Seasonal change of soil moisture in Mongolia: Its climatology and modelling

摘要

Mongolia has an arid and cold climate due to its geographical settings of inland and mid-latitude highlands. The soil moisture varies seasonally, depending mainly on the balance of precipitation and evapotranspiration as well as on winter soil-freezing and spring snowmelt. Soil moisture climatology (1986-2005) for Mongolia is presented with a focus on three vegetation zones: the forest steppe, steppe, and Gobi Desert. For this purpose, we used soil moisture observations based on the gravimetric method for the top 50-cm soil layer from 26 grass-covered field sites during April-October of the 20-year period. In general, there was a latitudinal gradient in soil moisture content, with the southwestern soils being drier than northeastern soils. The seasonal change in soil moisture was small and the seasonal pattern was similar throughout Mongolia. The seasonality was characterised by three major phases of the warm season: the spring drying, summer recharge, and autumn drying phases, although each phase differed somewhat in timing and length between zones. In the northernmost forest steppe zone, the recharge phase was longer than that in the southern steppe and Gobi Desert zones, while the two drying phases were shorter in the forest steppe zone. This difference had a significant effect on the plant phenological timings of Stipa spp.; these were earlier in the forest steppe zone and later in the Gobi Desert zone. A simple water balance model was developed to examine the observed soil moisture dynamics, which implicitly simulated snow accumulation and soil freezing. The model simulated the observed seasonal and inter-annual soil moisture variations reasonably well (r = 0.75, p < 0.05). The results demonstrated that the three phases of seasonal change were produced by a subtle balance between precipitation and evapotranspiration. This model will provide a useful tool for a reliable and timely monitoring of agricultural drought for decision-makers and herders in Mongolia.