Transformation of Rainfall by a Soybean Canopy

Abstract. The crop canopy significantly changes the distribution of rain water and irrigation water in the canopy and topsoil, thus potentially causing uneven distribution of surface soil water content and altering water use efficiency. In this study, simulated rainfall was used to measure soybean canopy stemflow and throughfall, and a spray method was used to observe canopy interception. The objectives were (1) to quantify stemflow, throughfall, and interception storage at different times during the soybean growing season and (2) to examine the spatial distribution of throughfall under a soybean canopy. Results showed that interception storage increased as soybean leaf area increased, reaching a maximum of 1.14 mm at the full-seed stage (R6). Stemflow, which was determined at rainfall intensities of 20 to 120 mm h-1, increased as soybean leaf area and rainfall intensity increased. Depending on leaf area and rainfall intensity, stemflow accounted for 3% to 23% of incident rainfall. Throughfall, which was only determined at a rainfall intensity of 40 mm h-1, decreased from 96% of incident rainfall at the second trifoliate (V2) stage to 75% of incident rainfall at the R6 stage. When soybean plants were small, throughfall was evenly distributed beneath the canopy. As the plants grew, throughfall was concentrated in the inter-row region. At the R6 stage, throughfall amounts in the center inter-row were up to 3.1 times those next to the soybean row. In summary, there were significant changes across time in the partitioning and spatial distribution of rainfall or sprinkler irrigation beneath a soybean canopy. This has important implications related to agrochemical applications as well as to erosion control and effective use of irrigation water on sloping land.