Acceleration of Horton overland flow and erosion by footpaths in an upland agricultural watershed in northern Thailand

Through field rainfall simulation experiments in an upland mountainous watershed of northern Thailand, we have identified two phenomena that increase the potential for Horton overland flow (HOF) generation on agricultural lands. First, there appears to be a transition period of 12–18 months, extending from the time of abandonment until the formation of a dense vegetation layer capable of intercepting rainfall and ponding surface water, during which HOF generation is accelerated. Simulation data indicate these recently abandoned fields may have runoff coefficients (ROCs) as high as 40% during large seasonal storms with wet antecedent soil moisture conditions. In comparison, actively cultivated lands and advanced (>16–18 months) fallow fields, the land surfaces existing before and after the threshold period, have ROCs≤4%. Secondly, compacted path surfaces initiate HOF within agricultural fields, which have saturated hydraulic conductivity (Ks) values that are 100–200 mm h−1 higher. In the study area, path/furrow networks, comprising 8–24% of field surface areas, are designed to provide walking access within fields and channel excess surface flow from the fields. Compared with hoed surfaces, path/furrows reduce the time to HOF generation by about 85% and have ROCs that are six times higher. Access paths have the lowest Ks values of all watershed surfaces, but conveyance efficiency of HOF generated on these surfaces is low. Even recently created field paths are capable of reducing runoff generation time by 40–90% and producing sixfold increases in ROCs. Collectively, the data suggest that agricultural erosion rates are accelerated during the 12–18-month threshold period following abandonment and during storms where path-generated HOF interacts with adjacent planting surfaces. Despite having periods of increased HOF generation, the total HOF contribution from agricultural fields to the basin stream hydrograph is similar in magnitude to that of unpaved roads, which occupy 95% less land area.