A modified landform development model for the topography of drained thermokarst lake basins in fine‐grained sediments

Permafrost degradation associated with the expansion of thermokarst lakes is commonly interrupted by catastrophic drainage. Subsequently, in tundra areas, permafrost aggradation in drained basins leads to uneven topography characterized by raised centres and wet, depressed margins. The genesis of such topography has been investigated in Old Crow Flats (OCF), a glaciolacustrine plain in the continuous permafrost of northern Yukon. The thermokarst lakes of OCF have a mean depth of only 1.5 m because excess ice is dominantly found only in the uppermost 10 m of the ground. Surface conditions were measured in three drained thermokarst lake basins, including relief, snow conditions, ground temperatures, near‐surface ground ice, and sediment stratigraphy. Four nearby lakes provided information on wave base, shore recession patterns, and bathymetry before drainage: the bottoms of these lakes were not raised in the centre. An elevation difference of up to 2 m was recorded between drained basin margins and centres but was not associated with variations in ice‐wedge density or segregated ice content. Hence basin topography was not controlled by differences in volumetric ground‐ice content between margins and centres. We propose that transport of fine sediment away from eroding lake margins during lake development is the primary mechanism for the genesis of depressed margins and raised centres in drained basins of OCF. Over time, the transport results in the deposition of more and finer sediment in the central parts of lakes, where the lake bottom has subsided below wave base, than at the shallow margins, where resuspension by wave action occurs frequently. This difference in sediment volume is revealed in the topography after drainage, when permafrost aggrades in the lake‐bottom sediment and underlying talik. Copyright © 2016 John Wiley & Sons, Ltd.     

The impacts of mountain pine beetle disturbance on the energy balance of snow during the melt period

Mountain snowpacks provide most of the annual discharge of western US rivers, but the future of water resources in the western USA is tenuous, as climatic changes have resulted in earlier spring melts that have exacerbated summer droughts. Compounding changes to the physical environment are biotic disturbances including that of the mountain pine beetle (MPB), which has decimated millions of acres of western North American forests. At the watershed scale, MPB disturbance increases the peak hydrograph, and at the stand scale, the ‘grey’ phase of MPB canopy disturbance decreases canopy snow interception, increases snow albedo, increases net shortwave radiation, and decreases net longwave radiation versus the ‘red’ phase. Fewer studies have been conducted on the red phase of MPB disturbance and in the mixed coniferous stands that may follow MPB‐damaged forests. We measured the energy balance of four snowpacks representing different stages of MPB damage, management, and recovery: a lodgepole pine stand, an MPB‐infested stand in the red phase, a mixed coniferous stand (representing one successional trajectory), and a clear‐cut (representing reactive management) in the Tenderfoot Creek Experimental Forest in Montana, USA. Net longwave radiation was lower in the MPB‐infested stand despite higher basal area and plant area index of the other forests, suggesting that the desiccated needles serve as a less effective thermal buffer against longwave radiative losses. Eddy covariance observations of sensible and latent heat flux indicate that they are of similar but opposite magnitude, on the order of 20 MJ m^?2 during the melt period. Further analyses reveal that net turbulent energy fluxes were near zero because of the temperature and atmospheric vapour pressure encountered during the melt period. Future research should place snow science in the context of forest succession and management and address important uncertainties regarding the timing and magnitude of needlefall events. Copyright © 2015 John Wiley & Sons, Ltd.     

Numerical simulation of agricultural sediment and pesticide runoff: RZWQM and PRZM comparison

Agricultural sediment and pesticide runoff is a widespread ecological and human health concern. Numerical simulation models, such as Root Zone Water Quality Model (RZWQM) and Pesticide Root Zone Model (PRZM), have been increasingly used to quantify off‐site agricultural pollutant movement. However, RZWQM has been criticized for its inability to simulate sedimentation processes. The recent incorporation of the sedimentation module of Groundwater Loading Effects of Agricultural Management Systems has enabled RZWQM to simulate sediment and sediment‐associated pesticides. This study compares the sediment and pesticide transport simulation performance of the newly released RZWQM and PRZM using runoff data from 2 alfalfa fields in Davis, California. A composite metric (based on coefficient of determination, Nash–Sutcliffe efficiency, index of agreement, and percent bias) was developed and employed to ensure robust, comprehensive assessment of model performance. Results showed that surface water runoff was predicted reasonably well (absolute percent bias <31%) by RZWQM and PRZM after adjusting important hydrologic parameters. Even after calibration, underestimation bias (?89% ≤ PBIAS  ≤ ?36%) for sediment yield was observed in both models. This might be attributed to PRZM's incorrect distribution of input water and uncertainty in RZWQM's runoff erosivity coefficient. Moreover, the underestimation of sediment might be less if the origin of measured sediment was considered. Chlorpyrifos losses were simulated with reasonable accuracy especially for Field A (absolute PBIAS  ≤ 22%), whereas diuron losses were underestimated to a great extent (?98% ≤ PBIAS  ≤ ?65%) in both models. This could be attributed to the underprediction of herbicide concentration in the top soil due to the limitations of the instantaneous equilibrium sorption model as well as the high runoff potential of herbicide formulated as water‐dispersible granules. RZWQM and PRZM partitioned pesticides into the water and sediment phases similarly. According to model predictions, the majority of pesticide loads were carried via the water phase. On the basis of this study, both RZWQM and PRZM performed well in predicting runoff that carried highly adsorptive pesticides on an event basis, although the more physically based RZWQM is recommended when field‐measured soil hydraulic properties are available.     

Short‐ and long‐term evapotranspiration rates at ecological restoration sites along a large river receiving rare flow events

Many large rivers around the world no longer flow to their deltas, due to ever greater water withdrawals and diversions for human needs. However, the importance of riparian ecosystems is drawing increasing recognition, leading to the allocation of environmental flows to restore river processes. Accurate estimates of riparian plant evapotranspiration (ET) are needed to understand how the riverine system responds to these rare events and achieve the goals of environmental flows. In 2014, historic environmental flows were released into the Lower Colorado River at Morelos Dam (Mexico); this once perennial but now dry reach is the final stretch to the mighty Colorado River Delta. One of the primary goals was to supply native vegetation restoration sites along the reach with water to help seedlings establish and boost groundwater levels to foster the planted saplings. Patterns in ET before, during, and after the flows are useful for evaluating whether this goal was met and understanding the role that ET plays in this now ephemeral river system. Here, diurnal fluctuations in groundwater levels and Moderate Resolution Imaging Spectroradiometer (MODIS) data were used to compare estimates of ET specifically at 3 native vegetation restoration sites during 2014 planned flow events, and MODIS data were used to evaluate long‐term (2002–2016) ET responses to restoration efforts at these sites. Overall, ET was generally 0–10 mm d^?1 across sites, and although daily ET values from groundwater data were highly variable, weekly averaged estimates were highly correlated with MODIS‐derived estimates at most sites. The influence of the 2014 flow events was not immediately apparent in the results, although the process of clearing vegetation and planting native vegetation at the restoration sites was clearly visible in the results.     

14C dating and palaeoenvironment of the historic ‘little ice age’ glacier advance of Nigardsbreen Southwest Norway

Moss and grass remains associated with a well-developed in situ palaeosol buried beneath a moraine ridge in front of Nigardsbreen (Jostedalsbreen ice cap, southern Norway) have been ¹⁴C-dated. Pollen preserved with the plant remains suggests the existence of an agricultural landscape prior to the deposition of the moraine. The calibrated dates and the pollen spectra are in close accord with historically-documented evidence for the timing and palaeoenvironment of the ‘Little Ice Age’ advance of the glacier. Considerable potential is indicated for estimating the maximum ages of moraine ridges and for palaeoenvironmental reconstruction where such documentary evidence does not exist.     

Remineralization Ratios in the Subtropical North Pacific Gyre

Based on a new mixing model of two end-members, the water column remineralization ratios of P/N/C_org - O₂ = 1/13 ± 1/135 ± 18/170 ± 9 are obtained for the Hawaii Ocean Time-series (HOT) data set at station ALOHA. The traditional Redfield ratios of P/N/C_org/–O₂ = 1/16/106/138 have standard deviations of more than 50%, when they are based on the average composition of phytoplankton. Apparently, the remineralization processes in the water column have smoothed out the observed large variability of plankton compositions. A new molar formula for the remineralized plankton may be written as ₁₃₅H₂₈₀O₁₀₅N₁₃P or C₂₅(CH₂O)₁₀₁(CH₄)₉(NH₃)₁₃(H₃PO₄). Oxidation of this formula results inC₂₅(CH₂O)₁₀₁(CH₄)₉(NH₃)₁₃(H₃PO₄) + 170O₂ 135CO₂ + 132H₂O + 13NO₃ ^- + H₂PO₄ ^- + 14H⁺.For comparison, remineralization using Redfield's formula gives:(CH₂O)₁₀₆(NH₃)₁₆(H₃PO₄) + 138O₂ 106CO₂ + 122H₂O + 16NO₃ ^-+ H₂PO₄ ^- + 17H⁺     

Evidence of ameliorated Middle Weichselian climate and sub‐arctic environment in the western Baltic region: coring lake sediments at Klintholm,Møn,Denmark

Coring through glaciotectonically stacked Quaternary sediments situated below sea level on the island of Møn, Denmark, recovered a succession of interstadial sediments of Middle Weichselian age. Plant and animal remains including insects found in laminated sand and mud indicate deposition in a lake surrounded by dwarf shrubs, herbs, mosses and rare trees. The insect fauna indicates a mean July temperature of 8–12 °C, suggesting an arctic to sub‐arctic environment, while winter temperatures around ?8 to ?22 °C suggest periglacial conditions with permafrost. Luminescence dating of sediment samples gave ages from 48–29 ka, and radiocarbon dating indicates deposition of plant fragments between 45 and 36 ka BP. The fossil assemblage from Møn shows close resemblance to those from other sites with similar ages found in the vicinity of the western Baltic Basin.