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Sediment regime disturbance of river catchments in a changing land cover context: Geoenvironmental and population dynamics

Periodic Reporting for period 1 - SEDILAND (Sediment regime disturbance of river catchments in a changing land cover context: Geoenvironmental and population dynamics)

Reporting period: 2019-09-01 to 2021-08-31

In a globally changing world, sediment regime disturbance has emerged as a dominant actor in the modification of river catchments. The sediment regime refers to the sediment budget (amount, type and timing of sediment inputs, outputs and storage) of a river system. Studies of sediment regimes in medium and heavily land cover modified catchments are scarce to the extent that even the likely direction of change of sediment yield compared to background conditions is unknown. Without understanding the changes in the sediments inputs of rivers we are potentially missing half the story. The main and novel research questions within the SEDILAND project are: how (and how rapidly) land cover changes impact on the sediment regime of a river catchment and ultimately on the river channel morphology?; Is there a critical tipping point that guarantees the reversibility of the sediment regime disturbance to a pre-developed scenario?; and if not, how can we assist in fostering a better environmental and territorial management in catchments that preserve natural processes (flow and sediment regime)?. These questions have been tackled within this project.
During the developmente of the project we have collected, homogenized and analyzed long-term (over 15 years) sediment transport records of four middle mountain experimental catchment managed by the IPE with a gradient of land cover modification: San Salvador (forest river basin), Arnás (abandoned field experimental catchment with progressive plant recolonization), Repoblación (afforested experimental catchment) and Araguás (badland experimental catchment). These four experimental catchments are located in the Spanish Pyrenees. They are completely equipped and they have cumulated data of sediment transport and channel morphology of over two decades. All four experimental catchments have rainfall driven flood hydrology. Current and past information on the sedimentary data (amount, type, timing of sediment inputs) have been analyzed to assess long-term trends and cycles between land cover and sediment regimen data.

We have used the method of wavelet transformation to identify the time-scale dependency of suspended sediment patterns concerning the temporally and spatially uneven transient processes of sediment production, accumulation, and transport. We have analyzed the temporal variation of concurrent discharge and suspended sediment fluxes in order to bridge the limitations of pure predictive models to learn from temporal data structures with the main purpose of identifying the mechanisms underpinning the suspended sediment patterns (e.g. climatic forces). Intraseasonal-to-seasonal, annual, and inter-annual dominant time-scales are thus identified. The short time-scales are driven by the quad-modal seasonal precipitation pattern specific to the climate of the region and provide a continuous supply of sediments to the river. The large time-scales, controlled by high magnitude flow events and within-reach sediment storage, display alternating periods of increasing and decreasing sediment fluxes; ultimately, they maintain the river channel within balance or within a moderate positive sediment accumulation process. This analysis and methodology help to understand temporal sediment dynamics, and ultimately to manage river catchments.
We expect the subset of the statistical techniques developed in this project to become tools ready to be used by practitioners. The develop algorithms are robust and reliable and will provide information at different sediment scales.
Comparison of cumulative precipitation and discharge disaggregated in four time-scales