Changes in Hydrothermal Conditions During the Spring Maize Growth Period in Inner Mongolia from 1961 to 2020 and Their Impact on the Meteorological Yield

Abstract

Climate change has led to significant changes in water and heat conditions in crop production areas, which have in turn affected the spring maize growth and yield. This study analyzed the spatiotemporal variation characteristics of the water and heat conditions, such as the growth degree-day (GDD), killing degree-day (KDD), sunshine hours (SD), effective precipitation (Pe), and irrigation water requirement (IR), of spring maize in Inner Mongolia based on data from 50 meteorological stations. The relationship between hydrothermal conditions and yield was revealed using methods that included stepwise regression analysis. The results showed that the GDD during the spring maize growth period ranged from 513 to 2011 °C, with high-value areas concentrated in western and southeastern regions of Inner Mongolia. The GDD showed an increasing trend during all the growth periods. High-KDD areas were mainly distributed in the Alxa League region in western Inner Mongolia, and the KDD showed an increasing trend during all periods except the rapid fertility period. The spatial distribution of the SD was consistent with that of the GDD, and the SD values for each reproductive period showed a decreasing trend. The average Pe and IR values in the last 60 years were 111 mm and 386 mm, respectively, and showed opposite spatial distribution trends. The overall Pe trend was decreasing and that of the IR was increasing, which will aggravate future water resource consumption in the region. Stepwise regression analysis showed that the Pe was the main factor affecting the spring maize yield. Overall, the spring maize fertility period in various regions of Inner Mongolia was extremely uneven in terms of the hydrothermal conditions. This study provides a basis for the regional spatial spring maize layout and the sustainable use of water and heat resources in the region.

1. Introduction

Climate change has become one of the most serious challenges to human society, with the continuous increase in the average global temperature and frequent occurrence of extreme meteorological disasters seriously restricting sustainable modern societal development [1,2,3]. Agriculture is among the sectors most affected by climate change, and food security issues are closely related to global climate change [4,5]. A Food and Agriculture Organization report indicates that human demand for food will increase by > 60% by 2050. Inner Mongolia is one of the most important grain-producing areas in China, ranking eighth in total output, and is also among the five provinces with net grain exports [6]. Spring maize is currently the most widely planted and high-yielding crop in Inner Mongolia and occupies a dominant position in local agriculture. Therefore, in the context of climate change, understanding the changes in meteorological conditions and yield-limiting factors during the spring maize growth period in the region is important for promoting stable growth in the spring maize yield and food security.

Understanding the relationship between changes in local water and heat conditions and crop growth is crucial to revealing the feedback mechanism between climate change and crops. Xing et al. analyzed the relationship between vegetation changes and hydrothermal conditions in Central Asia, and they pointed out that the contribution rate of climate warming to vegetation degradation in the region is over 44% [7]. Gudko et al. pointed out that precipitation is the main factor affecting the changes in the spring maize yield in the Rostov region [8]. Inner Mongolia is among the regions of China that have been significantly affected by climate change, which has significantly altered climate resources in Inner Mongolia and significantly impacted local agricultural production activities [9]. Research has shown that over the past 60 years, the average temperature increase rate in the region has been 0.31 °C/10 years (°C 10 a⁻¹), which is higher than the Chinese average of 0.22 °C 10 a⁻¹ [10,11]. Overall, precipitation shows an insignificant alternating decreasing trend [11]. Changes in hydrothermal conditions have increased opportunities and challenges for spring maize production in this region. Unsuitable temperature increases can reduce the maize yield. Research has found that under certain conditions, the spring maize yield decreases by 5.8% for every 1 °C increase in the average temperature [12]. Temperatures > 29 °C can cause heat stress and reduce the photosynthetic rate in maize [13]. The response of spring maize to increasing temperatures varies at different growth stages. In early growth stages, an increase in temperature increases the emergence and development rates of spring maize by 11% and 12%, respectively [14]. Excessive temperatures during the flowering period can affect maize flowering and silk emergence processes [15]. High temperatures can result in shortened vegetative and reproductive phases, thereby impacting maize growth [16]. In arid regions, precipitation is also an important factor affecting the spring maize yield, and some researchers believe that precipitation has a greater impact on yield than temperature [17]. Drought stress can affect the morphology and physiological characteristics of maize at different growth stages, resulting in a reduced spring maize yield. Water deficiency during the seedling stage can lead to decreases in the chlorophyll content and photosynthetic rate in leaves [18]. Water stress during the jointing stage affects plant growth [19]. Water shortage during the silk emergence stage has the greatest effect on the yield, affecting grain filling and fruit establishment [20]. The Inner Mongolia region is elongated from east to west (2400 km), exhibiting strong spatial heterogeneity and an uneven distribution of solar thermal resources. Over the past 60 years, the accumulated temperature (≥ 10 °C) in Inner Mongolia has shown a significant increasing trend at a rate of 75 °C 10 a⁻¹ [11]. Solar radiation shows a decreasing trend, reaching 39.32 MJ m⁻² 10 a^-1 [21]. Maize is a short-day plant, and an increasing sunshine time can prolong its flowering time and even prevent scioning [22]. The effect of the hydrothermal conditions on the yield varies at different spring maize growth stages; therefore, it is necessary to analyze the characteristics of the changes in hydrothermal conditions at different growth stages.

Most previous studies have focused on analyzing the local hydrothermal conditions during the growth period of spring maize, without considering the effects of hydrothermal conditions on the meteorological yield in the region. In this study, we collated the average cumulative temperature of ≥10 °C from 1961 to 2020 at 50 weather stations in Inner Mongolia. Based on the accumulated temperature, number of days in the growth period of different maturity types of spring maize, and analysis of hydrothermal conditions such as the growth degree-day (GDD), killing degree-day (KDD), effective precipitation (Pe), irrigation water requirement (IR), and sunshine hours (SD) at different spring maize growth stages, linear stepwise regression analysis was used to investigate the effects of water and heat conditions at different growth stages on the spring maize yield, with a view to providing a theoretical basis for the comprehensive utilization of hydrothermal resources and a high and stable spring maize yield in this region.

2. Materials and Methods

2.1. Experiment Location and Data Sources

Inner Mongolia is located on the northern border of China (37°24′–53°23′ N, 97°12′–126°04′ E), bordering Russia and Mongolia, with an area of 1.183 million km², accounting for 12.3% of the total land area of China [23]. Meteorological data were obtained from the China Meteorological Administration (CMA), which includes 50 meteorological stations. The meteorological elements included the daily average temperature, maximum temperature, minimum temperature, average relative humidity, wind speed (10 m), precipitation, sunshine hours, elevation, latitude, and longitude from 1961 to 2020. The research area is shown in Figure 1.

The 5-d sliding average method was used to organize the accumulated temperature in Inner Mongolia from 1961 to 2020 and divide the growth period of different maturity types of spring maize in Inner Mongolia based on the accumulated temperature [11], as shown in

Table 1. Growth period days and heat requirement of spring maize in Inner Mongolia.

Index	Maize Maturity Type
	Extreme Early Maturity	Early Maturity	Medium–Early Maturity	Medium Maturity	Medium–Late Maturity	Late Maturity
Accumulated temperature ≥ 10 (°C·d −1)	<2100	2100–2300	2300–2500	2500–2650	2650–2800	>2800
Number of days of fertility (d)	111	127	135	141	146	152

.

2.2. Research Methods

2.2.1. Climate Trends

Fitting meteorological elements using a linear equation reveals trends in their changes [24]. The calculation formula is as follows:

$$\mathrm{Y}=\mathrm{a}\mathrm{t}+\mathrm{b}$$

(1)

where a and b are regression coefficients, and ×10 was used to represent the climate tendency rate of meteorological elements every 10 years. A positive climate tendency rate indicates an increasing trend in the timescale of a meteorological element, whereas a negative value indicates a decreasing trend in the timescale.

2.2.2. Growing Degree-Day

Growing degree-days (GDDs) refer to the effective cumulative temperature experienced during the completion of a certain fertility stage, which characterizes the heat resources of a crop growth period. The suitable temperature range for growing maize is 10–30 °C [25,26]. The GDD is calculated using Equation (2):

$$\mathrm{G}\mathrm{D}\mathrm{D}=\left\{\begin{array}{c}{\mathrm{T}}_{\mathrm{ave}}-{\mathrm{T}}_{\mathrm{base}} (10\le {\mathrm{T}}_{\mathrm{a}\mathrm{v}\mathrm{e}}\le 30)\\ {\mathrm{T}}_{\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}}-{\mathrm{T}}_{\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{e}} ({\mathrm{T}}_{\mathrm{a}\mathrm{v}\mathrm{e}}>30)\\ 0 ({\mathrm{T}}_{\mathrm{a}\mathrm{v}\mathrm{e}}<{\mathrm{T}}_{\mathrm{b}\mathrm{a}\mathrm{s}\mathrm{e}})\end{array}\right.$$

(2)

where T_ave is the mean temperature (°C); T_base is the minimum threshold (10 °C) appropriate for maize growth; and T_high is the maximum temperature (30 °C) appropriate for maize growth.

2.2.3. Killing Degree-Day

When the temperature exceeds the suitable temperature for the maize growth period, it adversely affects maize growth. Killing degree-days (KDDs) are heat indices that characterize high-temperature disasters during crop growth [27]. The calculation formula is:

$$\mathrm{K}\mathrm{D}\mathrm{D}=\left\{\begin{array}{c}{\mathrm{T}}_{\mathrm{m}\mathrm{a}\mathrm{x}}-{\mathrm{T}}_{\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}} ({\mathrm{T}}_{\mathrm{m}\mathrm{a}\mathrm{x}}>{\mathrm{T}}_{\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}}) \\ 0 ({\mathrm{T}}_{\mathrm{m}\mathrm{a}\mathrm{x}}\le {\mathrm{T}}_{\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}})\end{array}\right.$$

(3)

where T_max is the highest daily temperature (°C) and T_high is the highest temperature suitable for maize growth (30 °C).

2.2.4. Effective Precipitation

The method recommended by the Soil Conservation Service of the United States Department of Agriculture was used to calculate the P_e [28] using the following formula:

$${\mathrm{P}}_{\mathrm{e}}=\left\{\begin{array}{c}P\left(4.17-0.2\mathrm{P}\right)/4.17\left(\mathrm{P}\le 8.3\mathrm{m}\mathrm{m}\right)\\ 4.17+0.1 \mathrm{P} (\mathrm{P}>8.3 mm) \end{array}\right.$$

(4)

where P is the daily precipitation (mm) and P_e is the effective precipitation (mm).

2.2.5. Crop Water Requirement and Irrigation Water Requirement

Using the single-crop coefficient method to calculate the daily water requirement during the maize growth period, the water requirement for each growth period was determined by accumulating the daily water requirements. The calculation formula is as follows:

$${\mathrm{E}\mathrm{T}}_{\mathrm{c}}={\mathrm{E}\mathrm{T}}_0\times {\mathrm{K}}_{\mathrm{c}}$$

(5)

where ET_c is the daily crop water requirement (mm); ET₀ is the daily reference crop evapotranspiration (mm); and K_c is the crop coefficient.

The ET₀ was calculated using the Penman-Monteith equation. The calculation formula is:

$${\mathrm{E}\mathrm{T}}_0={\displaystyle \frac{0.408∆\times \left({\mathrm{R}}_{\mathrm{n}}-\mathrm{G}\right)+\frac{900\mathsf{\gamma }\times {\mathrm{U}}_2\times \left({\mathrm{e}}_{\mathrm{s}}-{\mathrm{e}}_{\mathrm{d}}\right)}{\left(\mathrm{T}+273\right)}}{∆+\mathsf{\gamma }\times \left(1+0.34{\mathrm{U}}_2\right)}}$$

(6)

where ET₀ is the reference evapotranspiration (mm day⁻¹), T is the air temperature at 2 m height (°C), △ is the slope of the vapor pressure curve (KPa °C), Rn is the net radiation at the crop surface (MJ m⁻² day⁻¹), G is the soil heat flux density (MJ m⁻¹ d⁻¹), γ is the psychrometric constant (KPa °C⁻¹), es is the saturation vapor pressure (KPa), ed is the actual vapor pressure (kPa), and U₂ is the wind speed at 2 m height (m s⁻¹).

The IR refers to the amount of water that crops need to rely on for irrigation to supplement their growth process [29], which can characterize the degree of water shortage during crop growth stages. In arid areas such as Inner Mongolia, the IR is generally expressed as the difference between the crop water requirements and the Pe, and it is calculated as follows:

$$\mathrm{I}\mathrm{R}=\left\{\begin{array}{c}{\mathrm{E}\mathrm{T}}_{\mathrm{c}}-{\mathrm{P}}_{\mathrm{e}} ({\mathrm{E}\mathrm{T}}_{\mathrm{c}}>{\mathrm{P}}_{\mathrm{e}}) \\ 0 ({\mathrm{E}\mathrm{T}}_{\mathrm{c}}\le {\mathrm{P}}_{\mathrm{e}}) \end{array}\right.$$

(7)

In the present study, the entire maize growth period was divided into four stages. Early growth period (L_ini): sowing–seven leaves. Rapid growth period (L_dev): seven leaf–tassels. Middle growth stage (L_mid): tassel–milk. Late growth stage (L_late): milk–maturity. The GDD, KDD, SD, PE, and IR were counted at each stage, spatially interpolated, and plotted using the inverse distance weighting method in the ArcMap 10.2 toolbox.

2.2.6. Meteorological Yield Separation

Typically, the crop yield can be separated into three components: trend yield (Y_t), meteorological yield (Y_c), and random error [30]. Trend yields are primarily influenced by social production conditions and scientifically developed technologies. Meteorological yields are usually affected by climate change and are also known as fluctuating yields. The formula is as follows:

$$\mathrm{Y}={\mathrm{Y}}_{\mathrm{t}}+{\mathrm{Y}}_{\mathrm{c}}+{\mathrm{Y}}_{\mathrm{r}}$$

(8)

where Y_t is the trend yield; Y_c is the meteorological yield; and Y_r is the random error.

In this study, a 5-year sliding average method was used to separate the trend yield from the meteorological yield, in which the random error was negligible. The meteorological yield was obtained by removing the modeled trend yield from the actual yields. The formula is as follows:

$${\mathrm{Y}}_{\mathrm{c}}=\mathrm{Y}-{\mathrm{Y}}_{\mathrm{t}}$$

(9)

3. Results

3.1. Spatial and Temporal Variation Characteristics of the GDD During the Spring Maize Growth Period

Due to the different accumulated temperatures in various regions of Inner Mongolia, there were significant differences in the duration of the spring maize growth period. The number of growth period days and the temperature changes directly led to the different GDD performances of spring maize during different growth periods (Figure 2). In the early growth period, the GDD ranged from 89 to 419 °C, with an average of 243 °C; that during the rapid growth period ranged from 238 to 737 °C, with an average of 489 °C; and that during the middle growth period ranged from 166 to 566 °C, with an average of 376 °C. The average GDD in the late growth period was 153 °C, ranging from 21 to 289 °C. The GDD during the entire growth period ranged from 513 to 2011 °C, with an average of 1261 °C. On the time scale, the GDD generally showed an increasing and then decreasing trend. Regarding the spatial distribution, the distribution of high GDD values in different growth stages was similar, concentrated in the Alxa League in western Inner Mongolia and Tongliao and Chifeng in southeastern Inner Mongolia. The distribution of low GDD values in the early, late, and entire reproductive periods was similar and concentrated in the northeastern region of Inner Mongolia, Xilingol League, and the southern region of Ulanqab City. The low-GDD areas during the rapid and middle growth periods were mainly distributed in the northeast of Inner Mongolia, with a smaller distribution range in the middle-east.

The spatial distribution of the GDD climate trend rate at different spring maize growth stages in Inner Mongolia is shown in Figure 3a–e. From sowing to maturity, the GDD climate trend rate for each growth stage increased. The climate trend rate during the early growth period ranged from 3.85 to 17.32 °C 10 a⁻¹, with an average of 11.91 °C 10 a⁻¹. More than 57% of the stations had an above-average increasing trend in the GDD, mainly distributed in the western and middle-eastern regions of Inner Mongolia, with relatively small increase rates in the middle-southern and northeastern regions. The average climate trend rate during the rapid growth period was 13.20 °C 10 a⁻¹, with a significant increase in the GDD in the northern region, ranging from 13.51 to 21.10 °C 10 a⁻¹. The trend of the GDD increase in the southern region was relatively small (i.e., <13.51 °C 10 a⁻¹). The average climate trend rate during the middle growth period was 10.34 °C 10 a⁻¹, and the distribution range of high-value areas was relatively small, mainly distributed near the Xilingol League. Among the stations, 86% were maintained at <12.58 °C 10 a⁻¹. The average climate trend rate in the later growth period was 8.24 °C 10 a⁻¹, with a significant increase in the central and eastern regions. The increasing trend of the GDD throughout the entire growth period ranged from 16.91 to 68.32 °C 10 a⁻¹, with a larger trend in the central and eastern regions. Among them, 34% of the stations had an increasing trend > 47 °C 10 a⁻¹.

3.2. Spatial and Temporal Variation Characteristics of the KDD During the Spring Maize Growth Period

The spatial and temporal distribution characteristics of the KDD at different spring maize growth stages in Inner Mongolia are shown in Figure 4. The regions prone to high-temperature disasters during different growth stages were similar, and the areas with high KDD values were mainly distributed in the Alxa League region in western Inner Mongolia. In the early growth period, the KDD ranged from 1 to 36 °C, with an average of 8 °C; during the rapid growth period, it ranged from 2 to 182 °C, with an average of 37 °C. The spatial distribution was similar to that during the early growth stage, with the lowest KDD values in the central and northeastern regions. The spatial distributions of the KDD values in the middle and late growth periods were relatively similar, with the KDD values in the central and eastern regions being in the low range, with mean KDD values of 25 °C and 3 °C, respectively. The range of the KDD throughout the entire growth period ranged from 4 to 380 °C, with a spatial distribution similar to that in the early growth stages. The period of high-temperature disasters that are prone to occur during various growth periods was mainly concentrated in the rapid growth period.

The spatial distribution of the KDD climate tendency rates during different growth stages of spring maize in Inner Mongolia from 1961 to 2020 is shown in Figure 5a–e. From sowing to maturity, the KDD climate trend rate tended to increase in all the fertility periods except the rapid fertility period. During the early growth period, the increasing trend was smallest in the central and northeastern regions, and largest in the southeastern region. During the rapid growth period, all areas showed increasing trends except for the Chifeng and Baoguotu areas, which showed decreasing trends. Among them, the KDD in the western region showed a larger increasing trend (i.e., >4.65 °C 10 a⁻¹), and in most areas, the increasing trend ranged from 2.33 to 4.65 °C 10 a⁻¹. During the middle growth period, there was a significant increase in the KDD in the central-eastern and western regions, whereas the increase was relatively small in the central and northeastern regions. In the late growth period, areas with a significant increase in the KDD were mainly distributed in the Alxa League region in western Inner Mongolia, and the increasing trend in other areas was relatively small (i.e., <0.92 °C 10 a⁻¹). The KDD throughout the entire growth period ranged from 1.34 to 17.88 °C 10 a⁻¹, with the largest increasing trend in the western region.

3.3. Spatial and Temporal Variation Characteristics of the SD During the Spring Maize Growth Period

The spatial distribution of the SD during the spring maize growth period in Inner Mongolia is shown in Figure 6a–e. The spatial distribution of the SD in different growth stages was similar, showing a decreasing trend from southwest to northeast. The SD in the early growth period ranged from 315 to 477 h, with a mean of 404 h. The SD in the rapid growth period ranged from 281 to 495 h, with a mean of 395 h. The SD in the middle reproductive period ranged from 232 to 371 h, with a mean of 300 h. The SD in the late reproductive period ranged from 128 to 288 h, with a mean of 217 h. The SD in the full growth period ranged from 956 to 1631 h, with a mean of 1316 h. From sowing to maturity, the SD of spring maize showed a decreasing trend, and light resources were mainly concentrated in the early growth period.

The spatial distribution of the SD climate trend rates at different growth stages in Inner Mongolia from 1961 to 2020 is shown in Figure 7. From sowing to maturity, the spring maize SD mainly showed a decreasing trend in most regions of Inner Mongolia, with significant variability in the SD trends among regions. The average climate trend rate during the early growth period was −3.44 h 10 a⁻¹. Among the sites, 77% showed a decreasing trend in the SD values, and the sites showing an increasing trend were mainly concentrated in Chifeng and Alxa. The stations exhibiting an increasing trend in the rapid growth period were mainly concentrated in parts of Hulunbeier, with the remainder of the region showing mainly decreasing trends. The sites showing an increasing trend during the middle growth period were mainly concentrated in the middle-east regions of Inner Mongolia, with an average climate trend rate of −2.29 h 10 a⁻¹. The stations showing increasing trends during the late growth period were mainly concentrated in the northeastern part of Inner Mongolia. The climate trend rate of the SD throughout the entire growth period ranged from −41.39 to 11.98 h 10 a⁻¹, with an average of −12.87 h 10 a⁻¹.

3.4. Spatial and Temporal Variation Characteristics of the Pe During the Spring Maize Growth Period

The spatial variation in the Pe during different spring maize growth stages in Inner Mongolia from 1960 to 2020 is shown in Figure 8a–e. During the early growth period, the average annual Pe ranged from 3 to 38 mm, with an average of 19 mm. The spatial distribution showed a trend of a larger distribution in the northeastern region, followed by that in the central region, and a smaller distribution in the western region. The average Pe value during the rapid growth period was 41 mm, and the range of the second-highest values increased in the eastern region. During the middle growth period, the Pe ranged from 6 to 54 mm, with an average of 34 mm. The average Pe value in the late growth period was 16 mm, ranging from 3 to 28 mm. The spatial distributions of the Pe during the middle and late growth periods were similar. The entire growth period Pe ranged from 20 to 178 mm, with a general distribution trend of being highest in central-eastern and northeastern regions and lowest in other regions. From sowing to maturity, the Pe during the spring maize growth period was mainly concentrated in the rapid and middle growth periods, accounting for 67.9% of the entire growth period Pe.

The spatial distribution of the climate trend rate of the Pe during the different growth stages of spring maize is shown in Figure 9a–e. The climate trend rate of the Pe in the early stage of reproduction ranged from −0.26 to 0.35 mm 10 a⁻¹. Except for Guaizi Lake in Alxa League and East Ujimqin Banner, West Ujimqin Banner, Abaga Banner, and Sunite Left Banner in the Xilingol League, the climate inclination rate of the Pe in other areas showed an increasing trend. The climate trend rate of the Pe in the early growth period was between −0.26 and 0.35 mm 10 a⁻¹. Except for Guaizi Lake in the Alxa League, and East Ujimqin Banner, West Ujimqin Banner, Abaga Banner, and Sunite Left Banner in the Xilingol League, the climate trend rate of the Pe in other areas showed an increasing trend. The climate trend rate of the Pe during the rapid growth period ranged from −1.81 to 1.74 mm 10 a⁻¹, with an average of −0.24 mm/10 a. The middle-west regions showed an increasing trend, whereas other regions showed a decreasing trend. The Pe climate trend rate in the middle growth period ranged from −2.18 to 1.09 mm 10 a⁻¹. Regarding the spatial distribution of the Pe, the middle-east region showed a decreasing trend, whereas other regions showed an increasing trend. The average climate trend rate of the Pe in the late growth period was −0.21 mm 10 a⁻¹, showing an overall decreasing trend. Regarding the spatial distribution, the western region showed an increasing trend, and the middle-east and northeastern regions showed a decreasing trend. The Pe climate trend rate during the entire growth period ranged from −3.78 to 2.71 mm 10 a⁻¹, with a mean of −0.59 mm 10 a⁻¹, which was consistent with the late reproductive period in terms of the spatial distribution.

3.5. Spatial and Temporal Variation Characteristics of the IR During the Spring Maize Growth Period

The spatial and temporal distribution characteristics of the IR in different spring maize growth stages in Inner Mongolia are shown in Figure 10. The spatial distribution of the IR was similar in each growth period, with the high-IR areas mainly distributed in the Alashan region in western Inner Mongolia and the low-IR areas mainly distributed in the Hulunbeier region in western Inner Mongolia, which was opposite to the spatial distribution of the Pe. In the early growth stages, the IR was relatively low, ranging from 5 to 98 mm, with an average of 48 mm. The IR was higher in the rapid and middle-fertile stages, with mean values of 128 mm and 151 mm, respectively, accounting for 72.4% of the total IR. The IR began to decrease in the late reproductive stage, with a mean of 58 mm. The IR throughout the entire growth period ranged from 105 to 830 mm, with an average of 387 mm.

The spatial and temporal distribution characteristics of the IR climate trend rates for different growth stages of spring maize in Inner Mongolia were summarized, and the results are shown in Figure 11. The climate trend rate during the early growth stage ranged from −2.78 to 2.32 mm 10 a⁻¹; except for Sunite Left Banner, Abaga Banner, Xilinhot, Hohhot, and Guaizihu, which showed an increasing trend, all the other areas showed a decreasing trend. The climate trend rate of the IR during the rapid growth period ranged from −6.72 to 6.44 mm 10 a⁻¹, with an overall increasing trend in the central and northeastern regions and a decreasing trend in other regions. In the middle growth period, the IR climate variability ranged from −5.30 to 9.14 mm 10 a⁻¹, with a decreasing trend in the western region and an increasing trend in the central and eastern regions. The climatic trend rate in the late growth period ranged from −2.88 to 4.28 mm 10 a⁻¹, and the trend was similar to that in the middle growth period. The climate propensity rate for the entire growth period ranged from −17.07 to 19.83 mm 10 a⁻¹, with a mean climate trend rate of 2.12 mm 10 a⁻¹, and with an overall decreasing trend in the western sites and an increasing trend in the central and middle-eastern regions.

3.6. Effect of Hydrothermal Conditions on Yield During the Spring Maize Growth Period

The spatial distribution of the hydrothermal conditions during the spring maize growth period in Inner Mongolia varied significantly, which inevitably significantly impacted the spring maize yield in different regions. Statistical analysis of the sowing area and yield in 12 leagues and cities in Inner Mongolia revealed the following. Due to the small spring maize planting range in Wuhai, there were many time series discontinuities in the spring maize yield data in Xilingol, Ulanqab, Xing’an League, and Tongliao; therefore, we ultimately chose the other seven regions (Alxa League, Bayannur City, Ordos, Baotou, Hohhot, Chifeng, and Hulunbeier) to analyze the impact of the hydrothermal conditions on the yield. The results are summarized in

Table 2. Effect of the hydrothermal conditions on the meteorological yield during the spring maize growth period.

Typical Region	Regression Model	R2	p
Alxa	y = 1700.66 − 18.51 IR4	0.218	<0.05
Bayannur	*	*	*
Ordos	y = −1845.52 + 12.812 Pe5 + 15.394 KDD2	0.292	<0.05
Baotou	y = 2048.07 − 6.957 SD3	0.146	<0.05
Hohhot	y = −1025.71 + 7.40 Pe5	0.262	<0.05
Chifeng	y = −7494.89 + 21.456 Pe5 + 4.611 GDD1 + 2.31 GDD5	0.518	<0.05
Hulunbeier	y = −17,323.49 + 6.752 SD1 + 288.308 Pe1 + 215.05 IR1 + 5.10 GDD2	0.551	<0.05

Note: IR_i is the irrigation water requirement for the i-th growth stage (mm); Pe_i is the effective precipitation during the i-th growth period (mm); KDD_i is the killing degree-days of the i-th reproductive period (°C); SD_i is the sunshine duration of the i-th growth period (h); GDD_i is the growth degree-day of the ith growth period (°C). * not detected.

. The impact of the climatic conditions on the crop yield varied by region. The spring maize yield in Ordos and Hohhot was mainly affected by the Pe, that in the Alxa League was mainly affected by the IR, and that in Baotou was mainly affected by the SD. In the Chifeng region, the meteorological yields were mainly dependent on the Pe and GDD. In the Hulunbeier region, the meteorological yields were influenced by various factors, including the SD, Pe, IR, and GDD.

4. Discussion

The average and accumulated temperatures are important indicators for measuring the heat resources of a region, affecting the crop types, layout variety, and agricultural activities in the area [11]. Maize is a C4 crop that is highly dependent on the accumulated temperature changes. Climate change has significantly altered the heat resources during the spring maize growth period in Inner Mongolia. Regarding the spatial distribution, the high-GDD-value areas during each growth period were mainly concentrated in the western and southeastern regions of Inner Mongolia, and the distribution trend was consistent, with the stable accumulation of 10 °C in Inner Mongolia [31]. An increase in heat resources indicated that some areas had changed from unsuitable to suitable growth areas. It has been highlighted that as the temperatures and atmospheric CO₂ concentrations increase, the crop maturity and acreage change accordingly, with early maturing varieties migrating northwards [32,33,34,35]. The present study found that the GDD showed an increasing trend throughout the entire growth period from 1960 to 2020, with the highest rate of increase concentrated near Balinzuoqi, Erguna, and Sunitezuo Banner in northeastern Inner Mongolia, which may have led to changes in crop maturity in the region. When the temperature increases, the risk of crop growth may also increase. The KDD is a heat index that reflects the heat disaster during the spring maize growth period. Yang et al. [36] found that high temperatures can inhibit crop photosynthesis, leading to interruption to and termination of crop growth and development. Hawkins et al. [37] found that the maize yield in France decreases with an increase in the number of days with a maximum daily temperature >32 °C. There was significant spatial heterogeneity in the changes in the GDD and KDD in Inner Mongolia, whereas the western region had abundant heat resources but was also the area with the largest KDD. The rate of the KDD increase in the western region was significantly higher than in other areas, and the likelihood of being affected by high-temperature and drought disasters is likely to increase. Therefore, attention should be paid to the breeding of heat-tolerant varieties and timely watering during the planting period. The eastern region had insufficient heat resources, and the GDD increase trend was relatively large, whereas the trend of the KDD increase was relatively small. The temperature increase was more favorable for spring maize planting in the eastern region, and the planting range of spring maize can be appropriately expanded.

The spatial distribution of the SD values during the maize growth period in Inner Mongolia showed a gradually decreasing trend from southwest to northeast, and the overall SD values for each growth period showed a decreasing trend, which is consistent with the overall SD value changes in China [38]. A decrease in the SD value can lead to a reduction in solar radiation, which in turn affects the crop yield and quality, mainly during the maize kernel construction period [39,40]. The SD was relatively low in northeastern Inner Mongolia, and the decreasing trend of the SD may have a significant impact on crop growth in the area.

The spatial distribution of the Pe and IR is extremely uneven, showing opposite distribution trends. The western region of Inner Mongolia has a high effective accumulated temperature and has always been a late-maturing spring corn planting area. The crop growth period is relatively fixed, and there is no need to consider the impact of changes in the crop growth period on the crop water requirements. Spring maize in the western region had the smallest Pe and largest IR during the growth period; the Pe showed an increasing trend and the IR showed a decreasing trend. It has also been shown that the coupling of the Pe and crop water requirements in western Inner Mongolia was on an increasing trend in the time scale, which will help alleviate drought conditions in the region [11]. Therefore, in response to the scarcity of water resources in the area, the deployment of water-saving agricultural facilities should be expanded. Analyzing the impact of meteorological factors on the spring maize yield at the growth-stage scale, research has shown that rainfall is the main influencing factor in terms of the spring maize yield in the region, which is consistent with the results of the monthly scale analysis [11].

In the context of climate change, an increase in the cumulative temperature will inevitably change the maturity type and length of the fertility period of crops. In this study, the maturity type of the crops was determined based on the cumulative temperatures over the past 60 years, and the length of the reproductive period of the crops was relatively fixed. Therefore, the results of this study have some limitations. In addition, this study did not consider the effects of the soil texture and subsurface properties on the hydrothermal conditions. The groundwater depth is an indispensable part of the process of water transformation in farmlands [41]. There are evident regional differences throughout Inner Mongolia, and the locally available irrigation water is also a major constraint on agricultural production. Therefore, targeted measures need to be taken to make better use of local hydrothermal conditions and reduce the adverse effects of climate change.

5. Conclusions

Based on historical meteorological data and crop yield data, this study analyzed the spatial and temporal variations in the hydrothermal resources in Inner Mongolia over the past 60 years and revealed the influence of the hydrothermal conditions on the meteorological yields during the spring maize growth period. The main conclusions are as follows.

(1)

The spatial distribution of the GDD in the different growth stages of spring maize was relatively consistent, showing a trend of higher GDD in the western region and lower GDD in the northeastern region. The GDD showed an increasing trend at all growth stages. The KDD was mainly distributed in the western region of the Alxa League, where attention should be paid to preventing the impact of high temperatures on maize growth.

(2)

The spatial distribution of the SD during the spring maize growth period in Inner Mongolia showed a decreasing trend from west to east. Light resources were mainly concentrated in the early growth stages, and the SD for each growth period showed a decreasing trend. The Pe of spring maize ranged from 20 to 178 mm and was mainly concentrated in the rapid growth period. The high-value IR areas were mainly concentrated in the western region of Alxa League, and the low-value areas were mainly concentrated in the northeastern region, which was opposite to the spatial distribution trend of the Pe.

(3)

The effect of the hydrothermal conditions on the spring maize yield varied by region, with precipitation being the main factor affecting the spring maize yield.