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The use of water detection (WD) indices to infer daily discharge (Qd) has a great potential to enrich needed hydrological data for understanding fluvial processes driving the morphological changes of braided rivers. However, no consensus has been reached on which one stands out for use in mid-sized braided rivers. In this study, we compared the physical characteristics of three most commonly used WD indices, the Normalized Difference Water Index (NDWI), Modified Normalized Difference Water Index (MNDWI), and Normalized Difference Moisture Index (NDMI), for two mid-sized braided reach segments in the Qinghai-Tibet Plateau, China, that have different morphological structures. Relying on the Google Earth Engine web interface, we calculated the total mean water width (WWt) based on the detected surface-water areas (As) and braiding index (BI), as well as the mean values (m) of these indices over about four decades at the braided corridor scale (cs) (mNDWIcs, mMNDWIcs, and mNDMIcs). We then examined different responses of these indices to water and non-water features and their best threshold values for characterizing channel structures. Our analyses demonstrated that (1) NDWI and MNDWI perform well for detecting braided channel structures with the threshold of zero; (2) WWt is generally better correlated to Qd in a linear style than WD indices do, particularly when calculated from MNDWI; and (3) among WD indices calculated at the braided corridor scale, mMNDWIcs shows a better relationship with Qd than mNDMIcs does. Finally, we provided mechanisms that may explain these differences in terms of photometric discrepancies in calculating WWt and WD indices and the impact of image resolution on their calculations. [ABSTRACT FROM AUTHOR]

Neck cutoffs in meandering rivers have long been thought to occur when the neck width (b) approximates the mean width of the parent channel (W). Empirical evidence for this paradigm is scarce, however, because tracking late‐stage evolution of meander bends prior to cutoff at sufficient temporal resolutions is difficult in natural rivers. In this study, we captured field and photogrammetric data depicting temporal changes of the banklines forming the neck of one meander bend, and values of the narrowest neck width for 14 highly sinuous bends, in the Black River of the Qinghai–Tibet Plateau (China) over nearly four decades. Results show that the duration of bend evolution from the classic morphological threshold (b ≈ W) to the occurrence of neck cutoff could range from 52 to 161 years. This long period has hitherto been ignored by fluvial geomorphologists, and needs to be included in forthcoming kinematic and hydrodynamic models of meander evolution. Further analyses indicate that the neck narrowing process is autogenous at the local bend scale, neither controlled by the maximum bend sinuosity nor by the annual peak discharge. [ABSTRACT FROM AUTHOR]

Among braided rivers developed on the Qinghai-Tibet Plateau of China at very high elevations (>3,500 m), the middle and lower reaches of the Lhasa River have been affected by comprehensive human activities mainly involving dam construction, urbanization, farming, afforestation, and mining. In the current study, the impacts of these human activities on hydrology and morphology of the four braided reaches downstream of a cascaded of two dams are investigated. The study period was divided into 1985–2006 (P1), 2006–2013 (P2), and 2013–2019 (P3), representing the natural and changed flow regimes by dams. Using available daily discharge data at two stations within the four braided reaches, dam-induced hydrological alteration was analyzed based on the indicators of hydrologic alteration and range of variability approach and key discharge proxies were calculated. Remotely sensed images also were selected in the three periods and morphological metrics extracted from them were compared for the four reaches among these periods. Attenuated hydrological regimes were found for only two reaches. The total channel width (W c) and braiding intensity (BI t) followed different temporal trends among the four reaches. Annual average shift rates of the main channel in the four reaches were higher in the short (P2–P3) than in the long (P1–P2 and P1–P3) periods. The longitudinal changes of W c and the number of channels did not have any identifiable trend among the four reaches. By linking the morphological changes to quantified spatial and temporal patterns of various human activities, it was found that (1) the two dams had insignificant impact on channel morphology, suggesting that the studied braided river might have a short relaxation time and (2) the evolutional trajectories of morphological changes in most of the four reaches were similar, suggesting that temporal trends of morphological changes due to complex human activities are not affected by the different physiographic settings of the reaches. Continuous exploitation of the valley area requires comprehensive river management strategies for coordinating various human activities. [Display omitted] • A cascaded of two dams only changed the flow regime of the immediately downstream braided reach. • Channel morphology of the near-dam reach likely had a short relaxation time to recover from less intense disturbances. • Urbanization and farming constrain the space available for allow the braided river to change. • Historical trajectories of channel adjustment in response to human activities are similar among different braided reaches. [ABSTRACT FROM AUTHOR]

This study quantified the temporal properties of precipitation and actual evapotranspiration (ETa) in the Zoige basin of the Yellow River source region, China during the 1967–2011 period, as well as their influence to the area reduction of peatland. We extracted areas of different land-use and land-cover (LULC) types and obtained daily precipitation data. Then, we calculated annual precipitation (AP) and specific cumulative precipitation (SCP), which is the sum of precipitation to the date when an image was taken, and showed that the peatland areas were strongly affected by SCP. Using a modified Penman–Monteith equation, we calculated ETa for each LULC type and the area-weighted ones to show that the area-weighted total ETa was mainly contributed from grassland and peatland, which was between 450 and 550 mm. Temporal trends of the ratio of SCP to evapotranspiration showed that precipitation was generally greater than evapotranspiration rate not only during the summer but also over the 1967–2011 period. This trend failed to completely explain the continuous decrease of peatland area in the Zoige basin in decades. The draining effect of artificial ditches and natural gullies might play an additional role in causing peatland degradation but requires further process-based studies. [ABSTRACT FROM AUTHOR]

The carbon budget dynamics of peatlands plays a crucial role in both the response of peatland ecosystems to environmental change and the effect of environmental change on peatlands. Studies on northern peatlands have suggested that the reliable assessment of the carbon budget must include the carbon flux at the land‐stream interface. But this argument has not been proved in the Qinghai‐Tibet Plateau, the world's largest alpine system. To address the knowledge gap, in the growing season (May to October) of 2016, we chose Riganqiao peatland, a typical peatland in the Zoige Basin of the Qinghai‐Tibet Plateau which has been intensely drained via ditches since the 1960s. The objective of this study was to measure the organic carbon export from ditches, and to estimate what proportion of the carbon budget can be attributed to this export. Results showed that, during the growing season in 2016, total organic carbon flux in ditches was 2,345 t (8.6 g m−2), of which particulate organic carbon accounted for 12% and dissolved organic carbon for 88%. This flux decreased the carbon budget of Riganqiao peatland by 11.2% (from +76.8 g m−2 to +68.2 g m−2), making it a weakened carbon sink. This result revealed that organic carbon export from ditches plays an important role in the carbon balance in peatland of the Qinghai‐Tibet Plateau. [ABSTRACT FROM AUTHOR]

Peats have the unique ability of effectively storing water and carbon. Unfortunately, this ability has been undermined by worldwide peatland degradation. In the Zoige Basin, located in the northeastern Qinghai-Tibet Plateau, China, peatland degradation is particularly severe. Although climate change and (natural and artificial) drainage systems have been well-recognized as the main factors catalyzing this problem, little is known about the impact of the latter on peatland hydrology at larger spatial scales. To fill this gap, we examined the hydrological connectivity of artificial ditch networks using Google Earth imagery and recorded hydrological data in the Zoige Basin. After delineating from the images of 1392 ditches and 160 peatland patches in which these ditches were clustered, we calculated their lengths, widths, areas, and slopes, as well as two morphological parameters, ditch density (Dd) and drainage ability (Pa). The subsequent statistical analysis and examination of an index defined as the product Dd and Pa showed that structural hydrological connectivity, which was quantitatively represented by the value of this index, decreased when peatland patch areas increased, suggesting that ditches in small patches have higher degrees of hydrological connectivity. Using daily discharge data from three local gauging stations and Manning's equation, we back-calculated the mean ditch water depths (Dm) during raining days of a year and estimated based on Dm the total water volume drained from ditches in each patch (V) during annual raining days. We then demonstrated that functional hydrological connectivity, which may be represented by V, generally decreased when patch areas increased, more sensitive to changes of ditch number and length in larger peatland patches. Furthermore, we found that the total water volume drained from all ditches during annual raining days only took a very small proportion of the total volume of stream flow out of the entire watershed (0.0012%) and this nature remained similar for the past 30 years, suggesting that during annual rainfall events, water drained from connected ditches is negligible. This revealed that the role of connected artificial ditches in draining peatland water mainly takes effect during the prolonged dry season of a year in the Zoige Basin. [ABSTRACT FROM AUTHOR]