Study on Change Rules and Influencing Factors of Soil Moisture in Huaibei Plain

Lingjian KONG, Zhenlong WANG, Bing WANG, Lei WANG

Anhui & Huaihe River Institute of Hydraulic Research, Bengbu 233000, China; Anhui Province Key Laboratory of Water Conservancy and Water Resources, Bengbu 233000, China

Abstract [Objectives] To study the relationship between soil water, groundwater burial depth, and precipitation for summer maize in Huaibei Plain. [Methods] The atmospheric precipitation, soil water and groundwater for the growth period of summer maize in Huaibei Plain were analyzed using the 26-year long series of data from the Wudaogou Hydrological Experimental Station, combined with the hydrogen and oxygen stable isotope tracing method. [Results] The average soil moisture content of summer maize in different growth periods showed a trend of first decreasing, then increasing and then decreasing with the increase of soil depth. The average soil moisture content was the lowest at the surface soil layer. From the characteristic values of hydrogen and oxygen isotopes of atmospheric precipitation, soil water and groundwater, it can be known that the average values of δ18O and δD of soil water decreased with the increase of soil depth, indicating that soil moisture evaporation leads to the enrichment of soil heavy isotopes, and the degree of enrichment decreased from the surface layer to the deep layer of the soil. The seasonal variation of the stable isotope of hydrogen and oxygen in soil water declined with the increase of soil depth. The soil water changes at 30 cm and 50 cm soil depths were the most obvious. The soil was easily recharged by precipitation, and soil evaporation was relatively strong. [Conclusions] The research results are favorable for in-depth understanding of the regional water cycle process, and are expected to provide a certain scientific basis for realizing the efficient and sustainable use of regional groundwater.

Key words Soil water, Precipitation, Huaibei Plain, Hydrogen and oxygen stable isotopes

1 Introduction

Huaibei Plain, located in the south of Huang-Huai-Hai Plain and the transition area of southern and northern climatic zones of China, is a "sensitive area" of climate change in China. The per capita water resources in Huaibei Plain are less than a quarter of the national per capita water resources. It is an area where the supply and demand of water resources are seriously unbalanced. Huaibei Plain is an important agricultural economic zone and grain, cotton and oil production area in China, and shallow groundwater is the main source of agricultural water. With the increasing contradiction between the supply and demand of water resources, people are exploiting a large amount of groundwater resources, making the depth of groundwater present different patterns of change. Soil water, as an important part of a regional water resource, is a link connecting atmospheric precipitation, groundwater and plant water, and a key link that affects the regional water cycle. Soil water is affected by many factors such as precipitation, phreatic water evaporation, temperature and biological absorption and utilization, so it is more susceptible to change. Therefore, promptly grasping the amount of soil water, the reasonable regulation of shallow groundwater and soil water, the source and destination of soil water,

etc.

are essential for the growth of crops and the efficient use of field water resources.Traditional research methods are mainly to determine the amount of water in the soil and its energy state, such as monitoring of field soil water content, calculation of soil water flux, and observation of water budget using ground lysimeter,

etc.

All of these are determining the macroscopic changes of the amount of soil moisture and cannot grasp the specific source and destination. In recent years, hydrogen and oxygen stable isotope technology has gradually been used in the study of soil water migration. Tian Richang

et

al.

studied the soil water migration process of two different vegetation types by tracing the precipitation of the red soil hilly area and the hydrogen and oxygen isotopes of different depths of soil water stability. Du Kang

et

al.

studied the characteristics of hydrogen and oxygen isotopes in different water bodies in the loess hilly area and the recharge and transformation relationships between water bodies by collecting samples of precipitation, river water, groundwater and soil water, using isotope tracing technology, combined with a hybrid model. Using hydrogen and oxygen stable isotopes, Cheng Lipingstudied the contribution of different soil layers (especially deep soil water) to the water consumption of winter wheat in the Changwu loess plateau.

In this study, using the 26-year long series of data (1992-2017) from the Wudaogou Hydrological Experimental Station, combined with the hydrogen and oxygen stable isotope tracing method, based on the characteristic values of hydrogen and oxygen stable isotopes of atmospheric precipitation, soil water, and groundwater from July to October 2020, we analyzed the relationship between soil water and the groundwater burial depth and precipitation for summer maize in the Huaibei Plain.

2 Materials and methods

2.1 Overview of the research area

We carried out the experiment at Wudaogou Hydrological Experimental Station in July-October, 2020. The experimental station (117°21′ E, 33°09′ N) is located in Xinmaqiao Town, Guzhen County in Bengbu City of Anhui Province. The station is equipped with a runoff experiment field, a hydrometeorological multi-element observation field, and an automatic weighing ground lysimeter. The soil in the experimental area is mainly yellow fluvo-aquic soil and lime concretion black soil. The experimental area belongs to the warm temperate semi-humid monsoon climate zone. The annual average rainfall is 893 mm. The average rainfall in the flood season (June to September) accounts for 62% of the total annual rainfall. The annual average sunshine duration is 2 200-2 425 h. The average relative humidity is 73%, and it is the lowest in May-June and highest in July-August, the annual average wind speed is 3.0 m/s, and the annual average temperature is 13.5-14.9 ℃. The area belongs to the shallow water table (SWT), and the burial depth varies between 1.0-3.0 m.

2.2 Relationship between groundwater burial depth and soil water changes

For Wudaogou long series of experimental data, we selected the field soil water data at the vertical soil depth of 0-1.0 m during the summer maize growth period from 1992 to 2017. The growth process of summer maize during the experiment was basically divided into four growth periods: summer maize seedling stage-jointing stage (July 8to August 9), jointing-tasseling stage (August 10-18), tasseling-grain-filling stage (August 19-31) and grain-filling-harvesting stage (September 1-October 1). Under normal conditions, soil water was sampled every 5 d with soil drills (every 1and 6of every month), and the drying method is used to determine the average moisture content at each vertical soil layer of 0-0.1, 0.1-0.2, 0.2-0.3, 0.3-0.4, 0.4-0.5, 0.5-0.6, 0.6-0.8, 0.8-1.0, and 0-1.0 m. We selected the groundwater level data from the observation well data of Datian, and the meteorological data (rainfall, temperature, light, wind speed, relative humidity) from the meteorological field of the experimental station.

2.3 Relationship between soil water change and rainfall based on isotope tracing

Based on the automatic weighing ground lysimeter, we studied the relationship between the change of soil water moisture content and rainfall in different soil layers during the summer maize growth period in 2020. Combining with the long series of data of Wudaogou Experimental Station, we further explored the relationship between different groundwater burial depth conditions and changes in soil moisture content. The soil texture of the experimental lysimeter was undisturbed lime concretion black soil, with a land area of 2 mand a height of 4.4 m. The suitable groundwater burial depth for summer maize growth in Huaibei Plain was 0.8-2.0 m. The growth status mainly depends on the soil moisture in the active layer of the maize root system, and the soil moisture in the active layer of the maize root system mainly depends on precipitation and groundwater replenishment, so in this study, we controlled the groundwater burial depth to be 1.5 m in normal years. We monitored the soil moisture content using a three-parameter soil sensor, with depths of 0.3, 0.5, 1.0, and 1.3 m, and made a record every 5 d. The growth process of summer maize during the experiment in 2020 was basically divided into four growth periods: summer maize seedling stage-jointing stage (July 8to August 9), jointing-tasseling stage (August 10-18), tasseling-grain-filling stage (August 19-31) and grain-filling-harvesting stage (September 1-October 3).

During the maize growing period, we collected a total of 24 rainfall samples, 30 soil profile water samples, and 12 groundwater samples. The hydrogen and oxygen stable isotope analysis of water samples was carried out at the State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, and the analysis was performed using a laser liquid-water isotope analyzer (Picarro, L2130-i). Each sample was measured 6 times, and the average of the last 3 times was taken as the original value. Using three laboratory standards, we calibrated the original values of the water samples, and regularly calibrated the three laboratory standards are with two international standard samples VSMOW and GISP, purchased from the International Atomic Energy Agency. The standard deviation δO of repeated samples in instrument analysis is ±0.1‰, and δD is ±0.5‰. Both the δO and δD values are expressed in thousandths of a difference relative to V-SMOW.

3 Results and analysis

3.1 Effects of groundwater burial depth on soil water changes

The factors affecting soil water mainly include rainfall, groundwater level, evapotranspiration, and underlying surface conditions. The groundwater burial depth is deeply affected by rainfall. Through the analysis of the soil water data of the vertical soil layer at the depth of 0-1.0 m in the summer maize in growing period in Wudaogou Experimental Station from 1992 to 2017, we studied the effects of groundwater burial depth on soil water. The annual average soil water changes in each soil layer over time during the summer maize growth period are shown in Table 1.

From Table 1, it can be seen that the average soil moisture content of summer maize in different growth periods showed a trend of first decreasing, then increasing and then decreasing with the increase of soil depth. The average soil moisture content of the 0-0.2 m soil layer was the lowest, and the soil moisture content of the 0.3-0.5 m soil layer was the highest. From Table 1, it can be known that the average soil moisture content of the surface layer (0-0.1 m) and the deep layer (0.8-1.0 m) had the largest variation coefficient during the summer maize growth period, indicating that the soil moisture content has the largest variation during this period, mainly because the surface soil water after rain is greatly affected by evaporation, and rainfall infiltration raises the groundwater level, and deep soil water is affected by groundwater replenishment after rainfall, accordingly leading to a greater change in soil moisture content.

Table 1 Soil moisture content of each profile soil layer during the summer maize growth period

From Fig.1, it can be seen that the average soil moisture content of the 0-1 m soil layer at different growth periods of summer maize generally has a negative correlation with the groundwater burial depth. Specifically, during the seedling-jointing stage, which lasted about 30 d, the rainfall was high, the plant growth was vigorous, and the demand for water was large, the soil moisture content was relatively high, and the groundwater burial depth varied greatly, which was mainly affected by rainfall and the phreatic water evaporation. During jointing-tasseling stage, which lasted for a short period of time, there was only one time of rainfall and the soil moisture content was relatively low. During the tasseling-grain-filling stage, the water demand of summer maize growth was the largest, the soil moisture content was very high, and the groundwater burial depth was decreased. During the maize grain-filling-harvesting stage, the groundwater burial depth was decreased and the soil moisture content showed an increasing trend.

Fig.1 Curves for average soil moisture content of 0-1 m soil layer and groundwater burial depth during the summer maize growth period

3.2 Relationship between soil water change and rainfall based on isotope tracing

Soil water isotope is influenced by atmospheric precipitation and soil evaporation. The rules soil water migration can be explored by the characteristic values of soil water isotope. We made a statistic of the characteristic values of hydrogen and oxygen stable isotopic compositions of different types of water samples, and the statistical results are shown in Table 2.

According to Table 2, in general, the δO and δD characteristic values of atmospheric precipitation, soil water, and groundwater change were as follows: atmospheric precipitation>soil water>groundwater, which is due to the influence of factors such as temperature and humidity in the air during precipitation, as well as the influence of water vapor condensation and water vapor mass transportation methods. Specifically, the average value of soil water δO and δD decreased with the increase of soil depth, indicating that soil moisture evaporation leads to soil heavy isotope enrichment, and the degree of enrichment decreased from the soil surface to the deep layer, which is consistent with the conversion of rainfall infiltration to replenish soil water. At soil depths of 0.3 and 0.5 m, the soil was easy to receive precipitation replenishment, and soil evaporation was stronger, and the seasonal difference of hydrogen and oxygen stable isotopes was more obvious than that of deep soils. At the 0.5-1.0 m soil layer, the stable hydrogen and oxygen isotopic values of soil water varied greatly, mainly due to frequent exchanges between groundwater and soil water. At the 1.0-1.8 m soil layer, the stable isotopes of hydrogen and oxygen in soil water were relatively stable, which is due to the weak evaporation of deep soils and the replenishment of groundwater, making the soil moisture content relatively stable.

We performed a linear regression analysis on the hydrogen and oxygen stable isotopes of atmospheric precipitation during the summer maize growing period, and obtained the equation for the regional atmospheric precipitation line: δD=7.662 1δO+6.524 3(N=20,

R

=0.875 4). Then, we conducted a one-variable linear regression analysis on the stable isotopes of phreatic water in the study area, and obtained the equation for the regional phreatic evaporation trend line: δD=7.732 5δO +6.807 2 (N=42,

R

=0.900 7), as shown in Fig.2. From Fig.2, it can be seen that these water samples have the same water source characteristics, and they all come from atmospheric precipitation. The phreatic evaporation trend line always falls below the atmospheric precipitation line, reflecting that soil water and groundwater undergo different degrees of evaporation after receiving atmospheric precipitation. The water samples falling on different parts of the phreatic evaporation trend line reflect differences in the degree of unbalanced evaporation experienced by different water bodies. Especially, the surface soil water sample deviated greatly, indicating that it was more intensively subjected to evaporation; the groundwater sample was the most concentrated, indicating that it received weaker evaporation.

Table 2 Statistical characteristics of hydrogen and oxygen stable isotopes in different types of water samples

The isotope of soil water in different seasons of summer maize growth period was affected by atmospheric precipitation and soil evaporation. The relationship between soil water and atmospheric precipitation can be studied through the seasonal variation of soil water δO and δD, as shown in Table 3. From Table 3, it can be seen that due to the long duration of the maize seedling stage and the jointing stage, the rainfall was abundant, and the soil water was replenished by the precipitation after evaporation, resulting in obvious changes in the stable isotopes of hydrogen and oxygen; the water demand of maize gradually decreased from the jointing to the entire grain-filling stage, the soil moisture content gradually increased, and the rainfall and rainfall duration were different. With continuation of time, the rainfall was fractionated by isotopes, and heavy isotopes were gradually depleted, leading to generation of difference.

Fig.2 Relationship between atmospheric precipitation δD and δ18O

Table 3 Seasonal change of the average value of δ18O and δD of 30-180 cm soil water

4 Conclusions

(i) From the field soil water data at the vertical soil depth of 0-1.0 m during the summer maize growth period from 1992 to 2017 obtained from the long series data of Wudaogou Experimental Station, the average soil moisture content of summer maize in different growth periods showed a trend of first decreasing, then increasing and then decreasing with the increase of soil depth. The average soil moisture content of the 0-0.2 m soil layer was the lowest, and the soil moisture content of the 0.3-0.5 m soil layer was the highest.

(ii) From the characteristic values of hydrogen and oxygen isotopes of atmospheric precipitation, soil water and groundwater, it can be known that the average values of δO and δD of soil water decreased with the increase of soil depth, indicating that soil moisture evaporation leads to the enrichment of soil heavy isotopes, and the degree of enrichment decreased from the surface layer to the deep layer of the soil.

(iii) From the regional atmospheric precipitation line equation and the regional phreatic evaporation trend line equation, these water samples showed the same water source characteristics, all came from atmospheric precipitation. Both soil water and groundwater underwent different degrees of evaporation after receiving atmospheric precipitation. The water samples falling on different parts of the phreatic evaporation trend line reflect differences in the degree of unbalanced evaporation experienced by different water bodies.

(iv) From the relationship between the stable isotope values of hydrogen and oxygen in different soil layers and the seasonal variation of soil water δO and δD, it can be seen that the seasonal variation of the stable isotope of hydrogen and oxygen in soil water declined with the increase of soil depth. The soil water changes at 30 and 50 cm soil depths were the most obvious. The soil was easily recharged by precipitation, and soil evaporation was relatively strong. Rainfall and rainfall duration varied in different seasons. With continuation of time, the rainfall was fractionated by isotopes, and heavy isotopes were gradually depleted, leading to generation of difference.