Shallow Flow Research Articles

Influence of groundwater and topography on stream drying in semi‐arid headwater streams

AbstractNon‐perennial streams comprise over half of the global stream network and impact downstream water quality. Although aridity is a primary driver of stream drying globally, surface flow permanence varies spatially and temporally within many headwater streams, suggesting that these complex drying patterns may be driven by topographic and subsurface factors. Indeed, these factors affect shallow groundwater flows in perennial systems, but there has been only limited characterisation of shallow groundwater residence times and groundwater contributions to intermittent streams. Here, we asked how groundwater residence times, shallow groundwater contributions to streamflow, and topography interact to control stream drying in headwater streams. We evaluated this overarching question in eight semi‐arid headwater catchments based on surface flow observations during the low‐flow period, coupled with tracer‐based groundwater residence times. For one headwater catchment, we analysed stream drying during the seasonal flow recession and rewetting period using a sensor network that was interspersed between groundwater monitoring locations, and linked drying patterns to groundwater inputs and topography. We found a poor relationship between groundwater residence times and flowing network extent (R2 < 0.24). Although groundwater residence times indicated that old groundwater was present in all headwater streams, surface drying also occurred in each of them, suggesting old, deep flowpaths are insufficient to sustain surface flows. Indeed, the timing of stream drying at any given point typically coincided with a decrease in the contribution from near‐surface sources and an increased relative contribution of groundwater to streamflow at that location, whereas the spatial pattern of drying within the stream network typically correlated with locations where groundwater inputs were most seasonally variable. Topographic metrics only explained ~30% of the variability in seasonal flow permanence, and surprisingly, we found no correlation with seasonal drying and down‐valley subsurface storage area. Because we found complex spatial patterns, future studies should pair dense spatial observations of subsurface properties, such as hydraulic conductivity and transmissivity, to observations of seasonal flow permanence.

Unsaturated Flow Processes and the Onset of Seasonal Deformation in Slow‐Moving Landslides

AbstractPredicting rainfall‐induced landslide motion is challenging because shallow groundwater flow is extremely sensitive to the preexisting moisture content in the ground. Here, we use groundwater hydrology theory and numerical modeling combined with five years of field monitoring to illustrate how unsaturated groundwater flow processes modulate the seasonal pore water pressure rise and therefore the onset of motion for slow‐moving landslides. The onset of landslide motion at Oak Ridge earthflow in California’s Diablo Range occurs after an abrupt water table rise to near the landslide surface 52–129 days after seasonal rainfall commences. Model results and theory suggest that this abrupt rise occurs from the advection of a nearly saturated wetting front, which marks the leading edge of the integrated downward flux of seasonal rainfall, to the water table. Prior to this abrupt rise, we observe little measured pore water pressure response within the landslide due to rainfall. However, once the wetting front reaches the water table, we observe nearly instantaneous pore water pressure transmission within the landslide body that is accompanied by landslide acceleration. We cast the timescale to reach a critical pore water pressure threshold using a simple mass balance model that considers variable moisture storage with depth and explains the onset of seasonal landslide motion with a rainfall intensity‐duration threshold. Our model shows that the seasonal response time of slow‐moving landslides is controlled by the dry season vadose zone depth rather than the total landslide thickness.

Granular flow on a rotating and gravitating elliptical body

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Linear stability of shallow morphodynamic flows

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Borehole Heat Exchangers-Addressing the Application Gap with Groundwater Science.

Hydrogeologists and mechanical engineers approach the design of geoexchange systems, and the associated borehole heat exchanger (BHE) fields, in different ways, each focusing on their knowledge areas. While these differences have created a strong research base, with well-published innovations and designs that collectively allow for sustainable systems, industry has not embraced these recent advancements. Despite abundant research demonstrating how complex shallow groundwater flow and temperature conditions can influence BHE design and operation, the low-temperature geothermal industry remains largely fixed on simple analytical codes and assumed uniform ground conditions. Geoexchange system inefficiencies become masked via reduced heat pump performance and increased electricity consumption. Similarly, interactions between BHE fields and infrastructure in urban areas are slow to manifest and are often unrealized due to a lack of field temperature data. While regulations that include hydrogeological factors have been developed in some jurisdictions, they are largely voluntary or rudimentary and can be unapplied in industry. Addressing this application gap may be unreasonable as designing and installing thermally efficient geoexchange systems can put them out of the cost envelope of competing heating and cooling systems. Perhaps for hydrogeologists, the way forward lies in the use of BHE's to facilitate contaminated sites remediation, an area we are familiar with, and one that allows for innovative technologies to reduce cost envelopes. Following that path, hydrogeologists can help improve system efficiencies while fully considering the dynamic nature of advective and thermal transport by groundwater.

On steady alternate bars forced by a localized asymmetric drag distribution in erodible channels

Studying the effect of different in-stream fluvial turbines siting on river morphodynamics allowed us to witness the onset of a time-averaged, large-scale, alternate distortion of bed elevations, which could not be exclusively related to the turbine rotor blockage. The longitudinal profiles of this two-dimensional bathymetric perturbation resemble those of steady fluvial bars. In this contribution we generalize the problem addressing a spatially impulsive, asymmetric distribution of drag force in the channel cross-section. This is experimentally investigated through the deployment of differently sized grids perpendicular to the flow, and analytically explored as a finite perturbation of an open channel flow over an erodible sediment layer, as described by a coupled flow–sediment shallow water equation. The steady solutions of this fluvial morphodynamic problem, physically represented by alternate bars scaling with the channel width, highlight the importance of the resonant conditions in defining the spatial extent of the bed deformation. The equations further suggest that in very shallow flows any asymmetric obstruction may lead to an upstream propagation of the steady bars, consistent with previous studies on the effects of channel curvature. In broad terms, this study provides the preliminary framework to control the onset of river meandering through imposed finite perturbations of the cross-section. In a more applied sense, it provides a tool to predict non-local scour–deposition patterns associated with the deployment of energy converters or other flow obstructions.

Assessment of Critical Shear Stress and Threshold Velocity in Shallow Flow with Sand Particles

In this study, the incipient motion of four groups of sand, ranging from medium to very coarse particles, was experimentally examined using an acoustic Doppler velocimeter (ADV) in different water depths under the hydraulically transitional flow condition. The transport criterion of the Kramer visual observation method was used to determine threshold conditions. Some equations for calculating threshold average and near-bed velocities were derived. Results showed that the threshold velocity was directly proportional to both sediment particle size and water depth. The vertical distributions of the Reynolds shear stress showed an increase from the bed to about 0.1 of the water’s depth, after performing a damping area, then a decrease toward the water surface. By extending the linear portion of the Reynolds shear stress in the upper zone of the damping area to the bed, the critical shear stress, particle shear Reynolds number, and critical Shields parameter were calculated. Results showed that the critical Shields parameter was located under the Shields curve, showing no sediment motion. This indicates that the incipient motion of sediment particles occurred with smaller bed shear stress than that estimated using the Shields diagram in the hydraulically transitional flow region. The reason could be related to differences between the features of the present experiment and those of the experiments used in the development of the Shields diagram, including the approaches to determine and define threshold conditions, the accuracy of experimental tools to estimate critical shear stress, and sediment particle characteristics. Therefore, the change in the specifications of experiments from those on which the Shields diagram has been based led to the deviation between the estimation using the Shields diagram and that of real threshold conditions, at least in the hydraulically transitional flow region with sand particles.

Deciphering origins and pathways of low-enthalpy geothermal waters in the unconventional geothermal system of Juchipila graben (Central Mexico)

This work presents hydrochemical results for groundwater and dissolved gas samples collected from a thermal and cold aquifer in the Juchipila Basin, in southern Sierra Madre Occidental, central Mexico. Thermal springs in the Juchipila Basin reach temperatures of 60 °C, these manifestations are not related to recent or active volcanism as are all the known geothermal fields in Mexico. The thermal waters (>32 °C) are Na-HCO3 and Na-SO4 type, with an anomalous concentration of F, B, Li, and As. Their chemistry likely results from water-rock interaction processes. The cold waters (<32 °C) have a Ca-HCO3 composition typical of recent infiltration and shallow flow, but they have an anomalous concentration of NO3. The δ2H and δ18O indicate a common meteoric source for the warm and cold water plotting along an evaporation line. The waters have higher CO2 and He concentrations than the air-saturated water. The helium composition is mainly atmospheric and terrigenous with a mantle helium contribution of up to 14%. This suggests that faults affecting the region are deeply rooted, permitting mantle helium uprise. Geothermometry gives mean reservoir temperatures of 58–102 °C. Based on these results, we propose a model of hydrothermal circulation in the Juchipila Basin, in which rainwater infiltrates deeply through the graben edges fault system, dissolves ions and crustal helium, incorporates mantle helium, while heated by the geothermal gradient, and eventually surges and mixes with the cold, shallow aquifer along faults cutting the whole succession within the graben.

End member and Bayesian mixing models consistently indicate near‐surface flowpath dominance in a pristine humid tropical rainforest

AbstractThe impacts of forest conversion on runoff generation in the tropics have received much interest, but scientific progress is still hampered by challenging fieldwork conditions and limited knowledge about runoff mechanisms. Here, we assessed the runoff generation, flow paths and water source dynamics of a pristine rainforest catchment in Costa Rica using end member mixing analysis (EMMA) and a Bayesian mixing model (MixSIAR). Geochemical tracer data collected over a 4‐week field campaign were combined with tritium data used to assess potential deeper groundwater flow pathways to the perennial stream. The streamflow composition was best captured using three end‐members, namely throughfall, shallow (5–15 cm) and deeper (15–50 cm) soil water. We estimated the end‐member contributions to the main stream and two tributaries using the two mixing approaches and found good agreement between results obtained from EMMA and MixSIAR. The system was overwhelmingly dominated by near‐surface sources, with little evidence for deeper and older groundwater as tritium‐derived baseflow mean transit time was between 2.0 and 4.4 years. The shallow soil flow pathway dominated streamflow contributions in the main stream (median 39% and 49% based on EMMA and MixSIAR, respectively), followed by the deeper soil (32% and 31%) and throughfall (25% and 19%). The two tributaries had even greater shallow soil water contributions relative to the main stream (83% and 74% for tributary A and 42% and 63% for tributary B). Tributary B had no detectable deep soil water contribution, reflecting the morphology of the hillslope (steeper slopes, shallower soils and lower vegetation density compared to hillslope A). Despite the short sampling campaign and associated uncertainties, this study allowed to thoroughly assess runoff generation mechanisms in a humid tropical catchment. Our results also provide a first comparison of two increasingly used mixing models and suggest that EMMA and MixSIAR yield comparable estimates of water source partitioning in this tropical, volcanic rainforest environment.

Forward Modeling Helioseismic Signatures of One- and Two-cell Meridional Circulation

Abstract Using a 3D global solver of the linearized Euler equations, we model acoustic oscillations over background velocity flow fields of single-cell meridional circulation with deep and shallow return flows as well as double-cell meridional circulation with strong and weak reversals. The velocities are generated using a mean-field hydrodynamic and dynamo model—moving through the regimes with minimal parameter changes, counterrotation near the base of the tachocline is induced by sign inversion of the nondiffusive action of turbulent Reynolds stresses (Λ-effect) due to the radial inhomogeneity of the Coriolis number. By mimicking the stochastic excitation of resonant modes in the convective interior, we simulate realization noise present in solar observations. Using deep-focusing to analyze differences in travel-time signatures between the four regimes, as well as comparing to solar observations, we show that current helioseismology techniques may offer important insights about the location and strength of the return flow; however, it may not currently be possible to definitively distinguish between profiles of single-cell or double-cell meridional circulation.

Modeling shallow geological flows on steep terrains using a specific differential transformation

Shallow geological flows such as landslides, debris flows, and floods are frequently modeled mathematically in the depth-averaged framework. As the bed topography becomes steep and irregular, the classical shallow-water model loses accuracy. How to handle bed topography and gravity effects in depth-averaged models becomes important, and the validity of the model needs verifying. In this study, a differential transformation called the G2L transformation linking the global height to the local depth of the surface material is proposed. This transformation helps to reconstruct the landslide/avalanche surface for one-dimensional and two-dimensional terrains that are obviously curved as well as real landslide examples such as the Huangtian and Rongsong landslides with shallowness ratios as high as 1/4. With this G2L transformation and closures of the lateral and bed stress, two versions of a model governing the geological flow dynamics are developed to avoid difficulties in terrain treatment when using differential geometry. Version 1 of the model is a coupled hyperbolic–differential system; version 2 is a modified hyperbolic system. The two proposed models are verified by a simple one-dimensional granular flow problem on a steep slope. Both mitigate problems associated with computational differential geometry of geological flows and the singularity problem of shallow-water models. In particular, the reduced gravity of several current models in landslide dynamics is a special instance of version 2. The two proposed models are well able to mathematically model the gravity geological hazards and may provide the tools required for prediction and mitigation of natural geological hazards.

Tropical tufa and karst streams support unique and threatened assemblages of aquatic Diptera, Empididae in Thailand

AbstractAquatic Empididae (Diptera) in the subfamilies Hemerodromiinae and Clinocerinae were used as a model to provide the first appraisal of diversity, endemism, environmental preferences and anthropic threats to Diptera communities of calcareous streams and tufa ecosystems of tropical karst habitats of Thailand. Hemerodromia Mg. (Hemerodromiinae) was abundant and speciose with 17 spp., of which six were associated with tufa, two with calcareous streams, three occurred on both tufa and streams and six were eurytopic species also found away from karst. Clinocerinae were only represented by one rare species of Clinocera Mg. Local and regional‐scale endemism was apparent at species level in Hemerodromia. Assemblage similarity in tufa habitats decayed with distance but cluster analysis provided only weak support for community‐level endemism at any geographic scale. Ordination indicated that benthic substrate and canopy cover were the most important environmental variables for many species. The six obligate tufa species of Hemerodromia were absolutely associated with rapid shallow flows of water over tufa surfaces at waterfalls. Species richness and abundance varied through the monsoon cycle but there was little evidence of simple correlations with seasonal changes in water temperature, conductivity and pH. The hypothesis that cold‐adapted aquatic Empididae might have been marooned in cold emergent groundwaters at lower elevations was not supported. It is concluded that tropical tufa ecosystems in Thailand support distinctive, rich and often endemic populations of stenotopic species of considerable biological and conservation interest. Deteriorative changes linked to tourism, water abstraction and agricultural activity could threaten these unique communities.

Quantifying Shallow Overland Flow Patterns Under Laboratory Simulations Using Thermal and LiDAR Imagery

AbstractDesertification processes pose a global environmental threat, impacting 61 × 106 km2 of the terrestrial land area. Changes in overland flow patterns and consequent rainwater redistribution in drylands present a potential pathway to desertification, because vegetation often relies on water inputs from runoff to sustain growth under insufficient rainfall conditions. Of particular importance are the very shallow overland flows that redistribute water, nutrients, and biological matter within arid landscapes. However, characterizing overland flow patterns remains challenging, due to their very shallow depths, their distributed nature, and the poor understanding of how these flows interact with the underlying rough soil surface. This paper describes how coupling thermal images of shallow overland flows with light detection and ranging (LiDAR) scanning of the underlying soil surface in 1 m2 experimental trays allows spatial patterns of shallow overland flow to be quantified. Laboratory experiments were used to explore the behaviors of shallow overland flow as mean slope gradients and soil roughness were varied. The results show that these imaging techniques are able to capture differences in flow patterns arising across soil surfaces with varying slope, roughness, and spatial variation in infiltration properties. Several spatial indices characterizing overland flow patterns were found to correlate with runoff volume. The presence of high permeability soil patches substantially regulated overland flow. A next logical step would be to apply the thermal and LiDAR measurement techniques to the hillslope scale.

Modelling of flow pattern in an open channel with sidewall obstruction

To predict flow pattern of shallow obstructed open channel flows, laboratory experiments of flow around a vertical emergent sidewall abutment (lateral constriction) in an open channel flow is simulated numerically and results of water surface and vertical profile of velocity are compared with the numerical results. The turbulence kinetic energy at the separation zone and bed shear stresses are also investigated. A computational fluid dynamic (CFD) numerical tool and volume of fluid (VOF) were used to predict water surface profiles. Results showed a very good agreement between the numerical and the experimental results of the water surface profiles. The numerical findings highlighted an increase in the velocity around the structure of two times the average flow velocity. The maximum pressure was observed in front of the constriction increases with increasing discharges. The vertical velocity profiles upstream the sidewall abutment at the three selected locations showed reasonable results. The maximum turbulence kinetic energy is shown at the zone of separation when the flow passes around the nose with high velocity. In this research, numerically it was found that the estimated bed shear stresses are 2-3 times the mean bed-shear stress of incoming flow. While, previous researchers finding showed a maximum value of bed shear stress near the leading edge 3.63 to 5 times the bed shear stress of the incoming flow. Because of the wide rang variation in calculating bed shear stresses by the previous researchers, so our research aims to calculate it using CFD and compare it with others. Also, dynamics of the flow around the obstacle was considered in this research under uniform flow conditions for an aspect ratio ranged 5 to 10.

Assessing different imaging velocimetry techniques to measure shallow runoff velocities during rain events using an urban drainage physical model

Abstract. Although surface velocities are key in the calibration of physically based urban drainage models, the shallow water depths developed during non-extreme precipitation and the potential risks during flood events limit the availability of this type of data in urban catchments. In this context, imaging velocimetry techniques are being investigated as suitable non-intrusive methods to estimate runoff velocities, when the possible influence of rain has yet to be analyzed. This study carried out a comparative assessment of different seeded and unseeded imaging velocimetry techniques based on large-scale particle image velocimetry (LSPIV) and bubble image velocimetry (BIV) through six realistic but laboratory-controlled experiments, in which the runoff generated by three different rain intensities was recorded. First, the use of naturally generated bubbles and water shadows and glares as tracers allows unseeded techniques to measure extremely shallow flows. However, these techniques are more affected by raindrop impacts, which even lead to erroneous velocities in the case of high rain intensities. At the same time, better results were obtained for high intensities and in complex flows with techniques that use artificial particles. Finally, the study highlights the potential of these imaging techniques for measuring surface velocities in real field applications as well as the importance of considering rain properties to interpret and assess the results obtained. The robustness of the techniques for real-life applications yet remains to be proven by means of further studies in non-controlled environments.

Influence of plant types, bed media and feeding patterns on wastewater treatment performance of wetland roofs

This study investigated the effects of two local plants (Wedelia Trilobata - WT, Axonopus Compressus - AC), feeding patterns (continuous and intermittent), and types of bed media (sand and charcoal) on the treatment performance of shallow bed subsurface flow wetland roofs (WRs). Results showed that changing plant from AC into WT led to an increase in removal rate by 6–12 % for COD and 11–13 % for total nitrogen (TN). The removal rate accelerated by 8 % for COD, 45–64 % for TN and 125 % for total phosphorus (TP) when changing from sand to charcoal as bed media. Continuous feeding mode enhanced the removal rate by 17–19 % for COD and 11–13 % for TN, compared to intermittent feeding mode. The findings suggested that the treatment capacity of WR was significantly affected by the type of plant, bed material and feeding pattern.

Baseflow dynamics and multivariate analysis using bivariate and multiple wavelet coherence in an alpine endorheic river basin (Northwest China)

Baseflow is a component of streamflow derived from shallow and deep subsurface flows that is concurrently controlled by multiple factors. Rational estimation of baseflow is critical for understanding its spatiotemporal dynamics and influencing factors within a river basin. To address this, different filtering parameters were applied to separate the baseflow of the Heihe River Basin (HRB) in Northwest China using digital filtering methods. Moreover, using bivariate and multivariate wavelet coherences, multivariate relationships between baseflow and meteorological factors/large-scale circulation indices were identified for several factors, which explained most of the variations. Results showed annual average baseflow was 10.3–91.1 mm and that the baseflow index (BFI) varied between 0.50 and 0.72 (average: 0.62). This indicates that 62% of long-term streamflow likely originates from groundwater discharge and other delayed sources. Positive/negative Spearman correlation coefficients between baseflow and extreme climate indices were more significant at upstream (Yingluoxia, Liyuanbao-and Wafangcheng) stations in comparison with midstream (Suyukou, Shunhua) and downstream (Yangyangchi) stations. Correlation for the BFI was relatively weaker than for baseflow. Furthermore, bivariate wavelet coherences revealed that precipitation (six stations) and the Atlantic Multidecadal Oscillation (four stations) were the individual factors that best explained baseflow variations. Multiple wavelet coherence demonstrated that all meteorological factors/large-scale circulation indices had the highest percentage of the numbers of power significant at the 95% significance level that could best explain baseflow variations. However, the average power of wavelet coherence was not increased. Differences likely attributable to consideration of additional variables were diminished by collinearity effects among factors. Furthermore, baseflow at the midstream Zhengyxia and downstream Yangyangchi stations had significant positive and negative correlation with population and effective irrigation area, respectively. The findings indicate that development of regional hydrometeorological models should primarily consider the impact of climate change in the upstream HRB, whereas the effects of both climate change and human activities should be considered in the midstream and downstream HRB.

Permafrost Promotes Shallow Groundwater Flow and Warmer Headwater Streams

AbstractThe presence of permafrost influences the flow paths of water through Arctic landscapes and thereby has the potential to impact stream discharge and thermal regimes. Observations from 11 headwater streams in Alaska showed that July water temperatures were higher in catchments with more near‐surface permafrost. We apply a fully coupled cryohydrology model to investigate if the impact of permafrost on flow path depth could cause the same pattern in temperatures of groundwater discharging from hillslopes to streams. The model simulates surface energy and water balances, snow, and subsurface water and energy balances for two‐dimensional hillslope model cases with varying permafrost extent. We find that hillslopes with continuous permafrost have more shallow flow paths and twice as high rates of evapotranspiration, compared to hillslopes with no permafrost. For our simulated cases, 6.7% of the horizontal water flux moves through the top organic soil layers when there is continuous permafrost, while only 0.5% moves through organic layers without permafrost. The deeper flow paths in permafrost‐free simulations buffer seasonal temperature extremes, so that summer groundwater discharge temperatures are highest with continuous permafrost. Our results suggest that permafrost thawing alters groundwater flow paths and can lead to decreases in summer stream temperatures and reductions in evapotranspiration in headwater catchments. These changes are of potential importance for stream biotic components of ecosystems, however, the full impact remains unknown.

Combining Uranium, Boron, and Strontium Isotope Ratios (234U/238U, δ11B, 87Sr/86Sr) to Trace and Quantify Salinity Contributions to Rio Grande River in Southwestern United States

In semi-arid to arid regions, both anthropogenic sources (urban and agriculture) and deeper Critical Zone (groundwater with long flow paths and water residence times) may play an important role in controlling chemical exports to rivers. Here, we combined two anthropogenic isotope tracers: uranium isotope ratios (234U/238U) and boron isotope ratios (δ11B), with the 87Sr/86Sr ratios to identify and quantify multiple solute (salinity) sources in the Rio Grande river in southern New Mexico and western Texas. The Rio Grande river is a major source of freshwater for irrigation and municipal uses in southwestern United States. There has been a large disagreement about the dominant salinity sources to the Rio Grande and particularly significant sources are of anthropogenic (agriculture practices and shallow groundwater flows, groundwater pumping, and urban developments) and/or geological (natural groundwater upwelling) origins. Between 2014 and 2016, we collected monthly river samples at 15 locations along a 200-km stretch of the Rio Grande river from Elephant Butte Reservoir, New Mexico to El Paso, Texas, as well as water samples from agricultural canals and drains, urban effluents and drains, and groundwater wells. Our study shows that due to the presence of localized and multiple salinity inputs, total dissolved solids (TDS) and isotope ratios of U, B, and Sr in the Rio Grande river show high spatial and temporal variability. Several agricultural, urban, and geological sources of salinity in the Rio Grande watershed have characteristic and distinguishable U, Sr, and B isotope signatures. However, due to the common issue of overlapping signatures as identified by previous tracer studies (such as δ18O, δD, δ34S), no single isotope tracer of U, Sr, or B isotopes was powerful enough to distinguish multiple salinity sources. Here, combining the multiple U, Sr, and B isotope and elemental signatures, we applied a multi-tracer mass balance approach to quantify the relative contributions of water mass from the identified various salinity end members along the 200-km stretch of the Rio Grande during different river flow seasons. Our results show that during irrigation (high river flow) seasons, the Rio Grande had uniform chemical and isotopic compositions, similar to the Elephant Butte reservoir where water is stored and well-mixed, reflecting the dominant contribution from shallow Critical Zone in headwater regions in temperate southern Colorado and northern New Mexico. In non-irrigation (low flow) seasons when the river water is stored at Elephant Butte reservoir, the Rio Grande river at many downstream locations showed heterogeneous chemical and isotopic compositions, reflecting variable inputs from upwelling of groundwater (deeper CZ), displacement of shallow groundwater, agricultural return flows, and urban effluents. Our study highlights the needs of using multi-tracer approach to investigate multiple solutes and salinity sources in rivers with complex geology and human impacts.

Precipitation Characteristics and Land Cover Control Wet Season Runoff Source and Rainfall Partitioning in Three Humid Tropical Catchments in Central Panama

AbstractMechanisms of runoff generation in the humid tropics are poorly understood, particularly in the context of land‐use/land cover change. This study analyzed the results of 124 storm hydrographs from three humid tropical catchments of markedly different vegetation cover and land‐use history in central Panama during the 2017 wet season: actively grazed pasture, young secondary succession, and near‐mature forest. We used electrical conductivity to separate baseflow (old water) from storm‐event water (new‐water). In all three land covers, new‐water dominated storm runoff generation in 44% of the sampled storm events, indicating the dominance of fast shallow flow paths in the landscape. Activation of these flow paths was found to depend on a combination of maximum rainfall intensity and total storm rainfall, which, in turn, relates to markedly contrasting hydrograph separation results among land covers. Relationships between these rainfall characteristics and storm runoff generation were nonlinear, producing a threshold response with the exceedance of specific rainfall volumes and/or intensities. The pastoral catchment delivered order of magnitude more new‐water during storm events than the two forested catchments. Although new‐water contributed minimally (&lt;10%) to total wet season runoff in the forested catchments, 43% of runoff generation in the pasture came from five large rainfall events where a threshold response produced substantial increases in total runoff and new‐runoff efficiency. Based on our results, we propose a conceptual model of hydrologic flow paths in humid tropical systems that can explain previously observed disparities in seasonal storage and runoff with respect to land use/land cover.