Abstract Long-term monitoring of crop phenology is a critical process to understand changes in cropland vegetation dynamics related to climate and human management. During the last four decades, seasonal changes in vegetation have been measured by satellite instruments, the Advanced Very High Resolution Radiometers (AVHRRs). However, an acceptable detection of crop phenology has been prevented due to its coarse spatial resolution (~8 km) pixels in which croplands are frequently mixed with non-cropland. This study introduces a novel method for detecting the start of the growing season in cropland (SOSC) from the AVHRR Normalized Difference Vegetation Index (NDVI) to examine the long-term changes in SOSC at the 8-km pixel level across the core cropping regions of the Midwestern United States (the so-called ‘the Corn Belt’) for 1982–2015. The AVHRR-based SOSC was compared with the SOSC retrieved from the pixels covering mostly croplands of the 250-m Moderate Resolution Imaging Spectroradiometer (MODIS) NDVI. These two satellite-based SOSC shows reasonable correspondence in spatial patterns so that the annual values of the root mean square difference ranges from 3.6 to 7.4 days during the overlapping period of 2008–2015. Also, an acceptable correspondence was found between the interannual variations in the AVHRR SOSCs and in the state-level survey of crop stages. For sowing stages, correlation coefficients ranged from 0.43 to 0.68; for flowering stages those ranged from 0.51 to 0.83. By analyzing long-term SOSC changes with climate factors, we found a strong tendency that warmer pre-season condition is accompanied with earlier SOSC. In addition, larger pre-season precipitation tends to postpone SOSC as strongly as the pre-season temperature affects SOSC in a considerable portion of the core cropland. This study suggests that the postponing of sowing due to larger precipitation, which has rarely been highlighted in previous studies, requires more attentions in order for better understanding of vegetation dynamics in agriculture-climate studies.

中文翻译：

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从长期 AVHRR NDVI 改进美国玉米带作物生长季节开始的绘图和变化检测

摘要 作物物候的长期监测是了解与气候和人类管理相关的农田植被动态变化的关键过程。在过去的四年中，植被的季节性变化已通过卫星仪器测量，即高级超高分辨率辐射计 (AVHRR)。然而，由于其粗糙的空间分辨率（~8 公里）像素，其中农田经常与非农田混合，因此无法对作物物候进行可接受的检测。本研究介绍了一种从 AVHRR 归一化差异植被指数 (NDVI) 检测农田 (SOSC) 生长季节开始的新方法，以检查核心种植区 8 公里像素级别 SOSC 的长期变化1982-2015 年美国中西部地区（所谓的“玉米带”）。将基于 AVHRR 的 SOSC 与从覆盖 250 米中分辨率成像光谱仪 (MODIS) NDVI 的大部分农田的像素中检索到的 SOSC 进行了比较。这两个基于卫星的 SOSC 在空间模式上表现出合理的对应关系，因此在 2008 年至 2015 年的重叠期间，均方根差的年值范围为 3.6 至 7.4 天。此外，在 AVHRR SOSC 和州级作物阶段调查的年际变化之间发现了可接受的对应关系。对于播种阶段，相关系数范围为 0.43 至 0.68；对于开花阶段，这些范围从 0.51 到 0.83。通过分析长期 SOSC 随气候因素的变化，我们发现了一个强烈的趋势，即更暖的季前条件伴随着更早的 SOSC。此外，较大的季前降水倾向于推迟 SOSC，就像季前温度影响相当一部分核心农田的 SOSC 一样。这项研究表明，由于降雨量较大而推迟播种，这在以前的研究中很少被强调，需要更多的关注，以便更好地了解农业气候研究中的植被动态。