基于CNN-RNN网络的中国冬小麦估产

    Yield estimation of winter wheat in China based on CNN-RNN network

    • 摘要: 在大范围内快速、准确地预估作物产量,对作物管理、粮食安全、粮食贸易和决策有重要意义。遥感为大规模作物估产提供了便利,大多数研究者结合深度学习和遥感影像取得了较好的结果。然而,农作物生长状态随时间变化,其产量具有非线性时空特征,单一的深度学习方法无法充分利用影像信息。因此,该研究提出了一种基于卷积神经网络(Convolutional Neural Networks,CNN)和门控循环单元(Gated Recurrent Unit,GRU)混合神经网络估产模型(CNN-GRU),利用CNN从多光谱遥感影像中提取丰富的空间-光谱特征,在此基础上,结合GRU从多时相遥感影像中自适应学习冬小麦生育期各阶段之间的时间依赖,从多尺度融合冬小麦的生长特征并对其产量进行回归预测。该研究以全国冬小麦主产区为研究区,选取2001—2018年MODIS影像和冬小麦产量数据,构建了冬小麦估产数据集,并验证了CNN-GRU估产模型的性能。结果表明:1)以2016—2018年估产样本作为测试集,CNN-GRU估产模型的均方根误差(Root Mean Square Error,RMSE)年平均值为818.3 kg/hm2,相较于CNN、GRU、支持向量回归(Support Vector Regression,SVR)、随机森林(Random Forest,RF)和决策树(Decision Tree,DT)模型分别降低了20.13%、18.81%、29.51%、34.84%和36.57%;2)将冬小麦整个生育期划分为6个时间窗,CNN-GRU估产模型在灌浆-成熟期时精度最高,RMSE为817 kg/hm2,而抽穗-开花期的RMSE为823 kg/hm2,相较于灌浆-成熟期低0.7%。因此,该估产模型有能力提前2个月预测全国冬小麦主产区产量。

       

      Abstract: A rapid and accurate evaluation of crop yield at large scale is very critical to the planning and management of crop market. The prediction of crop yield includes the extensive manual sampling or the use of remote sensing images at present. Particularly, the remote sensing is preferred to predict the large-scale crop yield, providing a large number of satellite images with long time and high spatial resolution. Much effort has been made on the crop yield prediction using remote sensing images and deep learning. However, the yield generally presents nonlinear spatio-temporal characteristics, due mainly to the growth status of crops changes with the time. A single deep learning cannot make full use of crop growth characteristics in multi-temporal images. In this study, a Convolutional Neural Networks - Gated Recurrent Unit (CNN-GRU) estimation model was proposed to extract rich spatial-spectral features from multi-spectral remote sensing images. The time dependence in the phase remote sensing image was also adaptively learned among the various stages in the growth period of winter wheat. As such, the growth characteristics of winter wheat were integrated from multiple scales, further to predict the yield. Furthermore, the main production areas of winter wheat in the country were taken as the research area, including 1 713 districts and counties. The forecast dataset of winter wheat production was finally constructed using the statistical output of winter wheat, MODIS surface reflectance, day and night surface temperature images, and land cover data from 2001 to 2018, thereby to verify the performance of CNN-GRU production estimation model. The results showed that: 1) The annual average root mean square error (RMSE) and mean absolute error (MAE) of the CNN-GRU production estimation model from 2016 to 2018 were 818.3 and 560 kg/hm2, respectively, where the annual average RMSE was reduced by 20.13%, 18.81%, 29.51%, 34.84%, and 36.57%, respectively, compared with the CNN, GRU, Support Vector Regression (SVR), Random Forest (RF), and Decision Tree (DT). Therefore, the CNN-GRU yield estimation model can be expected to accurately predict the winter wheat yield, further to effectively extract the space-spectrum-time information in the entire growth period of winter wheat from multi-temporal remote sensing images. 2) Six time windows were divided during the whole growth period of winter wheat, including the sowing-emergence, tillering-overwintering, green-rising, jointing-booting, heading-flowering, and filling-maturity period. Among them, the highest accuracy of CNN-GRU estimation model was found in the filling-maturation period. Specifically, the annual averages of RMSE and MAE were 817 and 556 kg/hm2, respectively, and R2 was greater than 0.7. However, the annual averages of RMSE and MAE during the heading-flowering period were 823 and 560 kg/hm2, respectively, where the annual average RMSE was 0.7% lower than the filling-maturation period. Therefore, the proposed CNN-GRU yield estimation model can accurately predict the output of winter wheat in the middle and late stages of growth period, particularly for the national yield of winter wheat two months in advance.

       

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