Abstract
Heading and flowering are two key phenological stages in the growth process of winter wheat. It is of great significance for agricultural management and scientific research to accurately monitor and forecast the heading and flowering dates of winter wheat. However, the monitoring accuracy of existing methods based on remote sensing needs to be improved, and these methods cannot realize forecasting in advance. This study proposed an accumulated temperature method (ATM) for monitoring and forecasting the heading and flowering dates of winter wheat from the perspective of thermal requirements for crop growth. The ATM method consists of three key procedures: (1) extracting the green-up date of winter wheat as the starting point of temperature accumulation with the dynamic threshold method from remotely sensed vegetation index (VI) time-series data, (2) calculating the accumulated temperature and determining the thermal requirements from the green-up date to the heading date or the flowering date based on phenology observation samples, and (3) combining the satellite-derived green-up date, daily temperature data, and thermal requirements to monitor and forecast the heading date and flowering date of winter wheat. When applying the ATM method to winter wheat in the North China Plain during 2017–2019, the root mean square error (RMSE) for the estimated heading date was between 4.76 and 6.13 d and the RMSE for the estimated flowering date was between 5.30 and 6.41 d. By contrast, the RMSE for the heading and flowering dates estimated by the widely used maximum vegetation index method was approximately 10 d. Furthermore, the forecasting accuracy of the ATM method was also high, and the RMSE was approximately 6 d. In summary, the proposed ATM method can be used to accurately monitor and forecast the heading and flowering dates of winter wheat in large spatial scales and it performs better than the existing maximum vegetation index method.
Highlights
Heading and flowering are two important phenological stages in the growth process of winter wheat
The accumulated temperature method (ATM) proposed in this study assumes that the growth and development of winter wheat is mainly driven by temperature, and the accumulated value of the effective temperature required for its development from green-up date to specific growth stages is relatively fixed in a certain region
For both the heading date and the flowering date, the temperature indices based on P2 outperform the temperature indices based on P1 since the accumulated effective temperature (AET) based on P2 are much more concentrated in the frequency distribution and their coefficient of variation (CV) are much smaller
Summary
Heading and flowering are two important phenological stages in the growth process of winter wheat. The growth and development of winter wheat is very sensitive to factors such as water and heat conditions, soil fertility, and field management levels [2,3,4], and is a critical stage where agricultural practices and crop yield estimation research are concerned [5,6]. Crop phenological information is mainly derived through two methods: ground observation and retrieval from satellite time-series data. The advantages of ground observation are rooted in their simplicity, high precision, and long time span [7] This method is labor intensive and cannot provide spatially continuous phenological information at large scales. To solve these problems, the method of retrieving phenology from satellite time-series data is usually used. Most of the existing methods only focus on the start of the season (SOS) and the end of the season (EOS), lacking effective methods to monitor other crucial crop phenology such as the heading date and flowering date
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