Rainfall intensity in geomorphology: Challenges and opportunities

Abstract

Rainfall arrival at the land surface drives or influences many geomorphic processes. These range from the mechanisms through which vegetation transforms rain into erosive gravity drops or stemflow, infiltration and water partitioning at the soil surface, the drop-impact sealing of soil surfaces, splash, sheet, and gully erosion, and triggering of the various forms of mass movement including landslides and debris flows. Rainfall intensity is a key influence on many of these mechanisms but is not a straightforward parameter to quantify, partly owing to the customary aggregation of rainfall data to hourly or other clock-time totals. This aggregation conceals intensity fluctuations including erosive ‘intensity bursts’ as well as the intermittency of rainfall. Nevertheless, much research shows that rainfall intensity over short time periods – 10–30 minutes – has great explanatory power. Much of our understanding of the influence of rainfall intensity is based on rainfall simulation experiments, but the value of the findings is limited because simulation is normally carried out using a high and constant rainfall intensity, quite unlike natural rainfall. This has limited our ability to build an understanding of the other important aspects of rainfall intensity, including, critically, its time variation and changed character among different environments – arid, temperate, or tropical. Thus, significant challenges and opportunities remain in the exploration of rainfall intensity in relation to geomorphology and geomorphic processes.