An automated extraction of small- and middle-sized rice fields under complex terrain based on SAR time series: A case study of Chongqing

Abstract

Spatial assessment of rice cultivation area is a crucial activity, which is the foundation for the government to effectively improve the comprehensive rice production, promote the continuous increase of farmers' income and accelerate the construction of a modern rice industry system. In recent decades, large areas of arable land in Chongqing were converted to built-up land or restored to forests and grasses, resulting in a rapid decline in the rice planting area. Chongqing has an annual average of 104 cloudy and foggy days, the terrain is complex, and rice fields are generally fragmented with small- or middle-sized. How to timely obtain updated and accurate rice planting maps in Chongqing remains challenging. In this study, we processed 411 time series Sentinel-1A SAR scenes of vertical-horizontal polarization in 2020 over Chongqing, and analyzed the characteristics of rice phenological parameters under different terrain conditions, including rice transplanting date, mature grain date, length of growing season, rice agronomy flooding decline speed during sowing-transplanting period, and green-up speed during transplanting-mature period. On these bases, a decision tree algorithm integrating topographical features and rice phenological characteristics was proposed for collectively mapping patches of rice field. The identified rice results had user, producer, and overall accuracies of 0.96, 0.85 and 0.88, respectively. Meanwhile, SAR-derived rice area was compared against official rice statistical area at the county/district level with the correlation coefficient of 0.96. According to the rice map in 2020, there was a total area of 6125.70 km2 paddy rice in Chongqing, which was 6.27% lower than the data from the Chongqing statistical yearbook. Our study demonstrates the robustness and effectiveness of the proposed rice mapping method that comprehensively deliberate the topographical and rice phenological features in the complex landscapes with diverse crop types, small-medium sized and fragmented rice fields, and frequent cloudy and foggy weather.