【作者】 徐先英；

【导师】 孙保平；

【作者基本信息】 北京林业大学 ， 水土保持与荒漠化防治， 2008， 博士

【摘要】 石羊河下游的民勤绿洲,地表水资源短缺和地下水的过度开发,已造成绿洲-荒漠过渡带呈破碎状态,植被衰退、沙丘活化,已严重影响过渡带植被持续稳定地发挥固沙功能,因此,研究固沙植被生态水文效应,对于绿洲-荒漠过渡带的恢复与重建具有重要意义。本文运用生态水文学的理论,以民勤绿洲西北缘的过渡带为研究对象,通过野外调查和定位观测相结合的方法,对绿洲-荒漠过渡带地下水、降水时空变化对固沙植被的影响进行研究,初步研究结果如下:(1)民勤绿洲边缘地下水位经历从高到低渐变过程,但在空间纵向上,地下水位下降速率坝区﹥泉山区﹥湖区;在横向上,绿洲边缘﹥荒漠区,地下水位沿绿洲边缘向荒漠区方向,存在一个从低到高的水位梯度;时间上,地下水下降速率90年代﹥21th前6年﹥80年代﹥70年代﹥60年代;地下水位年内变化从60年代随季节变化,演变到目前全年几乎得不到回升。(2)随地下水位的下降,梭梭、柽柳和白刺群落盖度呈指数形式下降。当地下水位下降﹥5m,不同立地条件下固沙灌木退化速度为,固定沙丘﹥半固定沙丘﹥半流动沙丘。随地下水位的下降,天然植被从盐生草甸逐渐向荒漠植被演替;人工植被向沙生或更旱生植物演替,从发展趋势看,白刺沙包可能成为退化演替序列的顶级群落。地下水位的变化,导致以梭梭、柽柳和白刺分别为建群种的植物群落,在高水位和低水位条件下物种多样性减少,在中间水位条件下物种多样性增加;在半固定沙丘,白刺在过渡带均占有较高的生态位,当地下水位从7.5m下降到13.0m,白刺种群的生态位宽度逐渐增大,表明白刺对干旱环境的适应性高于柽柳和梭梭。(3)地下水埋深影响固沙灌木吸水根系的发育。当地下水位1.6m,梭梭和柽柳的吸水根在0～160cm层基本呈均匀分布;当地下水位为2.6m和3.6m,吸水根在地表以下0～120(160)cm和地下水位以上110cm范围内,呈高密度分布。随着树龄的增长,吸水根在地下水位影响的土层范围内大量发育,固沙灌木主要利用地下水;当地下水位下降到21～24m,梭梭、柽柳和白刺吸水根主要分布在地表以下0～120cm土层范围内,分别占垂直剖面总吸水根的67.6%、69.9和86.8%,固沙植物主要逐渐演变为利用天然降水。(4)降水强度和降水量影响固沙灌木土壤水分的入渗速率和入渗补给量,高降水强度有利于水分向深层渗透,但随入渗时间表现出滞后效应;具有物理结皮或生物结皮的灌木林地, 0～40(60)cm土壤层受降水影响最大,土壤水分波动明显;降水后水分在土壤中再分配与植被的蒸散同步进行,植物不但消耗入渗的水分,而且也消耗湿润峰以下土壤储存的水分。次降水在不同植被区入渗深度在0～200cm范围内。(5)不同固沙灌木冠层穿透水量随降水量增加而增加,穿透降水量与降水量的关系符合线形模型(p﹤0.01),梭梭、柽柳、正常生长白刺和衰退白刺冠层截留容量分别为0.9mm、0.8mm、0.6mm和0.3mm,群落冠层截留率分别为16.6%、33.1%、12.0%和2.7%。当降水强度﹥0.8mm.h-1时,梭梭和柽柳冠层截留率分别基本稳定在0.2～0.3和0.3～0.4之间;当降雨强度﹥0.5mm.h-1和﹥0.4mm.h-1,正常生长和衰退白刺冠层截流率分别维持在0.1～0.2之间和0.05～0.1之间。(6)干旱荒漠区沙地凝结水主要自大气中吸湿凝结,占总凝结量的83.8%,而土壤中水汽向上运移形成的凝结水只占16.2%;凝结水主要发生在0～10cm土层,其中0～2cm土层最高,占凝结水总量的38.1%,随土壤深度加深,凝结水出现时间存在滞后现象;凝结水在夜间2:00～4:00最高,平均占日凝结量的21.6%;三种灌木林地7～9月平均总凝结量14mm;地表温度、近地面气温、空气相对湿度和5cm层地温对土壤凝结水形成起主导作用。(7)梭梭的蒸腾耗水日变化表现为双峰型,而柽柳和白刺日变化则表现为多峰格型;3种灌木蒸腾耗水在中午存在“午休”现象,但“午休”不明显;不同灌木在夜间也保持一定的蒸腾速率,而且地茎越粗夜间蒸腾持续的时间越长,越细持续时间越短,不同地茎灌木在第二天液流启动之前,夜间液流都有一个明显的下降过程,甚至为0。固沙灌木茎干液流季节变化表明,梭梭和柽柳茎干液流8月份最高,7月份次之,6月与9月基本一致,而白刺7月份最高,8月次之。相近地茎3种灌木单位面积液流量为:白刺﹥柽柳﹥梭梭,梭梭耐旱性较高。在整个生长季,空气水汽压差或气温对梭梭起主导作用,而太阳辐射对柽柳和白刺起主导作用。(8)石羊河下游民勤绿洲防护体系每年实际生态需水量为1.49×108m3,占民勤绿洲7.64×108m3用水总量的19.5%,扣除0.35×108m3的有效天然降水,另外需要1.14×108m3的其它水资源,才能满足防护体系的生态需水要求。更多还原

【Abstract】 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.更多还原