With the rising air temperature and precipitation, water and sediment flux in the Source Region of the Yangtze River have increased significantly since 2000. Nonetheless, the response of braided river morphology to climate-driven water and sediment flux change is still unknown. Water bodies of nine large braided rivers from 1990 to 2020 were extracted based on Google Earth Engine platform, and impacts of climate change on activation indices of braided river morphology were quantified. The main results are presented that a new method of braided water body extraction by combining Lowpath algorithm and Local Otsu algorithm is firstly proposed, which reduces 59% of the root mean squared error of braiding intensity in comparison with the Global Otsu method. The braiding intensity has a parabolic variation trend with the water area ratio, and the average sandbar area ratio has a negative power law trend with the water area ratio. Intra-annual channel migration intensity has an obvious temporal scale effect, which increases rapidly when the time span is less than 5 years. The warming and wetting trend led to vegetation cover increasing significantly. With the increase of runoff, water area of each braided reach has increased in both flood and non-flood season. Intra-annual channel migration intensity shows three different trends of increasing, weakening, and unchanged over time. The response of migration intensity to climate warming can be classified into three patterns in the SRYR as follows: sediment increase constrained pattern, sediment increase dominated pattern, and runoff increase dominated pattern.
The spatiotemporal variability of groundwater level (GWL) is an important property of peatland hydrology that directly affects fluctuations of water storage. Nonetheless, current understanding of the variations of GWL in different time scales still remains unclear. In this study, two peatland watersheds (0.151 km 2 for W1 and 0.844 km 2 for W2) in the Zoige Basin in the Source Region of the Yellow River (SRYR) were selected for monitoring the temporal variability of GWL using self-recorded water loggers during 2017-2021. The main results demonstrate that: (1) GWL variations tended to be controlled by gully drainage in sites adjacent to the gully and be more synchronized with rainfall in sites distant from the gully. The GWL near the gully that cuts through the peat layer was lower than that near the gully without cutting through the peat layer, with a maximum difference between the former and the latter of 58.3 cm, indicating the effect of longitudinal attenuation of the GWL in W1. (2) Because rainfall had a lag effect on the GWL, the length of lag gradually decreased with increased rainfall intensity (i.e., the lag time of sites far away from the gully was about 18 min shorter than that of sites close to the gully in W1). (3) The peak values of the GWL occurred simultaneously with the maximum and minimum rainfall in W2, and the peak occurrence time was related to the ratio of precipitation to evaporation. In the downstream sites, GWL fluctuated more intensively than the upstream ones in W2. Moreover, the average GWL of the upstream sites was 14.3 cm higher than that of the middle ones, indicating a decreasing trend of water storage along the gully. (4) The GWL discrepancy between wet and dry seasons was explicit, but the difference was smaller in the upstream sites due to limited gully incision and higher water storage within the peat layer. Additionally, rainy days dominate the GWL change in wet and dry seasons, but the different rainfall intensity resulted in a stable GWL in the dry season and an oscillating GWL in the wet season in W2. This study uncovers the spatio-temporal variation of groundwater level in two peatland watersheds, which is of great significance for understanding runoff variation, ecohydrological processes, and wetland shrinkage in the SRYR.
In this study, we unveiled the lumped effects at the reach spatial scale over three decades in one of the braided rivers in the Qinghai-Tibet Plateau of China, the Upper Lancang River (ULR). Using Landsat images obtained in 13 years between 1989 and 2018, we extracted flowing and non-flowing channels, active channel widths (unvegetated bars and flowing channels), and calculated lateral shifting rates of the main channel for the 13 periods. We also developed an empirical equation between vegetation area (Av) calculated from the high-resolution ortho-photo derived from an Unmanned Aerial Vehicle survey and Normalized Difference Vegetation Index for pixels of the Landsat image obtained at the same time. This relationship allowed us to estimate Av for other 12 selected years. We found that (1) braiding intensity increases with low discharges, indicating that the ULR is a very well-connected braided system with groundwater providing a large set of aquatic habitats, (2) this braided system is very well-supplied and actively shifting in relation to peak flow and flood duration, and (3) The ULR supports a progressive vegetation encroachment, which seems to be linked to temperature rising. Our study showed several similar morphological patterns to those in other braided rivers, such as the ones observed in the European Alps but much more active, well-supplied and highly connected. These similarities suggest that similar morphodynamic processes might take effect in the braided rivers with very high elevations and potentially high spots of biodiversity, indicating the ULR may be a reference for this region similarly to the Tagliamento in the Alps, but it seems that this system can be very sensitive to global change due to vegetation encroachment following temperature rising and decreases of low flows.