Semi-humid Regions Research Articles

Mapping rainfall interception for assessing ecological restoration sustainability in China

Ecological restoration (ER) programs play an important role in local and global climate change and carbon management policy interventions. Water resource is a key criterion for assessing the sustainability of ERs. Herein, we explored the spatiotemporal patterns of rainfall interception (RI, an important component of ecosystem water budgets), and its drivers after ER implementation in China. Further, we assessed whether ERs are sustainable by analyzing the trends of RI and water supply. As expected, we found that ERs caused an increase in RI in China from 2001 to 2018 (0.64 mm yr−1, p < 0.01). Changes in the normalized difference vegetation index and leaf area index contributed to a higher change in RI compared with other drivers. The decrease in RI was mainly recorded in the Qinghai–Tibet Plateau in Southwest, northern North, and southern Central and Southern China. Conversely, an increasing trend of RI was recorded in the Loess Plateau in Northwest, Northeast, and East China. Moreover, ERs are not always unsustainable in China, especially in Northeast, East, Central and Southern, and high-latitude regions of northern North China. Even in the Loess Plateau, which was criticized by previous studies, the unsustainability occurred only in the semi-humid region. Future ERs should be prioritized in southern parts of Eastern, Central, and Southern China, and must be appropriately considered in the Northeast and high-latitude regions in North China. It should be alert to the pressures of ERs on water supply, and its demand remains vigilant in the Qinghai–Tibet Plateau and semihumid areas of the Loess Plateau. This study provides new ideas for accurately evaluating the impact of ERs on water security and the sustainability of ERs.

Spatio-temporal variations of vegetation cover and its influence on surface air temperature change over the Yellow River Basin, China

Abstract Northwest China has experienced dramatic changes in vegetation cover over the past few decades with the Yellow River Basin (YRB) being the most representative area. As the major climate-sensitive area in China, vegetation cover change is one reason for its impact on surface air temperature (SAT). This study uses the observation minus reanalysis (OMR) method to reveal the spatio-temporal variations of vegetation cover and its impact on SAT change over non-urban areas of the YRB from 1982 to 2015. The Global Inventory Modeling and Mapping Studies dataset, SAT derived from meteorological stations, and European Reanalysis (ERA)-interim reanalysis temperature data were used to analyze the relationship between normalized difference vegetation index (NDVI) and temperature variation caused by vegetation change. The NDVI trend (SlopeNDVI) of the entire YRB reached 1.11×10−2 decade−1, which indicated the recovery of vegetation in general. The impact of variation in vegetation conditions on SAT change during 1982–2015 was estimated to be 0.037 °C decade−1, which contributed 7.62% to the temperature change. The mean annual NDVI (MNDVI) and SlopeNDVI in the YRB were significantly negatively correlated (P&amp;lt;0.001) with OMR temperature variation. A negative correlation was exhibited in semi-arid and semi-humid regions, whereas a positive correlation was found in the arid region. The observed changes in vegetation and SAT in the YRB support the theory of the impact of vegetation variation on SAT in China.

Cost-Effectiveness Analysis of a Sponge City Construction Based on the Life Cycle Cost Theory—A Case Study of the Yanshan South Road Area of Qian’an City, China

In semi-humid regions of China, annual precipitation is not evenly distributed. Heavy, summertime rainfall with a short duration frequently causes urban flooding, and annual rainfall less than evaporation results in urban water scarcity. In 2014, Hebei Province’s Qian’an city was ranked among the first group of sponge city pilot cities. This paper investigates the historically flooded section of Yanshan South Road and its surrounding area in Qian’an, focusing on the cost of resolving an urban water problem. Using the storm flood management model (SWMM) and the life cycle cost (LCC) method, the waterlogging reduction effect and life-cycle cost of various low impact development (LID) scenarios were evaluated. Six rainfall design scenarios were simulated and calculated so that the hydrological performance and cost-effectiveness could be comprehensively evaluated to establish the economic value and effectiveness of implementing LID facilities. This study found that the cost-effectiveness values of sunken green space (SG), SG + infiltration ponds (IP) (3:1), SG + IP (1:1), and SG + IP (1:3) scenarios for infiltration LID schemes were relatively high, up to 2.10. In the infiltration-storage LID scheme, the cost-effectiveness of the SG + reservoirs (RE) (1:1) was grater, which was 1.84. In semi-humid regions, the regulation and storage of rainwater, regarding its collection and use, can be widely applied to the construction of sponge cities.

Evapotranspiration and land surface temperature of typical urban green spaces in a semi-humid region: Implications for green management

Ecological deterioration and water scarcity motivate people to seek a balance between ecological reserves and water consumption in cities located in Semi-humid regions. This study is designed to develop a method for evaluating cooling water-saving characteristics of green space structures. Land surface temperature (TS), vegetation quantity (VQ) and evapotranspiration (ET) of typical green vegetation structures in 50 plots with an average area of 10,000 m2 were studied. Parameters were obtained based on data from three temporal remote sensing images; the surface energy balance algorithm for land (SEBAL) model, single-channel algorithm, and normalized difference vegetation index (NDVI) were utilized in the calculation. The relationships between the TS, VQ, and ET of green land were explored using regression analysis. A comprehensive index (EWI) was proposed to evaluate the cooling and water-saving abilities of different green space structures. Based on assessment results, species were classified into three: good (EWI ≥ 0.795), common (0.795 &amp;gt; EWI ≥ 0.419), and weak (EWI &amp;lt; 0.419). The characteristics of 17 species or species compositions with good cooling and water-saving abilities were summarized as: 1) a mix of the arbor, shrub, and herb, and 2) complete cover of green space by shrub.

Is biodegradable film an alternative to polyethylene plastic film for improving maize productivity in rainfed agricultural areas? — Evidence from field experiments

Due to increasing concerns regarding plastic pollution throughout the world, biodegradable mulch is considered a promising alternative to polyethylene film and its suitability has been verified in some crops. However, it is unclear whether the long-term continuous use of biodegradable mulch as a substitute for polyethylene film is sustainable in rainfed agricultural areas, and whether its effect is consistent under different climatic conditions. Therefore, we conducted 7-year (2013–2019) and 4-year (2015–2018) field experiments in two regions with different precipitation levels (semiarid and semi-humid regions) on the Loess Plateau in China. We compared the effects of polyethylene film mulching (PM), biodegradable film mulching (BM), and traditional flat planting without film mulching (TF) on the soil temperature, soil moisture, grain yield, crop water productivity (WPc), and economic benefit (EB). The results showed that compared with TF, the grain yields under BM and PM increased by 2.1–93.3% and 7.6–34.6%, respectively, WPc increased by 2.1–93.3% and 7.6–34.6%, and EB increased by –16.6–131.5% and –7.8–27.0%. The differences in these indexes under the two mulching methods mainly occurred in the dry years with less precipitation in the semiarid region, whereas there were no significant differences in the grain yield, WPc, and EB under BM and PM in the semi-humid region. Therefore, the decision regarding whether BM can replace PM to improve maize productivity is significantly affected by the annual precipitation in rainfed areas. Furthermore, the univariate quadratic equation model was used to simulate that BM can obtain a higher crop yield and EB than PM when the precipitation is more than 600 mm and less than 800 mm, so as to achieve a win–win situation with continuous yield and income increases, and environmental friendliness in the Loess Plateau of China. However, the yield increase effect of plastic film mulching is also related to crop types, mulch types and farming practices except for climate. In addition, reducing the production cost will still be important for encouraging farmers to use biodegradable film instead of polyethylene film.

Simulation of soil desiccation effects and production water footprint of apple orchards in different precipitation areas of the Loess Plateau, China

To clarify the desiccation effect of deep soil and water footprint of crop production in two typical dry-farming apple orchards on the Loess Plateau of China, with Luochuan County in semi-humid region and Mizhi County in semi-arid region as two typical apple planting areas, we used the WinEPIC model to quantitatively simulate and analyze the variations of soil moisture from 0 to 15 m and water footprint of apple production during 1980-2020. The results showed that annual yield of mature orchards in Luochuan and Mizhi followed an "S" curve, with the average annual values being 24.64 and 18.42 t·hm-2, respectively. The average annual evapotranspirations of Luochuan and Mizhi were 623.82 and 458.97 mm, the average annual drought stress days of Luochuan and Mizhi were 20.4 and 52.73 days, and the water overconsumption were 167.94 and 121.15 mm, respectively. The available soil water contents in Luochuan and Mizhi showed a sharp decline from 1 to 25 years old and from 1 to 23 years old, respectively, with average annual soil desiccation rates being 64.6 and 68.03 mm·a-1, respectively. The dry layer of deep soil for the orchards of Luochuan and Mizhi appeared at the 13th and 7th years, and would be stable after 23 and 22 years, respectively. The formation and stability time of the soil dry layer in the area with high precipita-tion was later than that with low precipitation. Long-term soil moisture deficit would result in irreversible soil dry layer. Water footprint of apple production in Luochuan and Mizhi were low in the early stage and high in the later stage, with the average annual values being 0.187 and 0.194 m3·kg-1, respectively. Both apple yield and production water footprint were affected by precipitation. To ensure the sustainable and healthy development of apple industry, it was recommended that the optimal planting age of apple trees is about 23 years on the Loess Plateau where water sources is limited, and the maximum should not exceed 25 years.

Climatic and environmental changes in the Baidunzi wetland, Northwest China, over the last 800 years inferred from glycerol dialkyl glycerol tetraethers

To help understand the temperature and moisture variations in the semiarid and semi-humid regions of China that lie a transitional zone between the Westerlies and the East Asian monsoon, reconstruction of the regional climatic changes over the last millennium is useful. However, high-resolution time-series data related to the histories of climatic changes in these areas are limited. Therefore, this multiproxy analysis of a sediment core from the Baidunzi wetland, northwest China, was conducted using grain size, total organic carbon, glycerol dialkyl glycerol tetraethers (GDGTs), and molecular biomarkers. The results show that GDGTs can reliably reconstruct paleoclimate changes recorded in sediments in salt marsh environments. The climate and environment history of the past 800 years can be divided into the three stages: the end of the dry Medieval Warm Period (MWP) (1278–1366 CE), a humid Little Ice Age (LIA) (1366–1896 CE), and the humid Current Warm Period (CWP) (1896–1999 CE). The Baidunzi setting during the end of the MWP was a warm and dry wetland with high salinity, low water levels, and emersed aquatic plants, and the depositional environment transitioned from fluvial facies to shore lake facies. In contrast, the local climate during the LIA was cold and humid, and the era was characterized by the appearance of vascular flora and two short-term flooding events, that caused a rise in water levels. The lowest temperatures and highest salinities of the period were correlated, which was likely caused by seasonal freeze-thaw cycles. During the CWP, the local climate was warm and humid, with a higher water level than during the LIA, and emersed plants again developed in shallow lagoon facies. Generally, salinity was positively correlated with temperature, except for freezing and thawing salinization resulting from episodes of extremely low temperatures. The warm–dry to cold–humid climate changes of the area were like those of arid Central Asia, which indicates that the Baidunzi region was affected mainly by the Westerlies and was impacted by southward intrusions of that system. The area was also affected by the North Atlantic Oscillation and experienced diminished evaporation during periods of cold climate.

Stomatal opening ratio mediates trait coordinating network adaptation to environmental gradients.

A trait coordination network is constructed through intercorrelations of functional traits, which reflect trait-based adaptive strategies. However, little is known about how these networks change across spatial scales, and what drivers and mechanisms mediate this change. This study bridges that gap by integrating functional traits related to plant carbon gain and water economy into the coordination network of Siberian elm (Ulmus pumila), a eurybiont that survives along a 3800 km environmental gradient from humid forest to arid desert. Our results demonstrated that both stomatal density and stomatal size reached a physiological threshold at which adjustments in these traits were not sufficient to cope with the increased environmental stress. Network analysis further revealed that the mechanism for overcoming this threshold, the stomatal opening ratio, gratio , was represented by the highest values for centrality across different spatial scales, and therefore mediated the changes in the trait coordination network along environmental gradients. The mediating roles manifested as creating the highest maximum theoretical stomatal conductance (gsmax ) but lowest possible gratio for pathogen defense in humid regions, while maintaining the gratio 'sweet spot' (c. 20% in this region) for highest water use efficiency in semihumid regions, and having the lowest gsmax and highest gratio for gas exchange and leaf cooling in arid regions. These results suggested that the stomatal traits related to control of stomatal movement play fundamental roles in balancing gas exchange, leaf cooling, embolism resistance and pathogen defense. These insights will allow more accurate model parameterization for different regions, and therefore better predictions of species' responses to global change.

Растительность, как диагностический показатель серо-коричневых (каштановых) почв

The article talks about the role of vegetation in the formation of gray-brown (chestnut) soils. Depending on the conditions of occurrence, the nature of the soil-forming rocks and vegetation cover, this type of soil is divided into four subtypes: gray-brown dark, gray-brown ordinary, gray-brown light and gray-brown mud. Considering that different subtypes of gray-brown soils are formed in characteristic eco-climatic conditions, as well as under different vegetation, we conducted a comparative analysis of characteristic features between individual subtypes of gray-brown soils common in the semi-humid region of the Lankaran district of Azerbaijan. The vegetation cover of gray-brown (chestnut) soils was studied. The phytomass of natural and irrigated cenoses was determined, as well as the structure of natural phytocenosis with the identification of dominant families and genera of plants. On the basis of the collected material, the formation of subtypes of gray-brown soils under composite, cereal and caper formations was established. Each family includes separate genera of plants. The Compositae family includes Artemisia L. (sagebrush), the Poaceae family includes the genera: Andropogon L. (bluestem), Festuca Tourn. ex L. (fescue), Stipa L. (needlegrass), Capparaceae family includes Capparis L. genus (caper).

Effects of plant density, nitrogen rate and supplemental irrigation on photosynthesis, root growth, seed yield and water-nitrogen use efficiency of soybean under ridge-furrow plastic mulching

The ridge-furrow plastic mulching system has been reported to increase crop yield and water-nitrogen use efficiency in semi-arid and semi-humid regions, but the effects of plant density, nitrogen (N) rate and supplemental irrigation on chlorophyll, photosynthesis, root growth and seed yield of soybean under ridge-furrow plastic mulching are poorly understood. A two-year field experiment (2019 and 2020) was conducted on soybean in a split-split plot design, with two plant densities (D16: 160,000 plants ha−1; D32: 320,000 plants ha−1) as main plots, two N rates (N30: 30 kg N ha−1; N60: 60 kg N ha−1) as sub plots, and two irrigation levels (W0: rainfed; W30: supplemental irrigation of 30 mm at the flowering-podding stage) as sub-sub plots. The results showed that plant density, N rate and supplemental irrigation had significant effects on leaf chlorophyll content, chlorophyll fluorescence parameters, photosynthesis, aboveground/root growth and seed yield. Increasing plant density enhanced leaf area index (LAI) by 31.9% and aboveground biomass (AGB) by 59.4%, seed yield by 27.4%, water use efficiency of seed yield (WUEs) by 27.9% and nitrogen partial factor productivity (NPFP) by 26.6%. Increasing N rate increased leaf chlorophyll content by 11.9% and photosynthetic rate by 5.5%, thereby increasing LAI by 11.5%, AGB by 10.3%, seed yield by 8.9% and WUEs by 8.2%. Supplemental irrigation increased chlorophyll content by 7.6% and photosynthetic rate by 3.7%, thereby increasing LAI by 4.7%, AGB by 11.0%, seed yield by 4.7% and NPFP by 4.0%, while supplemental irrigation only increased WUEs under D32. Compared with D16N30W0, D32N60W30 enhanced seed yield by 44.0% and WUEs by 37.0%, but reduced NPFP by 28.8%. Compared with D32N30W30, D32N60W30 increased both seed yield and WUEs by 8.5%, but it reduced NPFP by 45.5% with 100% higher N input. LAI, AGB, WUEs, NPFP and some leaf physiological (EXC, Chla/Chlb, EWTleaf and Ls) and root (TRLD, TRVD and TRSD) indexes were closely related to soybean yield. This study provides a better understanding how plant density, N rate and supplemental irrigation improved soybean productivity and water-nitrogen use efficiency by regulating chlorophyll, photosynthesis and aboveground/root growth.

Canopy interception and water harvesting function of shrubs in urban green spaces of semi-humid region and their influencing factors

In the semi-humid region, developing innovative water conservation urban green space design and facilitating urban greening projects from high water consumption to water conservation are hot topics in research and practice. Using the simulated rainfall method, we explored the water interception and collection functions and their influencing factors of six shrub species (Ligustrum ×vicaryi, Euonymus japonicas, Buxus sinica var. parvifolia, Photinia ×fraseri, Juniperus chinensis and Platycladus orientalis) in urban green space in the semi-humid region. The results showed that canopy interception and water harvesting were two stages in hydrological processes. The canopy interception of coniferous shrubs was high, while their stemflow was low. When the rainfall intensity increased, throughfall rates and stem-flow rates of all shrub species increased significantly, while the interception rate relatively decreased. The throughfall and stem-flow rates of broad-leaved shrubs were significantly higher than those of coniferous shrubs. The canopy interception was significantly lower in broad-leaved shrub species than in coniferous ones. At the center of canopy projection, the throughfall rate was the lowest. The leaf area index (LAI) and throughfall rate decreased gradually from the center of the canopy projection area. When the rainfall intensity was small, the throughfall rate at the center of canopy projection area was low, and thus the interception rate and the stem-flow rate were higher. When the rainfall intensity was more elevated, throughfall at the center of canopy projection area was large, and thus the interception rate and the stem-flow rate were low. With increasing rainfall intensity, the funnel-shaped water collection system tended to shrink due to the increases of throughfall rate at the edge of canopy. Rainfall intensity and LAI were the most critical factors affecting water harvesting function. Planting broad-leaved shrubs under the forest may be more effective in water harvesting than planting coniferous shrubs.

Response of Ecohydrological Variables to Meteorological Drought under Climate Change

Drought is the most widespread climatic extreme that has negative impacts on ecohydrology. Studies have shown that drought can cause certain degrees of disturbances to different ecohydrological variables, but the duration and severity thresholds of drought that are sufficient to cause changes in ecohydrological variables remain largely unknown. At the same time, we should not ignore the dynamic variation of drought’s effect on ecohydrological variables under the condition of climate change. Here, we derived the thresholds of several ecohydrological variables in response to drought in a historical period (1982–2015), including evapotranspiration (ET), soil moisture (SM), the vapor pressure deficit (VPD) and the normalized difference vegetation index (NDVI), and we projected the occurrence probability’s change trend of drought events that cause changes in ecohydrological variables under future climate change. The results show that the impact of drought on ecohydrological variables is not dependent on drought indicators. ET and NDVI were expected to decrease in most parts of the world due to increases in radiation (RAD) and temperature (TEMP) and decreases in precipitation (PRE) during drought periods. SM decreased in most regions of the world (93.47%) during the drought period, while VPD increased in 85.41% of the globe. The response thresholds for different ecohydrological variables to drought in the same area did not differ significantly (especially for ET, SM and VPD). When a drought lasted for 8 to 15 months and the corresponding drought severity reached 10 to 15 (the inverse of the cumulative values of the drought index when the drought occurs), the drought caused changes in the ecohydrological variables in most regions of the world. Compared with arid and semiarid regions, ecohydrological variables are more sensitive to drought in humid and semihumid regions (p &lt; 0.05), and high-intensity human activities in different climatic conditions increased significantly the severity of drought processes. Between 2071 and 2100, more than half of the world’s ecohydrological variables are expected to be more susceptible to drought disturbances (regions with shorter return periods of drought events that cause significant changes in ET, SM, VPD and NDVI account for 60.1%, 64.4%, 59.6% and 54.5% of the global land area, respectively).

Environmental and Biophysical Effects of the Bowen Ratio over Typical Farmland Ecosystems in the Loess Plateau

The Bowen ratio (β) comprehensively reflects physical characteristics of the land-surface climate. In this study, eddy covariance systems installed at Dingxi and Qingyang were used to conduct energy distribution measurements and observations characteristic of semi-arid and semi-humid farmland ecosystems on the China Loess Plateau. We studied mechanisms by which eco-environmental factors influence β. Additionally, we investigated responses of physiological and ecological factors to water and heat exchange under seasonally dry and wet conditions within each farmland ecosystem. Our results showed that sensible heat flux in the semi-arid farmland was the main consumer of available energy. In the semi-humid area, latent heat flux in summer had the dominant role in energy distribution (mean β 0.71). The β in the semi-arid region was 1.5 times higher than that in the semi-humid region during the growing season. β increased with an increase in the vapor pressure deficit (VPD) and ground–air temperature difference (Ts − Ta), and decreased significantly with an increase in effective precipitation and soil moisture. The change in β with environmental factors was more clear-cut in semi-arid areas than in semi-humid areas. The Priestley–Taylor coefficient (α) and β satisfied a power function law in the growing season. There was a strong correlation between them, with the coefficients of determination for semi-humid and semi-arid areas being 0.62 and 0.72, respectively. β decreased with an increase in the normalized difference vegetative index (NDVI), with this phenomenon being more obvious in the semi-humid zone (R2 = 0.40). β responded more rapidly to NDVI in the semi-arid area than in the semi-humid area. There was a negative exponential relationship between canopy stomatal conductance (Gs) and β, which displayed a stronger declining trend with the increase in Gs in the semi-arid area than in the semi-humid area. This study provides an important reference for the determination of land-surface characteristics of semi-arid and semi-humid farmland ecosystems on the Loess Plateau and for improving parameterization of land-surface processes.

Evaluating the effects of dams and meteorological variables on riparian vegetation NDVI in the Tibetan Plateau

As the third pole of the world, the Qinghai-Tibet Plateau (QTP) has a very special climate and geographical environment. In the past 20 years, with the increasing demand for clean energy, more than ten hydropower stations have been built. The impacts of these hydropower stations on riparian vegetation (RV) have only been described qualitatively in previous studies, while the contribution of dams and meteorological variables to riparian vegetation has not been quantitatively assessed. This study selected eight representative large-scale hydropower stations in the QTP, calculated and analyzed the dynamics of the standardized difference vegetation index (NDVI) of the RV pre-and post the dams construction, combined with the measured temperature and precipitation data to explore the driving factors of RV changes. The results show that the dams promoted the growth of RV and they were the main contributor (>50%) while precipitation and temperature had relatively small impacts. The effect of dams varies for different regions, compared with the sub-cold regions, it was more significant in humid and semi-humid regions of temperate zone. The dams affected RV in an indirect way through regulating the microclimate, promoting precipitation and slowing down the rate of temperature rise and these effects may come from the increase of the upstream water surface area.

Context-Dependency in Relationships Between Herbaceous Plant Leaf Traits and Abiotic Factors.

Leaf traits are important indicators of plants’ adaptive strategy to environmental changes. It is an established fact that leaf traits are jointly regulated by climatic and edaphic factors besides genetic factors. However, the relative importance of these abiotic forces in determining the general patterns of herbaceous plant leaf traits across different climatic regions in China is far from clear. We collected 1,653 observations of 542 species of herbaceous plant leaf traits including leaf mass per area, leaf nitrogen, and leaf phosphorus from 316 sampling sites across four climatic regions. We found that the leaf mass per area in the arid region was apparently larger than the others, whereas the smallest mass-based leaf nitrogen and mass-based leaf phosphorus were found in the humid region. Increased growing season temperature and evapotranspiration consistently promoted a conservative growth strategy indicated by higher relative benefit of leaf mass per area, especially in the arid region. Solar radiation in growing season promoted an acquisitive growth strategy indicated by higher relative benefits of mass-based leaf nitrogen and phosphorus in the humid region, but opposite patterns were found in the arid region and semi-humid region. Of all the soil nutrients including soil organic matter, total nitrogen, total phosphorus, and available nitrogen, soil available nitrogen was the strongest predictor of relative benefits of leaf traits associated with a nutrient acquisitive strategy, except in the nutrient-rich semi-humid region. There was a relatively larger number of abiotic factors contributing to relative benefits of leaf traits in the arid and humid regions. We concluded that plant functionality could respond divergently to the same factor facing different habitat conditions. Moreover, the relative benefits of leaf traits tended to be more vulnerable to abiotic filtering in more stressful conditions. Our findings have important implications for understanding the context-dependency of plant functionality to environmental filtering and further improving the predictability of plant dynamics under global change.

Role of hydrological parameters in the uncertainty in modeled soil organic carbon using a coupled water-carbon cycle model

Soil organic carbon is the largest carbon pool in the terrestrial biosphere. Large uncertainties exist in the numerical simulations of soil organic carbon due to inaccuracies in their mathematical descriptions of hydrological processes. In this study, the upper limit of uncertainty in modeled soil organic carbon that is induced by hydrological parameter errors, which may stem from measurement or experiential errors, is estimated in China under four different arid and humid conditions. The study was conducted using a conditional nonlinear optimal perturbation related to parameters (CNOP-P) approach and a model of the coupled water-carbon cycle (the Lund-Potsdam-Jena Wetland Hydrology and Methane Dynamic Global Vegetation Model, LPJ-WHyMe). Uncertainties in hydrological processes resulted in the largest error (2.73 kg C m−2 yr−1, 20.2%) in the modeled soil organic carbon in the arid and semiarid regions of northern China, with errors of 1.20 kg C m−2 yr−1 (6.1%) in northeastern China, 0.45 kg C m−2 yr−1 (3.3%) in southern China, and -1.71 kg C m−2 yr−1 (13.7%) in the semihumid region of northern China. By analyzing the three components of soil organic carbon, the fast soil carbon pool was found to be the main cause of the uncertainties in modeled soil organic carbon in the four regions of China. Moreover, belowground litter was another cause of the uncertainties in the modeled soil organic carbon in northeastern China and in the semihumid region of northern China. Additional results indicated that the simulation and prediction abilities of soil organic carbon could be improved by reducing parameter errors in hydrological processes through observations or targeted observations. The parameter sensitivity test showed that the benefits to modeling soil organic carbon were similar when reducing the errors in the sensitive hydrological parameter subset, compared to the benefits of reducing the errors in all the hydrological parameters.

Using statistical functions and hydro-hydraulic models to develop human vulnerability curves for flash floods: The flash flood of the Taitou catchment (China) in 2016

The number of deaths due to flash floods in China has increased sharply in recent decades. Therefore, a quantitative assessment of loss of life at the potentially affected areas is highly recommendable, which can improve the strategies of disaster risk mitigation and reduction. Numerous papers published to date have focused on human vulnerability to river floods but seldom to flash floods because of the lack of data on loss of life or flood intensity matched with loss of life. This research aims at reconstructing the July 2016 flash flood event by combining hydro-hydraulic models with post-flood surveys to obtain the data of flood intensity and then developing human vulnerability curves for flash floods through statistical functions. The hydrological model HEC-HMS was used to obtain the hydrograph of the July 2016 flash flood, which was evaluated using the flow data obtained from references. Subsequently, the hydraulic model FLO-2D was used to reconstruct the July 2016 flash flood event to obtain the flood characteristics (e.g., water depth, flow velocity, depth-velocity product, and impact force). The evaluation of FLO-2D was carried out using post-flood surveys defining maximum flooded extent and flood depths. Then, statistical functions were used to fit the relationship between the flood characteristics and mortality calculated through post-flood surveys. The results show that the simulated peak discharge (548 m3/s) of the July 2016 flash flood for Taitou catchment was close to the estimated peak discharge (540 m3/s) by the existing literature. Likewise, the FLO-2D model showed a good performance in reconstructing the flash flood event (FitA = 80.89%, RMSE = 0.21, NSE = 0.89), indicating that the flood characteristics outputting from FLO-2D model can be able to develop the human vulnerability curves for flash floods. Furthermore, four different human vulnerability curves for flash floods were developed, which are all power functions of water depth, flow velocity, depth-velocity product, and impact force, respectively. The mortality caused by flash floods increased with all four flood characteristics; the growth rate of the velocity curve was the fastest. However, the fitting effect of the velocity curve was the poorest (R2 = 0.87, SSE = 0.155, RMSE = 0.18), while that of the water depth curve was the best (R2 = 0.97, SSE = 0.003, RMSE = 0.03). Our findings provide a quantitative way to quickly assess loss of life due to flash floods, which is more likely to have a good performance in arid and semi-humid regions of northern China.

Discrepancy in growth resilience to drought among different stand-aged forests declines going from a semi-humid region to an arid region

The growth resilience of forests to extreme drought event has become an urgent topic in global change ecology because of exacerbated water constraints upon trees’ growth over the last few decades. Yet, surprisingly little is empirically known about the contribution of stand age, a key factor influencing forest structure and ecological processes, to variation in growth resilience among stands. This study revealed discrepancies in the drought resilience of forests of different stand ages by analyzing an extensive tree-ring dataset from Qinghai spruce (Picea crassifolia Kom.), a typical moisture-sensitive tree species in northwestern China. We found that older growth Qinghai spruce forest stands have higher resistance to droughts than do younger growth ones. Conversely, however, the post-drought recoveries of these older growth forest stands are lower than those of the younger growth stands. Patterns in the variation of resilience indices were consistent between two contrasting hydrological niche regions, whereas the stand age-related discrepancies in drought resilience became significantly smaller going from the wetter region to the drier region. These findings imply that, instead of a “one-size-fits-all” strategy, more meticulous and more targeted strategies are needed to enhance forest management and strengthen forest conservation given the experienced and projected climate trends, which feature increasing precipitation but higher extreme-drought frequency across this spruce tree’s habitat and distribution in northwestern China.

Threshold effects of vegetation coverage on runoff and soil loss in the Loess Plateau of China: A meta-analysis

In the loess plateau, due to the vegetation recovery has achieved preliminary results, while it is controversial whether the vegetation cover can be increased unrestrictedly for a long time, thus defining the vegetation coverage threshold is gaining urgency. The purpose of this study was to define the vegetation coverage thresholds of runoff and soil erosion in the Loess Plateau, and quantify the effect of vegetation coverage changes on soil and water loss, and evaluate the effective vegetation coverage in different climatic regions. A total of 59 watersheds were involved in the meta-analysis, including 38 counties belonging to 6 provinces in the Loess Plateau. The vegetation coverage increased from 2.51% to 86.80%, the runoff modulus ranged from 155.7 to 780431.8 m3·km−2·a−1, and the soil erosion modulus ranged from 400 to 58285 t·km−2·a−1. Three specific vegetation coverage thresholds were identified for soil erosion: the lower threshold (0%–35%), the transition (35%–65%), and the upper threshold (65%–100%); four specific vegetation coverage thresholds were identified for runoff: the low threshold (0%–20%), the transition (20%–50%), the high threshold (50%–75%), and the upper threshold (75%–100%). In the Loess Plateau, the effective vegetation coverage in the cold and arid regions is 25.12%, in the semi-humid region is 51.02%, in the semi-arid region is 45.92%, and in the arid region is 26.53%, to which corresponding ecological management strategies should be adopted. Clarifying the impact of vegetation coverage on water and soil loss at the regional scale can provide insight into suitable management programs for the new pattern of runoff and soil erosion formed by the vegetation restoration in the Loess Plateau.

Estimation of maize evapotranspiration in semi-humid regions of northern China using Penman-Monteith model and segmentally optimized Jarvis model

Accurate estimation of maize evapotranspiration (ET) is of great significance for the improvement of crop water use efficiency and precision irrigation. The Penman-Monteith model (P-M) has been widely used to simulate crop ET. In the P-M model, the estimation accuracy of canopy resistance (rc) has a direct impact on ET. In this study, based on the eddy covariance system, large-scale lysimeter and meteorological station data from three sites (Yucheng, Yangling and Shangqiu) in semi-humid regions of northern China, the P-M model was applied to obtain canopy resistance (rc-PM) and correlation significances between rc-PM and different impact factors (Rn net radiation, T temperature, VPD saturated vapor pressure difference, θ soil moisture content, LAI leaf area index) were analysed. The whole growth period of maize was divided according to different LAI thresholds (0.1, 0.5, 1.0, 1.5, 2.0 and 3.0 m2 m−2). The Genetic Algorithms (GA) and Differential Evolution (DE) algorithms were used to optimize the empirical parameters of the Jarvis model, and the P-M model was applied to estimate ET under different LAI thresholds at the three stations. The correlation significances of rc-PM with different influencing factors followed the order Rn > LAI > θ >VPD >T, and it was extremely significant with Rn (P < 0.01) and significant with LAI and θ (P < 0.05). The GA and DE algorithm optimization results showed that the calculation accuracy of rc was highest when LAI=0.5 m2 m−2 at Yucheng station, with R2 of 0.80 and 0.81, respectively, and when LAI=1.0 m2 m−2, and the accuracy of rc was highest at Yangling station, with R2 of 0.87 and 0.89, respectively, and when LAI = 1.0 m2 m−2, and the accuracy of rc was highest at Shangqiu station, with R2 of 0.84 and 0.84, respectively. Combined with P-M model to calculate maize ET under different LAI thresholds, the simulation accuracy of ET was best when LAI = 0.5 m2 m−2 at Yucheng station, with averege R2 of 0.85, the order of simulation ET accuracy was: 0.5 >1.0 >1.5 >2.0 >3.0 >0.1 m2 m−2. When LAI = 1.0 m2 m−2, and the accuracy of maize ET was highest at Yangling and Shangqiu stations, with averege R2 of 0.83 and 0.85, respectively, the order of simulation ET accuracy was: 1.0>0.5>1.5>2.0>3.0>0.1 m2 m−2. ET accuracy calculated by the DE optimization algorithm was better than that of the GA optimization algorithm, with R2 of 0.40–0.84 and 0.58–0.86, respectively. This study suggests that the algorithm is of great importance to optimize the empirical parameters of the Jarvis model, of which DE optimization algorithm is recommended to simulate maize ET in semi-humid regions of northern China.