Abstract
Urbanization significantly impacts hydrological processes and the water environment in the urban area. Studies on soil moisture are necessary to explore the hydrological characteristics against the background of urbanization. A regional grassland with Bermudagrass in Yangzhou City, China, was adopted as the study area. An analysis of its response characteristics to rainfall infiltration was carried out for different rainfall events. The Penman–Monteith equation was used to estimate the hourly evapotranspiration (ET) of the soil layers at depths of 0–30 cm and 0–10 cm. The results indicate that the response time of the soil water content in the root layer (0–10 cm) decreases with the decrease of the soil water content in the topsoil. The rate of the increase of the soil water content increases as the rainfall intensity increases in the state of unsaturation. The soil water in the root layer provided more than 70% of the total ET. The Nash efficiency coefficient of the simulation results of the soil water content at different depths obtained using the Hydrus-1D model was more than 0.75. The rationality of the results for the different rainfall events and the infiltration depth were verified via numerical simulations using the Hydrus-1D model.
HIGHLIGHTS
Studies on soil moisture are necessary to explore hydrological characteristics in an urban region.
A regional Bermudagrass in Yangzhou City was adopted as the study area.
Main response characteristics of soil moisture to rainfall were clarified.
ET mainly provided by the soil water in the root layer was revealed.
The rationality for rainfall events and the infiltration depth were verified using the Hydrus-1D.
Graphical Abstract
INTRODUCTION
Urbanization leads to widely occurring land use and land cover changes (Zhang et al. 2020; Imran et al. 2021; Mohammad et al. 2021). Accelerated urbanization continues to convert natural land types to impervious surfaces, resulting in serious environmental impacts and affecting the growth of urban plants (Song et al. 2015). The urbanization process alters the hydrological performance of an area (Armson et al. 2013). Urbanization causes a range of environmental challenges as a direct result of the biochemical and physical changes to the hydrological systems (Fletcher et al. 2013; Zang et al. 2019). Compared with the original landform units, decreases in the water retention function of various artificially disturbed landform units due to urbanization activities are the main factor causing urban water and soil losses and the aggravation of urban floods under certain rainfall conditions and in specially designed drainage networks (Baek et al. 2015; Shi et al. 2016). Urban grasslands are expanding rapidly with urbanization, which is expected to increase at unprecedented rates in the near future. Compared with impervious areas, grassland is able to effectively delay runoff (Liu et al. 2020a). The large and increasing area of urban grasslands and their impact on water justify the need for a better understanding of these areas (Duran et al. 2013). Grassland is an indispensable part of the urban green space ecosystem. It plays an important role in improving the regional ecological environment, regulating the hydrological cycle, and reducing the loss of soil and water (Livesley et al. 2010; Xiong et al. 2014; Xu & Cheng 2019; Yang et al. 2020). Soil moisture plays a critical role in land surface–plant–atmosphere interactions, and it directly impacts food security, human health, and ecosystem functions (Huang & Shao 2019). Evapotranspiration (ET) is an important hydrological process in the water cycle and plays a key role in the energy budget and water balance of the earth–atmosphere system (Zhang & Shen 2007; Zhao et al. 2018). It is important to study the changes in the soil moisture and ET of grassland vegetation under the background of urbanization to improve the urban ecological environment and enhance the construction level of a sponge city. Many studies indicated that rainfall affects the change of soil moisture. Wiekenkamp et al. (2016) conducted research on the spatial and temporal occurrences of the preferential flow in a forested headwater catchment using the preferential flow model and found that rainfall has a significant influence on soil moisture. Liu et al. (2020b) analyzed the variation characteristics of soil moisture parameters during rainfall and proposed that the amount of rainfall is the most prominent rainfall feature controlling the soil moisture response. Chen et al. (2020) conducted research on the response of the soil moisture to rainfall events in black locust plantations at different stages of restoration in the hilly-gully area of the Loess Plateau, China, and found that rainfall infiltration mainly occurred at soil depths of 0–60 cm. The rainfall infiltration was mainly jointly influenced by the rainfall attributes and soil properties. In contrast, there are relatively few studies on the response characteristics of grassland to rainfall in urban areas in China. Determining the response characteristics of soil moisture to rainfall in urban grassland can provide a basis for the studies on soil moisture movement in grassland vegetation against the background of urbanization. This is of great significance for developing the grassland vegetation, conserving water resources, and improving the hydro-ecological environment in urban areas. In this study, a regional urban grassland was adopted as the study location to reveal the response characteristics of the soil moisture in grassland vegetation areas to rainfall against the background of urbanization. Analysis of the infiltration characteristics during different rainfall events was carried out, the ET over the grassland in the study area was calculated and evaluated, and the infiltration depths of the different rainfall events were verified via numerical simulation using the Hydrus-1D model. The results of this study provide a scientific basis for further studies of soil moisture in grasslands during urbanization and the improvement of the urban ecological environment based on the development of grasslands.
MATERIALS AND METHODS
General setting of the study area
Yangzhou City is located in the middle of Jiangsu Province on the southern part of the Jianghuai Plain. The study area has a subtropical humid monsoon climate. The annual average temperature is 16.1 °C. The annual average rainfall is about 1,000 mm, and 67% of the total rainfall is concentrated in the main rainy season from May to September (Zhou et al. 2019). Yangzhou City is one of the national ecological garden sites in China. There are many green spaces and parks in this urban area, with a green coverage rate of 44.03%, which plays an important role in improving the water circulation and reducing the waterlogging in the Yangzhou urban area.
According to the field survey, the 0–60 cm soil depth in the study area consists of mainly silty loam. The groundwater (phreatic water) level varies with the seasons and is about 2–5 m from the ground surface. There are many buildings and impervious pavement areas around the study area.
Data acquisition
An automatic weather station (mode: U30-nrc-10-s100-000; Onset Company, USA) was set up in the study area to record the hourly temperature, rainfall, solar radiation, wind speed, and relative humidity. The elevation of the observation point was 14 m, and the equipment height was 2 m (Figure 1(b)). The soil water contents were measured in two sites using soil moisture meters (mode: H21-002; Onset Company, USA), the observation depths of one point (P1) were 10, 25, and 40 cm, while those of the other point (P2) were 5, 15, 30, and 60 cm.
To evaluate the response of the soil moisture in the root layer of the Bermudagrass lawn to different rainfall processes, a simple soil experiment was carried out according to the weather forecasting information. The soil water content of the topsoil (0–5.5 cm) at point P1 was sampled and weighed before each rainfall event, and then it was weighed after drying to determine the initial water content of the topsoil. In addition, a tipping bucket rain gauge (mode: 7852M-L10, Onset Company, USA) was used to measure the rainfall at 10-min intervals, while the soil water content at point P1 (10, 30, and 60 cm) was measured every 10 min during the rainfall observations.
Additionally, the soil samples were collected from different depths in the study area. A laser particle size analyzer (model: Mastersizer 3000E; Malvern Company, UK) was used to measure the particle compositions of the soil samples, and the particle size distributions (clay, sand, and silt contents) were obtained.
ET calculation
Hydrus-1D model description
Basic equations
Definite solution conditions
Parameter calibration
According to the screening results of the soil particle size obtained from the particle composition of the soil samples (Table 1), the Rosetta module, which is based on a neural network, was used to preliminarily determine the saturated water content θs, the residual water content θr, and the saturated hydraulic conductivity Ks (Li et al. 2015). Based on this, comparisons of the Hydrus-1D calculated results and the measured soil water contents at various depths (5, 15, 30, and 60 cm) were performed. The adjustment of each parameter value and repeated model calculations were carried out to reduce the difference between the simulated results and the measured results to achieve the calibration of the parameters (Table 1).
Soil depth (cm) . | Sand grain % 50–2,000 μm . | Silt % 2–50 μm . | Clay % <2 μm . | θs cm3·cm−3 . | θr cm3·cm−3 . | α cm−1 . | n . | Ks cm·min–1 . | . |
---|---|---|---|---|---|---|---|---|---|
0–10 | 27.10 | 65.71 | 7.19 | 0.35 | 0.045 | 0.0047 | 1.23 | 2.08 | 0.5 |
10–20 | 21.78 | 69.76 | 8.48 | 0.37 | 0.051 | 0.0046 | 1.23 | 2.33 | 0.5 |
20–40 | 17.72 | 74.05 | 8.24 | 0.36 | 0.056 | 0.0048 | 1.33 | 1.85 | 0.5 |
40–60 | 19.33 | 71.23 | 9.45 | 0.37 | 0.053 | 0.0045 | 1.42 | 1.43 | 0.5 |
Soil depth (cm) . | Sand grain % 50–2,000 μm . | Silt % 2–50 μm . | Clay % <2 μm . | θs cm3·cm−3 . | θr cm3·cm−3 . | α cm−1 . | n . | Ks cm·min–1 . | . |
---|---|---|---|---|---|---|---|---|---|
0–10 | 27.10 | 65.71 | 7.19 | 0.35 | 0.045 | 0.0047 | 1.23 | 2.08 | 0.5 |
10–20 | 21.78 | 69.76 | 8.48 | 0.37 | 0.051 | 0.0046 | 1.23 | 2.33 | 0.5 |
20–40 | 17.72 | 74.05 | 8.24 | 0.36 | 0.056 | 0.0048 | 1.33 | 1.85 | 0.5 |
40–60 | 19.33 | 71.23 | 9.45 | 0.37 | 0.053 | 0.0045 | 1.42 | 1.43 | 0.5 |
RESULTS AND DISCUSSION
Response time of the soil moisture in the root zone to rainfall
Six periods with rainfall events from April to June in 2019 were chosen to analyze the changes in the soil water content during the different rainfall processes and to estimate the response time of the soil moisture in the root layer to the rainfall. The response time of the soil moisture in the root layer refers to the time from the beginning of the rainfall to the beginning of the soil water content change at a depth of 10 cm. The soil water content of the topsoil (5.5 cm) and the response time at a soil depth of 10 cm for each rainfall event are shown in Table 2.
No. . | Soil water content before rainfall (cm3·cm−3) . | Response time (min) . | |
---|---|---|---|
Topsoil/5.5 cm . | Root zone/10 cm . | ||
1 | 0.215 | 0.322 | 620 < t ≤ 630 |
2 | 0.021 | 0.201 | t ≤ 10 |
3 | 0.023 | 0.195 | t ≤ 10 |
4 | 0.045 | 0.254 | t ≤ 10 |
5 | 0.187 | 0.329 | 50 < t ≤ 60 |
6 | 0.131 | 0.310 | 30 < t ≤ 40 |
No. . | Soil water content before rainfall (cm3·cm−3) . | Response time (min) . | |
---|---|---|---|
Topsoil/5.5 cm . | Root zone/10 cm . | ||
1 | 0.215 | 0.322 | 620 < t ≤ 630 |
2 | 0.021 | 0.201 | t ≤ 10 |
3 | 0.023 | 0.195 | t ≤ 10 |
4 | 0.045 | 0.254 | t ≤ 10 |
5 | 0.187 | 0.329 | 50 < t ≤ 60 |
6 | 0.131 | 0.310 | 30 < t ≤ 40 |
Note: The response time is expressed by a time interval of 10 min as the unit time of rainfall and soil water content is set to 10 min.
The change processes of the soil water content at depths of 10, 25, and 40 cm from 21:00 on May 25th to 17:00 on May 27th are shown in Figure 2(b). A relatively concentrated rainfall event occurred on May 26th, with a rainfall amount of 12.6 mm and a maximum intensity of 0.20 mm · min−1. The response time of root layer to the rainfall occurred within 10 min, and the range of the increase in the soil water content in the root layer was significantly affected by the rainfall intensity. The soil water contents at 25 and 40 cm remained stable. The main reasons for these results are that the original soil water content of the topsoil was low before the beginning of the rainfall event (Table 2), the soil water content of the root zone did not attain saturation after the end of the rainfall event, and the soil water content below 25 cm was not recharged by the infiltration.
Figure 2(c) shows that a rainfall event occurred from 22:00 on June 5th to 11:00 on June 6th, with a rainfall amount of 30.4 mm and a maximum intensity of 0.46 mm · min−1. The water content of the topsoil was low before the rainfall event, and the response time to the rainfall was less than 10 min. The soil water content of the root zone increased slowly when the rainfall intensity was small and the rate of increase improved with the increasing rainfall intensity. The soil water content of the root zone increased significantly during 5:50–6:20 and 9:10–11:20 on June 6th since the maximum rainfall intensity reached 0.20 and 0.46 mm 10 min−1 in these two periods, respectively, which indicates that the rainfall intensity greatly affects the change in the soil water content of the root zone. The soil water content at 25 cm began to increase slowly from the time of the maximum rainfall intensity, while the soil water content at 40 cm remained stable.
Figure 2(d) shows the temporal changes in the soil water content at depths of 10, 25, and 40 cm from 18:00 on June 17th to 22:00 on June 18th. A rainfall event occurred from 20:00 on June 17th to 9:00 on June 18th, with a rainfall amount of 14 mm and a maximum intensity of 0.14 mm · min−1. The response time of the water content at a depth of 10 cm to the rainfall occurred within 10 min, and the soil water content at 25 cm increased slightly when the rainfall intensity reached the maximum, while the soil water content at 40 cm remained stable.
A relatively concentrated rainfall event occurred on June 20th (14:00–20:00), with a rainfall amount of 8 mm and a maximum intensity of 0.08 mm · min−1. The response time of the water content at 10 cm to the rainfall was 50–60 min, and the soil water contents at depths of 25 and 40 cm remained stable during this rainfall event (Figure 2(e)).
Figure 2(f) shows the temporal changes in the soil water content at depths of 10, 25, and 40 cm from 18:00 on June 17th to 22:00 on June 18th, and a rainfall event occurred from 22:00 on June 28th to 7:00 on June 29th, with a rainfall amount of 14 mm and a maximum intensity of 0.48 mm · min−1. The rainfall duration was relatively short, and the intensity was comparatively large. The response time of the water content at 10 cm to the rainfall was 30–40 min. The main reasons for these results are that the initial soil water contents of the topsoil and the root zone were relatively high before the rainfall event (Table 2), which resulted in a decreased infiltration rate.
To summarize the above analysis, the response time of the soil water content of the root zone to rainfall was affected by the actual rainfall process and the underlying surface conditions. In general, as the rainfall intensity increased, the response time of the soil water content of the root zone to the rainfall decreased. For similar rainfall intensities, the lower initial water content of the topsoil causes a shorter response time in the root layer to rainfall and a faster infiltration rate in the topsoil. The main reason for this is that the lower water content of the soil results in a lower soil water potential, so the suction of the water increases and the infiltration rate improves. From the beginning of the rainfall to a short time after the end of the rainfall, the water content below 25 cm could not receive or only received a small amount of effective infiltration recharge, while the soil water content at 40 cm remained relatively stable, which indicates that the grassland vegetation in the study area has a strong interception function. Before the soil water content of the root zone reaches saturation, the soil water content is significantly impacted by the rainfall intensity; the greater the rainfall intensity, the faster the soil water content increases. Yang et al. (2008) obtained similar results in another study conducted on the Loess Plateau, China. However, due to the limitations of the observed data, the quantitative evaluation of the relationship between the change in the rainfall intensity and the increment of the soil water content of the root zone was not possible in the current research.
Evapotranspiration
Amount of rainfall and infiltration depth
The eight rainfall events and the observed soil water contents on P2 (at depths of 5, 15, 30, and 60 cm) during the same periods were chosen to explore the relationship between the amount of rainfall and the infiltration depth. The grades of the eight rainfall events were classified (Table 3) according to the classification standard of the China Meteorological Administration.
Grade of rainfall event . | The periodic time . | Rainfall concentrated time . | Rainfall amount (mm) . | Infiltration depth (cm) . |
---|---|---|---|---|
Light rain | 10.08.2018–11.10.2018 | 10.09 14:00–16:00 | 4.8 | <5 |
30.05.2018–02.06.2018 | 05.30 22:00–05.31 7:00 | 8.9 | 5–15 | |
Moderate rain | 21.07.2018–24.07.2018 | 07.22 11:00–07.23 3:00 | 17.8 | 15–30 |
04.04.2018–07.04.2018 | 04.05 12:00–23:00 | 24.0 | ||
Heavy rain | 12.08.2018–15.08.2018 | 08.13 00:00–17:00 | 37.6 | 30–60 |
05.06.2019–08.06.2019 | 06.06 1:00–14:00 | 41.2 | >60 | |
Rainstorm | 09.08.2019–12.08.2019 | 08.10 2:00–08.11 1:00 | 55.0 | >60 |
23.05.2018–26.05.2018 | 05.24 22:00–05.25 20:00 | 107.0 | >60 |
Grade of rainfall event . | The periodic time . | Rainfall concentrated time . | Rainfall amount (mm) . | Infiltration depth (cm) . |
---|---|---|---|---|
Light rain | 10.08.2018–11.10.2018 | 10.09 14:00–16:00 | 4.8 | <5 |
30.05.2018–02.06.2018 | 05.30 22:00–05.31 7:00 | 8.9 | 5–15 | |
Moderate rain | 21.07.2018–24.07.2018 | 07.22 11:00–07.23 3:00 | 17.8 | 15–30 |
04.04.2018–07.04.2018 | 04.05 12:00–23:00 | 24.0 | ||
Heavy rain | 12.08.2018–15.08.2018 | 08.13 00:00–17:00 | 37.6 | 30–60 |
05.06.2019–08.06.2019 | 06.06 1:00–14:00 | 41.2 | >60 | |
Rainstorm | 09.08.2019–12.08.2019 | 08.10 2:00–08.11 1:00 | 55.0 | >60 |
23.05.2018–26.05.2018 | 05.24 22:00–05.25 20:00 | 107.0 | >60 |
Figure 5(a) shows the changes in the soil water contents at the four depths (5, 15 30, and 60 cm) during the period from October 8th to October 11th in 2018. The rainfall was mainly concentrated on October 9th, and the amount of rainfall was 4.8 mm (Table 3). The soil water contents at the four depths remained stable, that is, the rainwater infiltration did not reach a depth of 5 cm. The main reason for this is that the Bermudagrass lawn vegetation has a strong interception function so effective infiltration did not occur during this rainfall event. A relatively concentrated rainfall event occurred from 22:00 on May 30th to 7:00 on May 31st, with a rainfall amount of 8.9 mm. The soil water content at a depth of 5 cm increased significantly due to rainwater infiltration. It reached its peak on May 31, gradually decreased until June 2, and then returned to the initial state observed on May 30th. The soil water contents at 15, 30, and 60 cm remained stable during the 4 days (2018/05/30–06/02) (Figure 5(b)). The above two rainfall events were light rainfall events. The amount of rainfall during the second rainfall event (8.9 mm) was greater than that during the first event (4.8 mm), and its infiltration depth was also greater than that of the first event.
Two moderate rainfall events occurred from July 22nd to July 23rd and on April 5, 2018, and the amounts of the rainfall during these two events were 17.8 and 24.0 mm, respectively. The soil water contents at 5 and 15 cm increased significantly due to the infiltration caused by the two rainfall events. The changes in the water content at depths of 5 and 15 cm exhibited similar temporal patterns, that is, they gradually decreased after reaching their peak. The range of the change in the soil water content at 5 cm was greater than that at 15 cm, whereas the soil water contents at 30 and 60 cm remained stable. And the infiltration depth caused by moderate rain generally was less than 30 cm (Figure 5(c) and 5(d)).
Two heavy rainfall events occurred on August 13, 2018, and June 6, 2019, with rainfall amounts of 37.6 and 41.2 mm, respectively. The soil water contents at 5, 15, and 30 cm increased to different degrees due to infiltration. The soil water content at 60 cm remained relatively stable during the rainfall event and after the end of the rainfall on August 13, 2018, which indicates that the rainfall infiltration did not reach 60 cm (Figure 5(e)). Due to the infiltration effect of the rainfall event on June 6, 2019, the soil water content at 60 cm increased slightly (Figure 5(f)), that is, the infiltration due to this rainfall event reached 60 cm. The two heavy rainfall events both led to increases in the soil water contents at depths of 5, 15, and 30 cm, but the ranges of these increases were different. Generally, the range of the change in the soil water content decreased with increasing depth. (Figure 5(e) and 5(f)). The results also show that the infiltration depth of the heavy rainfall events exceeded 30 cm, and some reached 60 cm (Figure 5(e) and 5(f)).
Two rainstorms occurred from May 24th to 25, 2018, and from August 10 to 11, 2019, with rainfall amounts of 107 and 55 mm, respectively. The soil water contents at 5, 15, 30, and 60 cm visibly increased, and the infiltration depth exceeded 60 cm. However, the ranges of the changes in the soil water content and the water content processes were different at different depths. (Figure 5(g) and 5(h)).
There are a number of factors that affect infiltration such as topography, soil properties, and vegetation (Leonard & Andrieux 1998; Ribolzi et al. 2011; Huang et al. 2013; Nie et al. 2017). The topography of the study area is flat, with large areas having almost no slope (Figure 1(b)). The vegetation within the study area, as a regional artificial lawn, is a single species of grass (Bermudagrass) with uniform distribution. According to the screening results of soil particle composition experiment, the soil has good homogeneity, and the soil physical properties are in the similar conditions (Table 1). Therefore, the response characteristics of soil moisture to rainfall obtained in this study, such as the response time of the soil moisture in the root zone to rainfall, and the relationship between the amount of rainfall and infiltration depth, can appropriately represent the infiltration characteristics of grassland in the study area.
Validation of the Hydrus-1D model
In Equations (6)–(12), Si and Oi are the simulated value and the measured value, respectively; and devote the average of the simulated results and the measured results, respectively; r is the linear correlation coefficient between Si and Oi; α is a measure of relative variability in the simulated and measured values; β is the ratio between the average simulated value and the average measured value.
Depth (cm) . | 5 . | 15 . | 30 . | 60 . |
---|---|---|---|---|
RMSE | 0.034 | 0.022 | 0.015 | 0.012 |
NSE | 0.77 | 0.78 | 0.83 | 0.84 |
KGE | 0.714 | 0.903 | 0.930 | 0.945 |
LME | 0.690 | 0.906 | 0.932 | 0.967 |
Depth (cm) . | 5 . | 15 . | 30 . | 60 . |
---|---|---|---|---|
RMSE | 0.034 | 0.022 | 0.015 | 0.012 |
NSE | 0.77 | 0.78 | 0.83 | 0.84 |
KGE | 0.714 | 0.903 | 0.930 | 0.945 |
LME | 0.690 | 0.906 | 0.932 | 0.967 |
Model validation of infiltration depth
Figure 8(a) shows that during the light rainfall event on October 9, 2018, and before and after it, the measured soil water content at 5 cm remained stable, while the simulated results increased slightly during the concentrated rainfall period. In the process of the Hydrus-1D calculations, the rainfall caused water input at the model's upper boundary and the soil water content of the model increased, which resulted in a slight difference between the measured and simulated soil water contents at 5 cm. This confirms that the Bermudagrass in the study area has a strong interception effect. Figure 8(b) shows the change in the soil water content during another light rainfall event from May 30th to 31st in 2018 and before and after it. The measured and simulated soil water contents at 5 cm increased by a similar increment, whereas the measured and simulated soil water contents at 15 cm remained stable. Based on this, the rationality of the relationship between the amount of rainfall and the infiltration depth for light rainfall (Figure 5(b)) is valid.
The simulated soil water contents for the two moderate rainfall events (Table 3) show that the soil water content at 30 cm did not change, but it increased at 15 cm (Figure 8(c) and 8(d)). The simulated results are consistent with the analyzed results of the relationship between moderate rainfall and the infiltration depth (Table 3).
The simulated soil water contents for heavy rainfall show that the infiltration depth was less 60 cm for the heavy rainfall on August 13, 2018 (Figure 8(e)), while the infiltration depth reached 60 cm due to the heavy rainfall on June 6, 2019 (Figure 8(f)). These results validate the relationship between heavy rainfall and the corresponding infiltration depth (Table 3).
The simulated results of the soil water contents for the two rainstorms (Table 3) demonstrate that the soil water contents at different depths increased visibly (Figure 8(g) and 8(h)), and the downward release of water occurred at the lower boundary (i.e., the soil water flux was not 0 at a depth of 60 cm) in the model's calculation process, which indicates that the infiltration depth exceeded 60 cm. Thus, the rationality of the infiltration depth caused by the rainstorm is verified.
According to the above analysis, the Hydrus-1D model can be used to accurately estimate the infiltration depths for different rainfall events in the absence of measured soil water content data for the study area.
CONCLUSIONS
In this study, a regional urban grassland site with a single type of grass (Bermudagrass) was chosen as the study object. The response characteristics of the soil water content to the rainfall, including the relationship between the rainfall intensity and the response time of the soil moisture in the root zone, the relationship between the amount of rainfall and the infiltration depth, and the change in the daily ET, were evaluated. The rationality of the relationship between the amount of rainfall and the infiltration depth obtained from the data analysis was validated via simulations using the Hydrus-1D model. The main conclusions reached are as follows.
The initial soil water content of the topsoil and the rainfall intensity significantly affect the response time of the soil water in the root zone to the rainfall. A lower topsoil water content and a higher rainfall intensity generally lead to a shorter response time of the soil moisture in the root zone (depth of 10 cm) to rainfall.
Generally, for the regional urban Bermudagrass in this study area, the rainfall events at different grades can lead to different infiltration depths. The infiltration depths under the light or moderate rainfall events are mostly less than 15 or 30 cm, respectively. While those under heavy rainfall events can exceed 30 cm, and those under rainstorms even exceed 60 cm.
The soil water moisture in the root zone provides the main part (>70%) of the soil water consumed by ET.
The Hydrus-1D model is applicable to the simulation of the soil water content in the study area. The infiltration depths of different rainfall events can be accurately estimated using the Hydrus-1D model when there is no measured soil water content data for the study area.
This study revealed the characteristics of the response of the soil water content to rainfall and the ET characteristics of Bermudagrass in an urban area to a certain extent. Based on these results, the characteristics and laws of soil water movement in urban Bermudagrass in the same area will be further carried out. The results of this study also provide a methodological reference for research on soil moisture movement characteristics in different types of grasslands in urban areas.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (NSFC: 41271046), the 2018 Young Blue Project of Yangzhou University, China, and the 2019 Science and Technology Innovation Cultivation Fund of Yangzhou University, China (2019cxj069).
DATA AVAILABILITY STATEMENT
Data cannot be made publicly available; readers should contact the corresponding author for details.
CONFLICT OF INTEREST
The authors declare there is no conflict.