Investigating the stability of contour ridge system using rare earth element tracers under natural rainfall in the rocky mountain areas of Northern China

Abstract

Erratic and intensively erosive natural rainfall is becoming more frequent with the intensification of global climate warming, increasing uncertainty and unpredictable risks of the stability of contour ridge system. However, little attention has been paid to the in situ quantitative evaluation of contour ridge stability under natural rainfall. When water ponds in furrows, contour ridges tend to breach, inducing contour failure followed by ephemeral gully erosion, which greatly affects the stability of contour ridge system. To track sediment sources during ridge slope erosion, contour failure, and ephemeral gully erosion stages, ten rare earth element (REE) tracers were applied in three soil layers (total 0.3 m in depth) at four slope positions in-situ runoff plots under five rainfall levels. Runoff plots were conducted with contour ridgess for sweet potatoes (SP) and peanuts (PT). The stability of contour ridges was gradually weakened when erosion form shifted from ridge slope erosion to contour failure and then to ephemeral gully erosion. Ridge slope erosion controlled contour ridge stability, with contribution of 76.28–80.49 % to the total sediment yield. Sediment yield occupied 40.99 % of the total after contour failure appearing and 55.80 % after ephemeral gully formation. The lower-middle slope was the most fragile area for contour ridge system. Rainstorm (with precipitation 50–100 mm in 24 h) played a destructive role in weakening contour ridge stability, which generated the largest runoff (36.81–39.25 % to the total) and sediment yield (46.75–57.84 % to the total). Rainstorm made the greatest contribution to the occurrence of ridge slope erosion (51.64–59.99 %), contour failure (38.07–40.91 %), and ephemeral gully erosion (24.32–30.53 %). When using REE oxides to determine contour ridge stability, the tagging depth above 0.2 m and the uniform binding of oxides with < 0.05 mm particles should be considered. Thus, some modifications to lower the likelihood of ridge failures under rainstorm conditions may be effective to enhance the stability of contour ridge system.