Vegetation Carrying Capacity Of Soil Water In Steep Slope Of Loess Region In Northern Shaanxi

≥25°steep slopes of Loess region in northern Shaanxi , vegetation is sparse, soil drought and soil erosion is extremely serious, and it is the focus and difficulty in the reconstruction of vegetation and the control of soil erosion in the Loess Plateau.A long time,the problems of soil dry and low ecological and economic benefits were exist on steep slopes,it dued to shortage of rainfall and the lack basis of water balance in vegetation construction.Rational use of rain resources is the key of sustainable development in vegetation construction on steep slopes.To this reason, soil moisture ecological environment of artificial vegetations in steep slope , relationship between plant growth and soil moisture in steep slope, vegetation carrying capacity of soil water, and the efficient use of natural precipitation on steep slope has studied.The main conclusions are that:1.Soil water was extremely deficient under condition of perennial artificial vegetations in steep slope. soil water storage (0～10m) was only equal to 26.2% ~ 42.0% of field capacity in dry years, and in rainy years it was also only equal to 27.0%～43.3% of field capacity. The order of soil water deficit was: Caragana microphylla> locust> alfalfa> Chinese arbor-vitae> poplar> Chinese pine> wild land> apricot> Chinese date> farm land. Annual variations of soil moisture with same vegetation were weakened with soil depth increasing, and happened mainly in 0～200 cm soil layers. In same growth season, all CV (Coefficient of Variation) of soil moisture under different vegetations were bigger and concentrated comparatively in 0 ~ 120 cm soil layers, but difference of CV in different vegetations was small; below 120 cm soil layers, CV were smaller , but difference of CV in different vegetations was bigger. Permanent soil dry layers always happened under condition of perennial vegetations in steep slope , but the difference of soil aridization intensity was obvious among different vegetations and growth years. Soil water compensation and recovery depths in rainy season were 1.0 ~ 1.4 m , but the soil water storage increment and compensation degree in different vegetations were dramatically different. Soil water compensation depth in same vegetation in rainy years was increased over 60 cm than in dry years, while the soil water storage increment in 5 m soil layers was increased over 3 times. Under natural precipitation, the soil water deficit in artificial vegetations in steep slope cannot be reduced, soil aridization also can′t be relieved. 2. The dry strength of Vegetation on steep slopes can be indicated by soil desiccation index.Formula: SDI =(SM–WM)/(SSM–WM)×(SD-DT)/(SD)×100%,there, SM is soil moisture, WM is wilting moisture, SSM is the stable soil moisture, DT is the drying thickness, SD is the soil depth.The soil drying strength of vegetation on steep slopes can be divided into six levels: (1) SDI≥100%, non-drying; (2) 50%≤SDI <100%, mild desiccation; (3) 30%≤SDI <50%, moderate desiccation; (4)10%≤SDI <30%, serious desiccation; (5)0≤SDI <10%, strong desiccation; (6) SDI <0, extreme desiccation.3. Interception of Prunus armeniaca change in 0.30 ~ 9.5mm at each rain, rate of interception change in 2.6~ 67.6%, account for 21.10% of the total rainfall. Interception of Caragana korshinskii change in 0.21 ~ 3.2mm at each time, rate of interception change in 2.0 ~ 28.0%,account for 11.73% of the total rainfall. Rainfall frequency is more,or torrential rain is less, rate of interception account precipitation is bigger.4. Runoff is large on steep slopes.Runoff increases exponentially with the increase of rainfall in Alfalfa land, the relationship is: Y=0.002x2+0.1285x-0.2409(R2=0.9731),and runoff accounts for 12.41% of mean precipitation in Alfalfa land; runoff accounts for 11.41% of mean precipitation in Prunus armeniaca land; and runoff accounts for 16.27% of mean precipitation in Caragana korshinskii land.5. Under the condition of natural rain, the greatest infiltration depth is 140cm in alfalfa land in steep slope, the greatest infiltration depth is 160cm in apricot land in steep slope, the greatest infiltration depth is only 120cm in Caragana korshinskii land in steep slope.6. The main factors impact to soil moisture supplies in alfalfa land are natural precipitation, interception and surface runoff. In alfalfa land on 25°steep slope the relationship between soil moisture supply(Ys) with precipitation (P) is: Y up = 0.8003P +2.8568 (R2 = 0.987, n = 33), in 33°steep slope the relationship is: Ys = 0.7771 P +3.0411 (R2 = 0.985, n = 33). The slope is greater, the surface runoff is greater too, but soil moisture supply is less. In apricot land the relationship between rainfall (P) with soil water supply (SWS)is: SWS = 0.6299P +0.5901, correlation coefficient is 0.9829. In Caragana korshinskii land the relationship between rainfall (P) with soil water supply (SWS) is: SWS =0.5708P+28.579, correlation coefficient is 0.9658.7. In alfalfa land on steep slopes , the relationship between soil moisture supply (Ys) with aboveground biomass (Wd) showes a linear relationship,in 25°slope up to south the relationship is: Ys =0.0247W+275.52, R2 =0.9598; in 33°slope up to south the relationship is: Ys=0.0249 W +279.37, R2 =0.9767; in 25°slope down to south the relationship is: Ys =0.0348 W + 235.83, R2 = 0.9620; in 25°slope down to north the relationship is: Ys= 0.0304 W+247.31, R2=0.9727.In alfalfa land on steep slopes, the relationship between soil water consumption (Yc) with aboveground biomass (Wd) is the secondary function , in 25°slope up to south the relationship is: Yc=0.0001W2-0.4635W+854.72, R2=0.9595; in 33°slope up to south the relationship is: Yc =0.0001W2-0.3836W+659.16, R2=0.9805; in 25°slope down to south the relationship is: Yc=0.0001W2-0.4628W+805.53, R2= 0.9731; in 25°slope down to north the relationship is: Yc= 0.0001W2-0.5324W+ 991.67, R2=0.9514. Soil water consumption is enlarge with high biological production. Impact of aspect on soil water consumption is greater , on southern slope sun is strong, evaporation and soil water consumption is greater.On steep slopes the biggest carrying alfalfa production of soil water is 3992.2 ~ 4173.7 kg/hm2 by FAO method; and it is 2600 ~ 3500kg/hm2 by the principle of water balance,it is lower 16.07 ~ 33.52% than FAO method.because the application of meteorological factors in FAO method , increases the error, so in reality the principle of water balance should be based on whichever calculation results.8. The relationship between supply of soil moisture (Ys) with biomass (W)is: Ys=0.1226W-103.59, R2=0.9369. The relationship between consumption of soil moisture (Yc) and biomass(w) is: Yc=0.00001W2-0.0251W +195.61, R2 = 0.9282. Biomass of apricot that Soil moisture can carry is 2423kg/hm2, fruits production that Soil moisture can carry is 3063kg/hm2.9. In Caragana korshinskii land in steep slope the relationship between interception(I) with density(D)is:I=0.7359D0.4925, R2 =0.9642; the relationship between surface runoff(Run) with density(D) is: Run =- 0.021D + 152.53, R2=0.9509; the relationship between soil moisture supply(Ys) with density(D)is: Ys= 0.0145D +215.4, R2 =0.9582 ,and the relationship between soil water consumption(Yc) with density(D)is: Yc=0.00001D2-0.0089D +200.82. R2 = 0.9537. On steep slopes density that soil moisture can carry is 2852 clump/hm2.10. In loess region in North Shaanxi, the technique of plastic-catchment and promote-infiltration can increase survival rate, yield and quality and economic benefits of apricot and jujube, Rainwater use efficiency of 5 years-old apricot reached 2.92kg/m3, increased by 53.68% compared with CK, Rainwater use efficiency of 5 years-old jujube reached 3.45kg/m3 , increased by 53.33% compared with CK. During concentrated precipitation soil water recruitment below rhizosphere area (2-6m) was 31.1mm, it was almost equal to 40 times of CK. This part water was used by plant in arid season. This technique cost lowly and operated simply. The service life was long, The efficiency was high. It will have broad application prospect in the general drought and impoverished mountainous area of our country.