Variational-Based Data Assimilation to Simulate Sediment Concentration in the Lower Yellow River, China

Abstract

The heavy sediment load of the Yellow River makes it difficult to simulate sediment concentration using classic numerical models. In this paper, on the basis of the classic one-dimensional numerical model of open channel flow, a variational-based data assimilation method is introduced to improve the simulation accuracy of sediment concentration and to estimate parameters in sediment carrying capacity. In this method, a cost function is introduced first to determine the difference between the sediment concentration distributions and available field observations. A one-dimensional suspended sediment transport equation, assumed as a constraint, is integrated into the cost function. An adjoint equation of the data assimilation system is used to solve the minimum problem of the cost function. Field data observed from the Yellow River in 2013 are used to test the proposed method. When running the numerical model with the data assimilation method, errors between the calculations and the observations are analyzed. Results show that (1) the data assimilation system can improve the prediction accuracy of suspended sediment concentration; (2) the variational inverse data assimilation is an effective way to estimate the model parameters, which are poorly known in previous research; and (3) although the available observations are limited to two cross sections located in the central portion of the study reach, the variational-based data assimilation system has a positive effect on the simulated results in the portion of the model domain in which no observations are available.

Relationships between Soil Nutrients and Plant Diversity in Riparian Woodlands Along the Middle and Lower Reaches of the Yellow River, China

“…In contrast, rivers that run through flat agricultural areas have more gradual slopes and more alluvial development. This results in a slower transition from riparian zone to non-riparian zone, thus leading to a wider overall riparian zone [36,37,49]. Therefore, we cannot attribute differences among these studies to distance from river alone.…”

“…Topography, hydrology, vegetation, and soil conditions vary in response to distance to river in the riparian zone [25,[36][37][38]. Therefore, we surveyed soil nutrient contents and plant community characteristics in woodlands across four lateral subzones on both sides of the river.…”

“…The lower reaches of the Yellow River in China are famous as a perched river with heavy sediment deposition. As a result, the riverbed is 3 to 8 m higher than the surrounding land surface, and the water level is 3 to 5 m higher than ambient groundwater levels in the riparian zone and adjacent terrestrial areas [36,37]. This unique fluvial geomorphology makes the Yellow River act as a groundwater divide that continuously discharges surface water to groundwater aquifers [38].…”

“…Specifically, the width of the subzones in the present study was 0-1.5 km, 1.5-3.0 km, 3.0-4.5 km, and 4.5-6.0 km from the river, which was designed according to the unique geomorphologic characteristics of the study area. The lower Yellow River is a perched river with a main-channel width of 0.5 to 3.0 km and a riparian zone with width of 5 to 20 km that was restricted by the left and right levees [36,37]. As such, the riparian zone is larger than that used in the studies conducted by Hazlett et al [47] (5 m and 75 m from the lakeshore), Schilling et al [48] (1, 20, and 40 m from river), Cierjacks et al [49] (518±50 m from main channel), Smith et al [23] (transects 36 m long and separated by 6 m), Lidman et al [12] (4, 12 and 22 m from river), and de Sosa et al [45] (2 and 50 m from river).…”

“…These human disturbances can inevitably affect nutrient cycling and riparian plant habitat, which can, in turn, affect the relationship between soil nutrients and plant species diversity. With regard to hydrological processes, its impact on the relationship between soil nutrients and plant species diversity results from the combined effects of flood regime and groundwater discharge caused by the unique geomorphology of the study system [36,37]. On the one hand, flood regimes impose stress and disturbance on riparian plants and soils near the river and redistribute sediment and organic matter.…”