Hydrological cycle in relation to permafrost environment in forest-grassland ecotone in Mongolia

Abstract

In the mountainous regions of northern Mongolia, there are peculiar different patches of the landscape "ecotone" with north-facing forest slope and south-facing grassland slope. The slopes are distinguished as marked contrast of permafrost distribution as well. The present study revealed ecohydrological processes in terms of evapotranspiration and water balance based on continuous observation since 2004 in the ecotone area of Shiljiree river watershed, upper part of Tuul River in Khentii Mountains. Soil moisture amounts following rainfall during early growing season have primary importance on interannual variations in transpiration from larch forest slope. In addition, timings of snow disappearance and thawing active layer affect subsequent foliation of trees and their transpiration. The estimation of water balance during summer 2006 demonstrated that the larch forest slope with underlying permafrost suppressed the evapotranspiration with half of amounts in grassland slope. It means that precipitation and soil water in forest slope can be partitioned to evapotranspiration and river runoff. On the other hand, grassland at south-facing slope with no underlying permafrost has remarkably large amounts of evapotranspiration and therefore both precipitation and soil water was consumed entirely to evapotranspiration. The results depicted that the coexistence between permafrost and forest in mountain slope has an important role in sustaining water resource.

Temperature Regimes of the Active Layer and Seasonally Frozen Ground under a Forest‐Steppe Mosaic, Mongolia

“…The strong reduction in the amount of downward shortwave radiation on the forest floor (Figure ) is related to its modulation by the evolution of forest canopy and the slope, and is one of the factors that reduced ground surface temperatures in summer in the forested areas compared to the steppe (Figure a). The seasonal evolution of the forest canopy in north central Mongolia has been illustrated in other studies (Iijima et al ., ; Miyazaki et al ., ), where it has been observed that leaves emerge in May, mature in July and die in mid‐September. Permafrost beneath forested north‐facing slopes is impermeable to groundwater flow (Ishikawa et al ., ), and it reduces evapotranspiration from this slope (Iijima et al ., ).…”

“…The seasonal evolution of the forest canopy in north central Mongolia has been illustrated in other studies (Iijima et al ., ; Miyazaki et al ., ), where it has been observed that leaves emerge in May, mature in July and die in mid‐September. Permafrost beneath forested north‐facing slopes is impermeable to groundwater flow (Ishikawa et al ., ), and it reduces evapotranspiration from this slope (Iijima et al ., ). This results in wet ground at FN (Figure a), which generally has a higher thermal conductivity than dry soils (Farouki, ; Hinzman et al ., ) and should therefore contribute to heat transfer from the ground surface to deep soil layers, enabling a warm soil profile.…”

“…Since 2003, hydro‐meteorological components have been continuously measured at sites in northern Mongolia, with the goal of delineating the different hydrological characteristics of the mountain steppe on south‐facing slopes and those of the forest on north‐facing slopes (Ishikawa et al ., ; Iijima et al ., ). Permafrost in the region develops only under forested north‐facing slopes (Ishikawa et al ., ), and has an important role in sustaining local river water resources (Iijima et al ., ).…”

“…Since 2003, hydro-meteorological components have been continuously measured at sites in northern Mongolia, with the goal of delineating the different hydrological characteristics of the mountain steppe on south-facing slopes and those of the forest on north-facing slopes (Ishikawa et al, 2005;Iijima et al, 2012). Permafrost in the region develops only under forested north-facing slopes (Ishikawa et al, 2005), and has an important role in sustaining local river water resources (Iijima et al, 2012). The purpose of this paper is to describe 5-year records of comparable hydrometeorological parameters on these slopes, with particular focus on the distinctive features controlling the ground temperature regimes of the active layer and seasonally frozen ground on these contrasting slopes.…”

Thermal states, responsiveness and degradation of marginal permafrost in Mongolia

“…Permafrost fulfills a vital role in sustaining local ecosystem services, because near‐surface soils with shallow active layers remain wet despite low annual precipitation (eg, ). The areas with permafrost overlap considerably with forests and are major source of river water …”