Topographic Shading Research Articles

A novel approach suggestion for assessing the impact of topographic shading on the estimation of the floating photovoltaic technical potential

This study presents a novel approach for the implementation of floating photovoltaic (FPV) systems at the Ayvalı hydroelectric power plant (HPP) in Türkiye. The method proposed in this study accounts for dynamic changes in water levels to accurately calculate the shading effects induced by topography. First, the minimum reservoir surface for the FPV system was calculated using remote sensing (RS). The minimum reservoir surface area, which was determined as 504.69 ha using 60 Sentinel-2 satellite images, was calculated using machine learning algorithms on the Google Earth Engine (GEE) platform, support vector machines (SVM) and automatic water extraction index (AWEI). In the second stage, new digital elevation model (DEM) maps were produced by overlapping monthly changes in water height with ALOS PALSAR data and solar analysis was performed on them. An annual global horizontal irradiance (GHI) map was produced using these maps, and it was divided into five classes to emphasize differences in production potential. The results revealed that 1083.45 GWh of electricity can be produced annually by installing FPV in very high and high potential areas. However, as the moderate, low, and very low regions represent only 5.02% of the reservoir surface and there is a 1.68-fold difference in production potential between the highest and lowest areas due to topography-induced shading near the coastline, it was concluded that FPV installation would not be efficient in those regions. This study highlights the significance of incorporating topography-induced shading and emphasizes the importance of employing RS and geographic information system (GIS) techniques to achieve this objective.

Determination of the appropriate zone on dam surface for floating photovoltaic system installation using RS and GISc technologies

This study aims to reveal suitable places where floating photovoltaic-solar power plants (FPV-SPPs) can be installed on the dam surface using the possibilities of remote sensing (RS) and geographical information science (GISc) technologies. Past satellite images from Landsat and Sentinel platforms allow researchers to analyse shoreline changes in the dam surface. Shoreline extraction is a crucial process for the FPV-SPP to stay afloat despite external constraints. In this study, changes in dam water levels were determined by classifying 20-year satellite images and analysing a 32-year global surface water dynamics dataset. The water surface area was calculated as 1,562.40 ha using the random forest (RF) algorithm and the normalized differences water index (NDWI) on Google Earth Engine (GEE) cloud platform. In addition, solar analysis was carried out with GISc using annual solar radiation maps shuttle radar topography mission (SRTM) data, which directly affects the energy production of FPV-SPPs. It has been calculated that the solar radiation on the water surface varies between 1,554 kWh/m2-year and 1,875 kWh/m2-year. These calculated values were divided into five different classes, and it was observed that 88.5% of the dam surface had a very high level of solar radiation compared to other areas. Higher efficiency will be obtained from the FPV-SPP to be installed in this region compared to the systems to be installed in other regions. It has been observed that the radiation values in other parts of the water surface are lower due to topographic shading. These analyses revealed energy zones with high production potential, thereby easing the decision-making process for investors planning to establish FPV-SPPs.

Continuous Karakoram Glacier Anomaly and Its Response to Climate Change during 2000–2021

Glacier mass balance is one of the most direct indicators reflecting corresponding climate change. In the context of global warming, most glaciers are melting and receding, which can have significant impacts on ecology, climate, and water resources. Thus, it is important to study glacier mass change, in order to assess and project its variations from past to future. Here, the Karakoram, one of the most concentrated glacierized areas in High-Mountain Asia (HMA), was selected as the study area. This study utilized SRTM-C DEM and ICESat-2 to investigate glacier mass change in the Karakoram, and its response to climatic and topographical factors during 2000–2021. The results of the data investigation showed that, overall, the “Karakoram Anomaly” still exists, with an annual averaged mass change rate of 0.02 ± 0.09 m w.e.yr-1. In different sub-regions, it was found that the western and central Karakoram glaciers gained ice mass, while the eastern Karakoram glaciers lost ice mass in the past two decades. In addition, it was discovered that the increasing precipitation trend is leading to mass gains in the western and central Karakoram glaciers, whereas increasing temperature is causing ice mass loss in the eastern Karakoram glacier. Generally, decreasing net shortwave radiation and increasing cloud cover in the Karakoram restricts ice mass loss, while topographical shading and debris cover also have dominant impacts on glacier mass change.

Influence of Topographic Shading on the Mass Balance of the High Mountain Asia Glaciers

Most studies attribute the glacier mass balance within High Mountain Asia (HMA) to climate change, ignoring the influence of its complex terrain. Knowledge of the influence of this complex terrain is crucial for understanding the spatial variability in its mass balance. However, there is a lack of any systematic assessment of this influence across HMA. Therefore, in this study, we used the glacier outlines and raster data (SRTM DEM, slope and aspect) to calculate the topographic shading of all 97,965 glaciers within HMA during the ablation period, which is regarded as a major index of the influence of complex terrain on the mass balance. The results showed that 27.19% of HMA glacier area was subjected to topographic shading, and regional differences were significant with respect to both their altitudinal and spatial distributions. The topographic shading contributed to the protection of the smallest glaciers from solar illumination. Furthermore, we found a significant correlation between the topographic shading and mass balance in these small north-facing glaciers. However, these small glaciers were most prevalent in the north-facing orientation, especially in West Kunlun, East Kunlun, Inner Tibet Plateau and Qilian Shan, where shading was found to increase with decreases in the glacier area. This indicates that complex terrain can affect the spatial distribution of the mass balance by altering the solar illumination pattern.

Modeling the feedbacks between surface ablation and morphological variations on debris-covered Baltoro Glacier in the central Karakoram

Recent studies show that many debris-covered glaciers exhibit high-magnitude differential thinning despite the presence of supraglacial debris. Existing studies have not adequately explored the variability in surface ablation, morphology, and related feedback mechanisms that incorporate topographically controlled surface irradiance and debris transport. In this study, we address these issues using a radiation-driven surface ablation model that more fully characterizes ablation dynamics by accounting for temporally-linked radiative forcing, surface geomorphological evolution and gravitational debris flux. Simulation results based on Baltoro Glacier in the central Karakoram indicate the following: 1) A debris-covered glacier can exhibit high spatial variability in surface ablation due to heterogeneous debris thickness and debris transport. A bare-ice glacier given similar initial conditions, has higher overall ablation but exhibits much less variability. 2) The topographic influence on surface ablation is non-negligible because glacier-surface topography controls irradiance and gravitational debris flux. The overall ablation on a debris-covered glacier tends to increase in response to high-frequency topographic variations due to the larger area with thin debris cover. In contrast, a bare-ice glacier exhibits decreased overall ablation in response to high-frequency topographic variations due to its high sensitivity to topographic shading. 3) Gravity-driven surface debris flux plays an important role in local debris thickness redistribution, which regulates ablation rates over the ablation season. 4) Surface ablation dynamics on a debris-covered glacier is regulated by active system couplings and feedbacks between surface morphology, melt and debris transport. Consequently, certain locations on a debris-covered glacier may be more sensitive to radiative forcing than previously thought. Simulation results suggest that nonlinear feedback responses may permit debris-covered glacier subsystems to be very sensitive to radiative forcing, thereby causing significant morphological changes to the glacier surface.

Topographic correction of HiRISE and CaSSIS images: Validation and application to color observations of Martian albedo features

The topographic correction of satellite images has to be applied to both disentangle albedo features from illumination effects induced by local topography and perform spectrophotometric analyses of planetary surfaces. This work focuses on the modeling and removal of surface brightness variations induced by topography, referred as topographic shading, from high resolution images of Mars. Topographic shading can be modeled through functions of the surface illumination and observation angles called disk functions. We consider four disk functions that are widely used in planetary photometry: the Lambert, Lommel-Seeliger, Akimov and Minnaert disk functions. We test and evaluate their performances in removing topographic shading from High Resolution Imaging Science Experiment (HiRISE) and Colour and Surface Science Imaging System (CaSSIS) images. We here validate our method, moreover, we report scientific applications to single or multi-band datasets by analyzing topographically corrected HiRISE colour observations of Martian recurring slope lineae and dust devil tracks, as well as CaSSIS panchromatic observations.

Water Temperature Controls for Regulated Canyon‐Bound Rivers

AbstractMany canyon‐bound rivers have been dammed and downstream flow and water temperatures modified. In some regions, climate change is expected to cause lower storage in reservoirs and warmer release temperatures, which may further alter downstream flow and thermal regimes. To anticipate potential future changes, we first need to understand the dominant heat transfer mechanisms in canyon‐bound river systems. Toward this end, we adapt a dynamic process‐based river routing and temperature model to account for complex shading and radiation characteristics found in canyon‐bound rivers. We apply the model to a 362 km segment of the Colorado River in Grand Canyon National Park, USA to simulate temperature over an 18‐year period. Extensive temperature and flow data sets from within the canyon were used to assess model performance. At the most downstream gaging location, root mean square errors of hourly flow routing and temperature predictions were 11.5 m3/s and 0.93°C, respectively. We found that heat fluxes controlling temperatures were highly variable over space and time, primarily due to shortwave radiation dynamics and hydropeaking flow conditions. Additionally, the large differences between air and water temperature during summer periods resulted in high sensible and latent heat fluxes. Sensitivity analyses indicate that reservoir release temperatures are most influential above the RM88 gage (141 km below Glen Canyon Dam), while a combination of discharge, shortwave radiation, and air temperature become more important farther downstream. This study illustrates the importance of understanding the spatial and temporal variability of topographic shading when predicting water temperatures in canyon‐bound rivers.

Morphological changes to the terminus of a maritime glacier during advance and retreat phases: Fox Glacier/Te Moeka o Tuawe, New Zealand

ABSTRACT Fox Glacier/Te Moeka o Tuawe is a fast-responding maritime glacier that has undergone multiple advance and retreat phases during recent decades. Here we use a combination of repeat photography, Structure from Motion (SfM), and ice discharge measurement, to identify key morphological differences associated with these repeated phase changes, and assess how much of the current terminus is still dynamically active. Increasing surface-debris cover at the margins and topographic shading result in the asymmetry of the retreating terminus, with central portions receding faster than the margins. In 2019, the glacier is shorter than at any time in recorded history, and ice flux is insufficient to sustain the current glacier length, with a further ∼300 m of the glacier terminus region potentially vulnerable to retreat. However, due to the high climate sensitivity of this maritime glacier, even a slight shift towards increasing mass flux could see this trend reverse.

Surface Temperature Simulation of Lunar Dayside and Its Geological Applications: A Case in Sinus Iridum.

Lunar surface temperature is one of the fundamental thermophysical parameters of the lunar regolith, which is of great significance to the interpretation of remote-sensing thermal data. In this study, a daytime surface temperature model is established focusing on the lunar superficial layer with high spatial-temporal resolution. The physical parameters at the time of interest are adopted, including effective solar irradiance, lunar libration, large-scale topographic shading, and surrounding diffuse reflection. Thereafter, the 1/64° temperature distributions at five local times are quantitatively generated and analyzed in Sinus Iridum. Also, combined with Chang’E-2 microwave radiometer (CELMS) data and Diviner thermal infrared (TIR) data, the spectral emissivity distributions are estimated as a potential geological application of the simulated surface temperature. The results are as follows: (1) daytime surface temperature in Sinus Iridum is significantly affected by the local topography and observation time, and the influence of diffuse reflection energy is obvious; (2) the emissivity distributions provide a new way to understand the thermophysical properties difference of lunar regolith at different depths; (3) the influence of lunar orbiting revolution and precession on surface temperature should be analyzed carefully, which shows the importance of using the parameters at the time of interest.

Reanalysis of the US Geological Survey Benchmark Glaciers: long-term insight into climate forcing of glacier mass balance

AbstractMountain glaciers integrate climate processes to provide an unmatched signal of regional climate forcing. However, extracting the climate signal via intercomparison of regional glacier mass-balance records can be problematic when methods for extrapolating and calibrating direct glaciological measurements are mixed or inconsistent. To address this problem, we reanalyzed and compared long-term mass-balance records from the US Geological Survey Benchmark Glaciers. These five glaciers span maritime and continental climate regimes of the western United States and Alaska. Each glacier exhibits cumulative mass loss since the mid-20th century, with average rates ranging from −0.58 to −0.30 m w.e. a−1. We produced a set of solutions using different extrapolation and calibration methods to inform uncertainty estimates, which range from 0.22 to 0.44 m w.e. a−1. Mass losses are primarily driven by increasing summer warming. Continentality exerts a stronger control on mass loss than latitude. Similar to elevation, topographic shading, snow redistribution and glacier surface features often exert important mass-balance controls. The reanalysis underscores the value of geodetic calibration to resolve mass-balance magnitude, as well as the irreplaceable value of direct measurements in contributing to the process-based understanding of glacier mass balance.

Terrain Induced Biases in Clear-Sky Shortwave Radiation Due to Digital Elevation Model Resolution for Glaciers in Complex Terrain

Advancements in remote sensing, along with greater access to high spatial and temporal resolution imagery, have improved our ability to model glacier surface energy and mass balance in remote regions of complex terrain, such as High-mountain Asia (HMA). In general, net shortwave (SW) radiation accounts for the majority of energy available on a glacier surface during the summer months, suggesting that SW modeling errors can critically impact surface energy balance estimates. In this study, we model the clear-sky SW irradiance for a group of glaciers in the Everest region of HMA using a high-resolution (8-meter) digital elevation model (DEM) composite derived from commercial stereo satellite imagery. We then systematically downsample this DEM and considered the effect on incoming SW irradiance, with a sensitivity analysis for standard terrain attributes. The slope and aspect (combined) and topographic shading have the greatest impact on daily SW irradiance and also introduce a larger SW bias when DEM resolution is downsampled. Our results show that modeled incident SW is overestimated as resolution becomes coarser. For 10 selected glaciers in the Everest region, decreasing spatial resolution from 8 to 30 meters results in a range of average daily biases between +20 to +60 Wm-2 (or ~7 to 20%) at some high and low elevations, and an average bias of more than +100 Wm-2 (~33%) as resolution is coarsened to 500 meters. In order to determine the bearing these results have on surface melt, we explore the diurnal variability of this bias. Additionally, we compare our results with modeled incident SW using several global DEM products (ASTER, SRTM, and ALOS) to evaluate error introduced by lower resolution. Models using the 30-meter products show an overall average daily SW bias of +24 Wm-2 (or ~8%) across elevation with some elevations showing a bias up to +60 Wm-2 (~20%) on multiple glaciers. Taken together, our results demonstrate the value of high-resolution data to correct biases in modeled SW radiation and constrain uncertainties for glacier energy balance modeling in regions of complex terrain.

Impacts of topographic shading on direct solar radiation for valley glaciers in complex topography

Abstract. Topographic shading, including both shaded relief and cast shadowing, plays a fundamental role in determining direct solar radiation on glacier ice. However, shading has been oversimplified or incorrectly incorporated in surface energy balance models in some past studies. In addition, no systematic studies have been conducted to evaluate relationships between shading and other topographic characteristics. Here we develop a topographic solar radiation model to examine the variability in irradiance throughout the glacier melt season due to topographic shading and combined slope and aspect. We apply the model to multiple glaciers in high-mountain Asia (HMA) and test the sensitivity of shading to valley aspect and latitude. Our results show that topographic shading significantly alters the potential direct clear-sky solar radiation received at the surface for valley glaciers in HMA, particularly for north- and south-facing glaciers. Additionally, we find that shading can be extremely impactful in the ablation zone. Cast shadowing is the dominant mechanism in determining total shading for valley glaciers in parts of HMA, especially at lower elevations. Although shading can be predictable, it is overall extremely variable between glacial valleys. Our results suggest that topographic shading not only is an important factor contributing to surface energy balance but could also influence glacier response and mass balance estimates throughout HMA.

Effects of afforestation on soil and ambient air temperature in a pair of catchments on the Chinese Loess Plateau

Climate models and satellite remote sensing have been used to determine the effects of afforestation on soil temperature at regional and global scales. However, the coarse spatial resolution of both methods have made them insensitive to local topography, which is a controlling factor of the surface energy budget, especially in areas with complex topography. While typically applied only locally at representative sites, long-term field measurements can shed light on the role of topography on soil temperature after afforestation and provide direct evidence to verify and calibrate the results from modeling and satellite observation. In this study, a pair of neighboring catchments (one afforested and the other with natural regrowth of grasses) on the Chinese Loess Plateau (CLP) was selected to assess the effects of afforestation on soil and ambient air temperature at different slope positions with an entire year of continuous measurements collected every 10 min. The results showed that the uphill slope in both catchments experienced higher soil and air temperatures than the downhill gully, where less solar radiation was received at the ground surface due to canopy interception and topographic shading. For example, the annual average soil (10–100 cm depth) and air temperature in the uphill slope in the forestland catchment was 1.09 and 1.22 °C higher than in the downhill gully, respectively. The effects of topography on soil and air temperature varied at different times of the year, and these effects interacted with the growth status of vegetation. In winter and spring, topography significantly affected soil and air temperature due to varied solar radiation received at different slope positions. By contrast, in summer, the effect of vegetation increased along with higher evapotranspiration and more solar radiation interception by the canopy, especially in the forestland catchment, which showed cooler soils than the other catchment during both the days and nights at all investigated slope positions. However, compared with the grassland catchment, the uphill slope of the forestland catchment had cooler air temperatures during the day, but warmer temperatures at night. We have concluded that both topography and afforestation influence the spatial variation and temporal dynamics of soil and air temperature, and they thereby the surface energy balance of the CLP. More local studies are warranted in order to continue to calibrate regional models and remote sensing data.

Penumbra: A spatially distributed, mechanistic model for simulating ground-level incident solar energy across heterogeneous landscapes.

Landscape solar energy is a significant environmental driver, yet it remains complicated to model well. Several solar radiation models simplify the complexity of light by estimating it at discrete point locations or by averaging values over larger areas. These modeling approaches may be useful in certain cases, but they are unable to provide spatially distributed and temporally dynamic representations of solar energy across entire landscapes. We created a landscape-scale ground-level shade and solar energy model called Penumbra to address this deficiency. Penumbra simulates spatially distributed ground-level shade and incident solar energy at user-defined timescales by modeling local and distant topographic shading and vegetative shading. Spatially resolved inputs of a digital elevation model, a normalized digital surface model, and landscape object transmittance are used to estimate spatial variations in solar energy at user-defined temporal timesteps. The research goals for Penumbra included: 1) simulations of spatiotemporal variations of shade and solar energy caused by both objects and topographic features, 2) minimal user burden and parameterization, 3) flexible user defined temporal parameters, and 4) flexible external model coupling. We test Penumbra’s predictive skill by comparing the model’s predictions with monitored open and forested sites, and achieve calibrated mean errors ranging from -17.3 to 148.1 μmoles/m2/s. Penumbra is a dynamic model that can produce spatial and temporal representations of shade percentage and ground-level solar energy. Outputs from Penumbra can be used with other ecological models to better understand the health and resilience of aquatic, near stream terrestrial, and upland ecosystems.

Sensitivity of potential evapotranspiration to meteorological factors and their elevational gradients in the Qilian Mountains, northwestern China

Mountainous regions are important sources of freshwater. Measurement of actual evapotranspiration (ET) is difficult to obtain in high mountain regions because of the harsh natural environment, and potential ET (PET) is therefore a suitable term to describe the atmospheric water demand of land surfaces under given meteorological conditions in those high elevation areas. In situ measured meteorological data were collected in 2015 and 2016 from five meteorological stations at various elevations from 2980 m to 4484 m in the Qilian Mountains, northwestern China, and the meteorological factors changed markedly with elevation. PET calculated with the Penman method showed a significant elevational gradient, and decreased as the elevation increased. The sensitivity analysis indicated that over the whole period, PET in the research region was most sensitive to net radiation (RN), followed by relative humidity (RH), air temperature (T), wind speed (WS) and soil heat flux (G). When RN was positive, the sensitivity of PET to RN decreased as the elevation increased, and when RN was negative, the sensitivity increased as the elevation increased. When T was above 0 °C, the sensitivity of PET to T decreased as the elevation increased, and when T was below 0 °C, the sensitivity increased as the elevation increased. The higher the elevation, the greater the sensitivity of PET to both RH and WS. The topographic shading in mountainous regions affected meteorological factors, PET and its sensitivity to meteorological factors in those high elevation areas. The RN was relatively small at the sites with topographic shading because of the reduction in solar radiation, and resulted in less sensitivity of PET to RN and greater sensitivity of PET to other meteorological factors. This study can help us to understand PET in the Qilian mountains and in other mountain regions from which meteorological data are difficult to obtain and very sensitive to climate change.

Topographic Effects on Radiation in the WRF Model with the Immersed Boundary Method: Implementation, Validation, and Application to Complex Terrain

Abstract Topographic effects on radiation, including both topographic shading and slope effects, are included in the Weather Research and Forecasting (WRF) Model, and here they are made compatible with the immersed boundary method (IBM). IBM is an alternative method for representing complex terrain that reduces numerical errors over sloped terrain, thus extending the range of slopes that can be represented in WRF simulations. The implementation of topographic effects on radiation is validated by comparing land surface fluxes, as well as temperature and velocity fields, between idealized WRF simulations both with and without IBM. Following validation, the topographic shading implementation is tested in a semirealistic simulation of flow over Granite Mountain, Utah, where topographic shading is known to affect downslope flow development in the evening. The horizontal grid spacing is 50 m and the vertical grid spacing is approximately 8–27 m near the surface. Such a case would fail to run in WRF with its native terrain-following coordinates because of large local slope values reaching up to 55°. Good agreement is found between modeled surface energy budget components and observations from the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) program at a location on the east slope of Granite Mountain. In addition, the model captures large spatiotemporal inhomogeneities in the surface sensible heat flux that are important for the development of thermally driven flows over complex terrain.

Why Do Large‐Scale Land Surface Models Produce a Low Ratio of Transpiration to Evapotranspiration?

AbstractMost land surface models (LSMs) used in Earth System Models produce a lower ratio of transpiration (T) to evapotranspiration (ET) than field observations, degrading the credibility of Earth System Model‐projected ecosystem responses and feedbacks to climate change. To interpret this model deficiency, we conducted a pair of model experiments using a three‐dimensional, process‐based ecohydrological model in a subhumid, mountainous catchment. One experiment (CTRL) describes lateral water flow, topographic shading, leaf dynamics, and water vapor diffusion in the soil, while the other (LSM like) does not explicitly describe these processes to mimic a conventional LSM using artificially flattened terrain. Averaged over the catchment, CTRL produced a higher T/ET ratio (72%) than LSM like (55%) and agreed better with an independent estimate (79.79 ± 27%) based on rainfall and stream water isotopes. To discern the exact causes, we conducted additional model experiments, each reverting only one process described in CTRL to that of LSM like. These experiments revealed that the enhanced T/ET ratio was mostly caused by lateral water flow and water vapor diffusion within the soil. In particular, terrain‐driven lateral water flows spread out soil moisture to a wider range along hillslopes with an optimum subrange from the middle to upper slopes, where evaporation (E) was more suppressed by the drier surface than T due to plant uptake of deep soil water, thereby enhancing T/ET. A more elaborate representation of water vapor diffusion from a dynamically changing evaporating surface to the height of the surface roughness length reduced E and increased the T/ET ratio.

Influences of Topographic Shadows on the Thermal and Hydrological Processes in a Cold Region Mountainous Watershed in Northwest China

AbstractThe solar radiation incident in a mountainous area with a complex terrain has a strong spatial heterogeneity due to the variations in slope orientation (self‐shading) and shadows cast by surrounding topography agents (topographic shading). Although slope self‐shading has been well studied and considered in most land surface and hydrological models, topographic shading is usually ignored, and its influence on the thermal and hydrological processes in a cold mountainous area remains unclear. In this study, a topographic solar radiation algorithm with consideration for both slope self‐shading and topographic shadows has been implemented and incorporated into a distributed hydrological model with physically based descriptions for the energy balance. A promising model performance was achieved according to a vigorous evaluation. In a control model without considering the topographic shadows, the simulated solar radiation incident in the study area was about 14.3 W/m2 higher on average, which in turn led to a higher simulated annual mean ground temperature at 4 m (by 0.41 °C) and evapotranspiration (by 16.1 mm/a), and a smaller permafrost extent (reduced by about 8%), as well as smaller maximal snow depth and shorter snow duration. Although the simulation was not significantly improved for discharge hydrograph in the base model, higher river runoff peaks and an increased runoff depth were obtained. In areas with a rugged terrain and deep valleys, the influences of topographic shadows would even be stronger in reality than the presented results, which cannot be ignored in the simulation of the thermal and hydrological processes, especially in a refined model.

Red Squirrel (Tamiasciurus hudsonicus) Midden Site Selection and Conifer Species Composition

Reductions in whitebark pine (Pinus albicaulis) due to blister rust (Cronartium ribicola) and mountain pine beetle (Dendroctonus ponderosae) have prompted some forest managers to consider selective thinning and prescribed fire to reduce competition of whitebark pine with other conifer species. Whitebark pine is an important food source for grizzly bears (Ursus arctos horribilis) in the Greater Yellowstone Ecosystem, but most of the seeds are obtained by raiding red squirrel (Tamiasciurus hudsonicus) middens. Therefore, it is important to understand which attributes maximize red squirrel midden site selection. The objectives of this study were to estimate active midden site selection criteria and quantify the ideal conifer composition associated with red squirrel middens in the Cooke City Basin, MT. Active midden counts were collected in 810, 30-meter diameter circular plots, equally spaced along 27 transect lines in the basin. We used generalized linear mixed models to assess variables associated with middens. Midden occurrence probability was positively associated in a curvilinear relationship with conifer canopy cover and the amount of topographic shading. Midden occurrence increased as the percent of whitebark pine increased up to 44 percent. A conifer composition of 44 percent whitebark and 56 percent mixture of subalpine fir (Abies lasiocarpa) and Engelmann spruce (Picea engelmannii) was ideal for midden sites. Whitebark pine treatments to reduce fir and spruce competition should acknowledge the tradeoff to foraging grizzly bears.

Topographic shading influences cryoconite morphodynamics and carbon exchange

ABSTRACT Cryoconite holes are the most active and diverse microbial habitats on glacier and ice-sheet surfaces. In this article the authors demonstrate that the shape of cryoconite holes varies depending on ice-surface topography and that this has implications for the carbon cycling regime within. Net ecosystem production is shown to be controlled primarily by sediment thickness within holes. The authors show that irregular hole shapes are indicative of hole migration away from topographic shade, which promotes carbon fixation at the mesoscale on ice surfaces. A cellular automaton is used in conjunction with sediment-delivery experiments to show that migration is the result of simple sediment transfer processes, implying a relationship between ice-surface evolution and cryoconite biogeochemistry that has not previously been examined.