Late Quaternary sea‐level change and evolution of Belfast Lough,Northern Ireland: new offshore evidence and implications for sea‐level reconstruction

The interplay of eustatic and isostatic factors causes complex relative sea‐level (RSL) histories, particularly in paraglacial settings. In this context the past record of RSL is important in understanding ice‐sheet history, earth rheology and resulting glacio‐isostatic adjustment. Field data to develop sea‐level reconstructions are often limited to shallow depths and uncertainty exists as to the veracity of modelled sea‐level curves. We use seismic stratigraphy, 39 vibrocores and 26 radiocarbon dates to investigate the deglacial history of Belfast Lough, Northern Ireland, and reconstruct past RSL. A typical sequence of till, glacimarine and Holocene sediments is preserved. Two sea‐level lowstands (both max. ?40 m) are recorded at c. 13.5 and 11.5k cal a bp . Each is followed by a rapid transgression and subsequent periods of RSL stability. The first transgression coincides temporally with a late stage of Meltwater Pulse 1a and the RSL stability occurred between c. 13.0 and c. 12.2k cal a bp (Younger Dryas). The second still/slowstand occurred between c. 10.3 and c. 11.5k cal a bp . Our data provide constraints on the direction and timing of RSL change during deglaciation. Application of the Depth of Closure concept adds an error term to sea‐level reconstructions based on seismic stratigraphic reconstructions.     

Evaluation of NLDAS‐2 evapotranspiration against tower flux site observations

The North American Land Data Assimilation System project phase 2 (NLDAS‐2) has run four land surface models for a 30‐year (1979–2008) retrospective period. Land surface evapotranspiration (ET) is one of the most important model outputs from NLDAS‐2 for investigating land–atmosphere interaction or to monitor agricultural drought. Here, we evaluate hourly ET using in situ observations over the Southern Great Plains (Atmospheric Radiation Measurement/Cloud and Radiation Testbed network) for 1 January 1997–30 September 1999 and daily ET u‐sing in situ observations at the AmeriFlux network over the conterminous USA for an 8‐year period (2000–2007). The NLDAS‐2 models compare well against observations, with the National Centers for Environmental Prediction's Noah land surface model performing best, followed, in order, by the Variable Infiltration Capacity, Sacramento Soil Moisture Accounting, and Mosaic models. Daily evaluation across the AmeriFlux network shows that for all models, performance depends on season and vegetation type; they do better in spring and fall than in winter or summer and better for deciduous broadleaf forest and grasslands than for croplands or evergreen needleleaf forest. Copyright © 2014 John Wiley & Sons, Ltd.     

Limitations of suction‐controlled triaxial tests in the characterization of unsaturated soils

Behavior of unsaturated soils is influenced by many factors, and the influences of these factors are usually coupled together. Suction‐controlled triaxial (SCTX) tests are considered to allow researchers to investigate influences of individual variables on unsaturated soils under specified stress path with controls of stresses, pore water, and air pressures. In the past 50 years, SCTX testing method has been established as a standard approach to characterize constitutive behavior of unsaturated soils. Most important concepts for modern unsaturated soil mechanics were developed upon results from the SCTX tests. Among these, one of the most important contributions in the constitutive modeling of elasto‐plastic behavior for unsaturated soils is the Barcelona basic model (BBM) proposed by Alonso et al. in 1990. The BBM successfully explained many features of unsaturated soils and received extensive acceptance. However, the SCTX tests are designed based upon the divide‐and‐conquer approach in which an implicit assumption is used: soil behavior is stress‐path independent. However, it is well‐established that unsaturated soil behavior is elasto‐plastic and stress‐path dependent. It is found that the SCTX tests in fact cannot control the stress path of an unsaturated soil during loading. This incapability, in combination with complicated loading/collapse behavior of unsaturated soils, makes the SCTX tests for characterizing unsaturated soil questionable. This paper discusses the limitations of the SCTX tests in the characterization of unsaturated soils. A possible solution to the problem was proposed based on a newly developed modified state surface approach. The discussions are limited for isotropic conditions. Copyright © 2015 John Wiley & Sons, Ltd.     

Insights into the complexity of crustal differentiation: K2O‐poor leucosomes within metasedimentary migmatites from the Southern Marginal Zone of the Limpopo Belt,South Africa

Differentiation of the continental crust is the result of complex interactions between a large number of processes, which govern partial melting of the deep crust, magma formation and segregation, and magma ascent to significantly higher crustal levels. The anatectic metasedimentary rocks exposed in the Southern Marginal Zone of the Limpopo Belt represent an unusually well‐exposed natural laboratory where the portion of these processes that operate in the deep crust can be directly investigated in the field. The formation of these migmatites occurred via absent incongruent melting reactions involving biotite, which produced cm‐ to m‐scale, K₂O‐poor garnet‐bearing stromatic leucosomes, with high Ca/Na ratios relative to their source rocks. Field investigation combined with geochemical analyses, and phase equilibrium modelling designed to investigate some aspects of disequilibrium partial melting show that the outcrop features and compositions of the leucosomes suggest several steps in their evolution: (1) Melting of a portion of the source, with restricted plagioclase availability due to kinetic controls, to produce a magma (melt + entrained peritectic minerals in variable proportions relative to melt); (2) Segregation of the magma at near peak metamorphic conditions into melt accumulation sites (MAS), also known as future leucosome; (3a) Re‐equilibration of the magma with a portion of the bounding mafic residuum via chemical diffusion (H₂O, K₂O), which triggers the co‐precipitation of quartz and plagioclase in the MAS; (3b) Extraction of melt‐dominated magma to higher crustal levels, leaving peritectic minerals entrained from the site of the melting reaction, and the minerals precipitated in the MASs to form the leucosome in the source. The key mechanism controlling this behaviour is the kinetically induced restriction of the amount of plagioclase available to the melting reaction. This results in elevated melt H₂O and K₂O and chemical potential gradient for these components across the leucosome/mafic residuum contact. The combination of all of these processes accurately explains the composition of the K₂O‐poor leucosomes. These findings have important implications for our understanding of melt segregation in the lower crust and minimum melt residency time which, according to the chemical modelling, is <5 years. We demonstrate that in some migmatitic granulites, the leucosomes constitute a type of felsic refractory residuum, rather than evidence of failed magma extraction. This provides a new insight into the ways that source heterogeneity may control anatexis.     

Water level observations from unmanned aerial vehicles for improving estimates of surface water–groundwater interaction

Integrated hydrological models are usually calibrated against observations of river discharge and piezometric head in groundwater aquifers. Calibration of such models against spatially distributed observations of river water level can potentially improve their reliability and predictive skill. However, traditional river gauging stations are normally spaced too far apart to capture spatial patterns in the water surface, whereas spaceborne observations have limited spatial and temporal resolution. Unmanned aerial vehicles can retrieve river water level measurements, providing (a) high spatial resolution; (b) spatially continuous profiles along or across the water body, and (c) flexible timing of sampling. A semisynthetic study was conducted to analyse the value of the new unmanned aerial vehicle‐borne datatype for improving hydrological models, in particular estimates of groundwater–surface water (GW–SW) interaction. Mølleåen River (Denmark) and its catchment were simulated using an integrated hydrological model (MIKE 11–MIKE SHE). Calibration against distributed surface water levels using the Differential Evolution Adaptive Metropolis algorithm demonstrated a significant improvement in estimating spatial patterns and time series of GW–SW interaction. After water level calibration, the sharpness of the estimates of GW–SW time series improves by ~50% and root mean square error decreases by ~75% compared with those of a model calibrated against discharge only.     

Hydrologic responses to land cover change: the case of Jedeb mesoscale catchment,Abay/Upper Blue Nile basin,Ethiopia

The objective of this study was to quantify the impacts of land use/land cover (LULC) change on the hydrology of the Jedeb, an agricultural dominated mesoscale catchment, in the Abay/Upper Blue Nile basin, Ethiopia. Two methods have been used. First, the trends of certain daily flow variability parameters were evaluated to detect statistical significance of the change of the hydrologic response. Second, a conceptual monthly hydrological model was used to detect changes in the model parameters over different periods to infer LULC change. The results from the statistical analysis of the daily flows between 1973 and 2010 reveal a significant change in the response of the catchment. Peak flow is enhanced, i.e. response appears to be flashier. There is a significant increase in the rise and fall rates of the flow hydrograph, as well as the number of low‐flow pulses below a threshold level. The discharge pulses show a declining duration with time. The model result depicts a change in model parameters over different periods, which could be attributed to an LULC change. The model parameters representing soil moisture conditions indicated a gradual decreasing trend, implying limited storage capacity likely attributed to increasing agricultural farming practices in the catchment. This resulted in more surface runoff and less infiltration into the soil layers. The results of the monthly flow duration curve analysis indicated large changes of the flow regime. The high flow has increased by 45% between the 1990s and 2000s, whereas the reduction in low flows was larger: a 15% decrease between 1970s and 1980s, 39% between 1980s and 1990s and up to 71% between 1990s and 2000s. These results, could guide informed catchment management practices to reduce surface runoff and augment soil moisture level in the Jedeb catchment. Copyright © 2013 John Wiley & Sons, Ltd.     

Assessing the ‘two water worlds’ hypothesis and water sources for native and exotic evergreen species in south‐central Chile

Recent studies using water‐stable isotopes (δ¹⁸O and δ²H) have suggested an ecohydrological separation of water flowing to streams or recharging groundwater and water used by trees, known as the ‘two water worlds’ (TWW) hypothesis. In this study, we measured water isotopic composition in precipitation [open field and throughfall, i.e. local meteoric water line (LMWL)] and the mobile water compartment (i.e. stream and soil solution), bulk soil water and xylem water over a period of 1.5 years in two headwater catchments: NF, covered with old growth native evergreen forest (Aetoxicon punctatum, Laureliopsis philippiana and Eucriphya cordifolia), and EP, covered with 4 and 16‐year‐old Eucalyptus nitens stands. Our results show that precipitation, stream and soil solution plot approximately along the LMWL, while xylem waters from all studied tree species plot below the LMWL, supporting the TWW hypothesis. However, we also found evidence of ecohydrological connectivity during the wet season, likely controlled by the amount of antecedent precipitation. These observations hold for all investigated tree species. On both sites, a different precipitation source for stream and xylem water was observed. However, in EP, bulk soil showed a similar precipitation source as xylem water from both E. nitens stands. This suggests that E. nitens may use water that is recharging the bulk soil compartment. We conclude that under a rainy temperate climate, the TWW hypothesis is temporal and does not apply during wet seasons. Copyright © 2016 John Wiley & Sons, Ltd.     

Large‐scale poroelastic fractured reservoirs modeling using the fast multipole displacement discontinuity method

An effective approach to modeling the geomechanical behavior of the network and its permeability variation is to use a poroelastic displacement discontinuity method (DDM). However, the approach becomes rather computationally intensive for an extensive system of cracks, particularly when considering coupled diffusion/deformation processes. This is because of additional unknowns and the need for time‐marching schemes for the numerical integration. The Fast Multipole Method (FMM) is a technique that can accelerate the solution of large fracture problems with linear complexity with the number of unknowns both in memory and CPU time. Previous works combining DDM and FMM for large‐scale problems have accounted only for elastic rocks, neglecting the fluid leak‐off from the fractures into the matrix and its influence on pore pressure and stress field. In this work we develop an efficient geomechanical model for large‐scale natural fracture networks in poroelastic reservoirs with fracture flow in response to injection and production operations. Accuracy and computational performance of the proposed method with those of conventional poroelastic DDM are compared through several case studies involving up to several tens of thousands of boundary elements. The results show the effectiveness of the FMM approach to successfully evaluate field‐scale problems for the design of exploitation strategies in unconventional geothermal and petroleum reservoirs. An example considering faults reveals the impact of reservoir compartmentalization because of sealing faults for both geomechanical and flow variables under elastic and poroelastic rocks. Copyright © 2015 John Wiley & Sons, Ltd.     

Modelling groundwater/surface water interaction in a managed riparian chalk valley wetland

Understanding hydrological processes in wetlands may be complicated by management practices and complex groundwater/surface water interactions. This is especially true for wetlands underlain by permeable geology, such as chalk. In this study, the physically based, distributed model MIKE SHE is used to simulate hydrological processes at the Centre for Ecology and Hydrology River Lambourn Observatory, Boxford, Berkshire, UK. This comprises a 10‐ha lowland, chalk valley bottom, riparian wetland designated for its conservation value and scientific interest. Channel management and a compound geology exert important, but to date not completely understood, influences upon hydrological conditions. Model calibration and validation were based upon comparisons of observed and simulated groundwater heads and channel stages over an equally split 20‐month period. Model results are generally consistent with field observations and include short‐term responses to events as well as longer‐term seasonal trends. An intrinsic difficulty in representing compressible, anisotropic soils limited otherwise excellent performance in some areas. Hydrological processes in the wetland are dominated by the interaction between groundwater and surface water. Channel stage provides head boundaries for broad water levels across the wetland, whilst areas of groundwater upwelling control discrete head elevations. A relic surface drainage network confines flooding extents and routes seepage to the main channels. In‐channel macrophyte growth and its management have an acute effect on water levels and the proportional contribution of groundwater and surface water. The implications of model results for management of conservation species and their associated habitats are discussed. Copyright © 2015 John Wiley & Sons, Ltd.