Effects of afforestation on runoff and sediment load in an upland Mediterranean catchment

Evolution patterns and spatial sources of water and sediment discharge over the last 70 years in the Yellow River, China: A case study in the Ningxia Reach

The hydrology and sediment processes in large rivers play important roles in maintaining aquatic and coastal ecosystems and advancing civilization and production in human systems. Therefore, quantitatively analyzing the spatiotemporal variability and dynamics of water and sediment discharge in large rivers is essential for improving watershed management and sustainable development in the areas surrounding rivers, especially the Yellow River, which is one of the most sediment-laden rivers in the world. In this study, we analyzed the evolution patterns and spatial sources of water and sediment discharge in the Yellow River from 1951 to 2020 and determined the impacts of different factors on water and sediment discharge variations. The results showed that the annual water and sediment discharge significantly decreased (p < 0.05) over the past 70 years, with an abrupt change occurring in 1986. The first dominant periodicity of water discharge was approximately 29 years, while the first dominant periodicity of sediment discharge was approximately 28 years. In terms of the water and sediment discharge sources, the dominant factor affecting variations in water discharge was water diversion from 1951 to 2020, while the dominant factor affecting variations in sediment discharge was sediment aggradation from 1951 to 1985 and changed to tributary inflow sediment from 1986 to 2020. In addition, the water and sediment discharge changes were also affected by anthropogenic activities, such as water and sediment diversions, dams and reservoirs, and water and soil conservation measures. In particular, the water and sediment interception capabilities of the established soil and water conservation measures gradually became saturated over time. Specifically, the maximum water and sediment interception capabilities of the current soil and water conservation measures were 12.2 billion m³ and 1.9 Gt, respectively. Overall, the results of the present study can help tailor water and sediment regulation countermeasures in the future.

Zinc transport and partitioning of a mine-impacted watershed: An evaluation of water and sediment quality

Watershed systems influenced by mining waste products can persist for many years after operations are ceased, leading to negative impacts on the health of the surrounding environment. While geochemical behaviors of these trace metals have been studied extensively at the benchtop-scale, much fewer studies have looked at controls on their distributions at the watershed-level. In this study, trace metals (As, Cd, Cr, Cu, Ni, and Zn) were reported from water and stream bed sediments at eight sites between the years 2014-2018 along a watershed undergoing active remediation efforts. Zn was determined to be the only trace metal analyzed with concentrations above EPA and Kansas Department of Health guidelines for both water and sediment in the watershed, and thus was the primary focus for determining the health of the watershed system. Controls on trace metal pollution distribution over the watershed were investigated to determine where remediation efforts should be focused. Surface cover seemed to have the highest effectivity with pasture lands having a strong positive correlation to Zn concentrations. Initial remediation efforts were assessed by calculating the geoaccumulation index (I_geo) and the contamination factor (C_f-sediment) from sediments and contamination factor from water (C_f-water) after decades of chat pile removal efforts. Most of the sites showed significant reduction in metal concentration values compared to previous studies in the watershed for water and sediment, with four sites still reporting concentrations that reveal potential health risks. Results from this study will inform management and policy makers for areas to focus their remediation efforts on the Spring River Watershed as well as providing a framework for assessing pollution at a watershed scale.

Capability and Robustness of Novel Hybridized Artificial Intelligence Technique for Sediment Yield Modeling in Godavari River, India

Suspended sediment yield (SSY) prediction plays a crucial role in the planning of water resource management and design. Accurate sediment prediction using conventional models is very difficult due to many complex processes. We developed a fully automatic highly generalized accurate and robust artificial intelligence models for SSY prediction in Godavari River Basin, India. The genetic algorithm (GA), hybridized with an artificial neural network (ANN) (GA-ANN), is a suitable artificial intelligence model for SSY prediction. The GA is used to concurrently optimize all ANN’s parameters. The GA-ANN was developed using daily water discharge, with water level as the input data to estimate the daily SSY at Polavaram, which is the farthest gauging station in the downstream of the Godavari River Basin. The performances of the GA-ANN model were evaluated by comparing with ANN, sediment rating curve (SRC) and multiple linear regression (MLR) models. It is observed that the GA-ANN contains the highest correlation coefficient (0.927) and lowest root mean square error (0.053) along with lowest biased (0.020) values among all the comparative models. The GA-ANN model is the most suitable substitute over traditional models for SSY prediction. The hybrid GA-ANN can be recommended for estimating the SSY due to comparatively superior performance and simplicity of applications.

Evaporation in Brazilian dryland reservoirs: Spatial variability and impact of riparian vegetation

Evaporation is a major factor controlling the hydrological dynamics of surface water reservoirs in dry environments, therefore quantification with minimal uncertainties is desired. The aim of this paper is to assess the spatial variability and impact of riparian vegetation on reservoir evaporation by remote sensing. Eight reservoirs located in subhumid and semi-arid climates in the Brazilian Drylands were studied. Scenes from Landsat 5 and Landsat 8 satellites (1985 and 2018) supplied the data for four evaporation models. For reference evaporation, the Class A Pan and Piché Evaporimeter closest to the reservoirs were considered. The occurrence/density of riparian vegetation was associated with the Normalized Difference Vegetation Index (NDVI) and its influence on evaporation was assessed. The Surface Energy Balance System for Water (AquaSEBS) model presented the best average performance (Nash-Sutcliffe Efficiency coefficient 0.40 ± 0.19). Evaporation was observed to be higher at the reservoirs' margins and near the dams, due to the contact of exposed soil and rock/concrete, respectively, which transfer heat to the water. Marginal areas near the riparian forest presented low evaporation rates with decreases between 18% and 31% in relation to the average. This interdependence was evidenced by the high negative correlation (R² 0.87-0.96) between NDVI and evaporation; vegetation reduces radiation because of the shading of the reservoir margin and changes local aerodynamics, reducing evaporation. Depending on the spatial variability of evaporation, it was found that the volumes transferred to the atmosphere may have variations of up to 30%. On average, the evaporated volume in all the studied reservoirs is 450,000 m³/day, a quantity enough to supply more than two million people. Overall, the results of this study contribute not only to a better understanding of the spatial variability of evaporation in surface reservoirs, but also of the interdependence between riparian vegetation and evaporation rates.

Global Change and Forest Disturbances in the Mediterranean Basin: Breakthroughs, Knowledge Gaps, and Recommendations

Forest ecosystems in the Mediterranean Basin are mostly situated in the north of the Basin (mesic). In the most southern and dry areas, the forest can only exist where topography and/or altitude favor a sufficient availability of water to sustain forest biomass. We have conducted a thorough review of recent literature (2000–2021) that clearly indicates large direct and indirect impacts of increasing drought conditions on the forests of the Mediterranean Basin, their changes in surface and distribution areas, and the main impacts they have suffered. We have focused on the main trends that emerge from the current literature and have highlighted the main threatens and management solution for the maintenance of these forests. The results clearly indicate large direct and indirect impacts of increasing drought conditions on the forests of the Mediterranean Basin. These increasing drought conditions together with over-exploitation, pest expansion, fire and soil degradation, are synergistically driving to forest regression and dieback in several areas of this Mediterranean Basin. These environmental changes have triggered responses in tree morphology, physiology, growth, reproduction, and mortality. We identified at least seven causes of the changes in the last three decades that have led to the current situation and that can provide clues for projecting the future of these forests: (i) The direct effect of increased aridity due to more frequent and prolonged droughts, which has driven Mediterranean forest communities to the limit of their capacity to respond to drought and escape to wetter sites, (ii) the indirect effects of drought, mainly by the spread of pests and fires, (iii) the direct and indirect effects of anthropogenic activity associated with general environmental degradation, including soil degradation and the impacts of fire, species invasion and pollution, (iv) human pressure and intense management of water resources, (v) agricultural land abandonment in the northern Mediterranean Basin without adequate management of new forests, (vi) very high pressure on forested areas of northern Africa coupled with the demographic enhancement, the expansion of crops and higher livestock pressure, and the more intense and overexploitation of water resources uses on the remaining forested areas, and (vii) scarcity and inequality of human management and policies, depending on the national and/or regional governments and agencies, being unable to counteract the previous changes. We identified appropriate measures of management intervention, using the most adequate techniques and processes to counteract these impacts and thus to conserve the health, service capacity, and biodiversity of Mediterranean forests. Future policies should, moreover, promote research to improve our knowledge of the mechanisms of, and the effects on, nutrient and carbon plant-soil status concurrent with the impacts of aridity and leaching due to the effects of current changes. Finally, we acknowledge the difficulty to obtain an accurate quantification of the impacts of increasing aridity rise that warrants an urgent investment in more focused research to further develop future tools in order to counteract the negative effects of climate change on Mediterranean forests.

Spatiotemporal variation of nitrogen and phosphorus and its main influencing factors in Huangshui River basin

The foundation of managing excess nutrients in river is the identification of key physical processes and the control of decisive influencing factors. The existing studies seldom consider the influence of rainfall-runoff relationship and only focus on a few anthropogenic activities and natural attributes factors. To address this issue, a comprehensive set of influencing factors including rainfall-runoff relationship (represented by runoff coefficient), basic physical and chemical parameters of water quality, land use types, landscape patterns, topography, and socioeconomic development was constructed in this study. M-K test and cluster analysis were conducted to identify the temporal mutation and spatial clustering characteristics of NH₃-N and TP in Huangshui River basin, respectively. Partial least squares regression was used to elucidate the linkages between water contaminants and the factors. As shown in the results, the temporal mutations of NH₃-N and TP were obvious in the middle reaches, with 4 out of 7 catchments in the middle reaches have a larger number of mutations of NH₃-N than other catchments. The cluster analysis results of NH₃-N and TP among catchments were similar. This study also indicated that although the Huangshui River basin was located in the upper reaches of the Yellow River, the influences of rainfall-runoff relationship on spatiotemporal changes of NH₃-N and TP in its sub-basins were limited. Only the temporal change of NH₃-N in Jintan catchment in the upstream area was significantly affected by runoff coefficient. The indexes of proportion of water area (PWA), proportion of impervious area (PIA), and proportion of primary industry (PPI) were the top three influencing factors of temporal variation of NH₃-N and TP for most catchments in the middle reaches. The temporal change of NH₃-N in Jintan catchment in the upstream area was obviously affected by runoff coefficient. The spatial variation of NH₃-N and TP were all affected by PWA and proportion of secondary industry significantly. The results of this study can provide theoretical basis and technical support for the control and management of nitrogen and phosphorus pollution in upper reaches of rivers.

Water and sediment budgets unveiling contrasting hydro-sedimentary patterns in a mountainous Mediterranean catchment

Mountain regions have a key role in the generation of runoff, and in the production and transfer of sediments to fluvial networks, especially in Mediterranean catchments where these processes are affected by marked changes in climate and land use (i.e. global change). This paper presents the water and the sediment budgets of the Ribera Salada (224 km²), a meso-scale Mediterranean forested catchment located in the Southern Pyrenees. Field monitoring follows an integrated basins scheme (five nested sub-catchments), where hydrological and sediment transport data were collected continuously over a two-year period (2012-2013). Precipitation was obtained using radar images, which allowed the elaboration of rainfall maps used to characterize the spatial distribution of rainfall across multiple scales. Results indicate that the catchment is hydrologically divided in two areas which show contrasting fluvial regimes: the upper part of the catchment is considered wet and has a constant flow regime, supplying the majority of the water, while the lower part is drier, with ephemeral tributaries and water losses into the alluvial aquifer of the main river channel. In contrast to water yield, most of the suspended sediment load (i.e. 80%) is supplied by the driest part of the catchment where sediment availability was greater and where there is a greater connectivity between sediment sources and the channel network. The sediment yield of the whole catchment and the respective sub-catchments sits in the lower bounds of values reported for the Mediterranean region, indicating the generally low intensity of hydrological and geomorphic processes in the area. Once more the sediment budget approach matched to sound hydrological data proves efficient to characterize sediment dynamics in river basins, with special interest in areas such as the Mediterranean mountain catchments, where the effects of global change appear to be more acute.

The Impact of Socio-Economic Factors on Sediment Load: A Case Study of the Yanhe River Watershed

Under the influence of climate change and human activities, sediment load in rivers has changed significantly, which has a profound impact on the stability of ecosystems and the sustainable development of human beings. Taking the Yanhe River watershed as a case, this paper expounds the dynamic relationship among the Grain for Green Project, social and economic development, population migration, and sediment transport. The variability of sediment load was detected by Pettitt test, the double cumulative curve method, and the regression analysis method, and the effects of climate and human activities on sediment load were quantitatively analyzed. The results showed that 1) from 1956 to 2016, the precipitation of Yanhe River watershed rose slightly in the past 10 years, but the sediment load decreased significantly; 1996 was identified as the catastrophic year of the study period, when the contribution of climate change and human activity to reduced sediment load was 14.1% and 85.9%, respectively. 2) The Grain for Green Project increased the vegetation coverage of the study area from 40.6% to 78.5%. 3) The proportion of agricultural GDP in total GDP decreased from 52.26% to 7.3%, and the proportion of agricultural GDP was positively correlated with sediment transport and cultivated land area (p < 0.01). 4) Population migration resulted in the urbanization rate reaching 40.23%, and the urbanization rate is negatively correlated with sediment load and cultivated land area (p < 0.01), while the cultivated land area is positively correlated with sediment load (p < 0.01). The decrease of cultivated land area makes the sediment load gradually decrease. Therefore, socio-economic factors promote the sustainable development of the river basin.

Multiple Temporal Scales Assessment in the Hydrological Response of Small Mediterranean-Climate Catchments

Mediterranean-climate catchments are characterized by significant spatial and temporal hydrological variability caused by the interaction of natural as well human-induced abiotic and biotic factors. This study investigates the non-linearity of rainfall-runoff relationship at multiple temporal scales in representative small Mediterranean-climate catchments (i.e., <10 km2) to achieve a better understanding of their hydrological response. The rainfall-runoff relationship was evaluated in 43 catchments at annual and event—203 events in 12 of these 43 catchments—scales. A linear rainfall-runoff relationship was observed at an annual scale, with a higher scatter in pervious (R2: 0.47) than impervious catchments (R2: 0.82). Larger scattering was observed at the event scale, although pervious lithology and agricultural land use promoted significant rainfall-runoff linear relations in winter and spring. These relationships were particularly analysed during five hydrological years in the Es Fangar catchment (3.35 km2; Mallorca, Spain) as a temporal downscaling to assess the intra-annual variability, elucidating whether antecedent wetness conditions played a significant role in runoff generation. The assessment of rainfall-runoff relationships under contrasted lithology, land use and seasonality is a useful approach to improve the hydrological modelling of global change scenarios in small catchments where the linearity and non-linearity of the hydrological response—at multiple temporal scales—can inherently co-exist in Mediterranean-climate catchments.

The ecological consequences of the large quantities of trees planted in Northwest China by the Government of China

Rapid economic and population growth exacerbates water resource shortages and various associative ecological factors. Additionally, climate change makes it difficult to predict potential eco-environmental risks. The Government of China enacted a large-scale forestation campaign in the northwest to cope with the region's increasingly severe eco-environmental problems. This study applied GIS software to analyze areas where water resource changes have occurred and the reasons behind water shortages. Notwithstanding fluctuations, there was a general increase in water resource trends between 1980 and 2015. On a regional scale, we observed an increasing trend for provinces with large water resources, including Xinjiang, Qinghai, and Xizang, which accounted for 84.58% of the total increases observed between 1980 and 2015. The water resource trend for the region as a whole increased exponentially with increasing rainfall and decreasing evapotranspiration. Furthermore, water consumed by artificial forests in Northwest China reached 14 billion cubic meters, which is equivalent to 5.22% of its total annual water resources. In contrast, this study determined that under natural vegetation conservation practices, water consumed would have decreased to 10.13 billion cubic meters in 2015. Accordingly, this study concluded that the Government of China should change its policy from planting more trees to protecting natural vegetation.

Water quality modelling of the Mekong River basin: Climate change and socioeconomics drive flow and nutrient flux changes to the Mekong Delta

The Mekong delta is recognised as one of the world's most vulnerable mega-deltas, being subject to a range of environmental pressures including sea level rise, increasing population, and changes in flows and nutrients from its upland catchment. With changing climate and socioeconomics there is a need to assess how the Mekong catchment will be affected in terms of the delivery of water and nutrients into the delta system. Here we apply the Integrated Catchment model (INCA) to the whole Mekong River Basin to simulate flow and water quality, including nitrate, ammonia, total phosphorus and soluble reactive phosphorus. The impacts of climate change on all these variables have been assessed across 24 river reaches ranging from the Himalayas down to the delta in Vietnam. We used the UK Met Office PRECIS regionally coupled climate model to downscale precipitation and temperature to the Mekong catchment. This was accomplished using the Global Circulation Model GFDL-CM to provide the boundary conditions under two carbon control strategies, namely representative concentration pathways (RCP) 4.5 and a RCP 8.5 scenario. The RCP 4.5 scenario represents the carbon strategy required to meet the Paris Accord, which aims to limit peak global temperatures to below a 2 °C rise whilst seeking to pursue options that limit temperature rise to 1.5 °C. The RCP 8.5 scenario is associated with a larger 3-4 °C rise. In addition, we also constructed a range of socio-economic scenarios to investigate the potential impacts of changing population, atmospheric pollution, economic growth and land use change up to the 2050s. Results of INCA simulations indicate increases in mean flows of up to 24%, with flood flows in the monsoon period increasing by up to 27%, but with increasing periods of drought up to 2050. A shift in the timing of the monsoon is also simulated, with a 4 week advance in the onset of monsoon flows on average. Decreases in nitrogen and phosphorus concentrations occur primarily due to flow dilution, but fluxes of these nutrients also increase by 5%, which reflects the changing flow, land use change and population changes.

Parsimonious Modeling of Snow Accumulation and Snowmelt Processes in High Mountain Basins

The success of hydrological modeling of a high mountain basin depends in most case on the accurate quantification of the snowmelt. However, mathematically modeling snowmelt is not a simple task due to, on one hand, the high number of variables that can be relevant and can change significantly in space and, in the other hand, the low availability of most of them in practical engineering. Therefore, this research proposes to modify the original equation of the classical degree-day model to introduce the spatial and temporal variability of the degree-day factor. To evaluate the effects of the variability in the hydrological modeling and the snowmelt modeling at the cell and hillslope scale. We propose to introduce the spatial and temporal variability of the degree-day factor using maps of radiation indices. These maps consider the position of the sun according to the time of year, solar radiation, insolation, topography and shaded-relief topography. Our priority has been to keep the parsimony of the snowmelt model that can be implemented in high mountain basins with limited observed input. The snowmelt model was included as a new module in the TETIS distributed hydrological model. The results show significant improvements in hydrological modeling in the spring period when the snowmelt is more important. At cell and hillslope scale errors are diminished in the snowpack, improving the representation of the flows and storages that intervene in high mountain basins.

Simulated Runoff and Sediment Yield Responses to Land-Use Change Using the SWAT Model in Northeast China

Land-use change is one key factor influencing the hydrological process. In this study, the Hun River Basin (HRB) (7919 km2), a typical alpine region with only four gauge meteorological stations, was selected as the study area. The China Meteorological Assimilation Driving Datasets for the SWAT model (CMADS), widely adopted in East Asia, was used with the Soil and Water Assessment Tool (SWAT) model to simulate runoff and sediment yield responses to land-use change and to examine the accuracy of CMADS in the HRB. The criteria values for daily/monthly runoff and monthly sediment yield simulations were satisfactory; however, the validation of daily sediment yield was poor. Forestland decreased sediment yield throughout the year, increased water percolation, and reduced runoff during the wet season, while it decreased water percolation and increased runoff during the dry season. The responses of grassland and forestland to runoff and sediment yield were similar, but the former was weaker than the latter in terms of soil and water conservation. Cropland (urban land) generally increased (increased) runoff and increased (decreased) sediment yield; however, a higher sediment yield could occur in urban land than that in cropland when precipitation was light.

Effect of flow-sediment regime on benthic invertebrate communities: Long-term analysis in a regulated floodplain lake

Flow and sediment co-create habitat structure and dynamics within river and lake systems, of which the ecological effects are well-documented. But the temporal variability of aquatic community response to continuous flow-sediment alterations is less well-known. This paper explored aquatic biota response to the flow-sediment regime in a broader temporal context using long-term paired hydrological database and monitoring data of benthic invertebrates in Dongting Lake. The inter-annual variations of benthic invertebrates and certain flow and sediment variables were synchronous. Community diversity parameters showed different response to different hydrological events and particularly revealed a short-term peak in the first two years after the operation of the Three Gorges Dam. This finding affirmed the importance of the time series study. The overall flow-sediment regime, calculated from 15 flow and sediment characteristics, affected community structures and could also affect benthic invertebrates by affecting the water quality. The combination of variables, including sediment inputs load, sediment exports/inputs ratio and the duration of low water level, could best explain the variation of benthic invertebrate assemblages and should be given the greatest management concern. When individual components of the flow-sediment regime were analysed, the response patterns of community parameters to different environmental variables or to different gradients of the same variables were complex. Our results suggest that the sediment seemed to be a more influential stressor than flow, especially erosion, which could significantly reduce total abundance and species richness. The effect of the flow regime was reflected by the duration of the low water level. Stressor gradients and the response patterns of different taxa to different stressors should be considered by river and lake managers. The results would provide valuable information for the design of lake management strategies and upstream reservoir regulation rules.

Effects of soil conservation techniques on water erosion control: A global analysis

Water erosion control is one of the most important ecosystem services provided by soil conservation techniques (SCTs), which are being widely used to alter soil and water processes and improve ecosystem services. But few studies have focused on providing this service using various techniques across the world. Here, a comprehensive review was conducted to compare the effects of SCTs on water erosion control. We conducted a meta-analysis consisting of 1589 sample plots in 22 countries to identify SCTs, which we classified into three groups: biological techniques (BTs, such as afforestation and grain for green), soil management techniques (STs, such as no tillage and soil amendment), and engineering techniques (ETs, such as terraces and contour bunds). Our results were as follows: (1) The SCTs had significant positive effects on water erosion control, and they were generally more effective at reducing annual soil loss (84%) than at reducing annual runoff (53%). (2) The BTs (e.g., 88% for soil and 55% for runoff) were generally more effective at reducing soil and water loss than ETs (e.g., 86% for soil and 44% for runoff) and STs (e.g., 59% for soil and 48% for runoff). (3) On bare lands, the efficiency of water erosion control decreased as the terrain slope increased, but this value increased as the slope increased on croplands and orchards. Furthermore, the effects of SCTs on runoff and soil loss reduction were most efficient on 25°-40° slopes in croplands and on 20°-25° slopes in orchards. (4) The SCTs were more efficient on croplands and orchards in temperate climate zone (CZ), while those on bare lands were more effective in tropical CZ. (5) The SCTs in Brazil and Tanzania were more effective at reducing runoff and soil loss than those in the USA, China and Europe.

Accuracy Assessment of Digital Terrain Model Dataset Sources for Hydrogeomorphological Modelling in Small Mediterranean Catchments

Digital terrain models (DTMs) are a fundamental source of information in Earth sciences. DTM-based studies, however, can contain remarkable biases if limitations and inaccuracies in these models are disregarded. In this work, four freely available datasets, including Shuttle Radar Topography Mission C-Band Synthetic Aperture Radar (SRTM C-SAR V3 DEM), Advanced Spaceborne Thermal Emission and Reflection Radiometer Global Digital Elevation Map (ASTER GDEM V2), and two nationwide airborne light detection and ranging (LiDAR)-derived DTMs (at 5-m and 1-m spatial resolution, respectively) were analysed in three geomorphologically contrasting, small (3–5 km2) catchments located in Mediterranean landscapes under intensive human influence (Mallorca Island, Spain). Vertical accuracy as well as the influence of each dataset’s characteristics on hydrological and geomorphological modelling applicability were assessed by using ground-truth data, classic geometric and morphometric parameters, and a recently proposed index of sediment connectivity. Overall vertical accuracy—expressed as the root mean squared error (RMSE) and normalised median deviation (NMAD)—revealed the highest accuracy for the 1-m (RMSE = 1.55 m; NMAD = 0.44 m) and 5-m LiDAR DTMs (RMSE = 1.73 m; NMAD = 0.84 m). Vertical accuracy of the SRTM data was lower (RMSE = 6.98 m; NMAD = 5.27 m), but considerably higher than for the ASTER data (RMSE = 16.10 m; NMAD = 11.23 m). All datasets were affected by systematic distortions. Propagation of these errors and coarse horizontal resolution caused negative impacts on flow routing, stream network, and catchment delineation, and to a lower extent, on the distribution of slope values. These limitations should be carefully considered when applying DTMs for catchment hydrogeomorphological modelling.

Analyzing the Impacts of Climate Variability and Land Surface Changes on the Annual Water–Energy Balance in the Weihe River Basin of China

The serious soil erosion problems and decreased runoff of the Loess Plateau may aggravate the shortage of its local water resources. Understanding the spatiotemporal influences on runoff changes is important for water resource management. Here, we study this in the largest tributary of the Yellow River, the Weihe River Basin. Data from four hydrological stations (Lin Jia Cun (LJC), Xian Yang (XY), Lin Tong (LT), and Hua Xian (HX)) and 10 meteorological stations from 1961–2014 were used to analyze changes in annual runoff. The Mann–Kendall test and Pettitt abrupt change point test diagnosed variations in runoff in the Weihe River basin; the time periods before and after abrupt change points are the base period (period I) and change period (period II), respectively. Within the Budyko framework, the catchment properties (ω in Fu’s equation) represent land surface changes; climate variability comprises precipitation (P) and potential evapotranspiration (ET0). All the stations showed a reduction in annual runoff during the recording period, of which 22.66% to 50.42% was accounted for by land surface change and 1.97% to 53.32% by climate variability. In the Weihe River basin, land surface changes drive runoff variation in LT and climate variability drives it in LJC, XY, and HX. The contribution of land surface changes to runoff reduction in period I was less than that in period II, indicating that changes in human activity further decreased runoff. Therefore, this study offers a scientific basis for understanding runoff trends and driving forces, providing an important reference for social development, ecological construction, and water resource management.

Combined impacts of climate and socio-economic scenarios on irrigation water availability for a dry Mediterranean reservoir

The impacts of climate and associated socio-economic changes on water availability, including supply and demand, quality, and storage volume, were evaluated for the Vale do Gaio reservoir in southern Portugal, located in a dry Mediterranean climate and already under drought stress. The SWAT model was applied with 6 scenarios for 2071-2100, involving two storylines (A1B and B1) with individual changes in climate (-9% rainfall, increasing in winter by +28 to +30%), socio-economic conditions (an increase in irrigation demand by 11%, and a replacement of cereals and pastures by sunflower), and a combination of both. Most future scenarios resulted in lower water availability, due to lower supply (-19 to -27%) combined with higher irrigation demand (+3 to +21%). This resulted in more years with limited irrigation supplies (presently: 28%; scenarios: 37 to 43%), although limitations were mitigated by lower losses to excess discharge. Land-use changes also decreased quality by increasing P concentrations (+29 to +93%). Impacts were more severe in scenario A1B than in B1, and in combined changes than in climate or socio-economic changes only. Water availability was resilient to climate change, as impacts led only to a moderate aggravation of present-day conditions. Lower future water availability could be addressed by supply and demand management strategies and, in the most extreme scenario, by water transfers from regional water reserves; water quality issues could be addressed through land-use policies. Results also highlighted the importance of taking the characteristics of water supply systems into account when designing adaptation measures for future changes.

Impacts of converting low-intensity pastureland to high-intensity bioenergy cropland on the water quality of tropical streams in Brazil

In Brazil, the cultivation of bioenergy crops is expanding at an accelerated rate. Most of this expansion has occurred over low-intensity pasture and is considered sustainable because it does not involve deforestation of natural vegetation. However, the impacts on the water quality of headwater streams are poorly understood, especially with regard to the influence of land use patterns in the watershed. In this study, we investigated the effects of land-use conversion on the water quality of streams draining sugarcane fields and examined whether the preservation of forested areas at the top of the headwaters would help mitigate the negative impacts of intensive agriculture. Water samples were collected in two paired catchments in southeastern Brazil, which is one of the largest sugarcane production regions in the world. Our results show significant differences in the water quality of streams predominantly draining the pasture or the sugarcane field. Several parameters commonly used to indicate water quality, such as the concentrations of nitrate and suspended solids, were significantly higher in the sugarcane than in the pasture stream. Differences in water quality between the streams draining predominantly pasture or sugarcane fields were accentuated during the wet season. The preservation of forests surrounding the headwater streams was associated with overall better water quality conditions, such as lower nitrate concentrations and temperature of the stream water. We concluded that forest conservation in the headwater agricultural catchments is an important factor preventing water quality degradation in tropical streams. Therefore, we strongly recommend the preservation of robust riparian forests in the headwaters of tropical watersheds with intensive agriculture. More studies on the effects of best agricultural practices in bioenergy crops can greatly improve our capacity to prevent the degradation of water quality in the tropical waterways as intensive agriculture continues to expand in this region of the world.

Influences of anthropogenic activities and topography on water quality in the highly regulated Huai River basin, China

Our study analyzed the spatio-temporal trends of four major water quality parameters (i.e., dissolved oxygen (DO), ammonium nitrogen (NH₃-N), total phosphorus (TP) and permanganate index (COD_Mn)) at 17 monitoring stations in one of the most polluted large river basins, Huai River Basin, in China during 2005 to 2014. More concerns were emphasized on the attributions, e.g., anthropogenic actives (land cover, pollution load, water temperature, and regulated flow) and natural factors (topography) to the changes in the water quality. The seasonal Mann-Kendall test indicated that water quality conditions were significantly improved during the study period. The results given by the Moran's I methods demonstrated that NH₃-N and COD_Mn existed a weak and moderate positive spatial autocorrelation. Two cluster centers of significant high concentrations can be detected for DO and TP at the Mengcheng and Huaidian station, respectively, while four cluster centers of significant low concentrations for DO at Wangjiaba and Huaidian station in the 2010s. Multiple linear regression analysis suggested that water temperature, regulated flow, and load of water quality could significantly influence the water quality variations. Additionally, urban land cover was the primary predictor for NH₃-N and COD_Mn at large scale. The predictive ability of regression models for NH₃-N and COD_Mn declined as the scale decreases or the period ranges from the 2000s to the 2010s. Topography variables of elevation and slope, which can be treated as the important explanatory variables, exhibited positive and negative correlations to NH₃-N and COD_Mn, respectively. This research can help us identify the water quality variations from the scale-process interactions and provide a scientific basis for comprehensive water quality management and decision making in the Huai River Basin and also other river basins over the world.

Afforestation impacts microbial biomass and its natural (13)C and (15)N abundance in soil aggregates in central China

We investigated soil microbial biomass and its natural abundance of δ(13)C and δ(15)N in aggregates (>2000μm, 250-2000μm, 53-250μm and <53μm) of afforested (implementing woodland and shrubland plantations) soils, adjacent croplands and open area (i.e., control) in the Danjiangkou Reservoir area of central China. The afforested soils averaged higher microbial biomass carbon (MBC) and nitrogen (MBN) levels in all aggregates than in open area and cropland, with higher microbial biomass in micro-aggregates (<250μm) than in macro-aggregates (>2000μm). The δ(13)C of soil microbial biomass was more enriched in woodland soils than in other land use types, while δ(15)N of soil microbial biomass was more enriched compared with that of organic soil in all land use types. The δ(13)C and δ(15)N of microbial biomass were positively correlated with the δ(13)C and δ(15)N of organic soil across aggregates and land use types, whereas the (13)C and (15)N enrichment of microbial biomass exhibited linear decreases with the corresponding C:N ratio of organic soil. Our results suggest that shifts in the natural (13)C and (15)N abundance of microbial biomass reflect changes in the stabilization and turnover of soil organic matter (SOM) and thereby imply that afforestation can greatly impact SOM accumulation over the long-term.

Determining the hydrological responses to climate variability and land use/cover change in the Loess Plateau with the Budyko framework

Understanding and quantifying the impacts of land use/cover change and climate variability on hydrological responses are important to the design of water resources and land use management strategies for adaptation to climate change, especially in water-limited areas. The elasticity method was used to detect the responses of streamflow and runoff coefficient to various driving factors in 15 main catchments of the Loess Plateau, China between 1961 and 2009. The elasticity of streamflow (Q) and runoff coefficient (Rc) to precipitation (P), potential evapotranspiration (E0), and catchment characteristics (represented by the parameter m in Fu's equation) were derived based on the Budyko hypothesis. There were two critical values of m=2 and E0/P=1 for the elasticity of Q and Rc. The hydrological responses were mainly affected by catchment characteristics in water-limited regions (E0/P>1), and in humid areas (E0/P<1), climate conditions played a more important role for cases of m>2 whereas catchment characteristics had a greater impact for cases of m<2. The annual Q and Rc in 14 of the 15 catchments significantly decreased with average reduction of 0.87mmyr(-1) and 0.18%yr(-1), respectively. The mean elasticities of Q to P, E0 and m were 2.66, -1.66 and -3.17, respectively. The contributions of land use/cover change and P reduction to decreased Q were 64.75% and 41.55%, respectively, while those to decreased Rc were 75.68% and 32.06%, respectively. In contrast, the decreased E0 resulted in 6.30% and 7.73% increase of Q and Rc, respectively. The contribution of land use/cover changes was significantly and positively correlated with the increase in the percentage of the soil and water conservation measures area (p<0.05). The Rc significantly and linearly decreased with the vegetation coverage (p<0.01). Moreover, the Rc linearly decreased with the percentage of measures area in all catchments (eight of them were statistically significant with p<0.05).