Classification of field drainage and wetness in irrigated paddy rice and upland crop rotation area: Analysis using aerial images taken at three different times with varying soil surface moisture

Abstract

Effective field drainage and wetness information is crucial for farmers to plan cultivation strategies in irrigated rice paddy and upland crop rotation areas. In this study, we developed a method to create regional and field-specific maps by assessing five classes of field drainage and wetness through unsupervised classification of three low plant coverage aerial images that were shot at different times in February in a paddy rice and upland crop rotation area (500 ha) in Chikusei City, Ibaraki Prefecture. The study area, located in Kokaigawa Lowland, exhibited substantial intrafield and interfield variations in water retention and drainage characteristics. The classification results guided a field survey, which revealed the presence of a shallow gravel layer, a dense and low-permeability plow pan, and a muck-like layer in the subsoil, all of which contributed to field drainage and wetness. A comparative analysis of the classification results and the evaluation by farmers based on their knowledge of field drainage and wetness showed broadly match. Further analysis based on three groups of the crop cultivation (bare-after paddy rice, bare-after soybean, and wheat) indicated a lower Kappa coefficient for the bare-after paddy rice group, likely due to the presence of residues and a reduced pulverization ratio in the field surface after paddy rice cultivation.