Ecohydrology最新文献

Reservoir is a vital tool for human utilization of water resources, and the deterioration of its water quality can seriously threaten the water cycle and sustainable urban development. However, there are relatively few studies in academia that analyse and evaluate the water quality of multiple reservoirs at the same time. To address this knowledge deficit, we collected 108 water samples from three different reservoirs in Chuzhou City for 36 months (from 2019 to 2021), explored the drivers of changes in water quality parameters over time and the extent of eutrophication. Our results indicated that the water quality of the reservoirs was deteriorating during the study period, among which Huanglishu Reservoir and Shahe Reservoir reached mild eutrophic status, and both had higher eutrophication levels than Chengxi Reservoir. Secchi depth, total nitrogen and total phosphorus were the principal factors inducing eutrophication. The biassed utilization of reservoir functions was the major contributor to the discrepancy in the degree of eutrophication. Furthermore, Pearson correlation analysis revealed that there are significant correlations between many water quality parameters. Cluster analysis grouped the 12 months of each year into three clusters (stable water level period, rainy season high flow period and winter low flow period). Based on this, analysis of variance showed that most water quality parameters varied considerably between the clusters. Collectively, this study identified the actual water quality conditions of three reservoirs in Chuzhou City and provided guidance for local water quality management and environmental protection.

Global-scale estimates of carbon fluxes from satellite data-driven models are constrained by considerable uncertainties regarding Gross Primary Production (GPP) and the lack of the watershed-scale measurements required for model calibration. Recently conducted global modelling efforts indicate that semiarid ecosystems dominate the increasing trends and inter-annual variation of net CO₂ exchange with the atmosphere, but semi-arid regions have received little attention with regard to GPP estimation. In this study, we used the distinct isotope effect of transpiration and evaporation to calculate transpiration losses and subsequently CO₂ uptake by terrestrial vegetation through the water and carbon cycle using the water use efficiency of plants. By studying two Mediterranean watersheds with contrasted environmental conditions over several hydrological years, we found a strong dependence of GPP on annual and seasonal water availability. The results demonstrated that when compared to GPP values obtained in worldwide biomes using biological methods, our isotope approach was validated, highlighting the limitations of satellite-data-driven models like MODIS in capturing the impact of water stress on photosynthesis and GPP estimates. These results encourage investigation of GPP by the isotope mass balance approach where direct carbon flux measurements are rare or absent in order to help to substantiate, modify or shed doubt on interpolated GPP for those regions and achieve consensus on global GPP estimates. Given the relevant role of semi-arid ecosystems in the global carbon balance as well as the limitation of existing data sets, our improved method based on the isotope mass balance approach helps to obtain rapid and affordable estimates of GPP for semi-arid ecosystems.

In ecohydrology, water isotopologues are used to assess potential sources of root water uptake by comparing xylem water signatures with source water signatures. Such comparisons are affected by the variability and uncertainty of the isotope signatures of plant water and water sources. The tree-scale and stand-scale variabilities of the isotope signatures in stem xylem water are often unknown but are important for sampling design and uncertainty estimation in assessing the sources of tree water uptake. Here, we quantified tree-scale and stand-scale variabilities of xylem water isotope signatures in beech, oak and spruce trees in a mature forest on the Swiss plateau. For stem xylem water, sub-daily replicates and replicates in different cardinal directions showed no systematic differences, but we found systematic differences with sampling height. The observed variability of isotope signatures at different heights along the stem suggests that water residence times within trees need to be considered, along with their effects on the isotope signatures in different compartments (stem, branches, leaves). Further, concerning the hydrogen signatures, we found height- and species-specific offsets (SW-excess δ²H). Stem xylem water's tree-scale variability was similar in magnitude to its stand-scale variability and smaller than the variabilities in branch xylem and bulk soil water around each tree. Xylem water from stem cores close to the ground, therefore, can give a more precise estimate of the isotopic signal of the most recent root water uptake and facilitate more accurate source water attribution.

Riparian zones in drylands provide important refugia for plants but depend on groundwater and thus are subject to local temporal and spatial variability in abiotic controls. In lieu of costly field-based sampling, we used readily available data to establish site–scale interannual relationships among riparian plant health and the abiotic factors that control their water balance for a historically persistent wetland adjoining the Santa Clara River in southern California, USA. Non-linear generalized additive model (GAM) analysis of plant health, represented using the normalized difference vegetation index (NDVI), confirmed robust relationships among plant health and various geomorphological and hydrological factors over multi-decadal timeframes, including years since last high-flow event, intra-year groundwater elevation changes and magnitude of 2-year cumulative surface water inflows. Geomorphic controls are related to years with high flows that cause extensive scour and deposition that re-set riparian plant communities. Relationships with dry-season groundwater declines reflect direct plant access to sub-surface moisture. Hydrological dependence via cumulative inflow magnitude indicates the dependency of groundwater elevations on sufficient winter recharge to prevent precipitous groundwater decline. GAMs-based inflection point analysis of surface water inflows versus groundwater elevations confirmed that the cumulative magnitude of multi-year inflows is critical in avoiding catastrophic groundwater declines and that large flood events drive groundwater recovery. We show that abiotic controls on plant health can be derived from readily available data and that non-linear analysis better represents the complexity of these scalar controls. Our analysis has relevance for ecosystem management of human-altered rivers and climate change adaptation.

Stable isotopes of hydrogen and oxygen in water are common tools for investigating water uptake apportionment, but many of the existing methods rely on simple linear mixing approaches that do not mechanistically incorporate additional information about site physical properties and conditions. Here, we develop a ‘physically based root water uptake isotope mixing estimation’ model (PRIME) that combines a continuous and parametric probability density function for root water uptake with site physical data in a process-based linear mixing framework. To demonstrate the application of PRIME, water uptake patterns of boreal forest Pinus banksiana trees were estimated on four dates in 2019. To aid in validation, estimates were compared with that of the Bayesian linear mixing model framework, MixSIAR. The two approaches provided similar results, but due to its continuous and parametric nature, PRIME provided estimates of superior resolution, certainty, and model parsimony. Although both models incorporate additional physical information into their mixing frameworks, PRIME does so in a mechanistic manner, thereby reflecting the relevant hydrological processes more effectively than the purely empirical approach taken by MixSIAR. Furthermore, because PRIME uses a continuous function to describe the predicted uptake pattern, it allows users to quantify water uptake with essentially infinite resolution, through integration over the desired depth ranges. These findings demonstrate the advantages of utilizing a continuous, parametric, and process-based mixing model to estimate root water uptake apportionment, thus providing a relatively simple yet powerful tool with which to approach plant water sourcing.

Boreal forests cover vast stretches of land across all continents and represent a principal source area of clean water in the northern hemisphere. Increasingly, studies are conducted on the impact of changes in boreal forest cover on water yield; however, much remains unknown concerning the effects of forest structure changes on stream discharge over the course of multi-decadal forest harvest cycles. In this study, we analysed long-term hydrometeorological and forest dynamics data spanning from 1990 to 2016 from a typical boreal forest watershed in the Da Hinggan Mountains in northern China. Our objective was to quantify how changes in forest age and tree species composition affect mean annual streamflow and flow regimes in the context of a changing climate. To distinguish the effects of forest and climate changes on annual streamflow from one another, we employed a combination of a sensitivity-based method and a temporal trend analysis. Further, we evaluated the impact of forest changes on flow regimes using four indicators: magnitude, duration, frequency, and variability. The results indicated that mean annual streamflow increased by 55.8 mm, with forest changes contributing +61.4 mm compared to −5.6 mm due to climate change (negative effect). This increase occurred when approximately 20% of mature coniferous forests transitioned to mid-age broad-leaved forests, accompanied by a 10% increase in total stock volume during the later period. Finally, the effect of changes in forest structure on flow regime were not significant. Our results underscore that variations in forest structure affect streamflow differently depending on stand age and species proportions. Therefore, dynamic forest structure management can benefit not only carbon sequestration but also water supply capacity in boreal forested watersheds.

Tropical mountains such as the páramos of the Andes, which serve as ‘water towers’ for local communities and downstream cities, are important areas for early detection of climate change. Here, fog and low-intensity rainfall are very common and play a key role in ecohydrological processes. Although evapotranspiration (ET) represents an important part of the water cycle, how ET and fog processes interact and how they affect páramo vegetation and water resources availability are poorly understood. This study investigated the effects of foggy (fog only) and mixed (fog and rainfall) conditions on ET. To determine whether fog significantly reduces ET, we compared ET and meteorological data under these two conditions with those during dry days. We found that on foggy days, when fog was most prevalent in the early morning, ET declined on average by 4% and net radiation (Rn) by 9.2%. Under mixed conditions, daily ET declined by 42% and Rn by 33%. In the páramo, where mean annual precipitation and ET are 1210 and 635 mm, respectively, the estimated annual reduction in ET due to fog and rainfall combined is between 77 and 174 mm. We found that during fog and rainfall mixed conditions, solar radiation was reduced, consequently constraining the energy available for ET while sustaining high relative humidity, ultimately reducing water loss. Our findings, which suggest that the presence of fog and low-intensity rainfall restricts water losses by evaporative demand, contribute to a better understanding of the ecohydrological importance of these water inputs in the Andes.

Southern African savanna rangelands are facing a widespread degradation pattern called bush encroachment. This is associated with implications for various aspects of the water cycle and in particular canopy transpiration. At the individual-tree scale, it is estimated by scaling sap-flux density by sapwood area. However, the direct measurement of sapwood area is impracticable at landscape scale and general allometric equations of the West-Brown-Enquist (WBE) model relating sapwood area to primary size measures seem to fail for some species and climates. Therefore, we conducted intensive field measurements to establish species-specific allometric relationships between sapwood area and sizes (stem diameter, crown area) in six dominant shrub species involved in bush encroachment in Namibia (Colophospermum mopane, Senegalia mellifera, Vachellia reficiens, Dichrostachys cinerea, Vachellia nebrownii, Catophractes alexandri). We found strong allometric relationships between sapwood area and stem diameter as well as between sapwood area and crown area for all six species. These relations are largely in line with the WBE theory but still provide estimates that are more accurate. Only in D. cinerea, the sapwood area was significantly smaller than predicted by the WBE theory, which might be caused by a larger need for stabilizing heartwood. Our results are useful to estimate water loss via transpiration at a large scale using remote sensing techniques and can promote our understanding of the ecohydrological conditions that drive species-specific bush encroachment in savannas. This is particularly important in the light of climate change, which is considered to have major implications on ecohydrological processes in savannas.

As floodplains are inundated during floods in a compound channel, solutes in the surface water column reach the hyporheic zone and react with solutes upwelled from the groundwater. These biogeochemical reactive processes, such as aerobic respiration, nitrification, and denitrification, need more clarification. In this study, a 3D hydrodynamic model combined with a 2D groundwater and biogeochemical model was used to examine the influence of bank slope angle and ambient groundwater discharge on these processes. A denitrification zone was found under the interface between the main channel and the floodplain when the bank slope angle was 90°, while lower angles extended that zone horizontally. In addition, a lower bank angle decreased N entry into the streambed and enhanced nitrogen removal. A decrease in ambient groundwater had a negative impact on both aerobic respiration and denitrification. When the ambient groundwater discharge reached below −0.9 m/d, nitrification was dominant in the model domain, and the hyporheic zone turned into a NO₃⁻ source. The greatest removal efficiency, equal to 0.8, was attained at a discharge rate of −0.5 m/d for ambient groundwater and a bank slope angle of 30°. The hyporheic zone should lose its ability to remove N when ambient groundwater discharges exceed 0.25 m/d and removal efficiency fluctuates by 0. In conclusion, our findings indicate that bank slope angle and ambient groundwater discharge have a substantial impact on solute transport and biogeochemical activities in the hyporheic zone of a compound channel.

The term ‘succession’ was first proposed to describe the gradual development of plants from an initial stage such as a bare ground to a well-developed plant community, which at its peak, may reach a climax (primary succession). Accordingly, the earlier and fast growing stage (such as an annual plant community) may grant stability, organic matter and nutrients to the latter, high-biomass and slow-growing stages, such as trees. Commonly, reference to the different successional stages is also made once intact and disturbed communities (such as due to mechanical disturbance, tillage, fire, etc.) are compared (secondary succession). The concept was borrowed by many ecologists to describe variable biocrust types. Cyanobacterial or algal biocrust is regarded as an initial stage before turning to a later, more mature biocrust, whether composed of lichens or mosses. The underlain assumptions are that (a) the cyanobacteria provide essential stability and (b) nutrients that are required for the development of the later stages; (c) the initial biocrusts improve the water regime for the later successional stages; (d) cyanobacteria promote the lichen symbiosis; (e) due to substantial differences in the recovery time, a linear succession is inevitable, commonly from cyanobacterial/alga to lichen and lastly to moss; and (f) the cyanobacterial/algal biocrust is a temporary stage, just before being outcompeted by a later stage. It is argued hereafter that the above-mentioned assumptions are not necessarily correct. As with higher plants, unless a direct comparison between disturbed and intact crusts justifies a reference to successional stages, different types of biocrusts commonly reflect the abiotic conditions at their site, and as such, unless the abiotic conditions change, they reflect stable communities of variable crust types. This paradigm shift may have important implications regarding inoculation efforts and directions and may explain the low success thus far obtained following inoculation experiments once performed with the more developed biocrusts, lichens and mosses.