A cropland parcel is one of the most basic units in agricultural production. Its spatial structure has also changed significantly in China in recent years, particularly with the advancement of well-facilitated farmland construction. However, it remains unclear on the spatial morphological variations in the cropland parcels after well-facilitated farmland construction. It is still lacking in the evaluation of construction and long-term benefits. In this study, a systematic and efficient monitoring was proposed to timely detect the cropland changes. Several typical regions of the well-facilitated farmland construction were also selected in the middle reaches of the Yangtze River. Sentinel-2 and GF-2 imagery data were integrated with deep learning (Dual branch SpatioTemporal Fusion Network, DSTFNet). The vectorized boundaries of cropland parcels were extracted after construction. Six landscape metrics—mean parcel size, shape index, fractal dimension index, edge density, parcel density, and standard distance index—were employed to characterize the spatial morphological features of the cropland parcels after construction. Furthermore, the Cropland Parcel Spatial Morphology Index (CPSMI) was developed to evaluate the interannual variations in the cropland use intensification. There was a trend of “small parcels larger” after construction, according to the high-accuracy cropland parcel boundary data (with over half of the accuracy metrics exceeding 90%). The mean parcel size increased from 0.80 to 0.84 hm². Contiguous cropland facilitated faster land transfer and then enhanced labor efficiency throughout agricultural production. In addition, the geometric shape of the parcels was more regular: the area-weighted mean shape index decreased from 1.53 to 1.51, the mean fractal dimension declined from 1.48 to 1.46. These indicators collectively confirmed that the shape regularity and simplified boundaries were improved after the cropland consolidation. Mean parcel density also decreased from 0.82 to 0.72, indicating a more concentrated spatial distribution. While the mean standard distance index remained unchanged, its dispersion was reduced for intensive agricultural production. Overall, the spatial morphology of well-facilitated farmland parcels in the study area tended toward greater mechanization and intensification. The CPSMI of well-facilitated farmland parcels also decreased from 0.378 to 0.357. Moreover, several challenges were presented in the well-facilitated farmland construction in agricultural modernization. There was an urgent need to optimize the site selection of cropland parcels and supporting facilities within the construction region. Rational planning was essential to consider the natural and production conditions in the local regions. The supporting facilities were expanded to encroach on the cropland resources, leading to a reduction in cropland. It was crucial to strike a balance between the production efficiency of cropland parcels and supporting facilities, particularly for the stable and high grain production. In conclusion, the cropland parcel assessment—using multi-source remote sensing imagery, deep learning, and landscape metrics—can provide an effective tool to track the spatial variations in cropland morphology. The valuable data support and theoretical foundations can also be offered to monitor, regulate, and optimize the well-facilitated farmland construction. The finding can greatly contribute to more scientifically grounded and data-driven decision-making on modern agriculture.
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Due to frequent cloudy and rainy weather, it is challenging to map crop types over South China with fragmented agricultural landscapes. The Gaofen (GF) series satellites developed by China have high spatial-temporal resolution and good image quality. This study exploited the spatial and temporal advantages of multi-source GF images for fine crop classification in heterogeneous agricultural areas with frequent clouds and rain.
This study characterized the spatial geometry of agricultural fields based on GF-2 data with high spatial resolution, and cooperated with the encrypted observations from GF-1 and GF-6 time series to fully characterize the spectral seasonal variations of crops. By constructing three-dimensional classification features of spectrum-time-space, the crop classification based on random forest classifier were conducted, and the importance scores of different features were calculated. Furthermore, several classification scenarios were set based on different satellite combinations and different classification units, for further analyzing the performances of integrating different GF datasets on crop type mapping.
The overall accuracy of synergistic GF-1, GF-2 and GF-6 by object-oriented crop classification was 95.49% with Kappa of 0.94 in Qianjiang city; the overall accuracy in Zaoyang city was 93.78% with Kappa of 0.92. The accuracy of crop classification by integrating GF-2 and GF-6 was higher than that by GF-2 and GF-1. In addition, the object-oriented crop classification based on GF-2 outperformed the pixel-oriented one, in which the overall accuracy improved by 1.4% and 1.32% in Qianjiang and Zaoyang, respectively. Compared with the spectral and spatial features of GF-1 and GF-2, the GF-6 spectral bands had the largest contribution to crop type identification, and the cumulative importance score accounted for 82% (Qianjiang) and 77% (Zaoyang) of all spectral bands. Among them, the four new spectral bands of GF-6, namely, red-edge I band (B5), red-edge Ⅱ band (B6), purple band (B7) and yellow band (B8), accounted for 47% (Qianjiang) and 33% (Zaoyang) of all spectral bands for crop type identification.
Integrating multi-source GF images by taking advantages of their spectral, spatial and temporal features could not only alleviate the "mixed pixel" problem caused by the fragmented agricultural landscape, but also reduce the uncertainty of crop type identification in cloudy and rainy areas, providing great potential for accurate crop mapping in South China.
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