Runoff Fluxes Research Articles

Impacts of Climatic Fluctuations and Vegetation Greening on Regional Hydrological Processes: A Case Study in the Xiaoxinganling Mountains–Sanjiang Plain Region, Northeastern China

The Xiaoxinganling Mountains–Sanjiang Plain region represents a crucial ecological security barrier for the Northeast China Plain and serves as a vital region for national grain production. Over the past two decades, the region has undergone numerous ecological restoration projects. Nevertheless, the combined impact of enhanced vegetation greening and global climate change on the regional hydrological cycle remains inadequately understood. This study employed the distributed hydrological model ESSI-3, reanalysis datasets, and multi-source satellite remote sensing data to quantitatively evaluate the influences of climate change and vegetation dynamics on regional hydrological processes. The study period spans from 2000 to 2020, during which there were significant increases in regional precipitation and leaf area index (p < 0.05). The hydrological simulation results exhibited strong agreement with observed river discharge, evapotranspiration, and terrestrial water storage anomalies, thereby affirming the ESSI-3 model’s reliability in hydrological change assessment. By employing both a constant scenario that solely considered climate change and a dynamic scenario that integrated vegetation dynamics, the findings reveal that: (1) Regionally, climate change driven by increased precipitation significantly augmented runoff fluxes (0.4 mm/year) and water storage components (2.57 mm/year), while evapotranspiration trends downward, attributed primarily to reductions in solar radiation and wind speed; (2) Vegetation greening reversed the decreasing trend in evapotranspiration to an increasing trend, thus exerting a negative impact on runoff and water storage. However, long-term simulations demonstrated that regional runoff fluxes (0.38 mm/year) and water storage components (2.21 mm/year) continue to increase, mainly due to precipitation increments surpassing those of evapotranspiration; (3) Spatially, vegetation greening altered the surface soil moisture content trend in the eastern forested areas from an increase to a decrease. These findings suggested that sub-regional ecological restoration initiatives, such as afforestation, significantly influence the hydrological cycle, especially in areas with higher vegetation greening. Nevertheless, persistent increases in precipitation could effectively mitigate the moisture deficits induced by vegetation greening. The study’s outcomes provide a basis for alleviating concerns regarding potential water consumption risks associated with future ecological restoration and extensive vegetation greening projects, thereby offering scientific guidance for sustainable water resource management.

The use of in situ Gamma-Ray Spectrometry to Assess the Environmental Impacts of Intensive Agriculture in terms of Geochemical Mobility in soil and waters

The concentration and mobility of metals in the lithosphere and hydrosphere are led by many physical-chemical parameters and processes from natural and anthropogenic origins, the last one resulting in impacts over many ecosystems around the World, including wetlands. These transitional zones, often characterized by the presence of hydric soils, adapted vegetation, and seasonal or permanent presence of surface water, are commonly under human pressure in terms of land use conversion and contamination, notably in agricultural production areas, where excess of nutrients/organic matter, pesticides, salts, sediments, heavy metals and radionuclides (originated from inorganic fertilizers) can substantially alter the ecological balance of those ecosystems. Thus, this study aimed to evaluate the agricultural impact over a tropical geographically isolated wetland in the Brazilian Cerrado by the analysis of geochemical mobility and interaction between surface and groundwater through in situ gamma-ray spectrometry and hydraulic conductivity measurements. The results demonstrated that the margins of this diabase-derived soil wetland are one of the most important and critical compartments due to its capacity of metal immobilization and surface water infiltration, indicated especially by uranium concentrations. Thorium, in turn, was most related to colluvial transport from slopes to the center of wetland. It was also corroborated by low hydraulic conductivity zones as a result of soil compaction due to heavy agricultural machinery and increase in runoff fluxes. Thus, this methodology could be used as an initial fast screening method in wetlands under other climatic and geological/pedological contexts to evaluate the local hydrogeochemical dynamics and impacts of agricultura.

The Non‐Local Impacts of Antarctic Subglacial Runoff

AbstractLittle is known about Antarctic subglacial hydrology, but based on modeling, theory and indirect observations it is thought that subglacial runoff enhances submarine melt locally through buoyancy effects. However, no studies to date have examined effects of runoff on sea ice and oceanography on the continental shelf. Here we use modeled and observational estimates of runoff to force a regional model of the Amundsen Sea Embayment. We find that runoff enhances melt locally (i.e., within the ice‐shelf cavity), increasing melt at Thwaites ice shelf by up to 15 Gt/a given estimates of steady runoff, and up to 25 Gt/a if runoff is episodic as remote sensing measurements suggest. However runoff also has smaller non‐local effects on melt through freshwater influence on flow and stratification. We further find that runoff reduces summer sea‐ice volume over the continental shelf (by up to 10% with steady runoff but over 30% with episodic runoff). Furthermore runoff is much more effective at reducing sea ice than an equivalent volume of ice‐shelf meltwater—due in part to the latent heat loss associated with submarine melting. Results suggest that runoff may play an important role in continental shelf dynamics, despite runoff flux being small relative to ice‐shelf melting—and that runoff‐driven melt and circulation may be an important process missing from regional Antarctic ocean models.

Biochar or Straw Substituting Chemical Fertilizer Increase the Risk of Phosphorus Loss in Subsurface Runoff in Sloping Farmland

The purple soil slope farmland is an important agricultural land in southwest China but is also one of the main sources of agricultural non-point source pollution in the Three Gorges Reservoir area. Taking reasonable measures to control the loss of soil nutrients is of great significance to the treatment of non-point source pollution in the region. Here, a three-year (2018-2020) field runoff experiment was conducted to monitor and evaluate the phosphorus (P) loss in sloping farmland via surface runoff (i.e., surface flow, 0-20 cm) and subsurface runoff (i.e., subsurface flow, 20-60 cm), with five treatments including no fertilization (CK), conventional fertilization (CF), optimal fertilization (OF), biochar combined with 85% of OF (BF), and straw combined with 85% of OF (SF). The results showed that fertilization application reduced the sediment yields and surface runoff flux but increased the subsurface runoff flux. The total loss flux of phosphate (PO3-4-P), total phosphorus (TP), and particulate phosphorus (PP) in surface flow were the highest in the BF treatment and the lowest in the SF treatment. All fertilization applications increased the P loss fluxes in subsurface flow relative to that in CK. The highest PO3-4-P and TP loss flux in subsurface flow was found in the BF (213.88 g·hm-2 and 694.54 g·hm-2, respectively) treatment, followed by that in the OF and SF treatments. Redundancy analysis (RDA) results showed that surface runoff flux and biochar application were the main factors contributing to increased P loss in surface flow, and subsurface runoff flux was the main factor contributing to increased P loss in subsurface flow. In summary, the SF treatment reduced the amount of sediment yield and surface runoff flux in sloping farmland of purple soil and was the most effective for controlling P loss, whereas the risk of subsurface runoff flux requires further attention.

Effects of Soil Data Accuracy on Outputs of Irrigation Scheduling Tools

Highlights The effects of soil data accuracy on estimated water fluxes by an irrigation scheduling model were investigated. Free and frequently used web soil survey (WSS) soil textural data underestimated sand particles in 89% of cases. Forty-nine percent of the estimated differences in seasonal irrigation based on WSS and measured soil data were within ±25 mm. In most cases, use of WSS data resulted in larger evaporation, smaller deep percolation, and larger runoff compared to those based on measured soil data. Abstract. A widely used irrigation scheduling method is based on modeling soil water balance, which requires several key inputs, including soil data. Many scheduling tools developed using this method rely on publicly available soil data, such as the United States Department of Agriculture's Web Soil Survey (WSS). While soil survey data are a valuable source of information for general farm and natural resource planning and management at large scales, inaccuracies in soil conditions at field and subfield scales can hamper efficient agricultural water management through irrigation scheduling tools. To illuminate the implications of the localized inaccuracies, this study estimated the errors in WSS soil textural data at 18 sites in three regions of western Oklahoma through comparison with in-situ sampling (ISS) data. The effects of errors on estimated water fluxes were also investigated for dominant crops of each region over a 15-year (2006-2020) period. The findings demonstrated that WSS soil textures were finer than ISS at most sites and soil layers, resulting in generally greater root zone total available water estimates. Differences in seasonal irrigation demand estimates when WSS data were used instead of ISS reached 20% at one site but were within ±9% among the regions. Half of the estimated seasonal irrigation differences for all sites, years, and crops were within ±25 mm. Soil evaporation, deep percolation, and runoff fluxes were also impacted by soil data source, albeit to a smaller degree than irrigation, at levels and directions (over or underestimation) that were dependent on the sign and magnitude of WSS errors, as well as precipitation amounts and timing. Overall, errors in WSS data may not have a major impact at regional scales, but the effects on individual irrigated farms may be severe depending on the magnitude of difference between WSS data and true soil conditions. Keywords: Irrigation demand, Soil water balance, SSURGO, Water fluxes, Web soil survey.

δ26Mg, δ44Ca and 87Sr/86Sr isotope differences among bedrock minerals constrain runoff generation in headwater catchments: An acidified granitic site in Central Europe as an example

Magnesium, calcium and strontium isotope systematics were studied in a Central European headwater catchment underlain by granite. The Uhlirska catchment (Czech Republic) is recovering from acidification following 40 years of acid rain. A combination of isotope and non-isotope data was used to constrain the origin of base cations in runoff. Whole-rock δ26Mg, δ44Ca values and 87Sr/86Sr ratios were complemented by isotope analysis of apatite, biotite, plagioclase, orthoclase and titanite. Isotope composition of Mg, Ca and Sr in precipitation, throughfall and runoff was monitored for 12 months.Soil, soil solutions and Norway spruce tissues were also analyzed. Non-isotope data included a 24-year time-series of input/output Mg and Ca fluxes. Biotite was the likely main source of geogenic Mg. Apatite and plagioclase were the likely main sources of geogenic Ca, and plagioclase was the likely main source of geogenic Sr. Magnesium in biotite was isotopically too heavy to dominate runoff. Calcium in apatite and plagioclase was isotopically indistinguishable from whole-rock Ca and could play a major role in runoff generation. Plagioclase had a significantly lower 87Sr/86Sr ratio than bulk bedrock, close to the low 87Sr/86Sr ratio of runoff. Plagioclase weathering was consistent with a sizeable geogenic Sr contribution to runoff but if only bulk-rock 87Sr/86Sr was considered predominance of geogenic Sr in runoff would be unlikely. Higher Mg, Ca and Sr runoff fluxes, compared to deposition, suggested geogenic control of runoff. A decrease in runoff fluxes of Mg and Ca coincided with a decrease in deposition fluxes but there may not be a causal relationship pointing to a large atmospheric contribution of base cations to runoff. Decreasing fluxes of base cations via runoff were mostly related to decreasing sulfate export accompanying retreat of acidification. Mg/Ca/Sr isotope systematics at Uhlirska are discussed in light of analogous data from four other headwater catchments.

Distributions and Influencing Factors of Dissolved Manganese in Kongsfjorden and Ny-Ålesund, Svalbard

The contributions of polar glaciers to the global distribution of trace metals have attracted an increasing amount of attention due to global warming. To better understand the distributions and influencing factors of dissolved manganese (dMn) in the Arctic, samples were collected in Ny-Ålesund and Kongsfjorden in Svalbard in August 2013. dMn concentrations in the drainage basin of the Bayelva River, of which the average value was 216.4 ± 54.2 nM (n = 16) and higher than that in other polar rivers and lakes, showed no significant differences from upstream to downstream in August. dMn varied differently at certain stations in the mainstream of Bayelva River on different days due to complex factors in the polar region, e.g., topography, tributaries, groundwater, organic matter content, and runoff fluxes. It was estimated that the scale length (distance from the initial concentration to 1/e) of dMn in the coastal area of the Bayelva River was 1050 m, indicating that the influence of Bayelva River on the distributions of dMn in Kongsfjorden was insignificant. Affected by high-dMn concentration tidewater glacier meltwater (∼581.1 nM) in the inner fjord, dMn showed an increasing trend from the fjord entrance to the inner zone in the surface layer of Kongsfjorden. Furthermore, the longer scale length (4.25 km) and overlying relatively large discharge suggested a higher incidence of tidewater glaciers in Kongsfjorden. The vertical distributions of dMn presented obvious stratification, with higher dMn at the surface (80.9 ± 53.7 nM, n = 21) and lower dMn below the surface (12.6 ± 2.4 nM, n = 11). Combined with the water mass properties and flow field, the distributions of dMn in Kongsfjorden were mainly controlled by relatively high dMn concentrations, low-salinity surface water, relatively low dMn concentrations, and high-salinity Atlantic water mixing. The dMn concentrations in Ny-Ålesund and Kongsfjorden were significantly higher than those in the North Atlantic Ocean and Arctic Ocean around Svalbard. Tidewater glaciers (Kongsvegen, Kronebreen, and Kongsbreen) and glaciers of Svalbard were estimated to export 0.46 and 74 Mmol/year of dMn, which was comparable to other sources to the Arctic Ocean. The contributions of glaciers to the distributions of dMn in the Arctic and global Ocean were expected to be increasingly significant considering the increasing amount of glacier meltwater in the context of global warming.

δ53Cr values of catchment runoff exhibit seasonality regardless of bedrock Cr concentrations: Role of periodically changing chromium export fluxes

AbstractOne pre‐requisite for the construction of a global chromium isotope mass balance is detailed understanding of Cr isotope systematics in the critical zone where redox‐processes can modify the isotope signature of geogenic Cr input into the hydrosphere. A Cr isotope inventory of bedrock, soil, and runoff was performed in a Central European headwater catchment underlain by amphibolite, situated in the vicinity of two previously studied catchments underlain by different bedrock types (serpentinite and leucogranite). Fresh bedrock in the amphibolite catchment NAZ contained ~300 mg/kg Cr, serpentinite at PLB contained ~800 mg/kg Cr, and leucogranite at LYS contained ~2 mg/kg Cr. Monthly hydrochemical monitoring at all three sites revealed higher Cr(VI) export fluxes in winter than in summer. NAZ was characterized by a distinct seasonality in the δ53Cr values, with minima during winter/spring snowmelts (−0.35‰) and maxima during dry summers (0.40‰). Similar seasonality in δ53Cr values had been reported from PLB and LYS. Bedrock at all three sites had similar Cr isotope composition close to −0.10‰, a value indistinguishable from the δ53Cr value of bulk silicate Earth (BSE). Positive mean δ53Cr value of NAZ runoff indicated Cr‐isotope fractionations during weathering of geogenic Cr(III), combined with adsorption of the resulting Cr(VI) on soil particles during pedogenesis. However, the mass‐weighted mean δ53Cr of NAZ runoff was lower (−0.08‰), indistinguishable from the Cr isotope signature of bedrock. The same pattern of lower mass‐weighted mean δ53Cr values of runoff, compared to arithmetic mean δ53Cr values of runoff, were observed also at PLB and LYS. We suggest that elevated Cr runoff fluxes in winter remove some of the residual isotopically light Cr that accumulated in the soil during summer. Seasonality in runoff δ53Cr values appears to be a relatively widespread phenomenon, de‐coupled from Cr availability for chemical weathering.

The Late Quaternary Hydrological Changes in the Eastern Tarim Basin Inferred From 10Be Exposure Ages of River Terraces

AbstractReliable information on the dynamics of hydrological changes is crucial for the social and economic stabilities and sustaining developments in arid Central Asia. However, the few long‐term hydrological records limit understanding of the relationship between meltwater runoff and climate changes, because the short‐term observations show a paradoxical runoff trend with climate warming across Central Asia. Here, we made a systematic analysis on the 10Be exposure ages of the river terraces developed in the eastern Tarim Basin and reconstructed the hydrological regime changes during the past 170 ka. These data, together with the ages of the river terraces in the water source areas, suggest that the exposure ages of river terraces in the eastern Tarim Basin reflect the depositional time of alluvial sediments. The meltwater runoff displays an inverse relation with the North Hemisphere Summer Insolation and temperature changes on both orbital and millennial time scale, indicating that low temperatures favor high runoff fluxes. The observed correlation between the hydrological regimes and temperature changes suggests that the runoff will decrease and thus climate will become drying with global warming in arid Central Asia.

The annual cycle of terrestrial water storage anomalies in CMIP6 models evaluated against GRACE data

AbstractTerrestrial water storage anomaly (TWSA) is a critical component of the global water cycle where improved spatiotemporal dynamics would enhance exploration of weather and climate-linked processes. Thus, correctly simulating TWSA is essential not only for water-resource management but also for assessing feedbacks to climate through land-atmosphere interactions. Here we evaluate simulated TWSA from 25 climate models (from the Climate Model Intercomparison Project 6) through comparison with TWSA from GRACE satellite data (2003 – 2014) in 14 river basins globally and assess causes of discrepancies by examining precipitation (P), evapotranspiration (ET), and runoff (Roff) fluxes during recharge (increasing TWS) and discharge (decreasing TWS) cycles. Most models show consistent biases in seasonal amplitudes of TWS anomalies relative to GRACE output: higher modeled amplitudes in river basins in high northern latitudes, Parana, and Congo, and lower amplitudes in most mid-latitude basins and other tropical basins. This TWSA systematic bias also exists in the previous CMIP5 simulations. Models overestimate P compared to observed P datasets in 7 out of 14 basins, which increases (decreases) seasonal storage amplitude relative to GRACE in the recharge (discharge) cycle. Overestimation (underestimation) of runoff is another common contributing factor in the discharge phase that increases (decreases) TWSA amplitudes relative to GRACE in 5 river basins. The results provide a comprehensive assessment of the reliability of the simulated annual range in TWSA through comparison with GRACE data that can be used to guide future model development.

Mercury cycling during acid rain recovery at the forested Lesní potok catchment, Czech Republic

AbstractFrom 2011 to 2019, mercury (Hg) stores and fluxes were studied in the small forested catchment Lesní potok (LES) in the central Czech Republic using the watershed mass balance approach together with internal measurements. Mean input fluxes of Hg via open bulk deposition, beech throughfall and spruce throughfall during the periodwere 2.9, 3.9 and 7.6 μg m−2 year−1, respectively. These values were considerably lower than corresponding deposition Hg fluxes reported in the early years of the 21st century from catchments in Germany. Current bulk precipitation inputs at unimpacted Czech mountainous sites were lower than those in Germany. The largest Hg inputs to the catchment were via litterfall, averaging 22.6 and 17.8 μg m−2 year−1 for beech and spruce stands. The average Hg input, based on the sum of mean litterfall and throughfall deposition, was 23.0 μg m−2 year−1, compared to the estimated Hg output in runoff of 0.5 μg m−2 year−1, which is low compared to other reported values. Thus, only ~2% of Hg input is exported in stream runoff. Stream water Hg was only weakly related to dissolved organic carbon (DOC) but both concentrations were positively correlated with water temperature. The estimated total soil Hg pool averaged 47.5 mg m−2, only 4% of which was in the O‐horizon. Thus Hg in the O‐horizon pool represents 72 years of deposition at the current input flux and 3800 years of export at the current runoff flux. Age‐dating by 14C suggested that organic soil contains Hg from recent deposition, while mineral soil at 40–80 cm depth contained 4400‐year old carbon, suggesting the soil had accumulated atmospheric Hg inputs through millennia to reach the highest soil Hg pool of the soil profile. These findings suggest that industrial era intensification of the Hg cycle is superimposed on a slower‐paced Hg cycle during most of the Holocene.

A realistic Greenland ice sheet and surrounding glaciers and ice caps melting in a coupled climate model

Greenland ice sheet experienced an intensive melting in the last century, especially in the 1920s and over the last decades. The supplementary input into the ocean could disrupt the freshwater budget of the North Atlantic. Simultaneously, some signs of a recent weakening of the Atlantic Meridional Overturning Circulation (AMOC) have been reported. In order to better understand the possible impact of the increasing melting on the North Atlantic circulation, salinity and temperature trends, we construct an observation-based estimate of the freshwater fluxes spanning from 1840 to 2014. The estimate is based on runoff fluxes coming from Greenland ice sheet and surrounding glaciers and ice caps. Input from iceberg melting is also included and spatially distributed over the North Atlantic following an observed climatology. We force a set of historical simulations of the IPSL-CM6A-LR coupled climate model with this reconstruction from 1920 to 2014. The ten-member ensemble mean displays freshened and cooled waters around Greenland, which spread in the subpolar gyre, and then towards the subtropical gyre and the Nordic Seas. Over the whole period, the convection is reduced in the Labrador and Nordic Seas, while it is slightly enhanced in the Irminger Sea, and the AMOC is weakened by \(0.32 \pm 0.35\) Sv at \(26 ^\circ \) N. The multi-decadal trend of the North Atlantic surface temperature obtained with the additional freshwater forcing is slightly closer to observations than in standard historical simulations, although the two trends are only different at the 90% confidence level. Slight improvement of the Root Mean Square Error with respect to observations in the subpolar gyre region suggests that part of the surface temperature variability over the recent decades may have been forced by the release of freshwater from Greenland and surrounding regions since the 1920s. Finally, we highlight that the AMOC decrease due to Greenland melting remains modest in these simulations and can only explain a very small amount of the \(3\pm 1\) Sv weakening suggested in a recent study.

Quantifying the effects of land use and model scale on water partitioning and water ages using tracer-aided ecohydrological models

Abstract. Quantifying how vegetation mediates water partitioning at different spatial and temporal scales in complex, managed catchments is fundamental for long-term sustainable land and water management. Estimations from ecohydrological models conceptualising how vegetation regulates the interrelationships between evapotranspiration losses, catchment water storage dynamics, and recharge and runoff fluxes are needed to assess water availability for a range of ecosystem services and evaluate how these might change under increasing extreme events, such as droughts. Currently, the feedback mechanisms between water and mosaics of different vegetation and land cover are not well understood across spatial scales, and the effects of different scales on the skill of ecohydrological models needs to be clarified. We used the tracer-aided ecohydrological model EcH2O-iso in an intensively monitored 66 km2 mixed land use catchment in northeastern Germany to quantify water flux–storage–age interactions at four model grid resolutions (250, 500, 750, and 1000 m). This used a fusion of field (including precipitation, soil water, groundwater, and stream isotopes) and remote sensing data in the calibration. Multicriteria calibration across the catchment at each resolution revealed some differences in the estimation of fluxes, storages, and water ages. In general, model sensitivity decreased and uncertainty increased with coarser model resolutions. Larger grids were unable to replicate observed streamflow and distributed isotope dynamics in the way smaller pixels could. However, using isotope data in the calibration still helped constrain the estimation of fluxes, storage, and water ages at coarser resolutions. Despite using the same data and parameterisation for calibration at different grid resolutions, the modelled proportion of fluxes differed slightly at each resolution, with coarse models simulating higher evapotranspiration, lower relative transpiration, increased overland flow, and slower groundwater movement. Although the coarser resolutions also revealed higher uncertainty and lower overall model performance, the overall results were broadly similar. The study shows that tracers provide effective calibration constraints on larger resolution ecohydrological modelling and help us understand the influence of grid resolution on the simulation of vegetation–soil interactions. This is essential in interpreting associated uncertainty in estimating land use influence on large-scale “blue” (ground and surface water) and “green” (vegetation and evaporated water) fluxes, particularly for future environmental change.

Characteristics of Dissolved Organic Carbon Loss in Purple Soil Sloping Fields with Different Fertilization Treatments

The characteristics of dissolved organic carbon loss with different fertilization treatments were examined to derive the best nutrient management method for sloping farmland in the Three Gorges Reservoir area where maintaining the soil carbon balance and reducing environmental pollution caused by carbon loss is crucial. Experimental runoff plots were set up at the Experimental Station for Soil and Water Conservation and Environmental Research in the Three Gorges Reservoir Region, Chinese Academy of Sciences, involving the following five treatments:No fertilization (CK), conventional fertilization (conventional), optimum fertilization (optimum), biochar combined with 85% optimum fertilizer (biochar), and straw combined with 85% optimum fertilizer (straw). The effects of the five treatments on runoff flux, sediment yield, and soil organic carbon flux were monitored and evaluated. Results show that:①Subsurface flow accounted for 52.84%-92.23% of the runoff (both surface and subsurface flow) and the loss flux of dissolved organic carbon (DOC) in the subsurface accounted for 43.64%-87.35% of the total loss flux. Thus, in this sloping farmland, subsurface flow was the main pathway of runoff and dominated dissolved organic carbon transport. ②Compared with the optimum treatment, straw treatment reduced the surface runoff flux, sediment yield, surface loss flux of DOC, and loss flux of organic carbon in the sediment by 30.39%, 39.41%, 28.71%, and 23.97%, respectively, but increased the subsurface runoff flux and loss flux of DOC. Compared with the optimum treatment, the biochar treatment significantly increased the surface and subsurface runoff flux, sediment yield, loss flux of DOC in the surface and subsurface, and the loss flux of organic carbon in the sediment. ③The loss flux of DOC accounted for 99.31%-99.94% of the loss flux of soil organic carbon, and DOC was the major species of organic carbon in the organic carbon loss in this type of sloping farmland. The loss flux of DOC under the different fertilization treatments was ranked biochar > optimum > straw > conventional > CK. ④Compared to the optimum treatment, the straw treatment and biochar treatment increased the soil organic carbon (SOC) content by 95.79% and 32.16%, respectively. Based on these results, straw combined with 85% of optimum fertilizer is the best nutrient management method for this sloping farmland as it can reduce surface runoff flux, sediment yield, and the loss flux of soil organic carbon while increasing the soil organic carbon content.

The 2019–2020 Australian Drought and Bushfires Altered the Partitioning of Hydrological Fluxes

AbstractThough coarse in spatial resolution, the nearly all weather measurements from passive microwave sensors can help in improving the spatio‐temporal coverage of optical and thermal infrared sensors for monitoring vegetation changes on the land surface. This study demonstrates the use of vegetation optical depth (VOD) retrievals from the Soil Moisture Active Passive mission for capturing the vegetation alterations from the recent 2019 to 2020 Australian bushfires and drought. The impact of vegetation disturbances on terrestrial water budget is examined by assimilating the VOD retrievals into a dynamic phenology model. The results demonstrate that assimilating VOD observations lead to improved simulation of evapotranspiration, runoff, and soil moisture states. The study also demonstrates that the vegetation changes from the 2019 to 2020 Australian drought and fires led to significant modifications in the partitioning of evaporative and runoff fluxes, resulting in increased bare soil evaporation, reduced transpiration, and higher runoff.

Hydroclimatic variability drives submarine groundwater discharge and nutrient fluxes in an anthropogenically disturbed, semi-arid estuary

Nutrient budgets in semi-arid estuaries, with ephemeral freshwater inflows and limited nutrient sources, are likely incomplete if contributions from submarine groundwater discharge (SGD) are not included. Here, the relative importance of saline/recirculated SGD-derived nutrient fluxes spatiotemporal variability to the overall nutrient budget is quantified for Nueces Bay, Texas, U.S.A., across hydroclimatic conditions ranging from drought to normal, to flood. On average, 67% of the variance in water quality is due to temporal differences while 16% is explained by spatial differences. Principal component analysis (PCA) reveals three principal components: freshwater inflow (PC1 28.8%), saline/recirculated SGD and recycled nitrogen (PC2 15.6%), and total SGD and “new” nitrogen (PC3 11.2%). Total SGD porewater fluxes ranged from 29.9–690.3 mmol∙m−2d−1 for ammonium, 0.21–18.7 mmol∙m−2d−1 for nitrite+nitrate, 3.1–51.3 mmol∙m−2d−1 for phosphate, 57.1–719.7 mmol∙m−2d−1 for silicate, and 95.9–36,838.5 mmol∙m−2d−1 for dissolved organic carbon. Total and saline/recirculated SGD fluxes were on average 150–26,000 and 5.8–466 times, respectively, greater than surface runoff fluxes across all seasons. Nitrogen (N) enrichment in porewater occurs near the agricultural fields because of soil N flushing and percolation to groundwater, which facilitates N-rich groundwater fluxes. There were substantial “new” N inputs from terrestrial groundwater following precipitation while saline/recirculated SGD of recycled N accounts for only <4% of total SGD inputs. The “new” N inputs occur in the river and river mouth during flooding, and near the north shore where topography and hydraulic gradients are steeper during drought. Thus, while significant inputs of N may be associated with atmospheric deposition, or remineralization in the porewater, groundwater is the highest contributor to the nutrient budget in Nueces Bay. This result implies that nutrient management strategies should focus on land-use practices to reduce N contamination of shallow groundwater and subsequent contamination of estuaries.

Landsat-Based Indices Reveal Consistent Recovery of Forested Stream Catchments from Acid Deposition

Central European forests suffered from severe, large-scale atmospheric depositions of sulfur and nitrogen due to coal-based energy production during the 20th century. High deposition of acid compounds distorted soil chemistry and had negative effects on forest physiology and growth. Since 1994, continuous data on atmospheric deposition and stream runoff fluxes have provided evidence of ecosystem recovery from acidification. In this study, we combined for the first time mass budget data (sulfur deposition and total dissolved inorganic nitrogen (DIN) export) from the GEOMON monitoring network of headwater catchments with annual trajectories of vegetation indices derived from Landsat remote sensing observations. Time series of selected vegetation indices was constructed from Landsat 5, 7, and 8 using Google Earth Engine. Linear regression between the field data and vegetation indices was analyzed using R software. Biogeochemical responses of the forested catchment to declining acid deposition (driven by SO2 emission reduction) were consistent across all catchments covering various forest stands from different regions of the Czech Republic. Significant correlations were found with total sulfur depositions, suggesting that the forests are continuously and consistently prospering from reductions in acid deposition. Disturbance index (DI) was the only vegetation index that was well-related to changes in forest cover associated with salvage loggings (due to the forest decline) during the 1980s and 1990s. A significant relationship (R2 = 0.82) was found between the change in DI and DIN export in stream water. Regrowth of young forests in these highly affected areas tracks the most pronounced changes in total DIN export, suggesting a prominent role of vegetation in nitrogen retention. With the Landsat-derived DI, we could map decennial changes in forest disturbances beyond the small scale of the catchments to the regional level (demonstrated here for two protected landscape areas). This analysis showed the peak in forest disturbances to have occurred around the mid-1990s, followed by forest recovery and regrowth. Despite the improvement in forest ecosystem functioning over the past three decades in mountainous areas, emerging threats connected to changing climate will shape forest development in the near future.

Exploring the dynamics of Cu transported from the Yangtze River to the East China Sea

We used a section-segment method to determine the copper (Cu) fluxes from the Yangtze River to the East China Sea during 2012–2016 in this study, including the maximum, minimum and mean annual fluxes of dissolved and suspended Cu. The Cu fluxes exhibited a pronounced inter-annual variability. June was characterized with the highest monthly dissolved and suspended Cu fluxes, while the lowest monthly Cu fluxes was in February. The monthly Cu concentration and Cu fluxes both increased from January to June and decreased from July to December. The monthly Cu fluxes showed a positive correlation with the water flux and monthly mean Cu concentration; however, it had a negative correlation with the ratio of tidal flux to river runoff flux. Both natural (53%) and anthropogenic inputs (32.8%) were the main sources of Cu. The discharges of industrial and domestic sewage wastewater were the main anthropogenic factors affecting the entry of Cu.

Effects of biochar incorporation and fertilizations on nitrogen and phosphorus losses through surface and subsurface flows in a sloping farmland of Entisol

Nitrogen (N) and phosphorus (P) loss in runoffs has been a serious problem in China due to the amount of chemical fertilizer currently applied to farmland. Biochar has been used as a soil amendment for increasing nutrient retention, water holding capacity, and crop yield. However, little is known about the influence of biochar on the runoff erosion from both surface flow and subsurface flow, especially in sloping regions. Experimental runoff plots were, therefore, set up on Entisols in a sloping farmland, involving four treatments: control (CT), conventional NPK fertilization (CF, based on local practices), optimum NPK fertilization (OF, based on the recommendation from local agriculture committee), and biochar combined with 85% of optimum NPK fertilizer (BF). The effects of the four treatments on the runoff fluxes, sediment yields and the runoff coefficients, as well as the N (ammonium (NH4+), nitrate (NO3−) and total N (TN)) and P (phosphate (PO43-), total P (TP), dissolved TP (DTP), particulate P (PP)) losses via both surface runoff (i.e., surface flow, 0–20 cm, ploughed layer) and subsurface runoff (i.e., subsurface flow, 20–60 cm, non-ploughed layer) were all monitored and evaluated from April 2018 to September 2019. Results show that, in this sloping farmland, subsurface flow was the main process of runoff and dominated N transport, but surface flow dominated P transport. The three fertilization treatments reduced the runoff erosion by surface flow, while increasing it from subsurface flow (p < 0.05). Compared with two NPK fertilizations, BF treatment significantly increased the total sediment yield, subsurface runoff flux and subsurface runoff coefficient. Among all three fertilization treatments, the differences of grain yields are found not significant (except for the 4th crop). In surface flow, biochar addition increased the loss flux of NH4+ and P fractions, but reduced the loss flux of NO3− with the comparison of the CF and OF treatments. In subsurface flow, biochar addition didn’t significantly changed the runoff of N fractions, but increased P runoffs, especially for TP and PP runoffs (p < 0.05). Overall, the risk of N and P loss, and runoff erosion through subsurface flow in biochar incorporated Entisols sloping farmland needs be highlighted.

Calcium and strontium isotope dynamics in three polluted forest ecosystems of the Czech Republic, Central Europe

Calcium and strontium isotope ratios were used to identify Ca sources for bulk soil, soil solutions and runoff in the stressed forest ecosystems of Central Europe. All three study sites are underlain by Ca-poor crystalline bedrock (orthogneiss, leucogranite and serpentinite, respectively), but differ in historical rates of acid deposition. UDL, the spruce die-back affected site at the Czech–Polish border underlain by orthogneiss, received six times more acidifying compounds from the atmosphere than LYS and PLB, two paired catchments near the Czech–German border. The paired catchments near the Czech–German border, whose forests were only mildly damaged by industrial pollution, differ in their acid buffering capacity (extremely low for LYS leucogranite, extremely high for PLB serpentinite). At the spruce die-back affected catchment UDL, 19 years of monthly hydrochemical monitoring revealed six times higher atmospheric deposition and runoff fluxes of Ca, compared to the paired catchments LYS and PLB. Across the three sites, the mean δ44Ca values increased in the order: spruce bark (−0.84‰) < spruce xylem (−0.31‰) < spruce fine roots (0.0‰) < bedrock (0.08‰) < soil (0.11‰) < spruce needles (0.31‰) < open-area precipitation (0.68‰) < spruce throughfall (0.71‰) < runoff (0.74‰) < soil water (1.11‰). At all three sites, Ca from atmospheric deposition was isotopically similar to Ca in runoff, indicating export of recent atmospheric Ca. All three catchments had isotopically lighter Ca in silicate bedrock and exported isotopically heavier Ca via runoff. Trees mostly represented the isotopically lightest Ca pool, whereas soil water collected by lysimeters contained isotopically heavy Ca. The 87Sr/86Sr isotope ratios were nearly uniform at the serpentinite site PLB (0.710). The 87Sr/86Sr ratios across the two felsic sites (UDL and LYS) increased in the order: open-area precipitation (0.710) < spruce canopy throughfall (0.715) < spruce xylem (0.723) < spruce needles (0.725) < soil water (0.734) < runoff (0.740) < soil (0.862) < bedrock (0.881). Collectively, the δ44Ca and 87Sr/86Sr isotope systematics indicated that organic Ca cycling, along with atmospheric input of isotopically heavy Ca, largely contributed to the high-δ44Ca values of runoff. The studied silicate bedrock had significantly lower δ44Ca values than the reported world-wide average. While bulk soil Ca was likely affected by bedrock Ca even after the period of acid rain, the δ44Ca difference between soil and soil water indicated a major role of recycled organic Ca in supplying nutrition to the trees. Future Ca-Sr isotope research in headwater catchments should include isotope analysis of sequentially leached Ca fractions from weathered minerals in soil to better assess the geogenic Ca inputs to runoff. In catchments currently recovering from pollution, Ca-Sr isotope fingerprinting of dust emitted from coal-fired power plants will be needed. Isotope analysis of local coal can indicate to what extent Ca-Sr isotope composition of past deposition contributes to the isotope signatures of present-day runoff.