The evapotranspiration (ET) is coupled with photosynthesis, and plays a key role in the energy balance of land surface (Wu & Shukla, 2014), thereby affecting many biological and physical processes that occur at the ground surface (Sen, 2004). The ET at vegetation surfaces have important effects on several aspects of climate (Weveret al., 2002). In turn, ET is affected by the soil properties, vegetation characteristics, and weather (Suyker & Verma, 2008; Ding et al., 2013; Yang et al., 2013). More than 80% of total cultivated land is rainfed cropland (FAO, 2011), which plays a vital role in maintaining stable ecosystems and agricultural productivity (Zhang et al., 2016). It is critical, therefore, to study ET in rainfed agricultural ecosystems to model crop production and to elucidate the mechanisms for the hydrological and biogeochemical cycles.

In recent decades, the eddy covariance system is regarded as the standard method for observing turbulent f luxes between the surface and atmosphere (Taylor et al., 2013). This technique has also been widely used for accurate measurements of water vapor fluxes in different terrestrial ecosystems (Wu & Shukla, 2014; Yang et al., 2016). Published studies on ET in croplands focused mainly on irrigation croplands (Li et al., 2008; Yuan et al., 2014), while researchers have paid less attention to rainfed croplands (Suyker & Verma, 2009). Soil evaporation (E) and crop transpiration (T) constitute most ET in agricultural ecosystems; understanding these two components is important for optimizing agricultural management to increase plants water use efficiency (Kool et al., 2014). Considerable research on how abiotic and biotic factors control ET and its components is needed in rainfed croplands. The crop coefficient (Kc), and its components, soil water evaporation coefficient (Ke) and basal crop coefficient (Kcb), are the most important parameters for describing ET and its components (Alberto et al., 2014). Because Kc and its components can be used to estimate ET and its components (Zhao et al., 2015; Miao et al., 2016), they should be determined in rainfed agricultural ecosystems (Suyker & Verma, 2009; Liu et al., 2010a). ET in the non-growing season influences the annual water balance (Suyker & Verma, 2009; Zhang et al., 2016), as this period usually lasts more than half a year, but few studies have paid attention to this period in single-cropping systems in the drylands of northwest China.

The Loess Plateau has a population of ~90 million and covers an area of approximately 9 degrees of latitude and 11 degrees of longitude with a semiarid monsoon climate in northwest China (Liu et al., 2010b). As surface runoff is sparse and groundwater is deep, the Plateau is an important rainfed agricultural region, especially considering the rapid degradation of fragile ecosystems in China. Spring maize (Zea mays L.), the dominant crop on the Plateau, has an important influence on regional food security (Bu et al., 2013). However, low temperatures and limited water in April–June usually result in crop growth retardation (Liu et al., 2009; 2010b). Plastic film mulching has been widely used in spring maize croplands on the Plateau because it can significantly increase soil water and heat conditions during the early growing season and increase grain yields (Liu et al., 2009, 2010b; Bu et al., 2013). To date,there have been few studies using eddy covariance techniques for year-round ET in rainfed spring maize croplands under plastic film.

In this study, a rainfed spring maize field under plastic f ilm was investigated using the eddy covariance system combined with micro-lysimeters at Shouyang site in 2015 and 2016. The study objectives were limited to: (i) determine seasonal variations in ET and its components; and (ii) ascertain the key factors controlling ET and its components in the field on the Loess Plateau.

Site description

The experiments were conducted in a typical rainfed cropland on the eastern Loess Plateau, located in Shouyang County in Shanxi Province (N 37°45′, E 113°12′, altitude 1,202 m). The study area is characterized by a semi-arid temperate continental monsoon climate. Mean annual precipitation is 474.5 mm, with over 70% occurring from July-September; mean annual temperature is 8.2 ℃ and the mean annual frost-free period is 150 days. The soil is sandy–loamy, containing 54.9% sand, 29.5% silt and 15.6% clay; the soil bulk density is 1.34 g cm−3.The contents of soil organic matter, total nitrogen, total phosphorus, and total potassium are 9.00 g kg−1, 0.79 g kg−1, 0.72 g kg−1 and 19.61 g kg−1, respectively (Gao et al., 2017).

Spring maize was sown around May 1 at a row spacing of 0.5 m and a plant spacing of 0.3 m with plastic film mulching in 2015 and 2016. The mulching film and punch planter was used in the cropland, and one plastic film mulched three rows. The mulching area accounted for about 80% of the total in both years. The plastic film was a kind of transparent polyethylene film with a thickness of 8 μm, and did not visibly deteriorate until the harvest time. Base fertilization was consistent with the practice of local farmers (276 kg N ha−1, 144 kg P2O5 ha−1, and 60 kg K2O ha−1). Straw was chopped using automated machines and returned to the field at harvest time. Any straw that did not decompose during the non-growing season was completely mixed with the soil through tillage in late April of the following year (Gao et al., 2017).

Eddy covariance measurements

An eddy covariance system was installed near the central of the spring maize cropland under plastic film (100 m × 260 m). The information about the composition and installation of this eddy covariance system was shown in Gao et al. (2017). And the data processing of water vapor fluxes was described in Gao et al. (2018).

Other measurements

The measuring instruments of air temperature (Ta), relative humidity (Ha), net radiation (Rn), soil water con-tent at 10-cm depth (SWC), wind speed (U) and precipi-tation (P) were described in Gao et al. (2018). We used the manual method to measure the green leaf area index (GLAI) during the growing season (McKee, 1964; Gao et al., 2018). Daily E was measured by ten micro-lysi-meters made from polyvinyl chloride (PVC) tubes with a diameter of 10 cm and a height of 15 cm during the growing season, four micro-lysimeters were installed within bare soil between two films, and the remaining micro-lysimeters were installed under the film. The mi-cro-lysimeters were reinstalled within 1 day after preci-pitation or on the third day of continuous measurements. Daily E at each micro-lysimeter was obtained as the di-fference between the weights measured by an electronic scale with the precision of 0.1 g at sunset, and weigh-ted averaged as daily E in the spring maize field (Gao et al., 2017).

Non-mulching field

We also used eddy covariance system combined with micro-lysimeters to measure ET and its components in a non-mulching rainfed spring maize field in Shouyang site. The agricultural practices except for plastic film mulching in the non-mulching field were the same as the field under plastic film. The detailed information of ET and its com-ponents in the non-mulching field were shown in Gao et al. (2018).

Parameter calculation

Following Alberto et al. (2014), ET was calculated as follows:

where LE is the latent heat flux (W m−2) and Ta is the air temperature (°C). The ET0 was calculated according to Allen et al. (1998):

where Rn is the net radiation (MJ m−2 d−1), G is the soil heat flux (MJ m−2 d−1), U is the wind speed (m s−1), γ is the psychrometric constant (kPa °C−1), VPD is the vapor pressure deficit (kPa), and Δ is the slope of the water va-por pressure curve (kPa °C−1).

The crop coefficient (Kc), soil water evaporation coefficient (Ke), and basal crop coefficient (Kcb) were gi-ven by Allen et al. (1998):

where Ks is soil water stress coefficient, and calculated according to Ding et al. (2013).

Abiotic factors and GLAI

The abiotic factors and GLAI had similar general trends in both years, except for P and SWC (Fig. 1). The dynamics of daily Rn and Ta accorded with the quadratic curve, peaking at ~18 MJ m−2 d−1 and ~25 °C, respectively (Figs. 1a and 1b). With the coming of summer monsoon at the end of June, daily Ha, U, and VPD generally increa-sed, decreased, and decreased, respectively (Figs. 1c, 1d, and 1e). After each P, SWC increased suddenly and then decreased gradually, and total P was 386 and 461 mm in 2015 and 2016, respectively (Fig. 1f). The period from seed-sowing date (1 May in both years) to canopy dying date (7 October in 2015; 4 October in 2016) was defined as the growing season, and the GLAI peak marked the end of the canopy-increasing stage and the beginning of the canopy-decreasing stage. The peak values of GLAI were 4.17 m2 m−2 and 3.76 m2 m−2 on 24 July in 2015 and 31 July in 2016, respectively (Fig. 1g).

ET and its components

Seasonal variations in ET0, ET, E, and T in 2015 and 2016 are shown in Fig. 2. The daily ET0 displayed a para-bolic trend, peaking in the canopy-increasing stage (Fig. 2a). The daily E was relatively small during the growing season (Fig. 2b). The daily T increased dramatically in the canopy-increasing stage, and then declined significantly in the canopy-decreasing stage (Fig. 2c). The daily ET trend was similar to that of daily T during the growing season, and was very small during the non-growing sea-son (Fig. 2b). The daily ET and its components were dis-turbed by weather conditions; i.e., they decreased sharply on cloudy and rainy days, and increased dramatically after those days. The relationships between daily GLAI and E/ET were logarithmic equations in the canopy-increasing and -decreasing stages (Fig. 3). These relationships were used to interpolate the miss data of daily E and T in the spring maize field under plastic film (Figs. 2b and 2c).

The P and ET were much lower in the non-growing season than in the growing season (Table 1).E values in the growing season were 123 and 107 mm, and accounted for 36% and 33% of ET in the growing season in the field under plastic film in 2015 and 2016, respecti-vely. The water balance values were −5 and 78 mm in the growing season and −39 and −25 mm in the non-growing season in 2015 and 2016, respectively. These suggest that soil water increased only during the growing season in 2016. Water balance and ET partitions in the non-mul-ching spring maize field at Shouyang site was also shown in Table 1. Compared to the field under plastic film, E and T values in the growing season in the non-mulching field were higher and lower respectively. Over the 2 years, total ET in the non-mulching field was greater than that in the field under plastic film by 30 mm. And the annual water balance values in the non-mulching field were −28 and 24 mm in 2015 and 2016, respectively.

Seasonal variations in Kc, Ke, and Kcb in 2015 and 2016 are shown in Fig. 4. The daily Kc in the non-growing season was generally low, and increased sharply after P (Fig. 4a). The daily Kc and Kcb gradually increased in the canopy-increasing stage and then declined in the canopy-decreasing stage, and was disturbed by P (Figs. 4a and 4b). The peak values of daily Kc and Kcb during the growing season appeared earlier in 2016 than in 2015. The daily Ke was generally low during the growing season, and was also disturbed by P (Fig. 4a). P had a stronger effect on daily Kc and Ke than on daily Kcb in the growing season. The monthly average Kc, Ke, and Kcb in the growing season, and average Kc in the non-growing season in 2015 and 2016 are shown in Table 2.

Regardless of the effect of P on daily Kc, Ke, and Kcb, the effects of GLAI on daily Kc, Ke, and Kcb in the canopy-increasing and -decreasing stages in 2015 and 2016 are shown in Fig. 5. The data from rainy days or the three following days were removed from the database of daily Kc, Ke, whereas the data from rainy days and one following day were removed from the database of daily Kcb, mainly because water evaporated more quickly from plant surfaces than the soil surface after rain. The daily Kc in the canopy-increasing stage increased with increasing GLAI when GLAI was less than ~3 m2 m−2, whereas in the canopy-decreasing stage it decreased linearly with decreasing GLAI (Figs. 5a and 5b). The daily Ke in the canopy-increasing stage was insensitive to GLAI, but increased linearly with decreasing GLAI in the canopy-decreasing stage (Figs. 5c and 5d). There was an exponential relationship between daily Kcb and GLAI in the canopy-increasing stage, whereas in the canopy-decreasing stage daily Kcb decreased linearly with decreasing GLAI (Figs. 5e and 5f). At equivalent GLAI, daily Ke and Kcb were usually higher in the canopy-decreasing and -increasing stages, respectively.

Abiotic factors and GLAI controlling ET and its components

As shown in Table 3, the growing season daily ET and T were mainly controlled by Rn, followed by GLAI (p<0.001). And Ha and VPD also had an impact on growing season daily T (p<0.05). The growing season daily E was mainly controlled by Rn, followed by SWC (p<0.001), and had negative correlations with GLAI in 2016 (p<0.05). The non-growing season daily ET were mainly controlled by Rn (p<0.001), and was also influenced by SWC (p<0.001) and U (p<0.05) in 2015.

As shown in Table 4, the selected factors (those with p<0.001 in Table 3) predicted the growing season daily ET and T variations well with goodness-of-fit values of 0.79–0.80. Changes in selected factors also explained 4352% of the variations in the non-growing season daily ET and growing season daily E.

The growing season ET was 342 mm (daily average of 2.24 mm d−1) and 322 mm (daily average of 2.11 mm d−1) in the field under plastic film in 2015 and 2016, respectively (Table 1). Compared to irrigated spring maize fields under plastic film in northwest China (Ding et al., 2013; Li et al., 2013; Zhang et al., 2016), the water consumption was lower in this study. Suyker & Verma (2009) observed total ET values ranging from 502 to 586 mm and daily average ET values ranging from 3.02 to 3.62 mm d−1 in irrigated spring maize croplands in Nebraska, USA, those values were also higher than those in our study. The different agricultural practices and climate conditions in our study area could be the main reason that our results differed from those of published studies in which the eddycovariance system was also used to measure water flux. Previous studies of using the water balance method in rainfed spring maize croplands under plastic film on the Loess Plateau have reported the growing season ET va-lues of 358–373 mm (Liu et al., 2010b), 351–369 mm (Bu et al., 2013), and 331–376 mm (Zhou et al., 2009), which are comparable to the corresponding values in our study. Compared to the non-mulching field, the growing season ET in the field under plastic film was lower by 3 and 34 mm in 2015 and 2016 at our site, respectively (Table 1). Those results agree with previous studies on the Loess Plateau (Liu et al., 2010b; Bu et al., 2013), suggestingthat plastic film mulching could decrease ET in rainfed spring maize fields.

On an annual scale, water balance in the field under plastic film was −44 and 53 mm in 2015 and 2016, res-pectively (Table 1). Similar to our study, ET consumed almost all P (annual ET/P between 0.94 and 1.28) in a rainfed agroecosystem in Nebraska, USA (Suyker & Ver-ma, 2009). Although straw-returning had been used in the field under plastic film, the non-growing season soil water decreased by 39 and 25 mm in 2015 and 2016, respecti-vely (Table 1), implying that it may be critical to optimize agricultural practices for water management during the non-growing season. The non-growing season soil water in the non-mulching field also decreased by 23 and 26 mm in 2015 and 2016, respectively (Table 3), which are comparable to the field under plastic film, mainly becau-se plastic film was destroyed by the machine when the straw was returned to the field. Zhang et al. (2016) also proved that the non-growing season soil water decreased in irrigated spring maize croplands in northwest China. However, the non-growing season soil water increased by 43–169 mm in a rainfed maize–soybean agroecosystem (Suyker & Verma, 2009), where the non-growing season P (194–332 mm) was much greater than at our study site (Table 1). In addition, about 20% of annual ET occurred in the non-growing season at our site (Table 1), which was comparable to percentages for a rainfed maize–soybean agroecosystem (20–26%, Suyker & Verma, 2009), and irrigated spring maize fields (~15%, Zhang et al., 2016).The E consumed 36% and 33% of growing season ET in 2015 and 2016, respectively (Table 1), indicating that the growing season ET was mainly consumed by T in the field under plastic film. The growing season E/ET ratios were comparable to that in a winter wheat cropland (0.32, Liang et al., 2011), but lower than that in a summer mai-ze cropland in northwest China (0.44–0.53, Wang et al.,2007). Compared to the non-mulching field, the growing season E/ET, E, and T in the field under plastic film were lower, lower, and higher, respectively (Table 1). Those results indicate that plastic film decrease E, increase T, and promote the distribution of ET to the T, and finally stimulates the growth and development of crops, which is in agreement with the published studies (Liu et al., 2010b; Bu et al., 2013). Daily E/ET decreased logarithmically with increasing GLAI (Fig. 3), indicating that seasonal variations in GLAI controlled daily E/ET during the growing season. Similar relationships between GLAI and daily E/ET were also found in winter wheat and summer maize croplands in northwest China (Kang et al., 2003; Wang et al., 2007). At the same GLAI, daily E/ET was lower in the canopy-increasing stage than in the canopy-decreasing stage (Fig. 3); we suspect the higher transpiration rate of GLAI in the canopy-increasing stage may be responsible for this phenomenon.

The peak monthly average Kc values were 0.85 and 0.79 in August 2015 and July 2016, respectively (Table 2). Mid-season average Kc values were between 0.71 and 0.90 in a rainfed spring maize cropland in Nebraska, USA (Suyker & Verma, 2009), and from 0.85 to 0.90 in an irrigated cotton field in northwest China (Yang et al., 2016), values that are comparable to our results. However, average Kc values during periods of vigorous growth were usually > 1 in irrigated spring maize croplands in northwest China (Li et al., 2008; Jiang et al., 2014) and Nebraska, USA (Suyker & Verma, 2009). The daily Kc at our site increased with increasing GLAI until the GLAI threshold (~3 m2 m−2) in the canopy-increasing stage (Figs. 5a and 5b). These results are in agreement with those of Zhang et al. (2016) for irrigated spring maize croplands and Kang et al. (2003) for a summer maize field. These results indicate that ET is dramatically affected by GLAI when GLAI is below a certain threshold value in the canopy-increasing stage. The daily Kc at our site decreased linearly with decreasing GLAI in the canopy-decreasing stage (Figs. 5a and 5b). Yuan et al. (2014) and Suyker & Verma (2008) also reported nearly linear relationships between GLAI and daily Kc during the canopy-decreasing stage in a desert ecosystem and in an irrigated agricultural ecosystem, respectively. The curvature of the exponential curve between GLAI and daily Kc was relatively low in the canopy-increasing stage in 2015 (Fig. 5a), possibly because of soil water stress due to lower P in this stage (Fig. 1f), resulting in ET that was more dependent on GLAI and with a higher partial correlation coefficient (Table 3). The relationships between daily Kcb and GLAI in the canopy-increasing and -decreasing stages were similar to those between daily Kc and GLAI (Figs. 5e and 5), since T was the main component of ET, and there were similar seasonal variations in T and ET. E was relatively stable during the growing season (Fig. 2b); ET0 was also relatively stable during the canopy-increasing stage, butdecreased in the canopy-decreasing stage (Fig. 2a), resulting in daily Ke values that were insensitive to GLAI in the canopy-increasing stage but increased linearly with decreasing GLAI in the canopy-decreasing stage (Figs. 5c and 5d).

Because abiotic factors, such as Rn, Ta, U, and VPD calculated from Ta and Ha, are the variables to calculate ET0 (Allen et al., 1998), they have an important effect on seasonal variations in daily ET during the growing season reported in many previous studies (Li et al., 2008; Alberto et al., 2014; Zhang et al., 2016). In this study, daily ET and its components was most influenced by Rn, indicating the energy supply plays a key role in cropland water consumption on the Loess Plateau. As abiotic factors influence T by acting on green leaves, daily T and ET were also controlled by GLAI at our site (Table 3), which is in agreement with the results in a maize field in the Philippines (Alberto et al., 2014). We suspect higher GLAI combined with higher ET0, may be the main reason for the higher growing season ET value in 2015, although the growing season P was higher in 2016. The water sources for T and E are the root zone (0–100 cm) and surface soil (0–10 cm), respectively (Ding et al., 2013; Li et al., 2013). Consequently, SWC had a small but significant effect on daily T and E, respectively, in this study (Table 3). Although P had little influence on daily ET and its components (Table 3), the water in the soil and on plant surfaces evaporates very easily after a P event, resulting in a sharp increase in daily ET and its components, daily Kc and its components on sunny days following P events.

In this study, almost all P was consumed by ET, implying that the groundwater and runoff were not replenished from rainfed croplands on the Loess Plateau. The daily Kc and its components reacted differently to GLAI in the canopy-increasing and -decreasing stages, suggesting that crop growth had different effects on ET in different growth stages. Future studies should focus on the response of ET to climate change in relation to energy partitioning, carbon uptake and water use efficiency in rainfed croplands on the Loess Plateau.

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