Dry Summer Research Articles

Climate Change Impacts on Water Resources in the Danube River Basin: A Hydrological Modelling Study Using EURO-CORDEX Climate Scenarios

Climate change affects the hydrological cycle of river basins and strongly impacts water resource availability. The mechanistic hydrological model PROMET was driven with an ensemble of EURO-CORDEX regional climate model projections under the emission scenarios RCP2.6 and RCP8.5 to analyze changes in temperature, precipitation, soil water content, plant water stress, snow water equivalent (SWE) and runoff dynamics in the Danube River Basin (DRB) in the near (2031–2060) and far future (2071–2100) compared to the historical reference (1971–2000). Climate change impacts remain moderate for RCP2.6 and become severe for RCP8.5, exhibiting strong year-round warming trends in the far future with wetter winters in the Upper Danube and drier summers in the Lower Danube, leading to decreasing summer soil water contents, increasing plant water stress and decreasing SWE. Discharge seasonality of the Danube River shifts toward increasing winter runoff and decreasing summer runoff, while the risk of high flows increases along the entire Danube mainstream and the risk of low flows increases along the Lower Danube River. Our results reveal increasing climate change-induced discrepancies between water surplus and demand in space and time, likely leading to intensified upstream–downstream and inter-sectoral water competition in the DRB under climate change.

Eco-assessment of streams of Konya closed river basin (Turkey) using various ecoregional diatom indices.

Ecological assessment of freshwater ecosystems based on diatom metrics is an important issue for attaining environmental sustainability. The present study aimed to evaluate differences in the diatom-stressor relationship in relatively least disturbed streams in the Konya closed river basin using multivariate analyses and to bio-assess streams by the application of different ecoregional diatom indices. Cocconeis euglypta, Cymbella excisa, Cocconeis placentula, and Achnanthidium minutissimum are the most contributing species to the dissimilarity of sampling stations between rainy (spring) and dry (summer and fall) seasons and also between altitude (A2 800- < 1600m and A3 ≥ 1600m) groups. The first two axes of canonical correspondence analysis revealed a significant (82.8%) relationship between diatom species and stressors. Diatom species displayed distinct responses to environmental variables (electrical conductivity, Ni, Cu, B, and altitude) playing important roles on the distribution of species. Diatom indices indicate different ecological statuses of stations, from bad to high. European diatom indices except Duero Diatom Index (DDI) and Trophic Diatom Index (TDI) showed good responses to the eco-assessment of streams and indicated high ecological status for the least disturbed sampling stations symbolized as S16, S20, S24, S25, S27-29, S37, and S39. These results were also supported by abiotic evaluation. Although TIT was more competitive in the bio-assessment of streams among diatom indices, it is necessary to increase its species list by determining their trophic weights in future studies. Therefore, the use of ecoregion-specific diatom indices is suggested along with increasing the number of used species to correctly interpret the water quality.

Risk of Salinization in the Agricultural Soils of Semi-Arid Regions: A Case Study from Moldavian Plain (NE Romania)

In the present study, the salinization trends of different soil types from a small hydrographic basin situated in NE Romania (Roșior basin) are investigated. The climatic conditions are favorable for long dry summers and intensive evaporation. The soils are developed on immature evolved clay sediments, as revealed by geochemical analysis. The salinity varies among soil types, attaining maximum values for Solonetz with total dissolved salts (TDS) between 1165.75 to 1881.25 mg/L. The aqueous solutions are represented by natural water and soil solution. The hydrogeochemical facies of the natural waters change from HCO3−—Mg2+ in the upper basin to SO42−—Na+ in the middle or lower basin. The soil solution is moderately or strongly salinized and shows anionic variations from HCO3− to SO42− in the studied profiles, whereas Na+ is always the main cation. The concentrations of Na+ and SO42− evolve simultaneously. Raman spectroscopic exploration of the white efflorescences, which occur on topsoil, reveals the presence of thenardite as the dominant phase. The composition of soil solution results from both the ionic exchange and evaporation processes. The nature of soil solution mineralization and summer temperatures are two main factors that interact and promote the thenardite precipitation. The soil salinization induces negative effects on crop nutrition, impacting further the crop yields. The results of this study can be extrapolated to larger areas formed on Sarmatian sedimentary deposits affected by salinization processes.

Carbon uptake by Douglas-fir is more sensitive to increased temperature and vapor pressure deficit than reduced rainfall in the western Cascade Mountains, Oregon, USA

Understanding how trees respond to drought is critical to understanding forest sensitivity to global climate change, which can help inform forest policy and management decisions. However, mechanisms governing carbon fixation and water fluxes in response to increased temperatures and water limitation in regions with Mediterranean climates, with wet winters and dry summers, remain only partially understood. We tested the effect of increased vapor pressure deficit (VPD) and decreased rainfall on water and carbon fluxes of Douglas-fir (Pseudotsuga menziesii) trees using the Soil-Plant-Atmosphere model (SPA). We simulated a 50-year-old Douglas-fir stand on the western slopes of the Cascade Mountains in Oregon, USA. Simulation results showed that increasing the daily maximum VPD by 0.25–2.5 kPa during the summer increased cumulative transpiration by 1–3% and decreased cumulative gross primary production by 3–25%. In contrast, decreasing rainfall by 10–100% during the spring and summer decreased cumulative transpiration by 2–16% and decreased cumulative gross primary production by 0.5–4%. Transpiration was highly sensitive to decreases in rainfall, especially in late spring and early summer but much less sensitive to increases in maximum daily VPD. In contrast, gross primary productivity was much more sensitive to VPD, with summertime increases in VPD having a 5- to 6-fold greater effect on gross primary productivity than did decreasing the rainfall. These results suggested that temperature increases expected from climate change in combination with increases in VPD are likely to reduce forest productivity regardless of soil moisture availability.

Plant and herbivorous insect communities respond in complex ways to rainfall manipulation in an oak savanna grassland

Abstract Changes in precipitation due to climate change will have consequences for plant and herbivorous insect communities alike. Multiple hypotheses explain how changes in plant diversity and productivity can lead to changes in herbivore community composition. Yet as rainfall patterns change, the bottom‐up effects on the relationships between plant and herbivore communities are less well understood. Using a long‐term rainfall manipulation experiment in a remnant patch of Garry oak (Quercus garryana) savanna, we examined how plant diversity and productivity have responded to variation in soil moisture over 6 years. This highly endangered ecosystem is predicted to experience significantly wetter springs and drier summers by 2080. We also investigated plant‐mediated, indirect effects of manipulated rainfall on herbivore diversity and abundance, drawing on multiple hypotheses describing the relationships between plant and herbivore communities. For example, the more individuals hypothesis predicts that increased plant productivity results in increased herbivore abundance which, in turn, results in increased herbivore diversity. We found that plant productivity was influenced by soil moisture, but the direction and magnitude of the response varied across years, and found no support for plant diversity influencing productivity. We also found that the cover and productivity of grasses increased significantly with increasing precipitation. In addition to a direct negative effect on herbivore diversity, soil moisture had an indirect negative effect on herbivore abundance, via the negative effect of plant productivity on abundance, contradicting the more individuals hypothesis. Synthesis. Our results highlight that not only can drought result in significant reductions in plant productivity in this threatened ecosystem, but that these changes will also result in increases in herbivore abundance. In contrast, where soil moisture is higher, grasses will become more dominant resulting in decreased herbivore abundance. Ultimately, predicting how this system responds to changes in precipitation will depend on the ability to predict whether growing season soil moisture will be consistently drier or wetter in the future, a significant challenge. Going forward, investigating how variation in precipitation due to climate change affects the links between trophic levels, including how herbivores affect plant communities, remains critical for understanding ecosystem processes and stability.

Both climate sensitivity and growth trend of European beech decrease in the North German Lowlands, while Scots pine still thrives, despite growing sensitivity

Key messageBeech and pine respond differently to climate change. June precipitation is of particular influence for beech, and February/March temperature for pine.Climate warming exposes forests to increasing abiotic stress, demanding for difficult silvicultural decisions about the right choice of future timber species. Scots pine (Pinus sylvestris) and European beech (Fagus sylvatica) are major timber species in the North German Lowlands, which have suffered from recent hot droughts, thus raising concern about their suitability for future production forests in the region. We investigated the climate sensitivity of tree growth and long-term growth trends of ten paired beech and pine forests along a precipitation gradient in the North German Lowlands with the aim to compare the species’ climate sensitivity and to search for species-specific climatic thresholds. In the majority of beech stands, basal area increment (BAI) has lost its positive trend since the 1980s or growth declined since then, while the BAI of pine has continually increased. Long-term change in June precipitation is in the study region a more important determinant of beech growth trends than the amount of MAP, while pine growth is largely dependent on the warmth of February/March. Yet, pine growth is also sensitive to dry mid summers, with sensitivity increasing toward low MAP. Climate sensitivity of growth has significantly declined since the 1980s in beech, while the dominant drought signal of June persisted in pine. We conclude that recent climate change is affecting radial growth of beech and pine differently with both species revealing signs of vulnerability to hot droughts, suggesting for the drier part of the study region the preference of more drought-tolerant hardwood timber species over beech and pine.

Ecosystemic resilience of a temperate post-fire forest under extreme weather conditions

IntroductionThe effects of climate change are exacerbating the fire risk in forests worldwide. Conifer plantations in particular are especially vulnerable to fire outbreaks. At the end of the extraordinarily hot and dry summer of 2018, a forest pine plantation burned in Brandenburg, NE Germany. Different forestry interventions were carried out after the fire, while one area of the damaged plantation remained untouched.MethodsWe investigated the resilience of the forest ecosystem and the effectiveness of different active and passive forest restoration measures during the subsequent relatively warm and dry years 2019–2021.ResultsOne year after the fire, Populus tremula showed strong spontaneous colonization at all sites. In contrast, the majority of planted Pinus sylvestris plantlets died on the plots that had been salvage-logged after the fire. Three years after the fire, Populus tremula successfully established itself as the dominant tree species on all plots, with the highest abundance on the plot where the overstorey of the dead pines was left. Betula pendula, Salix caprea, and Pinus sylvestris showed lower abundance, with their proportion increasing with decreasing cover by dead trees. The distribution of regrowing trees is very heterogeneous across the different treatments and plots. In the clear-cut plots, the extreme microclimatic conditions expose the young trees to additional heat and drought, while the retention of deadwood measurably buffers the temperature and water stress.DiscussionThe resilience and adaptability of naturally regenerating forests that develop into ecosystems that are more diverse seem more promising than restoration through intervention. Apart from hampering restoration under extreme weather conditions, post-fire salvage logging contributes to soil degradation and loss of organic carbon.

Characterizing seasonal differences in hydrological flow paths and source water contributions to alpine tundra streamflow

AbstractAlpine headwaters collect larger volumes of precipitation per unit area than neighbouring lowlands, recharge regional aquifers, and generate a greater proportion of river discharge than their limited extent would suggest. Despite the importance of alpine headwaters, field observations and assessments of source water contributions to streamflow in alpine tundra settings are sparse throughout the subarctic and absent in the Taiga Cordillera ecozone specifically. As such, it remains uncertain how changes to seasonally specific hydrological processes control discharge of the larger rivers to which these alpine headwaters contribute. This study quantified variability in source water contributions and flow paths during the 2019 open water season within a Mackenzie Mountain alpine tundra basin based on measurements of stable water isotopes, specific conductivity (SPC), and water volumes during runoff generating events. During the freshet, large daily snowmelt volumes resulted in the greatest volume of streamflow, which was composed mainly of pre‐event water (~92%). As the summer progressed, evapotranspiration increased, and groundwater flow paths extended, resulting in reduced event water fractions and hydrograph amplitude, and an extended duration of streamflow response. A headwater subbasin within the larger study basin was both hydrologically and isotopically unresponsive to summer rains, and instead was characterized by a delayed hydrograph response and reduced event water fraction. Results indicate this portion of the catchment was regulated by discharge from groundwater springs capable of sustaining streamflow before snowmelt commenced and during the dry summer months. As climate change continues, greater precipitation volumes and a longer open water season will likely result in reduced runoff and stream discharge from alpine basins as greater evapotranspiration and channel bed infiltration occur. This study provides a valuable data set and observations of seasonally distinct runoff generation processes to inform prediction of changes in northern alpine tundra hydrology in response to a warming climate.

SPI-3 Analysis of Medjerda River Basin and Gamma Model Limits in Semi-Arid and Arid Contexts

The Standardized Precipitation Index SPI-3, associated with three months of rainfall accumulation, is a drought index for detecting immediate drought impacts. The two-parameter gamma distribution, recommended by the World Meteorological Organization as the underlying distribution for estimating SPI, has shown limits in semi-arid and arid conditions with respect to the normality test for the resulting SPI series. Our purpose was to evaluate its relevance for the Medjerda River Basin (Tunisia), a transboundary basin where the climate classes are temperate, dry, and hot summer, as well as arid hot desert and arid hot steppe. When analyzing the time series of 144 stations from 1950 to 2018, we found that the normality Shapiro–Wilk test was rejected in 17% of the cases, which agreed with the literature review results. The transition season (August, September, and October) had the highest rejection percentage. Three factors were identified to explain the deviation from normality. We first identified the rate of occurrence of completely dry (zero rain) three-month periods. The higher the rate of occurrence was, the higher that the probability was of its rejecting the normality test. High sample skewness was the second influencing factor. Finally, a series where the Grubbs’ test of identifying outliers was rejected was more likely to show the SPI-3 series deviating from normality.

Seasonal variations in surface processes and hydroclimate on an alpine lake, NE Tibetan Plateau

Koucha Lake, located in Qinghai Province, north-eastern Tibetan Plateau (NETP) at an elevation of 4530 m a.s.l. Occupies an ideal position to investigate the seasonal hydroclimatic variations and related surface processes during the Holocene. We re-investigated the lake and analysed a new sediment core from the deepest part of the lake. Grain size measurements and related endmember modelling analyses (EMMA) in combination with ostracod associations together with geochemical parameters and mineral compositions from core sediments were used as relevant proxies for hydro-climatic reconstruction.A comparison of δ18O/δD from various waters on the NETP indicates a clear depleted isotopic signature of non-monsoon moisture supply in comparison to summer rainfalls, highlighting the summer evaporation for δ18O enrichment, as a reference for interpreting δ18O values from lake sediments for various situations of hydrological balance. Based on the individual core chronology the multi-proxy data indicate a very low sedimentation rate over the entire period in accordance with the catchment topography and the absence of well-developed and constant river systems, limiting the flux of sediment supply to the lake. Thus, Koucha Lake remains almost a closed system since the Lateglacial, except for the early mid-Holocene part of the sequence. The coarser particle components and their representative endmembers for spring flood and permafrost thaw between 11.9 and 10.1 cal Ka BP and from 4.5 to ca. 1.0 cal Ka BP point to lower lake levels, influenced by long winter duration with dry summer interruptions, and lighter oxygen isotopes were preserved at low evaporations. Fine-grained sediments supplied by permafrost thaw associated with activating summer surface erosion led to lake level rise between 10.1 and 4.5 cal Ka BP and during the last 1000 years, with a maximum during 7.6–7.1 cal Ka BP. Their enriched δ18O values indicate extended summer duration and concomitant evaporation enhancement, albeit a positive hydrological balance. We suggest that the interannual latitudinal shifts of the intertropical convergence zone (ITCZ) and westerly Jet and attendant varying monsoon rain belt locations are responsible tracers for the variation of the hydrological budget of the lake and the variable spatio-temporal precipitation patterns in humid and (semi) arid regions in China.

Impact of Environmental Gradients on Phenometrics of Major Forest Types of Kumaon Region of the Western Himalaya

Understanding ecosystem functional behaviour and its response to climate change necessitates a detailed understanding of vegetation phenology. The present study investigates the effect of an elevational gradient, temperature, and precipitation on the start of the season (SOS) and end of the season (EOS), in major forest types of the Kumaon region of the western Himalaya. The analysis made use of the Normalised Difference Vegetation Index (NDVI) time series that was observed by the optical datasets between the years 2001 and 2019. The relationship between vegetation growth stages (phenophases) and climatic variables was investigated as an interannual variation, variation along the elevation, and variation with latitude. The SOS indicates a delayed trend along the elevational gradient (EG) till mid-latitude and shows an advancing pattern thereafter. The highest rate of change for the SOS and EOS is 3.3 and 2.9 days per year in grassland (GL). The lowest rate of temporal change for SOS is 0.9 days per year in mixed forests and for EOS it is 1.2 days per year in evergreen needle-leaf forests (ENF). Similarly, the highest rate of change in SOS along the elevation gradient is 2.4 days/100 m in evergreen broadleaf forest (EBF) and the lowest is −0.7 days/100 m in savanna, and for EOS, the highest rate of change is 2.2 days/100 m in EBF and lowest is −0.9 days/100 m in GL. Winter warming and low winter precipitation push EOS days further. In the present study area, due to winter warming and summer dryness, despite a warming trend in springseason or springtime, onset of the vegetation growth cycle shows a delayed trend across the vegetation types. As vegetation phenology responds differently over heterogeneous mountain landscapes to climate change, a detailed local-level observational insight could improve our understanding of climate change mitigation and adaptation policies.

Damage caused by an exceptionally warm and dry early summer on newly planted Norway spruce container seedlings in Nordic boreal forests

With climate change, drought and warm growth conditions in the spring and early summer are predicted to become more common in Nordic boreal zones in the future. Such conditions occurred in Finland in June and July 2021, offering a good opportunity to study the field performance of seedlings and which factors, including the weather, regeneration site, and operational or seedling level factors, affected the field performance of the seedlings. In the survey, 65 regeneration sites planted with Norway spruce (Picea abies (L.) Karst.) container seedlings from May to July 2021 were randomly selected from different parts of South and Central Finland (60–64°N), and several variables were assessed in sample plots from these sites in September–October 2021. On average, 6 % of the seedlings died, and 26 % were damaged during the first growing season. In most cases, the damage was caused by drought. The most important factors affecting drought risk were the planting period and packaging and storage methods of the seedlings: in freezer-stored seedlings, the probability of drought damage was <0.20 for all planting periods, except for July; when the seedlings overwintered outdoors and were delivered to the forest in open trays, the risk of damage was <0.25 when they had been planted in May and early June, but in late June, it was > 0.60. When the seedlings had overwintered outdoors and were then packed in closed packaging, the risk was >0.20, regardless of the planting period. A low previous-year height of the seedlings (especially in the outdoor stored seedlings), precipitation a week before planting, and a low average temperature two weeks after planting reduced the risk of drought damage. When the average air temperature was below 15 °C (i.e., during the May plantings), the shade of the nearby stand slightly reduced the drought risk, but in higher temperatures, shade did not affect it. The effect of the pre-planting precipitation was stronger in coarse-textured and peat soils than in medium-coarse soils. The risk of drought damage was lowest when the freezer-stored seedlings were planted in good quality mineral soil mounds, deep enough in medium-rich (MT type) and medium-coarse soils. It is impossible to completely prevent drought damage in newly planted Norway spruce seedlings, but by optimizing operational choices and measures, the development of full density fast-growing young Norway spruce forests can also be ensured in a warming climate.

Modes of summer temperature intraseasonal oscillations and heatwaves over the Indian region

Over the Indian region, the pre-monsoon (i.e. April–May) is a dry summer season. The heatwaves, as well as local temperature variations during this season, are not associated with significant large-scale convective heating like the monsoonal modes, and several studies identified several drivers of heatwaves. Heatwaves are extreme events. Are these extremes arising from low-frequency intraseasonal modes, in the same way, extreme rainfall occurs on a synoptic or intraseasonal mode during monsoon? Studies do not explicitly point out the existence of temperature intraseasonal modes during April–May over the Indian region, and it is not clear if some of the drivers of heatwaves can also explain the April–May temperature variations as derivative of some modes. This study identifies the dominant pair of the intrinsic mode of temperature intraseasonal oscillations, which can also explain the heatwave spikes. The empirical orthogonal function based modes are isolated in the detrended surface temperature data to remove the global warming mode. It was found that the subtropical jet acting as a Rossby wave guide drives the first mode with pan India spatial modal signature, while the second mode is driven by the extratropical Rossby wave modes originating from the latitudes of the eddy-driven jet. Another important result is that the first (second) mode principal component shows a significant decreasing (increasing) trend from 1981–2020 period. The observed spatial heterogeneity in warming and the trend in the spatial distribution of extreme temperature events in India could also be explained by the trend in the two modes of oscillation.

Land‐Atmosphere Cascade Fueled the 2020 Siberian Heatwave

AbstractA heatwave in Siberia starting in January 2020, initiated by a wave 5 pattern in the jet stream, caused the surface air temperature to reach 38°C in June with important impacts on ecosystems and water resources. Here we show that this dynamical setup started a chain of events leading to this long‐lasting and unusual event: positive temperature anomalies over Siberia caused early snowmelt, leading to substantial earlier vegetation greening accompanied by decreased soil moisture and browning in the summer. This soil moisture depletion and vegetation browning, in turn, increased the impact of the heatwave on the atmosphere through a land‐atmosphere feedback. This line of evidence suggests that large‐scale dynamics and land‐atmosphere interactions both contributed to the magnitude and persistence of this record‐breaking heatwave, in addition to the background global warming impact on mean temperature. Here, we describe a carry‐over effect in Siberia from a spring positive temperature anomaly into summer dryness and browning, with retroaction into the atmosphere. With the Arctic warming twice as fast as the global average, this event foreshadows the future of northern latitude continents and emphasizes the importance of both atmospheric dynamics and land‐atmosphere interactions in the future as the climate changes. More frequent similar events could have major consequences on the carbon cycle in these carbon‐rich northern latitude regions.

Impact of climate change on the degradation of rendered façades: Expectations for a dry and hot summer temperate climate

Climate change is accelerating and can seriously affect the built environment, if not properly acknowledged in the design and maintenance stages. Despite the globally expected warming, climate changes will differ depending on region and climate type. What implications will temperature, precipitation and wind changes have on the degradation of the building envelope, in Southern Europe? How will the performance of façade claddings be affected by the end of the century? The purpose of this research is to determine the expected impact of climate change on the degradation of rendered façades, in a dry and hot summer temperate climate. This study is based on the complex relationship between climate parameters projections for Portugal and the known effects of environmental agents on the degradation of renderings. Projections indicate the increase of mean temperature and heat waves, reduction of precipitation and wet days, intensification of rainfall in shorter periods and decrease of wind speed in most of the country. Notwithstanding the projections’ inherent uncertainty, more cracking and fewer stains are expected. Due to the cracking increase, more loss of adhesion is probable. Further research is necessary to assess the risks of intense rainfall events to rendered façades, considering the also projected precipitation decrease.

Suitability of Fagus orientalis Lipsky at marginal Fagus sylvatica L. forest sites in Southern Germany

European beech (Fagus sylvatica L.) is the most important tree species in Central Europe and is considered to be relatively resistant to climate warming. However, dry summers in the last five years led to considerable damage in beech stands in Southern Germany (SG). Assisted migration of drought resistant beech provenances including those of Oriental beech (Fagus orientalis Lipsky) may help to stabilise Central European beech forests under climate change. The focus of this study is to compare the climatic ranges of F. sylvatica and F. orientalis using quantile distribution of climatic variables based on WorldClim data at forest sites within their natural distribution area. Temperature, precipitation, and aridity quantile ranges showed that F. orientalis is better adapted to warmer and drier climate compared to F. sylvatica. The quantile distribution method was applied to the whole range of the species to map the habitat suitability for both species at marginal sites in the target region (SG) in the current climatic scenario (1970-2000) and in a warmer scenario (+2°C) using the climate marginality index (CMI), i.e., the distance of sites to the xeric edge at low-latitude and low-altitude distribution limits for the species. To this purpose we applied the simple BIOCLIM algorithm using annual temperature and precipitation as climatic variables. According to our results, F. orientalis seems a promising species with a high potential for future afforestation activities in Southern Germany, especially at marginal sites of European beech forests. However, before introducing F. orientalis on a larger scale in the study area, further research on the species ecology and genetics are needed. For further application of the quantile range method, we produced tables of the vigintiles of the climatic range for both species, which can be used for estimating CMI based on WorldClim data in other regions.

Characteristics of water vapor sources and precipitation contributions to drought and wet events on the Chinese Loess Plateau

In this paper, the standardized precipitation evapotranspiration index (SPEI) at the monthly scale was derived based on the ground observations to obtain different drought and wet events at the surface of the Loess Plateau in summer from 2000 to 2017. Using the National Centers for Environmental Prediction (NECP) Final Analysis (FNL) data for the same period, backward simulations with the FLEXPART model were performed to obtain the water vapor transport characteristics and potential evaporative sources corresponding to different dry and wet events. Finally, the contribution of water vapor sources to precipitation on the Loess Plateau was obtained. The results show that the primary sources of water vapor on the Loess Plateau during dry and wet summer events are concentrated in the Loess Plateau and the Yunnan-Kweichow Plateau regions and sporadically distributed in the Altay region of Xinjiang, as well as at the junction of China, Kazakhstan and Mongolia. The evaporation extent and intensity in northern Xinjiang and Mongolia increase when the surface experiences wet-normal-dry events. The local internal circulation on the Loess Plateau contributes to 31.3% and 36.8% of the water vapor in drought and wet events, respectively. Influenced by the westerly wind belt and southwest monsoon, the precipitation contributions of the Yunnan-Kweichow Plateau are 11.2% and 13.1%, respectively, and those of Mongolia are 14.6% and 11.9%, respectively. The water vapor contributions from the Arabian Sea and Bay of Bengal in the south are insignificant. The results aid in understanding the mechanisms of drought and wet events on the Loess Plateau.

Soil self-cleaning capacity: Removal of organic compounds during sub-surface irrigation with sewage effluent

Globally, the reuse of treated sewage effluent for irrigation purposes is increasingly encouraged as a practical solution against the mismatch between the demand for and availability of freshwater resources. The reuse of sewage effluent for sub-surface irrigation (SSI) in agriculture serves the dual purpose of supplying water to crops and diminishing emissions of contaminants of emerging concern (CoECs) into surface water. To investigate such reuse, in a real scale cropland with SSI using sewage effluent, from September 2017 to March 2019 including the extremely dry year 2018, residues were followed of 133 CoECs as related to their physicochemical properties and quantified by liquid chromatography coupled to high-resolution mass spectrometry. Of the 133 target CoECs, 89 were retrieved in the field, most non-detect CoECs have low persistency. During the growing season with sub-surface irrigation, CoECs spread to the shallow groundwater and rhizosphere. Significantly lower concentrations are found between infiltration pipes as compared to directly next to the pipes in shallow groundwater for all persistency-mobility classes. CoECs belonging to the class pm (low persistency and low mobility) or class PM (high persistency and high mobility) class show no change amongst their removal in the rhizosphere and groundwater in a dry versus normal year. CoECs belonging to the class pM (low persistency and high mobility) show high seasonal dynamics in the rhizosphere and shallow groundwater, indicating that these CoECs break down. CoECs of the class Pm (high persistency and low mobility) only significantly build up in the rhizosphere next to infiltration pipes. Climatic conditions with dry summers and precipitation surplus and drainage in winter strongly affect the fate of CoECs. During the dry summer of 2018 infiltrated effluent is hardly diluted, resulting in significantly higher concentrations for the CoECs belonging to the classes pM and Pm. After the extremely dry year of 2018, cumulative concentrations are still significantly higher, while after a normal year during winter precipitation surplus removes CoECs. For all persistency-mobility classes in the shallow groundwater between the pipes, we find significant removal efficiencies. For the rhizosphere between the pipes, we find the same except for Pm. Next to the pipes however we find no significant removal for all classes in both the rhizosphere and shallow groundwater and even significant accumulation for Pm. For this group of persistent moderately hydrophobic CoECs risk characterization ratio's were calculated for the period of time with the highest normalized concentration. None of the single-chemical RCRs are above one and the ΣRCR is also far below one, implying sufficiently safe ambient exposures. Overall the deeper groundwater (7.0–11.8 m below soil surface) has the lowest response to the sub-surface irrigation for all persistency-mobility. When adopting a SSI STP effluent reuse system care must be taken to monitor the CoECs that are (moderately) hydrophobic as these can build up in the SSI system. For the deeper groundwater and for the discharge to the surface water, we find significant removal for the pM and the PM class but not for other classes. In conclusion, relatively high removal efficiencies are shown benefiting the surface waters that would otherwise receive the STP effluent directly.

Factors Driving Autumn Quercus Flowering in a Thermo-Mediterranean Area

The flowering period of plants is a critical time since it determines their reproductive success. Flowering is controlled by different factors including genetic regulation and environmental conditions. In the Mediterranean area, favourable conditions usually occur in spring, when most plant species flower including those of the Mediterranean Quercus genus. This paper reveals and analyses an unusual and lesser-known phenomenon occurring in the two main Mediterranean agroforestry ecosystems of South Europe, the Mediterranean forest and “dehesa”, that is, a second flowering occurring in autumn for the species Quercus ilex subsp. ballota (holm oak). The continuous pollen monitoring of the atmosphere in the city of Cordoba (southern Spain) for 25 years, together with field phenological observations in the area, has indicated that, apart from the main pollination period in spring, secondary flowerings also occasionally occur in this area, specifically in autumn. The present work examines these uncommon pollination events detected in the autumns of certain years with the aim of determining the main environmental factors that influence and control them. During the 25-year study period, there were 7 years in which a secondary Quercus flowering was detected in the area from the second half of October until the end of November. The univariate statistical analysis of the influence of environmental variables determined that the meteorological conditions in September were the most influential. Low mean temperatures, together with record rainfall in that month, led to autumn flowering events. The phenological characteristics of the spring pollen season were also influential. In the years with a shorter spring, the Quercus pollen season tended to present autumn flowerings. A multivariate adaptive regression splines (MARS) model was built to explain the effects of the different variables on the occurrence of autumn pollination. The results indicated that the combined effect of three predicting variables, September rainfall, the length of the spring pollen season, and the end of the spring pollen season, explained 92% of the variance. The validation showed a strong relationship between the expected and the observed autumn pollen concentrations. Therefore, the present analysis of a long-term pollen database revealed that the main causes of this unusual second flowering in autumn were strongly related to climate change, i.e., strong dry summers and warm autumns. In addition, the results showed that the phenomenon was more frequent in the years with low pollination during spring due to different meteorological events potentiated by climate change, such as dryness or heavy rain episodes, as a way of ensuring acorn crops. The results explain how this unusual and lesser-known phenomenon in agroforestry dynamics is related to the adaptation to climate change and the main factors that are driving it, as well as the potential consequences for these important and endangered Mediterranean ecosystems.

Potential Nitrogen Contributions by Tropical Legume Summer Cover Crops in Mediterranean-Type Cropping Systems

Legume cover crops in temperate cropping systems can fix substantial amounts of nitrogen (N) and reduce N fertiliser requirements for subsequent crops. However, little is known about potential biological N2 fixation by summer cover crop legumes in the short summer fallow in Mediterranean-type cropping systems. Six legume species (balansa clover, barrel medic, mung bean, sunn hemp, lablab and cowpea) were grown for 8–9 weeks in the field in semi-arid southern Australia during the summer fallow, and in a glasshouse experiment, to estimate N2 fixation using the 15N natural abundance method. Cowpea, sunn hemp and lablab produced 1.2–3.0 t ha−1 biomass in the field while balansa clover and barrel medic produced &lt; 1.0 t ha−1. The percent of N derived from the atmosphere (%Ndfa) in the field ranged from 39% in barrel medic to 73% in sunn hemp, but only 15% (balansa clover) to 33% (sunn hemp) in the glasshouse experiment, likely due to higher soil mineral N availability in the glasshouse study. Biological N2 fixation of cowpea and sunn hemp in the field was 46–55 kg N ha−1, while N2 fixation in lablab and mung bean was lower (around 26 kg N ha−1). The N2 fixation in cowpea and sunn hemp of around 50 kg N ha−1 with supplementary irrigation in the field trial likely represents the upper limit of N contributions in the field in typically hot, dry summer conditions in Mediterranean-type climates. Given that any increase in summer cover crop biomass will have implications for water balances and subsequent cash crop growth, maximising N benefits of legume cover crops will rely on increasing the %Ndfa through improved rhizobium strains or inoculation technologies. This study provides the first known estimates of biological N2 fixation by legume cover crops in the summer fallow period in cropping systems in Mediterranean-type environments, providing a benchmark for further studies.