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Modelling ablation of glacier ice under a layer of mineral debris is increasingly important, because the extent of supraglacial debris is expanding worldwide due to glacier recession. Physically based models have been developed, but the uncertainty in predictions is not yet well constrained. A new one-dimensional model of debris-covered ice ablation that is based on the Simultaneous Heat and Water transfer model is introduced here. SHAW-Glacier is a physically based, vertically integrated, fully coupled, water and energy balance model, which includes the advection of heat by rainwater and lateral flow. SHAW-Glacier was applied to North Changri Nup, a high elevation alpine glacier in the monsoon-dominated Central Himalaya. Simulations were compared with observed debris temperature profiles, snow depth, and ablation stake measurements for debris 0.03–0.41 m thick, in a 2500 m2 study area. Prediction uncertainty was estimated in a Monte Carlo analysis. SHAW-Glacier simulated the characteristic pattern of decreasing ablation with increasing debris thickness. However, the observations of ablation did not follow the characteristic pattern; annual ablation was highest where the debris was thickest. Recursive partitioning revealed a substantial, non-linear sensitivity to the snow threshold air temperature, suggesting a sensitivity to the duration of snow cover. Photographs showed patches of snow persisting through the ablation season, and the observational data were consistent with uneven persistence of snow patches. The analyses indicate that patchy snow cover in the ablation season can overwhelm the sensitivity of sub-debris ablation to debris thickness. Patchy snow cover may be an unquantified source of uncertainty in predictions of sub-debris ablation.

Abstract Little is known about the neuropsychiatric and neurophysiological differences that characterize abnormal recovery following a concussion. The present study aimed to investigate the psycho-affective, cognitive, and neurophysiological profiles of symptomatic, slow-to-recover, concussed athletes, asymptomatic concussed athletes, and control athletes. Seventy-eight athletes (26 symptomatic, 26 asymptomatic, 26 control) completed the Beck Depression Inventory-II, Profile of Mood States, and 2-Back task. Additionally, event-related brain potentials were recorded during an experimental three-stimulus visual Oddball paradigm. Compared to asymptomatic and control groups, the symptomatic group reported greater depression symptoms and negatively altered mood states. Symptomatic athletes also exhibited poorer cognitive performance on the 2-Back task, indicated by more errors and slower reaction time. ERP analyses indicated prolonged P3b latency for both symptomatic and asymptomatic groups, but symptomatic athletes also exhibited reduced P3b amplitude compared to both asymptomatic and control groups. For the asymptomatic group, correlations were observed between time since last concussion and functioning, but no relations were observed within the symptomatic group for any measure. The current findings provide valuable information regarding the psycho-affective, cognitive, and neurophysiological profiles of athletes with and without persistent symptoms following a concussion and highlight the need to assess and treat symptomatic, slow-to-recover athletes from a multidimensional and integrative perspective.

Supraglacial debris is significant in many regions and complicates modeling of glacier melt, which is required for predicting glacier change and its influences on hydrology and sea-level rise. Temperature-index models are a popular alternative to energy-balance models when forcing data are limited, but their transferability among glaciers and inherent uncertainty have not been documented in application to debris-covered glaciers. Here, melt factors were compiled directly from published studies or computed from reported melt and MERRA-2 air temperature for 27 debris-covered glaciers around the world. Linear mixed-effects models were fit to predict melt factors from debris thickness and variables including debris lithology and MERRA-2 radiative exchange. The models were tested by leave-one-site-out cross-validation based on predicted melt rates. The best model included debris thickness (fixed effect) and glacier and year (random effects). Predictions were more accurate using MERRA-2 than on-site air temperature data, and pooling MERRA-2-derived and reported melt factors improved cross-validation accuracy more than including additional predictors such as shortwave or longwave radiation. At one glacier where monthly ablation was measured over 4 years, seasonal variation of melt factors suggested that heat storage significantly affected the relation between melt and energy exchange at the debris surface.

Abstract Background Anemia is common among people living with HIV infection (PLWH) and is associated with adverse health outcomes. Information on risk factors for anemia incidence in the current antiretroviral therapy (ART) era is lacking. Methods Within a prospective clinical cohort of adult PLWH receiving care at eight sites across the United States between 1/2010–3/2018, Cox proportional hazards regression analyses were conducted among a) PLWH free of anemia at baseline and b) PLWH free of severe anemia at baseline to determine associations between time-updated patient characteristics and development of anemia (hemoglobin

Most studies of gravel bed rivers present at least one bed surface grain size distribution, but there is almost never any information provided about the uncertainty in the percentile estimates. We present a simple method for estimating the grain size confidence intervals about sample percentiles derived from standard Wolman or pebble count samples of bed surface texture. The width of a grain size confidence interval depends on the confidence level selected by the user (e.g., 95 %), the number of stones sampled to generate the cumulative frequency distribution, and the shape of the frequency distribution itself. For a 95 % confidence level, the computed confidence interval would include the true grain size parameter in 95 out of 100 trials, on average. The method presented here uses binomial theory to calculate a percentile confidence interval for each percentile of interest, then maps that confidence interval onto the cumulative frequency distribution of the sample in order to calculate the more useful grain size confidence interval. The validity of this approach is confirmed by comparing the predictions using binomial theory with estimates of the grain size confidence interval based on repeated sampling from a known population. We also developed a two-sample test of the equality of a given grain size percentile (e.g., D50), which can be used to compare different sites, sampling methods, or operators. The test can be applied with either individual or binned grain size data. These analyses are implemented in the freely available GSDtools package, written in the R language. A solution using the normal approximation to the binomial distribution is implemented in a spreadsheet that accompanies this paper. Applying our approach to various samples of grain size distributions in the field, we find that the standard sample size of 100 observations is typically associated with uncertainty estimates ranging from about ±15 % to ±30 %, which may be unacceptably large for many applications. In comparison, a sample of 500 stones produces uncertainty estimates ranging from about ±9 % to ±18 %. In order to help workers develop appropriate sampling approaches that produce the desired level of precision, we present simple equations that approximate the proportional uncertainty associated with the 50th and 84th percentiles of the distribution as a function of sample size and sorting coefficient; the true uncertainty in any sample depends on the shape of the sample distribution and can only be accurately estimated once the sample has been collected.

This study focused on the effects of glacier wastage on streamflow in the Canadian portion of the Columbia River headwaters over the period 1977 to 2017. Between 1985 and 2013, glacier coverage decreased by up to 2% of catchment area for the 35 study catchments. The mean wastage flux contribution to streamflow had a positive relation with fractional glacier coverage and an inverse relation with catchment water yield. Glacier mass change estimates suggest that wastage flux contributions declined between 1985–1999 and 2000–2018, but the estimates are subject to substantial uncertainty. Annual wastage flux contributions over a four-year period for two study catchments ranged from 8 to 13% of annual water yield for a catchment with 17% glacier cover, with glaciers extending below treeline, and 9–19% for a smaller alpine catchment with 57% glacier cover. After accounting statistically for climatic forcing and non-glacial contributions to streamflow, August runoff from glacierized catchments decreased through time at a rate that was linearly related to loss of glacier cover. The analyses suggest that glacier-melt contributions to August runoff have already have passed peak water, and that these reductions have exacerbated a regional climate-driven trend to decreased August streamflow contributions from unglacierized areas.

Bridge Glacier is a lake-calving glacier in the Coast Mountains of British Columbia and has retreated over 3.55 km since 1972. The majority of this retreat has occurred since 1991. This retreat is substantially greater than what has been inferred from regional climate indices, suggesting that it has been driven primarily by calving as the glacier retreated across an overdeepened basin. In order to better understand the primary drivers of ablation, surface melt (below the equilibrium line altitude, ELA) and calving were quantified during the 2013 melt season using a distributed energy balance model (DEBM) and time-lapse imagery. Calving, estimated using areal change, velocity measurements, and assuming flotation were responsible for 23 % of the glacier's ablation below the ELA during the 2013 melt season and were limited by modest flow speeds and a small terminus cross-section. Calving and surface melt estimates from 1984 to 2013 suggest that calving was consistently a smaller contributor of ablation. Although calving was estimated to be responsible for up to 49 % of the glacier's ablation for individual seasons, averaged over multiple summers it accounted between 10 and 25 %. Calving was enhanced primarily by buoyancy and water depths, and fluxes were greatest between 2005 and 2010 as the glacier retreated over the deepest part of Bridge Lake. The recent rapid rate of calving is part of a transient stage in the glacier's retreat and is expected to diminish within 10 years as the terminus recedes into shallower water at the proximal end of the lake. These findings are in line with observations from other lake-calving glacier studies across the globe and suggest a common large-scale pattern in calving-induced retreat in lake-terminating alpine glaciers. Despite enhancing glacial retreat, calving remains a relatively small component of ablation and is expected to decrease in importance in the future. Hence, surface melt remains the primary driver of ablation at Bridge Glacier and thus projections of future retreat should be more closely tied to climate.

The role of spatial variability in water inputs on runoff dynamics has generally not received as much research attention as topography and soils; however, the influence of topography and forest cover on snow surface energy exchanges can result in asynchronous snowmelt throughout a catchment, complicating the space–time patterns of runoff generation. This study investigates temporal variation in the relative importance of spatial controls on the occurrence, duration, and timing of shallow groundwater response, utilizing a highly distributed monitoring network in a snowmelt-dominated montane catchment in western Canada. The study findings indicate that deep-soil hydraulic conductivity is a first-order control on the spatial distribution of sites that generate shallow groundwater response versus sites that experience only deep percolation. Upslope contributing area and slope gradient are first-order controls on the duration of groundwater response during peak-flow, recession-flow, and low-flow periods. Shallow runoff response areas expand and contract throughout these periods and follow the general spatial patterns of topographic convergence. However, spatial controls on the timing, intensity, and quantity of snowmelt and controls on vertical versus lateral flux partitioning in the soil overwhelm the influence of topographic convergence on runoff patterns during early spring freshet periods. The study findings suggest that various topographic indices and topography-based rainfall runoff models would not likely be good predictors of runoff patterns in snowmelt-dominated montane catchments during early phases of the spring freshet, but would increase in importance as the freshet and post-freshet periods proceed.

Stream temperature dynamics during winter are less well studied than summer thermal regimes, but the winter season thermal regime can be critical for fish growth and development in coastal catchments. The winter thermal regimes of Pacific Northwest headwater streams, which provide vital winter habitat for salmonids and their food sources, may be particularly sensitive to changes in climate because they can remain ice-free throughout the year and are often located in rain-on-snow zones. This study examined winter stream temperature patterns and controls in small headwater catchments within the rain-on-snow zone at the Malcolm Knapp Research Forest, near Vancouver, British Columbia, Canada. Two hypotheses were addressed by this study: (1) winter stream temperatures are primarily controlled by advective fluxes associated with runoff processes and (2) stream temperatures should be depressed during rain-on-snow events, compared to rain-on-bare-ground events, due to the cooling effect of rain passing through the snowpack prior to infiltrating the soil or being delivered to the stream as saturation-excess overland flow. A reach-scale energy budget analysis of two winter seasons revealed that the advective energy input associated with hillslope runoff overwhelms vertical energy exchanges (net radiation, sensible and latent heat fluxes, bed heat conduction, and stream friction) and hyporheic energy fluxes during rain and rain-on-snow events. Historical stream temperature data and modelled snowpack dynamics were used to explore the influence of transient snow cover on stream temperature over 13 winters. When snow was not present, daily stream temperature during winter rain events tended to increase with increasing air temperature. However, when snow was present, stream temperature was capped at about 5 °C, regardless of air temperature. The stream energy budget modelling and historical analysis support both of our hypotheses. A key implication is that climatic warming may generate higher winter stream temperatures in the rain-on-snow zone due to both increased rain temperature and reduced cooling effect of snow cover.

We describe a method to calculate regional snow line elevations and annual equilibrium line altitudes (ELAs) from daily MODIS imagery (MOD02QKM) on large glaciers and icefields in western North America. An automated cluster analysis of the cloud-masked visible and near-infrared bands at 250 m resolution is used to delineate glacier facies (snow and ice) for ten glacierized regions between 2000–2011. For each region and season, the maximum observed value of the 20th percentile of snow-covered pixels (ZS(20)) is used to define a regional ELA proxy (ELAest). Our results indicate significant increases in the regional ELA proxy at two continental sites (Peyto Glacier and Gulkana Glacier) over the period of observation, though no statistically significant trends are identified at other sites. To evaluate the utility of regional ELA proxies derived from MOD02QKM imagery, we compare standard geodetic estimates of glacier mass change with estimates derived from historical mass balance gradients and observations of ZS(20) at three large icefields. Our approach yields estimates of mass change that more negative than traditional geodetic approaches, though MODIS-derived estimates are within the margins of error at all three sites. Both estimates of glacier mass change corroborate the continued mass loss of glaciers in western North America. Between 2000 and 2009, the geodetic change approach yields mean annual rates of surface elevation change for the Columbia, Lillooet, and Sittakanay icefields of −0.29 ± 0.05, −0.26 ± 0.05, and −0.63 ± 0.17 m a−1, respectively. This study provides a new technique for glacier facies detection at daily timescales, and contributes to the development of regional estimates of glacier mass change, both of which are critical for studies of glacier contributions to streamflow and global sea level rise.

Glacier melt provides important contributions to streamflow in many mountainous regions. Hydrologic model calibration in glacier-fed catchments is difficult because errors in modelling snow accumulation can be offset by compensating errors in glacier melt. This problem is particularly severe in catchments with modest glacier cover, where goodness-of-fit statistics such as the Nash-Sutcliffe model efficiency may not be highly sensitive to the streamflow variance associated with glacier melt. While glacier mass balance measurements can be used to aid model calibration, they are absent for most catchments. We introduce the use of glacier volume change determined from repeated glacier mapping in a guided GLUE (generalized likelihood uncertainty estimation) procedure to calibrate a hydrologic model. This approach is applied to the Mica basin in the Canadian portion of the Columbia River Basin using the HBV-EC hydrologic model. Use of glacier volume change in the calibration procedure effectively reduced parameter uncertainty and helped to ensure that the model was accurately predicting glacier mass balance as well as streamflow. The seasonal and interannual variations in glacier melt contributions were assessed by running the calibrated model with historic glacier cover and also after converting all glacierized areas to alpine land cover in the model setup. Sensitivity of modelled streamflow to historic changes in glacier cover and to projected glacier changes for a climate warming scenario was assessed by comparing simulations using static glacier cover to simulations that accommodated dynamic changes in glacier area. Although glaciers in the Mica basin only cover 5% of the watershed, glacier ice melt contributes up to 25% and 35% of streamflow in August and September, respectively. The mean annual contribution of ice melt to total streamflow varied between 3 and 9% and averaged 6%. Glacier ice melt is particularly important during warm, dry summers following winters with low snow accumulation and early snowpack depletion. Although the sensitivity of streamflow to historic glacier area changes is small and within parameter uncertainties, our results suggest that glacier area changes have to be accounted for in future projections of late summer streamflow. Our approach provides an effective and widely applicable method to calibrate hydrologic models in glacier fed catchments, as well as to quantify the magnitude and timing of glacier melt contributions to streamflow.

No abstract available.