Effect of river channel planar complexity on riparian vegetation-river flow relationships in arid environments

Abstract

Understanding how changes in channel form affect the relationships between riparian vegetation and river flow is critical to scientific river regulation in arid environments, but relevant quantitative research is lacking. Aiming to quantify the effect of channel planar complexity, one of the most dynamic aspects of channel form, on riparian vegetation-flow relationships at annual and basin scales, a comparative study was conducted on two rivers in the lower Heihe River, China. Information on riparian vegetation and channel form was derived from multi-source remote sensing imagery and the Copernicus digital elevation model, and river flow and groundwater table data were obtained from field observations. Channel planar complexity was calculated as the ratio of the total length of all channels of a river to the straight-line distance between the two ends of the river, and the riparian vegetation-flow relationship was quantified by a multiple linear model that couples riparian vegetation degradation under zero-flow condition and riparian vegetation improvement by river flow. During 2002–2020, (1) one river was 1.5 and 1.04 times the other in terms of planar complexity and profile slope, respectively; (2) the water-year runoff of one river was 3.2 × 108 to 7.6 × 108 m3 and that of the other river was 0.8 × 108 to 4.8 × 108 m3; (3) the riparian vegetation condition indicator of the two rivers was 2202–4113 and 1242–3362, respectively; (4) the more complex river was 2.3 times the other in the efficiency of flow in improving riparian vegetation. The results indicate that planar complexity is positively correlated with flow's efficiency, with the underlying cause being changes in groundwater recharge rates, and that the change in efficiency can be greater than the change in planar complexity itself. The spatio-temporal variability in the effect of channel form on riparian vegetation-flow relationships merits further investigation and serious attention in river regulation.