CN116298184B - Tracing method for farmland soil evapotranspiration water vapor migration process by utilizing oxyhydrogen isotope - Google Patents
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Abstract
The invention discloses a tracing method for a water vapor migration process of farmland soil transpiration by utilizing oxyhydrogen isotopes, which is characterized in that the oxyhydrogen isotope ratio is analyzed by sampling farmland soil, planted crops and atmospheric water vapor, the isotope ratio of the soil transpiration water vapor and the crop transpiration water vapor is obtained by simulating a water isotope phase change fractionation mechanism, the component ratios of the transpiration water vapor and the soil vapor are calculated, the ratio of the total soil evaporation amount of each sampling period to the surface soil water and the total soil evaporation amount of the planted crops under the film covering condition are quantitatively obtained, then the quantitative evaluation is carried out on the soil evaporation and the crop transpiration amount according to the soil volume water content data, the microscopic water vapor transmission conversion process of bare soil and the in-film soil evaporation and the crop transpiration can be deeply revealed, the component ratios of the water vapor phase change process are quantitatively analyzed, the actual evaporation degree of soil and the actual transpiration degree of planted crops can be reflected more objectively and truly, and a new technology is provided for agricultural water resource evolution mechanism research.
Description
Technical Field
The invention belongs to the field of agricultural water resources, and particularly relates to a tracing method for a farmland soil evapotranspiration water vapor migration process by utilizing oxyhydrogen isotopes.
Background
Soil evaporation is an important link in hydrologic cycle, and is also an important component factor of water balance and heat balance. The accurate understanding of the farmland soil evaporation process has important significance for water circulation of farmland-atmosphere system, agricultural water resource evaluation and management and the like. In northwest arid regions of China, the soil is evaporated strongly, the continuous loss of surface soil moisture can affect the germination and emergence of seeds, and the moisture is an important limiting factor for crop yield. In arid and semiarid regions, in order to reduce soil evaporation loss and improve crop water utilization rate and yield, farmlands are widely covered with films. The physical process of surface soil moisture movement under the film covering condition, such as quantitative research of soil moisture evaporation and water vapor transmission and conversion process, is insufficient, how to accurately quantify soil moisture movement, crop moisture utilization, moisture transmission conversion components and the like under the farmland film covering condition is a problem to be solved in the research of agricultural high-utility water field.
The stable isotope technology is widely applied to hydrologic research, the environment stable isotope can be used as an effective natural tracer, particularly the oxyhydrogen isotope is the composition of water, and the determination method is quick and simple, has low cost and is very suitable for researching hydrologic cycle process. Under the influence of evaporation physical process, isotopes in the water body are fractionated, heavy isotopes in the residual water body are enriched, and lighter isotopes in the vapor are enriched, so that the hydrogen-oxygen isotopes of the evaporated vapor and the evaporated residual water are redistributed, the isotope ratio of the water body is determined by the degree of fractionation in the evaporation and condensation processes, and the dynamic change of the water isotope ratio is caused by the change of the fractionation degree. In the prior art, the quantitative research on the process of condensing under the film and secondarily evaporating to form water vapor after evaporating to form water vapor is less, as in the prior art 1, such as Wu Youjie, the mechanism and simulation [ D ] of the water vapor transmission mechanism of the laminated irrigation farmland SPAC based on stable isotopes are disclosed in the document, the theoretical basis and the model for quantifying the under-film soil evaporation by applying the stable isotope method are disclosed in the document, but the model is based on the isotope steady state on the assumption that the isotope evaporation fractionation change of the water vapor evaporated by the laminated soil and the water vapor evaporated by the farmland crops is not considered, and the change of the atmospheric humidity in the film is not considered, so that the water vapor concentration of the soil surface and the water vapor concentration in the film are not changed. In natural situations, the environmental factor is difficult to keep constant for a long time, so that isotope instability is normal, and therefore, surface soil moisture evaporation research based on isotope steady state is inaccurate. In addition, wu Youjie et al, estimation and differentiation of the evapotranspiration of corn fields based on oxygen isotopes [ J ]. Agricultural engineering journal, 2020, 36 (04), this prior art 2 proposes an estimation and differentiation method of the evapotranspiration of fields based on oxygen isotopes, but this method simulates the evapotranspiration of water vapor of crops by a numerical method, the basis of which is assumed is based on the past observed empirical relationship, and it is difficult to truly reflect the dynamic changes of evapotranspiration of crops at different stages.
Based on the method, the oxyhydrogen isotope ratio of the soil is monitored in real time, the situation of the unstable isotope is considered, the soil moisture is evaporated to form water vapor, then the water vapor is condensed under the film and is evaporated again to form water vapor for quantitative simulation, and the processes of soil moisture movement, crop moisture utilization, water vapor transmission and the like under the condition of farmland film covering can be accurately identified.
Disclosure of Invention
The invention aims to solve the problems that in the prior art, the surface soil moisture evaporation research based on the isotope steady state is inaccurate and the dynamic change of the evaporation of different stages of crops is difficult to truly reflect, and provides a tracing method for the farmland soil evaporation moisture migration process by utilizing oxyhydrogen isotopes, which is suitable for simulating the process that the evaporation of soil moisture forms moisture and then the condensation under a film and the secondary evaporation form the moisture.
The above object of the present invention is achieved by the following technical solutions:
a tracing method for farmland soil evapotranspiration water vapor migration process by utilizing oxyhydrogen isotopes comprises the following steps:
step 1, collecting bare soil outside a film, soil in the film and planted crop samples according to a certain frequency in the growth period stage of crops according to test requirements, and collecting atmospheric water vapor above the bare soil outside the film, above the soil in the film and around the crops on the film; measuring average hydrogen-oxygen isotope ratios of atmospheric water vapor around the bare soil outside the film and the soil water in the film, the crop water, the bare soil above the film, the soil above the film and the crops on the film, simultaneously measuring surface soil volume water contents of the bare soil outside the film and the soil in the film, and calculating the surface soil average volume water contents of the bare soil outside the film and the soil in the film at each growth period;
step 2, analyzing and etching the unbalance degree of the soil evaporation state in and outside the film in each sampling period according to meteorological elements of a test farmland, including temperature and relative humidity;
step 3, respectively simulating isotope ratios of soil evaporation water vapor and crop transpiration water vapor based on the oxyhydrogen isotope ratio of atmospheric water vapor;
step 4, respectively constructing isotope estimation models of soil evaporation outside and inside the film, and calculating the proportion of each component of the soil evaporation moisture phase change process under the conditions of bare soil outside the film and film coating, wherein the proportions comprise the proportion of water vapor formed by surface soil moisture evaporation of bare soil outside the film, the proportion of water vapor formed by surface soil moisture evaporation under the film coating condition, the proportion of condensed water formed by evaporation water vapor under the film coating condition and the proportion of water vapor diffused outside the film formed by secondary evaporation of the condensed water;
step 5, quantifying the ratio of the total soil evaporation amount to the surface soil water and the ratio of the transpiration amount of the planted crops to the total soil evaporation amount in each sampling period under the film covering condition;
and 6, quantitatively evaluating the soil evaporation and crop rising amount according to the soil volume water content data.
Further, in the step 1, the sampling operation of bare soil outside the film and soil inside the film and planted crops is as follows: three sampling points are respectively selected from bare soil outside the membrane and soil inside the membrane for each sampling, and soil with the surface layer of 0-10cm is obtained by using a soil sampler. Three crops are selected in the film, the root and stem parts of the crops with the depth of 5cm below the ground surface are adopted, and soil samples and the crops are respectively put into self-sealing bags for sealing and preservation. The samples of atmospheric water vapor above the farmland include atmospheric water vapor above bare soil outside the film, above soil in the film and around crops on the film, and the specific sampling operations are as follows: and collecting water vapor around the planted crops above bare soil outside the membrane, above soil in the membrane and on the membrane respectively by using a gas collection bag or a multi-channel atmospheric water vapor cold trap, and storing by using a gas sealing bag. All samples were stored at about 0 degrees celsius and protected from light.
In step 1, the moisture in the collected bare soil outside the film, the soil inside the film and the planted crop samples are extracted by adopting a plant soil moisture vacuum extraction system, all the extracted moisture samples are sealed and stored in a refrigerator, the temperature is controlled at about 4 ℃, and then the stable average oxyhydrogen isotope ratio in the bare soil outside the film, the soil inside the film, the planted crop and the atmospheric water vapor samples above farmlands is measured by using a liquid water isotope analyzer.
The stable oxyhydrogen isotope ratio in the bare soil outside the film and the soil inside the film, the planted crops and the atmospheric water vapor sample above the farmland comprises delta 18 O and delta 2 H, the following calculation can be performed using either or both of the isotopic compositions.
Further, in the step 2, the method for describing the unbalance degree of the soil evaporation state outside and inside the membrane in each sampling period is as follows:
1) Calculating the fractional distillation value of the vapor evaporation limit isotope above bare soil outside the membrane
Where h is the average air relative humidity, delta, for the sampling period A The ratio of the atmospheric oxyhydrogen isotopes above the bare soil outside the membrane;
α + is a function of the temperature T (c),
α + ( 18 O)＝exp[-7.685/10 -3 +6.7123/(273.15+T)-1666.4/(273.15+T) 2 +350410/(273.15+T) 3 ])
α + ( 2 H)＝exp[1158.8(273.15+T) 3 /10 12 )-1620.1×((273.15+T) 2 /10 9 )+794.84((273.15+T)/10 6 )-161.04/10 3 +2999200/(273.15+T) 3 ]
ε + is the equilibrium separation coefficient of isotope epsilon + ＝α + -1；
ε K Is the isotope power fractionation coefficient,
ε K ( 18 O)＝0.0277(1-h)
ε K ( 2 H)＝0.0245(1-h)
calculating the fractional distillation value of water vapor evaporation limit isotope above soil in membrane
h′＝h×W 2 /W 1
Wherein h 'is the average air relative humidity in the membrane at this stage, delta' A Is the ratio of atmospheric hydrogen and oxygen isotopes, W, above the soil in the film 1 、W 2 The water content of the soil surface layer soil volume of bare soil outside the membrane and in the membrane at the stage is respectively.
2) Calculating the water vapor balance isotope fractionation value delta above the bare soil outside the membrane S1 In-film vapor balance isotope fractionation value delta S2 ：
δ S1 ＝α + ×δ topsoil1 (1-h+ε K )+α + hδ A +α + ε K +ε +
δ S2 ＝α + ×δ topsoil2 (1-h′+ε K )+α + h′δ′ A +α + ε K +ε +
Wherein delta topsoil1 Average hydrogen-oxygen isotope ratio, delta, of bare soil sample outside membrane topsoil2 Is the average oxyhydrogen isotope ratio of the soil sample on the inner surface layer of the membrane.
3) Respectively calculating bare soil outside the membrane and soil steam inside the membraneDegree of imbalance sigma of hair state 1 、σ 2 ：
Further, in the step 3, the isotope composition of the soil evaporation water vapor and the crop transpiration water vapor is respectively simulated based on the oxyhydrogen isotope ratio of the atmospheric water vapor, and the method is as follows:
isotope ratio delta for evaporated water vapor above bare soil outside the membrane E1 ：
δ E1 ＝σ 1 ×((δ topsoil1 -ε + )/α + -hδ A -ε K )/(1-h+10 -3 ε K )
Isotopic composition delta for vaporized moisture above soil in a film E2 ：
δ E2 ＝σ 2 ×((δ topsoil2 -ε + )/α + -h′δ′ A -ε K )/(1-h′+10 -3 ε K )
Vapor isotope ratio delta for transpiration of crop plants T ：
δ T ＝(1-σ)δ ET -σδ P
Wherein delta ET Is the average hydrogen-oxygen isotope ratio delta of the atmospheric water vapor around crops on the film P The average hydrogen-oxygen isotope ratio of the root and stem moisture of the crops planted on the film.
Further, in the step 4, isotope estimation models of soil evaporation outside the film and inside the film are respectively constructed, and the proportion of each component of the soil evaporation moisture phase change process under the conditions of bare soil outside the film and the film coating is calculated, wherein the proportions comprise the proportion of water vapor formed by surface soil moisture evaporation of bare soil outside the film, the proportion of water vapor formed by surface soil moisture evaporation under the film coating, the proportion of condensed water formed by evaporation water vapor under the film coating and the proportion of water vapor diffusion outside the film formed by secondary evaporation of the condensed water. The specific calculation method is as follows:
m＝(h-10 -3 ×(ε K +ε + /α + ))/(1-h+10 -3 ε K )
wherein f 0 The proportion of water vapor formed by evaporating the soil moisture on the surface layer of bare soil outside the membrane; delta topsoil10 The average hydrogen-oxygen isotope ratio of the bare soil surface soil (0-10 cm) sample in the previous sampling stage; m is the isotope fractionation effect coefficient caused by evaporation of bare soil outside the membrane at this stage.
In the same way, the processing method comprises the steps of,
m′＝(h′-10 -3 ×(ε K +ε + /α + ))/(1-h′+10 -3 ε K )
wherein f 1 The proportion of water vapor formed by evaporating the surface soil moisture under the film covering condition; delta topsoil20 The average oxyhydrogen isotope ratio of the surface soil (0-10 cm) sample under the film covering condition of the previous sampling stage. m' is the isotope fractionation effect coefficient caused by surface soil evaporation under the film covering condition of the stage.
Solving the following ratio f of evaporated water vapor under the film forming condensed water under the film covering condition 2 ：
Wherein alpha is L-V Is liquid-gas fractionationCoefficients.
Solving the proportion f of water vapor diffusion out of the membrane formed by secondary evaporation of condensed water 3 ：
Inf 3 ＝(δ topsoil2 -δ topsoi20 )/10 3 (α L-V -1)
Further, in the step 5, the ratio of the total soil evaporation amount to the surface soil water and the ratio of the transpiration amount of the planted crops to the total soil evaporation amount in each sampling period under the film covering condition are quantified, and the specific method is as follows:
the total evaporation amount of soil accounts for the proportion FE of the surface soil water under the film covering condition:
FE＝f 1 ×(1-f 2 )×f 3
the ratio FT of the transpiration of the planted crops to the total evaporation of the soil:
FT＝(δ ET -δ E2 )/(δ ET -δ T )
further, in the step 6, the soil evaporation amount and crop rising amount are quantitatively evaluated according to the soil volume water content, and the specific method is as follows:
E1＝f 0 ×W 1
E2＝FE×W 2
T＝FT×E 2
wherein E1 is the evaporation capacity of bare soil outside the film in unit area, E2 is the evaporation capacity of soil in unit area under the condition of film covering, and T is the transpiration capacity of crops in unit area.
Compared with the prior art, the invention has the beneficial effects that:
the hydrogen-oxygen isotope ratio is analyzed by sampling farmland soil, planted crops and atmospheric water vapor, and the evaporation of farmland soil can be quantitatively inverted by utilizing a water isotope phase-change fractionation mechanism. Compared with the prior art 1 in the background art, which establishes the isotope evaporation fractionation change of the film-covered soil evaporation vapor and the farmland crop evaporation vapor in an isotope steady state, the invention describes and describes the isotope composition of the evaporation vapor in the soil outside and inside the film, and can more objectively and truly reflect the actual evaporation degree of the soil. In addition, the prior art 1 does not give out how to calculate the isotope composition of the water vapor evaporated from the water in the film, and the soil evaporation ratio in a certain period needs to be roughly determined by the change of the hydrogen-oxygen isotope ratio of the soil at the beginning. In fact, this operation is to attribute all the causes of the change in the oxyhydrogen isotope ratio of soil moisture to the fractionation of soil evaporation, which is contradictory to the basic equation of soil water balance, ignoring the change in the oxyhydrogen isotope ratio of soil due to the water absorption of the planted crop. According to the invention, the isotope ratio of soil evaporation water vapor and crop transpiration water vapor is obtained through simulation by measuring the hydrogen-oxygen isotope ratio of atmospheric water vapor, the proportion of each component of the soil evaporation water vapor phase change process under the conditions of bare soil outside a film and the film is calculated, the proportion of the total soil evaporation amount in each sampling period to the surface soil water under the conditions of the film and the ratio of the total plant crop transpiration amount to the total soil evaporation amount are quantitatively obtained, then the soil evaporation and crop transpiration amount are quantitatively evaluated according to the data of the water content of the soil volume, the microscopic water transfer conversion process of the bare soil and the soil evaporation in the film and the crop transpiration can be deeply revealed, the proportion of each component of the water vapor phase change process is quantitatively analyzed, and a new technology is provided for agricultural water resource evolution mechanism research.
Compared with the prior art 2, the method for further correcting the transpiration vapor isotope ratio of the planted crops by using the oxyhydrogen isotope ratio of the planted crops and the unbalanced degree of vapor fractionation under the actual vapor condition is more beneficial to correctly reflecting the actual transpiration degree of the planted crops because the basis of the simulation of the empirical formula is established on the basis of the past observed empirical relationship, the dynamic change of the transpiration of different stages of the crops is difficult to be truly reflected, and the transpiration needs to be continuously measured by a box-type system, so that the method is more ideal and is difficult to be realized in practical application.
Drawings
FIG. 1 shows the bare soil outside the film and the soil moisture under the film, the moisture of the planted crops, the bare soil above the film, the inside the film and the periphery of the crops in the exampleDelta of atmospheric vapor 18 And an O box diagram.
Fig. 2 is a bar graph showing evaporation of farmland soil (evaporation amount E1 per unit area of bare soil outside the film, evaporation amount E2 per unit area of soil under the film-covered condition) and crop emission amount (evaporation amount T per unit area of planted crops) at each stage of the whole growth period in the example.
Detailed Description
The technical solutions of the present invention will be clearly and completely described in connection with the embodiments, and it is obvious that the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by one of ordinary skill in the art without undue burden on the person of ordinary skill in the art based on embodiments of the present invention, are within the scope of the present invention.
Example 1
Taking sunflower test field of the permanent grain base in Wuyuan county of inner Mongolia as an example, soil outside and inside the film of the test field, sunflower crops and atmospheric water vapor in farmland are sampled and isotopically analyzed in the growing period of the sunflower in the period of 6 to 9 months of 2021, and meteorological elements and soil moisture characteristics are monitored.
A tracing method for farmland soil evapotranspiration water vapor migration process by utilizing oxyhydrogen isotopes comprises the following steps:
step 1, dividing the total growth period of crops into 4 periods: the method comprises the steps of collecting bare soil outside a film, soil inside the film and planted crop samples in each growth period, and collecting atmospheric water vapor above the bare soil outside the film, above the soil inside the film and around the crops on the film. The hydrogen-oxygen isotope ratio of atmospheric water vapor around the crops on the outside of the film, the water of the soil in the film, the water of the planted crops, the water of the soil above the outside of the film, the soil above the inside of the film and the soil in the film in each growth period is measured, and the average volume water content of the surface soil of the outside of the film, the bare soil in the film and the soil in the film is measured.
Soil and crop sampling operations are as follows: three sampling points are selected from bare soil outside the membrane and soil inside the membrane respectively for each sampling, and soil with the surface layer of 0cm to 10cm is sampled by a soil sampler; selecting three sunflower stems under the film, and taking root stems of crops with depth of 5cm below the ground surface; and respectively filling the soil samples and the planted crops into self-sealing bags for sealing and storing.
The sampling operation of the atmospheric water vapor above the farmland is as follows: and collecting water vapor around the crops planted on the outside of the membrane, the inside of the membrane and the soil by adopting a multi-channel atmospheric water vapor cold trap, and storing the water vapor by using a gas sealing bag. The preservation conditions of all soil, planted crops and water vapor samples are about 0 ℃ and are protected from light.
Extracting water in collected bare soil outside the film, soil in the film and planted crop samples by a plant soil water vacuum extraction system, sealing the extracted water samples, storing the water samples in a refrigerator, controlling the temperature to be about 4 ℃, and measuring average hydrogen-oxygen isotope ratios in the bare soil outside the film, the soil in the film, the planted crop and the atmospheric water vapor samples above farmlands by a liquid water isotope analyzer (Picaro, L-2130i, U.S.) by delta only shown here 18 The following calculation was performed as the moisture-stable oxyhydrogen isotope ratio of O, as shown in fig. 1.
And 2, analyzing and etching the imbalance degree of the soil evaporation states in and out of the film in four periods according to meteorological factors of a test farmland, including temperature and relative humidity.
The method for describing the unbalance degree of the soil evaporation state outside and inside the membrane in each sampling period is as follows:
1) Calculating the fractional distillation value of the vapor evaporation limit isotope above bare soil outside the membrane
Where h is the average atmospheric relative humidity, delta, over the bare soil for the sampling period A Is composed of an atmospheric oxygen isotope above bare soil,
α + is a function of the temperature T (c),
α + ( 18 O)＝exp[-7.685/10 -3 +6.7123/(273.15+T)-1666.4/(273.15+T) 2 +350410/(273.15+T) 3 ])
ε + is the equilibrium separation coefficient of isotope epsilon + ＝α + -1
ε K Is the isotope power fractionation coefficient epsilon K Is a function of the relative humidity h and,
ε K ( 18 O)＝0.0277(1-h)
secondly, calculating the fractional distillation value of the water vapor evaporation limit isotope in the membrane
h′＝h×W 2 /W 1
Wherein h 'is the average air relative humidity in the membrane at this stage, delta' A Is the hydrogen-oxygen isotope ratio, W, of the atmosphere above the soil in the film 1 、W 2 The average volume water contents of bare soil outside the membrane and surface soil inside the membrane at the stage are respectively.
2) Calculating the water vapor balance isotope fractionation value delta above the bare soil outside the membrane or in the membrane S ：
δ S1 ＝α + ×δ topsoil1 (1-h+ε K )+α + hδ A +α + ε K +ε +
δ S2 ＝α + ×δ topsoil2 (1-h′+ε K )+α + h′δ′ A +α + ε K +ε +
Wherein delta topsoil1 、δ topsoil2 Average hydrogen-oxygen isotope ratios of bare soil and membrane inner surface soil (0-10 cm) samples, respectively.
3) Calculating the unbalance degree sigma of the evaporation state of bare soil outside the membrane and soil inside the membrane respectively 1 、σ 2 ：
And 3, respectively simulating isotope ratios of soil evaporation water vapor and transpiration water vapor based on oxyhydrogen isotope ratios of atmospheric water vapor, wherein the method comprises the following steps:
for evaporated water vapor delta over bare soil outside the membrane E1 ：
δ E1 ＝σ×((δ topsoil1 -ε + )/α + -hδ A -ε K )/(1-h+10 -3 ε K )
Wherein delta topsoil The average oxyhydrogen isotope ratio of the sample of bare soil surface soil (0-10 cm) outside the membrane.
For evaporated water vapor delta above the soil in the film E2 ：
δ E2 ＝σ×((δ topsoil2 -ε + )/α + -h′δ′ A -ε K )/(1-h′+10 -3 ε K )
Wherein delta topsoil Is the average oxyhydrogen isotope ratio of a surface soil (0-10 cm) sample under the condition of film coating.
Transpiration water vapor delta for planted crops T ：
δ T ＝(1-σ)δ ET -σδ P
Wherein delta ET Average hydrogen-oxygen isotope ratio, delta, for water vapor collection around crops on film P The average hydrogen-oxygen isotope ratio of the root and stem moisture of the crops planted on the film.
And 4, respectively constructing isotope estimation models of soil evaporation outside the film and inside the film, and calculating the proportion of each component of the soil evaporation moisture phase change process under the conditions of bare soil outside the film and film coverage, wherein the proportion comprises the proportion of water vapor formed by surface soil moisture evaporation of bare soil outside the film, the proportion of water vapor formed by surface soil moisture evaporation under the film coverage, the proportion of condensed water formed by evaporation water vapor under the film coverage and the proportion of water vapor diffused outside the film formed by secondary evaporation of the condensed water.
The specific calculation method for the water vapor ratio formed by evaporating the soil moisture on the surface layer of bare soil outside the film is as follows:
m＝(h-10 -3 ×(ε K +ε + /α + ))/(1-h+10 -3 ε K )
wherein f 0 The proportion of water vapor formed by evaporating the soil moisture on the surface layer of bare soil outside the membrane; delta topsoil10 The average oxygen isotope ratio of the bare soil surface soil (0-10 cm) sample in the previous sampling stage.
In the same way, the processing method comprises the steps of,
m′＝(h′-10 -3 ×(ε K +ε + /α + ))/(1-h′+10 -3 ε K )
wherein f 1 The proportion of water vapor formed by evaporating the surface soil moisture under the film covering condition; delta topsoil20 The average oxygen isotope ratio of the surface soil (0-10 cm) sample under the film covering condition of the previous sampling stage.
Solving the following ratio f of evaporated water vapor under the film forming condensed water under the film covering condition 2 ：
Wherein alpha is L-V Is the liquid-gas fractionation coefficient.
Solving the following formula to obtain condensation water which is secondarily evaporated to form water vapor to be diffused intoProportion f outside the film 3 ：
Inf 3 ＝(δ topsoil2 -δ topsoi20 )/10 3 (α L-V -1)
And 5, quantifying the ratio of the total soil evaporation amount to the surface soil water and the ratio of the crop transpiration amount to the total soil evaporation amount under the film covering condition.
The proportion FE of the total soil evaporation amount in each period to the surface soil water under the film covering condition is quantified by the following formula:
FE＝f 1 ×(1-f 2 )×f 3
the ratio FT of the transpiration of the planted crops to the total evaporation of the soil:
FT＝(δ ET -δ E2 )/(δ ET -δ T )
the quantization results are shown in table 1:
TABLE 1 proportion of each component in the phase transition process of soil evaporation and crop transpiration under bare soil and film coating conditions
And 6, quantitatively evaluating the soil evaporation and crop emission according to the soil water content data.
E1＝f 0 ×W 1
E2＝FE×W 2
T＝FT×E 2
Wherein E1 is the evaporation capacity of bare soil outside the film in unit area, E2 is the evaporation capacity of soil in unit area under the condition of film covering, and T is the transpiration capacity of crops in unit area. Resulting in evaporation of farmland soil and crop emission at each stage of the total growth period, as shown in figure 2.
Although the basic principles, main features and advantages of the present invention have been shown and described, it will be understood by those skilled in the art that the present invention is not limited to the foregoing embodiments, but that various changes, modifications, substitutions and alterations can be made therein without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.
Claims (7)
1. The tracing method for the farmland soil evapotranspiration water vapor migration process by utilizing the oxyhydrogen isotope is characterized by comprising the following steps of:
step 1, collecting bare soil outside a film, soil in the film and planted crop samples according to a certain frequency in the growth period stage of crops according to test requirements, and collecting atmospheric water vapor above the bare soil outside the film, above the soil in the film and around the crops on the film; measuring average hydrogen-oxygen isotope ratios of atmospheric water vapor around the bare soil outside the film and the soil water in the film, the crop water, the bare soil above the film, the soil above the film and the crops on the film, simultaneously measuring surface soil volume water contents of the bare soil outside the film and the soil in the film, and calculating the surface soil average volume water contents of the bare soil outside the film and the soil in the film at each growth period;
step 2, analyzing and etching the unbalance degree of the soil evaporation state in and outside the film in each sampling period according to meteorological elements of a test farmland, including temperature and relative humidity; the method for describing the unbalance degree of the soil evaporation state outside and inside the membrane in each sampling period is as follows:
1) Calculating the fractional distillation value of the vapor evaporation limit isotope above bare soil outside the membrane
Where h is the average air relative humidity, delta, for the sampling period A Is composed of atmospheric oxyhydrogen isotope above bare soil, alpha + Is a function of temperature T (. Degree. C.);
α + ( 18 O)＝exp[-7.685/10 -3 +6.7123/(273.15+T)-1666.4/(273.15+T) 2 +350410/(273.15+T) 3 ]
α + ( 2 H)＝exp[1158.8×((273.15+T) 3 /10 12 )-1620.1×((273.15+T) 2 /10 9 )+794.84×((273.15+T)/10 6 )-161.04/10 3 +2999200/(273.15+T) 3 ]
ε + is the equilibrium separation coefficient of isotope epsilon + ＝α + -1；
ε K Is the isotope power fractionation coefficient,
ε K ( 18 O)＝0.0277(1-h)
ε K ( 2 H)＝0.0245(1-h)
calculating the fractional distillation value of the water vapor evaporation limit isotope in the membrane
h′＝h×W 2 /W 1
Wherein h 'is the average air relative humidity in the membrane at this stage, delta' A Is composed of oxyhydrogen isotopes of atmosphere above soil in the film, W 1 、W 2 The average volume water content of the bare soil outside the membrane and the soil on the inner surface layer of the membrane at the stage are respectively;
2) Calculating the water vapor balance isotope fractionation value delta above the bare soil outside the membrane S1 In-film vapor balance isotope fractionation value delta S2 ：
δ S1 ＝α + ×δ topsoil1 (1-h+ε K )+α + hδ A +α + ε K +ε +
δ S2 ＝α + ×δ topsoil2 (1-h′+ε K )+α + h′δ A ′+α + ε K +ε +
Wherein delta topsoil1 Average hydrogen-oxygen isotope ratio, delta, of bare soil sample outside membrane topsoil2 The average oxyhydrogen isotope ratio of the soil sample on the inner surface layer of the membrane;
3) Calculating the unbalance degree sigma of the evaporation state of bare soil outside the membrane and soil inside the membrane respectively 1 、σ 2 ：
Step 3, respectively simulating isotope ratios of soil evaporation water vapor and crop transpiration water vapor based on the oxyhydrogen isotope ratio of atmospheric water vapor;
step 4, respectively constructing isotope estimation models of soil evaporation outside and inside the film, and calculating the proportion of each component of the soil evaporation moisture phase change process under the conditions of bare soil outside the film and film coating, wherein the proportions comprise the proportion of water vapor formed by surface soil moisture evaporation of bare soil outside the film, the proportion of water vapor formed by surface soil moisture evaporation under the film coating condition, the proportion of condensed water formed by evaporation water vapor under the film coating condition and the proportion of water vapor diffused outside the film formed by secondary evaporation of the condensed water;
step 5, quantifying the ratio of the total soil evaporation amount to the surface soil water and the ratio of the transpiration amount of the planted crops to the total soil evaporation amount in each sampling period under the film covering condition;
and 6, quantitatively evaluating the soil evaporation and crop rising amount according to the soil volume water content data.
2. The tracing method for farmland soil transpiration water vapor migration process using hydrogen and oxygen isotopes of claim 1, wherein the specific practice of collecting bare soil outside the film, soil inside the film and planted crop samples in step 1 is as follows: when sampling is carried out each time, three sampling points are respectively selected from bare soil outside the membrane and soil inside the membrane, and soil with the surface layer of 0-10cm is obtained by using a soil sampler; three crops are selected in the film, the root and stem parts of the crops with the depth of 5cm below the ground surface are adopted, and soil samples and the crops are respectively put into self-sealing bags for sealing and preservation; and collecting water vapor around the planted crops above bare soil outside the membrane, above soil in the membrane and on the membrane respectively by using a gas collection bag or a multi-channel atmospheric water vapor cold trap, and storing by using a gas sealing bag.
3. The tracing method for farmland soil transpiration water vapor migration process using oxyhydrogen isotopes according to claim 1, wherein the specific practice of determining the oxyhydrogen isotope ratio of bare soil outside the film to soil water inside the film, water for planted crops, above bare soil outside the film, above soil inside the film and atmospheric water vapor around crops on the film in step 1 is as follows: extracting water in collected bare soil outside the film, soil in the film and planted crop samples by adopting a plant soil water vacuum extraction system, sealing all the extracted water samples, storing the water samples in a refrigerator, controlling the temperature to be about 4 ℃, and measuring average hydrogen-oxygen isotope ratios in the bare soil outside the film, the soil in the film, planted crops, the bare soil above the film, the soil above the film and atmospheric water vapor samples around the crops on the film by using a liquid water isotope analyzer.
4. The tracing method for the water vapor migration process of farmland soil transpiration using hydrogen and oxygen isotopes according to claim 1, wherein in the step 3, the isotope ratio of soil transpiration water vapor and crop transpiration water vapor is simulated based on the hydrogen and oxygen isotope ratio of atmospheric water vapor, respectively, and the method comprises the following steps:
isotope ratio delta for evaporated water vapor above bare soil outside the membrane E1 ：
δ E1 ＝σ×((δ topsoil1 -ε + )/α + -hδ A -ε K )/(1-h+10 -3 ε K )
Isotope ratio delta for vaporized moisture above soil in film E2 ：
δ E2 ＝σ×((δ topsoil2 -ε + )/α + -h′δ′ A -ε K )/(1-h′+10 -3 ε K )
Vapor isotope ratio delta for transpiration of crop plants T ：
δ T ＝(1-σ)δ ET -σδ P
Wherein delta ET Average hydrogen-oxygen isotope ratio, delta, for water vapor collection around crops on film P The average hydrogen-oxygen isotope ratio of the root and stem moisture of the crops planted on the film.
5. The tracing method for farmland soil transpiration water vapor migration process using hydrogen and oxygen isotopes of claim 4, wherein the calculation method for the proportion of water vapor formed by evaporating the water in the surface soil of bare soil outside the film in the step 4 is as follows:
m＝(h-10 -3 ×(ε K +ε + /α + ))/(1-h+10 -3 ε K )
wherein f 0 The proportion of water vapor formed by evaporating the soil moisture on the surface layer of bare soil outside the membrane; delta topsoil10 The average hydrogen-oxygen isotope ratio of a sample of 0-10cm of bare soil surface soil in the previous sampling stage;
the calculation method of the proportion of water vapor formed by evaporating the surface soil moisture under the film covering condition is as follows:
m′＝(h′-10 -3 ×(ε K +ε + /α + ))/(1-h′+10 -3 ε K )
wherein f 1 The proportion of water vapor formed by evaporating the surface soil moisture under the film covering condition; delta topsoil20 The average oxyhydrogen parity of 0-10cm sample of surface soil under the film covering condition of the previous sampling stageA prime ratio;
the proportion calculation method for evaporating water vapor under the film to form condensed water under the film covering condition is as follows:
wherein alpha is L-V Is the liquid-gas fractionation coefficient;
the proportion calculation method for the water vapor diffusion out of the membrane formed by the secondary evaporation of the condensed water is as follows:
Inf 3 ＝(δ topsoil2 -δ topsoi20 )/10 3 (α L-V -1)。
6. the tracing method for farmland soil vapor migration process using hydrogen and oxygen isotopes of claim 5, wherein in said step 5, the total amount of soil evaporation is the proportion FE of surface soil water under the condition of film coating:
FE＝f 1 ×(1-f 2 )×f 3
the ratio FT of the transpiration of the planted crops to the total evaporation of the soil:
FT＝(δ ET -δ E2 )/(δ ET -δ T )。
7. the tracing method for farmland soil vapor migration process using hydrogen and oxygen isotopes of claim 6, wherein in said step 6, the soil evaporation and crop emission are quantitatively evaluated according to the data of soil volume water content, and the specific method is as follows:
E1＝f 0 ×W 1
E2＝FE×W 2
T＝FT×E 2
wherein E1 is the evaporation capacity of bare soil outside the film in unit area, E2 is the evaporation capacity of soil in unit area under the condition of film covering, and T is the transpiration capacity of crops in unit area.
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