A one-dimensional local discontinuous Galerkin Richards’ equation solution with dual-time stepping

We present a compact, high-order Richards’ equation solver using a local discontinuous Galerkin finite element method in space and a dual-time stepping method in time. Dual-time stepping methods convert a transient problem to a steady state problem, enabling direct evaluation of residual terms and resolve implicit equations in a step-wise manner keeping the method compact and amenable to parallel computing. Verification of our solver against an analytical solution shows high-order error convergence and demonstrates the solvers ability to maintain high accuracy using low spatial resolution; the method is robust and accurately resolves numerical solutions with time steps that are much larger than what is normally required for lower-order implicit schemes. Resilience of our solver (in terms of nonlinear convergence) is demonstrated in ponded infiltration into homogeneous and layered soils, for which HYDRUS-1D solutions are used as qualitative references to gauge performance of two slope limiting schemes.

     

ADMESH:?An advanced, automatic unstructured mesh generator for shallow water models

In this paper, we present the development and application of a two-dimensional, automatic unstructured mesh generator for shallow water models called Admesh. Starting with only target minimum and maximum element sizes and points defining the boundary and bathymetry/ topography of the domain, the goal of the mesh generator is to automatically produce a high-quality mesh from this minimal set of input. From the geometry provided, properties such as local features, curvature of the boundary, bathymetric/topographic gradients, and approximate flow characteristics can be extracted, which are then used to determine local element sizes. The result is a high-quality mesh, with the correct amount of refinement where it is needed to resolve all the geometry and flow characteristics of the domain. Techniques incorporated include the use of the so-called signed distance function, which is used to determine critical geometric properties, the approximation of piecewise linear coastline data by smooth cubic splines, a so-called mesh function used to determine element sizes and control the size ratio of neighboring elements, and a spring-based force equilibrium approach used to improve the element quality of an initial mesh obtained from a simple Delaunay triangulation. Several meshes of shallow water domains created by the new mesh generator are presented.     

“Dead Zone” dynamics in Lake Erie: the importance of weather and sampling intensity for calculated hypolimnetic oxygen depletion rates

Calculated hypolimnetic oxygen depletion (HOD) rates depend not only on environmental factors but also logistical ones. In particular, lack of understanding of the effects of weather in addition to how sampling effort determines calculated HOD rates complicates ecological understanding and environmental management of lake ecosystems. To better determine the roles of weather and sampling effort, we combined (1) weekly measurements of temperature and dissolved oxygen (DO) concentrations from seven stations in the Sandusky subbasin of Lake Erie’s central basin during 2005, (2) contemporaneous measures of storm activity and tributary discharge, and (3) a two-dimensional coupled hydrodynamic, chemical, and biological model of Lake Erie to investigate (1) how increased storm activity and tributary discharge affected short- (daily) and long-term (seasonal) dynamics of hypolimnetic hypoxia, and (2) how spatial (number of sites sampled) and temporal (sampling frequency) sampling effort affected calculated HOD rates. Our model closely replicated field-observed DO dynamics. When comparing baseline modeled dynamics to those in a second simulation with twice the number of days with high winds, however, we found that with more storm activity (1) periods of entrainment became more frequent, (2) the hypolimnion was warmer, (3) thermal stratification occurred 1 month later, whereas autumnal turnover occurred at least 1 week earlier shortening the duration of stratification by 1–2 months, and (4) HOD rates increased 12%. Further, spatial and temporal sampling intensity also affected calculated HOD rates. Consequently, adequately quantifying actual HOD rates requires sufficient sampling effort and the particular role of weather should be assessed with rigorous field and simulation studies, especially if HOD rates are used to indicate management success.     

Climate influences on water and sediment properties of Genovesa Crater Lake,Galápagos

Genovesa Crater Lake is a remote, hypersaline lake in the northern Galápagos archipelago that contains a finely laminated sediment record. This sediment record has the potential to provide a high-resolution history of past climate variability in the eastern tropical Pacific. Here we present modern climate, lake, and sediment observations from 2009 to 2012 to explore how local climate variability influences Genovesa Crater Lake and its sediments. Surface lake temperature is strongly linked to air temperature and is highly seasonal. Temperature stratification is strongest during the warm season, whereas temperature becomes more uniform through the water column in the cool season. Deeper and earlier mixing occurred during the 2010 La Niña, which subsequently delayed 2011 cool season mixing and maximum warm season surface temperatures in 2011 and 2012. Lake salinity changes are influenced by precipitation, evaporation and persistent seawater influx. The largest declines in subsurface salinity follow months after the rainy season, when temperatures cool and fresher surface water from the previous warm/wet season mixes into the subsurface. Between 2009 and 2012, more calcium carbonate precipitated during a period of higher salinity. The period of highest calcium carbonate abundance measured in sediment records that span the late nineteenth to twentieth century coincides with the failure of two consecutive rainy seasons in 1988 and 1989 as well as the coldest monthly sea surface temperature measured at Puerto Ayora in 1989. More calcium carbonate-rich laminae from AD 1550 ± 70 to 1675 ± 90 may indicate a greater frequency of prolonged droughts or cooler temperatures, although enhanced productivity may also modulate carbonate precipitation. More Ca-rich laminae in Genovesa coincide with dry conditions inferred from other Galápagos sediment proxies, as well as prolonged dry and cool conditions inferred from reconstructions of the Southern Oscillation Index and NINO3 sea surface temperatures.     

Corrections to: Interpreting lacustrine bulk sediment δ15N values using metagenomics in a tropical hypersaline lake system

Journal of Paleolimnology -     

Timing and duration of hydrological transitions in Arctic polygonal ground from stable isotopes

Land surface models and Earth system models that include Arctic landscapes must capture the abrupt hydrological transitions that occur during the annual thaw and deepening of the active layer. In this work, stable water isotopes (δ²H and δ¹⁸O) are used to appraise hydrologically significant transitions during annual landscape thaw at the Barrow Environmental Observatory (Utqiaġvik, Alaska). These hydrologically significant periods are then linked to annual shifts in the landscape energy balance, deduced from meteorological data and described by the microclimatic periods: Winter, Pre-Melt, Melt, Post-Melt, Summer, and Freeze-Up. The tight coupling of the microclimatic periods with the hydrological transitions supports the use of microclimatic periods as a means of linking polygonal surface water hydrology to meteorological datasets, which provides a mechanism for improving the representation of polygonal surface water hydrology in process-based models. Rayleigh process reconstruction of the isotopic changes revealed that 19% of winter precipitation was lost to sublimation prior to melting and that 23% of surface water was lost to evaporation during the first 10 days post-melt. This agrees with evaporation rates reported in a separate study using an eddy covariance flux tower located nearby. An additional 17% was lost to evaporation during the next 33 days. Stable water isotopes are also used to identify the dominant sources of surface water to various hydrogeomorphological features prevalent in polygonal terrain (a lake, a low centre polygon centre, troughs within the rims of low centre polygons, flat centre polygon troughs, a high centre polygon trough, and drainages). Hydrogeomorphologies that retained significant old water or acted as snow drifts are isotopically distinct during the Melt Period and therefore are easily distinguished. Biogeochemical changes related to the annual thaw are also reported and coupled to the hydrological transitions, which provides insight into the sources and sinks of these ions to and from the landscape.     

Last glacial maximum ecology and climate from terrestrial gastropod assemblages in Peoria loess,western Kentucky

The Rocks loess section, in unglaciated western Kentucky, provides a high-resolution environmental record during the last glacial maximum onset. The Peoria Silt (9 m thick) contains 26 terrestrial gastropod species, with up to 15 species within a single 5 cm interval. Thirteen radiocarbon ages, using shells or charcoal, range between 30 and 24.5 cal ka; younger loess has been leached or eroded. Stratigraphic shifts in gastropod assemblages imply significant cooling, particularly ~27 cal ka, as solar insolation was decreasing and the southern Laurentide Ice Sheet rapidly advancing. Midwestern to southern species (e.g. Anguispira kochi, Gastrocopta pentodon, Hawaii miniscula, Helicodiscus parallelus, Vallonia perspectiva) occur only in the lowermost Peoria Silt (~30–27 cal ka). In contrast, cold-tolerant species (Columella alticola, Vertigo modesta, Vallonia gracilicosta) occur only in full glacial Peoria Silt (27–24.5 cal ka). Inferred mean July temperatures, from mutual climatic range methods, range from ~23 °C at 30 cal ka, cooling to ~18 °C by 26 cal ka; about 3–8 °C cooler than today (~26 °C). Superimposed on this cooling trend are multi-centennial variations in detrital carbonate, fossil shell concentrations, palaeotemperature estimates, and oxygen isotope values (Vertigo, Discus, Helicodiscus). The finer-scale variations imply relatively synchronous fluctuations in glacial sediment supply, loess sedimentation, and climate.     

Estrogenic Activity and Volume Fraction of Waste Water Origin in Monitoring Wells Along the Santa Cruz River, Arizona

The fate of estrogenic activity in waste water effluent was examined during surface transport and incidental recharge along the Santa Cruz River in Pima County, Arizona. Based on measurement of boron isotopes, the fractional contribution of reclaimed water in surface waters and ground water wells proximate to the river was determined for a contemporary sample set. Estrogenic activity decreased by −60% over the 25 mi length of the river below effluent discharge points in Tucson. In ground water samples obtained from monitoring wells that are proximate to the Santa Cruz River, both dissolved organic carbon ( p = 0.0003) and estrogenic activity ( p = 3 × 10⁻⁶) were highly correlated to fractional waste water content. Results indicate that proximate ground water quality is sensitive to incidental recharge of reclaimed water in the Santa Cruz River bed. In a few locations, little attenuation of estrogenic activity was apparent during percolation of effluent in the river channel to well withdrawal points.     

Size‐fecundity relationships in the smooth oreo,Pseudocyttus maculatus,and the black oreo,Allocyttus niger (Pisces: Oreosomatidae) (note)

Fecundity estimates were made from 72 smooth oreo (Pseudocyttus maculatus Gilchrist, 1906) and 77 black oreo (Allocyttus niger James, Inada & Nakamura, 1988) with oocytes in late vitellogenesis, captured from the south Chatham Rise during November 1986. Absolute fecundities ranged from 6000 to 84 000 for smooth oreo and 5000 to 62 000 for black oreo, with relative fecundities of 10 800 and 17 500 eggs per kg body weight for smooth and black oreos, respectively. Both species have absolute and relative fecundities within the same order of magnitude as orange roughy, but one to two orders of magnitude lower than six other commercially exploited fish species from the New Zealand continental shelf or slope region. The comparatively low fecundity of the oreo species could contribute to susceptibility to overfishing.