Eco-hydrological Responses On Dominated Sand-fixing Vegetations In The Transitional Zone From Oasis To Desert In The Lower Reaches Of Shiyang River

The lack of surface water and the excessive pumping of groundwater have resulted in the fixing-sand vegetation decline and the fixed dunes activation as well the breaking of transitional zone from oasis to desert, impacting seriously the fixing-sand functions of vegetations in Minqin oasis where located in the lower reaches of Shiyang river. However, to study the eco-hydrological responses of fixing sand shrubs, has an important meaning for the vegetative restoration and re-building in the transitional zone from oasis to desert in lower reaches of Shiyang river.The effects on the changes of groundwater level and precipitation in spatial and temporal to fixing-sand shrubs, and the response on the fixing-sand shrubs to soil water content have been done, by applying the theories of eco-hydrology in combination with the methods of field investigations and site monitoring. The primary conclusions were as follows:(1) The groundwater table at Minqin oasis fringe have suffered from a descending process from high water level to low water level since 1970, but the descending rate of groundwater was of difference in spatial and temporal, ranking as Baqu's﹥Quanshanqu's﹥Huqu's in spatial longitudinal direction and oasis fringe﹥desert region in spatial transverse direction, and ranking as 90s in 20th﹥10s middle in 21th﹥80s in 20th﹥70s in 20th﹥60s in 20th from fast to slow in temporal. The monthly changes of groundwater level showed that the groundwater level can be adjusted with seasonal changes of plant growing to renew to the level of beginning year before 1970, and can't be renew in all year now.(2) The results showed that the vegetation coverings of three fixing-sand shrub communities decreased by an exponent model with groundwater level. The degraded rate of three shrub communities depended on the site types of shrub growing after groundwater level dropped more than 5m, ranking as fixed dune﹥semi-fixed dune﹥semi-shifted dune from fast to slow. The result showed that the native vegetations growing in the transitional zone from to desert in arid areas would evolve gradually from halophyte meadow sere to eremophyte sere, while the artificial vegetations would evolve toward the succession direction of psammophyte or more xerophytic vegetation with groundwater level, implying that native N. tangutorum community may form the climax vegetation of degraded succession spectrum with groundwater level. The changes of groundwater level resulted in habitat heterogeneity, causing the changes of species diversity in fixing-sand shrub populations of H. ammodendron and T. ramosissima as well N.tangutorum that were the dominant community itself. The results showed that plant species numbers would decrease under high and low groundwater table, and increase under middle groundwater level. With the changes of underground water in different regions of Minqin oasis, N. tangutorum population had higher niche in semi-fixed dunes in transitional zone from oasis to desert in Minqin oasis. When the groundwater level descended from 7.5m to 13m, the niche breadth of N. tangutorum community distributing in fixed and semi-fixed dunes broadened gradually, implying that the adaptation of N. tangutorum community may be much strong than those of H. ammodendron and T. ramosissima communities under dry conditions.(3) The results showed that the distributions of fine root length density for H. ammodendron and T. ramosissima presented evenly in 0~160cm deep soil-layer when the groundwater table was 1.6m deep, and presented high density in 0~120cm deep soil-layer and in 110cm soil-layer above groundwater table when the groundwater tables were 2.6m and 3.6m deep in vertical direction. The plentiful development of fine roots in soil-layer of groundwater level effecting implied that the fixing-sand shrubs utilized mainly the groundwater to maintain their growths with the ages of fixing-sand shrubs. The fine roots of H. ammodendron and T. ramosissima as well N. tangutorum were concentrated in 0~120cm soil layer, accounting for 67.6%, 69.9% and 86.8% respectively of the total in 0~300cm soil layer in vertical direction when the groundwater table dropped at 21~24m deep, implying that the growing of fixing-sand shrubs depended on the precipitation.(4) The rainfall intensity and precipitation would impact the infiltration rate and infiltration accumulative in soil of different fixing-sand shrub woodlands. The infiltration under high rainfall intensity accorded with the source infiltration that made for the infiltration into deep soil, but the wetting front penetration had the hysteresis effects with infiltration periods in deep soil. The results showed that the water contents in 0~40 or 60cm deep soils changed obviously with the soil crust development in different shrub woodlands. The evapotranspiration and redistribution of precipitation in shrub woodlands were synchronous after rainfall. However, the shrub vegetations not only consumed the infiltration moisture in soils after rainfall, but consumed the storage moisture below the wetting front in soils, restricting the wetting front penetration. In general, the infiltration depths in soils of different shrub woodlands were less than 200cm deep for every rainfall event.(5) The results showed that the relationship between penetrable precipitation and rainfall would accord to a linear correlation(p﹤0.01), that was said the penetrable precipitation would increase with rainfall under different fixing-sand shrub canopies in transitional zone from oasis to desert. For H. ammodendron, T. ramosissima, normal N. tangutorum and degraded N. tangutorum communities, the interception storage capacity were 0.9mm, 0.8mm, 0.6mm and 0.3mm, respectively, and the average interception losses were 16.6%, 33.1%, 12.0% and 2.7% respectively of the total annual precipitation. The proportion of interception loss to gross rainfall trended to remain constant at about 0.2~0.3 for H. ammodendron and 0.3~0.4 for T. ramosissima when the rainfall intensity was more than 0.8mm.h-1. In contrast with normal N. tangutorum, the proportion of interception loss to gross rainfall trended to keep constant at about 0.1~0.2 for normal N. tangutorum and 0.05~0.1 for degraded N. tangutorum when the rainfall intensity was more than 0.5mm.h-1 and 0.4mm.h-1.(6) The result indicated that hygroscopic condensation was a dominated process, in which accounted for 83.8% of total condensation water from humidity air, only 16.2% from vapor of deeper soil. The condensation water occurred mainly in 0~10cm deep soil, in which the amount of condensation water in 0~2cm deep soil was highest taking 38.1% of daily total condensation waters, and the starting time of condensation water was delayed with soil depths as well the biggest amount of condensation water occurred during 2:00~4:00 at night accounting for 21.6% of daily condensation water. The result also showed that surface temperature, air temperature and relative humidity as well soil temperature under 5cm ground were the major weather factors affecting the formation of condensation water in soil. The mean total condensation water of 14mm in H. ammodendron, T. ramosissima and N. tangutorum woodlands from July to September implied that the effect of condensation water can not be neglected in the eco-hydrological process in arid desert regions.(7) The diurnal variations of sap flow showed a bi-peaked curve for H. ammodendron, and a multi-peaked curve for T. ramosissima and N. tangutorum. All of the three species had an ambiguous noon-depression phenomenon of the sap flow curves. There was sap flow occurred at night for the three shrubs, but the duration of the flow at night varied with stem diameters. The larger the stem diameter, the longer the duration of sap flow at night. The sap flow of the three shrubs at night had an obvious decreasing course and approached zero prior to the sap flow started next day. The seasonal variations of sap flow for the three shrubs showed a unimodal pattern, in which the highest value occurred in August for H. ammodendron and T. ramosissima, and in July for N. tangutorum. During the entire growing season, the unit area sap flow intensity for the three shrubs with similar stem diameters ranked as N. tangutorum﹥T. ramosissima﹥H. ammodendron from high to low, which implied that the drought tolerance of H. ammodendron was higher than that of N. tangutorum and T. ramosissima. The results showed that vapor pressure deficit or air temperature was the major weather factor affecting sap flow for H. ammodendron, and solar radiation for T. ramosissima and N. tangutorum, which indicated that the weather factors played different roles in affecting sap flow of the three shrubs.(8) The result showed that the actual total annual amount of ecological water requirements for 5 forestlands was 1.49×108 m3 accounting for 19.5% of annual total water utilization of 7.64×108m3 in Minqin oasis. The results also showed that 1.14×108m3 of extra water resources would be provide to ensure the ecological water requirements for those vegetative normal growing in Minqin oasis in each year after 0.35×108m3 of annual total effective precipitation for 5 woodlands was subtracted from the actual total annual amount of ecological water requirements.