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Overland flow resistance plays an important role in predicting hillslope hydrological and erosion processes on hillslopes. Soil surfaces covered with grass and/or gravel may increase hydraulic resistance to overland flow, but there is little information on the determination on these resistance components in a composite slopeland. In this study, the smooth and sand-bed surfaces covered with different gravel or/and grass (cover degree = 2.5%, 5%, 10%, 15%, 20%) under five slope gradients (3°, 6°, 9°, 12°, 15°) were subjected to inflows to investigate the hydraulic resistance and validate the sum law of resistance components. The results showed that, the overland flow mainly belonged to transition or turbulent and supercritical flow regime with 150

(1) Background: River health assessment provides the foundation for sustainable river development and management. However, existing assessments have no uniform standards and methods. (2) Methods: The combination weighting method was proposed, drawing on the advantages of subjective and objective weighting methods. To comprehensively investigate the river health level, an index system based on 16 indices selected from river morphology, river water environment, riparian condition, and social services level was established. The method and framework were applied to the Beijing section of Yongding River in China. (3) Results: The comprehensive weights of river morphology, river water environment, riparian condition, and social services are 0.1614, 0.3170, 0.4459, and 0.0757, respectively. The river health comprehensive index of Yongding River is 3.805; the percentages of excellent, healthy, sub-healthy, unhealthy, and sick river segments are 0%, 11%, 69%, 20%, and 0%, respectively. (4) Conclusions: The results indicate that Yongding River is in a sub-healthy state, and the riparian condition is the key factor that affects the river ecosystem health. Health level exhibited a remarkable spatial variation, mainly influenced by anthropogenic activities, and effective measures are needed to minimize the impact in fragile ecological areas.

Ecological water replenishment via interbasin water diversion projects provides opportunities for ecological river restoration. Untangling water quality changes, microbiota dynamics, and community functions is necessary for sustainable ecological management. Using the Yongding River as a case study, we monitored the water quality and applied genomic sequencing to investigate microbial communities of the river in different stages after ecological water replenishment. Our results showed that river water quality represented by chemical oxygen demand (COD), total nitrogen (TN), and chlorophyll-a (Chl-a) did not change significantly during months after water replenishment. The bacterial community composition varied in different months and river subsections. The Cyanobium_PCC-6307, CL500-29 marine group, and Pseudomonas were dominant in the later stages after water replenishment. Water temperature, pH, and nutrient levels significantly affected the microbial community composition, and ecological restoration may have the potential to influence nitrogen cycling in the river. Our results can provide ecological insights into sustainable water quality maintenance and river management following ecological restoration enabled by ecological water replenishment.

Understanding the water consumption mechanism of plantations is of great significance for the selection of afforestation trees and ecologically sustainable watershed management in semi-arid areas. In this study, Robinia pseudoacacia and Pinus tabulaeformis plantations, which have been widely planted in Chinese Loess Plateau, were selected to investigate the possible difference in water uptake modes. The spatial and temporal variations in precipitation, xylem and soil water stable isotope compositions (δ2H, δ18O) in 2019–2020 were analyzed, and the water uptake modes of plantations were quantified using the direct inference approach and MixSIAR model, with contrasting soil moisture dynamics. The results showed that δ18O values were positively correlated with air temperature and negatively correlated with precipitation volume. The δ18O content of surface soil (0–40 cm) closely related precipitation input, while those of deep soil layers (100–200 cm) remained stable. When compared with the direct inference approach, the MixSIAR model performed more effectively in quantifying water apportionment, especially in a drought year. In a drought year, R. pseudoacacia showed strong drought resilience to absorb water from deep soil, the soil layers of 0–40 cm and 40–200 cm contributed 32.9 % and 67.1 % to water absorption of R. pseudoacacia, and 62.2 % and 37.8 % to that of P. tabulaeformis, respectively, while there appeared to be minor differences in soil water uptake between the two plantations in a humid year. Generally, R. pseudoacacia consumed more water than P. tabulaeformis, especially in a humid year, and the former inclined to absorb soil layer with enriched soil moisture. The results indicated that R. pseudoacacia plantation may increase transpiration and cause dried deep soil layers when compared with P. tabulaeformis. This study improves our understanding of water uptake mechanisms of plantations and helps with selection of suitable plant species for ecological management in Chinese Loess Plateau.

Ephemeral rivers commonly occur in regions with a shortage of water resources, and their channel configuration tends to change substantially owing to long drying times and artificial sand extraction. During short-term water conveyance, water storage in large potholes and leakage along the dry riverbed retards the flow, which is detrimental for the river landscape and ecological water demand. The objective of this study is to evaluate the flow process corresponding to a certain release scheme. A coupled dynamic leakage loss and flood routing model was established to predict the flood routing distance for dry rivers with potholes and strong leakage. The model mainly includes three sub-models of flow dynamics, dynamic leakage loss and water balance along multiple cross sections of the river channel. The water head was dominated by flow velocity and the overflow from potholes. The model was applied to Yongding River, a typical ephemeral river in northern China, and the model parameters were calibrated and verified using monitoring data from ecological water releases into the Yongding River in 2019 and 2020, thus, making the model more stable and reliable. Finally, the model was used to evaluate the impact of cross section optimization and pothole treatment on the flow process. This study can provide scientific guidance for ecological water conveyance and the ecological restoration of ephemeral rivers.

Rainfall erosivity is an important factor to be considered when predicting soil erosion. Precipitation data for 1971–2010 from 39 stations located in the Loess Plateau of China were collected to calculate the spatiotemporal variability of rainfall erosivity, and the long-term tendency of the erosivity was predicted using data from the HadGEM2-ES model. Statistical analyses were done using Mann–Kendall statistic tests and ordinary Kriging interpolation. The results showed that the annual mean rainfall erosivity in the Loess Plateau decreased from 1,286.02 MJ mm hm−2 h−1 a−1 in 1971–1990 to 1,201.46 MJ mm hm−2 h−1 a−1 in 1991–2010 and mainly occurred in July to August. The rainfall erosivity decreased from the southeast to the northwest of the Loess Plateau and was closely related to the annual precipitation amount. However, the effect of annual precipitation on rainfall erosivity weakened under climate change: the annual precipitation increased and the rainfall erosivity decreased. Climate change, however, had little influence on the spatial variation in rainfall erosivity in the Loess Plateau. The results obtained can facilitate the prediction of spatial and temporal variations in soil erosion in the Loess Plateau. HIGHLIGHTS Both precipitation and rainfall erosivity showed an insignificant decreasing trend for 1971–2010.; The climate model predicts an increasing precipitation but decreasing rainfall erosivity.; South and southeast of the Loess Plateau are areas susceptible to rainfall erosion under climate change.

Although numerous studies have acknowledged that vegetation can reduce erosion, few process-based studies have examined how vegetation cover affect runoff hydraulics and erosion processes. We present field observations of overland flow hydraulics using rainfall simulations in a typical semi-arid area in China. Field plots (5 m × 2 m) were constructed on a loess hillslope (25°), including bare soil plot as control and three plots with planted forage species as treatments—Astragalus adsurgens (A. adsurgens), Medicago sativa (M. sativa) and Cosmos bipinnatus (C. bipinnatus). Both simulated rainfall and simulated rainfall + inflow were applied. Forages reduced soil loss by 55–85% and decreased overland flow rate by 12–37%. Forages significantly increased flow hydraulic resistance expressed by Darcy-Weisbach friction factor by 188–202% and expressed by Manning’s friction factor by 66–75%; and decreased overland flow velocity by 28–30%. The upslope inflow significantly increased overland flow velocity by 67% and stream power by 449%, resulting in increased sediment yield rate by 108%. Erosion rate exhibited a significant linear relationship with stream power. M. sativa exhibited the best in reducing soil loss which probably resulted from its role in reducing stream power. Forages on the downslope performed better at reducing sediment yield than upslope due to decreased rill formation and stream power. The findings contribute to an improved understanding of using vegetation to control water and soil loss and land degradation in semi-arid environments.

Ephemeral rivers commonly occur in regions with a shortage of water resources, and their channel configuration tends to change substantially owing to cultivation, tree planting and sand extraction. There is an urgent need to restore degraded river ecosystems. During short-term water conveyance, water storage in sand pits and leakage in dry riverbeds retards the flow of water, which is detrimental for ecological restoration of the riparian zone. A coupled dynamic leakage loss and flood routing model was established to predict the flow processes in the complex river channel of the Yongding River in China. The model mainly included three sub-models of flow dynamics, dynamic leakage loss, and water balance along multiple cross sections of the river channel. The complex section is reflected in the different infiltration properties for each section, and the existence of sand pits. The water head was dominated by flow velocity and the overflow from sand pits. Owing to the difference in landforms and the deposited sediment size of the riverbed bottom, the river channel was divided into 11 cross sections and a sand pit to ascertain the respective infiltration or leakage loss processes. The input parameters of the model came from field surveys of sand pits, river geometry and hydrogeology. The model was also calibrated and validated using monitoring data from ecological water releases into the Yongding River in 2019 and 2020. This coupled model can predict the water leakage loss and flow process of the water head and also provide important guidance for river reconstruction and ecological restoration.

Reclaimed water plays an important role in maintaining urban aquatic ecosystems, especially in areas with water shortages. However, there is little information on water quality dynamics and its driving mechanism in reclaimed water bodies. The simulated experiments were conducted to investigate the effect of flow exchange on water quality dynamics and soil microbial diversity for 100% reclaimed water and mixed water (50% reclaimed and 50% stream water), and the exchange periods ranged from 2 to 40 days. The results showed that the degradation coefficients (K) of CODMn and NH3–N were 0.015 d−1 and 0.001 d−1 for the mixed water, while their K values were negative for the reclaimed water. The flow exchange had little effect on water quality dynamics for the mixed water, which may be attributed to the relatively low concentration of TP in this reclaimed water. A small or great exchange period led to a relatively high fluctuation in K during the experimental period and corresponded to a worse soil microbial diversity. These results indicate that it is not recommended to fill an isolated urban lake with 100% reclaimed water and that a suitable flow exchange period of 5~10 days could help self-purify the water quality.

Grass strips can decrease erosion, trap sediment in silt-laden water flowing downhill, and control nonpoint source pollution. Determining the effects of different parts of grass strips on silt-laden overland flow will improve our understanding of sediment trapping by grass strips with different structures. Sediment trapping by grass strips was studied using a 5° slope, 30 L min−1 m−1 flow rate, 120 g L−1 sediment concentration, and different aboveground components of grass strips (complete grass, removed green grass, and removed green and withered grass). The whole overland flow process was monitored. Meanwhile, the runoff sediment samples at the outlet were collected and measured. Sediment trapping by aboveground grass parts was quantified at different stages. Of the soil bed surface, green grass, and withered grass, the soil bed surface dominated sediment trapping in the initial stage of the sediment-trapping process, contributing about 90% of total sediment deposition in the first 5 min. As the sediment-trapping process continued, the effect of the soil bed surface weakened, and the green grass played a major role at the later stage of sediment trapping. The ratio of the soil bed surface, green grass, and withered grass contributions to total sediment deposition at the stable stage of the experiments was approximately 3:5:2. The results will help assess the effects of vegetation restoration on sediment transport in entire watersheds.

Ephemeral gullies (EGs) are important erosion features that could account for as much as 70% of total erosion on the Loess Plateau of China. However, few quantitative studies have been conducted in the field on the impact and mechanism of upslope sediment runoff in EG erosion processes. A series of field rainfall simulation experiments were conducted on an artificial EG to investigate the impacts of upslope inflow with two flow rates (15 and 30 L min−1) and five sediment concentrations (20, 40, 80, 120, and 160 g L−1) on runoff and erosion processes within the EG. The results showed that the lower flow rate of 15 L min−1 increased soil loss by 26% compared with the higher inflow rate (30 L min−1). The lower inflow rate triggered greater sheet or rill erosion, whereas the higher rate mainly strengthened the incision of the main EG channel and led to increasing gully erosion. The channel shoulders expanded laterally very little, especially under the higher flow rate. Inflow sediment concentration had a major influence on EG erosion processes. With increasing sediment concentration, EG erosion and total slope erosion first increased, and then decreased. When sediment concentration exceeded 120 g L−1, the inflow sediment began to be deposited, and 120 g L−1 can be regarded as a critical value of EG erosion. These results are beneficial to establish a soil erosion model and to control ephemeral gully erosion in loess areas.

Litter accumulates yearly since vegetations were widely planted for reforestation, and it plays an important role in hydrologic cycling. There is little information on the effects of litter on re-allocation of rainfall processes. Eight runoff plots were established in the Pinus tabulaeformis stand with four litter (needle-leaf) masses (0, 0.6, 1.2, 1.8 kg/m2), and the surface runoff (R), evaporation (E), infiltration and soil moisture dynamics were measured throughout the mainly rainy season from August 4 to September 28 in the Loess Plateau. The results showed that, soil evaporation mainly occurred in daytime for bare soil, and decreased with increasing litter masses, and litter cover is prone to hinder the heat and water exchange between soil and atmosphere, especially for the soil layer 0~5 cm. Litter cover greatly decreased surface runoff, and it may hinder infiltration at the beginning of rainy season, but increasing soil water storage (SWS) with deeper infiltration depth for the long run, especially for the litter masses 1.2 and 1.8 kg/m2. With the litter covered, the ratio of R to precipitation (P) was less than 10%, no matter it was heavy rain or light rain. However, the proportion of R was amplified when the rainfall was intense for the bare soil. And the ratio of E to P was always below 10% for all treatments, except for light rainfall. With the increased litter masses, the proportion of R and E all decreased, and the SWS/P has well nonlinear positive relationship with litter masses, and it was proved that more than a half of rainfall was stored even for bare soil. This study may helpful to better understanding the effects of litter on hydrological response, and promotes practical measurements to the management of precipitation in a forest stand view.

Particle selectivity plays an important role in clarifying sediment transport processes in vegetative filter strips (VFS). 10-m long grass strips at slopes of 5○ and 15○were subjected to a series of silt-laden inflows experiments with different particle sizes to investigate the sediment transport and its response to overland flow hydraulics. The inflow sediments came from local soil, river-bed sand, and mixed, with median particle size d50 of 39.9, 207.9, and 77.4 μm, respectively. Three independent repeated experiments were carried for each treatment. The results show that when the sediment trapping lasted for a certain length of time, the re-entrainment of some small-sized particles was greater than the deposition; that is, negative deposition occurred, which was not erosion of the original soil. Negative deposition of particles is mainly determined by the particle diameter. The coarser the inflow sediment particles and/or the steeper the slope, the coarser the particles can be negatively deposited. Deposited sediment causes the VFS bed surface to become smooth and hydraulic resistance decrease exponentially. Stream power P is more suitable than shear stress τ of overland flow to be used to describe the process of sediment particle transport in VFS. The relationship between P and d50 of outflow sediment is very consistent with the form of power function with a constant term. These results are helpful to understand the physical process of sediment transport on vegetation hillslopes.

Due to the intensified influence of human activities, Yongding river presents a sharp decrease in water quantity and a trend of continuous deterioration of water environment, and the ecological environment is seriously damaged. Under this background, Yongding river ecological reconstruction project needs to be carried out urgently, and ecological water replenishment mode needs to be determined urgently. In order to explore the influence of multi-water source ecological replenishment mode on the evolution of river water quality and riverbank percolation water quality, this study conducted a simulation experiment to explore the influence of flow rate, temperature and soil percolation on reclaimed water replenishment water quality. The results show that the increase of flow velocity is beneficial to the degradation of pollutants. Compared with high temperature, the degradation capacity of pollutants at low temperature is significantly better than that at high temperature, indicating that low temperature is beneficial to the improvement of reclaimed water quality to some extent. Some water quality indexes of riverbank leachate improved to some extent, but the water passing through the soil was slightly eutrophication due to the aggregation and adsorption of river bottom sediments. The final results show that the velocity of flow has the greatest influence on the quality index of regenerated water.

Vegetation-restored hillslope surfaces not only reduce erosion but they also remove sediment from upslope silt-laden inflow. To investigate the sediment trapping effect of grassland, this study conducted a series of crossed sediment trapping experiments that examined various factors, such as slope (5°–20°), sediment concentration (40–160 g L−1), and unit flow rate (7.5–45.0 L min−1 m−1). The duration of each experiment was longer than required to reach the stable state of sediment trapping, so we measured and verified the individual sediment trapping capacity (Rm) by experiments. The results showed that gentler slopes generated higher instantaneous sediment trapping efficiency (ISTE) and greater Rm. As the sediment concentration of the silt-laden inflow increased, the impact of slope on Rm increased. Higher sediment concentration led to lower ISTE but greater Rm. Similar to the effect of sediment concentration, a larger unit flow rate led to lower ISTE and greater Rm. Thus, it is evident that interaction among these factors affects sediment trapping process. The experiments revealed the greatest sediment trapping effect of grass strips was concentrated mainly in the first 2-m width, and that 90% of sediment deposition occurred within half the time needed to reach the stable state. Slope and flow rate were found to have an effect on sediment trapping in each section of grass strips, whereas the effect of sediment concentration was concentrated primarily in the first 5-m width. Standard regression coefficients of a comprehensive regression analysis showed that the intensities of the influencing factors on Rm were as follows: slope (0.736) > grassland width (0.498) > unit flow rate (0.398) > sediment concentration (0.240). It was established that slope is the strongest influencing factor, and that sediment concentration and unit flow rate mainly affect Rm by changing the rate of sediment delivery. These results will help expand the theoretical basis regarding the effects of vegetation restoration on watersheds in soil erosion research.

Water quality prediction is an important technical means for preventing and controlling water pollution and is crucial in the formulation of reasonable water pollution prevention and control measures. The time series structure of natural water quality is complex and heteroscedastic, so it is difficult for the traditional prediction model to reflect the actual situation well. Hence, Markov-switching (MS) theory is applied to a water quality autoregression (AR) prediction model (MSAR) in this paper. Further, MSAR is improved by introducing the crow search algorithm to obtain model parameters (CSA-MSAR). Then existing water quality time series for CODMn was selected as the data for the CSA-MSAR model after a normality test and the Box–Cox normality transformation. The results show that the CSA-MSAR model for CODMn with (s, p) values of (3, 5) has the best performance. The improvement degree for selection criteria compared with AR model is as follows: Akaike information criterion for MSAR is 32.020% and 31.611% for CSA-MSAR; Bayesian information criterion for MSAR is 10.632% and 13.464% for CSA-MSAR; likelihood value for MSAR is 40.016% and 40.801% for CSA-MSAR; C for MSAR is 63.559% and 64.968% for CSA-MSAR. Moreover, the results show that the average prediction precision of the first- to fifth-order prediction is raised by 89.016% for MSAR and 89.340% for CSA-MSAR compared with AR, indicating that the introduction of MS makes the CSA-MSAR and MSAR models conform to the smoothness of the mean and variance in each state. The results also indicate that the introduction of CSA into the maximum likelihood estimation to obtain the parameters raise the model prediction precision (the average prediction precision of CSA-MSAR is higher than MSAR by 5.231% excluding the fifth-order prediction) and the CSA-MSAR model is scientifically valid and reasonable for water quality prediction.

The aim of this study was to investigate how the spatial distribution of grass influenced run-off and erosion from a hillslope with loess and cinnamon soils in the rocky area of Northern China. We set up a trial to test the two soils with different treatments, including bare soil (BS), grass strips on the upper (UGS) and lower (DGS) parts of the slope, grass cover over the entire slope (GS), and a grass carpet on the lower part of the slope (GC), under simulated rainfall conditions. The results showed that the run-off coefficients for the loess and cinnamon soils decreased by between 4% and 20% and by between 2% and 37%, respectively, when covered with grass. Grass spatial distribution had little effect on the run-off, but more effect on erosion than vegetation coverage degree. The most effective location of grass cover for decreasing hillslope erosion was at the foot, and the high efficiency was mainly due to controlling of rill formation and sediment deposition. The soil loss from GS, DGS, and GC on the loess and cinnamon soils was between 77% and 93% less and 55% and 80% less, respectively, compared with the loss from BS. However, the soil characteristics had little effect on soil erosion for well-vegetated slopes. The results highlight the importance of vegetation re-establishment at the foot of hillslope in controlling soil erosion.

The effects of vegetation cover on overland flow and erosion processes on hillslopes vary with vegetation type and spatial distribution and the different vegetation components, including the above- and below-ground biomass. However, few attempts have been made to quantify how these factors affect erosion processes. Field experimental plots (5 m × 2 m) with a slope of approximately 25° were constructed and simulated rainfall (60 mm hr−1) (Rainfall) and simulated rainfall combined with upslope overland flow (20 L min−1) (Rainfall + Flow) were applied. Three grass species were planted, specifically Astragalus adsurgens (A. adsurgens), Medicago sativa (M. sativa) and Cosmos bipinnatus (C. bipinnatus). To isolate and quantify the relative contributions of the above-ground grass parts (stems, litter cover and leaves) and the roots to reducing surface runoff and erosion, each of the three grass species was subjected to three treatments: intact grass control (IG), no litter or leaves (only the grass stems and roots were reserved) (NLL), and only roots remaining (OR). The results showed that planting grass significantly reduced overland flow rate and velocity and sediment yield, and the mean reductions were 21.8%, 29.1% and 67.1%, respectively. M. sativa performed the best in controlling water and soil losses due to its thick canopy and dense, fine roots. Grasses reduced soil erosion mainly during the early stage of overland flow generation. The above-ground grass parts primarily contributed to reducing overland flow rate and velocity, with mean relative contributions of 64% and 86%, respectively. The roots played a predominant role in reducing soil erosion, with mean contribution of 84%. Due to the impact of upslope inflow, overland flow rate and velocity and sediment yield increased under the Rainfall + Flow conditions. The results suggest that grass species on downslope parts of semi-arid hillslopes performed better in reducing water and soil losses. This study is beneficial for forage selection, allocation and management practices, such as forage harvesting, when implementing restoration strategies to control soil and water losses.

It is important to predict evolution processes of water quality in order to effectively protect the aquatic eco-system. We proposed an integrated autoregressive (AR) prediction model based on Markov-switching (MS) theory and the improved crow search algorithm (ICSA), and the integrated model of ICSA-MSAR was used to predict temporal evolution of water quality changes. The results showed that the MS and ICSA provided an optimized method to determine the model parameters, and the integrated model with (s, p) values of (3, 5) had a better performance. Compared with the traditional AR model, the integrated ICSA-MSAR model significantly raised Akaike information criterion by 32.0%, Bayesian information criterion by 14.2%, Log-likelihood value by 40.5%, C index ( C = AIC × BIC × LL ) by 65.6%, and reduced the prediction relative error by 73.3%. The better performance mainly attributed to its offset capacity and convergence capacity. The integrated model can well predict the changes of CODMn in Yongding River located of North China during 2008–2018, and it is worth extending this model to predict the changes of other ecological indicators in water bodies.

Limited information has isolated the impacts of rainfall on rill formation and erosion on steep hillslopes where upslope inflow simultaneously exists. Field simulation experiments were conducted on steep hillslopes (26°) under rainfall (60 mm h −1 ), inflow (6, 12, 18, 24, 30, 36 L min −1 m −1 ), and combination of rainfall and inflow to explore the impacts of rainfall on rill formation, and the interaction between rainfall and inflow on soil erosion. Rainfall decreased soil infiltration rate (10%–26%) mainly due to soil crust by raindrop impact. Rainfall strengthened rill formation, which behaved in the increment in rill width (5%–26%), length (4%–22%), and depth (3%–22%), but this increment decreased as inflow rates increased. Additionally, the contribution of rainfall on rill formation was most significant at the initial stage, followed by the final stage and active period of rill development. Rainfall increased rill erosion (8%–80%) and interrill erosion (36%–64%), but it played a dominant role in increasing interrill erosion under relatively high inflow rates. The most sensitive hydrodynamic parameter to soil erosion was shear stress and stream power under inflow and ‘inflow + rainfall’ conditions, respectively. For the lowest inflow rate, the reduction in soil loss by interaction between rainfall and inflow accounted for 20% of total soil loss, indicating a negative interaction. However, such interaction became positive with increasing inflow rates. The contribution rate to rill erosion by the interaction was greater than that of interrill erosion under relatively low inflow rates. Our results provide a better understanding of hillslope soil erosion mechanism.

The analysis of the effects of human activities and climate variability on the structural characteristics of streamflow series facilitates further exploring the regularity of streamflow cha...

Particle selectivity of the sediment deposited over vegetative barriers is of importance to predict sediment transport and particulate pollutant load into surface waters. Grassed barriers with 20%, 40%, 60%, 70% and 90% covers at 15° slope were subjected to silt-laden inflows in the presence and absence of simulated rainfalls to investigate the sediment deposition processes. The results show that re-grass of steep croplands can effectively trap eroded sediment from upslope, and the rowed grass barriers can strengthen sediment deposition. The deposition order of sediment particle sizes (μm) follows (>50)> (25-50)>(10-25)=( (2-10), and the particle selectivity weakens with increasing grass covers. Clay particles had a similar deposition efficiency to overall sediment, implying the effectiveness of re-grass in controlling soil nutrient loss. The contribution of grass to total overland flow resistance is almost equivalent to the percentage of grass cover. For steep grassed slopes, raindrop impact significantly decreases sediment deposition, but limitedly affects particle selectivity of deposited sediment and overland flow hydraulics. Both raindrop kinetic energy and stream power available for surface soil contribute to sediment deposition in net deposition areas of grass barriers. These imply that rainfall effect on sediment delivery over vegetated barriers derives from the additional raindrop energy, rather than the variation in runoff hydraulics. These results can help to clarify the effect of raindrop impact on sediment transport and to evaluate the benefit of re-vegetation in decreasing sediment yield and its particulate nutrient load into surface waters. This article is protected by copyright. All rights reserved.

It is still unclear how slope steepness (S) and revegetation affect resistance (f) to overland flow. A series of experiments on runoff hydraulics was conducted on granular surfaces (bare soil and sandpaper) and grassed surfaces, including grass plots (GP), GP with litter (GL), and GP without leaves (GS) under simulated rainfall and inflow (30

Vegetation-restored hillslopes not only reduce erosion but also trap sediment from uphill silt-laden inflow. To investigate the sediment trapping mechanism of grass strips, a series of crossed experiments with various factors, including slope (5–20°), sediment concentration (40–160 g L−1), and unit flow rate (7.5–45.0 L min−1 m−1) were conducted using a 10-m-long and slope adjustable soil plot. The duration of each experiment was longer than required time to reach the stable state of sediment trapping, which ensures that the whole process of sediment trapping can be monitored and the maximum sediment trapping yield (sediment trapping capacity, Rm) can be obtained. The results showed that gentler slopes generated higher instantaneous sediment trapping efficiency (ISTE) and greater Rm. Higher sediment concentration or larger unit flow rate led to lower ISTE but greater Rm. The impact of slope on Rm amplified when the sediment concentration increased, implying that the interaction among these factors affects sediment trapping process. The sediment trapping effect of the first 2-m width grass strips performed better than the subsequent strips. 90% of sediment deposition occurred within first half of the time needed to reach the stable state. Slope had similar effect intensity on sediment trapping in each section of grass strips, so did flow rate, whereas the effect of sediment concentration was concentrated primarily in the first 5-m width. The standard regression coefficients of the multiple curve regression equation showed that the intensities of the impacting factors on Rm were as follows: slope > grass strips width > unit flow rate > sediment concentration. It indicated that slope plays a key role in controlling Rm. These results are helpful in assessing the effects of vegetation restoration on sediment transport at a hillslope scale.

Vegetative filter strips (VFS) play an important role in reducing erosion, retaining runoff, and trapping sediment. The bed surface of a VFS is dynamic during continuous sediment deposition, which makes it difficult to determine the sediment transport capacity of overland flow in real time. This study described this process from a systematic perspective. Based on self-feedback theory, an ordinary differential equation was established with a self-inhibition correction factor and its analytical solution was obtained. A sediment-trapping-process model of VFS (STPMOD-VFS) was proposed. Then, the sediment trapping processes of three experiments under different conditions were predicted using the STPMOD-VFS, and the prediction was considered highly satisfactory. The uncertainty estimation results showed the STPMOD-VFS had significant ‘equifinality’ performance. The sensitivity of different parameters of the STPMOD-VFS was found significantly different. The parameters related to the sediment delivery rate of silt-laden inflow and those that control the attenuation coefficients of the instantaneous sediment trapping efficiency of the VFS were sensitive, while those parameters that control the initial sediment outflow rate were less sensitive. Finally, an equation describing the sediment trapping process by a VFS under a dynamic sediment inflow rate was built and solved, and a general expression for the VFS sediment trapping time-varying process was given. The findings of this study could help in evaluation of sediment processes on grassed hillslopes.

In the investigation of overland flow hydraulics, mean flow velocity (V) is frequently estimated using the measured surface flow velocity (Vs) multiplied by a correction factor, α. In total, 291 tests were performed in a flume with three beds [smooth glass (GL), sandpaper (SD), and plastic grass (GR)] to investigate α under submerged and non-submerged flows, and Vs was observed using dye-tracer method whilst V was calculated by the measured water depth and flow rate. For GL with 5.2% slope and 100

Constructing reservoirs can make more efficient use of water resources for human society. However, the negative impacts of these projects on the environment are often ignored. Optimal reoperation of reservoirs, which considers not only in socio-economic values but also environmental benefits, is increasingly important. A model of optimal reoperation of multi-reservoirs for integrated watershed management with multiple benefits was proposed to alleviate the conflict between water use and environmental deterioration. The social, economic, water quality and ecological benefits were respectively taken into account as the scheduling objectives and quantified according to economic models. River minimum ecological flows and reservoir water levels based on flood control were taken as key constraint conditions. Feasible search discrete differential dynamic programming (FS-DDDP) was used to run the model. The proposed model was used in the upstream of the Nanpan River, to quantitatively evaluate the difference between optimal reoperation and routine operation. The results indicated that the reoperation could significantly increase the water quality benefit and have a minor effect on the benefits of power generation and irrigation under different hydrological years. The model can be readily adapted to other multi-reservoir systems for water resources management.

Summary Sedimentation resulting from soil erosion degrades the surface water environment. This study determines the sediment trapping capacity (STC) of grass filter strips and how slope gradient and vegetation cover affect it. STC is defined as the maximum sediment trapped within vegetative filter strips (VFS) under given conditions. An exponential model was derived to mirror the relationship between instantaneous sediment trapping efficiency and runoff duration, with findings that STC is closely related to the initial efficiency and attenuation coefficients. The model effectively describes the sediment trapping process, and demonstrates that STC decreases with increasing slope and increases with vegetation cover. Grass strips have a small sediment trapping thickness measured in mm or cm, and grass stems and leaves have little influence on STC, which indicates that reasonable forage cutting may have little influence on sediment removal. STC is an intrinsic characteristic of VFS, and it can be recommended to assess the VFS performance in trapping sediment in severe soil erosion areas. A sediment trapping modulus at the watershed scale based on STC, can offer help to effectively evaluate soil and water conservation engineering and transformation of cropland into forest and grassland in China.

Little information is available concerning the performance of grass strips for erosion control from steep cropland. An experiment was conducted on 5-m-long grass strips with slopes of 3°∼15° that were subjected to silt laden runoff and simulated rainfall, to investigate the sediment trapping processes. The grass strips had three treatments including intact grass control (C), no litter (dead grass material covering the soil surface was removed) (NL), and no litter or leaves (only 2∼3 cm grass stems and roots were reserved) (NLL). Generally the grass strips had a high effectiveness in trapping sediment from steep cropland runoff. Sediment trapping efficiency (STE) decreased with increasing slope gradient, and even for a 15° slope, STE was still more than 40%. Most sediment deposited in the backwater region before each grass strips. The removal of grass litter or/and leaves had no significant influence on STE. The sediment median size (D50) in inflow was greater than that in outflow, and the difference (ΔD50) decreased with increasing slope. A positive power relationship between STE and ΔD50 can be obtained. Grass strips were more effective in trapping sediments coarser than 10 or 25 µm, but sediments finer than 1 µm were more readily removed from runoff than particles in the range of 2 to approximately 10 µm. Grass litter had less influence on flow velocity than leaves because the deposited sediment partially covered the litter layer. Mean flow velocity and its standard deviation were negatively correlated with STE, and they can help make good estimation of STE. Results from this study should be useful in planting and managing forage grass to effectively conserve soil loss by runoff from steep slopes on the Loess Plateau of China. Copyright © 2010 John Wiley & Sons, Ltd.

Evaluation of grass influence on soil erosion process can provide important information in soil and water conservation. The laboratory experiment was conducted to study runoff and sediment producing processes and runoff hydraulics in the grassplots with different covers (35%, 45%, 65% and 90%) and bare soil plot (control) at a slope of 15. The results showed that grass significantly reduced runoff and sediment. Compared with bare soil plot, the grassplots had a 14-25% less runoff and an 81-95% less sediment, and played a more important role in reducing sediment at the final stage of rainfall. There was a significantly negative logarithmic relationship between sediment yield rate (SDR) and cover (C): SDR = 1.077-2.911 ln(C) (R-2 = 0.999'"). Sediment yield rate of grassplots decreased with rainfall duration, and decreased linearly as runoff rate increased. Overland flow velocities deceased with increase in grass cover, and the cover had greater effect on lower slope velocity than upper one. Froude numbers decreased with increase in cover, and flow regimes of all treatments were laminar and tranquil. Darcy-Weisbach and Manning friction coefficients of grassplots increased as ground cover increased. Therefore, increase in grass coverage can efficiently reduce soil loss and improve ecological environments. (c) 2006 Elsevier B.V. All rights reserved.

It is important to evaluate the impacts of grasses on soil erosion process so as to use them effectively to control soil and water losses on the Loess Plateau. Laboratory-simulated rainfall experiments were conducted to investigate the runoff and sediment processes on sloped loess surfaces with and without the aboveground parts of grasses and moss (GAM: grass and moss; NGAM: no grass and moss) under slope gradients of 5 degrees, 10 degrees, 15 degrees, 20 degrees, 25 degrees and 30 degrees. The results show that runoff from GAM and NGAM plots increased up to a slope gradient of 10 degrees and decreased thereafter, whereas the runoff coefficients increased with gradient. The average runoff rates and runoff coefficients of NGAM plots were less than those of GAM plots except for the 5 degrees slope. This behaviour may be due to the reduction in water infiltration under moss. The difference between GAM and NGAM plots in average runoff rates varied from 1.4 to 8%. At the same gradients, NGAM plots yielded significantly (alpha = 0.05) more sediment than GAM plots. Average sediment deliveries for different slopes varied from 0.119 to 3.794 g m(-2) min(-1) from GAM plots, and from 0.765 to 16.128 g m(-2) min(-1) from NGAM plots. Sediment yields from GAM plots were reduced by 45 to 85%, compared with those from the NGAM plots. Plots at 30 degrees yielded significantly higher sediments than at the other gradients. Total sediments S increased with slope gradients G in a linear form, i.e. S = 9.25G - 39.6 with R-2 = 0.77*, for the GAM plots, and in an exponential model, i.e. S = 40.4 exp(0.1042G) with R-2 = 0.93**, for the NGAM plots. In all cases, sediment deliveries decreased with time, and reached a relative steady state at a rainfall duration of 14 min. Compared with NGAM plots, the final percentage reductions in sediment delivery from GAM plots were higher than those at the initial time of rainfall at all slopes. Copyright (c) 2006 John Wiley & Sons, Ltd.

There is little information on the performance of vegetative filter strips (VFS) in filtering high-concentration sediment from subcritical overland flow. Flume experiments on simulated grass strips were conducted using combinations of three slope gradients (3°, 9° and 15°), five 1-m-wide slope positions (from upslope to downslope), two flow rates (60 and 20 L min-1 m-1) and sediment concentrations of 100–300 kg m-3 under simulated rainfall and non-rainfall conditions. The results showed that sediment deposition efficiency increased with VFS width as a power function. Rainfall significantly reduced sediment deposited within VFS. Higher sediment concentration corresponded to a larger sediment deposition load but reduced deposition efficiency. Flow rate had a negative effect on deposition efficiency but no effect on deposition load. Sediments were more easily deposited at the upper slope position than downslope, and the upper slope position had a higher percentage of coarse sediments. The deposited sediment had significantly greater median diameters (D50) than the inflow sediment. A greater proportion of coarse sediments larger than 25 µm in diameter were deposited, and particles smaller than 1 µm and of 10–25 µm had a better deposition performance than particles of 1–10 µm. Rainfall reduced the deposited sediment D50 at a slope gradient of 3° and had no significant influence on it at 9° or 15°. A higher sediment concentration led to a smaller D50 of the deposited sediment. Rainfall had no significant effect on overland flow velocity. Both the deposited sediment load and D50 decreased with increasing flow velocity, and flow velocity was the most sensitive factor impacting sediment deposition. The results from this study should be useful to control sediment flowing into rivers in areas with serious soil erosion. Copyright © 2013 John Wiley & Sons, Ltd.

Based on the long-term observation and from the viewpoints of water balance and runoff-and sediment generation, this paper studied the generation processes of runoff and sediment on two typical woodlands, artificial P. tabulaeformis and secondary natural P. dadidiana, and uncultivated slope-land in Loess Plateau under hypo-rainfall. The results showed that within the range of 5.0 - 50.0 mm rainfall, the total interception of canopy and litter was 15.45 % - 56.80 % for P. tabulaeformis and 20.56% - 47.81% for P. dadidiana, and decreased with increasing rainfall. Woodlands had a higher soil water infiltration capacity than uncultivated slope-land, especially in 0-20 cm soil layer. Both the two woodlands did not generate runoff under regular rainfall. Under the assumed rainfall of 2.5 mm x min(-1) intensity and 30 min duration, P. dadidiana stand did not produce runoff, but the runoff velocity and sediment-carrying capacity of uncultivated slope-land were 23.5 times, and runoff shearing stress and energy were 8 times as much as P. tabulaeformis stand. The runoff-and sediment generation on P. tabulaeformis stand decreased by 87.6% and 99.4%, respectively, compared with those on uncultivated slopeland, which was well accorded with the average observed value in runoff plots during 1988 - 2000. The theoretical analysis on the generation mechanism of woodland runoff and sediment may be effective to evaluate the benefits of forest in soil and water conservation.

For the sustainable development and ecological construction in the loessial semiarid region, it is important to describe the variability of slopeland soil properties and the affecting factors of slopeland productivity. In this study, soil samples were taken from eroded steep slopeland, and leaf area index (LAI) and above-ground biomass (AGB) were measured at the sampling locations. The soil water content (WC) of 2 m depth at 20 cm intervals, and the soil organic matter (OM), total nitrogen (TN), total phosphorus (TP), available nitrogen (AN) and available phosphorus (AP) contents of 0-20 and 20-40 cm soil layers were determined in the laboratory. The results showed that the majority of the properties was normally distributed, and the nutrient contents were higher in 0-20 cm than in 20-40 cm layer, but the variations of soil nutrients were much smaller in 0-20 cm than in 20-40 cm layer. Soil nutrients had a significantly larger variation than soil moisture. Soil nutrient contents in 20-40 cm layer kept increasing from upslope to downslope, while those in 0-20 cm layer varied slightly. Slope topography had more obvious impact on soil organic matter, total nitrogen and available phosphorus than other affecting factors. Soil water and nutrient contents on the shallow gully trough were notably higher than those on the upslope, but above-ground biomass was less than that on the upslope. Though longitudinal slope (35 degrees-45 degrees) was obviously larger than the horizontal one (5 degrees-10 degrees), horizontal slope position had a greater influence on soil nutrients, but much weaker effect on soil moisture than longitudinal direction. There were significant correlations between 0-120 cm soil moisture and 20-40 cm soil nutrients, and among soil nutrients except 0-20 cm soil available phosphorus. Slopeland position had a great impact on soil moisture and nutrients, but soil moisture and/or nutrients had no significant impact on above-ground biomass.