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Effects of slope and rainfall intensity on runoff and soil erosion from furrow diking under simulated rainfall
Catena 177 (2019) 92–100
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Effects of slope and rainfall intensity on runoff and soil erosion from furrow diking under simulated rainfall
T
⁎
Yuxin Liu, Yan Xin, Yun Xie , Wenting Wang State Key Laboratory of Earth Surface Processes and Resource Ecology, Faculty of Geographical Science, Beijing Normal University, Beijing 100875, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Furrow diking Soil erosion Physical change in dike Soil and water conservation efficiency
Furrow diking is a type of conservation tillage practice that is widely used throughout the world and has been proved to be effective for preventing runoff and soil loss. However, it is not easy to maintain a stable conservation effect of furrow diking in various environments. Few studies have focused on the effects of rainfall and slope on the integrity and conservation efficiency of furrow diking. Therefore, simulated rainfall experiments were conducted with four rainfall intensities (30 mm/h, 60 mm/h, 90 mm/h and 120 mm/h) and three slopes (2, 4 and 8°) to study the effects of rainfall intensity and slope on soil and runoff production of furrow diking as well as its conservation efficiency. The conservation efficiencies were obtained by comparing furrow diking with updown slope ridges. The result indicated that as rainfall intensity and slope increased, furrow diking dams may suffer from runoff overtopping resulting in damage and ultimate collapse. During this process, runoff and soil loss increased rapidly, severe rill erosion developed on dams resulting in the loss of the ability of furrow diking to store rainwater and sediment and obvious declines of water and soil conservation efficiencies. This paper established empirical equations to estimate the conservation efficiency of furrow diking under undamaged and damaged conditions. An empirical equation was also presented to estimate the critical rainfall and slope conditions that may cause overtopping, damage or collapse of dams in order to guide the implementation of furrow diking and predict its conservation efficiency.
1. Introduction Furrow diking (or some form of tied ridge, basin tillage, basin listing, and micro-basin tillage), is one of the widely used tillage conservation technologies throughout the world, and creates a series of surface depression storage micro catchments between crop rows with small earthen dams over short intervals to more effectively catch and retain rainfall, thus promoting infiltration and preventing runoff and erosion (Jones and Stewart, 1990; Truman and Nuti, 2009, 2010; Silva, 2017). Furrow diking was first implemented in the US in the 1930s, but the practice was abandoned in the 1950s because of the slow operating speed of diking equipment and the limited benefit to crop yields (Peacock, 1931; Jones and Clark, 1987; Musick, 1981). The practice was proved to be efficient for soil conservation and crop yields in experimental grain sorghum and cotton fields after the diking equipment was improved by the agricultural engineers and then adopted by farmers again in 1975 (Lyle and Dixon, 1977; Clark and Jones, 1980). Furrow diking is now commonly used in arid and semiarid regions as
the basin-shaped furrow can retain water to increase rainwater infiltration and decreases soil erosion (Krishna, 1989; Nuti et al., 2009). The soil conservation effects of the special design of furrow diking have been documented in recent studies. However, the effects may decrease after the contour ridge's fail. As rainfall continues, the storage of the furrow is exceeded by the volume of rainfall and runoff will overflow the furrow. Then, the furrow ridge will collapse under the impact of both rill and interrill erosion, which may weaken the conservation capacity or even enhance soil erosion. Most studies have focused on the conservation effect of furrow diking but have neglected to estimate the adverse impact after furrow failure and the corresponding critical values. Therefore the effects of the changes in furrow diking geometry to soil conservation need to be studied. Furrow diking is influenced by rainfall, slope, soil types, furrow ridge size and crop row spacing, etc. It was indicated that furrow diking would effectively reduce soil erosion when the ridge height was constructed on nearly level fields or the furrow storage was greater than the volume of rainwater from the largest storm that was likely to occur plus
⁎ Corresponding author at: State Key Laboratory of Earth Surface Processes and Resource Ecology, Faculty of Geographical Science, Beijing Normal University, No.19 Xinjiekouwai Street, Haidian District, Beijing 100875, China. E-mail address: [email protected] (Y. Xie).
https://doi.org/10.1016/j.catena.2019.02.004 Received 10 July 2018; Received in revised form 23 January 2019; Accepted 6 February 2019 0341-8162/ © 2019 Published by Elsevier B.V.
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Fig. 1. Furrow diking in the field with equipment in Northeastern China.
2. Material and methods
infiltration during the storm (Hudson, 1971). The experience of Jones and Clark (1987) with furrow diking showed that dikes constructed on gentle slopes and clay loam soils could effectively control or prevent erosion from large storms. They also recommended that dikes could be used in conjunction with other runoff control practices, such as terracing when the slopes were higher than 2% to prevent the accelerated erosion caused by furrow overflow. Furrow diking is also widely used in China, especially in the northeastern region where there is a great degree of agricultural mechanization, a monsoon climate, smooth terrain, high land productivity and government support. Furrow diking tillage is recommended in this area on slopes < 5% and is implemented by special equipment with quick operation speeds and appropriate ridge sizes (Fig. 1). The conservation effects of furrow diking are mostly studied in fields under natural rainfall conditions. Worldwide global climate change is expected to exacerbate runoff and soil erosion by increasing the frequency of intense storms (Gao et al., 2018). Therefore, understanding and mastering the condition and efficiency of land management or soil conservation measures in some extremely climate situation such as intense storm is definitely more desirable and important. Simulated rainfall which is water applied in a form similar to natural rainfall has numerous advantages for many erosion studies and become an effective aid in soil erosion research. More rapid results, standardization of storms, control of plot preparation, these advantages of simulated rainfall have been discussed and demonstrated (Meyer, 1965). Hence, simulated rainfall experiment can be quite applicable and effective in understanding the complete process and corresponding changes for furrow diking in various rainfall conditions, especially in the intense storm situations. Therefore, this paper examined the effect of furrow diking on the runoff and soil loss in a soil flume under rainfall simulation experiments. The aims of the present study were to (1) estimate the effects of furrow diking compared with conventional tillage (up-down slope ridging) on runoff and soil loss under different rainfall intensities and slopes; (2) quantify the effects of the change in furrow ridge geometry on soil erosion and conservation under various rainfall events; and (3) determine the critical factor values when the furrow ridge geometry changes and establish equations for assessment of conservation effects.
2.1. Rainfall simulation experiments Laboratory experiments were conducted using soil flumes and rainfall simulators in the rainfall simulation laboratory of Beijing Normal University, China, in 2013 and 2014 (Fig. 2). Cinnamon soil (classified as Pedocals in the Chinese Soil Taxonomy, or Leptic Luvisols in the US Soil Taxonomy) (Gong et al., 1999; Shi et al., 2006) was selected for experiments with four simulated rainfall intensities (30 mm/h, 60 mm/h, 90 mm/h and 120 mm/h) under the furrow diking and conventional tillage (up-down slope ridging) flumes with three slopes (2, 4, 8°). Cinnamon soil was collected from a cultivated field in Yanqing county, Beijing. The characteristics of the soil properties are shown in Table 1. The rainfall intensities were designed based on the rainfall return periods of the north-eastern part of China according to the “Chinese Rainstorm Statistic Parameter Atlas” (2006). The maximum rainfall within 1 h and 6 h is 30 mm and 50–70 mm (choose 60 mm) respectively in multi-year average precipitation. The maximum rainfall within 1 h and 6 h is 80–100 mm (choose 90 mm) and 100–140 mm (choose 120 mm) respectively in hundred-year storm (Xin et al., 2016). The choice of slopes referred to the appropriate slope for furrow diking construction is no > 5° according to the “Techniques standard for comprehensive control of soil erosion in the black soil region” (2009). As consequence a suitable slope (4°) for furrow diking was chosen and relative lower and higher slopes (2° and 8°) were chosen as contrast. Each simulated rainfall event lasted 1 h. All treatments were replicated three times, and 36 rainfall runs were conducted in total. 2.1.1. Rainfall simulator and soil flume A trough spray-nozzle rainfall simulators by Beijing Normal University and Beijing Jiaotong University based on the portable rainfall simulator were used for the rainfall simulations (Zhang et al., 2007). It produced simulated rainfall that drop size distribution and drop velocity of fall near that of natural rainfall and kinetic energy at impact approximately 80% that of corresponding natural rainfall (Meyer, 1960). Five Veejet 80100 nozzles with 0.04 MPa water pressure were installed in the simulator set at 4 m height above the floor and produced a controlled rainfall intensity ranging from 20 to 140 mm/h 93
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Fig. 2. Rainfall simulation experiments.
simulators). The soil flume was a rectangular shaped steel unit that was 2 m long, 1 m wide and 0.35 m deep. The slope was adjusted by a stabilizer blade at the bottom of the flume. There were four wheels for transportation and a triangular water outlet ahead of the flume for free movement of runoff and sediment.
Table 1 Characters of the cinnamon soil used in the experiments. Bulk density (g/cm3)
Organic matter (%)
Sand (%)
Silt (%)
Clay (%)
1.3
0.4
39.2
43.7
17.1
with the rainfall uniformity coefficient > 89% (Xie et al., 2008). A combined use of all simulators was necessary in each rainfall event. There were 1.1 m and 1.5 m intervals between the two adjacent nozzles and simulators and thus produced a rainfall area of (n1) × 1.5 × 4.4 m2 under combined use (where n is the number of
2.1.2. Tillage treatment in the experimental flumes Cinnamon soil was first filled and layered in the flume to a depth of 0.3 m and a bulk density of 1.3 g/cm3. The contact surface of each layer was roughened to reduce the effect of soil stratification (Zhao et al., 2014). Then furrow diking tillage or up-down slope ridging were 94
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Fig. 3. Design of the two tillage (furrow diking and contour ridges) in the flumes.
constructed in the soil flume according to the techniques standard (SL 446-2009, 2009) and the actual size in the field. A 30 cm width V-type furrow was built at the centre of the cross section to construct the updown slope ridging. The furrow bottom was 15 cm deep at the depth of the flume outlet. Both ridge sides were leveled at widths of 35 cm. The furrow diking was constructed by first creating the furrow like the updown slope ridging treatment, and then three and one-half contour ridges between the furrow side were set at a fixed distance of 60 cm along the flume to form three basins (Fig. 3). The contour ridge was set as a ladder-type with a top width of 10 cm, a bottom width of 40 cm and a height of 10 cm. The designs of the two tillages are shown in Fig. 3, and each experiment was constructed at a fixed size for uniformity.
2.2. Data processing The effect of furrow diking compared to up-down slope ridging on soil conservation was calculated as follows,
Er = (1 − Rf / R c ) × 100%
(1)
Es = (1 − Sf / Sc ) × 100%
(2)
where Er and Es are the conservation efficiencies of furrow diking on runoff and soil loss. Rf and Rc are the measured runoff (mm) under furrow diking and conventional tillage, respectively, and Sf and Sc are the measured soil loss (t/ha) under furrow diking and conventional tillage, respectively. The recorded length, width and depth of erosion gully were used to calculate the quality of the rill detachment and treated as the removed soil caused by rill erosion. The interrill erosion was equal to the total soil erosion minus rill erosion. All data processing and analysis were performed with SPSS 20.0 software (IBM, USA) and Origin 8.0 software (OriginLab, USA). The means of two replicates were compared using the least significant difference test at a p < 0.05 significance level. The functions were regressed by a nonlinear curve or linear fitting techniques.
2.1.3. Measurements during simulated rainfall The runoff start time was recorded first in each rainfall event. As the rainfall continued, the contour ridge geometry changed, and the duration of change was recorded by observation. The time when the rainfall volume exceeded the basin storage and overtopped the contour ridge was recorded as “overtopping”. Then, the overflowed rainwater eroded the contour ridge, and the time when a rill was clearly observed on top of the dam was recorded as “damage”. As the rill continued cutting down on the dam, the dam completely collapsed, and this time was recorded as “collapse”. The runoff was collected in a bucket and two bottles at intervals of 5 min. The runoff volume was measured from both the bucket and bottles. The sediment was collected in the two bottles and deposited to separate it from runoff; then, the sediment was weighed after ovendrying at 105 °C for 24 h. The length, width and depth of distinct erosion gully on dams were measured using steel tapeline and recorded after rainfall.
3. Results 3.1. Runoff and soil loss under the furrow diking and up-down slope ridging The average runoff and soil production under the processes of furrow diking (FD) and conventional tillage (—up and down slope ridging) (CT) during rainfall events under different rainfall intensities and slopes are shown in Fig. 4. For runoff production, the trends of runoff variation for FD and CT were similar as the runoff rates first increased continually until they reached certain amounts, and then 95
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Fig. 4. Observed processes of runoff and soil production of FD and CT under four rainfall intensities and three slopes.
similar or slightly higher than those of CT. Increasing rainfall intensity and slope amplified the increasing rate of FD, which exhibited peak values and stable production of soil loss.
these rates stabilized until the rainfall ended. The increase in rainfall intensity and slope resulted in both a faster increase in the tillage runoff curve and a higher runoff level when the runoff rates stabilized. The runoff rates under FD were smaller than those under CT at first because FD produced runoff later. When rainfall intensities and slopes were small (below 60 mm/h with 2–4° slopes), the runoff curves of FD were lower than those of CT throughout the rainfall event. When the rainfall intensity was > 90 mm/h and the slope was 8°, the runoff curves under FD increased much faster at the beginning, and the subsequent stable runoff production was similar to the amount under CT. For soil loss production, slope ridging maintained a similar curve shape as runoff production – the curve first rose and then stabilized until the end of the rainfall event. Increasing rainfall intensities and slopes will increase the rising range of a soil loss curve and stablilize soil production. Nevertheless, the curves were different for furrow dikes when the rainfall intensity and slope increased. For rainfall events of 30 mm/h at 2–8° slopes and 60 mm/h at 2–4° slopes, the shape of the soil loss curve for FD was similar to its runoff production: the curve lines started later and rose slowly and maintained lower levels than those of CT. For rainfall events of 60 mm/h at 8° slopes, 90 mm/h at 2–8° slopes, and 120 mm/h at 2–8° slopes under FD, soil production increased rapidly at the beginning of the rainfall event, and the rate of increase was higher than that CT; then, the peak soil production was reached before it began to decline and its stable soil loss rates were
3.2. Effects of changing contour ridge geometry The reason for the different features of runoff and soil loss rate caused by rainfall and slope changes under furrow diking may be relevant to the changes in the contour ridges. Fig. 5 shows the time and corresponding rainfall amounts when overtopping, damage and collapse occurred on the contour ridges during every rainfall experiment. The 30 mm/h rainfall experiments are not presented in the figure because no overtopping or damaged occurred during these experiments. In addition, for the rainfall experiment at 60 mm/h and 2–4° slopes, both overtopping and damage occurred, but the contour did not collapse. Similar to the other rainfall with high rainfall intensities or slopes, the contour suffered both damage and collapse during rainfall. With the increasing of slope and rainfall intensity, furrow diking may suffer overtopping or damage in less time. Considering the rainfall amounts critical to the three kinds of contour geometry changes, the increase in slope tended to lower the critical rainfall amount, but there were no obvious critical rainfall patterns among the different rainfall intensities. Overall, the critical rainfall amounts for overtopping, damage and collapse were 34.3 mm, 46 mm and 59 mm, respectively for 96
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Fig. 5. Critical time and rainfall of contour ridge geometry changing for furrow diking.
an average value of 76.8%. Conservation efficiency began to clearly decrease after the contour was damaged. The water retention values decreased to 10.3–38.4% with an average value of 25.1%, which was an approximately 54% decline compared to the no damage period. The soil retention values declined to −16.3–22.3% with an average value of 1.2%, declining approximately 75.6% compared to the no damage period. Once damage or collapse of contour ridges occurred, furrow diking began to lose its ability to prevent runoff and soil loss, especially for the soil conservation, as the efficiencies reached negative values during some rainfall events (2°-120 mm/h, 4°-120 mm/h, 8°-90 mm/h and 8°-120 mm/h), which indicated the furrow diking had produced more soil than the up-down slope ridges at that time. The undamaged and damaged conditions of furrow were combined to evaluate the average conservation efficiency of furrow diking during all rainfall events; the water conservation efficiencies were 23.2–94.5% with an average value of 60.7% and the soil conservation efficiencies were 10.6–91.6% with an average value of 52.9%. There was a negative correlation between the water or soil conservation efficiencies of furrow diking and slope or rainfall intensity, but the integrities of the contour ridges also substantially affected the efficiency. When the contour ridges were undamaged, increasing slope and rainfall intensities had less effect on the conservation efficiency. Once the contour ridges were damaged, the increasing slope and rainfall intensities significantly decreased the conservation efficiency. To predict or evaluate the water and soil conservation efficiencies of furrow diking, a regression analysis was applied to its conservation efficiencies under both undamaged and damaged conditions. The
the 2° slope, and critical rainfall amounts were 28.3 mm, 39 mm and 47 mm for the 4° slope, respectively, and 25.3 mm, 32.7 mm and 45 mm for the 8° slope. Fig. 6 shows the average runoff and soil loss during the periods of four furrow diking geometry change for every rainfall event. The maximum runoff usually occurred during the “collapse → end” period followed by the “damaged → collapse” period, and the maximum average soil loss usually occurred during the “damage → collapse” period followed by the “collapse → end” period. The average ratios between these four periods under different rainfall intensities and slopes were 1:3.5:4.0:4.3 for runoff production and 1:4.5:7.8:4.8 for soil loss production.
3.3. The conservation efficiency of furrow diking on runoff and soil loss To estimate the effects of rainfall, slope and furrow diking contour failure on the ability of to prevent runoff and soil loss. This paper analyzed the water and soil conservation efficiency of furrow diking under intact and damaged condition. There were great differences in the water and soil conservation efficiencies of furrow diking between undamaged and damaged conditions (Fig. 7). The conservation efficiency maintained good values when the system was not damaged; as the rainfall intensity and slope increased, the efficiency declined but remained at a relatively high level as long as the contour remained in good conditions. When the furrow diking was undamaged, the water efficiencies were 66.3–94.5% with an average value of 79.1%, and the soil efficiencies were 50.9–91.6% with
Fig. 6. Furrow diking's average runoff and soil loss during its four geometry changing periods. 97
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Fig. 7. Water and soil conservation efficiencies of furrow diking under intact and damaged conditions.
like the dynamic of runoff and sediment trapping performance of vegetative filter strips illustrated by the Pan et al. (2017). So we give the evaluation of soil and water conservation efficiency under its damaged and undamaged conditions. The efficiency under undamaged condition can be seen as the stabilized soil and water preventing efforts of furrow diking during its lifecycles. While efficiency under damaged conditions demonstrated that the furrow diking had already lost its soil and water preventing efforts. In this situation it should be no longer regarded as a conservation tillage practice and need rebuilt. The average water and soil conservation efficiencies of furrow diking under undamaged condition were 60.7% and 52.9% respectively. This results were smaller than the previous studies such as Yang et al. (1994) and Truman and Nuti's (2009) study which showed that preventing water and soil efforts were normally from 70% to 97%, and this may be because of their natural rainfall experiments with small intensity and field conditions with some other runoff control practices.
relationship equations were assumed to be power equation as shown below:
Er (Es ) = α·Rβ ·S γ
(3)
where the Er and Es are the water and soil conservation efficiencies of furrow diking, R is the rainfall intensity (mm/h), S is the gradient of the slope (degree), α, β, and γ are constant coefficients of the equation. The results of the regression analysis of the water and soil conservation efficiencies under two conditions are shown in Table 2. 4. Discussion 4.1. Effects of furrow changing on runoff and soil loss The curves of runoff and soil loss for furrow diking have different features of variation trends under different rainfall intensities and slopes which may be due to the contour ridge changes during rainfall. According to the photos and records of the furrow diking conditions during simulated rainfall, at the beginning of the rainfall event, the furrow diking dams had good effects on soil and water loss prevention with a high conservation value compared with slope ridges. However, as the rainfall intensity or slope increased, the basins between the furrow diking dams quickly filled and the water that overflowed the dams caused overtopping, and then the striking of raindrops and erosion from runoff caused serious rill erosion cutting down the furrow diking dams (Fig. 8), which impacted the efforts of preventing runoff and soil loss. The dams completely collapsed when rill erosion continuously progressed, and at that time, the water previously stored in the ponds was immediately discharged, carrying large amounts of soil as well. After collapse, the undercutting of rill erosion on dams slowed down and as consequence soil erosion rate decreased slightly. However the contour ridges of furrow diking had already lost their ability to prevent runoff and soil as rainfall was no longer stored in the basins. From the comparison of soil and water conservation between undamaged and damaged condition, it can be seen that the conservation efficiency of furrow diking was a dynamic value during its lifecycles,
4.2. Effects of critical rainfall and slope on contour ridges The critical rainfall amounts when furrow diking suffered overtopping, damage or collapse were negatively correlated with slope variation, but a consistent change pattern was not apparent when the rainfall intensity changed. Because every basin between furrow diking dams had a certain volume, when the basin was filled with rainfall, the runoff overflowed across the dam naturally. Thus, the critical condition for the overtopping or damage to the contour ridge was related to the accumulated rainfall amounts, while rainfall intensity affected only how soon the contour ridges would be overtopped or damaged. The slope increase not only reduced the volumes of the basins between dams but also increased the energy of runoff striking and the erosion of dams. As a result, the changing conditions of the furrow diking contour ridges, such as overtopping, damage or collapse were related to the accumulated rainfall amounts, which were also significantly affected by slope. A regression analysis was applied to investigate the relationship between the rainfall amounts critical for overtopping, damage and collapse of furrow diking and slope as shown in Fig. 9 and Table 3. The three curve lines represent the probability of reaching the critical condition of rainfall amount and slope when overtopping, damage or collapse occurred to furrow diking. It can be seen that critical rainfall and slope are inversely correlated with each other, as less accumulated rainfall was needed when the slope increased. The relationship between the equations of three conditions is supposed to be: Sc = 3Sd = 9So. The three equations can be used to estimate risky topography areas that may cause furrow diking damage or collapse according to the existing precipitation data, moreover, they can also be used to estimate whether
Table 2 Empirical relationship equations of the water and soil conservation efficiencies of furrow diking.
Water conservation efficiency Soil conservation efficiency
Condition
Relationship equation
R2
Undamaged Damaged Undamaged Damaged
Er = 1.6 × R–0.131 · S–0.118 Er = 24 × R–0.844 · S–0.304 Es = 2.3 × R–0.246 · S–0.064 Es = 159 × R–1.435 · S–0.175
0.801 0.81 0.589 0.707
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Fig. 8. Furrow diking damage and collapse.
Fig. 10. Rill and interrill erosion of furrow diking under different slopes and rainfall intensities.
Fig. 9. The critical rainfall and slope conditions when furrow diking overtopping, damage and collapse occurred.
where β is the ratio of rill to interrill erosion, R is the rainfall intensity (mm/h), S is slope gradient (degree), a, b, and c are constant coefficients of the equation. Then, the established empirical equation between rainfall intensity, rainfall and ratio of rill to interrill erosion was:
Table 3 Relationship between critical rainfall amounts and slope under different conditions. Condition of furrow diking Overtopping Damage Collapse
Equation S = 80exp(−0.1P) S = 240exp(−0.1P) S = 720exp(−0.1P)
R2
β = 2 × 10−4 ·R1.534 ·S 0.471
2
R = 0.81 R2 = 0.88 R2 = 0.73
R = 0.921.Rill detachment has some efforts on erosion by affecting the slope length exponent. In RUSLE the slope length factor (L) showed that average erosion for the slope length λ(in m) varies as
L = (λ /22.13)m
some rainfall events may destroy furrow diking according to the implementation area as well to keep stable water and soil conservation efficiencies for furrow diking.
(6)
where m is the daily slope length exponent and it is a function of the ratio of rill to interrill erosion as equation:
m = β /(β + 1) 4.3. Rill and interrill erosion of furrow diking
(7)
where β is the ratio of rill to interrill erosion. It is indicated that the βvalue is related to the slope angle in RUSLE and it gives a constant m value of 0.5 for soil erosion by surface flow alone. In this paper it is showed that damaged and collapse condition of furrow diking may cause increased rill erosion and changes the ratio of rill to inerrill erosion, as well as the slope length exponent. The empirical Eq. (5) presented in this study demonstrated that the ratio of rill to inerrill β is highly related to slope and rainfall intensity and may be helpful to evaluate the revision coefficient of slope length exponent and erosion amounts under furrow diking system.
The reason for the rapidly increasing soil loss may be relevant to the increased rill erosion when the contour ridges were damaged or collapsed. Fig. 10 shows the rill and interrill erosion of the furrow diking under different rainfall intensities and slopes. As rainfall and slope increased, the rill erosion increased significantly, as well as their ratios, which indicated that the contour ridges were more seriously damaged and rill erosion contributed more to soil loss. A variance analysis was applied to investigate the effects of slope and rainfall intensities on the ratios of rill to interrill erosion. The results of the F-test result were F = 22.659, Sig = 0.001 for rainfall intensity and F = 6.463, Sig = 0.032 for slope which attests that both rainfall intensity and slope have significant effects on the ratios. Regression analysis was implemented to evaluate the relationship between rainfall intensity, slope and the ratio of rill to interrill erosion. The relationship equation was assumed to be power as shown below:
β = a · Rb · S c
(5)
2
5. Conclusion The effects of slope and rainfall intensity on furrow diking runoff and soil production, contour ridge maintenance and conservation efficiency were studied through simulated rainfall experiments. The dams of furrow diking would go through the process of “overtopping → damage → collapse” during rainfall. Through this process, rill erosion
(4) 99
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increased rapidly, resulting in damage or collapse of the dams, which seriously weakened the ability of the dams to prevent runoff and soil loss. Then, the production of soil and runoff consequently increased. The water and soil conservation efficiencies of furrow diking were obtained by comparing the slope ridges under three conditions—intact, damaged and the average value of the first two. The critical rainfall intensities and slopes that may cause overtopping, damage or collapse were analyzed and an empirical equation was developed to estimate the critical slope and rainfall intensity. The results and equations from this study may be different from the data collected from natural rainfall. For this reason, the experiments that compare furrow diking data between simulated rainfall and natural rainfall should be conducted to revise the relevant results or equations. Conservation efficiencies of furrow diking were obtained by comparing the results from slope ridges in order to reflect the abilities of dams and basins in furrows, while a subsequent analysis of efficiency that compares these values with those of bare land should be conducted as well. Besides, different kinds of vegetation growth can affect soil properties like infiltration capability or stability and thus may change efforts of soil and water conservation in furrow diking system. Such as Pan et al. (2017) reported the remarkable performance of vegetative filter strips on runoff and sediment trapping. Furrow diking indeed can combined with plant or crop growth in practical application and has better efforts on soil and water conservation. However, we didn't consider the efforts of vegetation growth in furrow diking system in this paper while it is indeed worthy intensive study in the future research.
Chinese). Hudson, N.W., 1971. Soil Conservation. Cornell University Press, Ithaca, New York 324pp. Jones, O.R., Clark, R.N., 1987. Effects of furrow dikes on water conservation and dryland crop yields. Soil Sci. Soc. Am. J. 51, 1307–1314. Jones, O.R., Stewart, B.A., 1990. Basin tillage. Soil Tillage Res. 18, 249–265. Krishna, J.H., 1989. Modeling the effects of tied-ridging on water conservation and crop yields. Agric. Water Manag. 16, 87–95. Lyle, W.M., Dixon, D.R., 1977. Basin tillage for rainfall retention. Trans. Am. Soc. Agric. Eng. 20, 1013–1017. Meyer, L.D., 1965. Simulation of rainfall for soil Erosion research. Trans. ASAE 63–65. Musick, J.T., 1981. Precipitation management techniques-new and old. In: Smith, D.D. (Ed.), Annual Groundwater Management District. Conference, 8th, Lubbock, TX, December 1981. High Plains Underground Water Conservation District No. 1, Lubbock, TX, pp. 13–19. Nuti, R.C., Lamb, M.C., Sorensen, R.B., Truman, C.C., 2009. Agronomic and response to furrow diking in irrigated and non-irrigated cotton. Agric. Water Manag. 96, 1078–1084. Pan, D.L., Gao, X.D., Miles, D., Song, Y.Q., Wu, P.T., Zhao, X.N., 2017. Dynamics of runoff and sediment trapping performance of vegetative filter strips: run-on experiments and modeling. Sci. Total Environ. 593-594 (2017), 54–64. Peacock, C.T., 1931. Patent Official Gazette. No. 408. U.S. Government Printing Office, Washington, DC, pp. 543. Shi, X.Z., Yu, D.S., Warner, E.D., Sun, W.X., Petersen, G.W., Gong, Z.T., Lin, H., 2006. Cross reference system for translating between genetic soil classifcation of China and soil taxonomy. Soil Sci. Soc. Am. J. 70, 78. Silva, L.L., 2017. Are basin and reservoir tillage effective techniques to reduce runoff under sprinkler irrigation in Mediterranean conditions? Agric. Water Manag. 191, 50–56. SL 446-2009, 2009. Techniques Standard for Comprehensive Control of Soil Erosion in the Black Soil Region. China Water and Power Press, Beijing. Truman, C.C., Nuti, R.C., 2009. Improved water capture and erosion reduction through furrow diking. Agric. Water Manag. 96, 1071–1077. Truman, C.C., Nuti, R.C., 2010. Furrow diking in conservation tillage. Agric. Water Manag. 97, 835–840. Xie, Y., Lin, X., Liu, Y., Zheng, Y., Liu, B., Zhang, G., 2008. Calibration of simulated rainfall intensity and its spatial distribution for trough rainfall simulator. J. Soil Water Conserv. 28, 1–6 (in Chinese). Xin, Y., Xie, Y., Liu, Y.X., Liu, H.Y., Ren, X.Y., 2016. Residue cover effects on soil erosion and the infiltration in black soil under simulated rainfall experiments. J. Hydrol. 543, 651–658. Yang, A.M., Shen, C.P., Liu, F., Yin, J.F., Deng, Y.F., 1994. Study on soil and water conservation benefit of contour check in sloping field. J. Soil Water Conserv. 8 (3), 52–58 (in Chinese). Zhang, G.H., Liu, B.Y., Li, P.K., 2007. Principles and properties of artificial trough rainfall simulator. Bull. Soil Water Conserv. 27 (6), 56–60 (in Chinese). Zhao, X.N., Wu, P., Gao, X.D., Tian, L., Li, H.C., 2014. Changes of soil hydraulic properties under early-stage natural vegetation recovering on the Loess Plateau of China. Catena 113, 386–391.
Acknowledgements This research was supported by the National Key R&D Program of China (Grant No. 2018YFC0507000). References Clark, R.N., Jones, O.R., 1980. Furrow dams for conserving rainwater in a semiarid climate. [in the Southern Great Plains; USA]. . Gao, X.D., Liu, Z.P., Zhao, X.N., Ling, Q., Huo, G.P., Wu, P.T., 2018. Extreme natural drought enhances interspecific facilitation in semiarid agroforestry systems. Agric. Ecosyst. Environ. 265, 444–453. Gong, Z.T., Chen, Z.C., Shi, X.Z., 1999. Chinese Soil Taxonomy. Science Press, Beijing (in
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Effects of slope and rainfall intensity on runoff and soil erosion from furrow diking under simulated rainfall
T
⁎
Yuxin Liu, Yan Xin, Yun Xie , Wenting Wang State Key Laboratory of Earth Surface Processes and Resource Ecology, Faculty of Geographical Science, Beijing Normal University, Beijing 100875, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Furrow diking Soil erosion Physical change in dike Soil and water conservation efficiency
Furrow diking is a type of conservation tillage practice that is widely used throughout the world and has been proved to be effective for preventing runoff and soil loss. However, it is not easy to maintain a stable conservation effect of furrow diking in various environments. Few studies have focused on the effects of rainfall and slope on the integrity and conservation efficiency of furrow diking. Therefore, simulated rainfall experiments were conducted with four rainfall intensities (30 mm/h, 60 mm/h, 90 mm/h and 120 mm/h) and three slopes (2, 4 and 8°) to study the effects of rainfall intensity and slope on soil and runoff production of furrow diking as well as its conservation efficiency. The conservation efficiencies were obtained by comparing furrow diking with updown slope ridges. The result indicated that as rainfall intensity and slope increased, furrow diking dams may suffer from runoff overtopping resulting in damage and ultimate collapse. During this process, runoff and soil loss increased rapidly, severe rill erosion developed on dams resulting in the loss of the ability of furrow diking to store rainwater and sediment and obvious declines of water and soil conservation efficiencies. This paper established empirical equations to estimate the conservation efficiency of furrow diking under undamaged and damaged conditions. An empirical equation was also presented to estimate the critical rainfall and slope conditions that may cause overtopping, damage or collapse of dams in order to guide the implementation of furrow diking and predict its conservation efficiency.
1. Introduction Furrow diking (or some form of tied ridge, basin tillage, basin listing, and micro-basin tillage), is one of the widely used tillage conservation technologies throughout the world, and creates a series of surface depression storage micro catchments between crop rows with small earthen dams over short intervals to more effectively catch and retain rainfall, thus promoting infiltration and preventing runoff and erosion (Jones and Stewart, 1990; Truman and Nuti, 2009, 2010; Silva, 2017). Furrow diking was first implemented in the US in the 1930s, but the practice was abandoned in the 1950s because of the slow operating speed of diking equipment and the limited benefit to crop yields (Peacock, 1931; Jones and Clark, 1987; Musick, 1981). The practice was proved to be efficient for soil conservation and crop yields in experimental grain sorghum and cotton fields after the diking equipment was improved by the agricultural engineers and then adopted by farmers again in 1975 (Lyle and Dixon, 1977; Clark and Jones, 1980). Furrow diking is now commonly used in arid and semiarid regions as
the basin-shaped furrow can retain water to increase rainwater infiltration and decreases soil erosion (Krishna, 1989; Nuti et al., 2009). The soil conservation effects of the special design of furrow diking have been documented in recent studies. However, the effects may decrease after the contour ridge's fail. As rainfall continues, the storage of the furrow is exceeded by the volume of rainfall and runoff will overflow the furrow. Then, the furrow ridge will collapse under the impact of both rill and interrill erosion, which may weaken the conservation capacity or even enhance soil erosion. Most studies have focused on the conservation effect of furrow diking but have neglected to estimate the adverse impact after furrow failure and the corresponding critical values. Therefore the effects of the changes in furrow diking geometry to soil conservation need to be studied. Furrow diking is influenced by rainfall, slope, soil types, furrow ridge size and crop row spacing, etc. It was indicated that furrow diking would effectively reduce soil erosion when the ridge height was constructed on nearly level fields or the furrow storage was greater than the volume of rainwater from the largest storm that was likely to occur plus
⁎ Corresponding author at: State Key Laboratory of Earth Surface Processes and Resource Ecology, Faculty of Geographical Science, Beijing Normal University, No.19 Xinjiekouwai Street, Haidian District, Beijing 100875, China. E-mail address: [email protected] (Y. Xie).
https://doi.org/10.1016/j.catena.2019.02.004 Received 10 July 2018; Received in revised form 23 January 2019; Accepted 6 February 2019 0341-8162/ © 2019 Published by Elsevier B.V.
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Fig. 1. Furrow diking in the field with equipment in Northeastern China.
2. Material and methods
infiltration during the storm (Hudson, 1971). The experience of Jones and Clark (1987) with furrow diking showed that dikes constructed on gentle slopes and clay loam soils could effectively control or prevent erosion from large storms. They also recommended that dikes could be used in conjunction with other runoff control practices, such as terracing when the slopes were higher than 2% to prevent the accelerated erosion caused by furrow overflow. Furrow diking is also widely used in China, especially in the northeastern region where there is a great degree of agricultural mechanization, a monsoon climate, smooth terrain, high land productivity and government support. Furrow diking tillage is recommended in this area on slopes < 5% and is implemented by special equipment with quick operation speeds and appropriate ridge sizes (Fig. 1). The conservation effects of furrow diking are mostly studied in fields under natural rainfall conditions. Worldwide global climate change is expected to exacerbate runoff and soil erosion by increasing the frequency of intense storms (Gao et al., 2018). Therefore, understanding and mastering the condition and efficiency of land management or soil conservation measures in some extremely climate situation such as intense storm is definitely more desirable and important. Simulated rainfall which is water applied in a form similar to natural rainfall has numerous advantages for many erosion studies and become an effective aid in soil erosion research. More rapid results, standardization of storms, control of plot preparation, these advantages of simulated rainfall have been discussed and demonstrated (Meyer, 1965). Hence, simulated rainfall experiment can be quite applicable and effective in understanding the complete process and corresponding changes for furrow diking in various rainfall conditions, especially in the intense storm situations. Therefore, this paper examined the effect of furrow diking on the runoff and soil loss in a soil flume under rainfall simulation experiments. The aims of the present study were to (1) estimate the effects of furrow diking compared with conventional tillage (up-down slope ridging) on runoff and soil loss under different rainfall intensities and slopes; (2) quantify the effects of the change in furrow ridge geometry on soil erosion and conservation under various rainfall events; and (3) determine the critical factor values when the furrow ridge geometry changes and establish equations for assessment of conservation effects.
2.1. Rainfall simulation experiments Laboratory experiments were conducted using soil flumes and rainfall simulators in the rainfall simulation laboratory of Beijing Normal University, China, in 2013 and 2014 (Fig. 2). Cinnamon soil (classified as Pedocals in the Chinese Soil Taxonomy, or Leptic Luvisols in the US Soil Taxonomy) (Gong et al., 1999; Shi et al., 2006) was selected for experiments with four simulated rainfall intensities (30 mm/h, 60 mm/h, 90 mm/h and 120 mm/h) under the furrow diking and conventional tillage (up-down slope ridging) flumes with three slopes (2, 4, 8°). Cinnamon soil was collected from a cultivated field in Yanqing county, Beijing. The characteristics of the soil properties are shown in Table 1. The rainfall intensities were designed based on the rainfall return periods of the north-eastern part of China according to the “Chinese Rainstorm Statistic Parameter Atlas” (2006). The maximum rainfall within 1 h and 6 h is 30 mm and 50–70 mm (choose 60 mm) respectively in multi-year average precipitation. The maximum rainfall within 1 h and 6 h is 80–100 mm (choose 90 mm) and 100–140 mm (choose 120 mm) respectively in hundred-year storm (Xin et al., 2016). The choice of slopes referred to the appropriate slope for furrow diking construction is no > 5° according to the “Techniques standard for comprehensive control of soil erosion in the black soil region” (2009). As consequence a suitable slope (4°) for furrow diking was chosen and relative lower and higher slopes (2° and 8°) were chosen as contrast. Each simulated rainfall event lasted 1 h. All treatments were replicated three times, and 36 rainfall runs were conducted in total. 2.1.1. Rainfall simulator and soil flume A trough spray-nozzle rainfall simulators by Beijing Normal University and Beijing Jiaotong University based on the portable rainfall simulator were used for the rainfall simulations (Zhang et al., 2007). It produced simulated rainfall that drop size distribution and drop velocity of fall near that of natural rainfall and kinetic energy at impact approximately 80% that of corresponding natural rainfall (Meyer, 1960). Five Veejet 80100 nozzles with 0.04 MPa water pressure were installed in the simulator set at 4 m height above the floor and produced a controlled rainfall intensity ranging from 20 to 140 mm/h 93
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Fig. 2. Rainfall simulation experiments.
simulators). The soil flume was a rectangular shaped steel unit that was 2 m long, 1 m wide and 0.35 m deep. The slope was adjusted by a stabilizer blade at the bottom of the flume. There were four wheels for transportation and a triangular water outlet ahead of the flume for free movement of runoff and sediment.
Table 1 Characters of the cinnamon soil used in the experiments. Bulk density (g/cm3)
Organic matter (%)
Sand (%)
Silt (%)
Clay (%)
1.3
0.4
39.2
43.7
17.1
with the rainfall uniformity coefficient > 89% (Xie et al., 2008). A combined use of all simulators was necessary in each rainfall event. There were 1.1 m and 1.5 m intervals between the two adjacent nozzles and simulators and thus produced a rainfall area of (n1) × 1.5 × 4.4 m2 under combined use (where n is the number of
2.1.2. Tillage treatment in the experimental flumes Cinnamon soil was first filled and layered in the flume to a depth of 0.3 m and a bulk density of 1.3 g/cm3. The contact surface of each layer was roughened to reduce the effect of soil stratification (Zhao et al., 2014). Then furrow diking tillage or up-down slope ridging were 94
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Fig. 3. Design of the two tillage (furrow diking and contour ridges) in the flumes.
constructed in the soil flume according to the techniques standard (SL 446-2009, 2009) and the actual size in the field. A 30 cm width V-type furrow was built at the centre of the cross section to construct the updown slope ridging. The furrow bottom was 15 cm deep at the depth of the flume outlet. Both ridge sides were leveled at widths of 35 cm. The furrow diking was constructed by first creating the furrow like the updown slope ridging treatment, and then three and one-half contour ridges between the furrow side were set at a fixed distance of 60 cm along the flume to form three basins (Fig. 3). The contour ridge was set as a ladder-type with a top width of 10 cm, a bottom width of 40 cm and a height of 10 cm. The designs of the two tillages are shown in Fig. 3, and each experiment was constructed at a fixed size for uniformity.
2.2. Data processing The effect of furrow diking compared to up-down slope ridging on soil conservation was calculated as follows,
Er = (1 − Rf / R c ) × 100%
(1)
Es = (1 − Sf / Sc ) × 100%
(2)
where Er and Es are the conservation efficiencies of furrow diking on runoff and soil loss. Rf and Rc are the measured runoff (mm) under furrow diking and conventional tillage, respectively, and Sf and Sc are the measured soil loss (t/ha) under furrow diking and conventional tillage, respectively. The recorded length, width and depth of erosion gully were used to calculate the quality of the rill detachment and treated as the removed soil caused by rill erosion. The interrill erosion was equal to the total soil erosion minus rill erosion. All data processing and analysis were performed with SPSS 20.0 software (IBM, USA) and Origin 8.0 software (OriginLab, USA). The means of two replicates were compared using the least significant difference test at a p < 0.05 significance level. The functions were regressed by a nonlinear curve or linear fitting techniques.
2.1.3. Measurements during simulated rainfall The runoff start time was recorded first in each rainfall event. As the rainfall continued, the contour ridge geometry changed, and the duration of change was recorded by observation. The time when the rainfall volume exceeded the basin storage and overtopped the contour ridge was recorded as “overtopping”. Then, the overflowed rainwater eroded the contour ridge, and the time when a rill was clearly observed on top of the dam was recorded as “damage”. As the rill continued cutting down on the dam, the dam completely collapsed, and this time was recorded as “collapse”. The runoff was collected in a bucket and two bottles at intervals of 5 min. The runoff volume was measured from both the bucket and bottles. The sediment was collected in the two bottles and deposited to separate it from runoff; then, the sediment was weighed after ovendrying at 105 °C for 24 h. The length, width and depth of distinct erosion gully on dams were measured using steel tapeline and recorded after rainfall.
3. Results 3.1. Runoff and soil loss under the furrow diking and up-down slope ridging The average runoff and soil production under the processes of furrow diking (FD) and conventional tillage (—up and down slope ridging) (CT) during rainfall events under different rainfall intensities and slopes are shown in Fig. 4. For runoff production, the trends of runoff variation for FD and CT were similar as the runoff rates first increased continually until they reached certain amounts, and then 95
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Fig. 4. Observed processes of runoff and soil production of FD and CT under four rainfall intensities and three slopes.
similar or slightly higher than those of CT. Increasing rainfall intensity and slope amplified the increasing rate of FD, which exhibited peak values and stable production of soil loss.
these rates stabilized until the rainfall ended. The increase in rainfall intensity and slope resulted in both a faster increase in the tillage runoff curve and a higher runoff level when the runoff rates stabilized. The runoff rates under FD were smaller than those under CT at first because FD produced runoff later. When rainfall intensities and slopes were small (below 60 mm/h with 2–4° slopes), the runoff curves of FD were lower than those of CT throughout the rainfall event. When the rainfall intensity was > 90 mm/h and the slope was 8°, the runoff curves under FD increased much faster at the beginning, and the subsequent stable runoff production was similar to the amount under CT. For soil loss production, slope ridging maintained a similar curve shape as runoff production – the curve first rose and then stabilized until the end of the rainfall event. Increasing rainfall intensities and slopes will increase the rising range of a soil loss curve and stablilize soil production. Nevertheless, the curves were different for furrow dikes when the rainfall intensity and slope increased. For rainfall events of 30 mm/h at 2–8° slopes and 60 mm/h at 2–4° slopes, the shape of the soil loss curve for FD was similar to its runoff production: the curve lines started later and rose slowly and maintained lower levels than those of CT. For rainfall events of 60 mm/h at 8° slopes, 90 mm/h at 2–8° slopes, and 120 mm/h at 2–8° slopes under FD, soil production increased rapidly at the beginning of the rainfall event, and the rate of increase was higher than that CT; then, the peak soil production was reached before it began to decline and its stable soil loss rates were
3.2. Effects of changing contour ridge geometry The reason for the different features of runoff and soil loss rate caused by rainfall and slope changes under furrow diking may be relevant to the changes in the contour ridges. Fig. 5 shows the time and corresponding rainfall amounts when overtopping, damage and collapse occurred on the contour ridges during every rainfall experiment. The 30 mm/h rainfall experiments are not presented in the figure because no overtopping or damaged occurred during these experiments. In addition, for the rainfall experiment at 60 mm/h and 2–4° slopes, both overtopping and damage occurred, but the contour did not collapse. Similar to the other rainfall with high rainfall intensities or slopes, the contour suffered both damage and collapse during rainfall. With the increasing of slope and rainfall intensity, furrow diking may suffer overtopping or damage in less time. Considering the rainfall amounts critical to the three kinds of contour geometry changes, the increase in slope tended to lower the critical rainfall amount, but there were no obvious critical rainfall patterns among the different rainfall intensities. Overall, the critical rainfall amounts for overtopping, damage and collapse were 34.3 mm, 46 mm and 59 mm, respectively for 96
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Fig. 5. Critical time and rainfall of contour ridge geometry changing for furrow diking.
an average value of 76.8%. Conservation efficiency began to clearly decrease after the contour was damaged. The water retention values decreased to 10.3–38.4% with an average value of 25.1%, which was an approximately 54% decline compared to the no damage period. The soil retention values declined to −16.3–22.3% with an average value of 1.2%, declining approximately 75.6% compared to the no damage period. Once damage or collapse of contour ridges occurred, furrow diking began to lose its ability to prevent runoff and soil loss, especially for the soil conservation, as the efficiencies reached negative values during some rainfall events (2°-120 mm/h, 4°-120 mm/h, 8°-90 mm/h and 8°-120 mm/h), which indicated the furrow diking had produced more soil than the up-down slope ridges at that time. The undamaged and damaged conditions of furrow were combined to evaluate the average conservation efficiency of furrow diking during all rainfall events; the water conservation efficiencies were 23.2–94.5% with an average value of 60.7% and the soil conservation efficiencies were 10.6–91.6% with an average value of 52.9%. There was a negative correlation between the water or soil conservation efficiencies of furrow diking and slope or rainfall intensity, but the integrities of the contour ridges also substantially affected the efficiency. When the contour ridges were undamaged, increasing slope and rainfall intensities had less effect on the conservation efficiency. Once the contour ridges were damaged, the increasing slope and rainfall intensities significantly decreased the conservation efficiency. To predict or evaluate the water and soil conservation efficiencies of furrow diking, a regression analysis was applied to its conservation efficiencies under both undamaged and damaged conditions. The
the 2° slope, and critical rainfall amounts were 28.3 mm, 39 mm and 47 mm for the 4° slope, respectively, and 25.3 mm, 32.7 mm and 45 mm for the 8° slope. Fig. 6 shows the average runoff and soil loss during the periods of four furrow diking geometry change for every rainfall event. The maximum runoff usually occurred during the “collapse → end” period followed by the “damaged → collapse” period, and the maximum average soil loss usually occurred during the “damage → collapse” period followed by the “collapse → end” period. The average ratios between these four periods under different rainfall intensities and slopes were 1:3.5:4.0:4.3 for runoff production and 1:4.5:7.8:4.8 for soil loss production.
3.3. The conservation efficiency of furrow diking on runoff and soil loss To estimate the effects of rainfall, slope and furrow diking contour failure on the ability of to prevent runoff and soil loss. This paper analyzed the water and soil conservation efficiency of furrow diking under intact and damaged condition. There were great differences in the water and soil conservation efficiencies of furrow diking between undamaged and damaged conditions (Fig. 7). The conservation efficiency maintained good values when the system was not damaged; as the rainfall intensity and slope increased, the efficiency declined but remained at a relatively high level as long as the contour remained in good conditions. When the furrow diking was undamaged, the water efficiencies were 66.3–94.5% with an average value of 79.1%, and the soil efficiencies were 50.9–91.6% with
Fig. 6. Furrow diking's average runoff and soil loss during its four geometry changing periods. 97
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Fig. 7. Water and soil conservation efficiencies of furrow diking under intact and damaged conditions.
like the dynamic of runoff and sediment trapping performance of vegetative filter strips illustrated by the Pan et al. (2017). So we give the evaluation of soil and water conservation efficiency under its damaged and undamaged conditions. The efficiency under undamaged condition can be seen as the stabilized soil and water preventing efforts of furrow diking during its lifecycles. While efficiency under damaged conditions demonstrated that the furrow diking had already lost its soil and water preventing efforts. In this situation it should be no longer regarded as a conservation tillage practice and need rebuilt. The average water and soil conservation efficiencies of furrow diking under undamaged condition were 60.7% and 52.9% respectively. This results were smaller than the previous studies such as Yang et al. (1994) and Truman and Nuti's (2009) study which showed that preventing water and soil efforts were normally from 70% to 97%, and this may be because of their natural rainfall experiments with small intensity and field conditions with some other runoff control practices.
relationship equations were assumed to be power equation as shown below:
Er (Es ) = α·Rβ ·S γ
(3)
where the Er and Es are the water and soil conservation efficiencies of furrow diking, R is the rainfall intensity (mm/h), S is the gradient of the slope (degree), α, β, and γ are constant coefficients of the equation. The results of the regression analysis of the water and soil conservation efficiencies under two conditions are shown in Table 2. 4. Discussion 4.1. Effects of furrow changing on runoff and soil loss The curves of runoff and soil loss for furrow diking have different features of variation trends under different rainfall intensities and slopes which may be due to the contour ridge changes during rainfall. According to the photos and records of the furrow diking conditions during simulated rainfall, at the beginning of the rainfall event, the furrow diking dams had good effects on soil and water loss prevention with a high conservation value compared with slope ridges. However, as the rainfall intensity or slope increased, the basins between the furrow diking dams quickly filled and the water that overflowed the dams caused overtopping, and then the striking of raindrops and erosion from runoff caused serious rill erosion cutting down the furrow diking dams (Fig. 8), which impacted the efforts of preventing runoff and soil loss. The dams completely collapsed when rill erosion continuously progressed, and at that time, the water previously stored in the ponds was immediately discharged, carrying large amounts of soil as well. After collapse, the undercutting of rill erosion on dams slowed down and as consequence soil erosion rate decreased slightly. However the contour ridges of furrow diking had already lost their ability to prevent runoff and soil as rainfall was no longer stored in the basins. From the comparison of soil and water conservation between undamaged and damaged condition, it can be seen that the conservation efficiency of furrow diking was a dynamic value during its lifecycles,
4.2. Effects of critical rainfall and slope on contour ridges The critical rainfall amounts when furrow diking suffered overtopping, damage or collapse were negatively correlated with slope variation, but a consistent change pattern was not apparent when the rainfall intensity changed. Because every basin between furrow diking dams had a certain volume, when the basin was filled with rainfall, the runoff overflowed across the dam naturally. Thus, the critical condition for the overtopping or damage to the contour ridge was related to the accumulated rainfall amounts, while rainfall intensity affected only how soon the contour ridges would be overtopped or damaged. The slope increase not only reduced the volumes of the basins between dams but also increased the energy of runoff striking and the erosion of dams. As a result, the changing conditions of the furrow diking contour ridges, such as overtopping, damage or collapse were related to the accumulated rainfall amounts, which were also significantly affected by slope. A regression analysis was applied to investigate the relationship between the rainfall amounts critical for overtopping, damage and collapse of furrow diking and slope as shown in Fig. 9 and Table 3. The three curve lines represent the probability of reaching the critical condition of rainfall amount and slope when overtopping, damage or collapse occurred to furrow diking. It can be seen that critical rainfall and slope are inversely correlated with each other, as less accumulated rainfall was needed when the slope increased. The relationship between the equations of three conditions is supposed to be: Sc = 3Sd = 9So. The three equations can be used to estimate risky topography areas that may cause furrow diking damage or collapse according to the existing precipitation data, moreover, they can also be used to estimate whether
Table 2 Empirical relationship equations of the water and soil conservation efficiencies of furrow diking.
Water conservation efficiency Soil conservation efficiency
Condition
Relationship equation
R2
Undamaged Damaged Undamaged Damaged
Er = 1.6 × R–0.131 · S–0.118 Er = 24 × R–0.844 · S–0.304 Es = 2.3 × R–0.246 · S–0.064 Es = 159 × R–1.435 · S–0.175
0.801 0.81 0.589 0.707
98
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Fig. 8. Furrow diking damage and collapse.
Fig. 10. Rill and interrill erosion of furrow diking under different slopes and rainfall intensities.
Fig. 9. The critical rainfall and slope conditions when furrow diking overtopping, damage and collapse occurred.
where β is the ratio of rill to interrill erosion, R is the rainfall intensity (mm/h), S is slope gradient (degree), a, b, and c are constant coefficients of the equation. Then, the established empirical equation between rainfall intensity, rainfall and ratio of rill to interrill erosion was:
Table 3 Relationship between critical rainfall amounts and slope under different conditions. Condition of furrow diking Overtopping Damage Collapse
Equation S = 80exp(−0.1P) S = 240exp(−0.1P) S = 720exp(−0.1P)
R2
β = 2 × 10−4 ·R1.534 ·S 0.471
2
R = 0.81 R2 = 0.88 R2 = 0.73
R = 0.921.Rill detachment has some efforts on erosion by affecting the slope length exponent. In RUSLE the slope length factor (L) showed that average erosion for the slope length λ(in m) varies as
L = (λ /22.13)m
some rainfall events may destroy furrow diking according to the implementation area as well to keep stable water and soil conservation efficiencies for furrow diking.
(6)
where m is the daily slope length exponent and it is a function of the ratio of rill to interrill erosion as equation:
m = β /(β + 1) 4.3. Rill and interrill erosion of furrow diking
(7)
where β is the ratio of rill to interrill erosion. It is indicated that the βvalue is related to the slope angle in RUSLE and it gives a constant m value of 0.5 for soil erosion by surface flow alone. In this paper it is showed that damaged and collapse condition of furrow diking may cause increased rill erosion and changes the ratio of rill to inerrill erosion, as well as the slope length exponent. The empirical Eq. (5) presented in this study demonstrated that the ratio of rill to inerrill β is highly related to slope and rainfall intensity and may be helpful to evaluate the revision coefficient of slope length exponent and erosion amounts under furrow diking system.
The reason for the rapidly increasing soil loss may be relevant to the increased rill erosion when the contour ridges were damaged or collapsed. Fig. 10 shows the rill and interrill erosion of the furrow diking under different rainfall intensities and slopes. As rainfall and slope increased, the rill erosion increased significantly, as well as their ratios, which indicated that the contour ridges were more seriously damaged and rill erosion contributed more to soil loss. A variance analysis was applied to investigate the effects of slope and rainfall intensities on the ratios of rill to interrill erosion. The results of the F-test result were F = 22.659, Sig = 0.001 for rainfall intensity and F = 6.463, Sig = 0.032 for slope which attests that both rainfall intensity and slope have significant effects on the ratios. Regression analysis was implemented to evaluate the relationship between rainfall intensity, slope and the ratio of rill to interrill erosion. The relationship equation was assumed to be power as shown below:
β = a · Rb · S c
(5)
2
5. Conclusion The effects of slope and rainfall intensity on furrow diking runoff and soil production, contour ridge maintenance and conservation efficiency were studied through simulated rainfall experiments. The dams of furrow diking would go through the process of “overtopping → damage → collapse” during rainfall. Through this process, rill erosion
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increased rapidly, resulting in damage or collapse of the dams, which seriously weakened the ability of the dams to prevent runoff and soil loss. Then, the production of soil and runoff consequently increased. The water and soil conservation efficiencies of furrow diking were obtained by comparing the slope ridges under three conditions—intact, damaged and the average value of the first two. The critical rainfall intensities and slopes that may cause overtopping, damage or collapse were analyzed and an empirical equation was developed to estimate the critical slope and rainfall intensity. The results and equations from this study may be different from the data collected from natural rainfall. For this reason, the experiments that compare furrow diking data between simulated rainfall and natural rainfall should be conducted to revise the relevant results or equations. Conservation efficiencies of furrow diking were obtained by comparing the results from slope ridges in order to reflect the abilities of dams and basins in furrows, while a subsequent analysis of efficiency that compares these values with those of bare land should be conducted as well. Besides, different kinds of vegetation growth can affect soil properties like infiltration capability or stability and thus may change efforts of soil and water conservation in furrow diking system. Such as Pan et al. (2017) reported the remarkable performance of vegetative filter strips on runoff and sediment trapping. Furrow diking indeed can combined with plant or crop growth in practical application and has better efforts on soil and water conservation. However, we didn't consider the efforts of vegetation growth in furrow diking system in this paper while it is indeed worthy intensive study in the future research.
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