Description: The Lena River Delta in Northern Yakutia forms one of the largest deltas in the Arctic and its catchment area (2 430 000 km2) is one of the largest in the whole of Eurasia. The Lena River distributes water and sediment in four main channels before discharging in total about 30 km3 of water through the delta into the Arctic Ocean every year and its discharge has been observed to be increasing. The goal of this presentation is to characterize the hydrologic processes that are strongly affected by a transient climate component- the permafrost. Permafrost plays a major role for storage and release of water to rivers and surface and subsurface water bodies. Conversely, the coupled water and heat fluxes in the atmosphere and below ground have a marked influence on the permafrost’s thermal regime. Our study site, the Lena River Delta, is also one of the coldest and driest places on Earth, with mean annual air temperatures of about -13 °C, a large annual air temperature range of about 44 °C and summer precipitation usually less than 150 mm. Very cold continuous permafrost of about −8.6 °C (11 m depth) underlays the area between about 400 and 600 m below surface and since 2006 the permafrost has warmed than 1 °C at 10.7 m. Roughly half of the land surface is dominated by wet surfaces, such as polygons, ponds and thermokarst lakes. This contribution summarizes past and ongoing research on hydrologic processes across spatial scales, from microtopographic processes of polygonal tundra to regional scale deltaic processes to assess short and long term changes in water fluxes. We quantify unfrozen water in soils, streams and river discharges and water bodies’ storage at larger scales. Water bodies were mapped using optical and radar satellite data with resolutions of 4 m or better, Landsat-5 TM at 30 m and the MODIS water mask at 250 m resolution. Ponds, i. e. water bodies with surface are smaller than 104 m, make over 95 % of the total number of water bodies and are not resolved in Landsat-scale land cover classifications. Ponds are generally well mixed and experience high water temperatures up to 23 °C during the summer and are, therefore, hotspots for biological activity and CO2 emission. The ponds in the study area freeze completely in winter, whereas the deeper thermokarst lakes do not freeze to the bottom, with implications for coupling of the permafrost to the atmosphere. These deep thermokarst lakes are thermally stratified during winter and reach maximum water temperatures of up to 19 °C during summer. The summer water balance at the catchment scale was found to be mainly controlled by vertical fluxes (precipitation and evapotranspiration). On the other hand, redistribution of storage water due to lateral fluxes takes place within the microtopography of polygonal tundra. The long-term summer storage (precipitation minus evapotranspiration) from 1958-2011 indicates a reasonably balance on the polygonal tundra with an average surplus of 5 mm, but it is also characterized by high interannual variability due to precipitation input. During negative water balance years where evapotranspiration exceeds precipitation, shallower water bodies dry out. The extent of wetlands and water bodies will shift with changes in vertical water fluxes as well as permafrost warming and thaw. Thus, water bodies can serve as sentinels of environmental change and we present applicable remote-sensing observations and upscaling methods

Description: 〈jats:title〉Abstract〈/jats:title〉 〈jats:p〉The Lena Delta in Siberia is the largest delta in the Arctic and as a snow-dominated ecosystem particularly vulnerable to climate change. Using the two decades of MODerate resolution Imaging Spectroradiometer satellite acquisitions, this study investigates interannual and spatial variability of snow-cover duration and summer vegetation vitality in the Lena Delta. We approximated snow by the application of the normalized difference snow index and vegetation greenness by the normalized difference vegetation index (NDVI). We consolidated the analyses by integrating reanalysis products on air temperature from 2001 to 2021, and air temperature, ground temperature, and the date of snow-melt from time-lapse camera (TLC) observations from the Samoylov observatory located in the central delta. We extracted spring snow-cover duration determined by a latitudinal gradient. The ‘regular year’ snow-melt is transgressing from mid-May to late May within a time window of 10 days across the delta. We calculated yearly deviations per grid cell for two defined regions, one for the delta, and one focusing on the central delta. We identified an ensemble of early snow-melt years from 2012 to 2014, with snow-melt already starting in early May, and two late snow-melt years in 2004 and 2017, with snow-melt starting in June. In the times of TLC recording, the years of early and late snow-melt were confirmed. In the three summers after early snow-melt, summer vegetation greenness showed neither positive nor negative deviations. Whereas, vegetation greenness was reduced in 2004 after late snow-melt together with the lowest June monthly air temperature of the time series record. Since 2005, vegetation greenness is rising, with maxima in 2018 and 2021. The NDVI rise since 2018 is preceded by up to 4 °C warmer than average June air temperature. The ongoing operation of satellite missions allows to monitor a wide range of land surface properties and processes that will provide urgently needed data in times when logistical challenges lead to data gaps in land-based observations in the rapidly changing Arctic.〈/jats:p〉