Medição e modelagem de voçorocas no bioma Caatinga: o caso da bacia representativa de Madalena, CE

Abstract

In contrast with inter-rill erosion, gullies can cause fast and severe changing on the landscape of a region. In the Brazilian semiarid region (a one-million km² area that coincides with the Caatinga biome), due to the shallow soils gully erosion processes are limited to small dimensions, being less representatives than sheet erosion, concerning total sediment yield. Nevertheless, gullies, even with small sizes, have a high influence on sedimentological processes, changing the sediment dynamics inside de watershed. Due to the land use change in the Caatinga, agricultural automation, deforestation for extensive cattle rising and development of infrastructure, as construction of new roads, gully occurrence has been becoming more frequent. This study focuses on the Madalena representative basin (124 km2, state of Ceará, Brazil), a land-reform settlement with 20 inhabitants per km², whose main economic activities are agriculture (especially Zea mays), livestock and fishing. In the catchment area, where there are 12 dams (with storage capacity ranging from 6.104 to 2.107 m³). Topographic surveys were performed using Total Station and UAV (Unmanned Aerial Vehicle), to obtain digital terrain models and assess the volume of soil eroded in the channel. Soil samples were collected in order to estimate the rill erodibility and the critic shear stress. The studied gullies, classified as small classical (or permanent) gullies have the following dimensions (μ±σ): projection area (317±165 m²), length (38±6 m), volume (42±25 m³) and eroded mass (61±36 Mg). The modelling pointed out an estimative of soil loss at 5±3 Mg.ano-1 in the first decade of the gully process, and almost 16 Mg.ha-1.ano-1 on the sub-basins studied, which corresponds to more the thrice the average of laminar erosion production in the State of Ceará. We used the Foster and Lane model (1986) for ephemeral gullies. Due to limitations of the original model to tackle the slope of the channel walls we proposed a correction factor of the cross-section area in function of the watershed geometry, obtaining good positive responses from the modelling (NSE = 0.75; for the Cross-section area), implying that the model can be applied as tool of volume prediction of gully volume in the semiarid, even for small classical gullies, once the analyzed gullies began around 58 years ago, due to the construction of a new road. As secondary result we identified that the 30-minute intensity is the most representative for the gully process, the same for sheet erosion proposed by Wischmeier and Smith (1978).