Water-limited Regions Research Articles

Optimizing irrigation for winter wheat to maximize yield and maintain high-efficient water use in a semi-arid environment

Optimizing irrigation scheduling is vital in maintaining high crop yield and sustainable water use in water-limited regions worldwide. In this study, a 4-year field experiment (2016–2020) was conducted to evaluate the effect of six irrigation schedules on the grain yield performance, soil water content, plant evapotranspiration, and irrigation water use efficiency of typical winter wheat production system in the North China Plain. The six irrigation schedules involved irrigation at different growth stages, including one critical growth stage (60 mm per stage, I60), two stages (I120), three stages (I180), four stages (I240), and five stages (I300). No irrigation was performed in the control field (I0). The results showed that the plant height and yield components of winter wheat (spike number per hectare, kernels per spike, thousand-kernel weight) increased with the increase in annual irrigation amount. The grain yield in each study year increased quadratically (P < 0.05) with the annual irrigation amount. The soil water content in the 0–90 cm soil layer (where the wheat roots are mainly distributed) increased significantly during the growing season with the increase in annual irrigation amount. In I240, the mean soil water content (0–90 cm) during the growing seasons (17.3–22.6 %) was 61.7–80.8 % of the field water holding capacity, sufficiently favoring wheat growth. The average soil water storage (0–90 cm) during the wheat growing season ranged from 241.1 to 283.2 mm in the irrigation applied treatments, which were 21.7–63.8 mm (9.9–29.1 %) higher than in no irrigation treatment (I0, 219.4 mm). The 4-year averages of wheat grain yield did not differ significantly between I240 (7909.1 kg ha-1) and I300 (7868.6 kg ha-1); however, the average irrigation water use efficiency decreased from 33.0 kg ha-1 mm-1 in I240 to 26.2 kg ha-1 mm-1 in I300. Therefore, we recommend an irrigation scheduling at the regreening, jointing, flowering, and filling stages (I240, 60 mm per stage, 240 mm in total) to maintain high grain yield and sustainable soil water use for winter wheat production in the North China Plain and similar semi-arid regions.

Mixed forest suffered less drought stress than pure forest in southern Siberia

Hotter droughts are becoming a major threat to forest growth in water-limited regions. Species interactions have long been acknowledged as an important component modulating ecosystem functions, including mediating their vulnerability to climate-related stresses. However, quantifying the impact of species interactions on forest growth in response to drought remains a challenge. Using the tree ring data of two species in two pure forests and two coniferous and broad-leaved mixed forests with different degrees of mingling in southern Siberia within the period of 1960–2017, we found that mixed forests with higher mingling have a higher ability to withstand the effect of extreme drought events, which is especially beneficial to the resistance and resilience of broad-leaved trees. Larch trees showed a remarkable consistency in their resistance and resilience to extreme drought events in both pure and mixed stands, suggesting their ability to promote the growth of neighboring white birch without reducing their own growth. The growth of white birch trees was enhanced in mixed forests and showed a significant trend of increasing resistance and resilience with increasing mingling. Our results provide further evidence of a decreasing duration of the legacy effect with increasing mingling for both coniferous and broad-leaved trees, which might explain the improved tolerance to drought of trees in mixed forests compared to those in pure forests in water-limited hemiboreal southern Siberia. Our study implies that changes in forest structure might help forests respond to hotter droughts in water-limited regions.

Investigating and predicting spatiotemporal variations in vegetation cover in transitional climate zone: a case study of Gansu (China)

Vegetation ecosystems are sensitive to large-scale climate variability in climate transition zones. As a representative transitional climate zone in Northwest China, Gansu is characterized by a sharp climate and vegetation gradient. In this study, the spatiotemporal variations of vegetation over Gansu are characterized using the satellite-based normalized difference vegetation index (NDVI) observations during 2000–2020. Results demonstrate that a significant greening trend in vegetation over Gansu is positively linked with large-scale climate factors through modulating the water and energy dynamics. As a climate transition zone, the northern water-limited and southern energy-limited regions of Gansu are affected by water and energy dynamics, differently. In the water-limited region, a weakening Asian monsoon along with colder Central Pacific (CP) and warmer North Pacific (NP) Oceans enhances prevailing westerlies which bring more atmospheric moisture. The enhanced atmospheric moisture and rising temperature promote the local vegetation growth. In contrast, large-scale climate variations suppress the southwest monsoon moisture fluxes and reduce precipitation in southern energy-limited regions. In these energy-limited regions, temperature has more effects on vegetation growth than precipitation. Therefore, the greenness of vegetation is because of more available energy from higher temperatures despite overall drying conditions in the region. Based on the above mechanism, future scenarios for climate impacts on vegetation cover over Gansu region are developed based on the two latest generation from coupled climate models (Coupled Model Intercomparison Project Phase 5 and Phase 6; CMIP5 and CMIP6). In the near-term future (2021–2039), the vegetation is likely to increase due to rising temperature. However, the vegetation is expected to decrease in a long-term future (2080–2099) when the energy-limited regions become water-limited due to increasing regional temperatures and lowering atmospheric moisture flux. This study reveals an increasing desertification risk over Gansu. Similar investigations will be valuable in climate transition regions worldwide to explore how large-scale climate variability affects local ecological services under different future climate scenarios.

Holocene upland and wetland dynamics in the Chihuahuan Desert, Cuatrociénegas Mexico

The Cuatrociénegas Basin, Coahuila MX, has the highest rate of endemism in North America and has been the source of a rich aridlands, paleoenvironmental history in the Chihuahuan Desert of North America. Understanding the recent past of the Cuatrociénegas Basin contextualizes how aridland terrestrial and wetland systems may respond to ongoing climate warming and the potential aridification in already water-limited regions. A 12.3 m long sediment core dated to ~13,500 cal yr BP was collected from Poza Cortador and examined for pollen to reconstruct the Holocene terrestrial and wetland history of the Basin. The terrestrial record indicates a transition from woodland to desert environments by ~7500 cal yr BP. This transition is characterized more by the decrease in conifer and oak pollen abundances relative to desert taxa rather than a sudden appearance of new pollen types. Wetlands appear more stable than the terrestrial record, possibly because of groundwater buffering of the wetland surface. Alternatively, there might be less obvious changes because of the overall pollen diversity of identified wetland pollen types selected in this study. Significant aridity in both the terrestrial and wetland environments is inferred between 5000 and 3000 cal yr BP based on poor pollen preservation. The record of the last 3000 cal yrs reflects the resilience of both the terrestrial and wetland systems to prolonged drought and provides hope that this unique system may have resiliency to some aspects of future climate change if the hydrologic connectivity can remain intact.

Automated real-time irrigation analytics inform diversity in regional irrigator behavior and water withdrawal and use characteristics

Effective agricultural water management requires accurate, continuous, and transparent accounting of water use in irrigated agroecosystems, especially in water-limited regions where moratoriums may be imposed. Advances in sensor technologies, networking, and data analytics can aid in fulfilling this task by automatically collecting, analyzing and reporting real-time data to infer irrigators’ practices and behaviors, crop water requirements, water applications and use. In this research, an automated irrigation water withdrawal and water use monitoring and data collection system was deployed to monitor actual irrigation dynamics for 31 commercial and large-scale agricultural production fields for three consecutive years. Production scale fields (representing a total of 1050 ha) included center pivot (P), gravity (surface/furrow) (G) and subsurface drip-irrigated (S) fields. On average, irrigation was initiated 40–70 days after planting (DAP) and terminated by 120–140 DAP. The proportion of irrigation systems operating simultaneously and peak water abstraction were highest (70–90 %) during July and August. Mean depth of water applied across all fields was 243, 264 and 284 mm in 2013, 2014 and 2015, respectively. Site-specific monitoring of precipitation, soil moisture, and evaporative demand and a soil-water balance model resulted in mean seasonal irrigation requirement estimates of 394, 242 and 184 mm, in 2013, 2014 and 2015, respectively; for maize; and 307, 163 and 219 mm in 2013, 2014 and 2015, respectively, for soybean. Despite reduction of calculated mean irrigation requirement in 2014 and 2015 by 40–50 %, actual irrigation applications by producers did not change considerably and irrigation applied exceeded irrigation water requirements in 80 % of the fields, suggesting needs for irrigation water management technology implementation and associated educational programs in the region. Some fields showed irrigation applications exceeding the mean annual allocated (moratorium) irrigation depth (305 mm), implying that irrigation decisions are still largely driven by non-scientific and/or technical methods. While substantial farm-to-farm heterogeneity makes it challenging to robustly benchmarking regional water footprint and irrigator behavior, it also creates an opportunity for developing and implementing methodologies and strategies for real-time monitoring of farm-level irrigation dynamics. New advances in technologies with telemetry capabilities as well as internet of things (IoTs) can be leveraged to effectively create databases, track and compare water usage to better plan, allocate, distribute, monitor and manage limited water resources for enhancing agricultural productivity. These processes can also be used for education and demonstration for irrigation professionals to enhance adoption of such technologies. This research has the potential for technology and strategy transfer for advanced water management to other regions in the United States and globally.

Allele mining of wheat ABA receptor at TaPYL4 suggests neo-functionalization among the wheat homoeologs

ABA receptors (PYR/PYL/RCAR) play a central role in the water loss control of plants. A previous report indicated that TaPYL4 is a critical gene in wheat that improves grain production under drought conditions and increases water use efficiency. In this study, we analyzed the sequence polymorphisms and genetic effects of TaPYL4s. Based on isolated TaPYL4 genes from chromosomes 2A, 2B and 2D, three haplotypes were detected in the promoter region of TaPYL4-2A, and two haplotypes were present in TaPYL4-2B and TaPYL4-2D, respectively. Marker/trait association analysis indicated that TaPYL4-2A was significantly associated with plant height in 262 Chinese wheat core collection accessions, as well as the drought tolerance coefficient (DTC) for plant height in 239 wheat varieties from Shanxi Province in multiple environments. However, the frequencies of favored drought-tolerant haplotype TaPYL4-2A-Hap2 were considerably low, accounting for only 10%, and lines with this certain Hap could be reserved in the breeding program. TaPYL4-2B was significantly associated with grain number, and the favored haplotype TaPYL4-2B-Hap1 was the dominant allele of above 90% in the collection. For TaPYL4-2D, there were no significant differences in these traits between the two haplotypes in either of the two panels. These results indicate that variation might lead to functional differentiation among the homoeologs and the haplotypes had undergone artificial selection during breeding. Two molecular markers developed to distinguish these haplotypes could be used for breeding in water-limited regions.

Effect of 45 full-scale WWTPs on tropical receiving water bodies in Brazil by partial least squares-discriminant analysis

Abstract Treated wastewater may affect water quality and thereby significantly alter physicochemical and biological water quality parameters. The impact of wastewater treatment plants (WWTPs) on receiving water bodies is a multivariate problem. In this study, we investigated the effect of 45 full-scale WWTPs on tropical receiving water bodies in Mato Grosso do Sul State, Brazil. Most of the Pantanal wetland area lies within Mato Grosso do Sul State, thus representing a region of great hydrological relevance. Partial least squares-discriminant analysis (PLS-DA) was employed to discriminate samples collected at four WWTPs monitoring sites: influent, final effluent, upstream, and downstream of the discharges. The model demonstrated excellent accuracy when discriminating the influent from the effluent samples, but poor accuracy when discriminating upstream and downstream samples, indicating the high dilution capacity of the receiving water bodies as a critical factor in the water resources management. The results demonstrate the great potential of the methodology for better water resources management, which can be used in even more complex WWTP databases, allowing the assessment of effluent disposals' impacts in detail. It is recommended to use this methodology in water-limited regions to determine the effect of disposals in areas with different characteristics.

Improved global evapotranspiration estimates using proportionality hypothesis-based water balance constraints

Accurate estimation of global evapotranspiration (ET) is critical to understand the water and energy cycles in the Earth system. Satellite-driven ET algorithms serve as an effective way to estimate the global ET. However, many algorithms have been designed independently of water balance constraints, which potentially limit their ability to estimate ET in water-limited and high interception regions. As ET remains one of the most uncertain terms in the global water budgets, incorporating water balance constraints into algorithms should improve the performance of ET estimates. In this study, we developed a general solution (denoted PEW) based on the proportionality hypothesis to incorporate available water control into the widely used Priestley Taylor-Jet Propulsion Laboratory (PT-JPL) ET algorithm. Simulated performances of the PEW model and PT-JPL algorithm were evaluated against 106 FLUXNET eddy covariance (EC) towers data at the site scale. Meanwhile, model results were compared at the global scale with the means of the widely used ET products. We found that the PEW model has smaller errors than the original PT-JPL algorithm, with the greatest improvements in water-limited regions and areas characterized by the high interception. Moreover, by incorporating the water balance constraints into the ET algorithm, the PEW model has the ability to distinguish variations in ET affected by El Nino-Southern Oscillation. In summary, our study offers a convincing evidence regarding the incorporation of water balance constraints into remote sensing algorithms for more accurately mapping global terrestrial ET with an enhanced understanding of ET variation under climate change. This model is the first of its kind among remote-sensing models to provide global land ET estimation with the proportionality hypothesis-based water balance constraints.

Effects of shrub-grass cover on the hillslope overland flow and soil erosion under simulated rainfall

Vegetation plays a vital role in regulating hydrological cycle and controlling soil erosion at multiple spatial and temporal scales. Establishing shrub-grass community is one of the widely adopted practices to increase rainfall infiltration and reduce soil erosion in water-limited and highland regions. To understand the effects of such vegetation communities on soil erosion and overland flow under different rainfall regimes at the hillslope scale, we conducted rainfall simulation experiments by setting up parallel plots at fixed slope of 15° including unvegetated (coverage 0%), shrub only (coverage 50%), grass only (coverage 50%), and shrub-grass covered (coverages 25%, 50%, 75%, and 100%) and constant rainfall intensities of 30, 60, and 90 mm h−1 rainfalls lasting 60 min each after the initiation of overland flow. Two native species Lespedeza bicolor and Carex giraldiana, distributed in the soil sampling region were planted on the plots to achieve designed coverages. We found that the overland flow and sediment load from vegetated slopes were reduced by 9%–58% and 27%–98%, respectively, compared with unvegetated slopes while the infiltration rate increased by over 45%. Shrub-grass community reduced the overland flow and sediment yield more significantly than shrub only and grass only treatments with the same coverage of 50% under three rainfall intensities. In addition, the overland flow rate linearly decreased while the mean sediment yield exponentially reduced against the increase in shrub-grass community coverage. Hydrodynamically, shrub-grass communities not only increased the critical hydrodynamic forces for the initiating soil erosion but also increased the resistance coefficient leading to reduce overland flow velocity, stream power, and thus soil erosion from the vegetative slope even under extreme rainfalls. Our research highlights the importance of developing the shrub-grass communities to reduce the quantity and energy of overland flow and control soil erosion on the hillslopes in water-limited and highland regions.

Quantifying nutrient stoichiometry and radiation use efficiency of two maize cultivars under various water and fertilizer management practices in northwest China

Precise nutrient diagnosis is the basis of accurate simulation of maize (Zea mays L.) grain yield and high-efficient utilization of resources under the changing climate. However, the combined effects of supplementary irrigation and fertilizer application on plant stoichiometry of various maize cultivars in water-limited and nutrient-deficient regions are still poorly understood. A three consecutive year field experiment (2015−2017) was conducted on two maize cultivars (QL14 and ZD958) in northwest China, with two irrigation treatments (RF: rain-fed and DI: drip irrigation) and two fertilization treatments (F0: no fertilization and F1: local fertilization practice). The results showed that the radiation use efficiency for biomass yield of QL14 with fertilization under drip irrigation was increased by 7.39% than that of ZD958. DI (70.19 kg kg−1) and ZD958 (68.55 kg kg−1) obtained significantly higher physiological efficiency of nitrogen fertilizer than RF (59.52 kg kg−1) and QL14 (61.17 kg kg−1), respectively. Both plant and grain N:P ratios under F1 were higher than those under F0. QL14 had significantly higher grain N:P ratio (4.64) than ZD958 (4.26) under DIF1. Grain yield was increased obviously when grain N:P ratio and plant N:K stoichiometry were greater than 4.0 and 1.2, respectively. The newly released maize cultivar (QL14) was highly recommended as forage and cereal crops, since QL14 obtained significantly higher leaf and stem dry matter even under severe drought and low soil fertility conditions. Our study provides an opportunity to achieve high-yielding maize systems in northwest China and maybe other regions with similar environments, and the data will help inform policies and strategic plans for the effective use of water and fertilizer resources.

Detecting the Spatial Mismatch of Water Resources and Grain Planting Pattern Changes in China Based on Satellite Data

China has achieved sustained growth in grain production and significant changes in grain patterns since the early 21st century. Meanwhile, the contradiction between the shortage of water resources and the development of agriculture is becoming more and more severe. This study introduced Gravity Recovery and Climate Experiment (GRACE) gravity satellite Total Water Storage (TWS) Product to indicate total water storage and calculated the Cumulated Normalized Difference Vegetation Index (CNDVI) of cropland as an indicator for grain growth. Based on the continuous satellite data, this paper revealed the spatial mismatch between water resources supply and grain growth pattern in China. The center of gravity of the CNDVI tends to move northwest, while the GRACE TWS data’s center of gravity is in the opposite direction. There were different relationships between GRACE-TWS and CNDVI changes in different zones. We calculated the pixel-wise spatial Pearson Correlation coefficients of TWS and CNDVI. The TWS data and CNDVI data show negative correlation trends in the water-limited areas such as the northern arid-semiarid region and northern China plain, while they show a positive correlation in relatively sufficient water resources in southeast China. According to the results, the changing pattern of grain production in China is likely to cause the depletion of grain production potential in the water-limited regions, while the southeastern regions with higher potential still have more capacity for agricultural production.

Integrating the Budyko framework with the emerging hot spot analysis in local land use planning for regulating surface evapotranspiration ratio

Land use planning regulates surface hydrological processes by adjusting land properties with varied evapotranspiration ratios. However, a dearth of empirical spatial information hampers the regulation of place-specific hydrological processes. Therefore, this study proposed a Local Land Use Planning framework for EvapoTranspiration Ratio regulations (ETR-LLUP), which was tested for the developments of spatially-varied land use strategies in the Dongjiang River Basin (DRB) in Southern China. With the first attempt at integrating the Emerging Hot Spots Analysis (EHSA) with the Budyko framework, the spatiotemporal trends of evapotranspiration ratios based on evaporative index and dryness index, from 1992 to 2018, were illustrated. Then, representative land-cover types in each sub-basin were defined using Geographically Weighted Principal Component Analysis, in two wet years (1998 and 2016) and three dry years (2004, 2009, and 2018), which in turn were identified using the Standard Precipitation Index. Finally, Geographically Weighted Regressions (GWRs) were used to detect spatially-varied relationships between land-cover proportions and evaporative index in both dry and wet climates. Results showed that the DRB was consistently a water-limited region from 1992 to 2018, and the situation was getting worse. We also identified the upper DRB as hotspots for hydrological management. Forests and croplands experienced increasingly water stress compared to other vegetation types. More importantly, the spatial results of GWR models enabled us to adjust basin land use by 1) expanding and contracting a combination of ‘mosaic natural vegetation’ and ‘broadleaved deciduous trees’ in the western and eastern parts of the basin, respectively; and 2) increasing ‘broadleaved evergreen trees’ in the upstream parts of the basin. These spatially-varied land use strategies based on the ETR-LLUP framework allow for place-specific hydrological management during both dry and wet climates.

Variations in water-balance components and carbon stocks in poplar plantations with differing water inputs over a whole rotation: implications for sustainable forest management under climate change

Understanding the long-term variations of stand water balance and carbon stocks under different water inputs is crucial for sustainable forest management under climate change. However, due to the lack of in-situ data, how forest plantations respond to variation in water inputs during stand development remains poorly understood. We varied water inputs with distinct irrigation amounts and measured the water-balance components, carbon stock growth, and water productivity during a whole rotation (2015–2019) in poplar plantations. Furthermore, in 2020, soil water contents in our stand and an adjacent 37-year-old poplar plantation were measured. Under rainfed conditions, soil water storage of different layers decreased greatly year by year, especially at the 2–3 m depth, such that transpiration was curtailed in 2019, a dry year. By 2019, the 0–2 m depth layer became periodically dried, and the 2–3 m was persistently dried, which was further confirmed by observations in 2020. Additionally, serious soil desiccation occurred throughout the 0–6 m soil depth in the 37-year-old poplar stand. Increasing the water inputs avoided stand water stress and decreased the drying rate of the deep soil. Furthermore, the highest water inputs treatment brought great increases in groundwater recharge, carbon stock growth, and water productivity. This treatment also led to 67% higher soil water storage in the 0–1 m soil layer and 23% higher soil water storage in the 1–6 m layer by the end of 2019, as compared to the rainfed treatment. However, these advantages were small or disappeared if the water inputs were insufficient. Our findings will be helpful to predict water relations and facilitate sustainable forest management under climate change in water-limited regions.

An improved method of using two-dimensional model to evaluate the carrying capacity of regional water resource in Inner Mongolia of China

The evaluation of regional water resource carrying capacity has been repeatedly conducted to provide a scientific basis for the local water resource management and the sustainable development, in particular in the water-limited regions. However, the definition of regional water resource carrying capacity and its evaluation method are still arguable. Through a case study of Inner Mongolia, located in the arid and semi-arid northern China, this paper developed an improved method to calculate regional water resource carrying capacity by the combination of the water supply-demand analysis and the S-shaped curve threshold analysis. The spatial and temporal patterns of the regional water resource carrying capacity in Inner Mongolia during 2000–2019 was evaluated at three scales, namely the province scale, the basin scale and the city scale. The results showed that the average regional water resource carrying capacity of the whole province was 0.25 (the full mark is 1.00); at the basin scale, the Yellow River Basin had the lowest regional water resource carrying capacity (0.17) among all the basins, showing that the utilization of the water resources was unreasonable; at the city scale, the average regional water resource carrying capacities in Hulunbuir and Xilingol were both over 0.25, while those in Alxa, BayanNur and Wuhai were below 0.1; Hulunbuir had 25.48 billion m3 water surplus, while BayanNur suffered from an average water deficit of 4.51 billion m3 from 2000 to 2019. This paper has provided a reasonable way to measure the regional water resource carrying capacity using an improved method by incorporating S-shaped curve threshold analysis, which may have a wider application for the clustering and optimization of regional water management. In addition, the spatial and temporal patterns of regional water carrying capacity are beneficial for policymakers in the implementation of the effective water usage.

A novel ecohydrological model by capturing variations in climate change and vegetation coverage in a semi-arid region of China

Variations in vegetation are influenced by regional climate regimes and, in turn, control the water balance behavior in water-limited regions. Owing to its role in ecohydrological processes, vegetation is an essential link in modeling the relationships among climate conditions, vegetation patterns, and dynamic water balance behavior. However, previous ecohydrological models have been empirical and complex, without physically significant parameters. Here, we propose a novel ecohydrological model (a Budyko model–coupled vegetation model) that combines the impacts of climate change and vegetation variations, featuring simple and deterministic parameters. In addition to accounting for the fundamental water balance model and its factors, mean precipitation, potential evapotranspiration, runoff, and variations in water storage (δS), the model showed better performance when incorporating δS (RMSE = 2.72 mm yr−1) and its parameter ε -, which is mechanically and quantitively subject to the vegetation coverage (R2 = 0.95, p < 0.01). This was estimated as a function of vegetation potential canopy conductance, mean rainstorm depth, mean time between storms, and potential rate of evapotranspiration in a semi-arid watershed with impulsive precipitation in China (R2 = 0.80, p < 0.01). The model also found that vegetation growth was mainly controlled by soil water content and decoupled the impact of the total amount of precipitation on vegetation in the northeastern area of the watershed. Hence, our method presents a new tool for building an ecohydrological model that includes deterministic parameters of mechanical significance.

Assessment of ecological restoration projects under water limits: Finding a balance between nature and human needs

Revegetation has significantly contributed to improvements in ecosystem services, such as carbon sequestration and soil retention. Yet, vegetation expansion in water-limited regions may generate conflict of water demand between nature and humans. Present studies are still lacking when it comes to identifying the permissible vegetation capacity, i.e. net primary productivity (NPP) threshold, based on the local water resources limits, and further proposing adjustment and optimization strategies to keep water use balanced in anthropogenic-biological systems. Under such a circumstance, this study assesses the difference between the actual NPP and NPP thresholds at regional and ecosystem scales in China. The results show that 8 out of 31 provinces have their provincial actual NPP above the regional NPP thresholds, mainly concentrated in northern China between 400 and 800 mm iso-precipitation line, i.e. North China Plain (Beijing-Tianjin-Hebei), the middle reaches of the Yellow River Basin (Shaanxi and Henan provinces), and the Northeast China (Heilongjiang, Jilin and Liaoning). Forest ecosystems dominate the difference between the actual total woodland and grassland ecosystems NPP and the permissible NPP thresholds in these regions, ranging from 67% (Beijing) to 99% (Tianjin). If the current vegetation intensity in these regions remains unchanged, the areas of woodland and grassland ecosystems should be optimized 0–48% and 0–100% of their present areas to balance the water demand between the ecosystems and humans, without considering the potential consequence of climate change and soil erosion. Although 23 provinces have their regional actual NPP below their permissible NPP thresholds, 6 out of 23 provinces still have their woodland and grassland ecosystems NPP above the corresponding NPP thresholds, mainly focusing on the Northwestern China north over the 400 mm iso-precipitation line, including Inner Mongolia, Qinghai, Hainan, Shanxi, Gansu and Xinjiang. Forest ecosystems also dominate the negative NPP differences in these regions, ranging from 91% (Inner Mongolia) to 46% (Gansu). These reveal the hidden and potential pressure in the 6 provinces to balance limited water resources in the local anthropogenic-biological system. This study provides a method to assess the water-resources permissible NPP threshold and further proposes the specific adjustment and optimization plans for the areas with actual NPP above the corresponding NPP thresholds, which can provide guidance for ecological restoration program implementations in a more sustainable way.

The Stem Sap Flow and Water Sources for Tamarix ramosissima in an Artificial Shelterbelt With a Deep Groundwater Table in Northwest China.

The shelterbelt forest between oases and the desert plays a vital role in preventing aeolian disasters and desertification in arid regions of northwest China. Tamarix ramosissima (T. ramosissima), a typical perennial and native xerophyte shrub in Northwest China, grows naturally and is widely used in building artificial shelterbelt forests. The balance between water consumption and the availability of water determines the survival and growth of T. ramosissima. How T. ramosissima copes with extremely low rainfall and a deep groundwater table remains unknown. To answer this, the transpiration and the water sources of T. ramosissima were investigated by the heat balance and oxygen isotopic analysis method, respectively. Our results show that the daily T. ramosissima stem sap flow (SSF) was positively correlated with air temperature (Ta), photosynthetically active radiation (PAR), and the vapor pressure deficit (VPD). We found no significant relationship between the daily SSF and soil moisture in shallow (0–40 cm) and middle (40–160 cm) soil layers. Oxygen isotope results showed that T. ramosissima mainly sources (>90%) water from deep soil moisture (160–400 cm) and groundwater (910 cm). Diurnally, T. ramosissima SSF showed a hysteresis response to variations in PAR, Ta, and VPD, which suggests that transpiration suffers increasingly from water stress with increasing PAR, Ta, and VPD. Our results indicate that PAR, Ta, and VPD are the dominant factors that control T. ramosissima SSF, not precipitation and shallow soil moisture. Deep soil water and groundwater are the primary sources for T. ramosissima in this extremely water-limited environment. These results provide information that is essential for proper water resource management during vegetation restoration and ecological reafforestation in water-limited regions.

Surface temperatures reveal the patterns of vegetation water stress and their environmental drivers across the tropical Americas.

Vegetation is a key component in the global carbon cycle as it stores ~450 GtC as biomass, and removes about a third of anthropogenic CO2 emissions. However, in some regions, the rate of plant carbon uptake is beginning to slow, largely because of water stress. Here, we develop a new observation-based methodology to diagnose vegetation water stress and link it to environmental drivers. We used the ratio of remotely sensed land surface to near surface atmospheric temperatures (LST/Tair ) to represent vegetation water stress, and built regression tree models (random forests) to assess the relationship between LST/Tair and the main environmental drivers of surface energy fluxes in the tropical Americas. We further determined ecosystem traits associated with water stress and surface energy partitioning, pinpointed critical thresholds for water stress, and quantified changes in ecosystem carbon uptake associated with crossing these critical thresholds. We found that the top drivers of LST/Tair , explaining over a quarter of its local variability in the study region, are (1) radiation, in 58% of the study region; (2) water supply from precipitation, in 30% of the study region; and (3) atmospheric water demand from vapor pressure deficits (VPD), in 22% of the study region. Regions in which LST/Tair variation is driven by radiation are located in regions of high aboveground biomass or at high elevations, while regions in which LST/Tair is driven by water supply from precipitation or atmospheric demand tend to have low species richness. Carbon uptake by photosynthesis can be reduced by up to 80% in water-limited regions when critical thresholds for precipitation and air dryness are exceeded simultaneously, that is, as compound events. Our results demonstrate that vegetation structure and diversity can be important for regulating surface energy and carbon fluxes over tropical regions.

Limited role of soil texture in mediating natural vegetation response to rainfall anomalies

Evidence suggests that the response of rainfed crops to dry or wet years is modulated by soil texture. This is a central tenet for certain agronomic operations in water-limited regions that rely on spatial distribution of soil texture for guiding precision agriculture. In contrast, natural vegetation in climatic equilibrium evolves to form a dynamic assemblage of traits and species adapted to local climatic conditions, primarily precipitation in water-limited regions. For undisturbed landscapes, we hypothesize that natural vegetation responds to rainfall anomalies irrespectively of local soil texture whereas rainfed crops are expected to respond to texture-mediated plant available water. Earth system models (ESMs) often quantify vegetation response to drought and water stress based on traditional agronomic concepts despite fundamental differences in composition and traits of natural vegetation and crops. We seek to test the hypothesis above at local and regional scales to differentiate natural vegetation and rainfed crops response to rainfall anomalies across soil types and better link them to water and carbon cycles. We employed field observations and remote sensing data to systematically examine the response of natural and rainfed cropped vegetation across biomes and scales. At local scales (field to ∼0.1 km), we used crop yields from literature data and natural vegetation productivity as gross primary productivity (GPP) from adjacent FLUXNET sites. At regional scales (∼102 km), we rely exclusively on remote-sensing-based GPP. Results confirm a lack of response of natural vegetation productivity to soil texture across biomes and rainfall anomalies at all scales. In contrast, crop yields at field scale exhibit correlation with soil texture in dry years (in agreement with conventional agronomic practices). These results support the hypothesis that natural vegetation is decoupled from soil texture, whereas rainfed crops retain dependency on soil texture in dry years. However, the observed correlation of crops with soil texture becomes obscured at larger scales by spatial variation of topography, rainfall, and uncertainty in soil texture and GPP values. The study provides new insights into what natural vegetation’s climatic equilibrium might mean and reveals the role of scale in expressing such sensitivities in ESMs.

Probabilistic assessment of vegetation vulnerability to drought stress in Central Asia

The increasing frequency and intensity of droughts in a warming climate are likely to exacerbate adverse impacts on ecosystems, especially for water-limited regions such as Central Asia. A quantitative understanding of the impacts of drought on vegetation is required for drought preparedness and mitigation. Using the Global Inventory Modeling and Mapping Studies NDVI3g data and Standardized Precipitation Evapotranspiration Index (SPEI) from 1982 to 2015, we evaluate the vegetation vulnerability to drought in Central Asia based on a copula-based probabilistic framework and identify the critical regions and periods. Furthermore, a boosted regression trees (BRT) model was also used to explore the relative importance of environmental factors and plant traits on vegetation response to drought. Additionally, we also investigated to what extent irrigation could alleviate the impacts of drought. Results revealed that months from June to September was the critical period when vegetated areas were most vulnerable to drought stress. The probabilities of vegetation loss below 20th quantile under extremely dry in these months were 68.7%, 69.4%, 71.0%, and 67.0%, respectively. Regarding vegetation-vulnerable regions, they shifted with different growth stages. During the middle of the growing season, semi-arid areas were the most vulnerable regions, whereas the highest drought-vulnerable regions were observed in arid areas during other periods. The BRT results showed that plant traits accounted for a large fraction (58.9%) of vegetation response to drought, which was more important than ambient soil environment (20.8%). The analysis also showed that mitigations from irrigation during July to September were smaller than in other months. The results of this paper provide insight into the influences of drought on vegetation and may contribute to drought mitigation and land degradation measures in Central Asia under accelerating global warming.