Spatial and seasonal variability of polygonal tundra water balance: Lena River Delta, northern Siberia (Russia)Variabilité saisonnière et spatiale du bilan d’eau de la toundra polygonale: Delta de la rivière Lena, Nord Sibérien (Russie)Variabilidad espacial y estacional del balance de agua de polígonos de tundra: Delta del Río Lena, norte de Siberia (Rusia)多边形冻原地下水均衡的空间和季节变化:以俄罗斯北西伯利亚的Lena河三角洲为例Variabilidade espacial e sazonal do balanço hídrico da tundra poligonal: Delta do rio Lena, norte da Sibéria (Rússia)

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.