| The soil desiccation which has been found widely in woodlands and grasslands on the loess plateau since 1980's and became more and more serious, not only led to deteriorating the ecological condition of soil water, the large-area reduction and even over parcels of vegetation dead, but also brought negative impacts to returning farmlands to forestlands and eco-environment construction. At present, the interaction between vegetation growth and change of soil water, the law on water consumption by vegetation, the bearing capacity of soil water to vegetation, the accurate evaluation about hydrographical function of vegetation have became the austere problem to be faced by vegetation construction on the Loess Plateau. In this paper, natural Quercus liaotungensis woodland, artificial acacia woodland, natural grassland and farmland in the loess hilly region of northern Shaanxi province were selected, and COUPMODEL (Coupled heat and mass transfer model for soil-plant-atmosphere system) was used to simulate the transfer of water and energy between soil, vegetation and atmosphere in soil-vegetation-atmosphere system, research the transfer characteristic of water and energy, and the water and heat environment and vegetation respond to it under different vegetation types, to provide theoretic foundation for vegetation construction in loess hilly region and similar areas. The main conclusions were as follows.(1)In dry year, the precipitation was consumed mainly through soil evaporation and vegetation transpiration, next was interception by canopy, and the evapotranspiration exceeded the homochronous precipitation and the soil water storage decreased in woodlands and grasslands. 1.81% of precipitation was stored in farmland soil because of short growing season and less water consumption. In wet year, the water income of soil-vegetation- atmosphere system was great than expense, and the precipitation of 17.07%-26.27% stored in soil, and. The supplementary water in soil from much to little was farmland, grassland on north-facing slope, Quercus liaotungensis woodland on north-facing slope, acacia woodland on north-facing slope, Quercus liaotungensis woodland on south-facing slope, grassland on south-facing slope, and acacia woodland on south-facing slope in correct order. (2)The south-facing and north-facing slope received solar radiation of 7169.19MJ·m-2, 6826.79MJ·m-2 in experimental plots respectively. The net radiation accepted by woodlands of 54.52%-62.73% came into vegetation, that of 37.27%-45.48% into soil; the net radiation accepted by grasslands of 40.00%-47.00% came into vegetation, that of 60.00%-53.00% into soil; the net radiation accepted by farmland of 30.74% came into vegetation, that of 69.26% into soil. The net radiation mainly released by latent heat flux accounted for 72.05%-81.41% of net radiation, secondly sensible heat flux accounted for 16.35%-26.37% of net radiation, and heat flux to soil in woodlands accounted for 1.39%-2.09% of net radiation was very little in woodland, grassland and farmland.(3)The evaporation and transpiration of water need consume some energy in soil-vegetation-atmosphere system and the exchange of water and transform of energy is a coupled process in this system. The heat flux to evaporate soil water was increased with the increase of soil evaporation, and to evaporate 1mm of water need 2.461MJ·m-2 of energy. The heat flux to evapotranspirate soil water and interception water by canopy was increased with the increase of evapotranspiration, and to evapotranspirate 1mm of water need 2.452MJ·m-2 of energy.(4)The soil water deficit in acacia woodland on south-facing slope, grassland on south-facing slope, acacia woodland on north-facing slope, Quercus liaotungensis woodland on south-facing slope, grassland on north-facing slope, Quercus liaotungensis woodland on north-facing slope, and farmland was 991.57mm, 941.21mm, 866.53mm, 815.89mm, 790.27mm, 745.20mm, and 325.55mm in correct order. The exchange depth of soil water in farmland, grassland on north-facing slope, Quercus liaotungensis woodland on north-facing slope, acacia woodland on north-facing slope, Quercus liaotungensis woodland on south-facing slope, grassland on south-facing slope, and acacia woodland on south-facing slope was 320cm, 240cm, 200cm, 160cm, 160cm, 140cm, and 120cm in correct order. In experimental period, about 10% of precipitation was stored in woodland soil, that of about 14% stored in grassland soil, and that of about 30% stored in farmland. The available water in 600cm deep soil reservoir in woodland and grassland only was 62.63-309.00mm, much less than that in farmland that was 728.65mm, and so the soil reservoir in woodland and grassland had less regulative capacity than farmland. The contribute ratio of climate drought and exhausting water by vegetation to soil desiccation were 70.74%, 29.26% respectively, thus drought of climate is the main reason of soil desiccation.(5)The soil temperature rose from February to July, and reduced from August to January. The soil temperature reduced gradually with soil depth from February to September, and rose with soil depth from October to January. The annual amplitude of surface temperature, soil temperature in 10cm and 20cm depth in farmland was greater than that in grassland, Quercus liaotungensis woodland, acacia woodland on north-facing slope; the annual amplitude of surface temperature, soil temperature in 10cm and 20cm depth in grassland was greater than that in Quercus liaotungensis woodland, acacia woodland on south-facing slope.(6)The soil water content in grassland on north-facing slope, Quercus liaotungensis woodland on north-facing slope, acacia woodland on north-facing slope, Quercus liaotungensis woodland on south-facing slope was greater than that in grassland on south-facing slope, and acacia woodland on south-facing slope, accordingly the former vegetation grew better than the latter. Under the double influence of water consumption of vegetation and water supply of soil, the dying branches appeared serious, even over entire trees died in acacia woodland on south-facing slope. There were the dying branches in acacia woodland on north-facing slope, but no dead entire trees. There existed dying branches in Quercus liaotungensis woodland, but very slight and the trees grew very exuberant. The date of leafing in grassland on south-facing slope and acacia woodland on south-facing slope was earlier 1 week than that in grassland on north-facing slope, Quercus liaotungensis woodland on south-facing slope, acacia woodland on north-facing slope, and earlier 12 days than in Quercus liaotungensis woodland on north-facing slope.(7)The artificial woodlands consumed soil water redundantly and the productivity of standing forest was lower, but the benefits in soil and water conservation of woodland more and more higher than farmland, and so restoring vegetation through forest planting still is a effective way in the region with serious soil and water loss. The higher interception and transpiration were the main causes that led to soil desiccation in artificial woodland, therefore the native tree species with less water consumption should be selected according to the law on vegetation succession and the measures that restore the vegetation under natural condition was main and artificial planting was auxiliary should be adopted in vegetation construction in the loess hilly region. The Quercus liaotungensis in the loess hilly region is climax community and holistic climate drought on the loess plateau led to its soil desiccation, but redundant soil water consumption by vegetation, and so it is not suitable to simply ascribe this soil desiccation to dried soil layers. Distinguishing dried soil layers should base on the biomass and soil water content of local stable natural vegetation communities.
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