Changes in fronts regulate nitrate cycling in Zhanjiang Bay: A comparative study during the normal wet season, rainstorm, and typhoon periods

Typhoons and rainstorms (>250 mm/day) are extreme weather events changing hydrological characteristics and thus nitrogen (N) cycle in coastal waters. However, responses of N cycle to rainstorms and typhoons and their underlying mechanisms need to be elucidated. In this study, we conducted an analysis of a comparative dataset encompassing concentrations of nitrate (NO3-), ammonium (NH4+), dissolved oxygen (DO), chlorophyll a (Chl a), hydrological parameters, dual isotopic composition of NO3- (δ15N-NO3- and δ18O-NO3-) in Zhanjiang Bay during three distinct periods: the normal wet season, rainstorm, and typhoon periods. After the rainstorm, the salinity front in Zhanjiang Bay was more weakened and steadier than that during the normal wet season, mainly because onshore wind and a large amount of freshwater was inputted into the ocean surface. This weakened and steady salinity front strengthened water stratification and provided a favorable condition for phytoplankton blooms. Correspondingly, evident NO3- deficits coincided with elevated δ15N-NO3- and δ18O-NO3- values indicated that sufficient NO3- sustained phytoplankton blooms, leading to NO3- assimilation during the rainstorm period. By contrast, due to the onshore wind induced by the typhoon, the salinity front in Zhanjiang Bay was more intensified and unsteady after the typhoon than the normal wet season. The salinity front after the typhoon was unsteady enough to enhance vertical mixing in the water column. Relatively high DO concentrations suggested that enhanced vertical mixing after the typhoon support freshly organic matter decomposition and nitrification via oxygen injection from the air into the water column. In addition, NO3- deficits coincided with elevated δ15N-NO3- values and δ18O-NO3- values demonstrated the coexistence of NO3- assimilation during the typhoon period. This study suggests that the changing processes involved in NO3- cycling after typhoons and rainstorms are associated with the stability and intensity of the salinity front altered by these weather events.

Recent decreasing precipitation and snowmelt reduce the floods around the Chinese Tianshan Mountains

Understanding and managing mountain floods has become increasingly urgent, with global climate change and human activities exacerbating flood risk. However, flood research in Tianshan Mountains, a typical flood-prone mountainous region in China, is still insufficient. Here, we customized a set of flood research methods based on rainstorms and extreme snowmelt events, including a new flood counting method that comprehensively considered the frequency and magnitude of floods and the methods of flood classification and change attribution. We found that floods around the Chinese Tianshan Mountains (CTM) increased from 2014 to 2016 but decreased rapidly from 2016 to 2021, with storm floods, snowmelt floods, and mixed floods accounting for 38.3 %, 26.5 %, and 34.6 % of total flood events, respectively. The variation of floods was most significantly correlated with the average and extreme precipitation, followed by the temperature-driven average snowmelt change. Furthermore, atmospheric circulation anomalies and water vapor input from the western boundary of CTM caused decreasing precipitation and storm floods. Meanwhile, the warming hiatus also greatly impacted declining flood frequency. Notably, flood frequency is projected to rebound soon because of the rising precipitation and temperature, infrastructure aging, and reservoir abandonment, implying the present flood decline unsustainable. Our research develops a strategy to investigate short-term flood anomalies under climate oscillations around the CTM, providing insights into flood research and prevention in global mountainous regions.

Impacts of extreme weather events on microplastic distribution in coastal environments

Microplastic pollution is a pressing environmental threat to the ecosystem, which can be influenced by varying weather factors. With arising weather extremes in recent years, it is crucial to assess the weather effects on coastal microplastic pollution. In this study, we conducted a year-long baseline survey on beach sediment and surface seawater in Hong Kong, and additional surveys after rainstorm and typhoon events. Our data reveals that microplastic abundance was 5 times higher in wet season over dry season. Yet, the seasonal variation was insignificant when considering only the baseline condition, suggesting the role of extreme weather as the main driver of the seasonal variation in microplastic distribution. Typhoons and rainstorms induced 5.1 to 36.4 times and 1.9 to 11.7 times more microplastics in beach sediment, respectively, and 3.5 to 6.0 times and 2.5 to 4.3 times more microplastics in surface seawater, respectively. The larger microplastic mass in beach sediment and the increased proportion of hard fragments under extreme weather conditions indicate the larger mobility of heavier plastic debris from a wider source. We identified positive correlations between plastic levels and multiple weather factors (including rainfall, wind and tide), suggesting the potential terrestrial inputs of microplastics via surface runoff and wind transport, and the potential redistribution of microplastics from deep to surface sediment via wave agitation. We also identified a strongly positive correlation between macro- and microplastic abundance in beach sediment, suggesting the potential plastic fragmentation under strong wave abrasion, which may intensify the coastal microplastic pollution. This study sheds light on the need for reinforced security of waste management systems to avoid terrestrial plastic inputs under extreme weather forces, as well as the timing of coastal cleanup work in terms of limiting plastic fragmentation and achieving better cleanup efficiency.

A comparative study on source of water masses and nutrient supply in Zhanjiang Bay during the normal summer, rainstorm, and typhoon periods: Insights from dual water isotopes

Typhoons and rainstorms (rainfall >250 mm day-1) are extreme weather events that seriously impact coastal oceanography and biogeochemical cycles. However, changes in the mixing of water masses and nutrient supply induced by typhoons and rainstorms can hardly be identified and quantified by traditional methods owing to the complex hydrological conditions in coastal waters. In this study, we analysed a comparative data set of dual water isotopes (δD and δ18O), hydrological parameters, nutrients, and chlorophyll-a from three periods (normal summer, rainstorm, and typhoon periods) in Zhanjiang Bay, a typical semi-enclosed mariculture bay in South China, to address this issue. The results revealed a significant increase in contributions from freshwater during rainstorms and typhoons. Correspondingly, nutrient supplies from freshwater during these periods remarkably increased compared to the normal summer, indicating that heavy rainfall can transport substantial amounts of terrestrial nutrients into the bay. Furthermore, disparities in hydrodynamic processes between typhoon and rainstorm periods were notable due to inconsistencies in freshwater diffusion paths. During rainstorms, freshwater primarily diffuses towards the outer bay in the upper layer due to strong stratification and cannot form an ocean front. However, under intense external forces caused by the typhoon, high-salinity water intruded into the bay, and enhancement of vertical mixing disrupted stratification. The massive influx of freshwater column during the typhoon mixed with higher salinity seawater column in the bay led to the formation of an ocean front, which could retain contaminants. This study suggests that although both rainstorms and typhoons can discharge large quantities of terrestrial nutrients into Zhanjiang Bay, the front formed during the typhoon period impedes the contaminant transportation to open sea thereby deteriorating water quality and affecting mariculture activities within the bay.

Response of nutrient loss to natural erosive rainfall events under typical tillage practices of contour ridge system in the rocky mountain areas of Northern China

Changes in natural rainfall characterized by heavy precipitation and high rainfall intensity would increase the risks and uncertainty of nutrients losses. Losses of nitrogen (N) and phosphorus (P) with water erosion from agriculture-related activities has become the principal nutrients resulting the eutrophication of water bodies. However, a little attention has been paid to the loss characteristic of N and P responding to natural rainfall in widely used contour ridge systems. To explore the loss mechanism of N and P in contour ridge system, nutrient loss associated with runoff and sediment yield was observed in in situ runoff plots of sweet potato (SP) and peanut (PT) contour ridges under natural rainfall. Rainfall events were divided into light rain, moderate rain, heavy rain, rainstorm, large rainstorm, and extreme rainstorm level, and rainfall characteristics for each rainfall level were recorded. Results showed that rainstorm, accounting for 46.27% of the total precipitation, played a destructive role in inducing runoff, sediment yield, and nutrient loss. The average contribution of rainstorm to sediment yield (52.30%) was higher than that to runoff production (38.06%). Rainstorm respectively generated 43.65-44.05% of N loss and 40.71-52.42% of P loss, although light rain induced the greatest enrichment value for total nitrogen (TN, 2.44-4.08) and PO4-P (5.40). N and P losses were dominated by sediment, and up to 95.70% of the total phosphorus and 66.08% of TN occurred in sediment. Nutrient loss exhibited the highest sensitivity to sediment yield compared to runoff and rainfall variables, and a significant positive linear relationship was observed between nutrient loss and sediment yield. SP contour ridge presented higher nutrient loss than that in PT contour ridge, especially for P loss. Findings gained in this study provide references for the response strategies of nutrient loss control to natural rainfall change in contour ridge system.

Analysis of urea nitrogen leaching under high-intensity rainfall using HYDRUS-1D

The leaching of nitrogen in farmland could lead to groundwater nitrogen pollution, while precipitation is the driving factor. Nevertheless, the influences of extreme precipitation on soil organic nitrogen leaching, such as urea, is not clear. In this study, we used soil column experimental data to construct a nitrogen migration and transformation model. The constructed model was applied to unsaturated zones in farmlands in the Dagu River Basin of China. Three rainfall scenarios, namely rainstorms, heavy rainstorms, and extreme rainstorms, were simulated to analyze the variation in urea nitrogen leaching to groundwater. As the rainfall intensity increased, the quantity of urea nitrogen leaching increased from 0.04% to 18.09%. The cumulative urea nitrogen leaching flux is related to the initial soil moisture content. The time interval between fertilization inversely influenced the urea nitrogen leaching flux. Applying urea-based fertilizers shortly before high-intensity rainfall significantly increases the soil-crop system urea leaching loss and decreases crop nitrogen utilization. These findings imply that urea leaching under extreme rainstorms should not be ignored, and based on weather forecasting, fertilizer application in farmland should be avoided before high-intensity rainfall.

Spatial and temporal variation of dissolved heavy metals in the Lijiang River, China: implication of rainstorm on drinking water quality

Lijiang River is an essential drinking water source and natural scenery in the Guilin City. For the first time, implications of rainstorm were taken into consideration by investigating spatial and temporal variation of dissolved heavy metals (HMs) in the Lijiang River water. A total of 68 water samples were collected during low flow (normal) season and high flow (rainstorm) season from 34 sampling sites. Dissolved HMs including Cr, Mn, Co, Cu, Zn, As, Cd, Sb, and Pb were found to meet the respective drinking water standards, while comparatively higher concentration was observed after the rainstorm season, except for Cr. Multivariate statistical analysis showed that Co, Cu, Cr, Zn, Sb, and Pb in normal season were mainly controlled by anthropogenic sources. Furthermore, higher concentrations of Mn, Cu, Cd, Pb, Co, and Zn during the high flow season were attributed to rainstorm. The water quality index (WQI) showed good grades and comparatively lower in rainstorm season. The results of health risk assessment revealed that HMs in Lijiang River posed limited health risk; however, As posed potential health risk specially in rainstorm season. It is suggested to adopt preventive measures for mining activities and industrial waste-water discharge at the river's upstream and downstream.

Occurrence and ecological risk of pharmaceutical and personal care products in surface water of the Dongting Lake, China-during rainstorm period

The pharmaceutical and personal care product (PPCP) residues in freshwater lakes are being highlighted around the world. The occurrence and ecological risk of 34 PPCPs classified as antibiotics, non-steroidal anti-inflammatory drugs (NSAID), cardiovascular drugs, psychotropic drugs, anti-inflammatory drugs, psychostimulants, and pesticides during rainstorm period in surface water of the Dongting Lake, China, were studied. Twenty-six out of thirty-four PPCPs were detected, and the total concentrations of antibiotics ranged from 0.15 to 214.75 ng L^-1 in surface water. The highest average concentration was observed for diclofenac, followed by diethyltoluamide (DEET). The PPCP concentrations were much lower in Dongting Lake compared to other rivers and lakes due to the strong dilution effect of rainstorm, while the detection rate remains high. Caffeine and DEET were detected with 100% frequency in Dongting Lake, and the detection rates of diclofenac, mefenamic acid, and roxithromycin were above 90%. The pollution levels of antibiotics decreased in the order of East Dongting Lake > South Dongting Lake > West Dongting Lake, which may be related to the distribution of aquaculture plants, sewage treatment plants, and population density. The risk quotient (RQ) method was used to evaluate ecological environment risk under the worst case and the results suggested that clarithromycin, diclofenac, roxithromycin, and erythromycin might pose a significant risk to aquatic organisms in Dongting Lake, especially clarithromycin. This study can provide data support for further research on the dilutive effect and mechanism of rainwater runoff on PPCPs in lakes on a large scale.

Global exposure to rainstorms and the contribution rates of climate change and population change

Quantifying global population exposure to rainstorms is a key component of population risk assessments for rainstorms and induced floods. Based on daily precipitation data from the NEX-GDDP dataset, rainfall from rainstorms is first calculated by a multi-model ensemble method for four periods from 1986 to 2100. Combined with population data from the SSP2 scenario, the global population exposure to rainstorms is then calculated and analyzed. Finally, the contribution rates of climate change effect, population change effect, and joint change effect on exposure change are quantitatively assessed. The results showed that (1) Population exposure to rainstorms shows a linear upward trend from base period to the late 21st century period in most regions, and the mid-21st century period compared with base period has the fastest rate of increase. (2) The spatial patterns of population exposure to rainstorms are very similar for the four periods and the areas with high exposure are mainly distributed in Asia, population exposure of Africa is gradually increasing. The countries with high exposure show little volatility, especially the top eight countries. (3) The change in total exposure is mainly due to population change. Based on the composition of the total exposure change for each country, the number of countries whose climate change effect is greater than that of population change is gradually increasing, and this number reaches more than a quarter of the total when the late 21st century period is compared with the mid-21st century period.

Rainfall erosivity in Europe

Rainfall is one the main drivers of soil erosion. The erosive force of rainfall is expressed as rainfall erosivity. Rainfall erosivity considers the rainfall amount and intensity, and is most commonly expressed as the R-factor in the USLE model and its revised version, RUSLE. At national and continental levels, the scarce availability of data obliges soil erosion modellers to estimate this factor based on rainfall data with only low temporal resolution (daily, monthly, annual averages). The purpose of this study is to assess rainfall erosivity in Europe in the form of the RUSLE R-factor, based on the best available datasets. Data have been collected from 1541 precipitation stations in all European Union (EU) Member States and Switzerland, with temporal resolutions of 5 to 60 min. The R-factor values calculated from precipitation data of different temporal resolutions were normalised to R-factor values with temporal resolutions of 30 min using linear regression functions. Precipitation time series ranged from a minimum of 5 years to a maximum of 40 years. The average time series per precipitation station is around 17.1 years, the most datasets including the first decade of the 21st century. Gaussian Process Regression (GPR) has been used to interpolate the R-factor station values to a European rainfall erosivity map at 1 km resolution. The covariates used for the R-factor interpolation were climatic data (total precipitation, seasonal precipitation, precipitation of driest/wettest months, average temperature), elevation and latitude/longitude. The mean R-factor for the EU plus Switzerland is 722 MJ mm ha(-1) h(-1) yr(-1), with the highest values (>1000 MJ mm ha(-1) h(-1) yr(-1)) in the Mediterranean and alpine regions and the lowest (<500 MJ mm ha(-1) h(-1) yr(-1)) in the Nordic countries. The erosivity density (erosivity normalised to annual precipitation amounts) was also the highest in Mediterranean regions which implies high risk for erosive events and floods.