Connectivity of sediment transport in a semiarid environment: a synthesis for the Upper Jaguaribe Basin,Brazil

Purpose

Hydrosedimentological studies conducted in the semiarid Upper Jaguaribe Basin, Brazil, enabled the identification of the key processes controlling sediment connectivity at different spatial scales (10⁰–10⁴ km²).

Materials and methods

Water and sediment fluxes were assessed from discharge, sediment concentrations and reservoir siltation measurements. Additionally, mathematical modelling (WASA-SED model) was used to quantify water and sediment transfer within the watershed.

Results and discussion

Rainfall erosivity in the study area was moderate (4600 MJ mm ha^?1 h^?1 year^?1), whereas runoff depths (16–60 mm year^?1), and therefore the sediment transport capacity, were low. Consequently, ～60 % of the eroded sediment was deposited along the landscape, regardless of the spatial scale. The existing high-density reservoir network (contributing area of 6 km² per reservoir) also limits sediment propagation, retaining up to 47 % of the sediment at the large basin scale. The sediment delivery ratio (SDR) decreased with the spatial scale; on average, 41 % of the eroded sediment was yielded from the hillslopes, while for the whole 24,600-km² basin, the SDR was reduced to 1 % downstream of a large reservoir (1940-hm³ capacity).

Conclusions

Hydrological behaviour in the Upper Jaguaribe Basin represents a constraint on sediment propagation; low runoff depth is the main feature breaking sediment connectivity, which limits sediment transference from the hillslopes to the drainage system. Surface reservoirs are also important barriers, but their relative importance to sediment retention increases with scale, since larger contributing areas are more suitable for the construction of dams due to higher hydrological potential.