ALBERT

Description: Projected decreases and changes in the seasonal distribution of precipitation will have profound impacts on southeastern Australia, including its ability to generate renewable hydroelectricity. Recent decreases in precipitation over the region may be significant in the context of instrumental records, but the question of whether these decreases are within long-term natural variability remains. To help address this issue, we present December-January streamflow and dam inflow reconstructions for southeastern Australia. These reconstructions for the Tasmanian west coast are based solely on local tree-ring chronologies and span up to 1600 years. Non-parametric estimates, however, indicate good model skill for the last 458 years (streamflow) and 478 years (dam inflow). The reconstructions indicate that 20 th century conditions were well within the range of historical variability, and were in fact relatively wet. The period from ca. 1600 – 1750 CE was one of enhanced variability and a high proportion of low and high flow events occurred in the 17 th century. There are significant relationships between streamflow and inflow reconstructions and large-scale ocean-atmosphere processes such as ENSO and the Southern Annular Mode. Critically, our two reconstructions rely heavily on new tree-ring chronologies based on properties such as tracheid radial diameter, cell wall thickness and density, underscoring the importance of these different types of chronologies in reconstructions. This article is protected by copyright. All rights reserved.

Description: Abstract Observatory‐scale data collection efforts allow unprecedented opportunities for integrative, multidisciplinary investigations in large, complex watersheds which can affect management decisions and policy. Through the National Science Foundation‐funded REACH (REsilience under Accelerated CHange) project, in collaboration with the Intensively Managed Landscapes‐Critical Zone Observatory, we have collected a series of multidisciplinary datasets throughout the Minnesota River Basin in south‐central Minnesota, USA, a 43,400 km2 tributary to the Upper Mississippi River. Post‐glacial incision within the Minnesota River valley created an erosional landscape highly responsive to hydrologic change, allowing for transdisciplinary research into the complex cascade of environmental changes that occur due to hydrology and land use alterations from intensive agricultural management and climate change. Datasets collected include water chemistry and biogeochemical data; geochemical fingerprinting of major sediment sources; high resolution monitoring of river bluff erosion; and repeat channel cross‐sectional and bathymetry data following major floods. The data collection efforts led to development of a series of integrative reduced complexity models that provide deeper insight into how water, sediment, and nutrients route and transform through a large channel network and respond to change. These models represent the culmination of efforts to integrate interdisciplinary datasets and science to gain new insights into watershed‐scale processes in order to advance management and decision making. The purpose of this paper is to present a synthesis of the data sets and models, disseminate them to the community for further research, and identify mechanisms used to expand the temporal and spatial extent of short‐term observatory‐scale data collection efforts.

Description: Modern hydrology places nearly all its emphasis on science-as-knowledge, the hypotheses of which are increasingly expressed as physical models, whose predictions are tested by correspondence to quantitative data sets. Though arguably appropriate for applications of theory to engineering and applied science, the associated emphases on truth and degrees of certainty are not optimal for the productive and creative processes that facilitate the fundamental advancement of science as a process of discovery. The latter requires an investigative approach, where the goal is uberty, a kind of fruitfulness of inquiry, in which the abductive mode of inference adds to the much more commonly acknowledged modes of deduction and induction. The resulting world-directed approach to hydrology provides a valuable complement to the prevailing hypothesis- (theory-) directed paradigm. This article is protected by copyright. All rights reserved.

Description: Along the river network, water, sediment, and nutrients are transported, cycled, and altered by coupled hydrological and biogeochemical processes. Our current understanding of the rates and processes controlling the cycling and removal of dissolved inorganic nutrients in river networks is limited due to a lack of empirical measurements in large, (non-wadeable), rivers. The goal of this paper was to develop a coupled hydrological and biogeochemical process model to simulate nutrient uptake at the network scale during summer baseflow conditions. The model was parameterized with literature values from headwater streams, and empirical measurements made in 15 rivers with varying hydrological, biological, and topographic characteristics, to simulate nutrient uptake at the network scale. We applied the coupled model to 15 catchments describing patterns in uptake for three different solutes to determine the role of rivers in network-scale nutrient cycling. Model simulation results, constrained by empirical data, suggested that rivers contributed proportionally more to nutrient removal than headwater streams given the fraction of their length represented in a network. In addition, variability of nutrient removal patterns among catchments was varied among solutes, and as expected, was influenced by nutrient concentration and discharge. Net ammonium uptake was not significantly correlated with any environmental descriptor. In contrast, net daily nitrate removal was linked to suspended chlorophyll a (an indicator of primary producers) and land use characteristics. Finally, suspended sediment characteristics and agricultural land use were correlated with net daily removal of soluble reactive phosphorus, likely reflecting abiotic sorption dynamics. Rivers are understudied relative to streams, and our model suggests that rivers can contribute more to network-scale nutrient removal than would be expected based upon their representative fraction of network channel length.

Description: To simplify the complex snow structures that occur in nature, polycrystalline ice spheres were produced and arranged vertically to model the sintering process. By controlling the temperatures on both the top and bottom of the ice sphere array, the effect of upward and downward vapor transfers was examined. The evolution of the neck areas between ice spheres was observed using X-ray Computed Micro-Tomography. As frequently observed under the basal part of a snow layer and previous experiments of snow temperature gradient metamorphism, depth hoar structures were formed along neck areas and their formation was found to be directly related to the vapor transfer direction. To model the TG inversion that can be induced in nature by daily cycles of radiative heating and cooling, we also performed sign-alternating temperature gradient experiments on the ice sphere arrays. The morphological evolution of the neck and the associated vapor transfer were examined through image analysis and 2D modeling. The final microstructures of the neck area turned out to be a symmetrical distribution of ice protrusions bridging neighboring ice spheres.

Description: Abstract Long‐term analysis indicates progressive and widespread salinization of freshwaters and increases are often associated with urbanization, yet knowledge of chemical dynamics during urbanization is limited and typically drawn from space‐for‐time studies. Thus, the potential role of stream chemistry in sharp biodiversity losses observed at low levels of urbanization is difficult to distinguish from other concurrent factors such as temperature, flow, or sediment. We used a 25‐y annual time series of impervious cover for the Baltimore—Washington, DC metropolitan area to interpret long‐term records from 12 watershed‐monitoring stations in the Mid‐Atlantic Piedmont USA from 1986‐2010 and explore stream conductivity under progressive urbanization. All 12 watersheds experienced variable but monotonic increases in impervious cover, which ranged from 〈1% to nearly 25% of contributing area. All monitoring stations exhibited elevated specific conductance relative to background concentrations. Proliferation of impervious cover led to seasonal shifts in monthly conductivity maxima, with progressive dominance of winter pulses and diminishing signal from evapotranspirative concentration in late summer. We found consistently steep increases in stream conductivity across years and seasons associated with incremental increases in low (0‐4.5%) levels of watershed impervious cover; moderate to low rates of increase, but distinct seasonal concentrations from 4.5‐13.8% impervious cover; and increasing predominance of pulses at high levels of impervious cover (〉13.8%), particularly when conditioned on winter storm events. Observed patterns may suggest distinct sources and different degrees of hydrologic connection. Despite ubiquitous increases, variability in conductivity trends across space and time underscores the need for more intensive monitoring as urbanization progresses.

Description: Seasonal variability is a significant source of uncertainty in projected changes to precipitation across southeastern Australia (SEA). While existing instrumental records provide seasonal data for recent decades, most proxy records (e.g., tree rings, corals, speleothems) offer only annual reconstructions of hydroclimate. We present the first cool-season (July – August) reconstruction of dam inflow (Lake Burbury) for western Tasmania in SEA based on tree-ring width ( Athrotaxis selaginoides ) and mean latewood cell wall thickness ( Phyllocladus aspleniifolius ) chronologies. The reconstruction, produced using principal component regression, verifies back to 1731 and is moderately skillful, explaining around 23% of the variance. According to the reconstruction, relatively low inflow periods occurred around 1860, the early 1900s and 1970, while relatively high inflows occurred in the 1770s and 1810s. Highest reconstructed inflows occurred in 1816, and lowest in 1909. Comparison with available documentary and instrumental records indicates that the reconstruction better captures high rather than low flow events. There is virtually no correlation between our reconstruction and another for December-January inflow for the same catchment, a result consistent with the relationship between seasonal instrumental data. This suggests that conditions in one season have not generally reflected conditions in the other season over the instrumental record, or for the past 277 years. This illustrates the value of obtaining reconstructions of regional hydroclimatic variability for multiple individual seasons in regions where dry and wet seasons are not strongly defined. The results also indicate that the hydroclimate of southeastern Australian region cannot be adequately represented by a single regional reconstruction. This article is protected by copyright. All rights reserved.

Description: Abstract Understanding controls of P movement through watersheds is essential for improved landscape management in intensively managed regions. Here, we analyze observational data from 104 gaged river sites and 176 non‐gaged river sites within agriculturally‐dominated watersheds of Minnesota, USA to understand the role of landscape features, land use practices, climate variability and biogeochemical processes in total, dissolved and particulate P dynamics at daily to annual scales. Our analyses demonstrate that factors mediating P concentration‐discharge relationships varied greatly across watersheds and included near‐channel sediment sources, lake and wetland interception, assimilation by algal P, and artificial land drainage. The majority of gaged sites exhibited mobilizing behavior for all forms of P at event (i.e., daily) time scales, and chemostatic behavior at annual time scales. The large majority of watershed P export (〉70%, on average) occurred during high flow conditions, suggesting that more frequent large storm events arising from climate change will drive increased P losses from agricultural watersheds without substantial management changes. We found that P export could be dominated by dissolved P, particulate P, or an even mix of the two forms, depending on watershed attributes. Implementation of management practices to control P losses must be guided by understanding of how local landscapes interact with current and future climate conditions. Managing for both dissolved and particulate P is required to reduce overall P load in many agricultural watersheds.