Vol.4, No.5B, 100-105 (2013) Agricultural Sciences
Soil water resources use limit in the loess plateau of
Ting Ning1, Zhongsheng Guo1,2*, Mancai Guo3, Bing Han1,2
1The State Key Laboratory of Soil Erosion and Dryland Farming in Loess Plateau, Institute of Soil and Water Conservation, CAS &
MWR, Yangling, China; *Corresponding Author: zhongshengguo@sohu.com
2Institute of Soil and Water conservation, Northwestern A & F University, Yangling, China
3College of Science, Northwestern A & F University, Yangling, China
Received 2013
Soil water is a key factor limiting plant growth in
water-limited regions. Without limit of soil water
used by plants, soil degradation in the form of
soil desiccation is easy to take place in the
perennial fores tland and gras sland with too higher
density or productivity. Soil water resources use
limit (SWRUL) is the lowest control limit of soil
water resources which is used by plant s in those
regions. It can be defined as soil water storage
within the maximum infiltration depth in which
all of soil layers belong to dried soil layers. In this
paper, after detailed discussion of characteristics
of water resources and the relationship between
soil water and plant growth in the Loess Plateau,
the definition, quantitative method, and practical
applications of SWRUL are introduced. Henceforth,
we should strengthen the study of SWRUL and
have a better underst anding of soil water resour-
ces. All tho se are of grea t import ance for design-
ing effective restoration project and sustainable
management of soil water resources in water-
limited regions in the future.
Keywords: Infiltration Depth; Dried Soil Layer;
Wilting Coefficient; Soil Water Resources Use Limit;
Initial Stage to Regulate the Relationship between
Soil Water and Plant Growth
Vegetation restoration is an effective measure to con-
serve soil and water and improve ecological environment.
Since 1960 s, large-scale afforestation has been carried
out on the Loess Plateau in order to control serious soil
erosion there [1]. Tree species, selected for their capacity
to extend deep roots and fast growth, have been planted
at initially high planting densities in order to rapidly
establish higher degrees of ground cover, biomass and
yields, and thereby to quickly realize ecological, eco-
nomic and social benefits during vegetation restoration.
To meet evapotranspiration needs, it is advantageous that
the roots of these plants can grow quickly and thus take
up water from considerable soil depths. Consequently,
the combination of increased water used by plants and
low water recharge rates has led to widespread soil dete-
rioration occurring on the Loess Plateau in the form of
excessive soil drying under both perennial grasses and
forests. Such soil deterioration can adversely affect the
stability of forest ecosystems and the ecological, eco-
nomical and societal benefits of forest and other plant
communities. Now trees and grasses species planted in the
Loess Plateau are generally suitable for the local cli-
mate[2]. Therefore, in order to regulate the relationship
between plant growth and soil water, the following two
issues should be solved: when we take effective meas-
ures to regulate the relationship and how much the
amount of trees and grass to be cut when regulating. This
paper aims to introduce the SWRUL in order to better
understand and use the soil water resources in wa-
ter-limited regions.
2.1. Characteristics of Precipitation on the
Loess Plateau
The Loess Plat eau, l ocate d in t he ce ntral of Chi na, largely
belongs to semi-arid area and waterlimited regions.
Precipitation plays an important role in the terrestrial
water cycle, especially in the Loess Plateau. This is be-
cause groundwater mostly lies 40 - 100 meters below the
surface and the infiltration of precipitation is almost the
only way to supplement soil water in the Loess Plateau
[3]. Owing to a little account of the snowfall in winter,
precipitation resources in this region can be represented
by the rainwater resources which usually be defined as
the sum of precipitation within a year in a place [4]. An-
Copyright © 2013 SciRes. Openly accessible at http:// www.scirp.org/journal/as/
T. Ning et al. / Agricultural Sciences 4 (2013) 100-105 101
nual rainfall in the Loess Plateau is limited, merely rang-
ing from 250 mm in the northwest (9.8 inch) to 600 mm
in the southeast (23.6 inch). Because of the monsoon
influence, rainfall here has great seasonal variability, and
about 70% of rainwater fell in the months from June to
September. At the same time, the relative variance of
annual rainfall is also between 20% - 30% respectively.
Furthermore, spatial variability of rainwater resources in
this region is strong, too. It shows a total decreasing
trend from southeast to northwest, which has a direct
relationship with the amount of rainfall [5].
2.2. Soil Water Resources in the Loess
Soil water resource is a kind of renewable fresh water
resource with the characteristics and properties of natu-
ral resources [6]. Both rainfall and gro undwater can only
be absorbed and used by plants after being transformed
into soil water as most of the water used by plant is ob-
tained from the soil by plant roots system. So under
rainfed conditions, soil water plays a key role in the
production of agriculture and forestry. Thinking highly
of the maintenance and use of soil water resources in the
Loess Plateau is of great significance.
In general, soil water resources in the Loess Plateau
have strong temporal and spatial variability. Multiple
factors, including meteorological factors (such as rainfall,
atmospheric evaporation) and soil factors (such as soil
texture, land-use patterns, and soil water holding capac-
ity) produce a combined effect on the distribution and
dynamic of soil water [7]. Of which, rainfall plays a key
role. Furthermore, groundwater of this region usually lies
40 - 100 m below the surface, leading to that groundwa-
ter recharge including the side stream recharge of
ground- water, is difficult to occur [8]. So water cycle in
this region is relatively simple, and water contents at
different time and depth depend on the redistribution of
rainwater resources after infiltration [9]. Limited rainfall
is intercepted by the topsoil firstly, then moving down
slowly under the soil water potential gradient [10]. It is
the reason why the infiltration depth is shallow, and gen-
erally the depth will not exceed 3 m [11].
Furthermore, soil in the Loess Plateau mostly belongs
to loam soil with a low bulk density of 1.0 - 1.3 g.cm-3.
The total porosity of th is loess soil can be up to 50% and
the water-holding por osity can also be up to 25% - 30%.
So the loess soil in this region is often regarded as “soil
reservoir” with a high water holding capacity. It is meas-
ured that topsoil in the 0 - 200 cm soil profile (78.7 inch)
can hold soil water of 551.1 mm (21.7 in ch) to 847.4 mm
(33.4 inch) [5], which almost equals to the annual pre-
cipitation. From another perspective, those features of
loess soil lead to its high evaporation in turn and soil
water can only be stayed for a short time. Of course, this
phenomenon has a direct relationship with abundant light
and heat of this region. Taking abandoned land for an
example, even in the rainy years, the evapotranspiration
calculated from water balance could account for about
80% of rainwater from natural rainfall [12]. Under par-
ticipation of vegetation , maintainin g plant normal gr owth
will require more water. This is the key reason why soil
water content in this region is often at a low level.
3.1. Soil Water Conditi ons and Plant Growth
Results of studies conducted in the Loess Plateau showed
that the soil water condition is a key index of plant pro-
ductivity [13, 14]. Soil water is divided into available
water and non-available water. Filed cap acity and wilting
coefficient are usually regarded as the upper and lower
limit of available water. Under normal cases, soil water
content in this region is always lower than the corre-
sponding field capacity. Actual available water conten t in
the Caragana(Caragana korshinskii Kom.) scrubland is
less than 1/2 or 1/3 of the potential available water con-
tent [15]. Then the non-available water storage equals to
the residual water storage in the soil when soil water
content is smaller than the wilting coefficient, and it can
be vividly called as the “dead” storage of the “soil reser-
voir”. In the light of available water, the optimum soil
water contents for different plants are different. For a cer-
tain plant, when the so il water conten t is within the range
from wilting coefficient to the corresponding optimum
soil water content, photosynthetic rate will increase to a
certain extent with the increase of water content. Simi-
larly, at the stage of soil drought, the increase of soil wa-
ter content can also result in the improvement of plant’s
leaf water content and then accelerate the transpiration
[16]. It is notable that there is a threshold for plants to
react to soil water deficit. Wilting coefficient is a small
range rather than a point [17], which upper limit and
lower limit are called as initial wilting coefficient and
permanent wilting coefficient respectively. When the soil
water content is less than the initial wiltin g coefficient, it
is difficult for plant to uptake soil water. In this case,
although it won’t immediately lead to the death of plant,
plant’s normal growth and development will be inhibited.
3.2. Soil Desiccation and Dried Soil Layer
The climate environment of “low rainfall” and “high
evaporation” suggested that soil desiccation is easy to
take place in most of the Loess Plateau [18]. Vegetation’s
participation will greatly accelerate this process. Dried
soil layer has been found in farmland, artificial gr assland
Copyright © 2013 SciRes. Openly accessi ble at http:// www.scirp.org/journal/as/
T. Ning et al. / Agricultural Sciences 4 (2013) 100-105
Copyright © 2013 SciRes. Openly accessible at http:// www.scirp.org/journal/as/
and forestland in semi-humid and semi-arid regions of
the Loess Plateau since 1960s [19]. It results from the
negative balance in soil water cycle directly. The charac-
teristics of the local plant resources, the features of the
underlying surface and the eco-climatic zones have a
combined effect on the formation of the dried soil layer
[8]. Among those, the decrease in rainfall and the in-
crease in temperature would be the direct reason, and
improper vegetation type and exorbitant density or pro-
ductivity would accelerate its development. The appear-
ance of dried soil layer would seriously deteriorate the
soil quality, and the self-regulating capacity of soil wou ld
also be weakened [20].
Concerning the assessment standards of DSL, no con-
sensus has been reached now. In most cases, it is often
estimated according to the standards that water content is
between stable water content and wilting coefficient [21].
As for the types of DSL, Li [22] d ivided it into temporary
type and permanent type. The former refers to those
dried soil layers which are located at the depth between
land’s surface and the maximum infiltration depth, and it
can be gradually restored by thinning, plowing and other
measures. But permanent type located below the depth of
soil affected by rainfall infiltration is relatively stable and
soil water in these soil layers cannot be restored unless
experiencing high-intensity precipitation. When DSL’s
depth equals to the maximum depth which can be sup-
plemented by rainfall infiltration, with the increasing
forest age, strong depletion of soil water by plants will
lead to serious soil degradation as the supplement amount
of rainfall is limited. From this point, forestland and
grassland will be further exacerbated by drou ght if effec-
tive human intervention and regulation measures are not
carried about. Thereby it will further affect the normal
growth of plant as well as vegetation’s ecological bene-
fits, even leading to the occurrence of desertification
4.1. Concept and Definition
In order to adapt to the dry climate coupled with low
soil water content, perennials in the Loess Plateau usu-
ally have highly developed root system which are able to
take root into deep soil layers quick ly to uptake more soil
water [24]. In the process of vegetation restoration, utili-
zation depth of soil water by plants will increase with the
age. For instance, result of study conducted in the semi-
arid Loess Hilly Area showed that the utilization depth of
soil water by Caragana increased from 2 cm (0.08 inch)
to 200 cm (7.9 inch) during the first year after sowing
[23]. The uneven distribution of plant roots in the soil
profile coupled with the strong depletion of soil water by
plants lead to the excessive consumption of soil water in
the rhizosphere layer. Without timely and sufficient wa-
ter supplement, soil water storage will reduce gradually
from a higher storage at the beginning of afforestation to
a very low storage at the adult Caragana scrubland be-
cause soil water content itself is limited, and dried soil
layer will take place eventually. In addition, the depletion
and utilization of soil water resources by plants origin-
nally are not unlimited. Non-available water of the soil
cannot be used by plants theoretically. “Dead” storage
capacity accounts for a considerable volume in the “soil
reservoir”, especially in the Loess Plateau. So there must
be an appropriate limit of soil water resources used by
plants during the process of the vegetation restoration,
which means soil water resources use limit (SWRUL)
[25]. As precipitation is the only source of soil water
supplement in this region, the maximum precipitation
infiltration depth is also the maximum depth of soil water
supplement. Dried soil layer and soil degradation will
inevitably take place once soil water storage within the
maximum infiltration depth lowered the limit. So, this
concept can be defined as the soil water storage within
the range from soil surface to the maximum infiltration
depth in which all soil layers belong to dried soil layers.
The standard of dried soil layer here is the initial wilting
coefficient expressed by indicator plant, objective tree and
grass species such as Caragana, Robinia pseudoacacia.
4.2. Quantitative Method
In order to determine the value of SWRUL in a certain
region, it is necessary to choose the indicator plant in the
local vegetation communities firstly. Here the indicator
plant is usually the constructive species of natural vege-
tation or the purpose plant species of artificial vegetation.
The maximum precipitation infiltration depth and the
wilting coefficient expressed by indicator plant are two
key parameters for determining the limit. The former
should be determined based on measurements of infiltra-
tion depth in forestland or grassland under rainfed con-
ditions for many years as inter-annual variability of pre-
cipitation in this region is quite strong [26 ]. Wilting coef-
ficient is the lowest limit of soil water use by plants, re-
flecting the minimum requirement of soil water by plants.
It can be got according to the direct field observations or
other indoor meth ods.
SWRUL numerically equals to the integral of soil wa-
ter storage along soil profile from soil surface to the maxi-
mum precipitation infiltration depth in which the soil
water content equals to indicator plant’s initial wilting
coefficient. Being similar to the calculation of soil water
storage, the value of SWRUL is generally got by strati-
fied calculation method. The corresponding theoretical
calculation formula is as follows:
T. Ning et al. / Agricultural Sciences 4 (2013) 100-105 103
where, L is the SWRUL, H is the maximum pr ecipitation
infiltration, n is the number of subdivisions of dried soil
layer, H(i) is the depth of a certain subdivided soil profile,
W(i) is the initial wilting coefficient in the certain soil
4.3. Significance and Application
4.3.1. The Standard of Measuring Whether the
Use of Soil Water Resources by Plants is
Excessive or Not
As a common physical phenomenon in the Loess Pla-
teau, DSL increasingly threatens achievements and stabil-
ity of vegetation restoration. However, under the back-
ground of climate drought and global warming, water
restoration of dried soil layer is quite difficult to realize.
The management of degraded land is facing with more
suffering and challenges. Study results showed that in the
semi-arid area of Loess Plateau, even the land use changed
from alfalfa to annualcrops for 12 years, its water con-
tent also couldn’t meet the needs of planting trees or
perennial leguminous plants for their normal growth [27].
Therefore, the sustainable use of soil water in this region
has an extremely important theoretical and practical sig-
nificance. It is necessary to choose a reasonable index to
evaluate local soil water conditions. SWRUL can be used
as the indicator.
It is mentioned that the essence of the dried soil layers
is the excessive depletion of soil water use by plants. The
depth and thickness of dried soil layer are increased with
the age of plants. For instance, soil water content at the
soil layer of 100 cm (39.4 inch) was smaller than the
wilting coefficient of Caragana scrubland in the third
year after sowing. After then, soil drying becoming more
and more serious with times going by, finally extending
to 60 cm (23.6 inch) to 300 cm (118.1 inch) at the fifth
year [28]. Hence, the use of soil water by plants began to
enter the “excessive use stage” as the maximum precipi-
tation infiltration depth was only 290 cm [23]. In fact, for
deep root plants, it often doesn’t need to take a long time.
To sum up, according to SWRUL, we can identify whet her
or not plants excessively use soil water re-sources at the
initial stage of vegetation restoration. This is of impor-
tant value for the sustainable use of soil water resources.
4.3.2. The Theoretic Foundation to Determine
Initial Stage of Regulating the Relationship
between Plant Growth and Soil Water
Generally, there are disorder relationship between plant
growth and soil water in forest ecosystems and grass
ecosystems in the water –limited region. Regulating this
relationship is an essential way to ensure healthy devel-
opment of ecosystems. In water-limited regions of the
Loess Plateau, the determination of the initial stage of
regulating relationship between plant growth and soil
water is critical. This is because if the time of regulating
the relationship is earlier than the mention ed initial stage,
it will result in the waste of soil water resources. Of
course, if the time of regulating is later than the initial
stage, it may lead to irreversible soil degradation [25].
SWRUL is the cordon of soil water use by plants and
the theoretic foundation to determine initial stage of re-
gulating the relationship between plant growth and soil
water. Once the depletion of soil water by plants reached
or was lowered than the limit, effective measures should
be taken to regulate the plant-water relationship. These
measures can be divided into the following two sorts: to
increase soil water storage according to plants’ require-
ment or to reduce evapotranspiration according to exist-
ing soil water conditions. The form er is di fficult to achieve
in water-limited regions and the later is mainly achieved
through trim or cut trees. It is reported that soil water
storage in maximum precip itation in filtration d epth in the
soil under the 5-year-old Caragana scrubland in Shang-
huang Ecoexperimental Station reached its limit, so the
initial stage of regulating the plant water relationship was
the fifth year [28]. Accordingly, SWRUL plays an im-
portant theoretical role in gu iding the regulatio n of plant-
water relationship at the population level.
4.4. Research Prospects
Given the importance of SWRUL in theory and prac-
tice, SWRUL in different site and vegetation types
should be paid more attention. As the theory is initially
proposed, many details remain to be improved, espe-
cially the following two points.
4.4.1. Determination of Maximum Precipitation
Infiltration Depth
In bare lands of the Loess Plateau, the maximum pre-
cipitation infiltration depth directly depends on the ini-
tial soil water content and the amount of rainfall [29,30].
Infiltration and redistribution of precipitation will be-
come complicated with vegetation’s participation [31].
On the one hand, the distribution of plant roots lead to non-
uniform consumption of soil water. Water in the soil
away from the rhizosphere area will flow to the rhizos-
phere under soil water potential gradient [32]. On the
other hand, vegetation coverage affects the characteristics
of soil infiltration by canopies interception and weaken-
ing the raindrop power hitting on the surface through
improving the nature of underlying surface [33]. The
vegetation coverage and biomass in a plant community
usually increase with time in the progress of vegetation
Copyright © 2013 SciRes. Openly accessi ble at http:// www.scirp.org/journal/as/
T. Ning et al. / Agricultural Sciences 4 (2013) 100-105
succession. During this process there are significant im-
provements of soil’s infiltration capacity. Biological holes
such as root holes in the recovery woodland is likely to
produce preferential flows, then the soil infiltration rate
and maximum precipitation infiltration depth will in-
crease to a certain extent [34]. Furthermore, watercon-
suming capacity in different vegetation types as well as
their influence on soil water redistribu tion is d ifferent. So
it is necessary to determine maximum precipitation infil-
tration depths in different vegetation types. Then the ap-
plication scale of soil water resources use limit can be
4.4.2. Vertical Variability of Wilting Coefficient in
the Soil Profile
Wilting coefficient is a key element in the calculation
of SWRUL. This valu e is usually ob tained by putting the
critical leaf water potential of indicator plant into soil-
water characteristic curve which describing the rela-
tionship between soil water and soil suction. Taking
Gardner empirical formula θ = a · Sb (θ is the volu-
metric soil water content, S the soil suction, a, b are pa-
rame- ters) to fit the curve, wilting coefficient can be ex-
pressed as W= a · 1.5 - b. In this formula, parameter a re-
flects soil’s water-holding capacity, and parameter b re-
flects the decreasing speed of soil water content with the
decrease of soil water suction [35]. Both parameter a and
parameter b are mainly influenced by soil texture,
or-ganic matter content and soil structure [36]. All fac-
tors mentioned change with soil depth, so it will lead to
vertical variability of wilting coefficient certainly. Re-
sults of studies carried out in Shanghuang Ecoexperi-
mental Station showed that wilting coefficient changed
indeed with soil depth, the fitted values (volumetric soil
water content) floated from 5.6% to 7.8%. The overall
trend was that the wilting coefficient at the land surface
was small and then increased to a certain level with the
in-crease of soil depth. The variability of adjacent soil
layers was quite strong within the depth that from soil
sur-face to the maximum precipitation infiltration depth.
In addition, the above analysis is based on the assump-
tion that water suction is considered to be 1.5 MPa for
temporary wilting coefficient in the Loess plateau [19].
Actually, the wilting coefficient has a relationship with
soil water absorption capacity by p lant, so wilting coeffi-
cients of different indicator plant types are different even
in the same soil environment [37]. Under such circum-
stances, researches on wilting coefficient of different
indicator plants should also be taken into consideration.
This project is supported by the National Science Foundation of
China (Project No: 41071193, 41271539).
[1] Wu, Q.X. and Yang W.Z. (1998) Vegetation construction
and sustainable development for the Loess Plateau. Sci-
entific Press, Beijing.
[2] Guo, Z.S. and Shao, M.A. (2013) Impact of afforestation
density on soil and water conservation of the semi-arid
Loess Plateau, China. Journal of Soil and water conser-
vation (in press).
[3] Mu, X.M. (2000) Interaction of soil and water conserva-
tion measures with soil water in the Loess Plateau in
China. Transactions of the Chinese Society of Agriculture
engineering, 16, 41-45.
[4] Feng, H., Shao, M.A. and Wu, P.T. (2001) Calculation
and assessment of developing potential of converting rain
water to resources in smallwatershed on the Loess Plateau.
Chinese Journal of Natural Resources, 16, 140-144.
[5] Yang, W.Z. (2001) Soil water resources and afforestation
in Loess Plateau. Chinese Journal of Natural Resources,
16, 433-438.
[6] Wang, H, Yang, G.Y., Jia, Y.W. and Wang, J.H.2006
Connotation and assessment index system of soil water
resources. Chinese Journal of Hydraulic engineering, 37,
[7] Hemmat, Ahmadi, A. and Masoumi, I.A. (2007) Water
infiltration and clod size distribution as influenced by
ploughshare type, soil water content and ploughing depth.
Biosystems Engineering, 97, 257-266.
[8] Li, Y.S.2001Effects of forest on water circle on the
Loess Plateau. Chinese Journal of Natural Resources, 16,
[9] Moribidelli, R.C., Corradini and Saltalippi, C. (2011) An
experimental hydrometeorological investigation to ad-
dress infiltration redistribution modelling.World Envi-
ronmental and Water Resources Congress. California:
American Society of Civil Engineers, 4759-4768.
[10] Meng, Q.Q., Wang, J., Wu, F.Q. and Zhang, Q.F. (2012)
Soil moisture utilization depth of apple orchard in Loess
Plateau. Transactions of the Chinese Society of Agricul-
tural Engineering, 28, 65-71.
[11] Mu, X.M., Xu, X.X., Wang, W.L., Wen, Z.M. and Du, F.
(2003) Impact of artificial forest on soil moisture of the
deep soil layer on Loess Plateau. Acta Pedologica Sinica,
40, 210-217.
[12] Chen, H.S., Shao, M.A. and Wang, K.L. (2005) Desicca-
tion of deep soil layer and soil water cycle characteristics
on the Loess Plateau. Acta Ecologica Sinica, 25,
[13] Li, Y.S and Huang, M.B. (2008) Pasture yield and soil
water depletion of continuous growing alfalfa in the
Loess Plateau of China. Agriculture, Ecosystems &
Environment, 124, 24-32. doi:10.1016/j.agee.2007.08.007
[14] Bescansa, P., Imaz, M.J. and Virto, I., Enrique, A. and
Hoogmoed, W.B. (2006) Soil water retention as affected
by tilla g e and r esi du e ma na g ement in semiarid Spain. Soil
and Tillage Research, 87, 19-27.
Copyright © 2013 SciRes. Openly accessible at http:// www.scirp.org/journal/as/
T. Ning et al. / Agricultural Sciences 4 (2013) 100-105
Copyright © 2013 SciRes. http://www.scirp.org/journal/as/ Openly accessible at
[15] Wang, M.B., Chai, B.F., Li, H.J. and Feng, C.P. (1999)
Soil water holding capacity and soil available water in
plantations in the Loess region. Scientia silvae sinicae, 35,
[16] Wang, M.B., Li, H.J. and Chai, B.F. (1999) A comparison
of transpiration, photosynthesis and transpiration effi-
ciency in four tree species in the Loess region. Acta Phy-
toecologica Sinica, 23, 401-410.
[17] Casadebaig, P., Philippe, D. and Jrmie, L. (2008) Thresh-
olds for leaf expansion and transpiration response to soil
water deficit in a range of sunflower genotypes. European
Journal of Agronomy, 28, 646-654.
[18] Chen, B.Q., Zhao, J.B. and Li, Y.H. (2009) Research on
causes of dried soil layer in the Loess Plateau, Geography
and Geo-Information Science, 25, 85-91.
[19] Chen, H.S., Shao, M.A. and Li, Y.Y. (2008) Soil desicca-
tion in the Loess Plateau of China. Geoderma, 143,
91-100. doi:10.1016/j.geoderma.2007.10.013
[20] Wang, L., Shao, M.A., Wang, Q.J. and Jia, Z. (2005)
Comparison of soil desiccations in natural and acacia
forests in the Ziwuling Mountain of the Loess Plateau.
Acta Bot. Boreal. –Occident. Sin., 25, 1279-1286.
[21] Wang, L., Shao, M.A. and Hou, Q.C. (2000) Preliminary
research on measured indexes of dried soil layer. Journal
of Soil and Water Conservation, 14, 87-90.
[22] Li, Y.S. (1983) The properties of water cycle in soil and
their effect on water cycle for land in the Loess Plateau.
Acta Ecologica Sinica, 3, 91-101.
[23] Guo, Z.S. and Shao, M.A. (2007) Dynamics of soil water
supply and consumption in artificial caragana scrubland.
Journal of Soil and Water Cconservation, 21, 119-123.
[24] Cheng, J., Hu, T.M., Cheng, J.M. and Wu, G.L. (2010)
Distribution of biomass and diversity of Stipa bungeana
community to climatic factors in the Loess Plateau of
northwestern ChinaDistribution of biomass and diversity
of Stipa bungeana community to climatic factors in the
Loess Plateau of northwestern China . African Journal of
Biotechnology, 9, 6733-6739.
[25] Guo, Z.S. (2010) Soil water resources use limit in semi
arid loess hilly area. Chinese Journal of Applied Ecology,
21, 3029-3035 .
[26] Qiu, Y., Fu, B.J., Wang, J. and Chen, L.D. (2003) Soil
moisture variation in relation to topography and land use
in a hillslope catchment of the Loess Plateau, China.
Journal of Hydrology, 240, 243-263.
[27] Wang, Z.Q., Liu, B.Y., and Lu, B.J. (2003) A study on
water restoration of dry soil layer in the semi-arid area of
Loess Plateau. Acta Ecologica Sinica, 23, 1944-1950.
[28] Guo, Z.S. and Li, Y.L. (2009) Initiation stage to regulate
the caragana growth and soil water in the semiarid area of
Loess Hilly Region, China. Acta Ecologica Sinica, 29,
[29] Du, J. and Zhao, J.B. (2007) Seasonal changes of soil
moisture content in dried soil later in artificial forest in
Gaoling of Xi’an. Scientia Geographica Sinica, 27,
[30] Li, Y.F. and Li, X.F. (2007) Study on precipitation infil-
tration recharge with groundwater depth variation. Jour-
nal of China Hydrology, 27, 58-60.
[31] Dong, S.X. (2004) Effect of Natural Vegetation Restora-
tion on Soil Infiltration in Slope Farmland of Loess Hilly
and Gully Region. Bulletin of Soil and Water Conserva-
tion, 24, 1-5.
[32] Millikil, C.S. and Bledsoe, C.S. (1999) Biomass and dis-
tribution of fine and coarse roots from blue oak (Quercus
douglasii) trees in the northern Sierra Nevada foot—hills
of California. Plant Soil, 214, 27-38.
[33] Zhao, H.Y., Wu, Q.X. and Liu, G.B. (2001) Mechanism
on soil and water conservation of forest vegetation on the
Loess Plateau. Scientia Silvae Sinicae, 37, 140-144.
[34] Wang, G.L. and Liu, G.B. (2003) The effect of vegetation
restoration on soil stable infiltration rates in small water-
shed of loess gully region. Journal of Natural Resources,
18, 529-535.
[35] Gardner, W. R. (1958) Some steady state solutions of the
unsaturated moisture flow equation with application to
evaporation from a water table. Soil Science, 85, 228-232.
[36] He, X. D., Gao, Y. B. and Ren, A.Z. (2003) Plant water
potential coefficient and its application in field experi-
ment. Acta Scientiarum Naturalium Universities
Nankaiensis, 36, 89-92.
[37] Lyman, J.B. and Shantz, H.L. (1912) The relative wilting
coefficients for different plants. Botanical Gazette, 53,
229-235. doi:10.1086/330752