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\begin{document}

\title{Soil Water Resources Utilization Limit by Red Plum Apricot}



\author[1]{zhongsheng guo}%
\affil[1]{Chinese Academy of Sciences and Ministry of Water Resources Institute of Soil and Water Conservation}%


\vspace{-1em}



  
  \date{June 17, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
:Red plum apricot is a deciduous fruit tree and the best cash forest in
semiarid loess hilly regions. Since 1995, the distribution area of red
plum apricot spreads in the most of the water-limited regions, China.
The yield, benefits and plantation area of red plum apricot increase
dramatically. But along with the growth of red plum apricot tree, soil
drying appeared and sometime become sever,which influence the
yield,quality and benefits of red plum apricot. At this time, the
relationship between the soil water and red plum apricot growth must be
regulated on Soil Water Resource Use Limitation by Plant and Soil
Moisture Carrying Capacity for Vegetation. However, there are few
studies on the regulation theory. In this study, daily precipitation,
soil water suctions at different soil water content were measured, and
the maximal infiltration depth and the soil water resource use
limitation by red plum apricot was estimated. The results show that
wilting coefficient varies with soil depth from 7.98 in surface soil to
7.1\% in 240 cm soil depth, and the maximal infiltration depth is 290
cm, and Soil Water Resources use limit by red plum apricot is 212.7 mm.
When the soil water resource in the maximal infiltration depth is lower
than the limit, the relationship between the soil water and red plum
apricot vegetative growth must be regulated in Red Plum Apricot
plantation for high quality production of red plum apricot.%
\end{abstract}%



\sloppy


Soil Water Resources Utilization Limit by Red Plum Apricot Zhongsheng
Guo1,2 1,Institute of Soil and Water Conservation, Northwestern A \& F
University, 2,Institute of Soil and Water Conservation, CAS \& MWR,
Yangling, China; 26, Xinong Road, Yangling, Shaanxi Province 712100, P.
R. China Tel. ++86-29-87012411 Fax. ++86-29-8701-2210 Corresponding
Author: zhongshenguo@sohu.com

ABSTRACT:Red plum apricot is a deciduous fruit tree and the best cash
forest in semiarid loess hilly regions. Since 1995, the distribution
area of red plum apricot spreads in the most of the water-limited
regions, China. The yield, benefits and plantation area of red plum
apricot increase dramatically. But along with the growth of red plum
apricot tree, soil drying appeared and sometime become sever,which
influence the yield,quality and benefits of red plum apricot. At this
time, the relationship between the soil water and red plum apricot
growth must be regulated on Soil Water Resource Use Limitation by Plant
and Soil Moisture Carrying Capacity for Vegetation. However, there are
few studies on the regulation theory. In this study, daily
precipitation, soil water suctions at different soil water content were
measured, and the maximal infiltration depth and the soil water resource
use limitation by red plum apricot was estimated. The results show that
wilting coefficient varies with soil depth from 7.98 in surface soil to
7.1\% in 240 cm soil depth, and the maximal infiltration depth is 290
cm, and Soil Water Resources use limit by red plum apricot is 212.7 mm.
When the soil water resource in the maximal infiltration depth is lower
than the limit, the relationship between the soil water and red plum
apricot vegetative growth must be regulated in Red Plum Apricot
plantation for high quality production of red plum apricot.

Keywords: Red plum apricot growth; water-limited regions;soil drying;
soil degradation; wilting coefficient; maximum infiltration depth; Use
Limit of Soil Water Resources by Red plum apricot; high quality
production of red plum apricot

INTRODUCTION

Soil water limits the ecological situation where plant roots grow,
especially in water-limited regions where climate and soil
characteristics set the limits of available water for plant growing.
Soil water equilibrium plays a vital role for restoration
rehabilitation. Therefore, soil water management is very important in
agricultural systems (Asgarzadeh et al. 2014).

In recent years, agricultural production activity has been strenthened
to meet the food need of an increasing population worldwide, the
intensification of agricultural activity coexisted negative
environmental influence (Maharjan et al. 2016). Along with population
increase in the water-limited regions, such as in the semiarid loess
hilly region of Loess plateau, People need a lot of food, fruit, fiber,
etc, and original vegetation has been destroyed and change into
farmland. As a result, forest and vegetation is scarcity, and the loss
of soil and water in the Loess Plateau had become a serious
environmental problem by 1949. Loss of soil and water eroded fertile
surface soil and led to soil fertility and crop productivity reduced,
which influence quality of human life. In order to conserve soil and
water loss, relief of destruction caused by sandstorms and haze weather,
increase crop productivity and the improvement of ecological
environment, the government has been taking many measures since 1950. In
particular, with the implementation of Three-North Shelter Forest
Program sponsored in 1978 for 50 years, large-scale afforestation and
fruit trees has been carried out on the Loess Plateau. As a result of
these efforts, great achievements have been made. The forest coverage
fast increased and annual sediment discharge on the Loess plateau has
been reduced from 1.6 billion tons in the 1970s to 0.31 billion tons in
recent years, and the runoff has been halved.

Because soil in this region is very deep and in the range of 30--80 m
from the surface (zhu et al. 1983), and the groundwater table is also
deep (Yang and Shao, 2000), and without irrigation, soil water mainly
comes from some precipitation penetrating~through the canopy. Along with
canopy and the roots development, the interception by canopies increase
and the roots of these plants grow quickly and thus take up water from
considerable soil depths, which reduces the soil water supply and
increases soil water consumption. Consequently, the increased water use
by plants and interception and low infiltration capacity and soil water
recharge rates has led to serious soil drying with times going by (Guo
and Li 2009). The dried soil layers(DSL)appeared and then its
thickness of DSL increased, and soil drying widespread (Li 1983; Chen et
al. 2007). Serious drying of soil eventually and poor self-regulation of
plant result in soil degradation, vegetation decline and agriculture
failure, which have adverse effects on sustainable use of soil water
resources and the stability of forest vegetation ecosystems (Guo and
Shao 2013). Thus we should take effective measures to regulate the
nonequilibrium relationship between soil water and plant growth (RBSWPG)
by reducing the population quantity or density of indicator plants in a
plant community on soil water carrying capacity for vegetation (SWCCV)
on the Loess Plateau to balance the soil water recharge and soil water
consumption in plantation (Guo and Shao 2003; Guo 2014,2021) because
soil in this region range from 30 to 80 m from the surface (zhu et al.
1983), and the groundwater table is also deep (Yang and Shao
2000),without irrigation.

The concept of soil water resources come in 1985 (Budagovski 1985;
Budagovski and Busarova 1992) after Lvovich proposed the concept of
overall soil moistening in 1980. Soil water resources have different
meaning in different field, such as Geology, Soil Science, Agriculture,
Forestry and Animal Husbandry. In order to meet the need of different
specialty, soil water resources can be classified into static soil water
resources and dynamic soil water resources. Static soil water resources
include generalized and narrow soil water resources. The generalized
soil water resources refers the water storage in the soil from surface
soil to water table, and narrow soil water resources refers the water
storage in the root zone soil, and dynamic soil water resources refers
the antecedent soil storage plus the soil water supply from
precipitation in the growing season for deciduous plant or a year for
evergreen plants. Soil water resources are renewable water resources, a
component of water resources (Guo 2014).

The state of vertical distribution of soil water in the root soil zone
space influence plant growth. Since drought is a recurring natural
phenomenon, and the soil in which plant root distribute resembles a
reservoirs and have the storage capacity of water, which have the
buffering effect of soil drying on plant growth, the effects of water
stress on plant growth vary with the their gravity in these regions.
Soil Water Resources Use Limit by Plant is the soil water storage in the
maximum infiltration depth(MID) when soil water content in all of the
soil layers of the MID equals wilting coefficient (Guo 2010, 2014 ).We
do not regulate the relationship as soil drought happens until the soil
water resources reduce to a degree (Soil Water Resources Use Limit by
Plant ) because when soil water resources in the maximum infiltration
depth equal Soil Water Resources Use Limit by Plant, soil water
seriously influence plant growth if the duration dry climate continue
surpass the key period of regulation of the relationship between soil
water and plant growth because plant have some self-regulation power .

Red plum apricot is a deciduous fruit tree and the best cash forest in
semiarid loess hilly regions. Since having been selected as good
varieties to popularize in 1995, the distribution area of red plum
apricot spreads from Guyuan county to the whole Ningxia, and then to
Gansu province and so on in the most of the water-limited regions,
China,the yield, benefits and planting area of red plum apricot
increase doubly. But along with the growth of red plum apricot and
precipitation, sometime soil drying become severer. Once serious drying
happens, which led to the change of red plum apricot tree leaf colour
from green to yellow or croci and drop earlier of the leaf. If serious
drying happens in the fruit expansion stage, the size of red plum
apricot fruit cannot expand to normal size, which influence the yield,
quality and economic benefits of red plum apricot forest. At this time,
the relationship between the soil water and red plum apricot growth must
be regulated on Soil Water Resources Use Limit by Plant and Soil Water
Carrying Capacity for Vegetation to reduce or evade the bad influence of
soil drought on the yield and benefits of red plum apricot (Guo
2014,2021). However, there are few studies of Use Limit of Soil Water by
red plum apricot.

In the present work, the study aims at achieving these objectives: (1)
the changes of cumulative infiltration depths with time and the maximum
cumulative infiltration depth in the red plum apricot forest ; (2)
Change of wilting coefficient with soil depth up to maximum infiltration
depth; and (3) Use Limit of soil Water by red plum apricot.

Methods

\textbf{Site description}

This study was conducted at National high quality red plum apricot
demonstration area,which is located at the Shanghuang Eco-experiment
Station in the semiarid Loess hilly region (35\selectlanguage{ngerman}°59\selectlanguage{english}'- 36deg02\selectlanguage{english}' N, 106deg26\selectlanguage{english}'-
106deg30\selectlanguage{english}' E) in Guyuan, Ningxia Hui Autonomous Region of China, Institute
of Soil and Water Conservation of Chinese Academy of Sciences, with the
altitude of the station ranges from 1,534 m to 1,824 m. Precipitation
here is absent in the periods from January to March and from October to
December, and the rainfall from June to September makes up more than
70\% of the annual precipitation. Mean rainfall measured between 1983
and 2001 was 415.6 mm with a maximum of 635 mm in 1984 and a minimum of
260 mm in 1991. The frost-free season is 152 days. The Huangmian soil
having developed directly from the loess parent materials, consists
mainly of loamy porous loess (Calcaric Cambisol, FAO1988) with wide
distribution in the semiarid hilly region of the Loess Plateau. Red plum
apricot tree is a kind of fine variety apricot (\emph{Armeniaca
vulgaris} Lam.). The experiment was conducted in 23-year-old red plum
apricot garden planted in 1996 and 1-year-old red plum apricot garden
planted 2018 .

Generally, some 2- year-old red plum apricot trees start to bear fruit.
3 -year-old red plum apricot forest has some yield and the yield of 4
-year-old red plum apricot forest have reached a certain level after
planting red plum apricot tree. An adult red plum apricot tree start to
bloom in the end of March and are in full bloom in the first ten-day
period of April, red plum apricot fruit is in the expansion period of
fruit from the second ten-day period of May to the second ten-day period
of June, and fruit mature in the first ten-day period of July, and leaf
drop in the middle and last ten-day period of September. Once serious
drying happens in the Fruit expansion stage, leaf of red plum apricot
tree change colour from green to yellow or croci, and drop earlier. The
size of red plum apricot fruit is smaller than normal fruit, which
influence the yield, quality and economic benefits of red plum apricot.

\textbf{Observation items and determination methods}

Rainfall at the study site was measured with standard rain gauges placed
in the center of the National first-class high-quality red plum apricot
Demonstration area, which was about 50 m from the Shanghuang
Eco-experiment weather station, as a part of Guyuan Eco-experiment
weather station under Institute of soil and water conservation of
Chinese Academy of Sciences. The study also included the determination
of the soil moisture content, plant root distributions, and other plant
growth parameters.

The experimental plots were located in the 23-year-old red plum apricot
forest planted in the bench terrace in 1996 and 1-year-old red plum
apricot forest planted in the bench terrace in 2018. The sampling pits
(soil profile) was dug in red plum apricot forest at the experimental
site for investigating soil profile and sampling purposes, whose
dimensions were 1m\textsuperscript{2} x 4 m depth on the red plum
apricot forest in April, 13, 2018. The undisturbed soil samples were
collected for 3 times at the depth of 0 to 5, 20 to 25, 40 to 45, 80 to
85, 120 to 125, 160 to 165, 200 to 205, 240 to 245 and 395 to 400 cm
with cutting rings (a 5 cm in high, 5 cm in inner diameter and 100
cm\textsuperscript{3} in cubage). At the same time\selectlanguage{english},the disturbed soil
of about 100g at each depth was collected for determination of soil
structure at the State Key Laboratory of Soil Erosion and Dryland
Farming on the Loess Plateau.

Cutting ring was used to measure the bulk density, total porosity,
capillary porosity, saturation moisture content. The core samples
(undisturbed soil sample) collected were used with cutting rings to
measure the soil bulk density, capillary porosity and noncapillary
porosity. The bulk density was determined by oven-drying the cores at
105-110\selectlanguage{english}, and the total porosity was calculated as 1-bulk density/soil
particles density, assuming that the density of soil particles was
2.65g/cm\textsuperscript{3}. Noncapillary porosity was the difference
between total porosity and capillary porosity. Soil particles were
measured with master sizer 2000 laser particle analyzer and grain size
was graded on the USA standard. Soil water contents at different soil
suctions ( 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 4.0, 6.0 bar, 1 bar =
1.0x10\textsuperscript{5} Pa ) were measured by a HITACHI centrifuge,
made by Instrument Co., Jappan. Because Huangmian soil had been
contracted when measuring with a centrifuge, the researchers measured
the shrink amount of soil samples in the cutting ring by vernier
callipers at different soil suctions and then calculated the volumetric
soil water content.

Select 1-year-old and 23-year-old red plum apricot tree with average
height and canopy as the sample of study. Two holes with 5.3 cm in
diameter were made by holesaw in the place about 40cm cm apart from the
1-year-old red plum apricot tree, and two 4-m long aluminum access pipes
were placed in the holes with an interval of 1 m between them. Another
two holes with 5.3 cm in diameter were made by holesaw in the middle of
the radius of red plum apricot tree canopy, about 2 m away from the tree
base (centre) to the exterior margin of the canopy in the 23-year-old
red plum apricot tree planted in the bench terrace in 1996. The
interspaces between access pipes and soil were filled with some fine
earth in case water might flow through the interspaces. A neutron probe,
CNC503A (DR), made by Beijing Nuclear Instrument Co., China, was used
for long-term monitoring of the field soil water content because of its
high precision in situ (Wang and others 2000; Evett and others 2012; Guo
and Shao, 2013). Before measuring the volumetric soil water content
(VSWC), the neutron probe was calibrated for the soil in the study area
by using standard methods (Hauser 1984). The calibration equation for
this soil at the site is \emph{y} = 55.76\emph{x} + 1.89, where \emph{y}
is VSWC, and \emph{x} is the ratio of the neutron count in the soil to
the standard count. The measuring depth ranged from 0 to 400 cm in the
period from April to October, 2018. Measurements were made with 15-day
intervals in time and 20 cm intervals in depth. Measurements were made
every 15 days to a depth of 4 m in increments of 20 cm starting at the 5
cm depth. When measuring soil water content at different soil depth,
first put the probe into the aluminum access pipes and change the
measuring line of the neutron probe to confirm the weather or not the
soil depth equal planned depth of determination according to the display
device of soil depth .Second, press the start button and then read and
record the numbers of soil water content at different soil depth on the
display screen of the neutron probe. The soil water content obtained for
each measuring depth was taken to be representative for the soil layer
that included the measuring point +- 10 cm depth, apart from that for the
5 cm depth, which was taken to represent the 0 to 10 cm soil. The
measurements were also made before and after each rain event in the red
plum apricot forest.

Height, diameter at the base and size of the canopy of the 1-year-old
red plum apricot tree growing on the plots were investigated and
measured, and estimate the maximal infiltration depth and Soil Water
Resources Use Limit by Plant. the relationship between the colour of
leaf or the size of fruit and the soil water was investigated and
estimate the suitable amount of leaf and vimen when the soil water
resources in the maximal infiltration depth is approach to or smaller
than Soil Water Resources Use Limit by Plant in 23-year-old red plum
apricot tree. The measurements of red plum apricot tree growth were
carried out in the time period from mid-April to October, and the
measurements of precipitation and soil water were carried out from
January to December in 2018 to 2020.

Mathmatical model

Depth of infiltration and maximal infiltration depth

Two curves method was found by Guo in 2004, and used to estimate the
depth of infiltration by Guo and Shao in 2009 and Guo in 2014, and named
by Guo in 2020. In this study, two curves method was used to estimate
the depth of infiltration and soil water supply for a rain event or some
days\selectlanguage{english},and a series of two curves methods for used to estimate the depth
of infiltration for a long time infiltration process, such as the time
period from mid-April to October in 2018.

When estimating the depth of infiltration and soil water supply for a
rain event or some days, first put the probe into the aluminum access
pipes and change the longth of measuring line connected with the neutron
probe sensor to the measuring soil depth according to the display device
of soil depth in the neutron probe and measure and record soil water
content at different soil depth and then draw the change of soil water
content with soil depth before a rain event and after the rain event
(two continuous soil water distribution curves or a series of soil water
distribution curves of soil water with soil depth at the same aluminum
access pipes and there is a cross location in the coordinate system in
the soil profile before a rain event and after the rain event (or an
infiltration process).The depth of infiltration during a rain event is
equal to the distance from the surface to the joint location between two
soil water distribution curves with soil depth . The MID, short for
maximal infiltration depth is equal to the distance from the surface to
the deepest joint location between two contiguous soil water
distribution curves with depth in the soil profile at the beginning and
the end of a period (Guo and Shao 2009, 2013; Guo 2017).

2 \textbf{The change of wilting coefficient with soil depth}

Because Gardner empirical formula can better describe the relationship
between w and S, the wilting coefficent can be estimated by the Gardner
empirical formula w=a \selectlanguage{english}[?] S \textsuperscript{-- b} (Guo and Shao
2009).First the Gardner empirical formula were transform into ln
(w)=alnS+b, and then used to fit the relationship between soil water
suction (S) and volumetric soil water content (w) at different soil
depth, and then established the relationship between ln (w) and ln (S)
by the least square method, and then estimate the wilting coefficent,
which is the volumetric soil water content (w) at 1.5Mpa.

\textbf{3 Soil Water Resources Use Limit by Plant}

The mathematical model for calculating Soil Water Resources Use Limit by
Plant (SWRULP) was showing as following:

SWRULP=

Here, SWRULP is Use Limit of Soil Water by Plant, expressed in mm. MID
is maximum infiltration depth. Ow is wilting coefficient at soil depth
of i, the symbol i is the number of soil layer and D is the thickness of
the soil at i soil layer

Statistical analysis

With the help of ANOVA coupled with SPSS 13.0 software, an analysis was
made concerning the significance of influence of the planting density on
all the parameters measured and the effect of pipe position, planting
density and soil depth on soil water content. A regression analysis was
then made to determine the different relationships, such as the
relationship between soil water content and moisture suction, the
relationship between the root density and soil depth using the least
square method. Data were transformed when necessary to obtain a linear
relationship.

\textbf{Results}

\textbf{1 The change of cumulative infiltration depths with time in} red
plum apricot forest

Infiltration is a process in which water enter soil. The water
infiltrated into soil have two function. One is to increase soil water
content in a soil layer, and another is to increase cumulative
infiltration depth. The two curve was used to estimate the depth of
infiltration before and after a rain event or an infiltration process or
several days. The infiltration depth for a rain event is equal to the
distance from the surface to the crossover point between two soil water
distribution curves with depth measured in the soil profile before and
after a rain event or several days. A lot of the crossover points at the
same height in the soil profile makes up the wetting peak. The annual
precipitation is 536.2 mm, which is 120.6 mm more than the mean
precipitation 415.6 mm and close to the maximum rainfall record of 634.7
mm in the National high quality red plum apricot demonstration area, see
fig.1. After two effective rain events, 9 mm in May 20 and 19.7 mm in
April 13, infiltration depth reach to 70 cm on April 28, 2018, see
fig.2.

As time goes on, the cumulative infiltration depth increased with time
in 2018 because the infiltration includes two stages: rainfall
infiltration generally happening during a rain event and the cumulative
infiltration (Chowdary and others 2006) or reinfiltration (Corradini and
others 2000), which occurs in the period between two

(Fig.1 Here)

\selectlanguage{english}(Figures 2 Here\selectlanguage{english})

rain events or a long term period in which there are more than two rain
events happens because there is a cumulative effect on the infiltration
process. After a heavy rain event, a high water-bearing soil layer
formed under land surface. With time going by, the soil water content in
the high water-bearing soil layer reduced because soil evaporate, plant
root water absorption, or infiltrate into deeper soil layer and form
another high water-bearing soil layer at deeper soil layer, and
cumulative infiltration depth increases in the soil profile (Guo and
Shao,2009; Guo 2017).When the soil water content in the upper layer of
wetting peak is equal to the lower layer of wetting peak, the cumulative
infiltration process stopped because there is not infiltration force,
and water potential difference between the upper layer of wetting peak
and the lower layer of wetting peak approaches to zero. At this time,
the cumulative infiltration depth is the maximum cumulative infiltration
depth (Guo 2020).That is to say, the maximum cumulative infiltration
depth is the maximum infiltration depth. The cumulative infiltration
depth arrives in 130 cm on May 28, up to 150 cm on June 16, get to 190
cm on July, 16 , and finally to 290 cm. So, the maximum infiltration
depth is 290 cm in red plum apricot forest, see Figure 3.

\selectlanguage{english}(Fig3 Here\selectlanguage{english})

\textbf{2 The change of wilting coefficient with soil depth}

Plants absorb water from the soil\selectlanguage{english},which cause soil water content root
reduce. Soil dry become severe and soil water stress in the soil layer
near root. At the same time, the water moves slowly from the from the
soil layer nearest root to the soil layer near root in the soil matrix
driven by gravity and water potential gradients. Wilting coefficient for
Huangmian loess soil is the water content at -1.5 MPa in a given soil
layer (Yang and Shao, 2000). In the terrace land, 23-years-old red plum
apricot tree root develops to a considerable soil depth and suck water
in the dry year on National high quality red plum apricot demonstration
area. Once a soil layer in which soil water content equals or less than
wilting coefficient, the soil layer become dried soil layer\textbf{,}The
dried soil layer happened in the soil layer deeper than maximum
infiltration depth is permanent dried soil layer in which the soil water
cannot be recovered. The permanent dried soil layer reduces the soil
moisture mobility and blocks up the intercourse between soil water in
the soil layer upper and below the permanent dried soil layer. So soil
water management should pay attention to soil water in the soil layers
from surface soil to maximum cumulative infiltration depth.

Plant root water absorption is a process in which plant root and soil
particle contends for soil moisture. Along with plant growth and root
water absorption, soil water content reduces and soil water stress
increase in the soil around root. When the soil water content in a soil
layer reduces to wilting coefficient, the soil water potential in a soil
layer surrounding the root reach balance with the water potential in
plant root cell, and plant cannot absorb the water from the soil layer
anymore. This balance point is wilting coefficient. The relationship
between volumetric soil water content, \emph{w,} and soil water suction,
S, is determined as: \emph{w} = a\emph{S} \textsuperscript{-b}, See
Figure 4. It can be seen that the volumetric soil water content dropping
with the increasing soil water suction from 0.01 Ba, 1Ba =
1x10\textsuperscript{5} Pa , to 15.0 Ba, such as in the 10 cm soil
layer, volumetric soil water content dropping from 38.37\% to 7.98\%
with the increasing soil water suction from 0.01x10\textsuperscript{5}Pa
to 15.0x10\textsuperscript{5} Pa,.

\selectlanguage{english}(Figuer 4 Here\selectlanguage{english})

Where, \selectlanguage{greek}\emph{θ} \selectlanguage{english}is soil water constant and S is soil water suction.
According to the relationship between \selectlanguage{greek}\emph{θ} \selectlanguage{english}and S, the wilting point
at the suction of -15 MPa can be estimated. The determination
coefficient, R\textsuperscript{2}, changes from 0.981 in 140-180 cm soil
layer to 0.991 in the 0-10 cm and 10 -30 cm soil layer. The change of
wilting coefficient with soil depth is shown in the Figure 5. It can be
seen from Figure 5 that field capacity at the suction of -0.33 MPa drops
from 28.11\% in 5 cm to 17.87 \% in 160 cm soil layer and then rises
gradually to 21.82\% in 400 cm with increasing soil depth. The wilting
coefficient at the suction of -15\selectlanguage{ngerman}×10\textsuperscript{5} Pa drops from
7.98 \% in 0-10 cm to 6.68 \% in the 120 cm.

\textbf{3 The Use Limit of Soil Water Resources by red plum apricot}

The state of vertical distribution of soil water in the root soil zone
space influence plant growth. Soil water stress influence root growth
and root water uptake. If the soil water content in the MID is equal to
wilting point and soil water content in the MID is equal to SWRULP, and
there is not enough water supply from precipitation, most red plum
apricot changes the colour of leaf from green to yellow or croci and
leaves fall earlier than usual time in the growing season. Red plum
apricot tree almost ceased growing, and red plum apricot fruit does not
expanding, which influence the yield\selectlanguage{english}, quality and economic benefits of
red plum apricot fruit even if the roots extended to a depth of more
than MID and could absorb some water from the soil layers more than 290
cm deep, suggesting that the total amount water that red plum apricot
roots absorbed from soil per unit time does not satisfy the need of
plant transpiration and fruit development in the semiarid loess hilly
region.

SWRULP is the limit of plants in using soil water resources.~It can be
defined as~the soil water storage in the MID when all of the soil layers
in the MID become~DSLs. The soil depth, representative soil layer and
Wilting coefficient see table1, and , the SWRLP in red plum apricot is
212.7mm, which is different from 252.8 in caragana shrubland at the
middle and the top of the Heici mountain and 220.8 mm in alfafa grass,
showed that the SWRULP changed with site condition and vegetation type.

\textbf{The control of Soil drought and the Use Limit of Soil Water
Resources by red plum apricot.}

As the air temperature increase in the spring, red plum apricot tree
planted in the spring begins to bloom on the last teen-day of March and
the first teen-day of April. Because of low temperature and frost, all
flowers of red plum apricot tree freeze to death on the morning of April
7. Red plum apricot tree germinates on April 30, and then spread and
growth. Because some water irrigation and the precipitation in 2018 is
536.2, is 120.6 mm higher than the average of 415.6 mm, see fig.1. and
the volumetric water content in the 0 to 290cm soil profile is more than
the wilting point, see fig.5 and soil water resources in in the MID is
more than the soil water resources use limit by plant, the red plum
apricot tree grow well. Up to June 16, new vimen grow up to 45 cm. By
the end of the growing season on the October, the width of 1-year-old
tree crown reached up to the range from 100 to 120 with an average 110
cm in width, and the longth of 1-year-old tree crown reached up to the
range from 120cm to 140 cm with average 130 cm, 1-years-old red plum
apricot tree grow well, which lay the foundation for the next years
blooming and fruiting.

\selectlanguage{english}(Fig.5 Here\selectlanguage{english})

The 23-years-old red plum apricot tree start to bloom in the end of
March and the flowers are in full bloom in the first ten-day period of
April. The fruit is in the expansion period from the second ten-day
period of May to the second ten-day period of June and mature in the in
the first ten-day period of July. Unfortunately\selectlanguage{english}, all of flowers wilt
and die because of serious cool temperature and frost on the April 7\selectlanguage{english},
2018. The 23-years-old red plum apricot tree begins to spreading leaf on
April 30, and true leaf develop up to June 16, and grow well, leaf drop
in the end of September because some water irrigation and the
precipitation is high, the volumetric water content in the 0 to 290cm
soil profile is more than the wilting point, and soil water resources in
the MID is more than the soil water resources use limit by plant. The
precipitation changes with time in 2019 (Fig.6),and the 24-years-old red
plum apricot tree grow well and red plum apricot mature because the soil
water resource in the MID is more than SWRULP.

\selectlanguage{english}(Fig.6 Here\selectlanguage{english})

Discussion

Governed by atmospheric demand, soil water and plant characteristics,
plant water relationship is dynamic, complicated, and important to
effective water management, particularly to the soil water management in
the water limited regions, such as Loess plateau in China. When planting
red plum apricot trees, soil water content and the soil water resources
in soil root zone are high because the size of canopy and the root
system of red plum apricot tree is small. As the trees grow, the leaf
area index increases and height growth increases. At the same time, the
amount of soil water took up by plant roots would keep rising, which
could cause rapid decline of soil water content and soil water resources
in the soil root zone even if there are some rise after a rain event,
led to the appearance of soil drying and the dried soil layers in the
soil profile (Guo and Shao 2003; Guo 2014). Because soil drying have
accumulates effect, as the soil drying develop, dried soil layers
developed. When the soil drying develop at stage, the permanent dried
soil layers, the dried soil layers appears in the soil layer below the
MID , the soil drying develop into severe desiccation of soil and red
plum apricot cannot extract enough water from the soil to meet the
transpiration of the plant, which ultimately resulted in soil
degradation and influence the quality and effective of red plum apricot
because permanent dried soil layers may cut off the link between soil
waters in the soil layers upper than MID and soil layers deep than MID,
and affect water circle severely in land (Tian 2010) and sustainable use
of soil water resources.

Because severe desiccation of soil and soil degradation does harm to
ecological and economy benefit of red plum apricot forest and it is not
good for sustainable use of soil water resources and sustainable produce
of red plum apricot forest in water-limited regions, we should interfere
and control the degree red plum apricot forest use soil water, and evade
the severe drying of soil and soil degradation and ensure health of red
plum apricot forest ecosystems in water limited regions. Before control
of soil degradation, we should select a suitable index to difference
severe drying of soil from soil drying before control soil degradation
because soil drought is a natural phenomenon, it often happens and we
have to make plant to accommodate. Severe drying of soil is a disaster,
which causes severe soil degradation and vegetation decline, we have to
control it.

There are some soil water deficit indices, such as crop moisture index
(Palmer 1968) , soil moisture de\selectlanguage{english}ficit index, evapotranspiration de\selectlanguage{english}ficit
index, plant water de\selectlanguage{english}ficit index (Shi and others 2015) .These drought
indices divides into meteorological, hydrological and agricultural
drought index (Mishra and Singh 2010).Because most of the drought
indices are based on meteorological variables (Mckee and others 1993) or
on a water balance equation, they do not account for water de\selectlanguage{english}ficit
accumulation or soil water storage (Martinez-Fernandez and others 2015),
they can not act as a suitable index for distinguishing severe drying of
soil from soil drought phenomenon in the red plum apricot forest in the
water-limited regions because soil drought is a nature phenomenon, a
water de\selectlanguage{english}ficit accumulation or a decrease in soil water storage in a given
soil depth. We have to develop a new index.

Because soil water resources are the soil water storage in soil root
zone and can account for soil accumulation drought (Guo 2014)\selectlanguage{english},we can
use soil water resources in the MID under extreme condition, soil water
use control limit by red plum apricot to express the severe drying of
soil and act as an suitable soil water management index, that is to say,
when the soil depth of DSL equals MID in which soil water resources
equal SWRULP, we reduce soil water use by plant to avoid the form of
permanent dried soil layers in the soil layers below MID.

Digging method can measure the infiltration depth in farmland, but it
cannot be used to determine the depth of infiltration and maximal
infiltration depth in the nature soil profile because it destroy soil
structure. Two curves method was used to estimate the depth of
infiltration and soil water supply for a rain event proposed by Guo
(2004), . A series of two curves methods for maximal infiltration depth
for a long time infiltration process (Guo and Shao 2009, Guo 2014,
2020).

SWRULP is one of the most important criteria for plants to use soil
water rationally (Guo 2010, 2014, 2020,2021a,2021b) , for it integrate
soil depth, infiltration depth, wilting point and soil water management
requirement and better difference the serious drying of soil from light
drying of soil in the forest land. When the soil water resources in the
MID equal to SWRULP, we should estimate the SWCCV , especially SWCCV in
the kay period of plant water relation regulation (Guo 2021a and 2021b)
and regulate the RBSWPG because the environment in which plants are
growing is complex, and roots distribution varies with soil depth, and
plants absorb water from different soil depth at the same time, and soil
water deficit index cannot describe this kind of severe drying of soil.
And soil water in the kay period of plant water relation regulation
seriously affects plant growth and maximum yield and beneficial results
when the soil water resources in the MID equals or smaller than SWRULP.

Conclusion

Soil desiccation is a natural phenomenon and often happens in
water-limited regions. Soil desiccation and infiltration has an
accumulative effect. When soil desiccation accumulates to a limit, which
severely influences the plant growth, causing soil degradation and
threatening the quality and economic benefits of red plum apricot. At
this time, we should regulate the RBSWPG. For better management of soil
water, control of soil degradation and realize sustainable use of soil
water resources and high quality production of red plum apricot, we must
have a better understanding of the difference between soil desiccation
and serious desiccation of soil and determine Soil Water Resources Use
Limit by Plant and SWCCV in the key period of plant water relationship
regulation and prepare to regulate the RBSWPG when soil desiccation
develops to serious desiccation of soil.

The SWRULP can serve as a standard to determine whether or not plant
have excessively used soil water resources as well as the theoretic
foundation to determine start time of regulating RBSWPG. The Soil Water
Resources Use Limit by red plum apricot is 212.7 mm. Because The annual
precipitation in 2018 is 536.2 mm, which is 120.6 mm more than the mean
precipitation 415.6 mm and the soil water resources is more than the
Soil Water Resources Use Limit by red plum apricot of 212.7 mm and red
plum apricot grow well, we do not need to regulating RBSWPG. When the
soil water resources in the MID of 290 cm has reached the limit, the use
of soil water resources by red plum apricot will reach their limit. We
have to consider regulate the RBSWPG and realize sustainable use of soil
water resources and high quality production of red plum apricot in
water-limited regions.

Because the interannual and seasonal variation of precipitation is great
in the study site, and the study of soil desiccation and soil
degradation effect on red plum apricot need to be continue.

Acknowledgements

This project is supported by National key Research \& Development
plan\selectlanguage{english}(Project No. 2016YFC0501702\selectlanguage{english})and the National Science Foundation of
China (Project No: 41071193) .

Additional Information

Data Availability Statement

The data that support the findings of this study are available on
request from the corresponding author. The data are not publicly
available due to privacy or ethical restrictions.

\textbf{Competing Financial Interests statement} :

There is not Competing Financial and non-financial interests

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