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Article

Actual and Reference Evapotranspiration in a Cornfield in the Zhangye Oasis, Northwestern China

1
Key Laboratory of Land Surface Process and Climate Change in Cold and Arid Regions (LPCC)/Nagqu Station of Plateau Climate and Environment (NPCE), Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China
2
State Key Laboratory of Cryospheric Science (SKLCS)/Cryosphere Research Station on Qinghai-Xizang Plateau (CRS), Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China
*
Author to whom correspondence should be addressed.
Water 2017, 9(7), 499; https://doi.org/10.3390/w9070499
Submission received: 28 April 2017 / Revised: 28 June 2017 / Accepted: 3 July 2017 / Published: 8 July 2017

Abstract

:
Evapotranspiration (ET) is an important component of the surface energy balance and water cycle, especially in arid and semiarid regions. The characteristics of the actual evapotranspiration (ETa), which was calculated using the eddy covariance method, and the reference evapotranspiration (ET0), which was estimated using the Food and Agriculture Organisation (FAO) Penman–Monteith method, were analysed. This work focussed on the seasonal variations in evapotranspiration and crop coefficient (Kc) above the heterogeneous canopy of an arid oasis ecosystem in a cornfield of the Zhangye oasis in northwestern China. The results showed that in 2008, the total net radiation (Rn) was 2457.73 MJ∙m−2 and that the rainfall was 117 mm. The average wind velocity, air temperature, and specific humidity, which were observed 2 m above the ground surface, were 1.23 m∙s−1, 7.07 °C, and 3.66 g∙kg−1, respectively. The total ETa and ET0 were 654.69 mm and 1039.92 mm, respectively; thus, the ET0 was higher than the ETa. The difference between the ET0 and ETa was high in summer and autumn, and low in winter and spring. The ETa was greatly influenced by irrigation events, whereas the ET0 was not influenced by irrigation. The ETa and ET0 were both greatly influenced by meteorological elements. The Kc values were less than 0.5 outside of the maize-growing stage and greater than 0.5 during the entire maize-growing stage (from 20 April to 22 September 2008). The Kc values were 0.63, 0.75, 0.78, 0.76, 0.61 and 0.71 at the seedling, shooting, heading, filling, and maturity stages and the entire growth stage, respectively.

1. Introduction

Evapotranspiration (ET) is important for water resource management, hydrometeorological forecasting, environmental conservation, and agricultural competitiveness [1,2,3]. ET is an indicator for the rate of change in the global water cycle, and it is a necessary variable for most numerical weather forecasting and global climate model simulations [2,4,5,6]. ET can represent a substantial portion of the regional water budget depending on the water availability, climate regime, and landscape conditions [2,7]. ET is a dominant controlling factor of climate and hydrology at the local and global scales, and it is also an important factor controlling energy and mass exchange between terrestrial ecosystems and the atmosphere. This issue has received considerable attention [8,9,10,11,12].
Many land surface experiments, such as the European Field Experiment in a Desertification-Threatened Area (EFEDA) [13], the Hydrologic Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) [14], the Heihe International Field Experiment (HEIFE) [15,16], the Inner-Mongolia Grassland Atmosphere Surface Study (IMGRASS) [17] and the Land-atmosphere Interaction Experiment in an Arid Region of Northwest China (NWC-ALIEX) [18], have been implemented in semiarid and arid regions. Numerous studies have analysed energy and water balances, water resource supply and demand, and water resource security in the irrigation regions of the Heihe River Basin in northwestern China [19,20,21]. Although ET and spring wheat irrigation in the middle reaches of the Heihe Basin have been previously evaluated [22,23], further investigation is necessary.
The Watershed Airborne Telemetry Experimental Research (WATER) project chose the Zhangye oasis as a key experimental area for an arid region hydrology experiment in the oasis-desert zone of the middle reaches of the Heihe River Basin [24]. The terrain is flat, with elevations ranging from 1500 m to 2000 m. The Zhangye oasis is located in the inland arid belt of northwestern China. The artificial oasis, the Gobi Desert, and the transitional zones between the oasis and desert are the dominant landscapes [25]. The total area of the Zhangye oasis is 4.19 × 104 km2, accounting for 32.23% of the total area in the Heihe River Basin. The vegetation coverage and oasis area accounted for 8.67% and 9.8% of the Zhangye oasis total area, respectively [26]. In such regions, ET is high, and the water availability is limited. In recent decades, increasing human activities and associated overexploitation or illogical water resource utilisation in the Zhangye oasis have resulted in a series of environmental problems arising in the lower reaches of the Heihe River Basin. Such problems include land desertification and salinisation as well as natural vegetation degeneration. As a result, environmental degradation has become a research focus in recent decades. An equitable partitioning of water resources among competing shareholders and ecosystems along the Heihe River Basin has been hampered by a lack of accurate water budgets, particularly a lack of accurate ET estimates [21].
Since Dalton introduced the ET formula in 1802, and with the development of observations and theories, various methods have been proposed for estimating ET, such as the Bowen ratio-energy balance method, aerodynamic method, eddy covariance (EC) method, Penman–Monteith model, and remote sensing method [1,27,28,29]. Actual evapotranspiration (ETa) is one of the key factors in land–atmosphere interactions. Apart from the incoming radiation, ETa is the most important component of the energy budget at the ground surface with sufficient moisture [30]. Reference evapotranspiration (ET0) is the basis for estimating crop evapotranspiration (ETc) and calculating crop irrigation requirements [31,32]. Increased ET0 estimation accuracy can result in the conservation of economic and water resources for both the planning and management of irrigated areas [33,34].
The objectives of this study were to estimate the ETa using the eddy covariance method and the ET0 using the Food and Agriculture Organisation (FAO) Penman–Monteith method over a cornfield in the Zhangye oasis region, and to analyse the seasonal variations in the ETa and ET0. The meteorological conditions and crop coefficients were also analysed.

2. Materials and Methods

2.1. Experimental Site and Instrumentation

The Yingke site (100.41° E, 38.86° N, and 1519 m in elevation) was chosen as the study site to measure the ETa and ET0 over a cornfield in the Zhangye oasis in the middle reaches of the Heihe River Basin, the second largest inland river basin in the arid region of northwestern China. The site was built for the WATER experiment project in November 2007 (Figure 1a). In the cornfield (Figure 1b,c), the male and female parents of FL-2 maize were sown on 20 and 28 April 2008, respectively, at 60 cm row spacing, with a distance of 25 cm between plants.
The monthly averages of the meteorological parameters observed at the Zhangye meteorological station from 1951 to 2000 are listed in Table 1. These data show that the annual averages of the wind velocity, air temperature and specific humidity were approximately 2.02 m∙s−1, 7.08 °C, and 4.84 g∙kg−1, respectively. The annual rainfall was approximately 70–210 mm, and the pan-measured annual evaporation was approximately 1500–2300 mm. The ET was substantially higher than the rainfall. The region is arid, and the local rainfall is inadequate for crop growth. Thus, irrigation is a major source of soil moisture for agricultural production.
The EC system (Figure 1b) and an automatic weather station tower (AWS-Tower) system (Figure 1c) were used during the observation period. The EC included a three-dimensional ultrasonic anemometer (CSAT3, Campbell Scientific, Inc., Logan, UT, USA) used to measure the wind velocity component and temperature fluctuations, an open-path infrared gas analyser (Li-7500, Li-COR Inc., Lincoln, NE, USA) used to measure the H2O and CO2 concentration, and a data logger (CR5000, Campbell Scientific, Inc., Logan, UT, USA) used to log the observation data continuously at a rate of 10 Hz. The AWS-Tower measured the wind velocity (010C, Vaisala Inc., Helsinki, Finland), air temperature, relative humidity (HMP45C, Vaisala Inc., Helsinki, Finland) at heights of 2 and 10 m, air pressure (CS100, Campbell Scientific, Inc., Logan, UT, USA), rainfall (52202, R.M. Young, Traverse, MI, USA), radiation budget (CM3/CG3, Campbell Scientific, Inc., Logan, UT, USA), soil temperature (109, Campbell Scientific, Inc., Edmonton, AB, Canada), soil moisture content (CS616, Campbell Scientific, Inc., Edmonton, AB, Canada) at depths of 10, 20, 40, 80, 120, and 160 cm, and soil heat flux (HFP01SC, Radiation and Energy Balance Systems, Seattle, WA, USA) at depths of 5 and 15 cm. The measurements were calculated continuously from 10-min averages. In this study, the data from November 2007 to January 2009 were analysed, and the time was based on Beijing local time.

2.2. Actual Evapotranspiration Estimates

In this study, actual evapotranspiration (ETa) values were mainly obtained by the eddy covariance (EC) method, which is considered an advanced technique for accurately capturing ET information over short-term periods (e.g., 10 min) in a large area [25]. The EC method does not include assumptions concerning the required eddy diffusivities. Disadvantages of the EC method include dew formation on the instruments during daybreak, which renders the instruments unreliable, and reduced instrument reliability during precipitation events [34]. EC system monitoring was occasionally interrupted during the observation period because of instrument failure and/or severe climatic conditions, and the missing data were filled in using meteorological data via the aerodynamic method, which is a conventional method for calculating the ETa [35].

2.2.1. Eddy Covariance (EC) Method

The EC technique measures turbulent fluxes according to the fluctuations around each block mean signal [36]. Corrections were performed for the data monitored by the EC system using a double coordinate rotation (DR) for each half hour [37]. DR corrections were widely used to process the data obtained through the EC system because of the convenience and accuracy of the technique. The DR corrections forced the mean horizontal wind direction to the X-direction and the mean vertical and lateral wind vectors to zero [38]. After the corrections, the ETa was calculated according to the following equations:
E T a = L E s L v ρ
L E = ρ L v w q v ¯
Lv = 2.5 × 106 − 2323 × t
where LE is the latent heat flux (w∙m−2); s is the time (s); ρ is the air density (kg∙m−3); Lv is the latent heat of vaporization (J∙kg−1); w is the vertical velocity (m∙s−1); qv is the specific humidity (g∙kg−1); and t is the air temperature (°C).
Energy closure is an important criterion used to evaluate the accuracy of the eddy covariance method [39], also for evaluate the accuracy of the evapotranspiration. The energy closure ratio (CR) is defined as follows [40]:
C R = R n G 0 H s + L E
where Rn is the net radiation (w∙m−2), G0 is the ground heat flux (w∙m−2); Hs is the sensible heat flux (w∙m−2); and LE is the latent heat flux (w∙m−2).
Figure 2 shows the energy closure status in 2008 using the daily average energy flux data at the Yingke site. The CR was 0.81 at the site, consistent with the results (0.82) of Wang et al. [41]. The energy unclosure may reflect the omission of other storage terms of heat in the biomass and air between the measurement height and ground surface, the amount of energy consumed by photosynthesis or released by respiration, and an underestimation of G0.

2.2.2. Aerodynamic Method

According to Oke [42], Malek [43] and Monteith and Unsworth [44], the modified aerodynamic equation for calculating the latent heat flux (LE, w∙m−2) is expressed as follows:
L E = 0.622 L v ρ k 2 [ e a ( z 2 ) e a ( z 1 ) ] [ u ( z 1 ) u ( z 2 ) ] P [ ln ( z 2 d z 1 d ) ] 2 ( Φ M Φ V ) 1
where ρ is the air density (kg∙m−3); k = 0.4 is the von Karman constant; d is the zero displacement height (m); P is the air pressure (hPa); ea (z1) and ea (z2) represent the actual vapour pressure (hPa), and u (z1) and u (z2) represent the wind velocity (m∙s−1) at heights of z1 = 2 m and z2 = 10 m, respectively; and ΦM and ΦV are stability functions for momentum and water vapour transport, respectively. The generalized stability factor F = [ΦMΦV)]−1 can be calculated for the stable atmosphere (Ri > 0) as follows:
ΦM = ΦV = (1 − 5R)−1
and
F = (1 − 5Ri) 2
For unstable atmospheres (Ri < 0), the generalized stability factor can be calculated as follows:
ΦM2 = ΦV = (1 − 16Ri)−1/2
and
F = (1 − 16Ri)3/4
Ri is the bulk Richardson number expressed as follows:
R i = g ( d θ d z ) T ( d u d z ) 2
where g is the acceleration because of gravity (m∙s−2); T is the average air temperature (K) over a height interval of dz (m); and θ is the potential temperature (K). Ri is negative, zero and positive under lapse (unstable) conditions, and neutral under inversion (stable) conditions, respectively.
The ETa was calculated according to Equation (1).

2.3. Reference Evapotranspiration Estimates: FAO Penman–Monteith Method

The FAO Penman–Monteith method is recommended as the standard ET0 method, and it clearly defines the ET of a hypothetical reference vegetated field [27]. This method provides consistent ET0 values in many regions and climates [32,45,46] and has long been accepted worldwide as an accurate estimator of ET0 compared with other methods, especially for daily calculations [31,47,48,49,50].
ET0 was estimated according to the following equations [50]:
E T 0 = 0.408 Δ ( R n G s f c ) + γ 900 T + 273 u 2 ( e s e a ) Δ + γ ( 1 + 0.34 u 2 )
e 0 ( T ) = 0.6108 e x p ( 17.27 T T + 237.3 )
e s = e 0 ( T max ) + e 0 ( T min ) 2
e a = e 0 ( T max ) R H min 100 + e 0 ( T min ) R H max 100 2
Δ = 4098 [ 0.6108 exp ( 17.27 T T + 237.3 ) ] ( T + 237.3 ) 2
γ = 0.665 × 10 3 P
where ET0 is the reference evapotranspiration (mm∙d−1); Rn is the net radiation (MJ∙m−2∙d−1); Gsfc is the soil heat flux at the ground surface (MJ∙m−2∙d−1); es is the saturation vapour pressure of the air temperature (KPa); ea is the actual vapour pressure (KPa); γ represents a psychometric constant (KPa∙°C−1); T and u are the mean daily air temperature (°C) and wind velocity (m∙s−1) at a height of 2 m, respectively; is the slope of the saturation vapour-pressure curve of the air temperature (KPa∙°C−1); RH is the relative humidity; and P is the air pressure (KPa).

3. Results and Discussion

3.1. Meteorological Conditions

Figure 3 shows the hourly net radiation (Rn) at the Yingke Site from November 2007 to January 2009; these data reflect the seasonal characteristics of the Rn. The maximum Rn of each season was 649.28 (spring), 777.93 (summer), 663.54 (autumn), and 473.36 w∙m−2 (winter). The maximum monthly total Rn was 409.90 MJ∙m−2 (June 2008), and the minimum monthly total Rn was 23.35 MJ∙m−2 (December 2008). The total Rn was 2547.73 MJ∙m−2 in 2008 (Table 2). Temperatures are influenced by the Rn. The air temperature (Ta) was observed 2 m above the ground surface, and the annual average value was 7.07 °C in 2008. The monthly average Ta reached a maximum (20.59 °C) in July and a minimum (−14.03 °C) in January (Table 3). The hourly average Ta was below 31 °C in spring, rose to a maximum of approximately 34 °C in summer, then fell below 28 °C in autumn, and descended to a minimum below 22 °C in winter. Except in January, February and August in 2008, the monthly and annual averages of Ta were lower than the values from 1951 to 2000, as shown in Table 1.
The monthly average wind velocity observed 2 m above the ground surface generally ranged between 1.0 and 2.0 m∙s−1 during the observation period (Table 3). The maximum hourly average wind velocity in 2008 was 9.35 m∙s−1. The average wind velocity in 2008 was 1.23 m∙s−1, which was lower than the average wind velocity from 1951 to 2000, as listed in Table 1.
The monthly average specific humidity observed 2 m above the ground surface reached a maximum (7.23 g∙kg−1) in July and a minimum (−1.07 g∙kg−1) in January (Table 3). The specific humidity was below 10 g∙kg−1 in spring, rose to a maximum of approximately 20 g∙kg−1 in summer, then fell below 15 g∙kg−1 in autumn and descended to a minimum below 5 g∙kg−1 in winter. The average specific humidity in 2008 was 3.66 g∙kg−1. Except in November and December 2008, the monthly and annual averages of the specific humidity were lower than the average specific humidity from 1951 to 2000 listed in Table 1.
Seasonal variations of rainfall are shown in Figure 4. The total rainfall in 2008 was approximately 117 mm, which was lower than the values from 1951 to 2000 (Table 1); the rainfall was concentrated (92% of the annual total) in summer and autumn (June to November 2008). During the observation period, the maximum monthly total rainfall was 37.20 mm (June 2008, Table 2). Rainfall did not occur in November and December 2008. The seasonal distribution of rainfall was 8.00 (spring), 64.50 (summer), 43.70 (autumn) and 0.70 mm (winter). The total rainfall was approximately 70 mm during the entire maize-growing stage (from 20 April to 22 September 2008).
The soil froze in winter, and the underlying land was seasonally frozen ground (Figure 5a). The maximum depth of the frost penetration reached up to approximately 100 cm. The average annual ground surface temperature (Tg) in 2008 was 7.00 °C. The seasonal variations in the soil moisture content are shown in Figure 5b. In winter, the soil moisture content was lower because of ground freezing. After March, the ground thawed, and the soil moisture content increased. Extreme soil moisture contents occurred after each irrigation event, which led to the high value centres at 20 cm. Generally, there was a high value belt at a depth of 120 cm. Vertically, the soil moisture content was lowest at 10 cm and highest at 120 cm (Table 4). As shown in Figure 5b, the soil moisture content peaked during each irrigation event except for the irrigation event on 25 August, because the soil moisture content data were missing. The monthly total soil heat flux ranged from negative to positive in March, and reached a maximum in May, a trend that was similar to that of the Tg. The total soil heat flux decreased to negative values again in July (Table 2) due to the oasis “wet island” effect [16,51], which indicates that the high amount of latent heat flux results in a cold land surface and decreases the sensible heat flux and the soil heat flux, even to negative values.

3.2. Seasonal Variations of Actual Evapotranspiration (ETa)

The seasonal variations in the ETa at the Yingke site from November 2007 to January 2009 are shown in Figure 6. The daily mean ETa was 1.49 in spring, 3.90 in summer, 1.41 in autumn and 0.22 mm∙day−1 in winter. In 2008, the total ETa was 654.69 mm, and the daily average ETa was 1.79 mm. The ETa observed in the Zhangye oasis cornfield was higher than the ETa observed in an arid oasis ecosystem of the Syrian desert in Palmyra [52] and a Tamarix ramosissima ecosystem in the extremely arid region of northwestern China [21].
In 2008, the emergence time of maize occurred on 6 May, and the shooting stage of maize began on 19 June. The heading stage of maize began on 20 July. The filling stage of maize occurred from 5 August to 10 September, and the maturity stage occurred from 11 September to 22 September. The crops were harvested on 22 September at the observation field [53]. During the entire maize-growing stage (from 20 April to 22 September 2008), the total ETa was approximately 500 mm with a daily average ETa of 3.33 mm∙day−1. As shown in Table 5, the total ETa values were 138.07 mm, 126.07 mm, 59.54 mm, 145.27 mm and 31.42 mm and their corresponding daily average ETa values were 2.30 mm∙day−1, 4.07 mm∙day−1, 3.72 mm∙day−1, 3.93 mm∙day−1 and 2.62 mm∙day−1 at the seedling, shooting, heading, filling and maturity stages, respectively. The rainfall was approximately 70 mm, and the amount of irrigation water was approximately 510 mm during the entire maize-growing stage, which was similar to the water loss. In the study area, the ETa was primarily derived from irrigation and was greatly influenced by irrigation events. The cropland was irrigated with approximately 150 mm on 3 June, 120 mm on 25 June, 120 mm on 28 July, 120 mm on 25 August, and 150 mm on 1 November, and the total irrigation water was approximately 660 mm in 2008 [54]. After the four intervals of irrigation in the maize-growing stage, the soil moisture content (smc) at a depth of 10 cm depth exhibited a peak (the smc data on 25 August when the fourth irrigation in the maize-growing stage were lost), and ETa also increased. After irrigation on November 1, after the maize harvest, the ETa decreased slightly due to the small Rn, and because of the lower temperatures, the water in the soil was frozen in winter and stored for the next spring sowing.
During the observation period, the ETa increased in spring, reached a maximum in summer, decreased in autumn and then reached a minimum in winter. This phenomenon may occur because the Rn was low in winter, and the Ta was negative when the soil was frozen. When these conditions occurred, the soil moisture content decreased to the lowest values. The water permeability of the frozen soil layer weakened, which led to a weak relationship between the frozen soil layer and thawed soil layer, and produced low ETa values. Thus, the specific humidity reached a minimum. In spring, however, the Rn was higher and the Ta was positive when the soil thawed. Under these conditions, the soil moisture content increased, which led to increased ETa. In summer, the growth of vegetation flourished, the Rn reached a maximum, the wind velocity was higher, and the cornfield was irrigated four times. Under these conditions, the ETa and the specific humidity reached a maximum. Although the overall rainfall amount was low, the rainfall amount was greater in summer, thereby contributing to the maximum ETa in summer. In autumn, the Rn and Ta decreased, and the surface was bare without vegetation; thus, the ETa began to decline.
During the observation period, the total rainfall was 117 mm. The ETa values were considerably higher than the rainfall, thus leading to arid conditions. The ETa was greatly influenced by the irrigation events and meteorological elements. When the site was irrigated, the ETa peaked the following day.
A regression analysis indicated that the ETa is closely related to the net radiation, wind velocity, air temperature and specific humidity (Figure 7) as follows:
E T a ( mm d - 1 ) = 0.22 × R n + 0.25 × W S 2 m + 0.01 × T   a 2 m + 0.06 × q   2 m -   0.43
where Rn is the net radiation (MJ∙m−2∙d−1), WS2m is the wind velocity 2 m above the ground surface (m∙s−1), Ta2m is the air temperature 2 m above the ground surface (°C), and q2m is the specific humidity 2 m above the ground surface (g∙kg−1). The multiplex correlation coefficient was approximately 0.91, whereas the number of cases was 313. Evapotranspiration was positively correlated with net radiation, wind velocity, air temperature and specific humidity. The regression formula (17) showed that wind velocity and net radiation play a significant role in evapotranspiration. When the relationships of evapotranspiration with meteorological factors were assessed in the upper [55] and middle [56] reaches of the Heihe River Basin, the effect of wind velocity was greatest, which is consistent with the results of this paper.

3.3. Seasonal Variations of the Reference Evapotranspiration (ET0)

The seasonal variations of the ET0 at the Yingke site from November 2007 to January 2009 are presented in Figure 8. The daily average ET0 values were 2.84 (spring), 5.27 (summer), 2.09 (autumn) and 0.64 mm∙day−1 (winter). In 2008, the total ET0 was 1039.92 mm, and the daily average ET0 was 2.85 mm. The ET0 observed in the Zhangye Oasis cornfield was similar to the values observed in a cornfield in the semiarid region of northern India [57], but was higher than the values observed in the Tanggula region of the Tibetan Plateau, except in winter [50].
During the observation period, the ET0 was slightly higher than the ETa, and the differences were large in summer and autumn, and small in winter and spring. Similar to the ETa, the ET0 increased in spring, reached a maximum in summer, decreased in autumn and reached a minimum in winter. During the entire maize-growing stage (from 20 April to 22 September 2008), the total ET0 was approximately 706 mm with a daily average ET0 of 4.53 mm∙day−1. As shown in Table 5, the total ET0 values were 219.56 mm, 167.97 mm, 76.55 mm, 190.08 mm and 51.76 mm, and their corresponding daily average ET0 values were 3.66 mm∙day−1, 5.42 mm∙day−1, 4.78 mm∙day−1, 5.14 mm∙day−1 and 4.31 mm∙day−1 at the seedling, shooting, heading, filling and maturity stages, respectively. The ET0 was primarily impacted by meteorological elements and was not influenced by irrigation.

3.4. Crop Coefficient (Kc)

The crop coefficient Kc was estimated according to FAO56 [27]:
K c = E T a E T 0
In 2008, the Kc ranged from 0.31 to 0.81 (Table 6), with the maximum values occurring in July, and the minimum values occurring in January. The annual average was 0.56, and the seasonal averages were 0.53 (spring), 0.74 (summer), 0.57 (autumn) and 0.38 (winter).
The Kc values were less than 0.5 outside of the maize-growing stage, because the cornfield was bare without vegetation and not irrigated, except for several rainfall events in April and one irrigation in November.
The Kc values were greater than 0.5 during the entire maize-growing stage (from 20 April to 22 September 2008) because the growth of corn flourished, and the cornfield was irrigated four times. As shown in Table 5, the Kc values were 0.63, 0.75, 0.78, 0.76, 0.61 and 0.71 at the seedling stage, shooting stage, heading stage, filling stage, maturity stage and the entire growth stage, respectively. Li et al. [25] reported that maize Kc values in Wuwei City, Gansu Province of northwestern China, at the seedling, shooting, heading, filling, and maturity stages were 0.44, 0.95, 1.46, 1.39, and 1.22, respectively, which generally were higher than our results, except at the seedling stage. The differences are mainly caused by two factors. (1) Kc is related to vegetation coverage [27]. In the study by Li et al. [25], maize was sown with 40 cm row spacing and 6.7 cm planting spacing, and thus the planting density was higher, consisting of approximately 374,800 plants ha−1. This higher density led to a higher Kc in the middle and late periods of the growing season. By contrast, in the present study, the maize was sown with a 60 cm row spacing and 25 cm planting spacing, resulting in a lower planting density of approximately 67,000 plants ha−1 and, consequently, a lower Kc in the middle and late periods of the growing season. (2) The ET0 estimated by the FAO56 model was underestimated in the middle and late maize-growing seasons in the study by Li et al. Kang et al. [58] observed a similar underestimation of ET0 based on FAO56 in the Loess Plateau, Shaanxi, China, and a higher Kc compared with the values given by Allen et al. [27]. This phenomenon also led to a higher Kc in the middle and late growing seasons.
In recent years, other methods of studying the Kc of maize have been used, such as models and remote sensing methods. Miao et al. [59] focused on the actual evapotranspiration, crop transpiration and crop coefficient using the SIMDualKc model which is a model for simulating soil water balance based on FAO56 double crop coefficient method in the Hetao irrigation district of the upper Yellow River basin, China, and a new modelling approach was developed for the basal crop coefficients (Kcb) of a relay-strip intercropping system. Kullberg et al. [60] compared several remote sensing methods to calculate crop evapotranspiration and Kcb in a deficit irrigation experiment for maize near Greeley, Colorado, and the results showed that remote sensing methods can inform users about the availability of certain data and irrigation levels. Models and remote sensing methods are important methods in regional evapotranspiration and Kc research [61,62,63].

4. Conclusions

Based on the EC system and the AWS-Tower data of the Yingke site from November 2007 to January 2009 in the Zhangye oasis cornfield of northwestern China, the characteristics of the ETa and ET0 and Kc were analysed and compared. The following conclusions have been drawn.
  • In 2008, the total ETa and ET0 were 654.69 mm and 1039.92 mm, respectively; the total rainfall was 117 mm, the irrigation water was approximately 660 mm, and these values were similar to the water loss value. The ET0 was slightly higher than the ETa. Differences between the two values were large in summer and autumn, and small in winter and spring. Both ETa and ET0 increased in spring, reached a maximum in summer, decreased in autumn and reached a minimum in winter.
  • During the observation period, both the ETa and the ET0 were substantially higher than the rainfall, which resulted in arid conditions. The ETa was primarily derived from irrigation and was greatly influenced by irrigation events and meteorological elements, whereas the ET0 was not influenced by irrigation and was primarily impacted by meteorological elements.
  • In 2008, the annual average Kc was 0.56, and the seasonal averages were 0.53 (spring), 0.74 (summer), 0.57 (autumn) and 0.38 (winter). The Kc values were less than 0.5 outside of the maize-growing stage, and were greater than 0.5 during the entire maize-growing stage (from 20 April to 22 September 2008). The Kc values were 0.63, 0.75, 0.78, 0.76, 0.61 and 0.71 at the seedling, shooting, heading, filling, and maturity stages, and the entire growth stage, respectively.
  • In 2008, the total Rn and rainfall were 2457.73 MJ∙m−2 and 117 mm, respectively; the average values for the wind velocity, air temperature, and specific humidity 2 m above the ground surface were 1.23 m∙s−1, 7.07 °C, and 3.66 g∙kg−1, respectively. The monthly and annual averages of the wind velocity and specific humidity were lower than the average wind velocity and specific humidity observed at the Zhangye meteorological station from 1951 to 2000. Except in January, February and August in 2008, the monthly and annual average air temperature were also lower than the average air temperature from 1951 to 2000.
In this paper, we mainly studied the variation characteristics of evapotranspiration and Kc at a single point; in the future, we hope to expand our research into the region. The Kc research of oasis in semiarid areas is not fully understood; future research will require the application of additional methods, such as modelling or remote sensing.

Acknowledgments

This research was supported by the National Natural Science Foundation of China (41575012, 41571069, 91337212, 91537101). This data set is provided by “Heihe Plan Science Data Center, National Natural Science Foundation of China”.

Author Contributions

Lianglei Gu contributed in literature search, study design, data collection, data analysis, figures, data interpretation, and article writing. Zeyong Hu contributed in study design and data collection. Jimin Yao contributed in literature search, data interpretation, and article writing. Genhou Sun contributed in data analysis and figures.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The Watershed Airborne Telemetry Experimental Research (WATER) experiment observation site locations (a), the eddy covariance (EC) system (b), and the automatic weather station tower (AWS-Tower) at the Yingke site (c).
Figure 1. The Watershed Airborne Telemetry Experimental Research (WATER) experiment observation site locations (a), the eddy covariance (EC) system (b), and the automatic weather station tower (AWS-Tower) at the Yingke site (c).
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Figure 2. Energy closure status using the daily average energy flux data at the Yingke site.
Figure 2. Energy closure status using the daily average energy flux data at the Yingke site.
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Figure 3. Hourly net radiation (Rn), monthly maximum (Rn_max), and monthly minimum (Rn_min) values of net radiation at the Yingke site from November 2007 to January 2009.
Figure 3. Hourly net radiation (Rn), monthly maximum (Rn_max), and monthly minimum (Rn_min) values of net radiation at the Yingke site from November 2007 to January 2009.
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Figure 4. Daily rainfall at the Yingke site from November 2007 to January 2009.
Figure 4. Daily rainfall at the Yingke site from November 2007 to January 2009.
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Figure 5. Seasonal variations in the soil temperatures (a, °C) and soil moisture content (b, m3∙m−3) at the Yingke site from November 2007 to January 2009.
Figure 5. Seasonal variations in the soil temperatures (a, °C) and soil moisture content (b, m3∙m−3) at the Yingke site from November 2007 to January 2009.
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Figure 6. Hourly actual evapotranspiration (ETa) and soil moisture content (smc) at 10-cm depth at the Yingke site from November 2007 to January 2009.
Figure 6. Hourly actual evapotranspiration (ETa) and soil moisture content (smc) at 10-cm depth at the Yingke site from November 2007 to January 2009.
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Figure 7. The comparison of the calculated results with the regression results of actual evapotranspiration (ETa).
Figure 7. The comparison of the calculated results with the regression results of actual evapotranspiration (ETa).
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Figure 8. Daily reference evapotranspiration (ET0) at the Yingke site from November 2007 to January 2009.
Figure 8. Daily reference evapotranspiration (ET0) at the Yingke site from November 2007 to January 2009.
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Table 1. Monthly averages of the meteorological elements observed at the Zhangye meteorological station from 1951 to 2000.
Table 1. Monthly averages of the meteorological elements observed at the Zhangye meteorological station from 1951 to 2000.
DateWind Velocity (m∙s−1)Air Temperature (°C)Specific Humidity (g∙kg−1)Evaporation (mm∙Day−1)Rainfall (mm∙Day−1)
January.1.70−9.841.161.160.06
February1.95−5.631.402.080.05
March2.442.012.314.480.13
April2.789.623.498.090.18
May2.5315.505.539.620.49
June2.1119.418.389.600.88
July1.9821.4610.479.191.17
August1.9220.319.948.451.20
September1.7014.527.216.260.62
October1.656.834.264.090.18
November1.79−1.282.451.970.07
December1.64−7.921.481.120.05
Table 2. Monthly total of the meteorological elements observed at the Yingke site from November 2007 to January 2009.
Table 2. Monthly total of the meteorological elements observed at the Yingke site from November 2007 to January 2009.
Month-YearNet Radiation (MJ∙m−2)Rainfall (mm)Soil Heat Flux a (MJ∙m−2)Soil Heat Flux b (MJ∙m−2)
November-200783.770.10-−11.71
December-200732.880.60−5.01−18.46
January-200831.320.00−18.08−4.08
February-200861.990.10−9.53−2.77
March-2008217.270.4016.4511.30
April-2008274.686.5018.0622.45
May-2008227.971.1017.4118.21
June-2008409.9013.205.303.55
July-2008379.6237.20−1.08−3.51
August-2008379.7014.10−1.94-
September-2008269.4633.206.91−7.09
October-2008172.6110.50−9.69−11.71
November-200899.860.00−10.49−13.74
December-200823.350.00−14.07−24.36
January-200924.510.10−9.88−17.15
Notes: a Observed 5 cm below the ground surface; b Observed 15 cm below the ground surface.
Table 3. Monthly averages of the meteorological elements observed at the Yingke site from November 2007 to January 2009.
Table 3. Monthly averages of the meteorological elements observed at the Yingke site from November 2007 to January 2009.
Month-YearAir Temperature * (°C)Wind Velocity * (m∙s−1)Specific Humidity * (g∙kg−1)
November-20070.731.372.62
December-2007−5.651.451.28
January-2008−14.031.231.06
February-2008−9.951.131.25
March-20084.75-1.87
April-200810.661.422.17
May-200817.312.012.48
June-200819.871.466.33
July-200820.591.179.42
August-200818.791.216.33
September-200814.401.005.78
October-20089.171.223.00
November-20080.011.402.61
December-2008−6.771.451.59
January-2009−9.671.401.17
Note: * Observed 2 m above the ground surface.
Table 4. Seasonal average and range in the soil moisture content (m3∙m−3) at various depths at the Yingke site from November 2007 to January 2009.
Table 4. Seasonal average and range in the soil moisture content (m3∙m−3) at various depths at the Yingke site from November 2007 to January 2009.
Soil Depth Season10 cm20 cm40 cm80 cm120 cm160 cm
Spring0.220.280.240.230.310.24
Summer0.250.310.290.300.410.33
Autumn0.270.320.290.290.390.31
Winter0.100.130.150.210.340.27
Average0.210.260.240.250.360.29
Range0.07–0.490.09–0.450.09–0.450.12–0.350.28–0.460.23–0.43
Table 5. The actual evapotranspiration (ETa), reference evapotranspiration (ET0) and crop coefficient (Kc) during different growth stages at the Yingke site in 2008.
Table 5. The actual evapotranspiration (ETa), reference evapotranspiration (ET0) and crop coefficient (Kc) during different growth stages at the Yingke site in 2008.
Growth StagePeriodDaysCumulative ET (mm)Daily Average ET (mm∙Day−1)Kc
EtaET0EtaET0
Seeding stage20 April–18 June60138.07219.56 2.30 3.66 0.63
Shooting stage19 June–19 July31126.07167.97 4.07 5.42 0.75
Heading stage19 July–4 August1659.5476.55 3.72 4.78 0.78
Filling stage5 August–10 September37145.27190.08 3.93 5.14 0.76
Maturity stage11–22 September1231.4251.76 2.62 4.31 0.61
Whole growth stage20 April–22 September156500.37705.91 3.33 4.66 0.71
Table 6. Monthly averages of the actual evapotranspiration (ETa), reference evapotranspiration (ET0) and crop coefficient (Kc) at the Yingke site in 2008.
Table 6. Monthly averages of the actual evapotranspiration (ETa), reference evapotranspiration (ET0) and crop coefficient (Kc) at the Yingke site in 2008.
JanuaryFebuaryMarchAprilMayJuneJulyAugustSeptemberOctoberNovemberDecember
ETa (mm∙day−1)0.170.381.171.371.993.833.993.892.501.090.900.25
ET0 (mm∙day−1)0.540.952.422.553.585.494.935.403.782.471.470.57
K c0.310.400.480.540.560.700.810.720.660.440.610.44

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Gu, L.; Hu, Z.; Yao, J.; Sun, G. Actual and Reference Evapotranspiration in a Cornfield in the Zhangye Oasis, Northwestern China. Water 2017, 9, 499. https://doi.org/10.3390/w9070499

AMA Style

Gu L, Hu Z, Yao J, Sun G. Actual and Reference Evapotranspiration in a Cornfield in the Zhangye Oasis, Northwestern China. Water. 2017; 9(7):499. https://doi.org/10.3390/w9070499

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Gu, Lianglei, Zeyong Hu, Jimin Yao, and Genhou Sun. 2017. "Actual and Reference Evapotranspiration in a Cornfield in the Zhangye Oasis, Northwestern China" Water 9, no. 7: 499. https://doi.org/10.3390/w9070499

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