Soil moisture is addressed as a key variable controlling hydrologic process and vegetation growth in the critical zone of earth surface (
Journal of Geographical Sciences, Volume. 30, Issue 6, 949(2020)
Influence of canopy and topographic position on soil moisture response to rainfall in a hilly catchment of Three Gorges Reservoir Area, China
Rainfall provides essential water resource for vegetation growth and acts as driving force for hydrologic process, bedrock weathering and nutrient cycle in the steep hilly catchment. But the effects of rainfall features, vegetation types, topography, and also their interactions on soil water movement and soil moisture dynamics are inadequately quantified. During the coupled wet and dry periods of the year 2018 to 2019, time-series soil moisture was monitored with 5-min interval resolution in a hilly catchment of the Three Gorges Reservoir Area in China. Three hillslopes covered with evergreen forest (EG), secondary deciduous forest mixed with shrubs (SDFS) and deforested pasture (DP) were selected, and two monitoring sites with five detected depths were established at upslope and downslope position, respectively. Several parameters expressing soil moisture response to rainfall event were evaluated, including wetting depth, cumulative rainfall amount and lag time before initial response, maximum increase of soil water storage, and transform ratio of rainwater to soil water. The results indicated that rainfall amount is the dominant rainfall variable controlling soil moisture response to rainfall event. No soil moisture response occurred when rainfall amounts was <8 mm, and all the deepest monitoring sensors detected soil moisture increase when total rainfall amounts was >30 mm. In the wet period, the cumulative rainfall amount to trigger surface soil moisture response in EG-up site was significantly higher than in other five sites. However, no significant difference in cumulative rainfall amount to trigger soil moisture response was observed among all study sites in dry period. Vegetation canopy interception reduced the transform ratio of rainwater to soil water, with a higher reduction in vegetation growth period than in other period. Also, interception of vegetation canopy resulted in a larger accumulated rainfall amount and a longer lag time for initiating soil moisture response to rainfall. Generally, average cumulative rainfall amount for initiating soil moisture response during dry period of all sites (3.5-5.6 mm) were less than during wet period (5.7-19.7 mm). Forests captured more infiltration water compared with deforested pasture, showing the larger increments of both soil water storage for the whole soil profile and volumetric soil water content at 10 cm depth on two forest slopes. Topography dominated soil subsurface flow, proven by the evidences that less rainfall amount and less time was needed to trigger soil moisture response and also larger accumulated soil water storage increment in downslope site than in corresponding upslope site during heavy rainfall events.
1 Introduction
Soil moisture is addressed as a key variable controlling hydrologic process and vegetation growth in the critical zone of earth surface (
Water balance and water movement of hilly catchments were mainly controlled by rainfall features, vegetation coverage, soil property, and topographic position (
Vegetation influenced soil moisture response to rainfall by canopy interception and evapotranspiration, which controlled the available rainwater amount for infiltration and output of soil water storage, respectively (
The importance of topographic position effects on soil moisture distribution was proposed by many researchers (
Soil hydraulic properties including porosity, permeability, and retention characteristics, which directly influence the transformation ratio of rainwater to soil water and water storage.
Compared with abundant studies of soil moisture in Loess Plateau and arid-semiarid area of China (
The objectives of this study were (1) to quantifying the independent and interacted effects of rainfall feature, vegetation type, and tomographic position on soil water dynamics and soil water infiltration processes and (2) to reveal the mechanism of rainfall, vegetation, and tomography effects on soil water movement.
2 Materials and methods
2.1 Study area and monitoring sites
This study area is located in a hilly catchment of Dalaoling National Forest Park (30°00′13′′N-31°28′30′′N, 100°51′8′′E-111°39′30′′E), which belongs to the Three Gorges Reservoir Area and is located 40 km upstream of the Three Gorges Dam (
Figure 1.
2.2 Soil moisture monitoring and soil property analysis
Three hillslopes with typical vegetation cover and topography were selected in the studied catchment in the May of 2017, which were covered with evergreen coniferous forest (EG), secondary deciduous forest mixed with shrubs (SDFS), and deforested pasture (DP), respectively (Figures 1b and 1c). Two soil moisture monitoring sites were established on each hillslope, one in the upslope position and the other in the downslope position. The local slope gradients are ~20° and ~29° in EG, ~27° and ~29° in SDFS, and ~25° and ~27° in DP for the upslope and downslope position, respectively. The facing directions of EG, SDFS, and DP were Northeast, East, and Southeast, respectively. The altitudes of the upslope position and downslope position for EG, SDFS and DP were 1308 m and 1275 m, 1264 m and 1250 m, and 1277 m and 1254 m, respectively. At each site, a vertical array of soil integrated moisture-temperature probes (5TM probes with a typical accuracy of ±2% without calibration, and a precision of ±0.08 vol.% in moisture; METER Group, Inc. U.S.A) were installed in the upslope-facing direction in a soil pit at different depths. Installed depths of sensors were determined by the depths of different horizons (A, E, B, C…). Five sensors were installed at different depths from A horizon to the deepest C horizon in each site, and the detailed sensor depths were 10, 20, 35, 50, and 70 cm in site 1; 10, 20, 40, 60, and 80 cm in site 2; 8, 20, 30, 45, and 60 cm in site 3; 10, 20, 30, 50, and 70 cm in site 4; 10, 20, 30, 45, and 60 cm in site 5; and 10, 20, 40, 60, 80 cm in site 6. In total, 30 sensors were installed in six monitoring sites of three vegetation coverage slopes (
Soil properties and sensor installation depths of six monitoring sites in the Three Gorges Reservoir Area
Soil properties and sensor installation depths of six monitoring sites in the Three Gorges Reservoir Area
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A 1 m length with 1 m width plot was excavated at 2 m downslope of the soil monitoring sensor installed pit in each site. Soil structure and soil property, including soil layering, color, aggregate, soil texture, bulk density, porosity, and saturated hydrologic conductivity, were detected by field observation or laboratory analysis. Undisturbed soil cores (100 cm3) with ten replicates and about 500 g disturbed soil samples were collected at a 10 cm depth interval from soil surface down to the bedrock. These undisturbed soil samples were used for soil bulk density (oven drying method) and saturated hydraulic conductivity (constant pressure head method) measurement. These disturbed samples were prepared for particle size distribution analyze with the pipette method, and soil organic matter content were determined with the approach of oxidation with potassium dichromate (Walkley and Black, 1934).
2.3 Precipitation measurement and event delineation
An automatic tippling bucket rain gauge (Spectrum Technologies Inc, USA, precision ± 0.2 m) was installed in an uncovered area in the studied catchment, which was close to downslope deforested pasture (
2.4 Soil moisture response to rainfall event
Different thresholds of the minimum increment of volumetric soil water content were used for determining the soil water response time of rainfall (
Parameters including wetting depth (indicated by the depth of the deepest probe that captured soil moisture response), soil moisture response time (the first appearance of an increase beyond 1% in soil moisture), infiltration velocity (dividing the depth by the response time), and maximum increment of soil moisture were calculated from the time series data of soil moisture during each rainfall event. These parameters were used to indicate the degree of soil water content response at a specific depth in different sites. Preferential flow occurred when subsurface horizon responded to rainfall earlier than horizons above it (
Soil water storage (SWS, mm) of each soil profile was calculated by Equations (1), (2), and (3).
where θ is the volumetric soil water content (cm3/cm3).
Transition of rainfall amount to soil water (RSP) during each rainfall event can be represented by ratio of the increment of soil water storage dividing accumulated rainfall amount.
2.5 Statistical analysis
Statistical analysis of soil moisture response features, including wetting depth, cumulative rainfall amount, lag time, maximum increment of soil water content, and SWS increase during 55 rainfall events were conducted. One-way ANOVA was used to test for differences in rainfall amount required to trigger all depths response rainfall events (AR), subsurface layer starting response events (SBR, deeper than 30 cm depth), and surface response events (SR, 10 cm soil depth). Statistical software SPSS 22.0 was used for the above analysis.
3 Results
3.1 Category of rainfall events and dynamics of soil moisture
The total rainfall amount during this period accumulated to 1235.3 mm, and 55 rainfall events were separated by an interval of 24 hr without rainfall, including 17 extremely light events with total amount <5 mm, 9 small rain events with total amount of 5-10 mm, 14 moderate rain events with total amount 10-25 mm, 10 heavy rain events with total amount 25-50 mm, 3 heavy rain events with total amount 50-100 mm, and 2 storms with 24 hours amount > 100 mm. The study period can be separated into two stages referring to the differences in rainfall amount and soil moisture response feature, including a wet period from May 1, 2018 to mid-November, 2018 and a dry period from mid-November, 2018 to May 1, 2019 (Figures 2a and 2b). The wet period and dry period are also corresponded to the growing and fallow season of DF. Due to the high correlations between total rainfall amount and other rainfall parameters, it was selected to categorize rainfall events in the subsequent soil moisture response analysis.
Time series of soil moisture with 30-minute interval showed that different patterns of soil moisture variation among five soil depths in six monitoring sites (Figures 2c-2h). The larger soil moisture variations among five depths were observed in EG-down, DP-down, and SDFS-up sites than in other three sites. Compared with corresponding upslope sites, the downslope sites of EG and DP were much wetter. While no distinct differences existed between the two sites in SDFS. Furthermore, the fluctuation of soil moisture was more distinct and frequent in wet period than in dry period for all monitoring sites. Except the DP-down site, surface soil horizons including 10 cm and 20 cm depth in other sites were more sensitive to rainfall than that of subsurface layers. Although the rainfall amount in wet period was significantly larger than in dry period, no obvious increase of soil water content was observed in wet period. And a 15-day period between June 3 and June 18 with little rainfall (4.2 mm) resulted in a sudden and steep decrease of soil moisture at the forest sites, especially at two EG sites.
3.2 Soil moisture response to rainfall events and features of rainwater infiltration at different sites
3.2.1 Wetting depth during various rainfall events with influence of rainfall features
The soil moisture responses were affected by the rainfall amount (
Figure 2.
Figure 3.
Other features of rainfall events also had effects on soil wetting depths (Figures 3b-3d). Both rainfall duration (
3.2.2 Cumulative rainfall amount and lag time for triggering soil moisture response of surface horizon
In 23 out of 55 rainfall events, the topsoil (0-10 cm) of all sites had soil moisture response (
Figure 4.
Differences in lag time for soil moisture response were also observed in different sites or during different rainfall periods. For the response lag time in wet period (
3.2.3 Surface soil moisture response to four selected rainfall events
Detailed information of cumulative rainfall amount and lag time for soil moisture response during four selected rainfall events (two in dry period and two in wet period) were shown in
Figure 5.
During two rainfall events in dry period, the cumulative rainfall amount required for the response of soil moisture in the surface soil (8 cm or 10 cm depth) of rainfall with lower intensity was greater than that of heavy rain (
Different soil water response patterns were observed between E15 and E49 rainfall events, although their total rainfall amounts were close to each other. A faster soil moisture response speed was found in E15 than in E49, while more accumulated rainfall amount was needed in E15 for the 1% increment of soil moisture. Those upslope sites and downslope sites respond simultaneously in both forests, while the downslope site responded earlier than the upslope site in DP.
3.2.4 Infiltration process during a heavy rainstorm event
Soil moisture response time and soil water percolation velocity in the heavy rainstorm event of E11 were showed in
Figure 6.
The preferential flow and lateral flow observed during E11 rainfall event presented in different soil moisture dynamics. In EG-up site, preferential flow at 20-70 cm depths led to an earlier soil moisture increase than at 10 cm soil depth. The lateral flow caused the large increments of soil moisture in 60 cm depth of SDFS-up site and 80 cm of DP-down site, and these increments were more than 17% and 20%, respectively. However, the soil moisture changes between SDFS-up site and DP-down site were different after the soil water content reached a peak value, characterized by a steeply decrease in SDFS-up site and a continuously high soil moisture in DP-down site. Except for 80 cm soil depth in DP-down site, the soil moisture increments of other depths in DP slope were less than 8%. However, the forest sites preserved more water than in DP sites, which was especially true for EG sites.
3.3 Maximum increment of soil moisture and accumulated soil water storage in both dry period and wet period
The maximum increase of soil moisture varied with rainfall amount and monitoring sites. The maximum increment of soil moisture at 10 cm soil depth was close to 20% in wet period (
Figure 7.
The RSP varied with accumulated rainfall amount and study sites (
Figure 8.
4 Discussion
Soil moisture response to rainfall event mostly related with two groups of variables, which included the first group of input rainfall features such as rainfall amount, rainfall duration, and rainfall intensity, and the second group of vegetation-topography-soil property combination governing the distribution and transition process from rainwater to soil water (
4.1 Rainfall features influence on soil moisture dynamics and infiltration processes
Among the first group variables, our studied results suggested that rainfall amount was the first order controlling factor on soil moisture respond process as it significantly influenced the wetting front depths (
The influence of rainfall on soil moisture was also proved by the significant correlation coefficients between rainfall features and maximum increment of soil water storage. Significant correlation between SWS increment and rainfall amount were observed in both wet period and dry period, especially during wet period with a coefficient larger than 0.8 (except for SDFS-down, which is 0.618, P<0.01). Although the soil moisture increment was also positively related with both rainfall duration and rain intensity in some period, the correlation coefficients were smaller than those with rainfall amount (
Correlation coefficients between maximum increment of soil water storage and rainfall features during both wet period and dry period of six monitoring sites in the Three Gorges Reservoir Area
Correlation coefficients between maximum increment of soil water storage and rainfall features during both wet period and dry period of six monitoring sites in the Three Gorges Reservoir Area
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4.2 Vegetation coverage influence on soil moisture response to rainfall event and soil water dynamics
Influence of vegetation on the transition of rainwater to soil water or overland flow has been widely discussed (
4.3 Topographic position influence on soil moisture dynamics and soil water movement
Topography controls soil moisture response to rainfall events through direct influence on lateral flow and indirect influence on soil property or evapotranspiration. In this study, the influences of topographic position on soil moisture response and SWS were presented in three aspects, including cumulative rainfall amount and lag time to trigger soil moisture response, soil water flow pattern, and SWS increase.
Firstly, the forest sites in the same position presented the similar soil moisture response features when the rainfall intensity was relative strong, indicating the dominant effects of topography on soil water movement. Also, the time for soil moisture response was earlier in downslope than in upslope for the high intensity characterized rainfall event (
Secondly, the occurrence of lateral flow along a hillslope is a direct evidence of topographic influence on soil water movement. During the rainfall event of E11, SWS in subsurface layers (i.e. 45, 60, and 80 cm soil depths, which are close to the low permeable bedrock) presented higher increment than in surface layers, and the increases of SWS at all the profiles were larger than corresponding cumulative rainfall amount. These observations suggested the existence of subsurface lateral flow which was recharged into the soil water in deep soil layers. Except two sites of EG and downslope of SDFS, all other sites detected subsurface lateral flow during E11 rainfall event. Subsurface lateral flow was also identified at the lower slope position in several studies (e.g.
Thirdly, lateral water flow resulted in high increase of surface soil moisture and profile SWS at downslope sites (i.e., EG-down and SDFS-down), especially at forest land or during wet period.
5 Conclusions
Three closely located hillslopes covered with EG, SDFS and DP in a hilly catchment were monitored at both upslope and downslope sites to collect soil moisture data at five soil depths from May 1, 2018 to May 1 2019. A total of 55 rainfall events were delineated and soil moisture responding to each event were analyzed with several parameters including cumulative amount and lag time to initiate soil moisture response, wetting depth, and maximum increase of soil moisture and SWS. The results indicate: (1) Rainfall amount was the most prominent rainfall feature for controlling soil moisture response as it had stronger correlation with soil moisture increment than other rainfall features. Average cumulative rainfall amounts for initial moisture response during dry period of all sites were less than those of wet period. During wet period, the cumulative rainfall amount to trigger surface soil response in EG-up site was significantly higher than in other five sites. In contrast, there is no significant differences in cumulative rainfall amounts among six sites during dry period. (2) Vegetation affected the transform ratio of rainfall to soil water by interception and transpiration, showing more rainfall amount and longer lag time was required to initial soil moisture response in vegetation growth period than in fallow period. More water usually infiltrated into forest sites compared with DP, indicated by a higher maximum increment of soil moisture at 10 cm depth and profile SWS in two forest sites. Topographic feature dominated soil subsurface flow, as less rainfall amount and less time were mostly needed in downslope site to trigger soil moisture increment compared with upslope site in a same slope. (3) Steep slope also leads the downslope sites to collect more rainfall, resulting in more lateral flow and sudden large increment during heavy rainstorm, which can partly explain the observation of large accumulated SWS in downslope sites compared with corresponding upslope sites during wet period. This study concluded that the integration of the rainfall features, vegetation cover, and topographic position govern the soil moisture response to rainfall events at the hilly area of the Three Gorges Reservoir Area of China.
Acknowledgements
We appreciate the assistance from the Governors of the Dalaoling Forest Park during the field experiment. Graduate students Lou Shulan, Yang Xiufeng, Li Xiangfu, and Yang Ye are acknowledged for their field work and laboratory measurements.
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Muxing LIU, Qiuyue WANG, Li GUO, Jun YI, Henry LIN, Qing ZHU, Bihang FAN, Hailin ZHANG. Influence of canopy and topographic position on soil moisture response to rainfall in a hilly catchment of Three Gorges Reservoir Area, China[J]. Journal of Geographical Sciences, 2020, 30(6): 949
Received: Sep. 20, 2019
Accepted: Mar. 5, 2020
Published Online: Sep. 30, 2020
The Author Email: GUO Li (lug163@psu.edu)