Ecohydrology最新文献

The formation of a single species, recalcitrant understory vegetation layer can limit tree regeneration and, in the long term, modify the composition of forests. Few studies have investigated how recalcitrant vegetation influences competition for water resources although the formation of a dense understory is likely to modify the forest water balance. In eastern North American hardwood forests, the development of a dense understory layer of American beech (Fagus grandifolia) has been observed in stands dominated by sugar maple (Acer saccharum), a phenomenon that shares many characteristics associated with recalcitrant vegetation. Given that water availability is generally negatively correlated with stand density, we hypothesized that the formation of a dense understory beech layer increased competition for water resources, thus leading to reduced water use by sugar maple trees in beech-dominated stands. Using thermal dissipation sensors, we measured sap flux density (F_d) of two sugar maple trees at three beech-dominated sites and three control sites. During the growing season, F_d of sugar maple trees was significantly larger at beech-dominated sites compared to control sites, indicating a greater rate of water use by sugar maples in stands with a dense understory beech layer. We provide two hypotheses to explain our results at the tree scale: (i) reduced cover by forest floor vegetation could limit transpiration by this layer, thus allowing increased water availability to supply transpiration by overstory trees, or (ii) increased tree transpiration rate could be a mechanism to satisfy nutrient requirements in beech-dominated stands often associated with lower soil fertility.

Tropical coastal rivers transport significant amounts of materials, and the dam's retention efficiency can affect hydrological processes generating impacts on reservoirs, such as eutrophication. Nevertheless, climate change projects uneven regional rainfall reduction, affecting surface water circulation and consequently the dam retention efficiency with possible effects on reservoir eutrophication. Here, we investigated the water, suspended particulate matter, and nutrient mass-balance budgets in a large reservoir under a lowest rainfall year and effects on reservoir eutrophication in a tropical coastal watershed. Under low rainfall condition, the annual water budget showed that the dam water retention was limited, but the dam retained around 75% of the suspended particulate matter fluxes from the rivers. In terms of nutrients, the dam exported TN, TP and D-Si while retaining PO₄³⁻ on average; however, these circumstances fluctuated depending on the sample event. The reservoir's trophic state varied from mesotrophic to eutrophic possible reflecting the reduced dam's nutrient retention efficiency under influence of the low rainfall condition. However, 2 years following our sampling period, supereutrophic conditions and algal bloom were measured. Because human activities account for the majority of N and P loads across the watershed, mainly to soils, this biological response has been attributed to a greater rainfall regime that transfers N and P from soils to the reservoir.

Flow resistance estimate is a challenging topic for establishing flooding propensity of streams, designing river restoration works, and evaluating the use of soil bioengineering practices. In this paper, flume measurements with rigid cylinders set in two arrangements (aligned, staggered) were used to evaluate the effect of rigid emergent vegetation on flow resistance. A well-known theoretical flow resistance equation was firstly reviewed. Then, it was calibrated and tested by measurements performed for these arrangements with six concentrations (0.53–11.62 stems dm⁻²). The analysis was conducted using three approaches: (i) distinguishing the experimental runs corresponding to different arrangements and stem concentrations; (ii) using only a scale factor representing the effect of the stem concentration; and (iii) joining all available data. The results demonstrated that the flow resistance equation gives an accurate estimate of the Darcy–Weisbach friction factor f, characterized, for the best approach among the tested ones, by errors less than or equal to ±5% for 95.9% of the examined cases for the aligned arrangement and for the staggered arrangement less than or equal to ±5% for 94.2% of the examined cases. For both arrangements, the measurements demonstrated that, for a given longitudinal distance between vegetation elements, flow resistance increases for decreasing values of the transverse distance and, for a given transverse distance, f decreases for increasing values of the longitudinal distance between elements. Finally, in the range of the investigated stem concentrations, the influence of the arrangement on flow resistance resulted negligible.

Recent drought, wildfires and rising temperatures in the western US highlight the urgency of increasing resiliency in overstocked forests. However, limited valuation information hinders the broader participation of beneficiaries in forest management. We assessed how historical disturbances in California's Central Sierra Nevada affected live biomass, forest water use and carbon uptake and estimated marginal values of these changes. On average, low-severity wildfire caused greater declines in forest evapotranspiration (ET), gross primary productivity (GPP) and live biomass than did commercial thinning. Low-severity wildfires represent proxies for prescribed burns and both function as biomass removal to alleviate overstocked conditions. Increases in potential runoff over 15 years post-disturbance were valued at $108,000/km² for commercial thinning versus $234,000/km² for low-severity wildfire, based on historical water prices. Respective declines in GPP were valued at −$305,000 and −$1,317,000/km², based on an average social cost of carbon. Considering biomass levels created by commercial thinning and low-severity fire as more-sustainable management baselines for overstocked forests, carbon uptake over 15 years post-disturbance can be viewed as a benefit rather than loss. Realizing this benefit upon management re-entry may require sequestering thinned material. High-severity wildfire and clearcutting resulted in greater declines in ET and thus greater potential water benefits but also substantial declines in GPP and live carbon. These lessons from historical disturbances indicate what benefit ranges from fuels treatments can be expected from more-sustainable management of mixed-conifer forests and the importance of setting an appropriate baseline.

It is commonly believed that nonrainfall water (NRW) serves as an important and even a central source of water for biocrusts in arid and semiarid regions. Known also as biological soil crusts, they mainly consist of cyanobacteria (that dwell within the upper 2 mm of the soil) and crustose chlorolichens that commonly protrude 1–2 mm above ground. It is commonly assumed that due to the prolonged dry season in deserts, a constant source of water is required for their survival. Overestimated, nonrealistic NRW values, whether due to the use of improper measurement devices or erroneous interpretation of the recorded values as well as instances during which distillation was regarded as dewfall, supported the notion that the amounts of NRW are sufficiently high to allow for biocrust activation. In the current commentary, a short account of the reasons that led scholars to believe that NRW may be utilized by biocrusts is presented, and the findings that led to this belief are critically analysed. It is argued that the amounts of NRW reaching the surface are below the critical thresholds of 0.05 and 0.1 mm required for net photosynthesis by the chlorolichens and cyanobacteria, respectively. Moreover, it is argued that even if these thresholds are seldom reached, their duration is too short to result in net carbon gain. As shown for the dewy Negev and may be applicable for other arid and semiarid regions, biocrusts rely on rainwater for their growth and survival.

Across the globe, environmental water has been allocated with the purpose of preserving the health and vitality of floodplain vegetation. However, the influences of environmental water volume and environmental water delivery strategies have not been studied widely because of shortage of on-ground monitoring data. Remotely sensed data can bridge this gap by providing long-term and continuous information; Landsat imagery from 1988 to 2020 was used in this research. We used the normalized difference vegetation index (NDVI) as an indicator of physiological condition of lake-fringing trees on the Hattah Lakes floodplain, south-east Australia. We employed the random forest (RF) regression method to model the relationship between NDVI and various climate and hydrological factors, such as the volume of water delivered to the connected lakes system as environmental water allocations or natural floods. The RF models performed well overall, with a mean R² value of 0.73. The analysis identified the monthly total of environmental water delivered 3 months prior to the Landsat image date as a more crucial factor than natural floods over the same period for driving vegetation condition. Environmental water from 3 months previously exerts a positive influence on NDVI until the volume reaches a specific threshold. We have observed significant improvements in floodplain vegetation through the current environmental water strategy, particularly since the construction of pumping infrastructure in 2013. We suggest that managers aim to inundate the lake fringing area every 3 years, specifically from August to September, by delivering environmental water up to the modelled threshold volume. Finally, the use of infrastructure has proven to be an effective and efficient method for irrigating floodplain lakes, leading to improvements in vegetation condition while conserving water resources.

Hydrological alteration has contributed to the global decline of stream frogs. Flows support stream frog reproduction, juvenile development, food resources, and maintain habitats for all life stages. At present, there is a lack of information regarding the specific water requirements necessary for the conservation of stream frogs. To address this gap, we developed a traits-based approach that serves as a valuable tool for grouping and prioritising water-dependent stream frog species to inform future research priorities and environmental flow design. In this study, we focussed on 53 Australian frog species and analysed eight species traits to develop water-dependent functional groups for stream frogs. We classified frogs based on their level of water dependency using an agglomerative hierarchical clustering analysis and a systematic review of water requirements and water management threats. The distinguishing traits that determined functional groups were: tadpole body type, egg clutch type, stream breeding habitat type, and documented association with flowing water and water permanence. Our study identified two distinct water-dependent groups: facultative stream spawners, capable of reproducing in both stream and non-stream habitats, and obligate stream spawners, restricted solely to stream habitats. Importantly, we highlight that the obligate stream spawners are the most sensitive group to within-channel flow alteration and should be prioritised for water management decisions in lotic environments. This study represents the first comprehensive overview of the importance of hydrology for stream frogs and identifies the critical need for additional research and validation to enhance our understanding of stream frog responses to flows regimes.

Stable isotopes δ¹⁸O and δ²H are used to infer vegetation water sources. In some studies, significant xylem water δ²H offsets from potential source waters have been observed. The offsets appear to be more prevalent with cryogenic vacuum distillation (CVD) of plant water. Hypothesized mechanisms for these offsets include changes during plant water uptake and transport, and methodological problems. We propose that a large portion of the offsets are due to hydrogen isotope exchange between xylem water and non-crystalline hydroxyl groups of wood cellulose and hemicellulose during CVD. We present a method for estimating the hypothesized isotopic exchange between wood tissues and water, which is the result of Rayleigh and equilibrium fractionation. To estimate the exchange, we use published wood properties for North American tree species and isotope chemical relationships as a function of moisture content, CVD temperature and water extraction efficiency. A simple model of exchange between xylem water and hydroxyl groups captures the range of observations in studies in which CVD and non-CVD methods were compared. To evaluate the model, we compared observed δ²H offsets (sw-excess) values from two field datasets (90°C, n = 364, and 170°C, n = 43) to δ²H offsets estimated with our chemical model. We found good agreement between observed and estimated δ²H offsets for samples extracted at 90°C (r² = 0.69) but not for samples extracted at 170°C (r² = 0.20). The offset may be eliminated by increasing the extraction temperature to 229°C or by adding a standard sufficient to raise the moisture content to >150%. A correction can also be approximated by applying a theoretical calculation based on the extraction temperature, moisture content and water extraction efficiency.

Despite their importance in carbon cycling and catchment runoff dynamics, the hydrology of temperate peat swamps in response to changing hydrometeorological conditions is largely understudied. We examined the importance of hydrogeomorphic settings in controlling hydrological connectivity and runoff in a temperate peat swamp in southern Ontario, Canada over two consecutive growing seasons with contrasting conditions (dry and wet years). We chose two different small-scale hydrogeomorphic settings to investigate: (i) a site with strong wetland-stream interactions (i.e., an unconfined stream channel; unconfined) and (ii) a site with limited wetland-stream interactions (confined).

During the wet year, the confined site exhibited a consistently gaining stream, maintaining lateral hydrological connectivity and yielding high runoff ratios, while during the dry year, the confined site lost water and experienced low runoff ratios during storm events. Overland flow at the unconfined site maintained a longitudinal hydrological connectivity delivering water to its sub-catchment outflow, as reinforced by hydrochemical observations. This connectivity was maintained in the wet year but ceased in the dry year despite consistent upstream sub-catchment water inflow due to high depression storage. Runoff ratios were reduced because of this hydrological disconnection.

We highlight the importance of small-scale hydrogeomorphic setting on peat swamp carbon storage as facilitated by the variation of within-site hydrological connectivity and runoff, which also has important implications for downstream water quality. The unconfined site maintained a higher water table position in both years and has much greater peat carbon stocks. We suggest peat swamp channelization either naturally or through drainage decreases carbon stocks.