A Pathway Analysis of Evapotranspiration Variation Characteristics and Influencing Factors of Summer Maize in the Haihe Plain

Abstract

The Haihe Plain in China is situated in the world’s largest groundwater funnel area, with per capita water resources far below the internationally recognized “extremely water-scarce” standard. To address the issue of water shortage in summer maize-planting areas of the Haihe Plain, we conducted research on the variation of summer maize evapotranspiration using a medium-sized lysimeter. This study aims to provide technical support for water-saving irrigation in summer maize fields. Through path analysis, direct and indirect influencing factors affecting the evapotranspiration of summer maize fields were determined. The results showed that the cumulative evapotranspiration of bare ground and farmland during the entire growth period of summer maize was 173.57 mm and 382.97 mm, respectively, with evapotranspiration intensities of 1.52 mm/d and 3.36 mm/d, respectively. Evapotranspiration during the maturity stage of summer maize was the least, accounting for only 1.82% of total evapotranspiration during the entire growth period. The period from the jointing to milk-ripening stage is when evapotranspiration in maize fields is at its highest. During this period, evapotranspiration in maize fields amounted to 265.58 mm, accounting for 69.35% of total evapotranspiration. The evapotranspiration intensity was 3.59 mm/day, which is 1.34 times higher than that of bare soil. The evapotranspiration intensities during each growth stage were ranked as jointing stage > tasseling-silking stage > seedling stage > milk maturity stage > maturity stage. The daily evapotranspiration of summer maize fields showed a “unimodal” curve with low values in the morning and evening, and high values at noon. Path analysis indicated that daily radiation and maximum daily temperature had the greatest impact on the evapotranspiration of maize fields, with the direct effect of maximum daily temperature being restrictive and the indirect effect being promotive, resulting in an overall promotive effect.

1. Introduction

The largest groundwater funnel area in the world is located in the North China Plain. The Haihe Plain is a part of the North China Plain and is one of the main maize-producing regions in China. In recent years, an increase in extreme weather events and long-term irrigation and groundwater extraction have led to uneven spatial and temporal distribution of precipitation and problems such as groundwater overexploitation. This has resulted in prominent water shortage issues in the Haihe Plain, causing tension in agricultural water use and occasional local or widespread agricultural droughts [1,2,3,4,5]. Therefore, conducting research on crop evapotranspiration water requirements and water-saving irrigation in the Haihe Plain region is of significant importance.

Scholars both domestically and abroad have mainly focused their current research on the relationship between maize transpiration and climate, regarding the issue of maize transpiration and its influencing factors. Howell et al. [6] used weighing lysimeters to study the relationship between maize evapotranspiration and yield, as well as water use efficiency. Frank et al. [7] found a close relationship between maize evapotranspiration and local climatic distribution types through their study on maize evapotranspiration in the Argentine Pampas grasslands. Tolk et al. [8] studied the relationship between evapotranspiration and yield of three maize varieties on the plateau using 48 weighing lysimeters. Meng et al. [9] discovered that large interannual differences in maize evapotranspiration in the black soil region of Northeastern China are mainly due to monsoon climate. According to Wang et al. [10], the dominant meteorological factors affecting evapotranspiration in the North China Plain are sunshine hours and wind speed, while relative humidity and temperature have less impact. Jiang et al. [11] found that the main meteorological factors affecting summer maize evapotranspiration in Xinjiang in 2015 were net radiation and relative humidity. Yang et al. [12] analyzed soil evaporation and maize field evapotranspiration in Huailai and found that the main factors influencing maize evapotranspiration were net radiation and soil moisture. Maize in China is mainly cultivated in regions such as Northeast China, North China, and mountainous areas in Southwest China, so there have been many studies on maize evapotranspiration in these regions. However, these studies are mostly based on model calculations. In contrast, this study is characterized by using lysimeter-measured data and focuses on the unique climatic conditions of the warm, temperate, semi-arid region of the Haihe Plain. The study aims to conduct field evapotranspiration experiments on summer maize in the Haihe Plain, identify variations in field evapotranspiration at different growth stages of summer maize, and determine direct and indirect influencing factors on field evapotranspiration. This will contribute to promoting the rational allocation of agricultural water in groundwater funnel areas and improving agricultural water use efficiency.

In this study, path analysis was used to investigate the changes in summer maize evapotranspiration and their relationship with meteorological elements in order to further clarify the dynamic changes and influencing factors of summer maize evapotranspiration in the Haihe Plain, providing a theoretical basis for water-saving irrigation in summer maize fields.

2. Materials and Methods

2.1. Experimental Area Overview

The experiment was conducted from June to October 2021 at the Raoyang National Climate Observatory in Hebei Province (115°44′35″ N, 38°13′23″ E). This location is situated in the southeastern part of Hebei Province and is part of the Haihe River alluvial plain. The area is characterized by flat and open terrain, with an elevation of 17.9 m above sea level. It has a continental monsoon climate with a warm, temperate, semi-arid zone. The climate characteristics of this place include significant differences between cold and warm, and dry and wet, four distinct seasons, abundant light and heat resources, and concentrated rainfall. The average annual sunshine hours, precipitation, temperature, and frost-free period are 2486 h, 464 mm, 13.0 °C, and 201 d, respectively.

2.2. Experimental Methods

The experiment used a set of medium-sized lysimeters (model GQZ-Z2) consisting of four lysimeter vessels for research. A lysimeter was installed within the Raoyang National Climate Observatory, with a mass of 5 T and a sensitivity of 0.01 mm. They were cylindrical in shape, with a diameter of 1.49 m and a depth of 2.6 m, with data collected 60 times/h.

The local summer maize (Latin name: Zea mays L.) variety “Denghai 618”, which is widely cultivated in the area, was selected for the experiment. Within a 1 m radius around each lysimeter vessel, summer maize was sown. The four lysimeter vessels were numbered, with Sets 1 and 2 left as bare ground without any crops, while Sets 3 and 4 were planted with summer maize. No mulching was applied to any of the sets. The maize was sown on 11 June 2021, and harvested on 3 October 2021, with a planting density of 70,000 plants per hectare. Two irrigation events were conducted (73 mm on 16 June and 63 mm on 7 July), with consistent irrigation applied to each lysimeter vessel. The backfilled soil inside the lysimeter matched the field soil conditions, and field management practices throughout the growth period were consistent with those of conventional field cultivation. The maize yield was 131.85 g per plant, which is equivalent to 9229.5 kg/ha.

Evapotranspiration data for summer maize from the years 2021 and 2023 were used in this study. The planting time, harvest time, planting density, and cultivation conditions for summer maize in 2023 were consistent with those of 2021. However, there were significant differences in meteorological conditions between the two years. In 2021, total precipitation during the growth period was 439.7 mm; total sunshine hours were 710.2 h; and the average temperature was 25.1 °C. In 2023, total precipitation during the growth period was 707.8 mm; total sunshine hours were 909.6 h; and the average temperature was 26.3 °C (daily average temperature, precipitation, and sunshine hours during the summer maize growth period are shown in Figure 1 and Figure 2). During the growth period of summer maize in 2023, there were multiple heavy rainfall events, resulting in short-term intense precipitation that caused water accumulation inside the lysimeter for several days. During this period, evaporation within the instrument was mainly from the water surface, rendering the experimental data from multiple days unusable. Data from 2021 were more comprehensive compared to 2023, hence only the data from 2021 were used for studying the characteristics of evapotranspiration variations in maize fields throughout the entire growth period and conducting path analysis on the influencing factors. Descriptive statistical analysis of the daily evapotranspiration data (unit: mm) for summer maize in 2021 reveals that the sample size is 114, with a mean of 3.36, a median of 3.2, a range of 8.0, a variance of 3.487, a standard deviation of 1.867, and a standard error of the mean of 0.175.

2.3. Meteorological Observation Data and Data Analysis Methods

The National Climatological Observatory is equipped with a standard meteorological station capable of recording various complete meteorological data, including sunshine duration (SS), maximum temperature (TMAX), minimum temperature (TMIN), average relative humidity (RH), average wind speed (V), average vapor pressure (E), precipitation (R), and other meteorological element data. Additionally, a radiometer is installed at the Hengshui Agricultural Meteorological Experimental Station, a sub-station located 33 km from the National Climatological Observatory, which automatically measures and records daily radiation irradiance (S).

Evapotranspiration from the lysimeter was calculated using a water balance equation. The hourly evapotranspiration for each of the four lysimeter vessels was computed, and the averages taken for different treatments to obtain the hourly evapotranspiration variations for bare ground and summer maize fields. Missing or significantly erroneous data were interpolated by taking the average of the preceding and succeeding data points. The accumulated hourly evapotranspiration yielded the daily evapotranspiration. The water balance equation is represented as follows:

$$\mathrm{E}\mathrm{T}={\mathrm{E}}_1-{\mathrm{E}}_2-{\mathrm{E}}_{\mathrm{g}}+\mathrm{P}+\mathrm{W}$$

(1)

where $\mathrm{E}\mathrm{T}$ is the evapotranspiration amount of the lysimeter during the stage time in mm; ${\mathrm{E}}_1$ and ${\mathrm{E}}_2$ represent the initial and final soil moisture content during the lysimeter testing period in mm; ${\mathrm{E}}_{\mathrm{g}}$ is the permeability of the lysimeter during that period; and $\mathrm{P}$ and $\mathrm{W}$ represent the precipitation and irrigation during that period in mm.

Due to the strong intercorrelation among various meteorological factors, it is difficult to reflect the individual impact of each meteorological factor on reference maize evapotranspiration through simple correlation analysis. Therefore, this study employs path analysis using SPSS 13 software to decompose the correlations between various meteorological factors and summer maize field evapotranspiration, determining the direct and indirect influencing factors on summer maize field evapotranspiration and accurately characterizing their influence levels [13].

3. Results and Analysis

3.1. Typical Daily Variation of Evapotranspiration in Maize Fields

From Figure 3, Figure 4 and Figure 5, it can be observed that the hourly distribution of evapotranspiration in maize fields exhibits a typical “unimodal” curve pattern with low values in the morning and evening, and higher values around noon. The variation in evapotranspiration is minimal from 19:00 to 7:00 the next day, remaining at low levels. Significant changes in evapotranspiration occur from 8:00 to 18:00, with a noticeable increase around 9:00, reaching maximum values from 12:00 to 16:00 (around 0.8 mm/h on sunny days and 0.3 mm/h on cloudy days). Subsequently, evapotranspiration gradually decreases, reaching its lowest point around 23:00 (typical minimum evapotranspiration values were 0 for clear days in 2021 and 0.17 mm/h in 2023). Evapotranspiration on cloudy days is relatively lower compared to sunny days, but the variation pattern remains similar. The evapotranspiration pattern in bare soil is consistent with that in maize fields, but the evapotranspiration in bare soil is significantly less than that in maize fields.

3.2. Continuous Variation Pattern of Evapotranspiration in Maize Fields during Key Growth Periods

Evapotranspiration data for four consecutive days during the critical tasseling-silking period of summer maize in 2021 and 2023 were selected for comparison. Hourly temperature, hourly relative humidity, and total radiation were analyzed simultaneously. Figure 6 shows that from 00:00 on 13 August to 00:00 on 17 August, the hourly evapotranspiration in maize fields in 2023 was consistently higher than that in 2021, with an average difference of 0.24 mm. There were noticeable patterns in meteorological conditions during this period (Figure 7): hourly temperatures in 2023 were consistently higher than in 2021, with an average difference of 2.8 °C; hourly relative humidity in 2023 was generally lower than in 2021, with an average difference of 3.2%; hourly total radiation in 2023 was generally higher than in 2021, with a total difference of 4492 W/m². During this period, the higher temperatures, lower humidity, and stronger solar radiation in 2023 resulted in higher evapotranspiration in maize fields (

Table 1. Statistical summary of meteorological data at the experimental site on 13–16 August 2021 and 2023.

	Mean Air Temperature at the Hour (°C)	Mean Relative Humidity at the Hour (%)	Total Radiation Flux (W/m²)	Total Evapotranspiration of Maize Fields (mm)
2021	25.7	78.8	23,459.0	13.6
2023	28.5	75.6	27,951.0	36.8

Note: the time range for data collection in this table is from 00:00 on 13 August to 00:00 on 17 August in 2021 and 2023.

), indicating that different meteorological conditions during the same growth period have an impact on evapotranspiration in maize fields.

3.3. Variation Pattern of Evapotranspiration in Maize Fields throughout the Entire Growth Period

The main growing season for summer maize in the Haihe Plain is from mid-June to early October. After processing the data according to the analysis method, the daily variations in evapotranspiration for bare soil and maize fields are shown in Figure 8. It can be observed that during the entire 114-day growth period of summer maize, the cumulative evapotranspiration for bare soil was 173.57 mm, with an intensity of 1.52 mm/d, while for maize fields, it was 382.97 mm, with an intensity of 3.36 mm/d, representing an increase in intensity of 120.64% compared to bare soil. Evapotranspiration in maize fields during the entire growth period exhibits a characteristic of low evapotranspiration in the early and late stages and relatively high evapotranspiration in the middle stage, which is closely related to the smaller size of the plants in the early stage, the increasing number and size of leaves in the middle stage, and weakening transpiration in the late stage when the leaves turn yellow.

Table 2. Characteristics of bare land evapotranspiration at different growth stages of summer maize in 2021.

	Seedling	Jointing	Tasseling-Silking	Milk Maturity	Maturity	Total
Days (d)	36	21	33	20	4	114
Evapotranspiration (mm)	57.26	44.86	51.96	16.83	2.67	173.57
Evapotranspiration Intensity (mm/d)	1.59	2.14	1.57	0.84	0.67	1.52
Stage Evapotranspiration Percentage (%)	32.99	25.84	29.93	9.69	1.54	100.00

and

Table 3. Characteristics of evapotranspiration in maize fields at different growth stages of summer maize in 2021.

	Seedling	Jointing	Tasseling-Silking	Milk Maturity	Maturity	Total
Days (d)	36	21	33	20	4	114
Evapotranspiration (mm)	110.42	94.39	129.25	41.94	6.97	382.97
Evapotranspiration Intensity (mm/d)	3.07	4.49	3.92	2.10	1.74	3.36
Stage Evapotranspiration Percentage (%)	28.83	24.65	33.75	10.95	1.82	100.00
Percentage Increase In Evapotranspiration Intensity (%)	92.83	110.43	148.78	149.26	160.98	120.64

show that evapotranspiration during the maturation period of summer maize is the lowest, accounting for only 1.82% of total evapotranspiration during the entire growth period. This is because maize almost stops growing during the maturation period, resulting in weak transpiration. From the beginning of the ear-emergence stage to the end of the milk-ripening stage is the period of maximum evapotranspiration in maize fields, accounting for 69.35% of total evapotranspiration in the maize fields. During this period, summer maize grows vigorously; vegetation coverage is high; and the difference in evapotranspiration intensity between maize fields and bare soil is significant, with 1.54 mm/d for bare soil and 3.59 mm/d for maize fields, representing an increase of 1.33 times relative to bare soil. Among them, the tasseling-silking stage has the highest evapotranspiration, accounting for 33.75% of total evapotranspiration.

Figure 9 illustrates the variation in evapotranspiration intensity during different growth stages of summer maize in 2021. It can be observed from the distribution of evapotranspiration intensity during each growth stage of summer maize in the figure that during the seedling stage, the crops are small, and evapotranspiration in maize fields mainly occurs between plants. The evapotranspiration intensity in bare soil is 1.59 mm/d, while in maize fields, it is 3.07 mm/d, representing a relative increase of 93.08% compared to bare soil. The jointing stage is a critical period for the growth of summer maize, during which the plants grow rapidly, and the leaves are fully expanded, representing the peak period of evapotranspiration for summer maize. During this stage, the evapotranspiration intensity in bare soil is 2.14 mm/d, while in maize fields, it is 4.49 mm/d, representing a relative increase of 110.43% compared to bare soil. The tasseling-silking stage is another important stage of growth for summer maize, during which the plants gradually enter the reproductive stage. During this period, the evapotranspiration intensity in bare soil is 1.57 mm/d, while in maize fields, it is 3.92 mm/d, representing a relative increase of 1.5 times compared to bare soil. As summer maize enters the milk-ripening stage, the grains gradually mature; the leaves turn yellow; and transpiration weakens. The evapotranspiration intensities of the two types of fields during the grain-filling period are 0.84 mm/d and 2.10 mm/d, respectively, with maize fields showing a relative increase of 1.5 times compared to bare soil. After entering the mature stage, the leaves turn yellow and almost lose their transpiration function. Evapotranspiration in maize fields mainly occurs between plants. At this time, the evapotranspiration intensity in bare soil is 0.67 mm/d, while in maize fields, it is 1.74 mm/d, representing a relative increase of 1.6 times compared to bare soil.

3.4. Path Analysis of Evapotranspiration-Influencing Factors in Maize Fields

The path analysis method was applied to analyze the true relationship between evapotranspiration (ET) of summer maize from 11 June to 3 October and its main influencing factors, namely, sunshine hours, daily radiation, daily average temperature, maximum temperature, minimum temperature, average relative humidity, average wind speed, and average vapor pressure, in order to calculate the correlation coefficient, direct path coefficient, and indirect path coefficient between maize field evapotranspiration and influencing factors.

Table 4. Path analysis of influencing factors of evapotranspiration variation in corn fields.

Factor	Correlation Coefficient	Direct Path Coefficient	Indirect Path Coefficient
			SS	S	TMAX	TMIN	RH	V	E	Total
SS	0.353	0.157	0.000	0.300	−0.191	−0.040	0.088	0.000	0.039	0.196
S	0.725	0.886	0.053	0.000	−0.342	−0.033	0.138	0.000	0.023	−0.161
TMAX	0.534	−0.438	0.069	0.693	0.000	−0.099	0.155	0.000	0.154	0.972
TMIN	0.226	−0.196	0.032	0.149	−0.222	0.000	0.017	0.000	0.446	0.422
RH	−0.255	−0.227	−0.061	−0.540	0.299	0.015	0.000	0.000	0.259	−0.028
V	−0.113	−0.002	−0.011	−0.024	0.005	−0.012	0.043	0.000	−0.111	−0.110
E	0.264	0.584	0.010	0.034	−0.116	−0.150	−0.100	0.000	0.000	−0.321

presents the path analysis of the influencing factors of evapotranspiration variation in maize fields throughout the entire growth period, and based on the correlation coefficients between each influencing factor, an ET change path analysis diagram was formed. Table 4 shows the direct and indirect contributions of each factor to ET. The influence of each factor on ET (correlation coefficient) is ranked from largest to smallest as S, TMAX, SS, E, RH, TMIN, and V, and both S and TMAX demonstrate a promoting effect on evapotranspiration in maize fields.

Among the factors directly affecting ET, the direct effect of S is the largest (direct path coefficient is 0.886), followed by TMAX (direct path coefficient is −0.438), and it is manifested as a limiting effect, while the direct effect of V is the smallest (direct path coefficient is −0.002). Through the analysis of the various indirect path coefficients, it is found that TMAX has the largest indirect effect on ET (the sum of indirect path coefficients is 0.972), which is manifested as a promoting effect, followed by TMIN (the sum of indirect path coefficients is 0.422), which is also manifested as a promoting effect.

4. Conclusions and Discussion

This study utilized a medium-sized lysimeter to analyze the variation patterns of evapotranspiration in summer maize fields and bare soil in the Haihe Plain. Additionally, the study employed path analysis to investigate the impact of different meteorological factors on evapotranspiration in maize fields, leading to the following main conclusions:

(1)

The diurnal variation patterns of evapotranspiration in summer maize fields and bare soil exhibit a “single-peak” curve characteristic, with lower values in the morning and evening, and higher values at noon. This is similar to the findings of You Debao et al. [14]. Their study compared the typical daily variation curves of evapotranspiration at different growth stages of maize and described the daily variation characteristics of evapotranspiration as “single-peak.” In contrast, our study added measured evapotranspiration data during key growth stages of maize for another year, revealing significant differences in nighttime transpiration between different years. On typical days in 2021, evapotranspiration in summer maize fields at night was close to 0, while in 2023, it was close to 0.2 mm/h. The reason for this is that in 2023, compared to the same period in 2022, total precipitation during the summer maize growing season was higher by 268.1 mm; total sunshine hours were longer by 199.4 h; and the average nighttime (20:00 to 08:00 the next day) temperature was higher by 0.8 °C, resulting in higher nighttime evapotranspiration in 2023.

(2)

In 2021, the total evapotranspiration of summer maize fields throughout the entire growth period was 382.97 mm, which is close to the evapotranspiration of maize fields after deficit irrigation treatments conducted by Liu Meihan et al. [13] in the Hetao Irrigation District of Inner Mongolia, China (359.21 mm) but significantly different from the evapotranspiration of maize under timely sowing conditions in the Adana region of Turkey, as reported by Deniz Levent KOÇ et al. [15] (618 mm). The experimental sites of these three studies are located in different climates: the Hetao region of Inner Mongolia is in a dry or semi-arid area; Adana is in the Mediterranean climate zone; and the experimental area of this study is in a temperate continental monsoon climate zone. The evapotranspiration of summer maize in these three climate zones presents distinct contrasts. This study measured the evapotranspiration of bare soil throughout the entire growth period and found it to be 173.57 mm, which is 45% of that of maize fields. Yanmin Yang et al. [16] also acknowledged that bare soil evapotranspiration consumes a considerable amount of water and compared the water-saving potential of different soil surface management measures through experiments. What sets this study apart is that it quantitatively observed the evapotranspiration of bare soil at various growth stages of summer maize for comparison with maize fields.

(3)

Path analysis indicates that daily radiation and maximum temperature have the greatest impact on evapotranspiration in maize fields, with direct effects of daily radiation and maximum temperature being the largest, and the indirect effect of maximum temperature on evapotranspiration being the largest among temperature factors. The direct effect of maximum temperature on evapotranspiration is restrictive, while the indirect effect is promotive, overall reflecting a promotive effect. Radiation and temperature play a crucial role in water–heat exchange, significantly affecting the variation of evapotranspiration. Similar conclusions were reached by Li et al. [17] and Yang et al. [18]. During critical growth periods when solar radiation is intense, maize fields are prone to drought stress, and relevant departments can issue timely drought warnings for maize. Furthermore, further exploration of methods to regulate indicators such as radiation and temperature in summer maize fields to improve water use efficiency and maximize water-saving and yield-increasing effects is warranted.

Based on the conclusions of this study, higher temperatures and intense solar radiation will significantly increase evapotranspiration in summer maize fields. During periods of high temperature, abundant precipitation, and long sunshine hours, nighttime evapotranspiration in summer maize fields may exceed 0. The conclusions of this study are not entirely consistent with those of some literature [19,20,21], which may be attributed to differences in research methods, research subjects, and climate and soil conditions in the study area. In addition, there are still many differences in this study, such as the fact that the lysimeter is thermally isolated from the surrounding farmland, which may lead to overestimation of evapotranspiration [12]; and the experimental data used in this article are relatively short and not sufficient for classification. Maize evapotranspiration is influenced by a variety of factors, so further exploration using multi-source data is needed in future research. By integrating measured data from lysimeters with various models [22,23,24], using the cropping system model to simulate the crop coefficients [25], estimating the water consumption requirements of summer maize fields in the Haihe Plain continental monsoon climate region will be the focus of our future research.