Effects of hydrological fluctuations induced by sluice operations on pollutant degradation coefficients in rivers of the chinese loess plateau during flood season.

Catchment-scale surface water-groundwater connectivity on China's Loess Plateau

In current land surface models or satellite remote sensing retrievals, clear‐sky surface albedo (α) is usually assumed to be symmetrical and relies only on the solar elevation angle (SEA). Based on 1‐min high‐resolution measurements of surface radiation fluxes, this study demonstrated that the diurnal variations of clear‐sky surface albedo exhibited a significant asymmetrical pattern in both summer and winter seasons over a semi‐arid grassland of the China's Loess Plateau. The results indicated that α values in the morning were generally larger than those in the afternoon at the same SEA, and diurnal asymmetry of surface albedo was distinctly prominent with SEA <40° in summer (before 9:30 a.m.) or SEA <20° in winter (before 10:00 am), and tended to diminish at midday. The averaged morning/afternoon albedo differences under sunny days were 0.05 (30.4%) and 0.09 (37.8%) in summer and winter seasons, respectively. Air relative humidity was positively correlated with the diurnal asymmetry of surface albedo, ascribed to probable formation of dew in the morning. Depression of the dew point was negatively linked to the morning/afternoon albedo differences, which was attributed to the strong scattering of incident sunlight by dewdrops could enhance the morning surface albedo. Such diurnal asymmetry of surface albedo should be included in the parameterization scheme of mesoscale and region‐scale climate models in the semi‐arid areas of China's Loess Plateau.

Vegetation plays an important role in gully headcut erosion in China's Loess Plateau, but little is known about the effects of root density on gully headcut erosion. This study attempted to investigate the effects of grass root density on gully headcut erosion and morphological development of the gully head using plots with different grass root densities (with bare land as the control). The experiment was performed on plots with different grass root densities, and the rainfall intensity and inflow discharge increased throughout the experiment. The results showed that headcut migration in bare land was mainly activated by upstream flow incision, headwall erosion, and plunge pool erosion, whereas headcut migration in grassland was driven by soil‐root matrix collapse at the headcut, headwall erosion, and plunge pool erosion. The soil loss rate (SLR), gully headcut retreat distance (GRD), gully area (GA), and upstream soil loss volume (SLVU) in grassland plots with different root densities were 45.64–68.45%, 66.97–85.38%, 69.26–78.18%, and 67.89–87.02% less than those in the bare land plots, respectively. However, as root density increased, the gully depth in the grassland plots increased from 19% to 59%, and the proportion of gully head soil loss volume (SLVGH) to the total soil loss volume increased from 63.37% to 88.37%. The SLR, GRD, GA, and SLVGH showed decreasing trends as the root density increased, as described by a Hill curve. Moreover, roots with diameters of 0–0.5 mm had greater effects on soil loss and morphological evolution than roots with larger diameters. Our results provide a reference for the reasonable selection of grass root density for controlling gully headcut erosion in China's Loess Plateau.

Sustainable value of degraded soils in China's Loess Plateau: An updated approach

Widespread land degradation has stimulated the implementation of a large‐scale ecological restoration programme in China's Loess Plateau—the Grain‐for‐Green Programme (GFGP). This programme has substantially increased vegetation cover and served to control soil erosion, but threatened regional food supply due to widespread cropland conversion. Consequently, a strategy balancing green and grain land uses is required. Here, we establish a dominance index of ecosystem services by quantifying the economic value of four key ecosystem services (net primary productivity, soil conservation, water yield, and food production), by combining spatially explicit datasets and census data. Using the dominance index, we identify the optimal areas to target for GFGP in the Loess Plateau. The identified areas (target areas) were the transition zone from low to high value of ecosystem services (ESV). These areas exhibited low grain productivity in addition to having the highest potential for soil conservation. Compared with other regions of the Loess Plateau, the loss of grain production due to cropland conversion in these target areas could decrease by 42%, whereas ESV could increase by 33%. Therefore, despite the fact that over the past 15 years (2000–2014) in these target areas more cropland was converted into ecological use (i.e. forest/grassland), there is still a need to strengthen ecological restoration in this region in the future. This study proposed a strategy for balancing green and grain from a spatial perspective, which could potentially solve land degradation issues and the tradeoff between ecosystem services in a more beneficial and targeted way.

The social–ecological trap is an emerging concept that describes situations in which self‐reinforcing social and ecological feedbacks maintain or push a social–ecological system towards an undesirable state and threaten the sustainability of human societies. Understanding a system's feedback loops and identifying the leading factors of such traps is essential to develop effective management strategies to warn, avoid and escape traps. To better understand the dynamics of social–ecological traps, we developed a quantitative diagnostic framework that combines the social–ecological network approach and composite system state index. We demonstrated the effectiveness of the framework by examining the rural social–ecological evolution in China's Loess Plateau (LP) from 1949 to 2020, an area once faced with severe social–ecological challenges such as soil erosion, land degradation and poverty. Our analysis identified three stages of trap dynamics in LP: locked in the trap (1949–1981), reacting to the trap (1981–2003) and escaping the trap (2003–2020). In the first stage, LP was locked into an undesirable trajectory where reinforcing feedback occurs between rapid population growth, limited livelihood opportunities, excessive reliance on agriculture and severe soil erosion. Our results also found that the LP has made significant progress in escaping this social–ecological trap during the 21st century through ecological restoration practices and socioeconomic development. Similar social–ecological traps are also observed in many other regions of the world, particularly in developing countries. Our analysis recommends three pathways for addressing social–ecological traps in the LP: (1) promoting urbanization and livelihood diversity, (2) implementing site‐specific engineering measures (e.g. terraces and check dams in the LP) and (3) investing in ecological restoration programs. Escaping the trap is not the end of the story, but could be an early stage of another trap. Policymakers and managers should keep assessing and monitoring the policy practices and outcomes to avoid entering new trap situations. Read the free Plain Language Summary for this article on the Journal blog.

Towards the progress of ecological restoration and economic development in China's Loess Plateau and strategy for more sustainable development

Unraveling carbon stock dynamics and their determinants in China's Loess Plateau over the past 40 years

Current health status of check dams and their flood control capacity amid frequent heavy rainfall remain unclear, partially due to lack of a comprehensive and efficient method for health assessment targeting potential heavy rainfall pressures. Thus, this study developed a health assessment framework for check dams under heavy rainfall scenarios, focusing on structural composition and integrity, siltation capacity, and flood control pressure. Indicators for these three aspects were calculated based on a real-scene 3D model and a high-precision DEM generated by unmanned aerial vehicle (UAV) tilt photography for a typical watershed on the Loess Plateau. Comprehensive health assessment of 138 check dams revealed that 14% were in excellent health, 16% in good health, 26% in fair health, and 44% in poor health. For those 44% check dams (61 in numbers) in poor health, corresponding actions were recommended based on the assessment results to reduce pressure from flooding, with 15 demanding prompt repairs, 12 requiring increased dam height, 16 needing spillway expansions, and the remaining 18 requiring multifaceted measures or the construction of new dams. At the sub-watersheds scale, more than 70% of the check dams in the Majiagou and Tianjiagou were destroyed or in poor health, which require special attention during flood seasons. In the framework developed in this study, indicators were easily accessible and highly accurate, making it suitable for practical application. This study provided a scientific basis and methodological support for decision-making regarding the reinforcement of existing and future planning of check dams in the Loess Plateau region.

Comparative assessment of environmental impacts, mitigation potentials, and economic benefits of rain-fed and irrigated apple production systems on China's Loess Plateau

Understanding hydrological effects of ecological restoration (ER) is fundamental to develop effective measures guiding future ER and to adapt climate change in China's Loess Plateau (LP). Streamflow (Q) is an important indicator of hydrological processes that represents the combined effects of climatic and land surface conditions. Here 14 catchments located in the LP were chosen to explore the Q response to different driving factors during the period 1961–2009 by using elasticity and decomposition methods based on the Budyko framework. Our results show that (1) annual Q exhibited a decreasing trend in all catchments (−0.30 ∼ −1.71 mm yr−2), with an average reduction of −0.87 mm yr−2. The runoff coefficients in flood season and nonflood season were both decreasing between two periods divided by the changing point in annual Q series; (2) the precipitation (P) and potential evapotranspiration (E0) elasticity of Q are 2.75 and −1.75, respectively, indicating that Q is more sensitive to changes in P than that in E0; (3) the two methods consistently demonstrated that, on average, ER (62%) contributing to Q reduction was much larger than that of climate change (38%). In addition, parameter n that entails catchment characteristics in the Budyko framework showed positive correlation with the relative area of ER measures in all catchments (eight of them are statistically significant with p < 0.05). These findings highlight the importance of ER measures on modifying the hydrological partitioning in the region. However, ER actions over the sloping parts of the landscape weakened the impact of those in channels (i.e., check‐dams) on Q, especially after the implementation of the Grain‐for‐Green project in 1999.

Afforestation and deforestation as human disturbances to vegetation have profound impacts on ecohydrological processes influencing both water and carbon cycles and ecosystem sustainability. Since 1999, large‐scale revegetation activities such as “Grain‐to‐Green Program” have been implemented across China's Loess Plateau. However, negative ecohydrological consequences, including streamflow decline and soil drying have emerged. Here we estimate the equilibrium vegetation cover over the Loess Plateau based on an ecohydrological model and assess the water balance under the equilibrium and actual vegetation cover over the past decade. Results show that the current vegetation cover (0.48 on average) has already exceeded the climate‐defined equilibrium vegetation cover (0.43 on average) in many parts of the Loess Plateau, especially in the middle‐to‐east regions. This indicates a widespread overplanting, which is found to primarily responsible for soil drying in the area. Additionally, both the equilibrium vegetation cover and soil moisture tend to decrease under future (i.e., 2011–2050) climate scenarios due to declined atmospheric water supply (i.e., precipitation) and increased atmospheric water demand (i.e., potential evapotranspiration). Our findings suggest that further revegetation on the Loess Plateau should be applied with caution. To maintain a sustainable ecohydrological environment in the region, a revegetation threshold is urgently needed to guide future revegetation activities.

Which slope aspect and gradient provides the best afforestation-driven soil carbon sequestration on the China's Loess Plateau?

To clarify changes of discharge (Q) and sediment yield (SSY) during flood events provide critical insights for flood disaster prevention and control. However, our understanding of the long‐term variations and driving factors of Q‐SSY relationships during flood events remains limited. This study examined the variations in Q, SSY, and sediment rating curves (SSY = aQb) during maximum one, three, and five flood events (ranked by peak discharge) across 15 catchments in the China's Loess Plateau during 1956–2019. We used the partial least squares‐structural equation modeling (PLS‐SEM) to quantitatively decouple the effects of driving factors (precipitation, soil, vegetation, topography, and soil and water conservation measures (SWCMs)) on Q‐SSY relationships. There was a significant declining trend in both Q and SSY during flood events across catchments, but their contributions to annual SSY significantly increased by 41.48%, underscoring the critical role of floods in sediment transport. The Q‐SSY relationship during flood events became weakened over time, with coefficient a decreased and index b increased. The five driving factors explained 44%–49% of the changes in coefficient a and 36%–51% in index b. Significant direct effects of vegetation (path coefficient (β) = −0.921) and precipitation (β = 0.616) on coefficient a were observed (p < 0.05). Index b was principally dominated by SWCMs and vegetation, and the effects diminished with increase in number of flood events. These findings highlight the importance of vegetation cover and SWCMs in mitigating sediment transport, offering valuable insights for sediment management strategies in the Loess Plateau and similar regions.

Evaluation of the AquaCrop model for simulating the impact of water deficits and different irrigation regimes on the biomass and yield of winter wheat grown on China's Loess Plateau