Moritz Langer

Subpixel heterogeneity of ice-wedge polygonal tundra: a multi-scale analysis of land cover and evapotranspiration in the Lena River Delta, Siberia

ABSTRACT Ignoring small-scale heterogeneities in Arctic land cover may bias estimates of water, heat and carbon fluxes in large-scale climate and ecosystem models. We investigated subpixel-scale heterogeneity in CHRIS/PROBA and Landsat-7 ETM+ satellite imagery over ice-wedge polygonal tundra in the Lena Delta of Siberia, and the associated implications for evapotranspiration (ET) estimation. Field measurements were combined with aerial and satellite data to link fine-scale (0.3 m resolution) with coarse-scale (up to 30 m resolution) land cover data. A large portion of the total wet tundra (80%) and water body area (30%) appeared in the form of patches less than 0.1 ha in size, which could not be resolved with satellite data. Wet tundra and small water bodies represented about half of the total ET in summer. Their contribution was reduced to 20% in fall, during which ET rates from dry tundra were highest instead. Inclusion of subpixel-scale water bodies increased the total water surface area of the Lena Delta from 13% to 20%. The actual land/water proportions within each composite satellite pixel was best captured with Landsat data using a statistical downscaling approach, which is recommended for reliable large-scale modelling of water, heat and carbon exchange from permafrost landscapes.

Small ponds with major impact: The relevance of ponds and lakes in permafrost landscapes to carbon dioxide emissions

Although ponds make up roughly half of the total area of surface water in permafrost landscapes, their relevance to carbon dioxide emissions on a landscape scale has, to date, remained largely unknown. We have therefore investigated the inflows and outflows of dissolved organic and inorganic carbon from lakes, ponds, and outlets on Samoylov Island, in the Lena Delta of northeastern Siberia in September 2008, together with their carbon dioxide emissions. Outgassing of carbon dioxide (CO2) from these ponds and lakes, which cover 25% of Samoylov Island, was found to account for between 74 and 81% of the calculated net landscape-scale CO2 emissions of 0.2–1.1 g C m�2 d�1 during September 2008, of which 28–43% was from ponds and 27–46% from lakes. The lateral export of dissolved carbon was negligible compared to the gaseous emissions due to the small volumes of runoff. The concentrations of dissolved inorganic carbon in the ponds were found to triple during freezeback, highlighting their importance for temporary carbon storage between the time of carbon production and its emission as CO2. If ponds are ignored the total summer emissions of CO2-C from water bodies of the islands within the entire Lena Delta (0.7–1.3 Tg) are underestimated by between 35 and 62%.

Modeling the impact of wintertime rain events on the thermal regime of permafrost

In this study, we present field measurements and numerical process modeling from western Svalbard showing that the ground surface temperature below the snow is impacted by strong wintertime rain events. During such events, rain water percolates to the bottom of the snow pack, where it freezes and releases latent heat. In the winter season 2005/2006, on the order of 20 to 50 % of the wintertime precipitation fell as rain, thus confining the surface temperature to close to 0 C for several weeks. The measured average ground surface temperature during the snow-covered period is−0.6C, despite of a snow surface temperature of on average−8.5C. For the considered period, the temperature threshold below which permafrost is sustainable on long timescales is exceeded. We present a simplified model of rain water infiltration in the snow coupled to a transient permafrost model. While small amounts of rain have only minor impact on the ground surface temperature, strong rain events have a long-lasting impact. We show that consecutively applying the conditions encountered in the winter season 2005/2006 results in the formation of an unfrozen zone in the soil after three to five years, depending on the prescribed soil properties. If water infiltration in the snow is disabled in the model, more time is required for the permafrost to reach a similar state of degradation.

Spatial and seasonal variability of polygonal tundra water balance: Lena River Delta, northern Siberia (Russia)

The summer water balance of a typical Siberian polygonal tundra catchment is investigated in order to identify the spatial and temporal dynamics of its main hydrological processes. The results show that, besides precipitation and evapotranspiration, lateral flow considerably influences the site-specific hydrological conditions. The prominent microtopography of the polygonal tundra strongly controls lateral flow and storage behaviour of the investigated catchment. Intact rims of low-centred polygons build hydrological barriers, which release storage water later in summer than polygons with degraded rims and troughs above degraded ice wedges. The barrier function of rims is strongly controlled by soil thaw, which opens new subsurface flow paths and increases subsurface hydrological connectivity. Therefore, soil thaw dynamics determine the magnitude and timing of subsurface outflow and the redistribution of storage within the catchment. Hydraulic conductivities in the elevated polygonal rims sharply decrease with the transition from organic to mineral layers. This interface causes a rapid shallow subsurface drainage of rainwater towards the depressed polygon centres and troughs. The re-release of storage water from the centres through deeper and less conductive layers helps maintain a high water table in the surface drainage network of troughs throughout the summer.RésuméLe bilan d’eau estival d’un bassin d’alimentation typique de la toundra polygonale sibérienne a fait l’objet d’investigations afin d’identifier la dynamique spatio-temporelle des ses principaux processus hydrologiques. Les résultats montrent que, à côté de la précipitation et de l’évapotranspiration, un flux latéral influence considérablement les conditions hydrogéologiques spécifiques du site. La microtopographie marquée de la toundra polygonale contrôle fortement l’écoulement latéral et la modalité d’emmagasinement du bassin objet de l’investigation. Les bords intacts des polygones centrés bas constituent des barrières hydrologiques, qui libèrent l’eau accumulée en été plus tard que les polygones à bords dégradés et dépressions au dessus des biseaux glacées dégradées. La fonction barrière des bordures est fortement contrôlée par le dégel du sol, qui ouvre de nouveaux chenaux d’écoulement en subsurface et en accroît la connectivité hydrologique. C’est pourquoi, la dynamique du dégel détermine l’instant et le volume d’émission d’eau en subsurface et la redistribution de la réserve dans le bassin versant. Les conductivités hydrauliques des bordures polygonales élevées diminuent brusquement avec la transition d’horizons organiques à des horizons minéraux. Ceci cause un drainage superficiel rapide de l’eau de précipitation vers les centres déprimés des polygones et goulottes. Le relargage de l’eau en réserve depuis les centres à travers des couches plus profondes et moins conductrices aide à maintenir élevée la surface libre de l’aquifère sous le réseau de drainage superficiel des goulottes durant tout l’été.ResumenSe investiga el balance de agua de verano en una típica cuenca siberiana de polígonos de tundra para identificar la dinámica espacial y temporal de sus principales procesos hidrológicos. Los resultados muestran que, además de la precipitación y evapotranspiración, el flujo lateral influye considerablemente las condiciones hidrológicas del sitio específico. La destacada microtopografía de los polígonos de tundra controla fuertemente el comportamiento del flujo lateral y almacenamiento de la cuenca investigada. Los bordes intactos de los polígonos bajos centrados constituyen barreras hidrológicas, que liberan el agua de almacenamiento del verano después que los polígonos con bordes degradados y canales por encima de cuñas degradadas del hielo. La función de barrera de los bordes está controlada fuertemente por el deshielo del suelo, que abre nuevas trayectorias al flujo subsuperficial y aumenta la conectividad hidrológica subsuperficial. Además, la dinámica de deshielo del suelo determina la magnitud y el tiempo de la salida subsuperficial y la redistribución del almacenamiento dentro de la cuenca. Las conductividades hidráulicas en los bordes elevados del polígono disminuyen drásticamente con la transición de capas orgánicas a minerales. Esto causa un rápido drenaje subsuperficial somero del agua de lluvia hacia el centro de los polígonos deprimidos y canales. La reliberación del agua del almacenamiento desde el centro a través de capas menos conductivas y más profundas ayuda a mantener un nivel freático alto en la red de drenaje superficial de canales durante todo el verano.摘要为确定它的主要水文过程的时空动态,本文研究了一个典型的西伯利亚多边形冻原流域在夏季的水均衡。结果显示,除了降水和蒸发蒸腾,侧向流在相当程度上影响着特定场地的水文条件。多边形冻原主要的微地形特征强烈控制着研究流域的侧向流和储存行为。在中心偏低的多边形的完整边缘上建造水文屏障,以此可利用退化冰楔之上退化的边缘和凹槽在夏天释放储存的水。边缘上屏障的作用受到土壤解冻强烈的控制,解冻作用打开了地下水流的流径,提高了地下的水文连通性。因此,土壤解冻的动态决定了流出的地下水的规模和时间,以及流域内所储存水量的重新分配。升高的多边形边缘从有机层过渡到矿物层,渗透系数急剧减小。这造成了浅部雨水向凹陷的多边形中心和洼地迅速排水。所储存的水通过更深部的低渗透岩层从中心重新释放,这有助于夏季在地表凹槽排水系统中保持一个较高的水位。ResumoÉ investigado o balanço hídrico estival de uma típica bacia hidrográfica de tundra poligonal para identificar as dinâmicas espaciais e temporais dos seus principais processos hidrológicos. Os resultados mostram que, para além da precipitação e da evapotranspiração, o escoamento lateral influencia consideravelmente as condições hidrológicas em cada local. A característica microtopografia da tundra poligonal controla fortemente o escoamento lateral e o comportamento do armazenamento na bacia investigada. Os bordos intactos dos polígonos com centro deprimido constituem barreiras hidrológicas que libertam a água armazenada mais tarde no verão do que nos polígonos com bordos degradados e fendas situadas acima das cunhas de gelo degradadas. A função de barreira dos bordos é fortemente controlada pelo descongelamento do solo que abre novos percursos de escoamento subterrâneo e incrementa a conectividade hidrológica subsuperficial. Consequentemente, a dinâmica do descongelamento do solo determina a magnitude e a temporização do escoamento e a redistribuição do armazenamento dentro da bacia. A condutividade hidráulica nos bordos poligonais elevados diminui drasticamente com a transição entre as camadas orgânicas a as camadas minerais. Isto provoca uma rápida drenagem subsuperficial pouco profunda da água de chuva em direção aos centros deprimidos e às fendas. A re-libertação da água armazenada a partir dos centros através de camadas mais profundas e menos condutivas ajudam a manter ao longo do verão um nível de água elevado na rede de drenagem superficial formada pelas fendas.

Latent heat exchange in the boreal and arctic biomes

In this study latent heat flux (λE) measurements made at 65 boreal and arctic eddy-covariance (EC) sites were analyses by using the Penman-Monteith equation. Sites were stratified into nine different ecosystem types: harvested and burnt forest areas, pine forests, spruce or fir forests, Douglas-fir forests, broadleaf deciduous forests, larch forests, wetlands, tundra and natural grasslands. The Penman-Monteith equation was calibrated with variable surface resistances against half-hourly eddy-covariance data and clear differences between ecosystem types were observed. Based on the modeled behavior of surface and aerodynamic resistances, surface resistance tightly control λE in most mature forests, while it had less importance in ecosystems having shorter vegetation like young or recently harvested forests, grasslands, wetlands and tundra. The parameters of the Penman-Monteith equation were clearly different for winter and summer conditions, indicating that phenological effects on surface resistance are important. We also compared the simulated λE of different ecosystem types under meteorological conditions at one site. Values of λE varied between 15% and 38% of the net radiation in the simulations with mean ecosystem parameters. In general, the simulations suggest that λE is higher from forested ecosystems than from grasslands, wetlands or tundra-type ecosystems. Forests showed usually a tighter stomatal control of λE as indicated by a pronounced sensitivity of surface resistance to atmospheric vapor pressure deficit. Nevertheless, the surface resistance of forests was lower than for open vegetation types including wetlands. Tundra and wetlands had higher surface resistances, which were less sensitive to vapor pressure deficits. The results indicate that the variation in surface resistance within and between different vegetation types might play a significant role in energy exchange between terrestrial ecosystems and atmosphere. These results suggest the need to take into account vegetation type and phenology in energy exchange modeling.

Permafrost – Physical Aspects, Carbon Cycling, Databases and Uncertainties

Permafrost is defined as ground that remains below 0°C for at least 2 consecutive years. About 24% of the northern hemisphere land area is underlain by permafrost. The thawing of permafrost has the potential to influence the climate system through the release of carbon (C) from northern high latitude terrestrial ecosystems, but there is substantial uncertainty about the sensitivity of the C cycle to thawing permafrost. Soil C can be mobilized from permafrost in response to changes in air temperature, directional changes in water balance, fire, thermokarst, and flooding. Observation networks need to be implemented to understand responses of permafrost and C at a range of temporal and spatial scales. The understanding gained from these observation networks needs to be integrated into modeling frameworks capable of representing how the responses of permafrost C will influence the trajectory of climate in the future.

Rapid degradation of permafrost underneath waterbodies in tundra landscapes—Toward a representation of thermokarst in land surface models

Waterbodies such as lakes and ponds are abundant in vast Arctic landscapes and strongly affect the thermal state of the surrounding permafrost. In order to gain a better understanding of the impact of small- and medium-sized waterbodies on permafrost and the formation of thermokarst, a land surface model was developed that can represent the vertical and lateral thermal interactions between waterbodies and permafrost. The model was validated using temperature measurements from two typical waterbodies located within the Lena River delta in northern Siberia. Impact simulations were performed under current climate conditions as well as under a moderate and a strong climate-warming scenario. The performed simulations demonstrate that small waterbodies can rise the sediment surface temperature by more than 10°C and accelerate permafrost thaw by a factor of between 4 and 5. Up to 70% of this additional heat flux into the ground was found to be dissipated into the surrounding permafrost by lateral ground heat flux in the case of small, shallow, and isolated waterbodies. Under moderate climate warming, the lateral heat flux was found to reduce permafrost degradation underneath waterbodies by a factor of 2. Under stronger climatic warming, however, the lateral heat flux was too small to prevent rapid permafrost degradation. The lateral heat flux was also found to strongly impede the formation of thermokarst. Despite this stabilizing effect, our simulations have demonstrated that underneath shallow waterbodies (<1 m), thermokarst initiation happens 30 to 40 years earlier than in simulations without preexisting waterbody.

Thaw Subsidence of a Yedoma Landscape in Northern Siberia, Measured In Situ and Estimated from TerraSAR-X Interferometry

In permafrost areas, seasonal freeze-thaw cycles result in upward and downward movements of the ground. For some permafrost areas, long-term downward movements were reported during the last decade. We measured seasonal and multi-year ground movements in a yedoma region of the Lena River Delta, Siberia, in 2013–2017, using reference rods installed deep in the permafrost. The seasonal subsidence was 1.7 ± 1.5 cm in the cold summer of 2013 and 4.8 ± 2 cm in the warm summer of 2014. Furthermore, we measured a pronounced multi-year net subsidence of 9.3 ± 5.7 cm from spring 2013 to the end of summer 2017. Importantly, we observed a high spatial variability of subsidence of up to 6 cm across a sub-meter horizontal scale. In summer 2013, we accompanied our field measurements with Differential Synthetic Aperture Radar Interferometry (DInSAR) on repeat-pass TerraSAR-X (TSX) data from the summer of 2013 to detect summer thaw subsidence over the same study area. Interferometry was strongly affected by a fast phase coherence loss, atmospheric artifacts, and possibly the choice of reference point. A cumulative ground movement map, built from a continuous interferogram stack, did not reveal a subsidence on the upland but showed a distinct subsidence of up to 2 cm in most of the thermokarst basins. There, the spatial pattern of DInSAR-measured subsidence corresponded well with relative surface wetness identified with the near infra-red band of a high-resolution optical image. Our study suggests that (i) although X-band SAR has serious limitations for ground movement monitoring in permafrost landscapes, it can provide valuable information for specific environments like thermokarst basins, and (ii) due to the high sub-pixel spatial variability of ground movements, a validation scheme needs to be developed and implemented for future DInSAR studies in permafrost environments.

Lake‐Atmosphere Heat Flux Dynamics of a Thermokarst Lake in Arctic Siberia

We conducted eddy covariance measurements from April to August 2014 on a Siberian thermokarst lake. The study site is located in the Lena River Delta and characterized as a floating ice lake. Heat fluxes differed in magnitudes, directions and temporal patterns depending on the lake surface conditions (“frozen” ice cover, ice cover melt, and open water). Significant heat release during frozen ice cover conditions highlighted the importance of lakes for the landscape heat budget and water balance. The energy balance was nearly closed during the open water period and highlighted the impact of melting energy on its closure during the ice cover period. Sensible and latent heat dynamics were driven by temperature and water vapor gradients scaled by wind speed, respectively. We calculated bulk aerodynamics transfer coefficients and evaluated the performance of the derived in situ and three independent heat flux parameterization schemes. We found that bulk transfer models perform moderately to poorly for the different lake surface conditions. During the open water period small‐scale temporal variability could not be represented by the models, particularly in case of latent heat flux. The model results were less sensitive to the specific model type than to the accuracy of the surface water temperature measurement, which is dependent on a well‐thought‐out measurement design. Our study stresses considerations that are crucial for similar campaigns in the future, in order to face the measurement challenges encountered on arctic lakes especially during the ice cover period.

A long-term (2002 to 2017) record of closed-path and open-patheddy covariance CO 2 net ecosystem exchange fluxes from theSiberian Arctic

Ground-based observations of land–atmosphere fluxes are necessary to progressively improve global climate models. Observed data can be used for model evaluation and to develop or tune process models. In arctic permafrost regions, climate–carbon feedbacks are amplified. Therefore, increased efforts to better represent these regions in global climate models have been made in recent years. We present a multi-annual time series of land–atmosphere carbon dioxide fluxes measured in situ with the eddy covariance technique in the Siberian Arctic (7222 N, 12630 E). The site is part of the international network of eddy covariance flux observation stations (FLUXNET; site ID: Ru-Sam). The data set includes consistently processed fluxes based on concentration measurements of closed-path and open-path gas analyzers. With parallel records from both sensor types, we were able to apply a site-specific correction to open-path fluxes. This correction is necessary due to a deterioration of data, caused by heat generated by the electronics of open-path gas analyzers. Parameterizing this correction for subperiods of distinct sensor setups yielded good agreement between openand closed-path fluxes. We compiled a long-term (2002 to 2017) carbon dioxide flux time series that we additionally gap-filled with a standardized approach. The data set was uploaded to the Pangaea database and can be accessed through https://doi.org/10.1594/PANGAEA.892751. Published by Copernicus Publications. 222 D. Holl et al.: Long-term eddy covariance CO2 fluxes from the Siberian Arctic