Temporal variability of climate, flooded and wet zones in Western Siberia from data stations, atmospheric reanalysis and satellite altimetry
It is difficult to make an assessment of climate variability and hydrological characteristics using only in situ data, especially on such vast and remote territories as Western Siberia where the network is sparse and has declined over the past decades. Therefore, atmospheric reanalysis and satellite altimetry provide regular and reliable data for the territory and are an additional source of information to in situ data. The main objective of this study is to assess the variability of climate parameters in different landscape zones from different data sets. First, we analyzed air temperature and precipitation of mid size watersheds of the Ob River system (the Poluy, Nadym, Pur, Taz, Nazym, Lyamin, Tromyugan, Vakh, Tym, Bol'shoy Yugan, Vasyugan, Severnaya Sosva, Konda, Tavda, Tura, Om Rivers, the inner watershed of the Ob- Irtysh interfluve area) from meteorological stations and ERA-40 reanalysis data (1958-2001). Data analysis revealed the increase of air temperature in the northern, central and southern parts of Western Siberia. The most pronounced warming was found in the north (the Nadym, Poluy, Pur, Taz Rivers). Increase of precipitation for the same period was observed mostly in the western part of the territory (the Severnaya Sosva, Konda, Tavda, Tura Rivers), central part (the Nazym and the Vakh) and northern part (the Taz). Variability of flooded and wet zones is difficult to estimate from in situ data. Moreover, the presence of wetlands in most of the Western Siberia significantly complicates its monitoring. Therefore, to estimate the variability of flooded and wet zones we applied satellite altimetry method for watersheds of the Ob River system. Although the primary mission of satellite altimetry was to measure the altitude of the object above the fixed level, it also provides information about the surface of different natural objects (wetlands, lakes, rivers, etc.). In this work, we used radar altimetry data from TOPEX/Poseidon mission (T/P) covering the period from 1993 to 2001. To estimate the flooded areas we analyzed the backscatter coefficient which is the ratio between the power reflected from the surface and the incident power emitted by the onboard radar, expressed in decibels (dB). As calm water provides a much higher return signal than land, a threshold approach was used and validated with available in situ data and maps. Next we classified the watersheds into 4 groups according to the difference in seasonal signal: 1) the central part of the Ob-Irtysh interfluve area and its inner watershed, 2) the western part of the territory and the northern part of the Ob-Irtysh interfluve area, 3) the Ob-Yenisey interfluve area, 4) Surgut swamp region and rivers in the north of Western Siberia territory. The proposed classification corresponds well to those proposed by Zemtsov (2003). Finally, a significant correlation between the flooded zones and summer precipitation from reanalysis data was found for several watersheds that validate good applicability of satellite altimetry data for estimation the variability of flooded and wet zones.