The permeability heterogeneity of cross-bedded sediment increases path lengths of river-groundwater mixing (hyporheic exchange) in riverbeds and modifies the distribution of residence times. For two ...case studies, we numerically simulated fluid flow and solute transport through immobile bed forms composed of heterogeneous sediment and equivalent homogeneous sediment in order to clarify how cross-bedded permeability structures impact hyporheic exchange. The two permeability fields are from the cross-bedded Massillon Sandstone and modern climbing ripple deposits of the Brazos River (Texas). In both cases, permeability heterogeneity creates long hyporheic exchange paths but only slightly increases the depth of exchange relative to equivalent homogeneous sediment. In the Massillon example, permeability heterogeneity increases the proportion of long hyporheic residence times (>3 days). In the Brazos example, permeability heterogeneity increases the proportion of short residence times (<17 h). We attribute the different responses in residence time distributions to differences in permeability patterns near the sediment-water interface. The tails of residence time distributions extend for tens of years and conform to a power law in both heterogeneous and homogeneous sediment. Current-bed form interactions are responsible for the long tails, as opposed to permeability heterogeneity.
We know little regarding how geomorphological features along the surface‐groundwater interface collectively affect water quality and quantity. Simulations of surface water‐groundwater exchange at ...increasing scales across bed forms, bars and bends, and basins show that groundwater has a power‐law transit time distribution through all these features, providing a purely mechanistic foundation and explanation for temporal fractal stream chemistry. Power‐law residence time distributions are almost always attributed to spatial variability in subsurface transport properties‐ something we show is not necessary. Since the different geomorphological features considered here are typical of most landscapes, fractal stream chemistry may be universal and is a natural consequence of water exchange across multifaceted interfaces.
Tracer approaches to estimate both porewater exchange (the cycling of water between surface water and sediments, with zero net water flux) and groundwater inflow (the net flow of terrestrially ...derived groundwater into surface water) are commonly based on solute mass balances. However, this requires appropriate characterization of tracer end‐member concentrations in exchanging or discharging water. Where either porewater exchange or groundwater inflow to surface water occur in isolation, then the water flux is easily estimated from the net tracer flux if the end‐member is appropriately chosen. However, in most natural systems porewater exchange and groundwater inflow will occur concurrently. Our analysis shows that if groundwater inflow (Qg) and porewater exchange (Qp) mix completely before discharging to surface water, then the combined water flux (Qg + Qp) can be approximated by dividing the combined tracer flux by the difference between the porewater and surface water concentrations, (cp – c). If Qg and Qp do not mix prior to discharge, then (Qg + Qp) can only be constrained by minimum and maximum values. The minimum value is obtained by dividing the net tracer flux by the groundwater concentration, and the maximum is obtained by dividing by (cp – c). Dividing by the groundwater concentration gives a maximum value for Qg. If porewater exchange and groundwater outflow occur concurrently, then dividing the net tracer flux by (cp – c) will provide a minimum value for Qp. Use of multiple tracers, and spatial and temporal replication should provide a more complete picture of exchange processes and the extent of subsurface mixing.
Key Points
Environmental tracer approaches to estimate surface water—groundwater exchange fluxes depend on appropriate characterization of end‐members
Extent of mixing of exchanging porewater and groundwater prior to discharge will affect choice of end‐members
Although absolute fluxes cannot always be determined, both porewater and groundwater fluxes can be constrained by tracer methods
Within water resources management, surface water area (SWA) variation plays a vital role in hydrological processes as well as in agriculture, environmental ecosystems, and ecological processes. The ...monitoring of long-term spatiotemporal SWA changes is even more critical within highly populated regions that have an arid or semi-arid climate, such as Iran. This paper examined variations in SWA in Iran from 1990 to 2021 using about 18,000 Landsat 5, 7, and 8 satellite images through the Google Earth Engine (GEE) cloud processing platform. To this end, the performance of twelve water mapping rules (WMRs) within remotely-sensed imagery was also evaluated. Our findings revealed that (1) methods which provide a higher separation (derived from transformed divergence (TD) and Jefferies–Matusita (JM) distances) between the two target classes (water and non-water) result in higher classification accuracy (overall accuracy (OA) and user accuracy (UA) of each class). (2) Near-infrared (NIR)-based WMRs are more accurate than short-wave infrared (SWIR)-based methods for arid regions. (3) The SWA in Iran has an overall downward trend (observed by linear regression (LR) and sequential Mann–Kendall (SQMK) tests). (4) Of the five major water basins, only the Persian Gulf Basin had an upward trend. (5) While temperature has trended upward, the precipitation and normalized difference vegetation index (NDVI), a measure of the country’s greenness, have experienced a downward trend. (6) Precipitation showed the highest correlation with changes in SWA (r = 0.69). (7) Long-term changes in SWA were highly correlated (r = 0.98) with variations in the JRC world water map.
Climate and land use change modify surface water availability in African savannas. Surface water is a key resource for both wildlife and livestock and its spatial and temporal distribution is ...important for understanding the composition of large herbivore assemblages in savannas. Yet, the extent to which ungulate species differ in their water requirements remains poorly quantified. Here, we infer the water requirements of 48 African ungulates by combining six different functional traits related to physiological adaptations to reduce water loss, namely minimum dung moisture, relative dung pellet size, relative surface area of the distal colon, urine osmolality, relative medullary thickness, and evaporation rate. In addition, we investigated how these differences in water requirements relate to differences in dietary water intake. We observed strong correlations between traits related to water loss through dung, urine and evaporation, suggesting that ungulates minimize water loss through multiple pathways simultaneously, which suggests that each trait can thus be used independently to predict water requirements. Furthermore, we found that browsers and grazers had similar water requirements, but browsers are expected to be less dependent on surface water because they acquire more water through their diet. We conclude that these key functional traits are a useful way to determine differences in water requirements and an important tool for predicting changes in herbivore community assembly resulting from changes in surface water availability.
Duration and timing of thermal stratification and the period of vertical mixing are crucial for internal chemical and biological processes of deep temperate lakes. Climatic changes induced a ...prolongation of thermal stratification in many lakes over the last decades. This study provides a comparative assessment of observed climate‐induced changes in thermal and limnological properties of three originally dimictic temperate lakes.
We analysed 41 years of vertical sampling data of Lake Irrsee (IR), Mondsee (MO) and Hallstätter See (HA) located in the Salzkammergut lake district of Austria. We examined temperatures in surface and bottom waters, quantified changes in thermal regimes and investigated the development of hypoxia, anoxia and phosphorus concentrations of the three lakes from 1975 to 2015.
Mean annual surface‐water temperatures increased substantially, whereas bottom‐water temperatures showed insignificant trends in IR and MO and a slight rising trend in HA. Resulting higher vertical temperature gradients caused stronger thermal stabilities of the lakes. Based on calculations of daily thermal stability, all three lakes showed a significant prolongation of the stratification period with +31 days (±7 days) in IR, +37 days (±7 days) in MO and +28 (±8 days) in HA. We observed greater changes in the timing of onset of stratification compared to the changes in the timing of stratification termination (IR onset −19 days versus termination +11 days, MO onset −21 days versus termination +16 days, HA onset −16 days versus termination +12 days).
We found that expanding seasonal hypoxia and anoxia were correlated to prolonged seasonal stratifications and to increasing phosphorus concentrations in bottom‐water layers. However, higher phosphorus concentrations in bottom waters did not raise the lakes′ volume‐weighted total phosphorus concentrations.
Under ongoing climate change, alterations in the thermal characteristics will inevitably lead to a transition of mixing regimes from a dimictic to a monomictic mixing pattern in originally ice‐covered temperate lakes. Our investigation revealed that this transition will be earlier in larger and deeper lakes.
The streambed is the critical interface between the aquatic and terrestrial systems and hosts important biogeochemical hot spots within river corridors. Although the streambed characteristics are ...significantly different from those of its surrounding soil, the streambed itself has not been explicitly represented in watershed models. Here, we explicitly incorporated a streambed layer into an integrated hydrologic model through model parameterization and discretization. We examined the hydrological effects of streambed characteristics, including hydraulic conductivity (K), layer thickness, and resolution, on the exchange fluxes across the streambed as well as the streamflow at the watershed outlet. The numerical experiments were performed in the American River Watershed, a headwater, mountainous watershed within the Yakima River Basin in central Washington. Despite having a negligible effect on the watershed streamflow, an explicit representation of the streambed with distinctive properties dramatically changed the magnitude and variability of the exchange flux. In general, a larger streambed K along with a thicker streambed layer induced larger exchange fluxes. The exchange flux was most sensitive to the streambed resolution. A finer streambed resolution increased exchange fluxes per unit area while reducing the overall exchange volumes across the entire streambed. The amount of baseflow decreased by 6% as the streambed resolution increased from 250 to 50 m. This finding is important because these hydrological changes may, in turn, affect the exchange of nutrients and contaminants between surface water and groundwater and the associated biogeochemical processes. Our work demonstrated the importance of representing streambeds in fully distributed, process‐based watershed models to better capture the exchange flow dynamics in river corridors.
Previous watershed models have not explicitly represented the streambed, despite its significantly different characteristics from the surrounding soil.
The researchers explicitly incorporate a streambed layer with various characteristics into an integrated hydrologic model through model parameterization and discretization.
The exchange flux across the streambed was most sensitive to the streambed resolution. A finer streambed resolution increased exchange fluxes per unit area while reducing the overall exchange volumes across the entire streambed.
Anthropogenic nitrate loads to rivers and coasts deteriorate coastal water quality. The primary sink for nitrate is denitrification in aquatic sediments. Here I show that nitrate removal rates in ...upwelling groundwater are as much as 60 times more efficient in heterogeneous than equivalent homogeneous aquatic sediments, even when travel times are the same. Coupled flow and reactive transport simulations were used to quantify the removal of groundwater‐borne nitrate in aquatic sediments with sand and silt structures that represent infilled burrows, rip‐up clasts, or other core‐scale features. In silt structures with greater organic carbon content and microbial biomass, aerobic respiration consumes oxygen, creating localized zones of denitrification that would not otherwise exist in homogeneous sediments. While hot spots of denitrification have previously been shown to form in organic‐rich aggregates in soils and sediments, this study is the first to quantify their potentially large influence on groundwater‐borne nitrate loads to surface waters.
Key Points
Core‐scale heterogeneity enhances denitrification in aquatic sediments
Organic matter distribution affects nitrogen removal as much as residence time
Homogeneous transport models may poorly represent nitrogen fate
Asia’s largest lake and the world’s foremost tropical lagoon, Chilika, stands as a testament to ecological diversity. The lake is diverse in biodiversity and is a sanctuary for over 400 distinct ...brackish and freshwater species. However, the lake is confronted by ceaseless pressures from a confluence of natural forces and anthropogenic activities. These challenges threaten to unleash ecological transformations that could reshape this ecological marvel. This study examined the spatial–temporal variation in the lake for the years 1988 to 2017 using archival remote sensing data. The Normalized Difference Water Index (NDWI) derived from Landsat 5-TM and Landsat 8-OLI was used to understand the expansion and contraction happening in the extent of the lake. To determine the water spread area, from each NDWI image, the minimum (Min.) pixel values, maximum (Max.) pixel values, and mean pixel values were extracted, and a yearly composite for was created the aforementioned years.
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