Drought grade classification of summer maize in Huang-Huai-Hai area

Drought is the main agrometeorological disaster of summer maize production in the Huang-Huai-Hai area in China. Establishing the drought grade indexes for more effective monitoring and evaluation is of great significance to the prevention and mitigation of agricultural disasters. There have been lots of published research results about the effects of different soil water conditions on growth, development, yield components and yield of summer maize. However, there are large differences about stage division and grade classification among different researches, and it is difficult to apply and compare in different regions. In this paper, soil relative moisture and crop water deficit index were selected to establish the drought grade indexes according to the growth process of summer maize. The whole growth duration of summer maize was divided into five stages, namely sowing to emergence, emergence to jointing, jointing to tasselling, tasselling to milking and milking to maturity. Drought was classified into five grades, including no drought, light drought, moderate drought, strong drought and severe drought. The drought grade indexes were determined for each stage and grade. The used data included crop phenology, soil water and daily weather record. By analysis and summary of the published results, a set of drought grade indexes based on soil relative moisture were determined. And this set of index included each stage of summer maize from sowing to maturity. The established index was convenient for application because its consistency among stages and grades. After the determination of grade indexes of soil relative moisture, the relationship models between soil relative moisture and crop water deficit index were set up using the observing data at 16 agrometeorological stations in Henan Province. Then, the grade indexes of crop water deficit index were calculated using the relationship models on the basis of the determined soil relative moisture indexes. According to the index of soil relative moisture, the critical index value at five stages of summer maize was 65%, 60%, 70%, 75% and 70% for light drought, respectively; it was 45%, 40%, 50%, 55% and 50% for strong drought and 40%, 35%, 45%,50% and 45% for severe drought, respectively. According to the index of crop water deficit index, the critical index value at five stages of summer maize was 35%, 40%, 20%, 10% and 35% for light drought, respectively; it was 50%, 65%, 55%, 45% and 65% for strong drought and 55%, 75%, 65%, 55% and 75% for severe drought, respectively. The two established drought grade indexes were validated in the Huang-Huai-Hai area by selecting typical stations. Validated results showed that the two indexes are well consistent with the consistency of 71%-91% for the identification of drought grades. Meanwhile, the identification results by the two indexes were basically consistent with the historical occurrences of drought disasters. Therefore, the drought grade indexes established in this paper could be used in the future monitoring and evaluation of drought in summer maize production in the Huang-Huai-Hai area.