Reconstructing rainfall and land-use conditions leading to the development of old gullies

Abstract

Knowledge of past erosion events and their controlling factors is an important key to understanding the impacts of environmental change (climate-land use) on the landscape. In this study, knowledge about erosion processes on the development of present-day ephemeral gullies is used for reconstructing conditions leading to the formation of old, permanent gullies. Empirical relations between flow hydraulics and channel geometry have been recently established for gullies. Hence, using measured bottom width Wbttom of old gullies as input, peak flow discharges (Qp) of these gullies can be estimated. In two forested areas in central Belgium, 52 old gullies were mapped. The old gullies had an average Wbottom ranging between 1.1 and 1.5 m. Corresponding calculated Qp values ranged between 0.04 and 0.07 m3/s. Rainfall intensities (I) were also deduced from Qp using the rational formula. By simulating various land use scenarios and thus various runoff coefficient (C) values, I and concentration time (Tc) could be calculated for each land-use class. Using I, Tc and intensity-duration-frequency tables for the study area, the recurrence interval (RI) of the rain events, needed to erode the observed gully channels was assessed. Although analysis of historical documents indicates that both areas have probably been under forest since the Middle Ages, it is unlikely that the old gullies originated under forest vegetation or even degraded forest vegetation, since R>I 200 years were obtained for these land-use scenarios. Cropland is the only land use that provides acceptable values of RI(11-128 years).