Vegetation response to large-scale mountain excavation and city construction projects on the Loess Plateau of China.

Since 2012, the "Mountain Excavation and City Construction" (MECC) project has been implemented extensively on the Loess Plateau of China, transforming gullies into flat land for urban sprawl by leveling loess hilltops to fill in valleys. However, this unprecedented human activity has caused widespread controversy over its unknown potential ecological impacts. Quantitative assessment of the impacts of the MECC project on the vegetation is key to ecological management and restoration. Taking the largest MECC project area on the Loess Plateau, Yan'an New District (YND), as the study area, this study investigated the spatiotemporal pattern of vegetation dynamics before and after the implementation of the MECC project using a multitemporal normalized difference vegetation index (NDVI) time series from 2009 to 2023 and explored the response of vegetation dynamics to the large-scale MECC project. The results showed that the vegetation dynamics in the YND exhibited significant spatial and temporal heterogeneity due to the MECC project, with the vegetation in the project-affected areas showing rapid damage followed by slow recovery. Vegetation damage occurred only in the project-affected area, and 84 % of these areas began recovery within 10 years, indicating the limited impact of the large-scale MECC project on the regional vegetation. The strong correlation between vegetation dynamics and the MECC project suggested that the destruction and recovery of vegetation in the project-affected areas was mainly under anthropogenic control, which highlights the importance of targeted ecological policies. Specifically, the MECC project induced local anthropogenic damage to the plant population structure during the land creation period, but regeneration and rational allocation of the vegetation were achieved through urbanization, gradually forming a new balanced ecological environment. These findings will contribute to a full understanding of the response of vegetation to such large-scale engineering activities and help local governments adopt projects or policies that facilitate vegetation recovery.

Research on the bearing behavior of single pile in self-weight collapsible loess areas.

The negative skin frictional caused by loess collapse will decrease the bearing capacity of single pile, which is essential to the design of pile foundations in loess areas. In this study, a method for estimating the subsidence of soil layer at any depth is firstly proposed based on the total self-weight collapse value. Secondly, a new load transfer constitutive model for pile-soil interface is developed, which considers the nonlinear stress-strain relationship and the ultimate shear strength of soil. Then, a load transfer calculation model for pile foundation is established, which can calculate the pile axial force, the pile skin frictional, neutral point position and the settlement of a single pile. The calculation results are compared with the test data that obtained from a pile foundation on-site immersion test and the effectiveness of the calculation method is verified well. This calculation method may be useful for designing pile foundations in collapsible loess regions.

Effects of land use on groundwater recharge of a loess terrace under long-term irrigation.

Understanding the relation between land use and groundwater recharge is of direct interest in areas that are prone to geohazards. This study was performed to characterize the effects of land use on groundwater table (GWT) distribution in the Heifangtai (HFT) loess terrace under long-term flood irrigation, as irrigation intensity was governed by the crop types. In light of the regional temperature-vegetable dryness index (TVDI) and optical images obtained in different seasons over four years, the dataset incorporated the growth cycles of the local crops, which in turn improved the workload and accuracy of land use detection. Irrigation intensities for different crops were recorded during field investigations. A total of 26 electrical resistivity tomography (ERT) profiles were conducted to estimate the GWT distribution of HFT terrace, which was further utilized to reveal the relation between land use (i.e. irrigation) and GWT fluctuations. The results indicated that high local GWT was associated with vegetable fields which had the highest irrigation intensity, and by contrast, low local GWT was attributed to the perennial fruit tree fields with the lowest irrigation intensity. The discharge was monitored intermittently for over three years to analyze the effect on the local GWT close to margin of HFT. The result showed a strong correlation between spring discharge and GWT fluctuations along the margin of HFT, indicating that groundwater recharge was affected by both irrigation and spring discharges. These results suggested that incorporating growth cycles of crops can effectively facilitate the interpretation of remote sensing data for land use detection, and the results can provide useful guidance for improving irrigation programs and for alleviating geohazards in regions that are under long-term irrigation.