The influence of ecohydrological and biogeomorphological seasonality in non-perennial rivers

Abstract

The interaction between riparian vegetation growth and river evolution is characterised by complex nonlinear feedback. When dealing with non-perennial rivers, this interaction presents growth patterns directly affected by the temporality of the flow, which controls water availability. Dry periods reduce the diversity and coverage of herbaceous species along the channel and riparian area. When prolonged, it modifies/prevents ecological succession in geomorphic units, such as bars and islands and riverbanks, delaying the growth process. The long periods of gentle precipitation allow moisture to permeate the channel, allowing the maintenance of herbaceous strata. On the other hand, extreme flood events act by removing the riparian vegetation and structurally modifying the bio-geomorphological patterns. To contribute to these analyses, the present work evaluated how ecohydrological and bio-geomorphological seasonality affects the morphology of non-perennial sandy channels in Northeast Brazil. For this purpose, DEMs and orthomosaics, generated from five UAV surveys in three stretches of the basin that present a different hydrological behaviour between October 2021 and September 2022, were analysed, allowing the analysis of morphological changes in the coverage of the watershed. Vegetation. Simultaneously, we installed field quadrants to monitor the coverage and growth of some riparian vegetation species of some geomorphic units of the stretches. This information was related to data from six pluviometric pluviometric stations. The rainfall volume of the hydrological year was between 400mm and 500mm, with an average of 46 days of rain and two daily events of extreme rainfall (above 50mm/day) during the year. The wettest period was between March 20th and April 9th, when rainfall accumulated from 56mm to 118mm around the watershed and had two daily extreme rainfall events. The surveys between January 7th and March 19th identified insignificant bio-geomorphological changes in the stretches; at the beginning of the first precipitation events, erosion processes were identified in the sandy bars, which soon after were stabilised by the growth of herbaceous species. The rain events between March 20th and April 9th generated an accumulation of precipitation between 90mm and 117mm and a low flow in the channel in the monitored sections. In one of the stretches, an increase in erosive processes on the banks was identified, and in all stretches, there was the growth of herbaceous vegetation in the alluvial bed. In general, there was a significant increase in vegetation cover, mainly herbaceous, in all quadrants; some stood out for showing a 70% increase in area coverage and a 100% increase in the number of bushy species (Jatropha mollissima ). It is essential to highlight that this quadrant is located in a sandy bar formed from the accumulation of sediment from an invasive tree individual (Prosopis juliflora) that was removed and relocated in an anterior extreme event and regrowth in the bed. These results highlight that the growth and maintenance of riparian vegetation and its spatial location strongly depend on the hydrological regime. The presence of vegetation associated with the deposition of sediments, stabilisation of banks and changes in morphology are vital features to river dynamic understanding.