Partitioning of rainfall and sprinkler-irrigation by crop canopies: A global review and evaluation of available research

The role of crop canopies in the global water cycle is a topic of increasing international interest. How much rain and sprinkler-irrigation water are returned to the atmosphere or reach the soils beneath crop canopies, and the pathways of those water inputs at the soil, are linked to agricultural productivity and sustainability. This concise-format review synthesized and evaluated the available, limited, observational data (138 studies) on cropland throughfall, stemflow, and/or interception for >60 crop species covering all major climate types to obtain a global analysis of rainfall and sprinkler-irrigation partitioning by crop canopies. Partitions normalized per unit rain/sprinkler-irrigation (relative fractions, %) vary greatly across crop types with the interquartile range of throughfall, stemflow, and interception being 58–83%, 2–26%, and 11–32%, respectively. Stemflow data distribution across crop types is more often different than for throughfall and interception, contributing to overall variations in the partitioning of rain and irrigation observed to date. Partitions per storm also differ depending on the magnitude of rain or sprinkler-irrigation events and the stage of crop growth. Furthermore, throughfall and stemflow input patterns at the soil surface and subsurface may erode soils through different physical processes (i.e., throughfall droplet impact/splash versus scouring by stemflow); however, more research is needed to elucidate the underlying mechanisms and overall impacts. Finally, comparative analyses of partitions among croplands, shrublands, and forests indicate that crop canopies partition rain inputs differently and that there is a lack of studies in croplands. Hence, we suggest that future effort should be directed to the partitioning of rainfall and sprinkler-irrigation by canopies in agricultural settings.