Erosion assessment in the desertification site of Gilbues, Brazil

The Gilbués Desertification Site (GDS) is an 8,000-km² area located in the Northeast of Brazil. It comprises large continuous areas with deep (up to 30 m), wide (up to 50 m), and long (up to 6 km) gullies, as well as severe inter-rill erosion. Inside the GDS there is an experimental site, in which almost 100 check dams were constructed a decade ago to assess their feasibility as a soil-restoration initiative. For two years (2018 and 2019) we have monitored a 15-ha watershed that contains 52 check dams so as to estimate the main erosion-related parameters as well as to assess the effectiveness of the check dams. The monitoring program consisted of (i) a climate station; (ii) four hillslopes with pins every m², measured monthly to quantify gross erosion; (iii) five flights with an accurate unmanned aerial vehicle (UAV) to identify the siltation of the check dams and to parameterize the rainfall-runoff behavior; (iv) 92 soil samples in the hillslopes and inside the check dams; and (v) four infiltration experiments. The results show that (i) the gross erosion is 8 mm.yr^-1, or 10² Mg.ha^-1.yr^-1, a value ten times higher than the region average; (ii) based on the silting of the check dams, the sediment yield averaged 85 Mg.ha^-1.yr^-1, 20 times higher than the regional mean value, which is partially explained by the small size of the watersheds (10²-10³ m²); (iii) the Wischmeier vegetation C factor is 0.9, showing high degree of vegetative-cover degradation; and (iv) the sediment delivery ratio was 0.8, which could be satisfactorily represented by the Maner equation. These results show that, although the GDS corresponds to only 10% of the Boa Esperança (5,000 hm³) hydroelectric power plant basin, it may cause 60% of the reservoir silting. The GDS soil has also shown specific properties: 71% of the soil mass has a diameter of ~ 0.1 mm; there is a high rate of open macro-pores when the soil is dry (they close shortly after a moderate rainfall event ~ 40 mm); and it is prone to form gravel-like particles that silt in the reservoir delta (despite its fine diameter). Last, we observed that the check dams – as they were built – are not a sustainable solution: after a decade, nearly 10% are spilling due to the high siltation rates, causing dam-wall erosion and instability; and three dams have presented piping, with discharges (0.2 – 0.7 L.s^-1) one thousand times higher than the expected percolation flow through the dams.