Explore the vast range of titles available.

Headwaters play a crucial role in maintaining forest biodiversity by providing unique habitats and are important for the regulation of water temperature and oxygen levels for downstream river networks. Approximately 90% of the total length of streams globally originate from headwaters and these systems are discussed to be especially vulnerable to impacts of climate change. This study uses an integrated hydrological model (HydroGeoSphere) in combination with 23 downscaled ensemble members from representative concentration pathways (RCPs) 2.6, 4.5 and 8.5 to examine how climate change affects water availability in a headwater catchment under baseflow conditions. The simulations consistently predict increasing water deficits in summer and autumn for both the near (2021–2050) and far future (2071–2099). Annual mean water deficits were estimated to be 4 to 7 times higher than historical levels. This is mainly due to a projected reduction in precipitation inputs of up to – 22%, while AET rates remain similar to those observed during the historical reference period (1992–2018). The declining groundwater storage reserves within the catchment are expected to result in a significant decline in surface water availability during summer and autumn, with a reduction in mean annual stream discharge by up to 34% compared to the reference period. Due to declining groundwater levels, upstream reaches are predicted to become intermittent in summer leading to a reduction of the total stream flow length by up to 200 m. Findings from this study will enhance our understanding of future water availability in headwater systems and may aid in the development of effective management strategies for mitigating local impacts of climate change and preserving these vulnerable ecosystems.

This article is a study of headwater catchments and streams that aims at a first morphological typology of such catchments for Corsica island. Such typology offers a base for various studies or assessments regarding, for instance, water management, geological hazards assessments or faunal and/or floral species conservation. Corsica island comprises 120 peaks over 2000 m above sea-level. Herein, the characteristics and developments of 83 Corsican headwater catchments found above 1200 m above sea-level are evaluated using a digital elevation model, a principal component analysis and a hierarchical cluster analysis based on 14 morphometric parameters. It appears that 65.31% of the data’s total variance is clarified by the first three principal components. On the first principal component maximum elevation, mean elevation, mean slope and Melton ratio are found opposed to area and perimeter. Moreover, a latitudinal gradient on the first principal component is confirmed by the supplementary variable latitude’s coordinate. On the second principal component the form factor is opposed to the Lemniscate ratio. The most representative catchment points are best highlighted in terms of the catchments of the seven main streams. The cluster analysis allows to divide the catchments into four groups or types, unequivocally reflecting the first principal component and emphasising the comparable results for the two analyses. These four types of catchments are numerically characterised by four main parameters, which are area, Melton ratio, mean elevation and mean slope. A first morphological typology for Corsican headwater catchments is proposed in accordance to both principal components and cluster analyses.

To improve flood prediction in headwater catchments, hydrologists need to know initial soil moisture conditions that precede rain events. In torrential hydrology, soil moisture mapping provides a valuable tool for investigating surface runoff generation processes. In these mountainous environments, soil moisture prediction is challenging because of highly heterogeneous land cover and soil properties. This survey propose a methodology to study spatial soil moisture variations in the mountainous and torrential environment of the Draix Bléone experimental site—Laval 0.86 km2. This approach associates water content measurements at the plot scale with spatialized soil bulk electrical conductivity (ECa) measurements combined in a multivariate statistical analysis based on topographical parameters. Between the summer of 2015 and winter of 2016, four geophysical surveys were conducted under various moisture conditions and along the same pathway, using the Slingram electromagnetic induction (EMI) technique (EM31 device) in horizontal dipole to identify changes in soil properties to a depth of 3 m. These results were analyzed to determine water dynamics in this mountainous catchment. Temporal variations of ECa vary among land cover types (forest, grassland, and black marl). A significant relationship was observed between ECa and soil water content (SWC) measured with capacitive sensors in forest and grassland. A multiple linear regression produced using the spatial interpolation code LISDQS shows a significant correlation between ECa and landform units depicted on a high-resolution DEM. ECa variations decrease with distance to talwegs. Riparian zones appear as potential hydrological contributing areas with patterns varying according to moisture status. This study shows that multiple linear regression analysis and EMI make it possible to fill gaps between SWC plot measurements, over wide areas that are steep and that present numerous obstacles due to vegetation cover.

Groundwater zone formation in the soil layers of a headwater catchment is an important factor that controls volumetric and chemical changes in streamflow; it also induces shallow landslides. Previous studies have suggested that the groundwater zone in soil layers generally forms transiently atop low-permeability layers in response to rainfall. This study focused on an unchanneled hollow in a serpentine headwater catchment, where a semi-perennial to perennial groundwater zone was observed in thin organic soil layers (OSLs) overlying thick clay mineral soil layers (CMLs), even during dry periods. We conducted detailed observations in this catchment to clarify the formation processes of the semi-perennial to perennial groundwater zone. The results showed that water is supplied from the CMLs to the OSLs as unsaturated upward flow in areas where the OSLs are dry. This water then accumulates in the downslope hollow, which sustains the groundwater zone in the OSLs during dry periods. The frequent and long-term occurrence of upward flow can be attributed to differences in the hydraulic properties of OSLs and CMLs. This process prevents the OSLs in the hollow from drying, presumably causes volumetric and chemical changes in streamflow, and reduces the stability of OSLs.

Headwaters are hotspots of carbon dioxide (CO2) evasion from rivers. While emerging evidence suggests that groundwater contributes disproportionately to CO2 in headwater streams, the processes of CO2 delivery to streams and subsequent evasion to the atmosphere remain largely unknown. Here we show the variability of CO2 input and evasion fluxes based on coupled measurements of dissolved CO2 along streams and in adjacent groundwater from two headwater catchments of the tropical and temperate zones. We find that the processes can be highly localized in both space and time. Spatially, they are significantly influenced by heterogeneities in the subsurface and stream landscape; temporally, they predominately occur during the transient activation of connected subsurface water flows. We highlight sharp increases and decreases in the stream CO2 flux, and suggest that current models fail to capture the true magnitude of CO2 evasion. The high spatial and temporal variability of CO2 input from groundwater and evasion to the atmosphere makes accurate assessment of CO2 evasion fluxes difficult, and will require a collaborative effort by catchment hydrologists and aquatic ecologists to fully understand the contribution of groundwater to stream CO2 emissions.

This paper provides a summary of the assessment report of the World Meteorological Organization (WMO) Expert Team on Weather Modification that discusses recent progress on precipitation enhancement research. The progress has been underpinned by advances in our understanding of cloud processes and interactions between clouds and their environment, which, in turn, have been enabled by substantial developments in technical capabilities to both observe and simulate clouds from the microphysical to the mesoscale. We focus on the two cloud types most commonly seeded in the past: winter orographic cloud systems and convective cloud systems. A key issue for cloud seeding is the extension from cloud-scale research to water catchment–scale impacts on precipitation on the ground. Consequently, the requirements for the design, implementation, and evaluation of a catchment-scale precipitation enhancement campaign are discussed. The paper concludes by indicating the most important gaps in our knowledge. Some recommendations regarding the most urgent research topics are given to stimulate further research.

Flowing stream networks dynamically extend and retract, both seasonally and in response to precipitation events. These network dynamics can dramatically alter the drainage density and thus the length of subsurface flow pathways to flowing streams. We mapped flowing stream networks in a small Swiss headwater catchment during different wetness conditions and estimated their effects on the distribution of travel times to the catchment outlet. For each point in the catchment, we determined the subsurface transport distance to the flowing stream based on the surface topography and determined the surface transport distance along the flowing stream to the outlet. We combined the distributions of these travel distances with assumed surface and subsurface flow velocities to estimate the distribution of travel times to the outlet. These calculations show that the extension and retraction of the stream network can substantially change the mean travel time and the shape of the travel time distribution. During wet conditions with a fully extended flowing stream network, the travel time distribution was strongly skewed to short travel times, but as the network retracted during dry conditions, the distribution of the travel times became more uniform. Stream network dynamics are widely ignored in catchment models, but our results show that they need to be taken into account when modeling solute transport and interpreting travel time distributions.

Inter‐catchment groundwater flow (IGF) plays an essential role in streamflow generation and water quality in forested headwaters. Multiple factors are thought to contribute to IGF, including climate, topographical, and geological factors. However, studies have not clarified the relationships between IGF and catchment properties in the headwater catchments due to the lack of observational data at scales smaller than 100 ha. This study examined possible factors influencing IGF using random forest analysis based on annual water balance data from 152 forested catchments ranging from 0.09 to 9400 ha in Japan. The results showed that catchment scale had the greatest influence on IGF, and IGF tended to decrease with increasing catchment area at scales of less than 10 ha. The average IGF stabilized around zero in catchments greater than 10 ha. The averaged IGF trend with catchment scale indicated more outward groundwater flow in catchments smaller than 10 ha, but no relationship between IGF and catchment size in catchments larger than 10 ha. The variability in IGF decreased with catchment size and was lowest at 10–100 ha. The decrease in variability in catchments less than 100 ha was mainly due to river confluence and the increased variability in catchments larger than 100 ha indicated potential observation errors increase in catchments of this size. Plain Language Summary Inter‐catchment groundwater flow (IGF) refers to groundwater flux across surface topographic boundaries. Recent studies have clarified that IGF significantly affects streamflow generation and water quality. However, direct observation of IGF is difficult, and the mechanisms underlying IGF are not fully understood. This study examined the factors influencing IGF, as well as its scale dependence and variability based on water balance data from 152 forested catchments ranging from 0.09 to 9400 ha in Japan. This showed that catchment area had the greatest influence on IGF. Groundwater tended to flow out of catchments smaller than 10 ha, and stabilized in those larger than 10 ha. Variability in the IGF decreased with catchment size at less than 100 ha. These results suggest that on the scale of hillslope and headwater catchments, IGF has a great influence on streamflow generation. Key Points We examined the factors influencing inter‐catchment groundwater flow (IGF) using water balance data for 152 forest catchments in Japan Catchment scale was the factor with the greatest influence on the IGF Groundwater tends to flow out of catchments smaller than 10 ha and IGF approaches zero when the catchment area exceeds 10 ha

Headwater catchments have strong impacts on downstream waterways, near‐shore ecosystems, and the quality of water available for growing human populations. Thus, understanding how water and solutes are exported through these upland landscapes is critically important. A growing body of literature highlights the interaction of topography, climate, and the critical zone structure as a key control on streamflow and chemical export. However, more focused work is needed to pinpoint how variability in subsurface structure across lithologically complex regions impacts streamflow and chemical signals at catchment outlets. Here, we aim to better understand how lithology and subsurface critical zones modulate streamflow response and solute export patterns in two central coastal California headwater catchments that are similar in topography, vegetation, and climate but have different lithologies. We monitored streamflow and collected surface water samples at the catchment outlets for dissolved major ions and organic carbon (DOC) for two consecutive water years. The catchment with mélange bedrock displayed much flashier hydrologic behavior with 7.8 times higher peak flow values and 1.9 times higher mean event concentrations of DOC, suggesting shorter and shallower hydrologic flow paths that likely arise from regions of shallower bedrock. Despite distinct hydrologic behavior and DOC export, dissolved major ion concentrations were broadly similar and chemostatic, which may be driven by rapid chemical reactions in the critical zone of both catchments. Our work contributes to building an integrated understanding of how subtle differences in catchment structure can have profound impacts on how water and solutes are routed through headwater catchments. Key Points Bedrock lithology drives differences in streamflow and carbon export in adjacent catchments, controlling variability in shallow flow paths Both catchments display similar in‐stream major ion concentrations and chemostatic behavior, likely driven by rapid chemical reactions Individual storm events have an outsized effect on annual chemical export patterns

Deforestation can considerably affect transpiration dynamics and magnitudes at the catchment scale and thereby alter the partitioning between drainage and evaporative water fluxes released from terrestrial hydrological systems. However, it has so far remained problematic to directly link reductions in transpiration to changes in the physical properties of the system and to quantify these changes in system properties at the catchment scale. As a consequence, it is difficult to quantify the effect of deforestation on parameters of catchment-scale hydrological models. This in turn leads to substantial uncertainties in predictions of the hydrological response after deforestation but also to a poor understanding of how deforestation affects principal descriptors of catchment-scale transport, such as travel time distributions and young water fractions. The objectives of this study in the Wüstebach experimental catchment are therefore to provide a mechanistic explanation of why changes in the partitioning of water fluxes can be observed after deforestation and how this further affects the storage and release dynamics of water. More specifically, we test the hypotheses that (1) post-deforestation changes in water storage dynamics and partitioning of water fluxes are largely a direct consequence of a reduction of the catchment-scale effective vegetation-accessible water storage capacity in the unsaturated root zone (SU, max) after deforestation and that (2) the deforestation-induced reduction of SU, max affects the shape of travel time distributions and results in shifts towards higher fractions of young water in the stream. Simultaneously modelling streamflow and stable water isotope dynamics using meaningfully adjusted model parameters both for the pre- and post-deforestation periods, respectively, a hydrological model with an integrated tracer routine based on the concept of storage-age selection functions is used to track fluxes through the system and to estimate the effects of deforestation on catchment travel time distributions and young water fractions Fyw. It was found that deforestation led to a significant increase in streamflow accompanied by corresponding reductions of evaporative fluxes. This is reflected by an increase in the runoff ratio from CR=0.55 to 0.68 in the post-deforestation period despite similar climatic conditions. This reduction of evaporative fluxes could be linked to a reduction of the catchment-scale water storage volume in the unsaturated soil (SU, max) that is within the reach of active roots and thus accessible for vegetation transpiration from ∼258 mm in the pre-deforestation period to ∼101 mm in the post-deforestation period. The hydrological model, reflecting the changes in the parameter SU, max, indicated that in the post-deforestation period stream water was characterized by slightly yet statistically not significantly higher mean fractions of young water (Fyw∼0.13) than in the pre-deforestation period (Fyw∼0.12). In spite of these limited effects on the overall Fyw, changes were found for wet periods, during which post-deforestation fractions of young water increased to values Fyw∼0.37 for individual storms. Deforestation also caused a significantly increased sensitivity of young water fractions to discharge under wet conditions from dFyw/dQ=0.25 to 0.36. Overall, this study provides quantitative evidence that deforestation resulted in changes in vegetation-accessible storage volumes SU, max and that these changes are not only responsible for changes in the partitioning between drainage and evaporation and thus the fundamental hydrological response characteristics of the Wüstebach catchment, but also for changes in catchment-scale tracer circulation dynamics. In particular for wet conditions, deforestation caused higher proportions of younger water to reach the stream, implying faster routing of stable isotopes and plausibly also solutes through the sub-surface.