Hydrology under change: long-term annual and seasonal changes in small agricultural catchments in Norway

In agricultural catchments, hydrological processes are highly linked to particle and nutrient loss and can lead to a degradation of the ecological status of the water. Global warming and land use changes influence the hydrological regime. This effect is especially strong in cold regions. In this study, we used long-term hydrological monitoring data (22–26 years) from small agricultural catchments in Norway. We applied a Mann–Kendall trend and wavelet coherence analysis to detect annual and seasonal changes and to evaluate the coupling between runoff, climate, and water sources. The trend analysis showed a significant increase in the annual and seasonal mean air temperature. In all sites, hydrological changes were more difficult to detect. Discharge increased in autumn and winter, but this trend did not hold for all catchments. We found a strong coherence between discharge and precipitation, between discharge and snow water equivalent and discharge and soil water storage capacity. We detected different hydrological regimes of rain and snow-dominated catchments. The catchments responded differently to changes due to their location and inherent characteristics. Our results highlight the importance of studying local annual and seasonal changes in hydrological regimes to understand the effect of climate and the importance for site-specific management plans.

Forecasting and identifying the meteorological and hydrological conditions favoring the occurrence of severe hazes in Beijing and Shanghai using deep learning

Severe haze or low-visibility events caused by abundant atmospheric aerosols have become a serious environmental issue in many countries. A framework based on deep convolutional neural networks containing more than 20 million parameters called HazeNet has been developed to forecast the occurrence of such events in two Asian megacities: Beijing and Shanghai. Trained using time-sequential regional maps of up to 16 meteorological and hydrological variables alongside surface visibility data over the past 41 years, the machine has achieved a good overall performance in identifying haze versus non-haze events, and thus their respective favorable meteorological and hydrological conditions, with a validation accuracy of 80 % in both the Beijing and Shanghai cases, exceeding the frequency of non-haze events or no-skill forecasting accuracy, and an F1 score specifically for haze events of nearly 0.5. Its performance is clearly better during months with high haze frequency, i.e., all months except dusty April and May in Beijing and from late autumn through all of winter in Shanghai. Certain valuable knowledge has also obtained from the training, such as the sensitivity of the machine's performance to the spatial scale of feature patterns, that could benefit future applications using meteorological and hydrological data. Furthermore, an unsupervised cluster analysis using features with a greatly reduced dimensionality produced by the trained HazeNet has, arguably for the first time, successfully categorized typical regional meteorological–hydrological regimes alongside local quantities associated with haze and non-haze events in the two targeted cities, providing substantial insights to advance our understandings of this environmental extreme. Interesting similarities in associated weather and hydrological regimes between haze and false alarm clusters or differences between haze and missing forecasting clusters have also been revealed, implying that factors, such as energy-consumption variation and long-range aerosol transport, could also influence the occurrence of hazes, even under unfavorable weather conditions.

Quantitative assessment of hydrological alteration over multiple periods caused by human activities at the Jingjiang Three Outlets, China

The Jingjang Three Outlets (JTO) play a major role in the water-sediment transport from the Yangtze River to Dongting Lake. The hydrological regimes at the JTO (Songzi, Taiping, and Ouchi) had been changed due to the Jingjiang Cutoffs (JC), the Gezhou Dam (GD), and the Three Gorges Dam (TGD). Based on hydrological data from 1955 to 2019, the variation trend in annual streamflow was detected using three techniques, and the hydrological alteration was assessed with the Range of Variability Approach. Conclusions are as follows: (1) the inflection points consistent with human activities and the time series were divided into periods of P1 (1955–1971), P2 (1972–1985), P3 (1986–2002), and P4 (2003–2019); (2) human activities made a greater contribution to streamflow change than climate change; (3) the hydrological alteration degree caused by the JC, GD, and TGD projects were 56%, 47%, and 52% at the Songzi outlet; 57%, 41%, and 57% at the Taiping outlet; and 57%, 41%, and 57% at the Ouchi outlet; and (4) the ability of division from the Yangtze River to Dongting Lake is weakening and the hydrological regimes at the JTO are deteriorating due to the JC, GD, and TGD, resulting in negative impacts on the biotic composition, structure, and function of riparian ecosystems. This study provides useful insight for ecosystem protection under hydrological alteration. HIGHLIGHT Quantitative assessment of hydrological alteration over multiple periods. Differentiated the influence of cutoffs projects and dam construction on hydrological alteration. Discussed the negative impacts of hydrological alteration on ecosystems and the countermeasures for future.

Particulate and Dissolved Black Carbon in Bohai and Laizhou Bays, China: Distributions, Sources, and Contrasts Under Two Distinct Fluvial Hydrological Regimes

Coastal seas, including coastal bays, are the geographically critical transitional zone that links terrestrial and open oceanic ecosystems. Organic carbon cycling in this area is a dynamic and disproportionally key component in the global carbon cycle and budget. As the thermally-transformed organic carbon produced exclusively from the incomplete combustion of biomass and fossil fuels, the recalcitrance and resultant longer environmental residence times result in important implications of black carbon (BC) in the global carbon budget. However, the environmental dynamics of BC in coastal seas have not well been constrained. In this study, we conducted one seawater sampling campaign in the high-intensity BC emission influenced Bohai Bay (BHB) and Laizhou Bay (LZB) in 2013, and quantified both particulate and dissolved BC (PBC and DBC). We elaborated the distributions, sources, and associated influencing factors of PBC and DBC in BHB and LZB in 2013, and simultaneously contrasted the PBC and DBC quantity and quality under two distinct fluvial hydrological regimes of 2013 and 2014 [discussed in Fang et al. (Environ. Sci. Technol., 2021, 55, 788–796)]. Except for the overwhelmingly high PBC in northern BHB caused by anthropogenic point-source emission, horizontally, both PBC and DBC showed a seaward decreasing trend, suggesting that riverine discharge was the major source for PBC and DBC. Vertically, in contrast to the uniform concentrations of DBC between surface and bottom waters, the PBC levels in bottom waters was significantly higher than that in surface waters, which was primarily resulted from the intense sediment re-suspension process during this sampling period. The nearly simultaneous investigations in 2013 and 2014 revealed consistent spatial patterns of PBC and DBC quantity and quality. But significantly lower PBC and DBC quantity and quality were found in 2014 than in 2013, which were largely due to the significantly different climatic conditions (including the watershed hydrology and sunlit radiation) between these 2 years.

Decline in Iran’s River Flows

This study examined changes in Iran’s river flows by applying regression and analysis of variance methods to long-term ground-truth data. Evaluations were performed for the country’s data-rich rivers, covering almost 97% of all rivers and including more than 35 years of measurements. The results showed that about 56% of Iran’s rivers have experienced a negative trend in mean annual flow that is approximately 2.5 times greater than that reported for world’s rivers, leading to a shift from perennial to intermittent for about 20% of rivers in Iran’s sub-basins. This reflects surface freshwater shortages in Iran caused by natural and, more importantly, anthropogenic disturbances. It may even indicate the development of new hydrological regimes which can have significant implications for future surface water storage in Iran. This research improves understanding of changes in Iran’s river flows and provides beneficial information for sustainable water resources management in the country.

Future changes in annual, seasonal and monthly runoff signatures in contrasting Alpine catchments in Austria

Hydrological regimes of alpine catchments are expected to be strongly affected by climate change, mostly due to their dependence on snow and ice dynamics. While seasonal changes have been studied extensively, studies on changes in the timing and magnitude of annual extremes remain rare. This study investigates the effects of climate change on runoff patterns in six contrasting Alpine catchments in Austria using a process-based, semi-distributed hydrological model and projections from 14 regional and global climate model combinations for two representative concentration pathways, namely RCP4.5 and RCP8.5. The study catchments represent a spectrum of different hydrological regimes, from pluvial–nival to nivo-glacial, as well as distinct topographies and land forms, characterizing different elevation zones across the eastern Alps to provide a comprehensive picture of future runoff changes. The climate projections are used to model river runoff in 2071–2100, which are then compared to the 1981–2010 reference period for all study catchments. Changes in the timing and magnitude of annual maximum and minimum flows, as well as in monthly runoff and snowmelt, are quantified and analyzed. Our results indicate a substantial shift to earlier occurrences in annual maximum flows by 9 to 31 d and an extension of the potential flood season by 1 to 3 months for high-elevation catchments. For low-elevation catchments, changes in the timing of annual maximum flows are less pronounced. Magnitudes of annual maximum flows are likely to increase by 2 %–18 % under RCP4.5, while no clear changes are projected for four catchments under RCP8.5. The latter is caused by a pronounced increase in evaporation and decrease in snowmelt contributions, which offset increases in precipitation. In the future, minimum annual runoff will occur 13–31 d earlier in the winter months for high-elevation catchments, whereas for low-elevation catchments a shift from winter to autumn by about 15–100 d is projected, with generally larger changes for RCP8.5. While all catchments show an increase in mean magnitude of minimum flows by 7–30% under RCP4.5, this is only the case for four catchments under RCP8.5. Our results suggest a relationship between the elevation of catchments and changes in the timing of annual maximum and minimum flows. For the magnitude of the extreme flows, a relationship is found between catchment elevation and annual minimum flows, whereas this relationship is lacking between elevation and annual maximum flow.