ALBERT

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: Using well-established procedures for paleoflood hydrology and employing optically stimulated luminescence (OSL) geochronology, we analyzed a very well preserved natural record of 44 Upper Colorado River extreme floods with discharges ranging from 1800 to 9200 m 3 s -1 . These are the largest floods occurring during the last 2140 ± 220 years, and this natural record indicates that large floods are much more frequent than can be estimated by extrapolation from the stream gaging record that extends back to 1914. Most of these large floods occurred during the last 500 years, and the two largest floods in the record both exceeded the probable maximum flood (PMF) estimated at 8500 m 3 s -1 (300,000 cfs) for nearby Moab, Utah. Another 4 floods, with discharges greater than 7000 m 3 s -1 , occurred during the last two millennia. Flood frequency analyses using the FLDFRQ3 model yields the following values, depending on the Manning n roughness coefficients: 100-yr flood – 4670-4990 m 3 s -1 ; 500-yr flood – 6675-7270 m 3 s -1 ; 1000-yr flood – 7680-8440 m 3 s -1 . The presumed PMF discharge (8500 m 3 s -1 ) gets assigned a recurrence interval of about 1000 years, and the largest historical 1884 flood (3540 m 3 s -1 ) – a recurrence interval of 〈100 years. Flood frequency analysis for the Moab Valley based on the gaged record (1914-2012) yield 2730 m 3 s -1 for the 100-yr flood and 3185 m 3 s -1 for the 500-yr flood. This underestimation of the frequency of large floods from the gage data results from effects on that record by modern regulation of upstream river flow and associated water extraction for agriculture.

Description: The semi-theoretical universal calibration function (UCF) for estimating soil moisture using cosmic-ray neutron sensors was tested by comparing to field measurements made with the same neutron detector across a range of climates, soil, latitude, altitude and biomass. There was a strong correlation between neutron intensity and the total amount of hydrogen at each site; however, the relationship differed from that predicted by the UCF. A linear fit to field measurements explained 99% of the observed variation and provides a robust empirical means to estimate soil moisture at other sites. It was concluded that measurement errors, neutron count corrections and scaling to remove altitudinal and geomagnetic differences were unlikely to explain differences between observations and the UCF. The differences may be attributable to the representation of organic carbon, biomass or detector geometry in the neutron particle code, or to differences in the neutron energy levels being measured by the cosmic-ray sensor and modelled using the particle code. The UCF was derived using simulations of epithermal neutrons; however lower energy thermal neutrons may also be important. Using neutron transport code we show the differences in response of thermal and epithermal neutrons to the relative size of the hydrogen pool. Including a thermal neutron component in addition to epithermal neutrons in a modified UCF provided a better match to field measurements; however thermal neutron measurements are needed to confirm these results. A simpler generalized relationship for estimating soil moisture from neutron counts was also tested with encouraging results for low biomass sites.

Description: The form and functioning of peatlands depend strongly on their hydrological status, but there are few data available on the hydraulic properties of tropical peatlands. In particular, the saturated hydraulic conductivity ( K ) has not previously been measured in Neotropical peatlands. Piezometer slug tests were used to measure K at two depths (50 and 90 cm) in three contrasting forested peatlands in the Peruvian Amazon; Quistococha, San Jorge, and Buena Vista. Measured K at 50 cm depth varies between 0.00032 and 0.11 cm s -1 , and at 90 cm it varies between 0.00027 and 0.057 cm s -1 . Measurements of K taken from different areas of Quistococha showed that spatial heterogeneity accounts for c. 20 % of the within-site variance, and that depth is a good predictor of K . However, K did not vary significantly with depth at Buena Vista and San Jorge. Statistical analysis showed that c. 18 % of the variance in the K -data can be explained by between-site differences. Simulations using a simple hydrological model suggest the relatively high K values could lead to lowering of the water table by 〉 10 cm within c. 48 m of the peatland edge for domed peatlands, if subjected to a drought lasting 30 days. However, under current climatic conditions, even with high K , peatlands would be unable to shed the large amount of water entering the system via rainfall through subsurface flow alone. We conclude that most of the water leaves these peatlands via overland flow and/or evapotranspiration. This article is protected by copyright. All rights reserved.

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: Hydraulic conductivity is one of the most critical and at the same time one of the most uncertain parameters in many groundwater models. One problem commonly faced is that the data are usually not collected at the same scale as the discretized elements used in a numerical model. Moreover, it is common that different types of hydraulic conductivity measurements, corresponding to different spatial scales, coexist in a studied domain, which have to be integrated simultaneously. Here we address this issue in the context of Image Quilting, one of the recently developed multiple-point geostatistics methods. Based on a training image that represents fine-scale spatial variability, we use the simplified renormalization upscaling method to obtain a series of upscaled training images that correspond to the different scales at which measurements are available. We then apply Image Quilting with such a multi-scale training image to be able to incorporate simultaneously conditioning data at several spatial scales of heterogeneity. The realizations obtained satisfy the conditioning data exactly across all scales, but it can come at the expense of a small approximation in the representation of the physical scale relationships. In order to mitigate this approximation, we iteratively apply a kriging-based correction to the finest scale that ensures local conditioning at the coarsest scales. The method is tested on a series of synthetic examples where it gives good results and shows potential for the integration of different measurement methods in real-case hydrogeological models. 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.