Montane Watersheds Research Articles

Persistent and lagged effects of fire on stream solutes linked to intermittent precipitation in arid lands

Increased occurrence, size, and intensity of fire result in significant but variable changes to hydrology and material retention in watersheds with concomitant effects on stream biogeochemistry. In arid regions, seasonal and episodic precipitation results in intermittency in flows connecting watersheds to recipient streams that can delay the effects of fire on stream chemistry. We investigated how the spatial extent of fire within watersheds interacts with variability in amount and timing of precipitation to influence stream chemistry of three forested, montane watersheds in a monsoonal climate and four coastal, chaparral watersheds in a Mediterranean climate. We applied state-space models to estimate effects of precipitation, fire, and their interaction on stream chemistry up to five years following fire using 15 + years of monthly observations. Precipitation alone diluted specific conductance and flushed nitrate and phosphate to Mediterranean streams. Fire had positive and negative effects on specific conductance in both climates, whereas ammonium and nitrate concentrations increased following fire in Mediterranean streams. Fire and precipitation had positive interactive effects on specific conductance in monsoonal streams and on ammonium in Mediterranean streams. In most cases, the effects of fire and its interaction with precipitation persisted or were lagged 2–5 years. These results suggest that precipitation influences the timing and intensity of the effects of fire on stream solute dynamics in aridland watersheds, but these responses vary by climate, solute, and watershed characteristics. Time series models were applied to data from long-term monitoring that included observations before and after fire, yielding estimated effects of fire on aridland stream chemistry. This statistical approach captured effects of local-scale temporal variation, including delayed responses to fire, and may be used to reduce uncertainty in predicted responses of water quality under changing fire and precipitation regimes of arid lands.

Changes in Surface Runoff and Temporal Dispersion in a Restored Montane Watershed on the Qinghai–Tibetan Plateau

Surface runoff is a major component of the hydrological cycle, and it is essential for supporting the ecosystem services provided by grassland and forest ecosystems. It is of practical importance to understand the mechanisms and the dynamic processes of runoff in a river’s basin, and in this study, we focused on the restored montane Pailugou Basin in the Qilian Mountains, Gansu Province, China, since its water status is extremely important for the large arid area and local economies therein. Our purpose was to determine the annual variation in the surface runoff in the Pailugou Basin because it is important to understand the influence of climate fluctuations on surface water resources and the economy of the basin. In addition, little is known about the annual variations in precipitation and runoff in this region of the world. Daily atmospheric precipitation, air temperature and runoff data from 2000 to 2019 were analyzed by the calculation of the uneven annual distribution of surface runoff, the calculation of the complete adjustment coefficient, and the vector accumulation expressed by the concentration degree. We also used the cumulative anomaly approach to determine the interannual variation trend of runoff, while the change trend was quantified by the sliding average method. Finally, we used the Mann–Kendall mutation test method and regression analysis to establish the time-series trend for precipitation and runoff and to determine the period of abrupt runoff changes. The results indicated concentrated and positive distributions of surface runoff on an annual basis, with a small degree of dispersion, and an explicit concentration of extreme flows. The relative variation ranges exhibited a decreasing trend, and the distribution of the surface runoff gradually was uniform over the year. The runoff was highest from July to September (85% of the annual total). We also determined that annual surface runoff in the basin fluctuated over the 20-year period but showed an overall increasing trend, increasing by 3.94 × 105 m3, with an average increase rate of 0.42 × 105 m3 every ten years. From 2005 to 2014, the annual runoff and the proportion of runoff in the flood season (July to September) to the annual runoff fluctuated greatly. The correlation between the runoff and precipitation was significant (r = 0.839, p < 0.05), whereas the correlation between air temperature and surface runoff was low (r = 0.421, p < 0.05).

Spatial and temporal dynamics of base flow in semi-arid montane watersheds and the effects of landscape patterns and topography

Base flow (BF) is harder to predict than other hydrological signatures. The lack of hydrologically relevant information or adequately broad spectrum of typically selected catchment attributes (particularly landscape and topography) hinders the explanatory power. Our goals were to identify the most influential controls on base flow spatially and temporally and to elucidate the response relationships. Base flow in 19 semi-arid sub-watersheds was separated by digital filtering. One hundred and fourteen sub-watershed attributes were related to base flow using random forest regression. The main results were as follows: (1) Annual BF significantly declined since 1999 due to decreased precipitation, increased air temperature, afforestation, urban expansion, and increasing water consumption. Annual base flow index (BFI), varying between 0.319 and 0.695, showed less noticeable temporal trends. (2) Precipitation (P) and underlying carbonate rocks primarily controlled the spatial variation of annual BF and total flow (TF), with the impacts being positive. Landscape was less influential. After the abrupt runoff decline, landscape composition rather than configuration exerted greater impacts on spatial BF and TF, and the importance of forest increased, whereas landscape configuration was decisive for BFI during the whole observation period. The absence of significant links between landscape configuration and water quantity may result from a scale issue. Concave profile curvatures were found to be topographic variables more important than slopes. The impact of soil was the least. This study would benefit the selection of catchment attributes and spatial extents to quantify these attributes in building BF predicting models in future studies.

Modeling and Risk Analysis of Dam-Break Flooding in a Semi-Arid Montane Watershed: A Case Study of the Yabous Dam, Northeastern Algeria

The risk related to embankment dam breaches needs to be evaluated in order to prepare emergency action plans. The physical and hydrodynamic parameters of the flood wave generated from the dam failure event correspond to various breach parameters, such as width, slope, and formation time. This study aimed to simulate the dam breach failure scenario of the Yabous dam (northeast Algeria) and analyze its influence on the related areas (urban and natural environments) downstream of the dam. The simulation was completed using the sensitivity analysis method to assess the impact of breach parameters and flooding on the dam break scenario. The flood wave propagation associated with the dam break was simulated using the one-dimensional HEC-RAS hydraulic model. This study applied a sensitivity analysis of three breach parameters (slope, width, and formation time) on five sites selected downstream of the embankment dam. The simulation showed that the maximum flow of the flood wave recorded at the level of the breach was 8768 m3/s, which gradually attenuated along the river course to reach 1972.7 m3/s at about 8.5 km downstream the dam. This study established the map of flood risk areas that illustrated zones threatened by the flooding wave triggered by the dam failure due to extreme rainfall events. The sensitivity analysis showed that flood wave flow, height, and width revealed positive and similar changes for the increases in adjustments (±25% and ±50%) of breach width and slope in the five sites. However, flood wave parameters of breach formation time showed significant trends that changed in the opposite direction compared to breach slope and width. Meanwhile, the adjustments (±25% and ±50%) of the flood hydrograph did not significantly influence the flood parameters downstream of the dam. In the present study, the HEC-RAS 1-D modeling demonstrated effectiveness in simulating the propagation of flood waves downstream of the dam in the event of dam failure and highlighted the impact of the breach parameters and the flood hydrographical pattern on flood wave parameters.

Strontium isotope dynamics reveal streamflow contributions from shallow flow paths during snowmelt in a montane watershed, Provo River, Utah, USA

AbstractQuantifying the routing of snowmelt to surface water is critical for predicting the impacts of atmospheric deposition and changing land use on water quality in montane catchments. To investigate solute sources and streamflow in the montane Provo River watershed (Utah, USA), we used time‐series 87Sr/86Sr ratios sampled at three sites (Soapstone, Woodland and Hailstone) across a gradient of bedrock types. Soils are influenced by aeolian dust contributions, with distinct 87Sr/86Sr ratios relative to siliciclastic bedrock, providing an opportunity to investigate shallow versus deeper flow paths for controlling water chemistry. At the most upstream site (Soapstone), Sr concentrations averaged ~17 μg/L with minimal dilution during snowmelt suggesting subsurface flow paths dominated streamflow. However, a decrease in 87Sr/86Sr ratios from ~0.717 during baseflow to as low as ~0.713 during snowmelt indicated the activation of shallow flow paths through dust‐derived soils. In contrast, downstream sites receiving water inputs from Sr‐rich carbonate bedrock (Woodland and Hailstone) exhibited strong dilution of Sr from ~120 to 20 μg/L and an increase in 87Sr/86Sr ratios from ~0.7095 to ~0.712 during snowmelt. A three‐component mixing model using 87Sr/86Sr ratios and Sr concentrations at Soapstone showed water inputs were dominated by direct snowmelt and flushed soil water during runoff and groundwater during baseflow. At Woodland and Hailstone, a two‐component mixing model showed that the river was a mixture of groundwater and up to 75% upstream channel water during snowmelt. Our findings highlight the importance of flushed soil water for controlling stream water discharge and chemistry during snowmelt, with the signal from the upstream site propagating downstream in a nested catchment. Further, aeolian dust contributes to the solute chemistry of montane streams with potential impacts on water quality along shallow flow paths. Potential contaminants in these surface soils (e.g., Pb deposition in dust) may have significant impacts on water quality during snowmelt runoff.

Dynamic Hillslope Soil Moisture in a Mediterranean Montane Watershed

AbstractVariations in hillslope soil moisture control forest hydrologic fluxes and storage pools, yet sparse observations combined with the complexity and heterogeneity of water movement and storage in the vadose zone can make temporal and spatial patterns and processes difficult to predict. We used two years of field observations of volumetric soil moisture at three depths (15, 30, and 100 cm) across five topographic positions (riparian, toeslope, sideslope, shoulder, and ridge) along three hillslope transects to better understand how soil moisture changes with hillslope position and through time. As expected, we found higher values of soil moisture at all depths at the riparian and toeslope positions. Unexpectedly, we found that ridges were particularly wet during the wet winter months and dried quickly during the summer months, indicating that topography alone cannot account for mean wet season soil moisture in our Mediterranean climate field site. The variability in soil moisture across all soil depths and topographic positions was greatest when soils were dry and decreased under wet soil conditions; this variability remained high in the deeper soil horizons, regardless of season. Lastly, event analysis suggests that the response to early season rainfall was highly variable along the hillslopes and was likely dominated by localized controls such as microtopography and vegetation as well as soil texture, antecedent moisture conditions, and rainfall characteristics. Our results suggest that the drivers of wet and dry season soil moisture dynamics can vary across topographic positions along a hillslope and do not always follow topographic controls.

Modeling and comparing streamflow simulations in two different montane watersheds of western himalayas

The montane watersheds are often data-scarce, creating difficulty in modeling the crucial hydrological simulations. A good modeling approach with the least prediction uncertainity is important in predicting the streamflow simulations in such data scarce watersheds. In this study , we applied SWAT model in two high-altitude mountainous watersheds (Liddar-the glaciated and Sandran-non glaciated) of Western Himalayas. Our results indicate that calibration at different sites responds with different simulated streamflow varing spatially from site to site as well as from watershed to watershed. Our analysis suggests that runoff processes, orographic controls, and snow processes are more dominant processes in the glaciated watershed. However groundwater processes are significantly similar in both the watersheds. Temporally, simulated streamflow processes in these watersheds respond with steep rising and falling limbs from May to September but show a flatter recession from October to April. This results from the onset of snowmelt discharge, creating peak spikes during spring and summer flows. This is followed by low flow condition (baseflow) reducing the peak flow from autumn to winter. During the extremely low flow conditions and flood events the model overestimates and underestimates the streamflow respectively. The parameter uncertainty in terms of P_factor and R_factor was relatively lower in the glaciated watershed compared to the non-glaciated watershed. However, this multi-site approach resulted in more realistic parameter values which better represents the spatially influenced hydrological simulations. The results suggest to spatially calibrate and validate the SWAT model in large mountainous watersheds to improve the hydrological simulations.

Spatiotemporal Drivers of Hydrochemical Variability in a Tropical Glacierized Watershed in the Andes

AbstractThere is a critical knowledge gap about how glacier retreat in remote and rapidly warming tropical montane watersheds will impact solute export, which has implications for downstream geochemical cycling and ecological function. Because tropical glacierized watersheds are often uniquely characterized by year‐round ablation, upslope vegetation migration, and significant groundwater flow, baseline understanding is needed of how spatiotemporal variables within these watersheds control outlet hydrochemistry. We implemented a recently developed reactive transport watershed model, BioRT‐Flux‐PIHM, for a sub‐humid glacierized watershed in the Ecuadorian Andes with young volcanic soils and fractured bedrock. We found a unique simulated concentration and discharge (C‐Q) pattern that was mostly chemostatic but superimposed by dilution episodes. The chemostatic background was attributed to large simulated contributions of groundwater (subsurface lateral flow) to streamflow, of which a notable fraction (37%) comprised infiltrated ice‐melt. Relatively constant concentrations were further maintained in the model because times and locations of lower mineral surface wetting and dissolution were offset by concentrating effects of greater evapotranspiration. Ice‐melt did not all infiltrate in simulations, especially during large precipitation events, when high surface runoff contributions to discharge triggered dilution episodes. In a model scenario without ice‐melt, major ion concentrations, including Na+, Ca2+, and Mg2+, became more strongly chemostatic and higher, but weathering rates decreased, attenuating export by 23%. We expect this reduction to be exacerbated by higher evapotranspiration and drier conditions with expanded vegetation. This work brings to light the importance of subsurface meltwater flow, ecohydrological variability, and interactions between melt and precipitation for controlling hydrochemical processes in tropical watersheds with rapidly retreating glaciers.

Stable isotopes of water and specific conductance reveal complimentary information on streamflow generation in snowmelt-dominated, seasonally arid watersheds

The utility of a particular tracer to perform hydrograph separations depends on the dominant watershed properties combined with meteorological patterns; therefore, drawing conclusions from one tracer can be misleading. Combining information from multiple tracers can reveal complimentary insights that advance our knowledge of runoff generating processes. We performed hydrograph separations during spring snowmelt and for one summer rain event at two seasonally arid, montane watersheds in southeastern Wyoming using two independent tracer systems: Stable water isotopes (18O and 2H; 1–4 h resolution) and specific conductance (SC; 15 min resolution). Event water dominated streamflow generation during snowmelt using both tracers, but much lower uncertainty in hydrograph separations were achieved during this period with SC than with stable isotope tracers; the stable isotopic composition of pre-event and event water were too similar which led to large degrees of uncertainty. Stable isotopes of stream water did not vary as much as SC throughout the year and indicated the dominance of stream water derived from snowmelt. During the main snowmelt period, high frequency stream water isotopic composition was remarkably consistent, despite considerable variability measured in snowpack and snowmelt samples. Stable isotope measurements were useful for partitioning streamflow during a summer rain event when source waters were more isotopically distinct and suggested the dominance of pre-event water at generating streamflow. Despite the inability to partition event and pre-event streamflow during snowmelt, relationships between isotopic composition, SC, and watershed properties helped to understand mechanisms for streamflow generation. For instance, line conditioned excess (lc-excess) on the rising limb of the main snowmelt period was significantly positively correlated to the amount of rapid diurnal snowmelt contributions, the fastest moving compartment of event flow, supporting evidence that freshly melted snow, with relatively higher lc-excess, preferentially contributed to rapid diurnal snowmelt contributions. During fall low flow stream conditions, the isotopic value of streamflow was significantly positively correlated to elevation, where the highest-elevation watersheds had the highest delta values. This relationship was contradictory to what was expected (i.e. “elevation effect”) and may be explained by extensive snowpack fractionation occurring later in the season in deeper snowpacks located at higher elevations, or by a larger amount rainfall, with enriched isotopic composition relative to snowmelt, occurring at higher elevations. Through synthesizing patterns in two different natural tracers in conjunction with watershed properties, insights into the dominant controls on streamflow generation were gained from seasonally arid, snowmelt-dominated, headwater watersheds.

Comparing Trace Elements (As, Cu, Ni, Pb, and Zn) in Soils and Surface Waters among Montane, Upland Watersheds and Lowland, Urban Watersheds in New England, USA

Trace element biogeochemistry from soils to rivers is important for toxicity to aquatic ecosystems. The objective of this study was to determine whether trace element exports in contrasting watersheds are controlled by their abundance in soil, current land uses in the watershed, or geologic processes. Upland soils and river water samples were collected throughout the Deerfield watershed in southern Vermont and western Massachusetts and in the Quinebaug and Shetucket watersheds of eastern Connecticut. Soil concentrations were only an important predictor for dissolved Fe export, but no other trace element. Soil pH was not correlated with normalized dissolved exports of trace elements, but DOC was correlated with normalized dissolved Pb and Ni exports. The limited spatial and depth of soil sampling may have contributed to the poor correlation. Surprisingly, linear regressions and principal component analysis showed that human development was associated with higher soil trace metal concentrations but not significantly correlated with dissolved trace elements export. Instead, forest abundance was a strong predictor for lower Cu, Pb, and Zn soil concentrations and lower As, Fe, Ni and Pb dissolved exports across the watersheds. Dissolved exports of Al, K, and Si suggest that enhanced mineral dissolution in the montane watersheds was likely an important factor for matching or exceeding normalized pollutant trace element exports in more urbanized watersheds. Further studies are needed to evaluate subsurface/hyporheic controls as well as soil–surface water interface to quantify exchange and transport.

Trace Element Export From the Critical Zone Triggered by Snowmelt Runoff in a Montane Watershed, Provo River, Utah, USA

The Critical Zone is an important source of trace elements to headwater streams during the snowmelt runoff period, yet the mechanisms of trace element release are poorly characterized. To evaluate changes in water chemistry in response to snowmelt, we measured trace element and major ion concentrations at three sites in the upper Provo River in northern Utah, USA, over a five–year period spanning years with below– and above–average discharge. We also sampled snowpack, ephemeral streams, and soil water to investigate trace element sources. The river drains siliciclastic bedrock above the upper site, carbonate rocks between the upper and middle sites, and volcanic rocks between the middle and lower sites, with minimal anthropogenic impacts in the watershed. Concentrations of specific trace metals (Be, Al, Cu, and Pb) and rare earth elements (represented by La and Y) increased during snowmelt runoff each year at all three sites, with decreasing concentrations from upstream to downstream. In contrast, major ion concentrations, including Ca2+ and SO42-, were similar year–round at the upper site and were diluted during snowmelt at the lower sites, with increasing concentrations from upstream to downstream. The snowmelt runoff period (April–June) accounted for >84% of the annual trace element loading, with most trace element inputs occurring above the upper sampling site. Concentration–discharge (C–Q) relationships revealed variations in solute behavior from upstream to downstream. For example, at the upstream site the trace elements had a slight positive slope in log C–log Q space, while Ca2+ and SO42- had zero slope. At the downstream sites, the trace elements had a strong positive slope and Ca2+ and SO42- had a negative slope. Trace element concentrations were relatively low in snowpack but elevated in ephemeral streams and soil water, suggesting that flushing of shallow soils by snowmelt causes increased trace element concentrations with positive C–Q relationships in the upper part of the river. Trace element loads propagate downstream where concentrations are diluted by groundwater inputs from carbonate bedrock. Our results demonstrate that soil water flushing in the Critical Zone at the headwaters of mountain streams is an important control on downstream water chemistry.

Forest Vegetation Change and Its Impacts on Soil Water Following 47 Years of Managed Wildfire

Managed wildfire is an increasingly relevant management option to restore variability in vegetation structure within fire-suppressed montane forests in western North America. Managed wildfire often reduces tree cover and density, potentially leading to increases in soil moisture availability, water storage in soils and groundwater, and streamflow. However, the potential hydrologic impacts of managed wildfire in montane watersheds remain uncertain and are likely context dependent. Here, we characterize the response of vegetation and soil moisture to 47 years (1971–2018) of managed wildfire in Sugarloaf Creek Basin (SCB) in Sequoia-Kings Canyon National Park in the Sierra Nevada, California, USA, using repeat plot measurements, remote sensing of vegetation, and a combination of continuous in situ and episodic spatially distributed soil moisture measurements. We find that, by comparison to a nearby watershed with higher vegetation productivity and greater fire frequency, the managed wildfire regime at SCB caused relatively little change in dominant vegetation over the 47 year period and relatively little response of soil moisture. Fire occurrence was limited to drier mixed-conifer sites; fire-caused overstory tree mortality patches were generally less than 10 ha, and fires had little effect on removing mid- and lower strata trees. Few dense meadow areas were created by fire, with most forest conversion leading to sparse meadow and shrub areas, which had similar soil moisture profiles to nearby mixed-conifer vegetation. Future fires in SCB could be managed to encourage greater tree mortality adjacent to wetlands to increase soil moisture, although the potential hydrologic benefits of the program in drier basins such as this one may be limited.

Effects of montane watershed development on vulnerability of domestic groundwater supply during drought

Climate change is expected to reduce recharge to montane aquifers in the western United States, but it is unclear how this will impact groundwater resources in watersheds where intensive surface-water development has disrupted the natural hydrologic regime. To better understand sources of recharge and associated vulnerabilities of groundwater supply in this setting, we made a detailed geochemical survey of domestic wells finished in fractured bedrock throughout the Yuba and Bear River watersheds (Sierra Nevada foothills, northern California) during historic drought (2015–2016). Stable isotopes of water and noble gas recharge temperatures closely tracked atmospheric lapse rates across a broad elevation gradient (100–2000 m), indicating groundwater inputs are dominated by local precipitation that rapidly recharges fractured bedrock during the winter wet-season. However, nearly one-quarter of wells had water isotopes that were fractionated by evaporation and warm recharge temperatures, indicative of mixing with dry-season recharge by surface water. Monte Carlo mixing models suggest evaporation-impacted groundwater samples are mixtures of local rain with an average of 28% ± 13% from diverted surface water that can recharge bedrock aquifers during the dry-season by either irrigation return flow or seepage from extensive distribution infrastructure. Wells that received recharge subsidies from diverted surface water had elevated levels of nitrate and coliform bacteria compared to those replenished exclusively by local precipitation, which are more vulnerable to supply shortage during drought. It is important to consider the impacts of increased surface-water development on the quantity and quality of groundwater recharge in rapidly developing montane watersheds.

Mercury and dissolved organic matter dynamics during snowmelt runoff in a montane watershed, Provo River, Utah, USA

The mechanisms of Hg and dissolved organic matter (DOM) transport from watersheds to streams remain unclear, especially in snowmelt dominated montane systems. We characterized total Hg concentrations and DOM characteristics during snowmelt by weekly and/or monthly sampling at three locations in the upper Provo River, northern Utah, over two water years (2016 and 2017). Total Hg concentrations increased from <1 ng/L during baseflow to >7 ng/L during the snowmelt period (April–June), with decreasing concentrations from upstream to downstream. Filtered THg concentrations accounted for 65–75% of the unfiltered concentrations, suggesting that Hg is primarily transported in the dissolved phase. Annual THg loading in the upper Provo River was approximately 1 kg/yr, with ~90% of the flux occurring during the snowmelt period. Filtered THg concentrations were strongly correlated with in-situ fluorescence DOM (fDOM) measurements, allowing for the development of high-resolution proxy THg concentrations. Further, DOM characteristics, evaluated using excitation-emission matrices (EEMs) and parallel factor analysis (PARAFAC), identified the dominance of soil humic and fulvic acid fractions of DOM during runoff. Total Hg concentrations were low in snowpack but elevated in ephemeral streams during snowmelt runoff, indicating that Hg originated from shallow soil water rather than snow. Concentration—discharge relationships revealed clockwise hysteresis patterns, suggesting that Hg was flushed from soils on the rising limb of the hydrograph. Our results demonstrate that a majority of Hg transport occurs with a flux of DOM during the short snowmelt season as shallow soils are flushed by meltwater. The snowmelt-driven Hg pulse is a substantial source to downstream reservoirs and potentially contributes to a fish consumption advisory.

Peatland Contribution to Stream Organic Carbon Exports From a Montane Watershed

AbstractMountains contain many small and fragmented peatlands within watersheds. As they are difficult to monitor, their role in the water and carbon cycle is often disregarded. This study aims to assess the stream organic carbon exports from a montane peatland and characterizes its contribution to the water chemistry in a headstream watershed. High frequency in situ monitoring of turbidity and fDOM were used to quantify respectively particulate organic carbon (POC) and dissolved organic carbon (DOC) exports at the inlet and outlet of a peatland over three years in a French Pyrenean watershed (1,343 m.a.s.l.). The DOC and POC signals are both highly dynamic, characterized by numerous short peaks lasting from a few hours to a few days. Forty‐six percent of the exports occurred during 9% of the time corresponding to the highest flows monitored at the outlet. Despite its small area (3%) within the watershed, the peatland contributes at least 63% of the DOC export at the outlet. The specific DOC flux ranges from 16.1 ± 0.4 to 34.6 ± 1.5 g m2year−1. POC contributes 17% of the total stream organic carbon exports from the watershed. As the frequency of extreme climatic events is expected to increase in the context of climate change, further studies should be conducted to understand the evolution of underestimated mountainous peatland carbon fluxes and their implication in the carbon cycle of headwaters.

Laboratory assessment of alternative stream velocity measurement methods.

Understanding streamflow in montane watersheds on regional scales is often incomplete due to a lack of data for small-order streams that link precipitation and snowmelt processes to main stem discharge. This data deficiency is attributed to the prohibitive cost of conventional streamflow measurement methods and the remote location of many small streams. Expedient and low-cost streamflow measurement methods used by resource professionals or citizen scientists can provide scientifically useful solutions to this data deficiency. To this end, four current velocity measurement methods were evaluated in a laboratory flume: the surface float, rising body, velocity head rod, and rising air bubble methods. The methods were tested under a range of stream velocities, cross-sectional depths, and streambed substrates. The resulting measurements provide estimates of precision and bias of each method, as well as method-specific insight and calibration formulas. The mean values of the coefficient of variation, a measure of precision, were 10% for the surface float, 10% for the velocity head rod, 14% for the rising body, and 9% for the air bubble method. The values of scaled mean error, a measure of bias, were -8% for the surface float, -4% for the velocity head rod, -1% for the rising body, and 4% for the air bubble. The velocity head rod and surface float methods were the easiest methods to use, providing greater precision at large (> = 0.6 m/s) and small (<0.6 m/s) velocities, respectively. However, the reliance on a velocity ratio for each of these methods can generate inaccuracy in their results. The rising body method is more challenging to execute and of lower precision than the former two methods but provides low bias measurements. The rising air bubble method has a complex instrument assembly that is considered impractical for potential field user groups.

Hysteretic Response of Solutes and Turbidity at the Event Scale Across Forested Tropical Montane Watersheds

Concentration-discharge relationships are a key tool for understanding the sources and transport of material from watersheds to fluvial networks. Storm events in particular provide insight into variability in the sources of solutes and sediment within watersheds, and the hydrologic pathways that connect hillslope to stream channel. Here we examine high-frequency sensor-based specific conductance and turbidity data from multiple storm events across two watersheds (Quebrada Sonadora and Rio Icacos) with different lithology in the Luquillo Mountains of Puerto Rico, a forested tropical ecosystem. Our analyses include Hurricane Maria, a category 5 hurricane. To analyze hysteresis, we used a recently developed set of metrics to describe and quantify storm events including the hysteresis index (HI), which describes the directionality of hysteresis loops, and the flushing index (FI), which can be used to infer whether the mobilization of material is source or transport limited. We also examine the role of antecedent discharge to predict hysteretic behavior during storms. Overall, specific conductance and turbidity showed contrasting responses to storms. The hysteretic behavior of specific conductance was similar across sites, displaying clockwise hysteresis and a negative flushing index indicating proximal sources of solutes and consistent source limitation. In contrast, the directionality of turbidity hysteresis was significantly different between watersheds, although both had strong flushing behavior indicative of transport limitation. Overall, models that included antecedent discharge did not perform any better than models with peak discharge alone, suggesting that the magnitude and trajectory of an individual event was the strongest driver of material flux and hysteretic behavior. Hurricane Maria produced unique hysteresis metrics within both watersheds, indicating a distinctive response to this major hydrological event. The similarity in response of specific conductance to storms suggests that solute sources and pathways are similar in the two watersheds. The divergence in behavior for turbidity suggests that sources and pathways of particulate matter vary between the two watersheds. The use of high-frequency sensor data allows the quantification of storm events while index-based metrics of hysteresis allow for the direct comparison of complex storm events across a heterogeneous landscape and variable flow conditions.

Forest productivity varies with soil moisture more than temperature in a small montane watershed

Mountainous terrain creates variability in microclimate, including nocturnal cold air drainage and resultant temperature inversions. Driven by the elevational temperature gradient, vapor pressure deficit (VPD) also varies with elevation. Soil depth and moisture availability often increase from ridgetop to valley bottom. These variations complicate predictions of forest productivity and other biological responses. We analyzed spatiotemporal air temperature (T) and VPD variations in a forested, 27-km2 catchment that varied from 1000 to 1650 m in elevation. Temperature inversions occurred on 76% of mornings in the growing season. The inversion had a clear upper boundary at midslope (∼1370 m a.s.l.). Vapor pressure was relatively constant across elevations, therefore VPD was mainly controlled by T in the watershed. We assessed the impact of microclimate and soil moisture on tree height, forest productivity, and carbon stable isotopes (δ13C) using a physiological forest growth model (3-PG). Simulated productivity and tree height were tested against observations derived from lidar data. The effects on photosynthetic gas-exchange of dramatic elevational variations in T and VPD largely cancelled as higher temperature (increasing productivity) accompanies higher VPD (reducing productivity). Although it was not measured, the simulations suggested that realistic elevational variations in soil moisture predicted the observed decline in productivity with elevation. Therefore, in this watershed, the model parameterization should have emphasized soil moisture rather than precise descriptions of temperature inversions.

Beech versus spruce deadwood as forest microhabitat: does it make any difference to bryophytes?

The presence data of 153 bryophyte species, recorded on beech (BDW) and spruce (SDW) deadwood in the managed forests within a montane watershed, were analysed by controlling for different plot size, substrate pH and elevation. Only three pleurocarpous species, which were probably remnant epiphytes from live wood, showed relative specificity for BDW, but none to SDW. The few species displaying preference for BDW or SDW were generally true epixylic, mosses and hepatics, respectively. The chances of occurrence of epiphytic (early successional) and epixylic (mid-late successional) taxa on either deadwood species increased and respectively, decreased with the substrate pH. There was no consistent relationship between bryophyte preference to BDW/SDW and elevation. Neither total nor threatened species richness was related to deadwood species identity. However, at 100 cm2 resolution and within certain extension range, the cumulative number of bryophyte species was significantly higher on BDW than on SDW, despite no significant difference in species pool size. Threatened species richness increased monotonically towards lower elevations, suggesting that valley bottoms might act as refugia with high air humidity. The number of threatened species and in particular, the presence of Lophozia ascendens, were good predictors of total bryophyte richness regardless of deadwood species identity.

Recent Patterns in Climate, Vegetation, and Forest Water Use in California Montane Watersheds

California has recently experienced one of the worst droughts on record, negatively impacting forest ecosystems across the state. As a major source of the region’s water supply, it is important to evaluate the vegetation and water balance response of these montane forested watersheds to climate variability across the range of rain- to snow-dominated precipitation regimes. The Standardized Precipitation Index (SPI) and the Standardized Runoff Index (SRI) were used to capture the hydrologic drought signal, and MODIS vegetation indices (i.e., the normalized difference vegetation index and the enhanced vegetation index) were used to evaluate the vegetation and evapotranspiration response in three headwater catchments. The study catchments comprised a low elevation rain-dominated site (Caspar Creek) on the northern California coast, a mid-elevation site with a mix of rain and snow (Providence Creek) in the California Sierra Nevada, and a high elevation snow-dominated site (Bull Creek) in the Sierra Nevada. Lowest SPI values occurred in the third drought year of 2014 for all sites. Lowest SRI was in 2014 for Caspar, but in 2015 for Providence and Bull, reflecting differences in snowpack-delayed runoff and subsurface storage capacity between the lower and higher elevation watersheds. The most accurate water balance closure using evapotranspiration estimates from vegetation indices was within 10% of measured precipitation at snow-dominated Bull. The rain-dominated Caspar watershed had the highest vegetation index values and annual evapotranspiration, with the lowest variability over the previous 13 years (2004–2016). Vegetation index values and annual evapotranspiration decreased with increasing elevation and snow contribution to precipitation. Both snow-influenced Sierra Nevada watersheds showed elevated vegetation and evapotranspiration responses to interannual climate variability. There remains a need for institutional support to expand long-term observations in remote forested mountain watersheds to monitor and research these changing and extreme environmental conditions in source watershed regions.