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Role of groundcover management in controlling soil erosion under extreme rainfall in citrus orchards of southern China
Journal Pre-proofs Research papers Role of groundcover management in controlling soil erosion under extreme rainfall in citrus orchards of southern China Jian Duan, Yao-Jun Liu, Jie Yang, Chong-Jun Tang, Zhi-Hua Shi PII: DOI: Reference:
S0022-1694(19)31025-X https://doi.org/10.1016/j.jhydrol.2019.124290 HYDROL 124290
To appear in:
Journal of Hydrology
Received Date: Revised Date: Accepted Date:
1 August 2019 24 October 2019 25 October 2019
Please cite this article as: Duan, J., Liu, Y-J., Yang, J., Tang, C-J., Shi, Z-H., Role of groundcover management in controlling soil erosion under extreme rainfall in citrus orchards of southern China, Journal of Hydrology (2019), doi: https://doi.org/10.1016/j.jhydrol.2019.124290
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Role of groundcover management in controlling soil erosion under extreme rainfall in citrus orchards of southern China Jian Duana,b,c,1, Yao-Jun Liua,b,c,1, Jie Yanga,b,*, Chong-Jun Tanga,b, Zhi-Hua Shic,d a Jiangxi
Institute of Soil and Water Conservation, Nanchang, Jiangxi 330029, PR China
b Jiangxi
Provincial Key Laboratory of Soil Erosion and Prevention, Nanchang, Jiangxi 330029, PR China
c Research
Center of Soil and Water Conservation and Ecological Environment, Chinese Academy of Sciences and
Ministry of Education, Yangling, Shaanxi 712100 China d College
1
of Resources and Environment, Huazhong Agricultural University, Wuhan, Hubei 430070, PR China
These authors contributed equally to this work.
*Corresponding to: Jie Yang, Jiangxi Institute of Soil and Water Conservation, 330029, China. Email:[email protected]
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Abstract: Extreme rainfall is becoming more frequent and intense due to climate change and increasing human activity. Changes in rainfall will increase the risks and uncertainty of water erosion from orchards. Groundcover management is an important factor affecting hydrological and erosive processes of orchards. However, the roles of different groundcover managements for controlling soil water erosion caused by extreme rainfall are not fully understood. In this study, four groundcover managements (i.e., control bare land, orchard with no cover, orchard with grass cover and orchard with cover crops) were used to analyze runoff and erosion characteristics related to extreme and ordinary rainfalls in the red soil region of China. Based on the rainfall-runoff data measured on runoff plots, 356 natural rainfall events from 2001 to 2012 were analyzed, and rainfall depths and maximum 30-min rainfall intensities were used to select extreme rainfall events, according to the criteria of the World Meteorological Organization for extreme rainfall. There were 25 extreme rainfall events with a probability of 7.0% during the study period. Extreme rainfall events played the destructive role in inducing soil loss. The average extreme runoff coefficient and soil loss amount were 2.8 and 11.1 times higher than the values from ordinary rainfall, respectively. The contribution of extreme rainfall to sediment yield was 44.2%, which far higher than those to runoff generation (15.8%). Moreover, the runoff coefficients and erosion amounts varied greatly among the different groundcover managements under extreme rainfall, and were ranked in the following order: control bare land > orchard with no cover > orchard with cover crops > orchard with grass cover. However, the highest percentage of soil loss amount caused by extreme rainfalls were 59.0% in orchard with cover crops, and the lowest in orchard with grass cover (26.0%). Therefore, extreme rainfall should cause more attention in improving the groundcover management strategy for orchard soil erosion control, and 2
groundcover like grass cover was extremely needed even after interplanting crops for the serious soil loss. Keywords: water erosion; extreme rainfall; groundcover management; citrus orchards; red soil region
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1. Introduction Water erosion is mainly responsible for global land degradation and productivity declines (Rodrigo et al., 2016; Wei et al., 2017; Bonetti et al., 2019). For a long period, the red soil region of southern China has been facing severe soil and water loss due to high precipitation, hilly landforms and unsustainable farming practices (Barton et al., 2004; Liang et al., 2010; Shi et al., 2014; Fang et al, 2017). In recent decades, with the rapid growth of human population (40% of China's population) and heavy pressure on productive soil resources, increasing amounts of barren land with a secondary community on the slopes have been transformed into orchards (Zuazo et al., 2005; Xu et al., 2012; Li et al., 2014; Chen et al., 2019). Hilly citrus orchards are widely distributed in this region due to their additional economic benefits. Meanwhile, citrus orchard lands are exposed to serious risks of soil and water loss due to the intense soil disturbances from large-scale mechanized excavation and the lack of surface vegetation cover (Durán Zuazo et al., 2005; Prosdocimi et al., 2016; Comino et al., 2018). Soil and water conservation measures on soil surfaces are rarely implemented for orchard production. Relevant groundcover measures are very infrequent, and further research is necessary to assess the sustainable groundcover management strategies. Groundcover management is an important factor that affects water erosion processes in orchards. Vegetation-free groundcover management, such as herbicides spraying and mechanical weeding, have been widely implemented to maximize commercial production in conventional orchards (Novara et al., 2011; Keesstra et al., 2016; Prosdocimi et al., 2016). The management method with bare soil year round decreased soil infiltration and favored runoff production and sediment yields (Francia Martinez et al., 2006; Cerdà et al., 2009). In contrast, groundcover vegetation provided 4
mechanical soil protection by increasing water infiltration and by reducing splash erosion and concentrated flow erosion (Kinnell, 2005; Martinez Raya et al., 2006; Novara et al., 2011; Ferreira et al., 2018). Although the effects of groundcover management on runoff and erosion have been discussed in previous studies, comparisons between different groundcover management strategies in citrus orchards have yet to be examined. Soil and water loss processes are strongly influenced by factors other than groundcover. Among these factors, rainfall characteristics are expected to be the most frequently mentioned (Kinnell, 2005; Ran et al., 2012; Nearing et al., 2017; Huang et al., 2018). Rainfall is the initial and essential driving force resulting for water erosion. Variations in rainfall parameters, such as depth, intensity and erosivity, significantly influence the occurrence and intensity of soil erosion (Liu et al., 2016; Higley and Conroy, 2019). Due to global climate change and environmental change, an important consequence of rainfall change has been the increased frequency and intensity of extreme rainfall (Easterling et al., 2000; Wei et al., 2009a; Westra et al., 2014; Vallebona et al., 2015; Sarhadi and Soulis, 2017; Sharma and Mujumdar, 2019). The increase in extreme rainfall events will aggravate the uncertainty and complexity of the water erosion processes and their control (Wei et al., 2009b; Wang et al., 2018; Farsi and Mahjouri, 2019; Solano-Rivera et al., 2019; Zhang et al., 2019). Many studies have shown that runoff and erosion amounts due to extreme rainfall are far greater than those arising from ordinary rainfall (Coppus and Imeson, 2002; Xu et al., 2007; Wei et al., 2009b; Chen et al., 2015; Zhao et al., 2019). As a consequence, the severe water erosion and disaster caused by extreme precipitation have gained widespread attention. However, the responses of runoff and sediment loss to extreme rainfall events in the hilly citrus orchards remain scarce in southern China, especially for different groundcover management 5
systems. The principal objectives of this study are as follows: (a) to study the effects of different groundcover managements on runoff and erosion from citrus orchards; (b) to analyze the responses of water erosion characteristics to extreme rainfall events in citrus orchards; and (c) to determine the role of different groundcover managements for control of soil and water loss during extreme rainfall events. The results derived from this study will be helpful for improving groundcover management measures for extreme water erosion control in citrus orchards of the red soil region, southern China. 2. Materials and Methods 2.1. Study area The study was conducted at the Jiangxi Ecological Park of Soil and Water Conservation in the Yangou watershed (29°16′N to 29°17′N, 115°42′E to 115°43′E), which is the part of Poyang Lake watershed and is located in De’an County, Jiangxi province (Fig. 1). This region is dominated by a subtropical monsoon climate with an average annual temperature of 16.7 ℃ . The mean annual precipitation is 1,469 mm, of which more than 70% occurs from April to September (i.e., mainly in the spring and summer), and the average annual sunshine ranges from 1,700 to 2,100 h. The frost-free period of the region is between 245 and 260 d. The altitude within this watershed ranges from 30 to 90 m. The dominant soil type in the watershed is the red clay soil, produced by the weathering of Quaternary sediments. The soil depth in the study area is about 100 cm, consisting of approximately 15.32±3.24% sand, 68.28±3.12% silt and 16.40±2.23% clay (Liu et al., 2016; Dai et al., 2018). Natural resources, land use types and population densities in the Yangou watershed are typical of the hilly mountain regions in southern China. The local landscape is characterized by ranges of 6
hilly mountains, in which sloping fields are common. The slopes of the fields within the watershed range from 8 degrees to 15 degrees and have been mostly planted with citrus because these fruit trees generate additional economic benefits for the local farmers. These citrus orchards are the main type of agricultural activity in the studied watershed. Insert Figure 1 2.2. Experimental plot design Four in-situ runoff plots were constructed in the study watershed, each with a size 20 m × 5 m (length × width) on a grassland slope with a 12° gradient, and the soil conditions among the four runoff plots were similar. The plots were adjacent and parallel to the slope, and cement ridges were inserted vertically 50 cm into the soil and were 20 cm above the ground at the borders of each plot; there were constructed to separate adjacent plots and to restrict hydrological interference. Two-year seedlings of Citrus reticulate were planted with a spacing of 3.0 m × 2.5 m in the runoff plots except for the control bare land. The treatments for the four runoff plots were control bare land (BLck), orchard with grass cover (OCgc), orchard with cover crops (OCcc) and orchard with no cover (OCnc) (Fig.1). In this study, bare land was employed as the control treatment, and the weeds in the runoff plot were manually removed every month, without tillage and loosen on the plot. For the orchard with grass cover treatment, a local grass, Cynodon dactylon, was strip contour planted between the citrus trees with a strip width of 0.5 m, and the grass remained without reseeding or cutting during the study period. This grass is a perennial grass species with a typical height of 0.3 m and is widely used for soil erosion control in the red soil region. Interplanting crops beneath the fruit trees is an economical treatment to improve orchard income, and furthermore, the crops provided 7
some surface cover which was beneficial for soil erosion control, especially for the young orchards where the fruit trees were small. For this research, soybeans (summer) and radishes (winter) were contour cultivated between the citrus trees. The harvest time of soybeans and radishes were conducted on every August and March, respectively. During the crops harvest processes, the soybeans roots and radishes tubers were manually exported from the field, and were shaken gently to remove excess soil. The crop residues were removed from the orchards after each harvest. Weeds grow easily in the red soil region due to the high precipitation and are harmful to the fruit trees, and herbicide spraying is widely used in orchard management by local farmers, resulting in a bare land surface and heavy soil erosion during the rainy season. To test the generation soil loss and runoff under such orchard management practices, herbicides were sprayed bimonthly on the plot to remove weeds, and the tillage and loose soil were absent from the plot (OCnc). To collect the surface runoff and sediments from the red soil with different groundcover managements, runoff gathering trenches and storage containers were constructed at the bottoms of the runoff plots, and the storage containers were connected to the trenches by plastic pipes (Fig. 2). The construction and experimental measures for runoff plots were accomplished in 2000, and the runoff and sediment measurements began in 2001. Insert Figure 2 2.3. In-situ measurements The surface runoff and soil loss from each plot were collected and measured after each rainfall event from 2001 to 2012. The runoff and sediment yield of each plot were collected and transferred to their corresponding runoff storage containers. A water-level gauge was installed in 8
each container to record runoff levels from each rainfall event. The runoff volumes of each rainfall event were calculated by taking the product of the container water level and its base area, and the runoff depth was measured by dividing the runoff volume by the plot area. An erosional rainfall event was not considered independent unless the intervals between other rainfall events exceeded 6 h. After each rainfall-water level measurement, the sediment grains in the runoff gathering trench were washed with the clear liquid deposited from the surface runoff collections and then transferred into the runoff storage containers through plastic pipes. Five runoff samples mixed sediment were then collected in 1,000-ml measuring flasks after thoroughly mixing runoff water and sediment in each runoff container, and were then transported to the laboratory for measuring the sediment concentrations by oven drying at 105 °C to a constant weight. The amount of soil loss for each rainfall event was measured by calculating the product of the runoff volume and the sediment concentration. After each runoff and sediment sampling episode, all runoff storage containers were washed completely with clean water and were then used to collect runoff and sediment from the next rainfall event. Several rainfall variables, including rainfall depth, maximum rainfall intensity in 30 min and rainfall duration of each event were automatically recorded at 5-min intervals by a meteorological station located near the plots in this study area. 2.4. Definition of extreme rainfall Extreme rainfall events generally refer to the occurrence of lower probability rainfall events compared with the same period, which are characterized by high hazard and sudden occurrences. In this study, the criteria of the World Meteorological Organization (WMO) were chosen to define the extreme rainfall. If the difference between the rainfall variables and corresponding multiyear average were greater than the double variance of variable, the rainfall events were defined as 9
being extreme rainfall events (CCCIN, 2002; Wei et al., 2009b). In addition, rainfall depths and maximum intensities in 30 min (I30) were used to define extreme rainfall, and the corresponding calculation formula was as follows: φ > 2θ where φ refers to the specific value of rainfall variable, such as depth or I30, and and θ are the multiyear mean value and variance of the relevant variable, respectively. According to the equation mentioned above, the extreme rainfall was determined by two specific criteria. (1) Rainfall depth was greater than its multiyear average, and the I30 value exceeded the sum of its perennial average value and the double variance. (2) If I30 failed to meet the WMO criterion, the rainfall depth must exceed the relevant WMO requirements. Rainfall events satisfying either of the above two criteria belong to an extreme rainfall event. 2.5. Statistical analysis In-situ observations of runoff and sediment data from twelve consecutive years (2001-2012) were used to analyze the runoff and erosional characteristics of the red soil in this study. The runoff coefficient (ROC) and soil loss amount (SLA, Mg ha-1) were determined by following formulae: ROC =
RD × 100% PD
where RD was the runoff depth (mm) and PD was the precipitation depth (mm). SLA =
SL PA
where SL was the sediment loss amount (Mg) and PA was the and runoff plot area (ha). One way ANOVA was performed to determine the significance of groundcover managements and rainfall patterns (extreme and ordinary rainfall) for the runoff coefficients and soil loss amounts. Duncan's tests were used to separate the means at a probability level of 5%. The Pearson 10
correlations between the runoff coefficients and the soil loss amounts and rainfall variables were analyzed. Statistical analyses of all data were performed using SPSS version 17.0 for Windows, and all graphics were created using Origin Pro 9.0 software. 3. Results 3.1. Rainfall patterns According to the continuous observation data and the WMO criteria, the minimum thresholds of extreme rainfall depth and I30 were 84.2 mm and 46.2 mm h-1 in this study, respectively. On the basis of the threshold criteria, 25 extreme rainfall events were selected from 356 erosive rainfall events in the study area from 2001 to 2012, and the remaining rainfall events were assigned to the ordinary rainfall category (Table 1). The occurrences of extreme rainfall and ordinary rainfall had probabilities of 7.0% and 93.0%, respectively. As shown in Table 1, the mean depth (87.5 mm) and I30 (51.4 mm h-1) of extreme rainfall were far greater than the values (25.7 mm and 13.3 mm h-1, respectively) from ordinary rainfall. The minimum and maximum values of I30 for extreme rainfall were 9.6 and 90.6 mm h-1, respectively. The extreme rainfall durations ranged from 48 to 3,478 min with a highest variable coefficient of 1.02. Considering seasonal distributions, extreme rainfall events occurred 14 times in summer (from June to August) with the highest probability of 56.0%, 6 times in spring (from March to May) with a probability of 24.0%, and 4 and 1 times in autumn and winter with probabilities of 16.0 and 4.0%, respectively. These findings indicated that the spring and summer were the periods of high frequency of occurrence of extreme rainfall events. Insert Table 1 3.2. Average runoff coefficients and erosion amounts for different groundcover managements 11
Significant differences in the average runoff coefficients were identified between the different plots (F=167.25, p0.001). On average, the runoff coefficients were significantly higher on the BLck plot (22.57%) than for the OCnc plot (11.99%), the OCcc plot (5.36%) and the OCgc plot (2.38%) (Fig. 3A). The values ranged from 2.39 to 65.70% in the BLck plot, from 1.63 to 44.71% in the OCnc plot, from 1.20 to 16.63% in the OCcc plot, and from 0.77 to 4.46% in the OCgc plot. Likewise, significant differences among treatments were also observed in the average soil loss amounts (F=23.30, p0.001). These were highest in BLck plot (1.09 Mg ha-1), followed by the OCnc plot (0.36 Mg ha-1) and the OCcc plot (0.18 Mg ha-1), and were lowest in the OCgc plot (0.003 Mg ha-1) (Fig. 3B). The BLck plot values ranged between 1.2×10-3 and 21.25 Mg ha-1; the values for the OCnc plot ranged between 5.0×10-4 and 8.84 Mg ha-1; the OCcc plot values ranged between 1.0×10-4 and 9.45 Mg ha-1; and the OCgc plot values ranged between 4.0×10-5 and 0.14 Mg ha-1. The soil and water losses for orchards with no cover, grass cover and cover crops were sharply lower compared to the control bare land. The OCgc, OCcc and OCnc plots had the lower runoff and soil loss compared with the BLck plot with a reduction of 89.4%, 76.3% and 46.9% in runoff coefficient, 99.7%, 83.5% and 66.8% in soil loss amount. Insert Figure 3 3.3. Runoff and erosion characteristics under different rainfall patterns The runoff and erosion characteristics for all groundcover managements under different rainfall patterns are shown in Fig. 4. Significant differences were observed in the runoff coefficients (F=131.14, p0.001) and soil loss amounts (F=278.38, p0.001) between the extreme rainfall and ordinary rainfall events. Fig. 4 clearly shows that the extreme rainfall events produced far more runoff and sediment than ordinary rainfall events under all groundcover managements. The 12
average runoff coefficients under extreme rainfall and ordinary rainfall ranked in the order of BLck (58.92 and 19.82%, respectively) > OCnc (30.28 and 10.65%, respectively) > OCcc (14.66 and 4.69%, respectively) > OCgc (2.52 and 2.42%, respectively). The average soil loss amounts under extreme rainfall and ordinary rainfall events were highest in the BLck plot (6.75 and 0.66 Mg·ha-1, respectively), followed by the OCnc plot (2.45 and 0.20 Mg·ha-1, respectively), the OCcc plot (1.47 and 0.08 Mg·ha-1, respectively) and the OCgc plot (0.01 and 0.002 Mg·ha-1, respectively). Insert Figure 4 The ratios of the extreme runoff coefficients and the erosion amounts to the mean ordinary runoff coefficients and erosion amounts are shown in Fig. 5. The differences in magnitude of the erosion amounts were far higher than those of the runoff coefficients between the extreme rainfall and ordinary rainfall events. The average ratios of the extreme erosion amounts to the mean ordinary erosion amounts were highest in the OCcc plot (18.73), followed by the OCnc plot (12.04), the BLck plot (10.19) and the OCgc plot (4.39), whereas the average ratios of the extreme runoff coefficients to the ordinary runoff coefficients ranged from 1.04 to 3.13. The ratios of the extreme erosion amounts to the mean ordinary erosion amounts ranged from 0.10 to 54.76 in the OCcc plot, 0.27 to 32.81 in the OCnc plot, 0.81 to 23.40 in the BLck plot and 0.53 to 6.36 in the OCgc plot. These findings indicate that orchard erosion with cover crops showed high sensitivity to extreme rainfall, whereas orchard erosion with grass cover showed lower sensitivity to extreme rainfall. Insert Figure 5 4. Discussion 4.1. Effects of groundcover management on water erosion Orchard lands are important land use types in red soil region. Due to increases in vegetation 13
coverage, soil and water losses were reduced significantly (46.9 and 66.8%, respectively) in orchards compared to the case of bare land (Fig. 3). However, there currently exist serious risks of soil and water losses in the orchards that often have extensive areas of bare soil, due to unreasonable management practices, such as the extensive use of herbicides. Experimental results have shown that the multiyear average of the soil erosion rate in orchards with no cover was 11.40 Mg ha-1 a-1. These results were consistent with other research results in the red soil region. Durán Zuazo et al. (2005) measured an average annual soil loss of 9.1 Mg ha-1 a-1 from orchard terraces. Fang et al. (2017) measured average annual erosion amounts of 15.33 and 10.20 Mg ha-1 a-1 for bare land and orchard land, respectively. The erosion rates for orchards with no cover in this study were much higher than the tolerable erosion rate of 5.00 Mg ha-1 a-1 in the red soil region, according to the soil erosion standards for classification and gradation given by the Ministry of Water Resources of China (SL190-2007). Bare soil surfaces with soil compaction might be responsible for the high erosion rate for orchards with unprotected soil, which means that such soil management practices are unsustainable. Groundcover vegetation is a good soil conservation practice for orchards, because of raindrop interception and runoff energy absorption on the soil surface (Roce et al., 2003; Francia Martinez et al., 2006; Feng et al., 2018). In this study, the runoff coefficients and soil loss amounts differed significantly between the orchards with and without vegetation cover. The multi–year average runoff coefficients and soil erosion rates were significantly lower (2.38 and 5.36%, 0.06 and 2.25 Mg ha-1 a-1, respectively) in the orchards with grass cover and cover crops than those with no cover. The experimental results indicated that the erosion rates in orchards with vegetation cover were far lower than the tolerable erosion rate for this region. The differences in runoff and erosion 14
among the different groundcover managements were similar to those reported in previous studies. Francia Martinez et al. (2006) reported that cover barley (Hordeum vulgare) in olive orchards on a 30% slope substantially decreased soil and water loss amounts (19.8 mm a-1 and 2.1 Mg ha-1 a-1, respectively). Keesstra et al. (2016) found that the lowest runoff and erosion amounts were identified in covered plots, and the highest values were seen in uncovered plots. Gómez et al. (2009) determined that cover crops effectively reduced runoff and erosion to tolerable levels, while Atucha et al. (2013) found that the runoff and erosion were reduced in a hillside avocado orchard with grass striped or as a complete cover compared to orchards with no cover. These findings confirmed that groundcover vegetation treatments reduced runoff and erosion more efficiently than for the case of no cover in orchards. Fig. 6 shows the annual runoff coefficients and soil erosion amounts under different groundcover managements from 2001 to 2012. The interannual variations in the runoff coefficients and erosion amounts were significantly affected by the annual rainfall changes for the control bare land. However, the runoff and erosion variations in different years were significantly lower in the other orchard plots. Compared with grass cover, the cover crops had higher annual variations in runoff and erosion due to rotation tillage practices. Overall, the vulnerability of water erosion to rainfall was mitigated by orchard development (tree cover and surface vegetation cover). This is explained by the sharp reduction or halting of rainfall erosivity with increasing vegetation coverage (Francia Martinez et al., 2006; Feng et al., 2018). As shown in Fig. 6B, the highest erosion rates were seen in the first year for the control bare land and the orchard with no cover (69.18 and 65.00 Mg ha-1 a-1, respectively), accounting for 20.0% and 55.8%, respectively of the total erosion over 12 years. The erosion rates in the first year 15
reached the criterion of intense erosion, and similar observations were reported by Cerdà et al. (2009) and Antucha et al. (2013) for new orchards. This was explained by the serious disturbance of the surface soil by large scale mechanized excavation that caused a lack of vegetation cover in this period, and by the low tree coverage in the newly planted hillside orchards. The severe soil erosion rates from newly planted orchards are not sustainable under current farming practices where the use of groundcover vegetation is completely avoided. In this study, the average erosion rate reductions in the first year were 64.81 and 64.52 Mg ha-1 in orchards with grass cover and cover crops compared to orchards with no cover, respectively (Fig. 6B). These results confirmed that severe water erosion is easily induced in the initial stage of orchard establishment. Groundcover vegetation, once established, reduced runoff and sediment yield more efficiently than a no cover treatment in young orchards. Insert Figure 6 4.2. Effects of extreme rainfall on water erosion Many studies have shown that rainfall variables, such as depth and intensity, were confirmed as the main drivers of the degree of surface hydrology and soil erosion (Ran et al., 2012; Liu et al., 2016; Nearing et al. 2017). As shown in Table 1, the mean rainfall depths and maximum rainfall intensities during 30 min of extreme rainfall were 3.4 and 3.9 times those of ordinary rainfall, respectively. Extreme rainfall with high intensity exceeded the soil infiltration capacity and much rainfall was lost as surface runoff. As a consequence, the extreme rainfall events with large magnitudes and low frequencies were assumed to dominate with respect to soil loss compared to ordinary rainfall events. In this study, the mean extreme runoff coefficients and erosion amounts for all treatments were 2.8 16
and 11.1 times the values from ordinary rainfall, respectively (Fig. 4). The results were similar to those reported in previous studies. Solano-Rivera et al. (2019) also showed that extreme precipitation events have an overriding impact on surface runoff and erosion. Wei et al. (2009b) reported that the mean runoff amounts and erosion rates of extreme rainfall were 2.7 and 53.2 times those of ordinary rainfall, respectively, in a semiarid hilly loess area in NW China. The lower erodibility of loess was responsible for the greater extreme erosion rates in the research results of Wei et al., (2009b). Fig. 5 clearly shows that sediment yields were more sensitive to extreme rainfall compared to runoff production. The ratios of extreme erosion to ordinary erosion were 3-6 times the values of extreme runoff to ordinary runoff in all plots. Meanwhile, the contribution of extreme rainfall to sediment yield (44.2%) is far higher than this to runoff production (15.8%) (Fig.7). The reason for these results is that higher sediment concentrations were found during heavy rainfall (Aksoy and Kavvas 2005; Dai et al., 2018) and were mainly because of higher soil detachment capacities under
strong rainfall erosivity and runoff energy (Fan et al., 2010). Furthermore, the Pearson correlation analysis shows the different effects of the rainfall variables on the runoff coefficients and on the soil loss amounts between extreme rainfall and ordinary rainfall (Table 2). For example, the maximum intensity in 30 min had a more significant correlation with soil erosion amounts than with rainfall depth, whereas the rainfall depth had a significantly stronger relationship with the runoff coefficient. The results confirmed that the maximum intensity in 30 min contributed more to soil erosion than did the rainfall depth. Insert Table 2 4.3. Role of various groundcover managements on extreme water erosion control 17
The extreme runoff coefficients and soil loss amounts varied significantly among the different groundcover managements (Fig. 4). Vegetation covers effectively reduced extreme runoff and erosion compared to those orchards with no cover. The differences in runoff and sediment reduction rate between grass cover and cover crops were very clear, especially for control of sediment loss. Sediment reduction rate for the grass cover treatment (99.6%) was two orders of magnitude greater than that measured for the cover crops treatment (40.0%) (Fig. 4B). The literature shows that concentrated flow was easily generated on sloping land under extreme rainfall with high intensity and depth (Boardman, 2015; Wang et al., 2018). Compared to the cover crops treatment, the denser stems and higher biomass of grass cover on the soil surface provided a more effective buffer, which decreased the overland flow velocity and energy, and decreased soil erosion (Roce et al., 2003; Pan and Shanguan, 2010; Mu et al., 2019), and the stem parts of the grass intercepted a portion of the sediment particles carried by the runoff (Dunkerley et al., 2001). The accumulated contribution rates of extreme rainfall and ordinary rainfall to the runoff and erosion are shown in Fig. 7. The highest erosion contribution of 59.0% was found in orchard with cover crops, followed by orchard with no cover (47.9%), bare land (43.8%) and orchard with grass cover (26.0%). The results indicated that 59.0% of the total soil erosion was caused by a few extreme rainfall events, with a probability of 7.0% (Table 1) in orchards with cover crops, and the lowest soil erosion occurred in orchards with grass cover during the study period. The results were similar to the research of Martı́nez-Casasnovas et al. (2002), who reported that soil erosion caused by a few extreme rainfall events often accounted for 60-90% of the total annual erosion amount. This was because the grass used on the grass cover plots with high vegetation coverage was 18
Cynodon dactylon, a warm-season perennial grass species grown without reseeding and cutting, and the soil erosion for the cover crop plots was especially vulnerable to tillage measures with yearly crop rotations. Due to the low grass strip, the grass cover without cutting did not affect the orchard production management. But for other more tall grass species, the groundcover should be suitably mowed and the cutting residues mulch on soil surface during winter for the least precipitation and low soil erosion. Cover crops with rotation have usually been viewed as the primary cause of soil erosion (Ramos et al., 2010; Keesstra et al., 2016; Fang et al., 2017). O'Neal et al. (2005) determined that lower soil losses were found for continuous crop cultivation than for crop rotation. In this study, crop rotations with soybeans and radishes were used in the orchards. The soybeans were grown from April to August, and the radishes were grown from September to March in each year. The crops harvest season occurred in spring and summer with high occurrence frequencies of extreme rainfall events (80% in the study area). Severe soil erosion amounts were easily induced due to the high rainfall erosivity and low surface vegetation coverage. Loose soil and soil adhering to the crops were exported from field together with these crops during crops harvesting processes. Ruysschaert et al. (2004) indicated that soil losses due to crop harvesting may rise from a few Mg to a few tens of Mg per hectare per harvest. Moreover, the frequent tillage with crop rotation increased soil erodibility and the probability of severe erosion in the crop growth season. From the perspective of soil and water conservation, the severe soil erosion caused by extreme rainfall should receive more attention when interplanting crops beneath fruit trees. The crop straw mulch and no-till or minimum tillage after harvesting were recommended as effective strategy to prevent extreme soil erosion in citrus orchard with cover crops. 19
Insert Figure 7 5. Conclusions In this paper, based on rainfall depth and I30, extreme rainfall and ordinary rainfall were determined by the continuous observation data and criterion of WMO. The minimum thresholds for rainfall depths and I30 values for extreme rainfall were determined to be 84.2 mm and 46.2 mm·h-1, respectively. 25 extreme rainfall events were selected from 356 erosive rainfall events from 2001 to 2012. The mean rainfall depth and I30 of extreme rainfall were threefold more than those of ordinary rainfall events. Extreme rainfall events caused severer soil and water loss compared with ordinary events. The average extreme runoff and erosion amount were 2.8 and 11.1 times higher than the values from ordinary rainfall, respectively. Meanwhile, the contribution of extreme rainfall to sediment yield (44.2%) was far higher than the contribution of extreme rainfall to runoff production (15.8%). Obviously, extreme rainfall events played the destructive role in inducing sediment loss. Runoff coefficient and soil loss amount differed significant among the groundcover managements under extreme rainfall. The size order was as follow: control bare land > orchard with no cover > orchard with cover crops > orchard with grass cover. However, the contribution of extreme rainfall to sediment yield differed greatly from the above rule. The highest erosion contribution was found in orchard with cover crops, and the lowest in orchard with grass cover under extreme rainfall. For orchards with cover crops and grass cover, 59.0 and 26.0% of total soil loss amount were caused by a few extreme rainfalls with a probability of 7.0%. These findings are meaning for controlling water erosion caused by extreme rainfall in citrus orchards of southern China. The soil and water losses caused by extreme rainfall should deserve 20
more attention when interplanting crops beneath fruit trees. Grass cover between crops is recommended as a groundcover management strategy to effectively fight against extreme water erosion. Acknowledgements This study was funded by the National Key Research and Development Program of China (2017YFC0505405), the National Natural Science Foundation of China (41877084, 41761065 and 41867016) and the Scientific Major Project of Water Conservancy in Jiangxi Province (KT201717 and 201821ZDKT17). Reference Aksoy, H., Kavvas, M.L., 2005. A review of hill slope and watershed scale erosion and sediment transport models. Catena 64, 247-271. Atucha, A., Merwin, I.A., Brown, M.G., Gardiazabal, F., Mena, F., Adriazola, C., Lehmann, J., 2013. Soil erosion, runoff and nutrient losses in an avocado (Persea americanaMill) hillside orchard under different groundcover management systems. Plant and Soil 368(1-2), 393-406. Barton, A.P., Fullen, M.A,, Mitchell, D.J., Hockinga, T.J., Liu, L.G., Bo, Z.W., Zheng, Y., Xia, Z.Y., 2004. Effects of soil conservation measures on erosion rates and crop productivity on subtropical Ultisols in Yunnan Province, China. Agriculture Ecosystems and Environment 104(2), 343-357. Boardman, J., 2015. Extreme rainfall and its impact on cultivated landscapes with particular reference to Britain. Earth Surface Processes and Landforms 40, 2121-2130. Bonetti, S., Richter, D.D., Porporato, A., 2019. The effect of accelerated soil erosion on hillslope morphology. Earth Surface Processes and Landforms, doi.org/10.1002/esp.4694. CCCIN (China Climate Change Information Network), 2002. Climate change: observation facts and future 21
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Solano-Rivera, V., Geris, J., Granados-Bolaños, S., Brenes-Cambronero, L., Artavia-Rodríguez, G., Sánchez-Murillo, R., Birke, C., 2019. Exploring extreme rainfall impacts on flow and turbidity dynamics in a steep, pristine and tropical volcanic catchment. Catena 182, 104-118. Vallebona, C., Pellegrino, E., Frumento, P., Bonari, E., 2015. Temporal trends in extreme rainfall intensity and erosivity in the Mediterranean region: a case study in southern Tuscany, Italy. Climatic Change 128(1-2), 139-151. Wang, L.J., Zhang, G.H., Wang, X., Li, X.Y., 2018. Hydraulics of overland flow influenced by litter incorporation under extreme rainfall. Hydrological Processes 33(5), 1-11. Wei, S.C., Zhang, X.P., McLaughlin, N.B., Chen, X.W., Jia, S.X., Liang, A.Z., 2017. Impact of soil water erosion processes on catchment export of soil aggregates and associated SOC. Geoderma 294, 63-69. Wei, W., Chen, L.D., Fu, B.J., 2009a. Effects of rainfall change on water erosion processes in terrestrial ecosystems: a review. Progress in Physical Geography 33(3), 307-318. Wei, W., Chen, L.D., Fu, B.J., Lü, Y.H., Gong, J., 2009b. Responses of water erosion to rainfall extremes and vegetation types in a loess semiarid hilly area, NW China. Hydrological Processes 23(12), 1780-1791. Westra, S., Fowler, H.J., Evans, J.P., Alexander, L.V., Berg, P., Johnson, F., Kendon, E.J., Lenderink, G., Roberts, N.M., 2014. Future changes to the intensity and frequency of short-duration extreme rainfall. Reviews of Geophysics 52(3), 522-555. Xu, Z.X., Li, J.Y., Liu, C.M., 2007. Long-term trend analysis for major climate variables in the Yellow River basin. Hydrological processes 21, 1935-1948. Xu, Q.X., Wang, T.W., Cai, C.F., Li, Z.X., Shi, Z.H., 2012. Effects of soil conservation on soil properties of citrus orchards in the Three-Gorges Area, China. Land Degradation and Development 23(1), 34-42. Zhang, Y., Zhao, Y.Y., Liu, B.Y., Wang, Z.Q., Zhang, S., 2019. Rill and gully erosion on unpaved roads under 26
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Figure legends: Fig.1. Location of study area in the red soil region of southern china. The study site was 15 km distance away from Poyang Lake, which is the biggest freshwater lake in China. The photos down below show the four runoff plots: control bare land (A), and orchards with groundcover managements including no cover (B), grass cover (C) and cover crops (D). Fig.2. Schematic diagrams of the runoff plot and the surface flow and sediment collection system. Fig.3. The average runoff coefficient (A) and soil loss amount (B) for different groundcover managements (n=356). The different letters are significantly different among the groundcover types according to one way ANOVA and Duncan's test (P<0.05). BLck, OCnc, OCgc and OCcc are control bare land and orchard with no cover, grass cover and cover crops, respectively. Fig.4. Runoff coefficient (A) and soil loss amount (B) under extreme rainfall (n=25) and ordinary rainfall (n=331) for different groundcover managements. Solid and dashed lines in this figure represent the median and mean, respectively. The box boundaries represent the 75% and 25% quartiles, the whisker caps represent the 90% and 10% quartiles, and the circles represent the 95% and 5% quartiles. Within each panel, the means followed by different letters are significantly different between extreme rainfall and ordinary rainfall according to one way ANOVA and Duncan's test (p<0.05). BLck, OCnc, OCgc and OCcc are control bare land and orchard with no cover, grass cover and cover crops, respectively. Fig.5. REr (A) and REe (B) values of each extreme rainfall event under different groundcover managements from 2001 to 2012 (n=25). REr and REe are the ratios of extreme runoff coefficient and soil loss amount to mean ordinary runoff coefficient and soil loss amount, respectively. Solid and dashed lines in this figure represent the median and mean, respectively. The box boundaries represent the 75% and 25% quartiles, the whisker caps represent the 90% and 10% quartiles, and the circles represent the 95% and 5% quartiles. Different letters represent 28
significant differences among different groundcover managements according to one way ANOVA and Duncan's test (p<0.05). BLck, OCnc, OCgc and OCcc are control bare land and orchard with no cover, grass cover and cover crops, respectively. Fig.6. Annual runoff coefficient (A) and soil loss amount (B) under different groundcover managements from 2001 to 2012. BLck, OCnc, OCgc and OCcc are control bare land and orchard with no cover, grass cover and cover crops, respectively.
Fig.7. Accumulated contribution rate of extreme rainfall and ordinary rainfall in runoff coefficient (A) and erosion amount (B) for different groundcover managements. The shaded and unshaded areas in this figure indicate the contribution rate of extreme rainfall and ordinary rainfall, respectively. BLck, OCnc, OCgc and OCcc are control bare land and orchard with no cover, grass cover and cover crops, respectively.
29
30
31
32
33
34
35
36
Table 1 Statistical features of extreme rainfall and ordinary rainfall variables. I30, SD and CV refer to maximum rainfall intensity in 30 min, standard deviation and variable coefficient, respectively. Rainfall patterns Extreme rainfall Ordinary rainfall
Eigenvalue
Min
Max
Mean
SD
Sum
CV
Rainfall depth (mm)
32.6
253.4
87.5
51.9
2188.6
0.59
9.6
90.6
51.4
20.9
—
0.41
Rainfall duration (min)
48
3478
1043
1064
26078
1.02
Rainfall depth (mm)
3.6
81.8
25.7
17.4
8163.0
0.68
1.8
63.0
13.3
10.5
—
0.79
18
3710
836
667
276797
0.80
I30
I30
(mm·h-1)
(mm·h-1)
Rainfall duration (min)
37
Frequency (Times) 25
331
Table 2 Pearson correlation analysis between rainfall parameters and runoff coefficients and soil erosion amounts. BLck, OCnc, OCgc and OCcc are control bare land and orchard with no cover, grass cover and cover crops, respectively. *p0.05, **p0.01. Variables
BLck
Rainfall
OCnc
OCgc
OCcc
parameters
P
R2
Rainfall depth
0.620**
0.000
0.478**
runoff
I30
0.460**
0.000
0.342**
0.005
0.278*
0.015
0.479*
coefficient
Rainfall duration
0.183
0.381
0.199
0.340
-0.254
0.221
0.307
0.136
Rainfall depth
0.587**
0.000
0.344**
0.000
0.218
0.442
0.257**
0.003
runoff
I30
0.272*
0.021
0.300*
0.045
0.262
0.407
0.219
0.292
coefficient
Rainfall duration
-0.143
0.109
-0.013
0.815
-0.008
0.885
0.034
0.535
Extreme
Rainfall depth
0.578**
0.00
0.329**
0.002
0.232*
0.011
0.325**
0.002
erosion
I30
0.650**
0.000
0.489**
0.000
0.375**
0.000
0.491**
0.000
amount
Rainfall duration
-0.284*
0.016
-0.003
0.990
-0.068
0.746
0.073
0.731
Ordinary
Rainfall depth
0.277*
0.013
0.245
0.061
0.076
0.167
0.119
0.569
erosion
I30
0.487**
0.002
0.315**
0.007
0.015
0.943
0.317**
0.008
amount
Rainfall duration
-0.150
0.169
-0.043
0.435
0.129
0.059
0.001
0.988
Extreme
Ordinary
38
P
R2 0.002
P
R2
P
R2
0.470**
0.008
0.537**
0.000 0.000
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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40
Highlights: 356 natural rainfalls showed a destructive role of extreme rainfall in soil erosion. Extreme rainfall had higher capability in soil loss rather than runoff generation. Grass cover was useful in controlling extreme water erosion from orchards.
41
S0022-1694(19)31025-X https://doi.org/10.1016/j.jhydrol.2019.124290 HYDROL 124290
To appear in:
Journal of Hydrology
Received Date: Revised Date: Accepted Date:
1 August 2019 24 October 2019 25 October 2019
Please cite this article as: Duan, J., Liu, Y-J., Yang, J., Tang, C-J., Shi, Z-H., Role of groundcover management in controlling soil erosion under extreme rainfall in citrus orchards of southern China, Journal of Hydrology (2019), doi: https://doi.org/10.1016/j.jhydrol.2019.124290
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Role of groundcover management in controlling soil erosion under extreme rainfall in citrus orchards of southern China Jian Duana,b,c,1, Yao-Jun Liua,b,c,1, Jie Yanga,b,*, Chong-Jun Tanga,b, Zhi-Hua Shic,d a Jiangxi
Institute of Soil and Water Conservation, Nanchang, Jiangxi 330029, PR China
b Jiangxi
Provincial Key Laboratory of Soil Erosion and Prevention, Nanchang, Jiangxi 330029, PR China
c Research
Center of Soil and Water Conservation and Ecological Environment, Chinese Academy of Sciences and
Ministry of Education, Yangling, Shaanxi 712100 China d College
1
of Resources and Environment, Huazhong Agricultural University, Wuhan, Hubei 430070, PR China
These authors contributed equally to this work.
*Corresponding to: Jie Yang, Jiangxi Institute of Soil and Water Conservation, 330029, China. Email:[email protected]
1
Abstract: Extreme rainfall is becoming more frequent and intense due to climate change and increasing human activity. Changes in rainfall will increase the risks and uncertainty of water erosion from orchards. Groundcover management is an important factor affecting hydrological and erosive processes of orchards. However, the roles of different groundcover managements for controlling soil water erosion caused by extreme rainfall are not fully understood. In this study, four groundcover managements (i.e., control bare land, orchard with no cover, orchard with grass cover and orchard with cover crops) were used to analyze runoff and erosion characteristics related to extreme and ordinary rainfalls in the red soil region of China. Based on the rainfall-runoff data measured on runoff plots, 356 natural rainfall events from 2001 to 2012 were analyzed, and rainfall depths and maximum 30-min rainfall intensities were used to select extreme rainfall events, according to the criteria of the World Meteorological Organization for extreme rainfall. There were 25 extreme rainfall events with a probability of 7.0% during the study period. Extreme rainfall events played the destructive role in inducing soil loss. The average extreme runoff coefficient and soil loss amount were 2.8 and 11.1 times higher than the values from ordinary rainfall, respectively. The contribution of extreme rainfall to sediment yield was 44.2%, which far higher than those to runoff generation (15.8%). Moreover, the runoff coefficients and erosion amounts varied greatly among the different groundcover managements under extreme rainfall, and were ranked in the following order: control bare land > orchard with no cover > orchard with cover crops > orchard with grass cover. However, the highest percentage of soil loss amount caused by extreme rainfalls were 59.0% in orchard with cover crops, and the lowest in orchard with grass cover (26.0%). Therefore, extreme rainfall should cause more attention in improving the groundcover management strategy for orchard soil erosion control, and 2
groundcover like grass cover was extremely needed even after interplanting crops for the serious soil loss. Keywords: water erosion; extreme rainfall; groundcover management; citrus orchards; red soil region
3
1. Introduction Water erosion is mainly responsible for global land degradation and productivity declines (Rodrigo et al., 2016; Wei et al., 2017; Bonetti et al., 2019). For a long period, the red soil region of southern China has been facing severe soil and water loss due to high precipitation, hilly landforms and unsustainable farming practices (Barton et al., 2004; Liang et al., 2010; Shi et al., 2014; Fang et al, 2017). In recent decades, with the rapid growth of human population (40% of China's population) and heavy pressure on productive soil resources, increasing amounts of barren land with a secondary community on the slopes have been transformed into orchards (Zuazo et al., 2005; Xu et al., 2012; Li et al., 2014; Chen et al., 2019). Hilly citrus orchards are widely distributed in this region due to their additional economic benefits. Meanwhile, citrus orchard lands are exposed to serious risks of soil and water loss due to the intense soil disturbances from large-scale mechanized excavation and the lack of surface vegetation cover (Durán Zuazo et al., 2005; Prosdocimi et al., 2016; Comino et al., 2018). Soil and water conservation measures on soil surfaces are rarely implemented for orchard production. Relevant groundcover measures are very infrequent, and further research is necessary to assess the sustainable groundcover management strategies. Groundcover management is an important factor that affects water erosion processes in orchards. Vegetation-free groundcover management, such as herbicides spraying and mechanical weeding, have been widely implemented to maximize commercial production in conventional orchards (Novara et al., 2011; Keesstra et al., 2016; Prosdocimi et al., 2016). The management method with bare soil year round decreased soil infiltration and favored runoff production and sediment yields (Francia Martinez et al., 2006; Cerdà et al., 2009). In contrast, groundcover vegetation provided 4
mechanical soil protection by increasing water infiltration and by reducing splash erosion and concentrated flow erosion (Kinnell, 2005; Martinez Raya et al., 2006; Novara et al., 2011; Ferreira et al., 2018). Although the effects of groundcover management on runoff and erosion have been discussed in previous studies, comparisons between different groundcover management strategies in citrus orchards have yet to be examined. Soil and water loss processes are strongly influenced by factors other than groundcover. Among these factors, rainfall characteristics are expected to be the most frequently mentioned (Kinnell, 2005; Ran et al., 2012; Nearing et al., 2017; Huang et al., 2018). Rainfall is the initial and essential driving force resulting for water erosion. Variations in rainfall parameters, such as depth, intensity and erosivity, significantly influence the occurrence and intensity of soil erosion (Liu et al., 2016; Higley and Conroy, 2019). Due to global climate change and environmental change, an important consequence of rainfall change has been the increased frequency and intensity of extreme rainfall (Easterling et al., 2000; Wei et al., 2009a; Westra et al., 2014; Vallebona et al., 2015; Sarhadi and Soulis, 2017; Sharma and Mujumdar, 2019). The increase in extreme rainfall events will aggravate the uncertainty and complexity of the water erosion processes and their control (Wei et al., 2009b; Wang et al., 2018; Farsi and Mahjouri, 2019; Solano-Rivera et al., 2019; Zhang et al., 2019). Many studies have shown that runoff and erosion amounts due to extreme rainfall are far greater than those arising from ordinary rainfall (Coppus and Imeson, 2002; Xu et al., 2007; Wei et al., 2009b; Chen et al., 2015; Zhao et al., 2019). As a consequence, the severe water erosion and disaster caused by extreme precipitation have gained widespread attention. However, the responses of runoff and sediment loss to extreme rainfall events in the hilly citrus orchards remain scarce in southern China, especially for different groundcover management 5
systems. The principal objectives of this study are as follows: (a) to study the effects of different groundcover managements on runoff and erosion from citrus orchards; (b) to analyze the responses of water erosion characteristics to extreme rainfall events in citrus orchards; and (c) to determine the role of different groundcover managements for control of soil and water loss during extreme rainfall events. The results derived from this study will be helpful for improving groundcover management measures for extreme water erosion control in citrus orchards of the red soil region, southern China. 2. Materials and Methods 2.1. Study area The study was conducted at the Jiangxi Ecological Park of Soil and Water Conservation in the Yangou watershed (29°16′N to 29°17′N, 115°42′E to 115°43′E), which is the part of Poyang Lake watershed and is located in De’an County, Jiangxi province (Fig. 1). This region is dominated by a subtropical monsoon climate with an average annual temperature of 16.7 ℃ . The mean annual precipitation is 1,469 mm, of which more than 70% occurs from April to September (i.e., mainly in the spring and summer), and the average annual sunshine ranges from 1,700 to 2,100 h. The frost-free period of the region is between 245 and 260 d. The altitude within this watershed ranges from 30 to 90 m. The dominant soil type in the watershed is the red clay soil, produced by the weathering of Quaternary sediments. The soil depth in the study area is about 100 cm, consisting of approximately 15.32±3.24% sand, 68.28±3.12% silt and 16.40±2.23% clay (Liu et al., 2016; Dai et al., 2018). Natural resources, land use types and population densities in the Yangou watershed are typical of the hilly mountain regions in southern China. The local landscape is characterized by ranges of 6
hilly mountains, in which sloping fields are common. The slopes of the fields within the watershed range from 8 degrees to 15 degrees and have been mostly planted with citrus because these fruit trees generate additional economic benefits for the local farmers. These citrus orchards are the main type of agricultural activity in the studied watershed. Insert Figure 1 2.2. Experimental plot design Four in-situ runoff plots were constructed in the study watershed, each with a size 20 m × 5 m (length × width) on a grassland slope with a 12° gradient, and the soil conditions among the four runoff plots were similar. The plots were adjacent and parallel to the slope, and cement ridges were inserted vertically 50 cm into the soil and were 20 cm above the ground at the borders of each plot; there were constructed to separate adjacent plots and to restrict hydrological interference. Two-year seedlings of Citrus reticulate were planted with a spacing of 3.0 m × 2.5 m in the runoff plots except for the control bare land. The treatments for the four runoff plots were control bare land (BLck), orchard with grass cover (OCgc), orchard with cover crops (OCcc) and orchard with no cover (OCnc) (Fig.1). In this study, bare land was employed as the control treatment, and the weeds in the runoff plot were manually removed every month, without tillage and loosen on the plot. For the orchard with grass cover treatment, a local grass, Cynodon dactylon, was strip contour planted between the citrus trees with a strip width of 0.5 m, and the grass remained without reseeding or cutting during the study period. This grass is a perennial grass species with a typical height of 0.3 m and is widely used for soil erosion control in the red soil region. Interplanting crops beneath the fruit trees is an economical treatment to improve orchard income, and furthermore, the crops provided 7
some surface cover which was beneficial for soil erosion control, especially for the young orchards where the fruit trees were small. For this research, soybeans (summer) and radishes (winter) were contour cultivated between the citrus trees. The harvest time of soybeans and radishes were conducted on every August and March, respectively. During the crops harvest processes, the soybeans roots and radishes tubers were manually exported from the field, and were shaken gently to remove excess soil. The crop residues were removed from the orchards after each harvest. Weeds grow easily in the red soil region due to the high precipitation and are harmful to the fruit trees, and herbicide spraying is widely used in orchard management by local farmers, resulting in a bare land surface and heavy soil erosion during the rainy season. To test the generation soil loss and runoff under such orchard management practices, herbicides were sprayed bimonthly on the plot to remove weeds, and the tillage and loose soil were absent from the plot (OCnc). To collect the surface runoff and sediments from the red soil with different groundcover managements, runoff gathering trenches and storage containers were constructed at the bottoms of the runoff plots, and the storage containers were connected to the trenches by plastic pipes (Fig. 2). The construction and experimental measures for runoff plots were accomplished in 2000, and the runoff and sediment measurements began in 2001. Insert Figure 2 2.3. In-situ measurements The surface runoff and soil loss from each plot were collected and measured after each rainfall event from 2001 to 2012. The runoff and sediment yield of each plot were collected and transferred to their corresponding runoff storage containers. A water-level gauge was installed in 8
each container to record runoff levels from each rainfall event. The runoff volumes of each rainfall event were calculated by taking the product of the container water level and its base area, and the runoff depth was measured by dividing the runoff volume by the plot area. An erosional rainfall event was not considered independent unless the intervals between other rainfall events exceeded 6 h. After each rainfall-water level measurement, the sediment grains in the runoff gathering trench were washed with the clear liquid deposited from the surface runoff collections and then transferred into the runoff storage containers through plastic pipes. Five runoff samples mixed sediment were then collected in 1,000-ml measuring flasks after thoroughly mixing runoff water and sediment in each runoff container, and were then transported to the laboratory for measuring the sediment concentrations by oven drying at 105 °C to a constant weight. The amount of soil loss for each rainfall event was measured by calculating the product of the runoff volume and the sediment concentration. After each runoff and sediment sampling episode, all runoff storage containers were washed completely with clean water and were then used to collect runoff and sediment from the next rainfall event. Several rainfall variables, including rainfall depth, maximum rainfall intensity in 30 min and rainfall duration of each event were automatically recorded at 5-min intervals by a meteorological station located near the plots in this study area. 2.4. Definition of extreme rainfall Extreme rainfall events generally refer to the occurrence of lower probability rainfall events compared with the same period, which are characterized by high hazard and sudden occurrences. In this study, the criteria of the World Meteorological Organization (WMO) were chosen to define the extreme rainfall. If the difference between the rainfall variables and corresponding multiyear average were greater than the double variance of variable, the rainfall events were defined as 9
being extreme rainfall events (CCCIN, 2002; Wei et al., 2009b). In addition, rainfall depths and maximum intensities in 30 min (I30) were used to define extreme rainfall, and the corresponding calculation formula was as follows: φ > 2θ where φ refers to the specific value of rainfall variable, such as depth or I30, and and θ are the multiyear mean value and variance of the relevant variable, respectively. According to the equation mentioned above, the extreme rainfall was determined by two specific criteria. (1) Rainfall depth was greater than its multiyear average, and the I30 value exceeded the sum of its perennial average value and the double variance. (2) If I30 failed to meet the WMO criterion, the rainfall depth must exceed the relevant WMO requirements. Rainfall events satisfying either of the above two criteria belong to an extreme rainfall event. 2.5. Statistical analysis In-situ observations of runoff and sediment data from twelve consecutive years (2001-2012) were used to analyze the runoff and erosional characteristics of the red soil in this study. The runoff coefficient (ROC) and soil loss amount (SLA, Mg ha-1) were determined by following formulae: ROC =
RD × 100% PD
where RD was the runoff depth (mm) and PD was the precipitation depth (mm). SLA =
SL PA
where SL was the sediment loss amount (Mg) and PA was the and runoff plot area (ha). One way ANOVA was performed to determine the significance of groundcover managements and rainfall patterns (extreme and ordinary rainfall) for the runoff coefficients and soil loss amounts. Duncan's tests were used to separate the means at a probability level of 5%. The Pearson 10
correlations between the runoff coefficients and the soil loss amounts and rainfall variables were analyzed. Statistical analyses of all data were performed using SPSS version 17.0 for Windows, and all graphics were created using Origin Pro 9.0 software. 3. Results 3.1. Rainfall patterns According to the continuous observation data and the WMO criteria, the minimum thresholds of extreme rainfall depth and I30 were 84.2 mm and 46.2 mm h-1 in this study, respectively. On the basis of the threshold criteria, 25 extreme rainfall events were selected from 356 erosive rainfall events in the study area from 2001 to 2012, and the remaining rainfall events were assigned to the ordinary rainfall category (Table 1). The occurrences of extreme rainfall and ordinary rainfall had probabilities of 7.0% and 93.0%, respectively. As shown in Table 1, the mean depth (87.5 mm) and I30 (51.4 mm h-1) of extreme rainfall were far greater than the values (25.7 mm and 13.3 mm h-1, respectively) from ordinary rainfall. The minimum and maximum values of I30 for extreme rainfall were 9.6 and 90.6 mm h-1, respectively. The extreme rainfall durations ranged from 48 to 3,478 min with a highest variable coefficient of 1.02. Considering seasonal distributions, extreme rainfall events occurred 14 times in summer (from June to August) with the highest probability of 56.0%, 6 times in spring (from March to May) with a probability of 24.0%, and 4 and 1 times in autumn and winter with probabilities of 16.0 and 4.0%, respectively. These findings indicated that the spring and summer were the periods of high frequency of occurrence of extreme rainfall events. Insert Table 1 3.2. Average runoff coefficients and erosion amounts for different groundcover managements 11
Significant differences in the average runoff coefficients were identified between the different plots (F=167.25, p0.001). On average, the runoff coefficients were significantly higher on the BLck plot (22.57%) than for the OCnc plot (11.99%), the OCcc plot (5.36%) and the OCgc plot (2.38%) (Fig. 3A). The values ranged from 2.39 to 65.70% in the BLck plot, from 1.63 to 44.71% in the OCnc plot, from 1.20 to 16.63% in the OCcc plot, and from 0.77 to 4.46% in the OCgc plot. Likewise, significant differences among treatments were also observed in the average soil loss amounts (F=23.30, p0.001). These were highest in BLck plot (1.09 Mg ha-1), followed by the OCnc plot (0.36 Mg ha-1) and the OCcc plot (0.18 Mg ha-1), and were lowest in the OCgc plot (0.003 Mg ha-1) (Fig. 3B). The BLck plot values ranged between 1.2×10-3 and 21.25 Mg ha-1; the values for the OCnc plot ranged between 5.0×10-4 and 8.84 Mg ha-1; the OCcc plot values ranged between 1.0×10-4 and 9.45 Mg ha-1; and the OCgc plot values ranged between 4.0×10-5 and 0.14 Mg ha-1. The soil and water losses for orchards with no cover, grass cover and cover crops were sharply lower compared to the control bare land. The OCgc, OCcc and OCnc plots had the lower runoff and soil loss compared with the BLck plot with a reduction of 89.4%, 76.3% and 46.9% in runoff coefficient, 99.7%, 83.5% and 66.8% in soil loss amount. Insert Figure 3 3.3. Runoff and erosion characteristics under different rainfall patterns The runoff and erosion characteristics for all groundcover managements under different rainfall patterns are shown in Fig. 4. Significant differences were observed in the runoff coefficients (F=131.14, p0.001) and soil loss amounts (F=278.38, p0.001) between the extreme rainfall and ordinary rainfall events. Fig. 4 clearly shows that the extreme rainfall events produced far more runoff and sediment than ordinary rainfall events under all groundcover managements. The 12
average runoff coefficients under extreme rainfall and ordinary rainfall ranked in the order of BLck (58.92 and 19.82%, respectively) > OCnc (30.28 and 10.65%, respectively) > OCcc (14.66 and 4.69%, respectively) > OCgc (2.52 and 2.42%, respectively). The average soil loss amounts under extreme rainfall and ordinary rainfall events were highest in the BLck plot (6.75 and 0.66 Mg·ha-1, respectively), followed by the OCnc plot (2.45 and 0.20 Mg·ha-1, respectively), the OCcc plot (1.47 and 0.08 Mg·ha-1, respectively) and the OCgc plot (0.01 and 0.002 Mg·ha-1, respectively). Insert Figure 4 The ratios of the extreme runoff coefficients and the erosion amounts to the mean ordinary runoff coefficients and erosion amounts are shown in Fig. 5. The differences in magnitude of the erosion amounts were far higher than those of the runoff coefficients between the extreme rainfall and ordinary rainfall events. The average ratios of the extreme erosion amounts to the mean ordinary erosion amounts were highest in the OCcc plot (18.73), followed by the OCnc plot (12.04), the BLck plot (10.19) and the OCgc plot (4.39), whereas the average ratios of the extreme runoff coefficients to the ordinary runoff coefficients ranged from 1.04 to 3.13. The ratios of the extreme erosion amounts to the mean ordinary erosion amounts ranged from 0.10 to 54.76 in the OCcc plot, 0.27 to 32.81 in the OCnc plot, 0.81 to 23.40 in the BLck plot and 0.53 to 6.36 in the OCgc plot. These findings indicate that orchard erosion with cover crops showed high sensitivity to extreme rainfall, whereas orchard erosion with grass cover showed lower sensitivity to extreme rainfall. Insert Figure 5 4. Discussion 4.1. Effects of groundcover management on water erosion Orchard lands are important land use types in red soil region. Due to increases in vegetation 13
coverage, soil and water losses were reduced significantly (46.9 and 66.8%, respectively) in orchards compared to the case of bare land (Fig. 3). However, there currently exist serious risks of soil and water losses in the orchards that often have extensive areas of bare soil, due to unreasonable management practices, such as the extensive use of herbicides. Experimental results have shown that the multiyear average of the soil erosion rate in orchards with no cover was 11.40 Mg ha-1 a-1. These results were consistent with other research results in the red soil region. Durán Zuazo et al. (2005) measured an average annual soil loss of 9.1 Mg ha-1 a-1 from orchard terraces. Fang et al. (2017) measured average annual erosion amounts of 15.33 and 10.20 Mg ha-1 a-1 for bare land and orchard land, respectively. The erosion rates for orchards with no cover in this study were much higher than the tolerable erosion rate of 5.00 Mg ha-1 a-1 in the red soil region, according to the soil erosion standards for classification and gradation given by the Ministry of Water Resources of China (SL190-2007). Bare soil surfaces with soil compaction might be responsible for the high erosion rate for orchards with unprotected soil, which means that such soil management practices are unsustainable. Groundcover vegetation is a good soil conservation practice for orchards, because of raindrop interception and runoff energy absorption on the soil surface (Roce et al., 2003; Francia Martinez et al., 2006; Feng et al., 2018). In this study, the runoff coefficients and soil loss amounts differed significantly between the orchards with and without vegetation cover. The multi–year average runoff coefficients and soil erosion rates were significantly lower (2.38 and 5.36%, 0.06 and 2.25 Mg ha-1 a-1, respectively) in the orchards with grass cover and cover crops than those with no cover. The experimental results indicated that the erosion rates in orchards with vegetation cover were far lower than the tolerable erosion rate for this region. The differences in runoff and erosion 14
among the different groundcover managements were similar to those reported in previous studies. Francia Martinez et al. (2006) reported that cover barley (Hordeum vulgare) in olive orchards on a 30% slope substantially decreased soil and water loss amounts (19.8 mm a-1 and 2.1 Mg ha-1 a-1, respectively). Keesstra et al. (2016) found that the lowest runoff and erosion amounts were identified in covered plots, and the highest values were seen in uncovered plots. Gómez et al. (2009) determined that cover crops effectively reduced runoff and erosion to tolerable levels, while Atucha et al. (2013) found that the runoff and erosion were reduced in a hillside avocado orchard with grass striped or as a complete cover compared to orchards with no cover. These findings confirmed that groundcover vegetation treatments reduced runoff and erosion more efficiently than for the case of no cover in orchards. Fig. 6 shows the annual runoff coefficients and soil erosion amounts under different groundcover managements from 2001 to 2012. The interannual variations in the runoff coefficients and erosion amounts were significantly affected by the annual rainfall changes for the control bare land. However, the runoff and erosion variations in different years were significantly lower in the other orchard plots. Compared with grass cover, the cover crops had higher annual variations in runoff and erosion due to rotation tillage practices. Overall, the vulnerability of water erosion to rainfall was mitigated by orchard development (tree cover and surface vegetation cover). This is explained by the sharp reduction or halting of rainfall erosivity with increasing vegetation coverage (Francia Martinez et al., 2006; Feng et al., 2018). As shown in Fig. 6B, the highest erosion rates were seen in the first year for the control bare land and the orchard with no cover (69.18 and 65.00 Mg ha-1 a-1, respectively), accounting for 20.0% and 55.8%, respectively of the total erosion over 12 years. The erosion rates in the first year 15
reached the criterion of intense erosion, and similar observations were reported by Cerdà et al. (2009) and Antucha et al. (2013) for new orchards. This was explained by the serious disturbance of the surface soil by large scale mechanized excavation that caused a lack of vegetation cover in this period, and by the low tree coverage in the newly planted hillside orchards. The severe soil erosion rates from newly planted orchards are not sustainable under current farming practices where the use of groundcover vegetation is completely avoided. In this study, the average erosion rate reductions in the first year were 64.81 and 64.52 Mg ha-1 in orchards with grass cover and cover crops compared to orchards with no cover, respectively (Fig. 6B). These results confirmed that severe water erosion is easily induced in the initial stage of orchard establishment. Groundcover vegetation, once established, reduced runoff and sediment yield more efficiently than a no cover treatment in young orchards. Insert Figure 6 4.2. Effects of extreme rainfall on water erosion Many studies have shown that rainfall variables, such as depth and intensity, were confirmed as the main drivers of the degree of surface hydrology and soil erosion (Ran et al., 2012; Liu et al., 2016; Nearing et al. 2017). As shown in Table 1, the mean rainfall depths and maximum rainfall intensities during 30 min of extreme rainfall were 3.4 and 3.9 times those of ordinary rainfall, respectively. Extreme rainfall with high intensity exceeded the soil infiltration capacity and much rainfall was lost as surface runoff. As a consequence, the extreme rainfall events with large magnitudes and low frequencies were assumed to dominate with respect to soil loss compared to ordinary rainfall events. In this study, the mean extreme runoff coefficients and erosion amounts for all treatments were 2.8 16
and 11.1 times the values from ordinary rainfall, respectively (Fig. 4). The results were similar to those reported in previous studies. Solano-Rivera et al. (2019) also showed that extreme precipitation events have an overriding impact on surface runoff and erosion. Wei et al. (2009b) reported that the mean runoff amounts and erosion rates of extreme rainfall were 2.7 and 53.2 times those of ordinary rainfall, respectively, in a semiarid hilly loess area in NW China. The lower erodibility of loess was responsible for the greater extreme erosion rates in the research results of Wei et al., (2009b). Fig. 5 clearly shows that sediment yields were more sensitive to extreme rainfall compared to runoff production. The ratios of extreme erosion to ordinary erosion were 3-6 times the values of extreme runoff to ordinary runoff in all plots. Meanwhile, the contribution of extreme rainfall to sediment yield (44.2%) is far higher than this to runoff production (15.8%) (Fig.7). The reason for these results is that higher sediment concentrations were found during heavy rainfall (Aksoy and Kavvas 2005; Dai et al., 2018) and were mainly because of higher soil detachment capacities under
strong rainfall erosivity and runoff energy (Fan et al., 2010). Furthermore, the Pearson correlation analysis shows the different effects of the rainfall variables on the runoff coefficients and on the soil loss amounts between extreme rainfall and ordinary rainfall (Table 2). For example, the maximum intensity in 30 min had a more significant correlation with soil erosion amounts than with rainfall depth, whereas the rainfall depth had a significantly stronger relationship with the runoff coefficient. The results confirmed that the maximum intensity in 30 min contributed more to soil erosion than did the rainfall depth. Insert Table 2 4.3. Role of various groundcover managements on extreme water erosion control 17
The extreme runoff coefficients and soil loss amounts varied significantly among the different groundcover managements (Fig. 4). Vegetation covers effectively reduced extreme runoff and erosion compared to those orchards with no cover. The differences in runoff and sediment reduction rate between grass cover and cover crops were very clear, especially for control of sediment loss. Sediment reduction rate for the grass cover treatment (99.6%) was two orders of magnitude greater than that measured for the cover crops treatment (40.0%) (Fig. 4B). The literature shows that concentrated flow was easily generated on sloping land under extreme rainfall with high intensity and depth (Boardman, 2015; Wang et al., 2018). Compared to the cover crops treatment, the denser stems and higher biomass of grass cover on the soil surface provided a more effective buffer, which decreased the overland flow velocity and energy, and decreased soil erosion (Roce et al., 2003; Pan and Shanguan, 2010; Mu et al., 2019), and the stem parts of the grass intercepted a portion of the sediment particles carried by the runoff (Dunkerley et al., 2001). The accumulated contribution rates of extreme rainfall and ordinary rainfall to the runoff and erosion are shown in Fig. 7. The highest erosion contribution of 59.0% was found in orchard with cover crops, followed by orchard with no cover (47.9%), bare land (43.8%) and orchard with grass cover (26.0%). The results indicated that 59.0% of the total soil erosion was caused by a few extreme rainfall events, with a probability of 7.0% (Table 1) in orchards with cover crops, and the lowest soil erosion occurred in orchards with grass cover during the study period. The results were similar to the research of Martı́nez-Casasnovas et al. (2002), who reported that soil erosion caused by a few extreme rainfall events often accounted for 60-90% of the total annual erosion amount. This was because the grass used on the grass cover plots with high vegetation coverage was 18
Cynodon dactylon, a warm-season perennial grass species grown without reseeding and cutting, and the soil erosion for the cover crop plots was especially vulnerable to tillage measures with yearly crop rotations. Due to the low grass strip, the grass cover without cutting did not affect the orchard production management. But for other more tall grass species, the groundcover should be suitably mowed and the cutting residues mulch on soil surface during winter for the least precipitation and low soil erosion. Cover crops with rotation have usually been viewed as the primary cause of soil erosion (Ramos et al., 2010; Keesstra et al., 2016; Fang et al., 2017). O'Neal et al. (2005) determined that lower soil losses were found for continuous crop cultivation than for crop rotation. In this study, crop rotations with soybeans and radishes were used in the orchards. The soybeans were grown from April to August, and the radishes were grown from September to March in each year. The crops harvest season occurred in spring and summer with high occurrence frequencies of extreme rainfall events (80% in the study area). Severe soil erosion amounts were easily induced due to the high rainfall erosivity and low surface vegetation coverage. Loose soil and soil adhering to the crops were exported from field together with these crops during crops harvesting processes. Ruysschaert et al. (2004) indicated that soil losses due to crop harvesting may rise from a few Mg to a few tens of Mg per hectare per harvest. Moreover, the frequent tillage with crop rotation increased soil erodibility and the probability of severe erosion in the crop growth season. From the perspective of soil and water conservation, the severe soil erosion caused by extreme rainfall should receive more attention when interplanting crops beneath fruit trees. The crop straw mulch and no-till or minimum tillage after harvesting were recommended as effective strategy to prevent extreme soil erosion in citrus orchard with cover crops. 19
Insert Figure 7 5. Conclusions In this paper, based on rainfall depth and I30, extreme rainfall and ordinary rainfall were determined by the continuous observation data and criterion of WMO. The minimum thresholds for rainfall depths and I30 values for extreme rainfall were determined to be 84.2 mm and 46.2 mm·h-1, respectively. 25 extreme rainfall events were selected from 356 erosive rainfall events from 2001 to 2012. The mean rainfall depth and I30 of extreme rainfall were threefold more than those of ordinary rainfall events. Extreme rainfall events caused severer soil and water loss compared with ordinary events. The average extreme runoff and erosion amount were 2.8 and 11.1 times higher than the values from ordinary rainfall, respectively. Meanwhile, the contribution of extreme rainfall to sediment yield (44.2%) was far higher than the contribution of extreme rainfall to runoff production (15.8%). Obviously, extreme rainfall events played the destructive role in inducing sediment loss. Runoff coefficient and soil loss amount differed significant among the groundcover managements under extreme rainfall. The size order was as follow: control bare land > orchard with no cover > orchard with cover crops > orchard with grass cover. However, the contribution of extreme rainfall to sediment yield differed greatly from the above rule. The highest erosion contribution was found in orchard with cover crops, and the lowest in orchard with grass cover under extreme rainfall. For orchards with cover crops and grass cover, 59.0 and 26.0% of total soil loss amount were caused by a few extreme rainfalls with a probability of 7.0%. These findings are meaning for controlling water erosion caused by extreme rainfall in citrus orchards of southern China. The soil and water losses caused by extreme rainfall should deserve 20
more attention when interplanting crops beneath fruit trees. Grass cover between crops is recommended as a groundcover management strategy to effectively fight against extreme water erosion. Acknowledgements This study was funded by the National Key Research and Development Program of China (2017YFC0505405), the National Natural Science Foundation of China (41877084, 41761065 and 41867016) and the Scientific Major Project of Water Conservancy in Jiangxi Province (KT201717 and 201821ZDKT17). Reference Aksoy, H., Kavvas, M.L., 2005. A review of hill slope and watershed scale erosion and sediment transport models. Catena 64, 247-271. Atucha, A., Merwin, I.A., Brown, M.G., Gardiazabal, F., Mena, F., Adriazola, C., Lehmann, J., 2013. Soil erosion, runoff and nutrient losses in an avocado (Persea americanaMill) hillside orchard under different groundcover management systems. Plant and Soil 368(1-2), 393-406. Barton, A.P., Fullen, M.A,, Mitchell, D.J., Hockinga, T.J., Liu, L.G., Bo, Z.W., Zheng, Y., Xia, Z.Y., 2004. Effects of soil conservation measures on erosion rates and crop productivity on subtropical Ultisols in Yunnan Province, China. Agriculture Ecosystems and Environment 104(2), 343-357. Boardman, J., 2015. Extreme rainfall and its impact on cultivated landscapes with particular reference to Britain. Earth Surface Processes and Landforms 40, 2121-2130. Bonetti, S., Richter, D.D., Porporato, A., 2019. The effect of accelerated soil erosion on hillslope morphology. Earth Surface Processes and Landforms, doi.org/10.1002/esp.4694. CCCIN (China Climate Change Information Network), 2002. Climate change: observation facts and future 21
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Figure legends: Fig.1. Location of study area in the red soil region of southern china. The study site was 15 km distance away from Poyang Lake, which is the biggest freshwater lake in China. The photos down below show the four runoff plots: control bare land (A), and orchards with groundcover managements including no cover (B), grass cover (C) and cover crops (D). Fig.2. Schematic diagrams of the runoff plot and the surface flow and sediment collection system. Fig.3. The average runoff coefficient (A) and soil loss amount (B) for different groundcover managements (n=356). The different letters are significantly different among the groundcover types according to one way ANOVA and Duncan's test (P<0.05). BLck, OCnc, OCgc and OCcc are control bare land and orchard with no cover, grass cover and cover crops, respectively. Fig.4. Runoff coefficient (A) and soil loss amount (B) under extreme rainfall (n=25) and ordinary rainfall (n=331) for different groundcover managements. Solid and dashed lines in this figure represent the median and mean, respectively. The box boundaries represent the 75% and 25% quartiles, the whisker caps represent the 90% and 10% quartiles, and the circles represent the 95% and 5% quartiles. Within each panel, the means followed by different letters are significantly different between extreme rainfall and ordinary rainfall according to one way ANOVA and Duncan's test (p<0.05). BLck, OCnc, OCgc and OCcc are control bare land and orchard with no cover, grass cover and cover crops, respectively. Fig.5. REr (A) and REe (B) values of each extreme rainfall event under different groundcover managements from 2001 to 2012 (n=25). REr and REe are the ratios of extreme runoff coefficient and soil loss amount to mean ordinary runoff coefficient and soil loss amount, respectively. Solid and dashed lines in this figure represent the median and mean, respectively. The box boundaries represent the 75% and 25% quartiles, the whisker caps represent the 90% and 10% quartiles, and the circles represent the 95% and 5% quartiles. Different letters represent 28
significant differences among different groundcover managements according to one way ANOVA and Duncan's test (p<0.05). BLck, OCnc, OCgc and OCcc are control bare land and orchard with no cover, grass cover and cover crops, respectively. Fig.6. Annual runoff coefficient (A) and soil loss amount (B) under different groundcover managements from 2001 to 2012. BLck, OCnc, OCgc and OCcc are control bare land and orchard with no cover, grass cover and cover crops, respectively.
Fig.7. Accumulated contribution rate of extreme rainfall and ordinary rainfall in runoff coefficient (A) and erosion amount (B) for different groundcover managements. The shaded and unshaded areas in this figure indicate the contribution rate of extreme rainfall and ordinary rainfall, respectively. BLck, OCnc, OCgc and OCcc are control bare land and orchard with no cover, grass cover and cover crops, respectively.
29
30
31
32
33
34
35
36
Table 1 Statistical features of extreme rainfall and ordinary rainfall variables. I30, SD and CV refer to maximum rainfall intensity in 30 min, standard deviation and variable coefficient, respectively. Rainfall patterns Extreme rainfall Ordinary rainfall
Eigenvalue
Min
Max
Mean
SD
Sum
CV
Rainfall depth (mm)
32.6
253.4
87.5
51.9
2188.6
0.59
9.6
90.6
51.4
20.9
—
0.41
Rainfall duration (min)
48
3478
1043
1064
26078
1.02
Rainfall depth (mm)
3.6
81.8
25.7
17.4
8163.0
0.68
1.8
63.0
13.3
10.5
—
0.79
18
3710
836
667
276797
0.80
I30
I30
(mm·h-1)
(mm·h-1)
Rainfall duration (min)
37
Frequency (Times) 25
331
Table 2 Pearson correlation analysis between rainfall parameters and runoff coefficients and soil erosion amounts. BLck, OCnc, OCgc and OCcc are control bare land and orchard with no cover, grass cover and cover crops, respectively. *p0.05, **p0.01. Variables
BLck
Rainfall
OCnc
OCgc
OCcc
parameters
P
R2
Rainfall depth
0.620**
0.000
0.478**
runoff
I30
0.460**
0.000
0.342**
0.005
0.278*
0.015
0.479*
coefficient
Rainfall duration
0.183
0.381
0.199
0.340
-0.254
0.221
0.307
0.136
Rainfall depth
0.587**
0.000
0.344**
0.000
0.218
0.442
0.257**
0.003
runoff
I30
0.272*
0.021
0.300*
0.045
0.262
0.407
0.219
0.292
coefficient
Rainfall duration
-0.143
0.109
-0.013
0.815
-0.008
0.885
0.034
0.535
Extreme
Rainfall depth
0.578**
0.00
0.329**
0.002
0.232*
0.011
0.325**
0.002
erosion
I30
0.650**
0.000
0.489**
0.000
0.375**
0.000
0.491**
0.000
amount
Rainfall duration
-0.284*
0.016
-0.003
0.990
-0.068
0.746
0.073
0.731
Ordinary
Rainfall depth
0.277*
0.013
0.245
0.061
0.076
0.167
0.119
0.569
erosion
I30
0.487**
0.002
0.315**
0.007
0.015
0.943
0.317**
0.008
amount
Rainfall duration
-0.150
0.169
-0.043
0.435
0.129
0.059
0.001
0.988
Extreme
Ordinary
38
P
R2 0.002
P
R2
P
R2
0.470**
0.008
0.537**
0.000 0.000
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Highlights: 356 natural rainfalls showed a destructive role of extreme rainfall in soil erosion. Extreme rainfall had higher capability in soil loss rather than runoff generation. Grass cover was useful in controlling extreme water erosion from orchards.
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