This study investigates trends in bed surface and substrate grain sizes in relation to reachscale hydraulics using data from more than 100 gravel-bed stream reaches in Colorado and Utah. Collocated measurements of surface and substrate sediment, bankfull channel geometry and channel slope are used to examine relations between reach-average shear stress and bed sediment grain size. Slopes at the study sites range from 0·0003 to 0·07; bankfull depths range from 0·2 to 5 m and bankfull widths range from 2 to 200 m. The data show that there is much less variation in the median grain size of the substrate, D 50s , than there is in the median grain size of the surface, D 50 ; the ratio of D 50 to D 50s thus decreases from about four in headwater reaches with high shear stress to less than two in downstream reaches with low shear stress. Similar trends are observed in an independent data set obtained from measurements in gravel-bed streams in Idaho. A conceptual quantitative model is developed on the basis of these observations to track differences in bed load transport through an idealized stream system. The results of the transport model suggest that downstream trends in total bed load flux may vary appreciably, depending on the assumed relation between surface and substrate grain sizes. Figure 1. Changes in bed load transport rate and median grain size for two rivers in Idaho, USA.load transport equations, differences in mobility of small and large particles are accounted for by a so-called hiding function, f(D i /D 50 ), where D i is an individual grain size and D 50 is the median grain size. Let us assume for the moment that D i and D 50 are characteristic grain sizes of the bed load and bed surface, respectively, and that D i < D 50 , as noted above. Let us also assume that the substrate is the primary source of the bed load, thus D i is equivalent to the substrate median grain size, D 50s . In a channel network, both D 50 and D 50s should become finer downstream due to selective transport, deposition and/or abrasion. If the two sizes fine at the same rate, then the ratio of D 50 to D 50s is constant, and the effects of hiding and exposure stay the same in a relative sense. Alternatively, if the two sizes fine at different rates, the hiding-exposure effects may offset changes in shear stress and limit (or enhance) the mobility of the bed load as it moves through the network. Both hypotheses are reasonable; however, conditions favoring one versus the other have not been explored, nor have the implications with respect to models of downstream fining or drainage basin evolution.In this paper, we examine interactions between reach-scale flow properties and trends in surface and substrate grain sizes. We have amassed a large data set from field studies of gravel-and cobble-bed rivers in Colorado and Utah that allows us to examine relations between shear stress, armoring and bed load transport intensity over a broad range of scales. Additional data from studies conducted elsewhere in the USA are included to assess the a...

Baseflow is essential for stream ecosystems and human water uses, particularly in areas with Mediterranean climates. Yet the factors controlling the temporal and spatial variability of baseflow and its sources are poorly understood. Measurements of oxygen and hydrogen isotopic composition (δ18O and δ2H) were used to evaluate controls on baseflow in the stream network of a 64‐km2 catchment in western Oregon. A total of 607 water samples were collected to contrast baseflow in a year of near average precipitation (2016) to a year with low winter snowpack and subsequent summer drought conditions (2015). Spatial autocorrelation structures and relationships between surface water isotopic signatures and geologic and topographic metrics throughout the network were determined using Spatial Stream Network models. Isotope values varied widely in space and between years, indicating disparate baseflow water sources. During average flow conditions, the spatial variation in δ18O was primarily related to elevation, reflecting the influence of prior precipitation and input of water from snowmelt at higher elevation. In contrast, during drought conditions, the spatial variation in δ18O was also related to terrain slope and roughness—proxies for local water storage in deep‐seated earthflows and other Quaternary deposits. A prominent spring‐fed tributary with high unit baseflow discharge illustrated the importance of subsurface water storage in porous volcanic bedrock. As drought increases in a warming climate, baseflow in mountain catchments may become more dependent on storage in geologic and geomorphic features.

Abstract. This study focuses on the investigation of the mean transit time (MTT) of water and its spatial variability in a tropical high-elevation ecosystem (wet Andean páramo). The study site is the Zhurucay River Ecohydrological Observatory (7.53 km 2 ) located in southern Ecuador. A lumped parameter model considering five transit time distribution (TTD) functions was used to estimate MTTs under steadystate conditions (i.e., baseflow MTT). We used a unique data set of the δ 18 O isotopic composition of rainfall and streamflow water samples collected for 3 years (May 2011 to May 2014) in a nested monitoring system of streams. Linear regression between MTT and landscape (soil and vegetation cover, geology, and topography) and hydrometric (runoff coefficient and specific discharge rates) variables was used to explore controls on MTT variability, as well as mean electrical conductivity (MEC) as a possible proxy for MTT. Results revealed that the exponential TTD function best describes the hydrology of the site, indicating a relatively simple transition from rainfall water to the streams through the organic horizon of the wet páramo soils. MTT of the streams is relatively short (0.15-0.73 years, 53-264 days). Regression analysis revealed a negative correlation between the catchment's average slope and MTT (R 2 = 0.78, p < 0.05). MTT showed no significant correlation with hydrometric variables, whereas MEC increases with MTT (R 2 = 0.89, p < 0.001). Overall, we conclude that (1) baseflow MTT confirms that the hydrology of the ecosystem is dominated by shallow subsurface flow; (2) the interplay between the high storage capacity of the wet páramo soils and the slope of the catchments provides the ecosystem with high regulation capacity; and (3) MEC is an efficient predictor of MTT variability in this system of catchments with relatively homogeneous geology.

Abstract:Stream water temperature (t s ) is a critical water quality parameter for aquatic ecosystems. However, t s records are sparse or nonexistent in many river systems. In this work, we present an empirical model to predict t s at the site scale across the USA. The model, derived using data from 171 reference sites selected from the Geospatial Attributes of Gages for Evaluating Streamflow database, describes the linear relationship between monthly mean air temperature (t a ) and t s . Multiple linear regression models are used to predict the slope (m) and intercept (b) of the t a -t s linear relation as a function of climatic, hydrologic and land cover characteristics. Model performance to predict t s resulted in a mean Nash-Sutcliffe efficiency coefficient of 0.78 across all sites. Application of the model to predict t s at additional 89 nonreference sites with a higher human alteration yielded a mean NashSutcliffe value of 0.45. We also analysed seasonal thermal sensitivity (m) and found strong hysteresis in the t a -t s relation. Drainage area exerts a strong control on m in all seasons, whereas the cooling effect of groundwater was only evident for the spring and fall seasons. However, groundwater contributions are negatively related to mean t s in all seasons. Finally, we found that elevation and mean basin slope are negatively related to mean t s in all seasons, indicating that steep basins tend to stay cooler because of shorter residence times to gain heat from their surroundings. This model can potentially be used to predict climate change impacts on t s across the USA.

[1] Recent studies of catchment hydrologic response are incorporating increasingly complex datasets to investigate model representation of spatial and temporal variability. In this paper, catchment rainfall-runoff and stable isotope tracer response were modeled using a lumped conceptual model that integrates the unit hydrograph and isotope hydrograph separation methodologies. The model was applied across eight nested catchments (7 to 147 ha) for four rainstorms collected between summer and fall in 2001-2002, generating a usable 23 rainstorm datasets ranging from 1.2 to 10.3 h in length and spanning variability in environmental conditions related to storm characteristics (size and intensity) and antecedent moisture. Monte Carlo simulations were run for four model structures of varying complexity and evaluated using a Generalized Likelihood Uncertainty Estimation (GLUE) approach. We found that a model of intermediate complexity was adequate to model all catchment-storm pairs. Relationships between the parameters of the best model and catchment and storm characteristics were sought. We found that the fraction of effective rainfall routed as event water was correlated to rainstorm size but insensitive to catchment size, indicating that it is controlled by environmental conditions such as storm intensity and size. The mean transit time of event water decreased with increasing rainstorm size, indicating increased connectivity during larger rainstorms. Finally, a linear relation was found between the mean transit time of event water and catchment size suggesting that the time it takes for event water to be transferred to the stream is directly related to catchment size, particularly for catchments greater than 30 ha.Citation: Segura, C., A. L. James, D. Lazzati, and N. T. Roulet (2012), Scaling relationships for event water contributions and transit times in small-forested catchments in Eastern Quebec, Water Resour. Res., 48, W07502,

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This study investigated spatial-temporal variations of shear stress and bed load transport at three gravel bed river reaches of the Williams Fork River, Colorado. A two-dimensional flow model was used to compute spatial distributions of shear stress (τ) for four discharge levels between one third of bankfull (Q bf ) and Q bf . Results indicate that mean τ values are highly variable among sites. However, the properties of the mean-normalized distributions of τ are similar across sites for all flows. The distributions of τ are then used with a transport function to compute bed load transport rates of individual grain size fractions. Probability distributions of the instantaneous unit-width transport rates, q b , indicate that most of the bed load is transported through small portions of the bed with high τ. The mean-normalized probability distributions of q b are different among sites for all flows except at Q bf , when the distributions overlap. We also find that the grain size distribution (GSD) of the bed load adjusts with discharge to resemble the grain size distribution of the subsurface at Q bf . We extend these results to 13 locations in the basin, using the mean-normalized distributions of shear stress and measured subsurface grain sizes to compute bed load transport rates at Q bf . We found a remarkably similar shape of the q b distribution among sites highlighting the basin-wide balance between flow forces and GSD at Q bf and the potential to predict sediment flux at the watershed scale.

Abstract. Although most field and modeling studies of river corridor exchange have been conducted a scales ranging from 10’s to 100’s of meters; results of these studies are used to predict their ecological and hydrological influences at the scale of river networks. Further complicating prediction, exchange are expected to vary with hydrologic forcing and the local geomorphic setting. While we desire predictive power, we lack a complete spatiotemporal relationship relating discharge to the variation in geologic setting and hydrologic forcing that are expected across a river basin. Indeed, Wondzell’s [2011] conceptual model predicts systematic variation in river corridor exchange as a function of (1) variation in discharge over time at a fixed location, (2) variation in discharge with location in the river network, and (3) local geomorphic setting. To test this conceptual model we conducted more than 60 solute tracer studies collected in a synoptic campaign in the 5th order river network of the H. J. Andrews Experimental Forest (Oregon, USA). We interpret the data using a series of metrics describing river corridor exchange and solute transport, testing for consistent direction and magnitude of relationships relating these metrics to discharge and local geomorphic setting. We confirmed systematic decrease in river corridor exchange space through the river networks, from headwaters to the larger mainstem. However, we did not find systematic variation with changes in discharge through time, nor with local geomorphic setting. While interpretation of our results are complicated by problems with the analytical methods, they are sufficiently robust for us to conclude that space-for-time and time-for-space substitutions are not appropriate in our study system. Finally, we suggest two strategies that will improve the interpretability of tracer test results and help the hyporheic community develop robust data sets that will enable comparisons across multiple sites and/or discharge conditions.