Description: The focus of this study is on the climatology of atmospheric rivers (ARs) over the central United States using six atmospheric reanalysis products. This climatology is used to understand the long-term impacts of ARs on annual precipitation, precipitation extremes, and flooding over the central United States. The relationship between the frequency of ARs and three prominent large-scale atmospheric modes [Pacific-North American (PNA) teleconnection, Artic Oscillation (AO), and North Atlantic Oscillation (NAO)] is investigated, and the results are used to statistically model the frequency of ARs at the seasonal scale. AR characteristics (e.g., frequency, duration) are generally robust across the different reanalysis products. ARs exhibit a marked seasonality, with the largest activity in winter (more than ten ARs per season on average), and the lowest in summer (less than two ARs per season on average). Overall, the duration of most ARs is less than three days, but exceptionally persistent ARs (more than six days) are also observed. The year-to-year variations in the total annual precipitation over the central United States are largely explained by the variations in AR-related precipitation. Moreover, 40% of the top 1% daily precipitation extremes are associated with ARs, and more than 70% of the annual instantaneous peak discharges and peaks-over-threshold floods are associated with these storms, in particular during winter and spring. The seasonal frequency of ARs can be described in terms of large-scale atmospheric modes, with PNA playing a major role in particular in winter and spring. This article is protected by copyright. All rights reserved.

Description: The Complementary Relationship (CR) between actual and potential evaporation offers an attractive framework for estimating actual evaporation of drying land surfaces from simple meteorological measurements. Land surfaces are often heterogeneous with variable soil types, land cover, and local hydrologic conditions that give rise to spatially variable evaporation dynamics. The main aim is to incorporate effects of spatial heterogeneities on estimates of actual evaporation in the CR framework. The study extends the physically based approach of Aminzadeh et al. [2016] and proposes upscaling schemes for land-atmosphere interactions affecting reference evaporation from heterogeneous surfaces comprised of vegetation and bare soil patches. For small-scale surface heterogeneity relative to the extent of convective boundary layer (CBL), area averaged atmospheric boundary conditions were imposed over the domain of interest to integrate contributions from patches with different dynamics. For large-scale heterogeneity (large patches relative to the scale of the mean CBL), fluxes from each patch were weighted by their respective areas. Preliminary results are in reasonable agreement with available field measurements and illustrate various effects of heterogeneous surface evaporative fluxes on the CR response. The results also highlight hidden dynamics not captured by standard CR, such as ability of vegetated patches to support steady evaporative fluxes until the onset of water stress while bare soil has already dried out. The study provides new insights into the roles of different vegetation types, land cover fraction, and atmospheric conditions on regional CR behavior hence advancing predictive capabilities of actual evapotranspiration from spatially heterogeneous land surfaces. This article is protected by copyright. All rights reserved.

Description: A flume experiment was conducted to study channel adjustment to episodic sediment supply in mountain streams. The bulk sediment used for the bed and feed included grain sizes 0.5-64 mm with geometric mean D g (bulk) of 5.7 mm. Water discharge was held constant for 40 h, and 300 kg of sediment was supplied through a range of scenarios. Bed slope, sediment storage, sediment transport and bed surface texture responded to sediment supply. During the first of seven runs, bed slope decreased from 0.022 m/m (flume slope) to 0.018 m/m due to sediment starvation. Bed slope increased beginning in the second run as the bed aggraded due to preferential storage of grains 〉8 mm. Transport rate and bed-surface particle size were significantly affected by magnitude–frequency of sediment feed. Under constant feed, transport rate increased gradually and D g (surface) ranged between 12-15 mm. Instead, sediment pulses caused a pronounced increase in sediment transport rate and surface fining, trends that were inverted as sediment evacuated. At the run-scale, sediment transport and storage behaved as with constant feed if pulse relaxation time exceeded time between pulses. The increase in transport rate and surface fining were proportional to pulse size. After the 300 kg pulse, transport rate reached 100 g m −1 s −1 and D g (surface) was 〈10 mm. After 75 kg pulses transport rate reached ∼10g m −1 s −1 and D g (surface) was 〉12 mm. Textural differences on the initial bed surface influenced the patterns of sediment transport. Channel adjustment was controlled by magnitude-frequency of sediment feed and not by total feed. This article is protected by copyright. All rights reserved.

Description: Data assimilation can help to ensure that model results remain close to observations despite potential errors in the model, parameters and inputs. In this study, we test whether assimilation of snow depth observations using the particle filter, a generic data assimilation method, improves the results of a multi-layer energy-balance snow model, and compare the results against a direct insertion method. At the field site Col de Porte in France, the particle filter reduces errors in SWE, snowpack runoff and soil temperature when forcing the model with coarse resolution reanalysis data, which is a typical input scenario for operational simulations. For those variables, the model performance after assimilation of snow depths is similar to model performance when forcing with high quality, locally observed input data. Using the particle filter, we could also estimate a snowfall correction factor accurately at Col de Porte. The assimilation of snow depths also improves forecasts with lead-times of, at least, seven days. At further forty sites in Switzerland, the assimilation of snow depths in a model forced with numerical weather prediction data reduces the root-mean-squared-error for SWE by 64% compared to the a model without assimilation. The direct-insertion method shows similar performance as the particle filter, but is likely to produce inconsistencies between modelled variables. The particle filter, on the other hand, avoids such limitations without loss of performance. The methods proposed in this study efficiently reduces errors in snow simulations, seems applicable for different climatic and geographic regions and are easy to deploy. This article is protected by copyright. All rights reserved.

Description: In this study, we investigate the impact of single stream discharge events on water exchange, solute transport and reactions in the hyporheic zone below a natural in-stream gravel bar. We set up a reactive transport groundwater model with stream flow scenarios that vary by event duration and peak discharge. A steady ambient groundwater flow field is assumed that results in losing, neutral, or gaining stream conditions depending on the stream stage. Across the streambed dissolved oxygen, organic carbon and nitrate are transported into the subsurface. Additional nitrate is received from upwelling groundwater. Aerobic respiration and denitrification are simulated for scenarios with different stream solute concentrations. Results show that hyporheic exchange flux, solute transport and consumption increase during events. However, their intensities depend highly on the interplay between event characteristics and ambient groundwater conditions. During events where reversals in the hydraulic gradient occur stream water and solutes infiltrate deeper into the aquifer where they have more time to react. For those events, the reactive efficiency of the hyporheic zone (solute consumption as fraction of influx) for aerobic respiration and denitrification is up to 2.7 and 10 times higher compared to base flow conditions. The fraction of stream nitrate load consumed in the hyporheic zone increases with stream discharge (up to 150 mg/m 2 /hour), but remains below the value under base flow conditions for weak events. Events also increase denitrification of groundwater borne nitrate, but groundwater nitrate flux to the stream decreases by up to 33% due to temporary gradient reversals. This article is protected by copyright. All rights reserved.

Description: Although the water management sector is often characterized as resistant to risk and change, urban areas across the United States are increasingly interested in creating opportunities to transition toward more sustainable water management practices. These transitions are complex and difficult to predict – the product of water managers acting in response to numerous biophysical, regulatory, and political factors within institutional constraints. Gaining a better understanding of how these transitions occur is crucial for continuing to improve water management. This paper presents a replicable methodology for analyzing how urban water utilities transition toward sustainability. The method combines standardized quantitative measures of variables that influence transitions with contextual qualitative information about a utility's unique decision making context to produce structured, data-driven narratives. Data-narratives document the broader context, the utility's pre-transition history, key events during an accelerated period of change, and the consequences of transition. Eventually, these narratives should be compared across cases to develop empirically-testable hypotheses about the drivers of and barriers to utility-level urban water management transition. The methodology is illustrated through the case of the Miami-Dade Water and Sewer Department (WASD) in Miami-Dade County, Florida and its transition towards more sustainable water management in the 2000s, during which per capita water use declined, conservation measures were enacted, water rates increased, and climate adaptive planning became the new norm. This article is protected by copyright. All rights reserved.

Description: This paper demonstrates improved retrieval of snow water equivalent (SWE) from spaceborne passive microwave measurements for the sparsely-forested Upper Kern watershed (511 km 2 ) in the southern Sierra Nevada (USA). This is accomplished by assimilating AMSR-E 36.5 GHz measurements into model predictions of SWE at 90-m spatial resolution using the Ensemble Batch Smoother (EnBS) data assimilation framework. For each water year (WY) from 2003 to 2008, SWE was estimated for the accumulation season (October 1 st to April 1 st ) with the assimilation of the measurements in the dry snow season (December 1 st to February 28 th ). The EnBS SWE estimation was validated against snow courses and snow pillows. On average, the EnBS accumulation season SWE RMSE was 77.4 mm (13.1%, relative to peak accumulation), despite deep snow (average peak SWE of 545 mm). The prior model estimate without assimilation had an accumulation season average RMSE of 119.7 mm. After assimilation, the overall bias of the accumulation season SWE estimates was reduced by 84.2%, and the RMSE reduced by 35.4%. The assimilation also reduced the bias and the RMSE of the April 1st SWE estimates by 80.9% and 45.4%, respectively. The EnBS is expected to work well above treeline and for dry snow. This article is protected by copyright. All rights reserved.

Description: In coastal rivers, tidal pumping enhances the exchange of oxygen-rich river water across the sediment-water interface, controlling nitrogen cycling in riverbed sediment. We developed a one-dimensional, fluid flow and solute transport model that quantifies the influence of tidal pumping on nitrate removal and applied it to the tidal freshwater zone (TFZ) of White Clay Creek (Delaware, USA). In field observations and models, both oxygenated river water and anoxic groundwater deliver nitrate to carbon-rich riverbed sediment. A zone of nitrate removal forms beneath the aerobic interval, which expands and contracts over daily timescales due to tidal pumping. At high tide when oxygen-rich river water infiltrates into the bed, denitrification rates decrease by 25% relative to low tide. In the absence of tidal pumping, our model predicts that the aerobic zone would be thinner, and denitrification rates would increase by 10%. As tidal amplitude increases towards the coast, nitrate removal rates should decrease due to enhanced oxygen exchange across the sediment-water interface, based on sensitivity analysis. Denitrification hot spots in TFZs are more likely to occur in less permeable sediment under lower tidal ranges and higher rates of ambient groundwater discharge. Our models suggest that tidal pumping is not efficient at removing surface water nitrate but can remove up to 81% of nitrate from discharging groundwater in the TFZ of White Clay Creek. Given the high population densities of coastal watersheds, the reactive riverbeds of TFZs play a critical role in mitigating new nitrogen loads to coasts. This article is protected by copyright. All rights reserved.

Description: The permeability architecture of the critical zone exerts a major influence on the hydrogeochemistry of the critical zone. Water flowpath dynamics drive the spatio-temporal pattern of geochemical evolution and resulting streamflow concentration-discharge (C-Q) relation, but these flowpaths are complex and difficult to map quantitatively. Here, we couple a new integrated flow and particle tracking transport model with a general reversible Transition-State-Theory style dissolution rate-law to explore theoretically how C-Q relations and concentration in the critical zone respond to decline in saturated hydraulic conductivity ( K s ) with soil depth. We do this for a range of flow rates and mineral reaction kinetics. Our results show that for minerals with a high ratio of equilibrium concentration ( C eq ) to intrinsic weathering rate ( R max ), vertical heterogeneity in K s enhances the gradient of weathering-derived solute concentration in the critical zone and strengthens the inverse stream C-Q relation. As decreases, the spatial distribution of concentration in the critical zone becomes more uniform for a wide range of flow rates, and stream C-Q relation approaches chemostatic behaviour, regardless of the degree of vertical heterogeneity in K s . These findings suggest that the transport-controlled mechanisms in the hillslope can lead to chemostatic C-Q relations in the stream while the hillslope surface reaction-controlled mechanisms are associated with an inverse stream C-Q relation. In addition, as decreases, the concentration in the critical zone and stream become less dependent on groundwater age (or transit time). This article is protected by copyright. All rights reserved.

Description: The management of large-scale water resource systems with surface and groundwater resources requires considering stream-aquifer interactions. Optimization models applied of large-scale systems have either employed deterministic optimization (with perfect foreknowledge of future inflows, which hinders their applicability to real-life operations) or stochastic programming (in which stream-aquifer interaction is often neglected due to the computational burden associated with these methods). In this paper, stream-aquifer interaction is integrated in a stochastic programming framework by combining the Stochastic Dual Dynamic Programming (SDDP) optimization algorithm with the Embedded Multireservoir Model (EMM). The resulting extension of the SDDP algorithm, named Combined Surface-Groundwater SDDP (CSG-SDDP), is able to properly represent the stream-aquifer interaction within stochastic optimization models of large-scale surface-groundwater resources systems. The algorithm is applied to build a hydroeconomic model for the Jucar River Basin (Spain), in which stream-aquifer interactions are essential to the characterization of water resources in the system. Besides the uncertainties regarding the economic characterization of the demand functions, the results show that the economic efficiency of the operating policies under the current system can be improved by better management of groundwater and surface resources. This article is protected by copyright. All rights reserved.