Method for estimating actual evapotranspiration in a desert photovoltaic power station area

Abstract

The application discloses a kind of desert photovoltaic power station area actual evapotranspiration estimation method, steps include:1) collection relevant data;2) calculate daily scale reference evapotranspiration;3) calculate monthly scale reference evapotranspiration ET o ;4) calculate monthly scale soil water storage change ΔS;5) calculate monthly scale precipitation P;6) calculate monthly scale actual evapotranspiration ET;7) calculate crop coefficient K c ;8) calculate daily scale actual evapotranspiration ET, namely, this application method, only need meteorological observation equipment and soil observation equipment to collect basic data, can accurately obtain the dynamic characteristics of daily actual evapotranspiration on long time sequence;It can also monitor meteorological and soil element data simultaneously, facilitate overall water balance analysis, to clarify the water cycle process in the background of climate warming in northwest arid region, to provide strong support for further exploration of arid region water cycle research.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of desert evapotranspiration measurement, and relates to a method for estimating actual evapotranspiration in a desert photovoltaic power station area. BACKGROUND

[0002] As an important part of the core process of the climate system, evapotranspiration is an important link between vegetation, soil and climate, and has a close relationship with agriculture, hydrology, meteorology and ecological environment. The accurate monitoring of evapotranspiration is a common scientific problem in the research of geophysics, biology and even environmental processes. As an interface phase change phenomenon, in the vertical direction, the atmospheric state is the upper boundary condition of the evapotranspiration process, and the soil or vegetation constitutes the lower boundary condition, involving the atmospheric boundary layer, the surface cover condition and the water state change and other processes. In recent years, the growing demand for low-carbon energy has promoted the development of solar photovoltaic energy. Because the construction of photovoltaic parks requires a large amount of available land and strong total solar radiation, most photovoltaic parks are built in semi-arid and arid regions. At present, the evapotranspiration characteristics of photovoltaic power station areas are still poorly understood.

[0003] The prior art uses Bowen ratio energy balance method, soil moisture balance method, aerodynamic method, eddy correlation method and other monitoring methods to analyze the evapotranspiration of farmland, grassland and forest ecosystems, mainly focusing on the dynamic change of evapotranspiration and the influencing factors, and some studies compare the evapotranspiration calculation and determination methods in different regions.

[0004] However, the above evapotranspiration observation methods have different degrees of limitations. For example, the calculation accuracy of the Bowen ratio energy balance method depends on the accuracy of temperature difference and humidity difference observation, and requires obvious temperature and humidity gradient distribution, uniform underlying surface and no influence of advection. The water balance method is relatively simple to calculate, and the estimation is relatively accurate in arid and complex terrain areas, but the estimation uncertainty is large at short time scales. The aerodynamic method has empirical parameters in the flux-profile relationship, which can easily lead to deviations in the calculation results. The eddy correlation method has good data continuity and stability, but the instrument cost is high, and there is a problem of energy non-closure. In addition, in different ecological systems, the evapotranspiration is often affected by the interaction of multiple complex factors, and most studies only focus on a single influencing factor, which is far from enough. The all-weather real-time monitoring of multiple ecological elements is also one of the key problems to be solved for further research on evapotranspiration dynamics. SUMMARY

[0005] The purpose of the present application is to provide a method for estimating actual evapotranspiration in a desert photovoltaic power station area, which solves the problem of insufficient reliability and accuracy of detection and calculation results caused by the fact that the existing evapotranspiration observation methods do not comprehensively observe the evapotranspiration data in arid regions, the selected methods are not appropriate, and the observation time is discontinuous.

[0006] The technical scheme adopted by the present application is a method for estimating actual evapotranspiration in a desert photovoltaic power station area, based on a hardware setting and a calculation model, and implemented according to the following steps:

[0007] Step 1, collecting relevant data,

[0008] Step 2, calculating daily reference evapotranspiration ETo day ,

[0009] Step 3, calculating monthly reference evapotranspiration ETo month ,

[0010] The daily reference evapotranspiration ETo day of each month is added to obtain the monthly reference evapotranspiration ETo month .

[0011] Step 4, calculating the change in monthly soil water storage ΔS,

[0012] Step 5, calculating the monthly precipitation P,

[0013] The monthly precipitation P is obtained by adding the precipitation measured by the rain gauge in a month;

[0014] Step 6, calculating the monthly actual evapotranspiration ET month ,

[0015] Step 7, calculating the crop coefficient K c ,

[0016] Step 8, calculating the daily actual evapotranspiration ET day , i.e.

[0017] The present application has the beneficial effects that, compared with the existing evapotranspiration observation method, the dynamic characteristics of the daily actual evapotranspiration over a long time sequence can be accurately obtained, which provides strong support for further exploration of water cycle research in arid regions; at the same time, only meteorological observation equipment and soil observation equipment are needed to collect basic data, without the need for other complex evapotranspiration equipment to participate in observation; in addition, the present application method can also monitor meteorological and soil element data at the same time while obtaining actual evapotranspiration data, which is convenient for overall water balance analysis. Therefore, based on meteorological data and water balance principle, the present application method can completely estimate the daily actual evapotranspiration in a desert photovoltaic power station area, and realize high-quality acquisition of actual evapotranspiration data over a long time sequence in the field, so as to clarify the water cycle process in the arid region of northwest China under the background of climate warming. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is a flow chart of the present application method for estimating daily actual evapotranspiration based on multi-ecological parameter observation;

[0019] Figure 2 is a schematic diagram of a hardware device arrangement for actual evapotranspiration on a daily scale used in the method of the present application;

[0020] Figure 3 is a flow chart of data processing of actual evapotranspiration on a daily scale in the method of the present application;

[0021] Figure 4 is a comparative schematic diagram of actual evapotranspiration on a daily scale in a desert area in May 2022 calculated by the estimation method of actual evapotranspiration and the eddy correlation method respectively in Embodiment 1 of the present application;

[0022] Figure 5 is a curve of daily net radiation, air temperature, reference evapotranspiration and actual evapotranspiration in a desert area in May 2022 in Embodiment 1 of the present application;

[0023] Figure 6 is a comparative schematic diagram of actual evapotranspiration on a daily scale in a desert area in July 2022 calculated by the estimation method of actual evapotranspiration and the eddy correlation method respectively in Embodiment 2 of the present application;

[0024] Figure 7 is a curve of daily net radiation, air temperature, reference evapotranspiration and actual evapotranspiration in a desert area in July 2022 in Embodiment 2 of the present application;

[0025] Figure 8 is a comparative schematic diagram of actual evapotranspiration on a daily scale in a desert area in September 2022 calculated by the estimation method of actual evapotranspiration and the eddy correlation method respectively in Embodiment 3 of the present application;

[0026] Figure 9 is a curve of daily net radiation, air temperature, reference evapotranspiration and actual evapotranspiration in a desert area in September 2022 in Embodiment 3 of the present application.

[0027] In the figure, 1. soil moisture sensor, 2. soil heat flux plate, 3. rain gauge, 4. wind speed sensor, 5. air temperature and humidity sensor, 6. net radiation sensor, 7. atmospheric pressure sensor, 8. data collector. DETAILED DESCRIPTION

[0028] The present application will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] Reference Figure 2The hardware part relied on by the method of the present application mainly comprises a soil moisture sensor 1, a soil heat flux plate 2, a rain gauge 3, a wind speed sensor 4, an air temperature and humidity sensor 5, a net radiation sensor 6, an atmospheric pressure sensor 7 and a data collector 8, the relevant data are acquired in real time by the soil moisture sensor 1, the soil heat flux plate 2, the rain gauge 3, the wind speed sensor 4, the air temperature and humidity sensor 5, the net radiation sensor 6 and the atmospheric pressure sensor 7 respectively, and the data collector 8 is used for collecting, calculating and storing data;

[0030] The soil moisture sensor 1 is installed in the measured soil at certain intervals on a vertical gradient to monitor the soil moisture of multiple gradients and acquire dynamic data of the soil moisture content in real time all day round;

[0031] The soil heat flux plate 2 is installed in the soil of the monitoring area of the photovoltaic power station to acquire the value of the soil heat flux of the monitoring area in real time;

[0032] The rain gauges 3 are uniformly distributed on the ground of the monitoring area of the photovoltaic power station, the rainfall enters the mechanical device through the funnel, and when the rainfall fills to the calibration line, the rainwater is automatically tilted and poured out, the increment of 0.1mm rainfall can be accurately measured, and the precipitation of the monitoring area is acquired;

[0033] The wind speed sensor 4 is arranged at a position 2m above the vegetation of the monitoring area of the photovoltaic power station to acquire the real-time wind speed 2m above the observed vegetation;

[0034] The air temperature and humidity sensor 5 is arranged at a position 2m above the vegetation of the monitoring area of the photovoltaic power station to acquire the air temperature data and the relative humidity data at a position 2m above the observed vegetation area;

[0035] The net radiation sensor 6 is arranged above the vegetation of the monitoring area of the photovoltaic power station to acquire the net radiation data of the observed vegetation area;

[0036] The atmospheric pressure sensor 7 is used for measuring the atmospheric pressure of the monitoring area of the photovoltaic power station;

[0037] All the sensors are respectively connected with the data collector 8 through data lines, and the data collector 8 calculates and stores the evapotranspiration data in real time after collecting the real-time observation data measured by each sensor.

[0038] Reference Figure 3 The calculation model of the evapotranspiration data calculated by the method of the present application is as follows:

[0039] Firstly, the daily scale reference evapotranspiration ET is calculated o_day The daily scale reference evapotranspiration is calculated by using meteorological element data and according to the FAO Penman Monteith model, and the expression is as follows:

[0040] (1)

[0041] In formula (1), R n is net radiation, unit: MJ / m 2 / d; G is soil heat flux, unit: MJ / m 2 / d; Δ is the slope of saturated water vapor pressure to air temperature, unit: kPa / ℃; γ is the psychrometer constant, unit: kPa / ℃; u2 is the wind speed at 2m height, unit: m / s; e s is saturated water vapor pressure, unit: kPa; e a is actual water vapor pressure, unit: kPa; T is daily average air temperature, unit: ℃.

[0042] Secondly, calculate the change of soil water storage ΔS each month:

[0043] (2)

[0044] In formula (2), S0 is the soil water storage at the beginning of each month, unit: mm; W S0 is the soil water storage at the beginning of each month, unit: mm; W t S is the soil water storage at the end of each month, unit: mm; then the expression of soil water storage W in the measurement month is as follows:

[0045] (3)

[0046] In formula (3), W is soil water storage, unit: mm; n is the number of soil moisture sensors in the soil moisture gradient; θ i is the volumetric water content of the i-th layer of soil; D i is the thickness of the i-th layer of soil.

[0047] Thirdly, according to the water balance equation, determine each item of the water balance of the monitored ecosystem within a period of time, so as to indirectly obtain the evapotranspiration, the expression of water input and output of the ecosystem is as follows:

[0048] (4)

[0049] In formula (4), the left side is the water input term, P is the precipitation measured by the rain gauge, unit: mm; N is the water that capillary water rises to the soil layer, unit: mm; the right side is the water output term, ΔS is the difference of soil water content at the beginning and end of the observation period, unit: mm; D is the water that seeps below the soil layer, unit: mm; ET is the actual evapotranspiration, unit: mm; R is the surface runoff, unit: mm.

[0050] In the desert arid area, due to its low groundwater depth, the capillary water and seepage water can be ignored; The daily precipitation is rare, and the surface runoff can also be ignored, so that the water input and output are respectively dependent on the precipitation and the soil vegetation evapotranspiration, and the formula (4) is simplified as the following formula:

[0051] (5)

[0052] According to the above water balance formula, the actual evapotranspiration can be simulated, and in general, the actual evapotranspiration estimated by the water balance method is relatively accurate in seasonal scale (monthly scale) or annual scale, and has smaller uncertainty, but the estimation effect in daily scale is still unclear; In order to obtain more accurate daily scale actual evapotranspiration, another method can be used to calculate, that is, according to the daily scale reference evapotranspiration ET o and the crop coefficient K c of a certain growth stage of vegetation, the actual evapotranspiration is calculated, and the expression is as follows:

[0053] (6)

[0054] In formula (6), ET represents the actual evapotranspiration, and the unit is mm; K c represents the crop coefficient of a certain growth period; ET o represents the reference evapotranspiration, and the unit is mm;

[0055] Fourthly, the crop coefficient will change with the change of the growth period of vegetation in a year, and the method divides the growth period in months, and since the water balance method is more accurate in seasonal scale (or monthly scale) to estimate evapotranspiration, according to formula (6), the monthly crop coefficient can be estimated according to the monthly scale reference evapotranspiration and the monthly scale actual evapotranspiration calculated by the water balance method, and then the daily scale actual evapotranspiration is calculated according to formula (6) according to the monthly crop coefficient and the daily scale reference evapotranspiration of the month.

[0056] With reference to Figure 1 , the actual evapotranspiration estimation method of the desert photovoltaic power station region of the present application is based on the above hardware setting and calculation model, and is implemented according to the following steps:

[0057] Step 1, collecting related data,

[0058] Install hardware equipment, set soil moisture sensor 1, soil heat flux plate 2, rain gauge 3, wind speed sensor 4, air temperature and humidity sensor 5, net radiation sensor 6 and atmospheric pressure sensor 7 in the predetermined area, connect all the sensors with the data collector 8 reliably, and ensure all-weather operation, see Figure 2 ;

[0059] Start real-time monitoring of soil moisture, soil heat flux, precipitation, wind speed, air temperature and humidity, net radiation and atmospheric pressure data; all the relevant data output by the sensor is continuously collected by the data collector 8, and the corresponding time of the relevant data is recorded and saved, see Figure 1 ;

[0060] Step 2, calculate the daily scale reference evapotranspiration ETo day ,

[0061] Using the obtained net radiation, soil heat flux, air temperature, wind speed and atmospheric pressure data, the daily total amount of net radiation, soil heat flux, daily average air temperature, saturated water vapor pressure, actual water vapor pressure, and the slope of the saturated water vapor pressure to the air temperature are calculated, and then the daily scale reference evapotranspiration ETo is calculated according to formula (1) day ;

[0062] Step 3, calculate the monthly scale reference evapotranspiration ETo month ,

[0063] Add the daily scale reference evapotranspiration ETo day of each month to obtain the monthly scale reference evapotranspiration ETo month ;

[0064] Step 4, calculate the monthly scale soil water storage change ΔS,

[0065] According to the soil moisture sensor measured volume water content of each soil layer, the monthly scale soil water storage change ΔS is calculated by using formula (2) and formula (3);

[0066] Step 5, calculate the monthly scale precipitation P,

[0067] Add the precipitation measured by the rain gauge in a month to obtain the monthly scale precipitation P of this month;

[0068] Step 6, calculate the monthly scale actual evapotranspiration ET month ,

[0069] According to formula (5), the monthly scale actual evapotranspiration ET is calculated from the monthly scale water storage change ΔS and the monthly scale precipitation P month ;

[0070] Step 7, calculate the crop coefficient K c ,

[0071] According to formula (6), the crop coefficient K is calculated by the monthly scale reference evapotranspiration ETo month and the monthly scale actual evapotranspiration ET month ; c ;

[0072] Step 8, calculate the daily scale actual evapotranspiration ETday ,

[0073] According to formula (6), the daily scale reference evapotranspiration ETo is calculated by the daily scale reference evapotranspiration ETo day and the crop coefficient K c . day .

[0074] Example 1

[0075] Referring to Figure 4 , Figure 5 , according to the foregoing step process of the present application, the daily scale actual evapotranspiration of a desert area in a certain place in the early growing season of May 2022 is calculated by the estimation method of actual evapotranspiration, and compared with the eddy correlation method. In this embodiment, y = 1.1044x + 0.1011, R 2 = 0.6935, Figure 4 The small squares in the figure represent data points with measured data as the X axis and estimated data as the Y axis. Some of the relevant data involved in this embodiment 1 are shown in Table 1.

[0076] Table 1, daily net radiation, air temperature, reference evapotranspiration and actual evapotranspiration of desert area in Example 1

[0077]

[0078] From the daily net radiation, air temperature, reference evapotranspiration and actual evapotranspiration of the desert area in May 2022 shown in Figure 5 , it can be seen that compared with the eddy correlation method, the results obtained by the method of the present application are closer to the measured data, have better accuracy, and are more real and reliable.

[0079] Example 2

[0080] Referring to Figure 6 , Figure 7 , according to the foregoing step process of the present application, the daily scale actual evapotranspiration of a desert area in a certain place in the middle of the growing season of July 2022 is calculated by the estimation method of actual evapotranspiration, and compared with the eddy correlation method. In this embodiment, y = 0.87x + 0.3519, R 2 = 0.7172, Figure 6 The small squares in the figure represent data points with measured data as the X axis and estimated data as the Y axis. Some of the relevant data involved in this embodiment 2 are shown in Table 2.

[0081] Table 2, daily net radiation, air temperature, reference evapotranspiration and actual evapotranspiration of desert area in Example 2

[0082]

[0083] From the daily net radiation, air temperature, reference evapotranspiration and actual evapotranspiration of the desert area in May 2022 shown in Figure 7The desert area daily net radiation, air temperature, reference evapotranspiration and actual evapotranspiration in July 2022 shown in the figure can be seen that, compared with the eddy correlation method, the result obtained by the method of the application is closer to the measured data, and the accuracy is better, and it is real and reliable.

[0084] Example 3

[0085] Referring to Figure 8 , Figure 9 , according to the foregoing step process of the application, the actual evapotranspiration of a certain desert area in September 2022 at the end of the growing season is calculated by the actual evapotranspiration estimation method, and compared with the eddy correlation method, wherein y = 1.0133x + 0.1117, R 2 = 0.778, Figure 8 The small squares in the figure represent data points with measured data as the X axis and estimated data as the Y axis, and part of the relevant data involved in this embodiment 3 is shown in Table 3.

[0086] Table 3, actual evapotranspiration of desert area daily net radiation, air temperature, reference evapotranspiration and actual evapotranspiration in Example 3

[0087]

[0088] By Figure 9 The desert area daily net radiation, air temperature, reference evapotranspiration and actual evapotranspiration in September 2022 shown in the figure can be seen that, compared with the eddy correlation method, the result obtained by the method of the application is closer to the measured data, and the accuracy is better, and it is real and reliable.

[0089] In summary, as Figure 4 , Figure 6 , Figure 8 , respectively, the actual evapotranspiration of the desert area at the daily scale in May, July and September 2022 calculated by the actual evapotranspiration estimation method and the eddy correlation method in the embodiments of the application are compared respectively. As Figure 5 , Figure 7 , Figure 9 The results show that: the actual evapotranspiration calculated by the method of the application and the calculation result based on the eddy correlation method have a very high correlation, R 2 respectively set to 0.6935, 0.7172 and 0.778, which fully illustrates that the method of the application has very high precision and reliability in estimating the actual evapotranspiration of the desert area at the daily scale.

Claims

1. A method for estimating the actual evapotranspiration in a desert photovoltaic power station area, characterized in that, Based on a hardware setup and computational model, the following steps are performed: Step 1: Collect relevant data. The specific process involves installing hardware equipment, setting up soil moisture sensors, soil heat flux plates, rain gauges, wind speed sensors, air temperature and humidity sensors, net radiation sensors, and atmospheric pressure sensors in a predetermined area, reliably connecting all sensors to the data acquisition unit, and ensuring 24 / 7 operation. The system begins real-time monitoring and acquisition of data on soil moisture content, soil heat flux, precipitation, wind speed, air temperature and humidity, net radiation, and atmospheric pressure. All relevant data output from the sensors are continuously collected by the data acquisition unit, and the corresponding time of the relevant data is recorded and saved. Step 2: Calculate the daily reference evapotranspiration ET oday , The specific process involves using the obtained data on net radiation, soil heat flux, air temperature, wind speed, and atmospheric pressure to calculate the daily total net radiation, daily total soil heat flux, daily average air temperature, saturated vapor pressure, actual vapor pressure, and the slope of saturated vapor pressure relative to air temperature. Then, the daily-scale reference evapotranspiration ET is calculated according to equation (1). oday ; Step 3: Calculate the monthly reference evapotranspiration ET omonth ; Step 4: Calculate the monthly soil water storage change ΔS. The specific process is to calculate the monthly soil water storage change ΔS based on the volumetric water content of each soil layer measured by the soil moisture sensor and then using equations (2) and (3). Step 5: Calculate monthly precipitation P; Step 6: Calculate the actual monthly evapotranspiration ET month , The specific process is as follows: the actual monthly evapotranspiration ET is obtained by calculating the monthly water storage change ΔS and the monthly precipitation P according to equation (5). month ; Step 7: Calculate the crop coefficient K c , The specific process is as follows: according to equation (6), the monthly scale reference evapotranspiration ET is used. omonth Actual evapotranspiration on a lunar scale (ET) month The crop coefficient K was calculated. c ; Step 8: Calculate the actual daily evapotranspiration ET day , The specific process is as follows: according to equation (6), the daily-scale reference evapotranspiration ET is used. oday and crop coefficient K c The actual daily evapotranspiration ET was calculated. day .

2. The method for estimating the actual evapotranspiration in a desert photovoltaic power station area according to claim 1, characterized in that: The hardware setup includes a soil moisture sensor, a soil heat flux plate, a rain gauge, a wind speed sensor, an air temperature and humidity sensor, a net radiation sensor, an atmospheric pressure sensor, and a data acquisition unit. The soil moisture sensor, soil heat flux plate, rain gauge, wind speed sensor, air temperature and humidity sensor, net radiation sensor, and atmospheric pressure sensor acquire relevant data in real time, and then the data acquisition unit collects, calculates, and stores the data. Soil moisture sensors are installed at regular intervals along a vertical gradient in the soil being tested to acquire dynamic data on soil moisture content in real time, around the clock. Soil heat flux plates are installed in the soil of the photovoltaic power station monitoring area to obtain the soil heat flux value of the monitoring area in real time. Rain gauges are evenly distributed on the ground in the monitoring area of ​​the photovoltaic power station. Rainwater enters the tipping bucket of the mechanical device through the funnel. When the rainwater is filled to the calibration line, it automatically tilts to pour out the rainwater and obtains the precipitation in the monitoring area. The wind speed sensor is set 2m above the vegetation in the monitoring area of ​​the photovoltaic power station to obtain the real-time wind speed 2m above the observed vegetation. An air temperature and humidity sensor is set up 2m above the vegetation in the monitoring area of ​​the photovoltaic power station to obtain air temperature and relative humidity data at a height of 2m in the vegetation area. The net radiation sensor is installed above the vegetation in the monitoring area of ​​the photovoltaic power station to obtain net radiation data of the observed vegetation area. Atmospheric pressure sensors are used to measure the atmospheric pressure in the monitoring area of ​​photovoltaic power plants; All sensors are connected to the data acquisition unit via data cables. After collecting the real-time observation data measured by each sensor, the data acquisition unit immediately calculates and stores the evapotranspiration data.

3. The method for estimating the actual evapotranspiration in a desert photovoltaic power station area according to claim 1, characterized in that: The computational model is as follows: First, calculate the daily-scale reference evapotranspiration ET. oday The daily-scale reference evapotranspiration is calculated using meteorological data and based on the FAO Penmanmonteith model, and the expression is as follows: （1） In equation (1), Rn is the net radiation, with units of MJ / m². 2 / d; G is soil heat flux, in MJ / m 2 / d; Δ is the slope of saturated vapor pressure with respect to air temperature, in kPa / ℃; γ is the hygrometer constant, in kPa / ℃; u2 is the wind speed at a height of 2m, in m / s; e s This is the saturated vapor pressure, in kPa; e a The actual water vapor pressure is expressed in kPa; T is the daily average temperature in °C. Second, calculate the monthly change in soil water storage ΔS: （2） In equation (2), W 0 represents the soil water storage at the beginning of each month, in mm; Wt Let W be the soil water storage at the end of each month, in mm; then the expression for the soil water storage W for that month is as follows: （3） In equation (3), W represents soil water storage in mm; n represents the number of soil moisture sensors in the soil moisture gradient. D represents the volumetric water content of the i-th soil layer; i Let be the thickness of the i-th soil layer; Third, based on the water balance equation, the various components of the ecosystem water balance within the monitoring range are measured over a period of time to indirectly obtain evapotranspiration. The expressions for ecosystem water input and output are as follows: （4） In equation (4), the left side represents the water input item, where P is the precipitation measured by the rain gauge in mm; N is the water content of the soil layer reached by capillary water in mm; and the right side represents the water output item. The difference in soil moisture content between the beginning and end of the observation period is expressed in mm; D represents the water that seeps into the soil layer, expressed in mm; ET represents the actual evapotranspiration, expressed in mm; and R represents the surface runoff, expressed in mm. In arid desert regions, water input and output depend on precipitation and soil / vegetation evapotranspiration, respectively. Therefore, the simplified expression of equation (4) is: （5） Based on daily reference evapotranspiration ETo day The actual evapotranspiration is calculated using the crop coefficient Kc at a certain growth stage of the vegetation, as shown in the following expression: （6） In formula (6), ET represents the actual evapotranspiration in mm; Kc represents the crop coefficient at a certain growth stage; ETo represents the reference evapotranspiration in mm. Fourth, according to formula (6), the crop coefficient for the month can be estimated by using the monthly scale reference evapotranspiration and the actual monthly scale evapotranspiration calculated by the water balance method. Then, the actual daily scale evapotranspiration can be calculated by using the crop coefficient for the month and the daily scale reference evapotranspiration for the month according to formula (6).

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