Event-scale flood types and controlling mechanisms in tile-drained agricultural watersheds

Study region This study focuses on 118 tile-drained agricultural watersheds in Iowa, USA, a region characterized by intensive agricultural management and widespread subsurface drainage systems. Study focus Hourly precipitation and discharge data from 2002 to 2023 were used to identify 7621 flood events. Based on hydrograph characteristics, these events were classified into three flood types: small-fast floods, sustained low-crest floods, and high-crest, high-volume floods. Interpretable machine-learning models were employed to identify the climatic, soil, hydrological, and topographic controls on flood-type occurrence, peak discharge, and runoff volume. Correlation and multiple regression analyses were further used to examine how flood-type frequency and runoff contributions vary across tile-drainage gradients. New hydrological insight Flood-type occurrence reflected the combined influences of climatic, soil, hydrological, and topographic factors, with precipitation duration emerging as one of the strongest controlling factors. Short-duration events were primarily associated with small-fast floods, whereas longer-duration events increasingly favored sustained low-crest floods. Most importantly, flood-type composition varied systematically across tile-drainage gradients. Watersheds with higher tile-drainage percentages exhibited greater volume (R = 0.50) and frequency (R = 0.55) contributions from sustained low-crest floods, but lower contributions from high-crest, high-volume floods (R = −0.34 and −0.43, respectively). These relationships remained significant after controlling for climatic, topographic, and soil-related factors. Together, these findings demonstrate the importance of considering flood-type-specific characteristics when developing watershed-management strategies in tile-drained agricultural landscapes.