Nonperennial streams dominate global river networks and are increasing in occurrence across space and time. When surface flow ceases or the surface water dries, flow or moisture can be retained in ...the subsurface sediments of the hyporheic zone, supporting aquatic communities and ecosystem processes. However, hydrological and ecological definitions of the hyporheic zone have been developed in perennial rivers and emphasize the mixing of water and organisms from both the surface stream and groundwater. The adaptation of such definitions to include both humid and dry unsaturated conditions could promote characterization of how hydrological and biogeochemical variability shape ecological communities within nonperennial hyporheic zones, advancing our understanding of both ecosystem structure and function in these habitats. To conceptualize hyporheic zones for nonperennial streams, we review how water sources and surface and subsurface structure influence hydrological and physicochemical conditions. We consider the extent of this zone and how biogeochemistry and ecology might vary with surface states. We then link these components to the composition of nonperennial stream communities. Next, we examine literature to identify priorities for hydrological and ecological research exploring nonperennial hyporheic zones. Lastly, by integrating hydrology, biogeochemistry, and ecology, we recommend a multidisciplinary conceptualization of the nonperennial hyporheic zone as the porous subsurface streambed sediments that shift between lotic, lentic, humid, and dry conditions in space and time to support aquatic–terrestrial biodiversity. As river drying increases in extent because of global change, we call for holistic, interdisciplinary research across the terrestrial and aquatic sciences to apply this conceptualization to characterize hyporheic zone structure and function across the full spectrum of hydrological states.
One central issue affecting the health of native fish species in the Pacific Northwest is water temperature. In situ observation methods monitor point temperatures, while thermal infrared (TIR) ...remote sensing captures spatial variations. Satellite-based TIR sensors have the ability to view large regions in an instant. Four Pacific Northwest river reaches were selected to test the ability of both satellite-based and moderate resolution aircraft-based TIR remote sensing products to measure river temperatures. Images with resolutions of 5, 15, and 90 meters were compared with instream temperature observations to assess how along stream radiant temperatures are affected by resolution, reach width, and sensor platform. Where the stream reach can be resolved by the sensor, all sensors obtain water temperatures within +/- 2 degrees C of instream observations. Along stream temperature variations of up to +/- 5 degrees C were also observed. Trends were similar between two sets of TIR images taken several hours apart, indicating that the sensors are observing actual temperature patterns from the river surface. If sensor resolution is sufficient to obtain fully resolved water pixels in the river reach, accurate temperatures and spatial patterns can be observed. The current generation of satellite-based TIR sensors is, however, only able to resolve about 6 percent of all Washington reaches listed as thermally impaired.
Emitted thermal infrared radiation (TIR, lambda = 8 to 14 micrometer) can be used to measure surface water temperatures (top approximately 100 micrometer). This study evaluates the accuracy of stream ...(50 to 500 m wide) and lake (300 to 5,000 m wide) radiant temperatures (15 to 22 degrees C) derived from airborne (MASTER, 5 to 15 m) and satellite (ASTER 90 m, Landsat ETM+ 60 m) TIR images. Applied atmospheric compensations changed water temperatures by -0.2 to +2.0 degrees C. Atmospheric compensation depended primarily on atmospheric water vapor and temperature, sensor viewing geometry, and water temperature. Agreement between multiple TIR bands (MASTER - 10 bands, ASTER - 5 bands) provided an independent check on recovered temperatures. Compensations improved agreement between image and in situ surface temperatures (from 2.0 to 1.1 degrees C average deviation); however, compensations did not improve agreement between river image temperatures and loggers installed at the stream bed (from 0.6 to 1.6 degrees C average deviation). Analysis of field temperatures suggests that vertical thermal stratification may have caused a systematic difference between instream gage temperatures and corrected image temperatures. As a result, agreement between image temperatures and instream temperatures did not imply that accurate TIR temperatures were recovered. Based on these analyses, practical accuracies for corrected TIR lake and stream surface temperatures are around 1 degrees C.
•Predictability challenges related to soil moisture variability are addressed.•Coupling between near/deep soil moisture controls hydrologic non-uniqueness.•Availability of mobile particles controls ...geomorphic non-uniqueness.
This study advances mechanistic interpretation of predictability challenges in hydro-geomorphology related to the role of soil moisture spatial variability. Using model formulations describing the physics of overland flow, variably saturated subsurface flow, and erosion and sediment transport, this study explores (1) why a basin with the same mean soil moisture can exhibit distinctly different spatial moisture distributions, (2) whether these varying distributions lead to non-unique hydro-geomorphic responses, and (3) what controls non-uniqueness in relation to the response type. Two sets of numerical experiments are carried out with two physically-based models, HYDRUS and tRIBS+VEGGIE+FEaST, and their outputs are analyzed with respect to pre-storm moisture state. The results demonstrate that distinct spatial moisture distributions for the same mean wetness arise because near-surface soil moisture dynamics exhibit different degrees of coupling with deeper-soil moisture and the process of subsurface drainage. The consequences of such variations are different depending on the type of hydrological response. Specifically, if the predominant runoff response is of infiltration excess type, the degree of non-uniqueness is related to the spatial distribution of near-surface moisture. If runoff is governed by subsurface stormflow, the extent of deep moisture contributing area and its “readiness to drain” determine the response characteristics. Because the processes of erosion and sediment transport superimpose additional controls over factors governing runoff generation and overland flow, non-uniqueness of the geomorphic response can be highly dampened or enhanced. The explanation is sediment composed by multi-size particles can alternate states of mobilization or surface shielding and the transient behavior is inherently intertwined with the availability of mobile particles. We conclude that complex nonlinear dynamics of hydro-geomorphic processes are inherent expressions of physical interactions. As complete knowledge of watershed properties, states, or forcings will always present the ultimate, if ever resolvable, challenge, deterministic predictability will remain handicapped. Coupling of uncertainty quantification methods and space-time physics-based approaches will need to evolve to facilitate mechanistic interpretations and informed practical applications.
•Measured stream stage and water content beneath hyper-arid ephemeral streams.•Streams flowed ⩽5 times over three years.•Rain events did not produce a water content response at depths >1m.•Flow ...percolated rapidly and produced transient saturation in channel alluvium.•Saturation persisted after flow only where fine sediments impeded percolation.
In hyper-arid regions, ephemeral stream channels are important sources of subsurface recharge and water supply for riparian vegetation, but few studies have documented the subsurface water content dynamics of these systems. This study examines ephemeral channels in the hyper-arid western Sonoran Desert, USA to determine how frequently water recharges the alluvial fill and identify variables that affect the depth and persistence of recharge. Precipitation, stream stage, and subsurface water content measurements were collected over a three-year study at six channels with varying contributing areas and thicknesses of alluvial fill. All channels contain coarse alluvium composed primarily of sands and gravels, and some locations also have localized layers of fine sediment at 2–3m depth. Rain alone contributed 300–400mm of water input to these channels over three years, but water content responses were only detected for 36% of the rain events at 10cm depth, indicating that much of the rain water was either quickly evaporated or taken up by plants. Pulses of water from rain events were detected only in the top meter of alluvium. The sites each experienced ⩽5 brief flow events, which caused transient saturation that usually lasted only a few hours longer than flow. These events were the only apparent source of water to depths >1m, and water from flow events quickly percolated past the deepest measurement depths (0.5–3m). Sustained saturation in the shallow subsurface only developed where there was a near-surface layer of finer consolidated sediments that impeded deep percolation.
We use Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER) data to estimate spatial energy flux and evaporation distributions at the Salar de Atacama, a playa in Northern Chile. ...Our approach incorporates ASTER surface kinetic temperature, emissivity, and reflectance data, ground-based meteorological measurements, and empirical parameters. Energy flux distributions are estimated using either spatially constant or spatially distributed values of model parameters, with spatially distributed parameters assigned separately to each land cover category in an image classification. We test the sensitivity of energy budget calculations to state variable and parameter values by conducting Monte Carlo simulations for regions with ground energy budget measurements. Results show that assigning spatially distributed model parameters via land cover classifications yields significant improvements to ground and sensible heat flux predictions. Latent heat fluxes cannot, however, be predicted with sufficient accuracy to allow estimation of area-integrated evaporative moisture loss at this low-evaporation playa.
Rivers that cease to flow are globally prevalent. Although many epithets have been used for these rivers, a consensus on terminology has not yet been reached. Doing so would facilitate a marked ...increase in interdisciplinary interest as well as critical need for clear regulations. Here we reviewed literature from Web of Science database searches of 12 epithets to learn (Objective 1-O1) if epithet topics are consistent across Web of Science categories using latent Dirichlet allocation topic modeling. We also analyzed publication rates and topics over time to (O2) assess changes in epithet use. We compiled literature definitions to (O3) identify how epithets have been delineated and, lastly, suggest universal terms and definitions. We found a lack of consensus in epithet use between and among various fields. We also found that epithet usage has changed over time, as research focus has shifted from description to modeling. We conclude that multiple epithets are redundant. We offer specific definitions for three epithets (non-perennial, intermittent, and ephemeral) to guide consensus on epithet use. Limiting the number of epithets used in non-perennial river research can facilitate more effective communication among research fields and provide clear guidelines for writing regulatory documents.
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