A novel index for vegetation drought assessment based on plant water metabolism and balance under vegetation restoration on the Loess Plateau.

Vegetation is vital to the ecosystem, contributing to the global carbon balance, but susceptible to the impacts of climate change. Monitoring vegetation drought remains challenging due to the lack of widely accepted drought indices. This study focused on vegetation, and simulated the vegetation suitable water demand and soil available water supply (calculated by Remote-sensing-based Water Balance Assessment Tool model). The standardized Vegetation Water deficit Index (SVWDI) was established by calculating the vegetation water deficit, which reflects the response of vegetation to drought. We examined the spatiotemporal evolution of vegetation drought on the Loess Plateau and evaluated the applicability of standardized vegetation water deficit index. Our findings revealed that the standardized vegetation water deficit index demonstrated an overall upward trend across different time scales from 1991 to 2020. Drought conditions were concentrated in the first 20 years of the study period, but vegetation drought on the Loess Plateau has been alleviated in the past decade. Moreover, as the time scale extended, the trend of SVWDI generally decreased, with approximately 49.50 % (1-month scale), 46.66 % (3-month scale), 47.08 % (12-month scale), and 32.16 % (24-month scale) of the grid areas experiencing increased SVWDI. The correlation between SVWDI and tree-ring width index (TRWI) performed well under all precipitation gradients, but the Palmer drought severity index (PDSI) was only highly correlated with TRWI in regions with low precipitation. In terms of the relationship with vegetation health, SVWDI demonstrated the highest correlation with the normalized difference vegetation index (NDVI) across different time scales, followed by PDSI and standardized precipitation evapotranspiration index (SPEI). This study provides insights into the evolution of vegetation drought in response to climate change. The findings can guide initiatives such as returning farmland to forest and grassland on the Loess Plateau to aid climate change adaptation strategies.

Virtual water output intensifies the water scarcity in Northwest China: Current situation, problem analysis and countermeasures.

With 80% water resources in the south and 65% arable land in the north, China is facing a rigorous challenge due to the spatial mismatch between water distribution and food & energy production to make a balanced development of economy and ecosystem. In the past decades, the northwest has played a prominent role in maintaining national food and energy security. However, the lack of water resources in this region poses a great threat to sustainable development. Based on this, this study quantitatively analyzed the evolution trend of water footprint (WF) of major crops and energy products in Northwest China from 2000 to 2015 and revealed the virtual water (VW) transfer pattern with commodity trade and its water resource stress caused by the virtual water output. The results show that, although the improvement of technology has greatly reduced the WF per unit production, the northwest region is still a net VW output area, whose net VW output associated with food and energy trade is increasing sharply from 287.2 × 108 m3 (2000) to 328.5 × 108 m3 (2015) with a growth rate of 14.4%, seriously aggravating the local water resource pressure. To ensure the water, food and energy safety of the northwest, we proposed countermeasures and suggestions on technological development and strategic planning, including water-saving technology promotion, industrial and agricultural structure optimization, and the coordinated management of physical and virtual water. The above findings provide a scientific reference to ensure the sustainable development of Northwest China.

Water stress assessment integrated with virtual water trade and physical transfer water: A case study of Beijing, China.

With the increase in population and economic development, urban water demand has increased significantly over the past decades, and physical transfer water (i.e., freshwater transported from water source regions to water intake regions through channels or pipelines) and virtual water (i.e., freshwater used in the production of goods and services along their supply chains, abbreviated as VW) have gradually become viable water sources for many cities to relieve water stress. This study used Beijing as the research object and systematically analyzed the impact of physical transfer water, VW, and local water on urban water stress from 2000 to 2016. The results show that VW inflow related to food trade has increased from 3.55 billion m3 in 2000 to 16.76 billion m3 in 2016, and that energy's VW inflow increased from 52.76 million m3 to 137.47 million m3 over the same period. Before 2011, Beijing's water demand was largely met by local water resources; however, after 2011, external water resources (including physical transfer water and VW) accounted for majority of the city's water demand, and VW's contribution increased from 47% in 2011 to 53% in 2016. Although Beijing has significantly reduced local water use in favor of external sources, its water stress index in 2016 remained considerable, far exceeding the upper limit of available water. This study also proposes some measures to ensure Beijing's water security based on the sustainability of external water supply.

The spatial and temporal evolution of the actual evapotranspiration based on the remote sensing method in the Loess Plateau.

Due to the joint impact of climate change and human activities, vegetation cover area in Loess Plateau has been tremendously improved. However, vegetation restoration will lead to high water use, which might pose a great threat to the water sustainability of regional ecosystems. In order to analyze the spatiotemporal characteristics of water consumption, this study estimated the actual evapotranspiration (AET) based on the SEBAL model. The study found that precipitation and AET increased significantly (5% confidence level) with the rising of vegetation coverage in the period of 1990-2015. The increase rate of precipitation was 1.91 mm/a, higher than that of AET (1.68 mm/a). From a spatial perspective, the AET of Loess Plateau is increasing from northwest to southeast. The high-AET area (AET > 400 mm) was also extremely enlarged from 39% (1990) to 73% (2015) with the vegetation recovery. In terms of the intra-annual variability, the AET from March to April is usually much higher than the precipitation in different hydrological years, which suggests that the spring drought is a potential threat for vegetation growth in Loess Plateau. At last, this study introduced an index of rainwater utilization potential indicator (IRUP) to analyze the water supply and demand in Loess Plateau. It found that IRUP values are positively correlated with NDVI (Normalized Difference Vegetation Index), indicating that the available water for vegetation growth is seemingly increasing with the vegetation cover area enlarged in the whole region.

Combination and placement of sustainable drainage system devices based on zero-one integer programming and schemes sampling.

The combination and placement of sustainable drainage systems (SuDS) devices is important for system design, but differing site characteristics and device properties can make this a challenging task. Opinion-based and optimization-based approaches have the disadvantages of subjectivity and excessive computational burden respectively. This paper presents a new framework for SuDS device combination and placement in system design. It integrates zero-one integer programming, random sampling, scheme filtering and cost-effectiveness analysis. The effectiveness of the framework is tested with a SuDS device placement design case in Chizhou city, Anhui province, China. The proposed framework will help to objectively choose the best SuDS device combination and placement scheme for cost-effective implementation.

Drought-Induced Carbon and Water Use Efficiency Responses in Dryland Vegetation of Northern China.

Given the context of global warming and the increasing frequency of extreme climate events, concerns have been raised by scientists, government, and the public regarding drought occurrence and its impacts, particularly in arid and semi-arid regions. In this paper, the drought conditions for the forest and grassland areas in the northern region of China were identified based on 12 years of satellite-based Drought Severity Index (DSI) data. The impact of drought on dryland vegetation in terms of carbon use efficiency (CUE) and water use efficiency (WUE) were also investigated by exploring their correlations with DSI. Results indicated that 49.90% of forest and grassland experienced a dry trend over this period. The most severe drought occurred in 2001. In general, most forests in the study regions experienced near normal and wet conditions during the 12 year period. However, grasslands experienced a widespread drought after 2006. The forest CUE values showed a fluctuation increase from 2000 to 2011, whereas the grassland CUE remained steady over this period. In contrast, WUE increased in both forest and grassland areas due to the increasing net primary productivity (NPP) and descending evapotranspiration (ET). The CUE and WUE values of forest areas were more sensitive to droughts when compared to the values for grassland areas. The correlation analysis demonstrated that areas of DSI that showed significant correlations with CUE and WUE were 17.24 and 10.37% of the vegetated areas, respectively. Overall, the carbon and water use of dryland forests was more affected by drought than that of dryland grasslands.

Study on Water Suitability of Apple Plantations in the Loess Plateau under Climate Change.

With the implementation of the Grain for Green Project, the apple plantation area is increasing in Loess Plateau. However, due to severe water scarcity, the sustainability of apple tree growth is threatened. In this paper, we used meteorological data (1990⁻2013) and forecasted climate data (2019⁻2050) to estimate water demand and establish a water suitability model to study the water balance between available water and water consumption of the apple trees. The results show that: (i) the order of the average water demand of apple plantation in each subarea is Shaanxi Province > Yuncheng area > Gansu Province > Sanmenxia Region, ranging from 500 to 950 mm; (ii) the temporal variability of water suitability from 1990 to 2013 is large, and the higher values are concentrated in the late growth stage of the apple trees and the lower values are concentrated in the early growth stage; (iii) the temporal and spatial distribution of water suitability is relatively stable and even in the Loess Plateau in the period of 2019⁻2050; (iv) the water suitability is mainly affected by effective precipitation and reference evapotranspiration and the reference evapotranspiration is mainly affected by the solar radiation (36%) and average temperature (38%). Furthermore, due to the joint influence of precipitation increases and solar radiation (average temperature) increases, the future water suitability of the apple plantation area in the Loess Plateau is showing a non-significant downward trend under RCP4.5 scenario.

The impacts of land conversion and management measures on the grassland net primary productivity over the Loess Plateau, Northern China.

In the 1990s, the Chinese government began implementation of a series of national-scale restoration programs to combat environmental degradation. As one of most important arid and semiarid regions of China, the Loess Plateau has attracted attention related to the effectiveness of these initiatives. The present study analyzed land use and cover change (LUCC) of the grassland in the Loess Plateau and the consequent change in net primary productivity (NPP) based on a consecutive land use data derived from the European Space Agency Climate Change Initiative land cover maps and the CASA (Carnegie-Ames-Stanford Approach) model driven by MODIS-NDVI data. The contributions of climate variation and human activities (including land conversion and management measures) to these changes were also quantitatively differentiated. The results indicated that the area of the Loess Plateau grassland experienced a net increase of 0.43 × 104 km2 over the study period. The total NPP of the Loess Plateau grassland increased by 11,325.13 Gg C·yr-1, of which the human activities and climate variation were responsible for 78.45% and 21.55%, respectively. The land conversion reduced the grassland NPP by 308.60 Gg C·yr-1, whereas management measures increased the NPP by 9197.97 Gg C·yr-1 in the otherwise unmodified grassland. Overall, ecological restoration programs have effectively increased grassland NPP in the Loess Plateau. However, human activities played both positive and negative impacts in this process.

Temporal and spatial evolution of the standardized precipitation evapotranspiration index (SPEI) in the Loess Plateau under climate change from 2001 to 2050.

Loess Plateau has great uncertainty on drought occurrence due to climate change. This paper analyzes the evolution of precipitation, potential evapotranspiration and standardized precipitation evapotranspiration index (SPEI) based on the Coupled Model Inter-comparison Project Phase 5 (CMIP5) data and regional downscaling model (RegCM4.0). Results indicate that, under RCP2.6 Scenario, the precipitation will increase significantly (5% confidence level) at the rate of 16.40mm/10a. However, the potential evapotranspiration is showing non-significant decreasing trend at the rate of 2.16mm/10a. Moreover, the SPEI will decrease in the south and northernmost area and increase in the central northern area of Loess Plateau. Under RCP8.5 Scenario, the precipitation will increase significantly (5% confidence level) at the rate of 19.12mm/10a. The potential evapotranspiration will non-significantly decrease at the rate of 2.16mm/10a and the SPEI is showing increasing trend almost in the whole Loess Plateau. Generally, Loess Plateau is becoming wetter in the central part under RCP2.6 Scenario and the wet area will be enlarged to almost the whole plateau under RCP8.5 Scenario. Based on the results, the water resources will increase under global warming, which may alleviate the water scarcity issue in the Loess Plateau.

Actual ET modelling based on the Budyko framework and the sustainability of vegetation water use in the loess plateau.

Jointly influenced by the natural factors and the artificial protection measures, the ecological environment of Loess Plateau has been significantly improved in recent years, but which has already brought about some water-related problems. To maintain the balance between precipitation and water consumption is an important foundation for sustainable development of the ecology remediation. This study used Budyko Framework to simulate the actual water consumption of 161 sub-basins from 1990 to 2014. Based on the simulation results, the research also analyzed the evolution characteristics of water balance in Loess Plateau from 1990 to 2014. Results show that, with the increase of vegetation coverage, the regional precipitation and actual evapotranspiration were both showing a significant increasing trend, and the increasing rate of precipitation was 1.91mm/a on average, which was greater than the increasing rate of actual evapotranspiration of 1.34mm/a. To further demonstrate the water balance regime in Loess Plateau, the evapotranspiration coefficient (ECC) was used to quantitatively indicate the ratio of the vegetation water consumption and the total precipitation. The average values of ECC were 0.868, 0.863, 0.851 and 0.837 respectively in four sub-periods of 1990-1999, 2000-2004, 2005-2009 and 2010-2014. The above analyses indicate that with the vegetation recovery and ecological restoration, the percentage of evapotranspiration in the total precipitation is keeping decreasing and in turn the percentage of water yield in the total precipitation is keeping increasing. Consequently, it seems more sustainable for vegetation water use in most areas of Loess Plateau currently.