheat in Hebei Province

Based on the daily meteorological data of 23 meteorological stations during 1960-2013 across the study area and its surroundings, the spatiotemporal variation of drought frequency of winter wheat is investigated by using the crop water deficit index (CWDI). The results show that during the entire growth period, the sequence of drought frequency is extreme drought > severe drought > moderate drought > light drought. The frequencies of moderate and severe droughts presented increasing trends after 1995, whereas light and extreme droughts showed decreasing trends during the same period. In addition, the drought frequency decreased from seeding to maturation, and the fluctuation of drought frequency tended to intense with an increase in the level of drought, especially during green-up to jointing. During that period, the frequency of extreme drought presented a decreasing trend after 1995, whereas an increasing trend was detected in severe drought conditions. Moreover, significant spatial differences in the trend of drought frequency were detected. For example, severe drought presented an increasing trend in the northwest parts of the study area and a decreasing trend in the east. Increasing and decreasing trends in extreme drought were detected in the northern and southern regions of the study area, respectively. Further, the drought frequency was characterized as a decreasing trend during seeding to overwinter. The frequencies of light, moderate, and severe droughts presented decreasing trends, whereas extreme drought showed an increasing trend during green-up to jointing. On the contrary, a reverse trend was detected during tasselling to maturation.