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Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization (ARO), The Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel
Corresponding author (vwguy{at}volcani.agri.gov.il)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTION1. . Moisture Redistribution...2. . Consolidation of...3. . Surface Seal...MATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: HC, hydraulic conductivity • IR, intake rate
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTION1. . Moisture Redistribution...2. . Consolidation of...3. . Surface Seal...MATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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45% (Wolters, 1992). Surge irrigation is the intermittent application of surface irrigation water (Stringham, 1988). It has the potential to increase infiltration uniformity of surface irrigation application by (i) increasing the advance rate, which decreases cross-field differences in infiltration opportunity time, and (ii) decreasing the IR at the upstream end of the furrows to compensate for the longer infiltration opportunity times at these locations (Kemper et al., 1988). The infiltration decrease caused by surge flow is highly variable, is not fully understood, and is difficult to predict (Izuno et al., 1985; Kemper et al., 1988; Trout, 1991; Samani et al., 1985). Many studies have been conducted to determine the mechanisms taking place during the intermittent off period of surge flow irrigation. Several basic phenomena have been recognized:
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1. . Moisture Redistribution in the Soil Profile |
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TOPNOTESABSTRACTINTRODUCTION1. . Moisture Redistribution...2. . Consolidation of...3. . Surface Seal...MATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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2. . Consolidation of the Soil near the Furrow Perimeter |
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TOPNOTESABSTRACTINTRODUCTION1. . Moisture Redistribution...2. . Consolidation of...3. . Surface Seal...MATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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3. . Surface Seal Formation |
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TOPNOTESABSTRACTINTRODUCTION1. . Moisture Redistribution...2. . Consolidation of...3. . Surface Seal...MATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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In addition, other mechanisms related mainly to bed load have been proposed to explain the effects of surge irrigation on furrow IR: (i) filling of cracks that develop during flow interruption with bed load during the following surge (Kemper et al., 1988); (ii) greater sediment detachment and movement caused by more rapid advance of the surge stream front (Kemper et al., 1985; Trout, 1991); (iii) forced deposition (and consolidation) of suspended sediment on the furrow perimeter when the water supply is interrupted (Kemper et al., 1985); and (iv) air entrapment (Seymour, 1990) and its expansion upon rewetting (Jalali-Farahani et al., 1993).
Miniflumes have been used to evaluate the interactive effects of flow characteristics, soil properties and water quality on rill erosion in the laboratory (Shainberg et al., 1994, 1996). Rill erodibility data obtained with miniflumes agreed well with field data (Shainberg et al., 1994). Miniflume studies were also found to simulate well the effect of polyacrylamide (PAM) on furrow erosion in the field (Lentz et al., 1992; Shainberg et al., 1994). Miniflumes were used by Shainberg et al. (1996) to study rill erosion in a loess and a clay soil; it was found that (i) rill erosion decreased with aging of several hours, (ii) the decease in erosion was more pronounced in the clay soil, and (iii) erosion depended on water content in the soil. These researchers postulated that aging and water tension enhanced clay to clay contacts and increased cohesive forces between soil particles, thus leading to the observed reduction in erosion. Application of these mechanisms to surge irrigation suggests that the water tension that builds up during the off period of the surge may cause an enhanced reduction in erosion.
It is hypothesized that interrupted flow will affect both soil IR and rill erosion, and that it can be evaluated from laboratory miniflume studies. Thus, the objectives of our study were (i) to study the effects of continuous and interrupted flow on the IR and on rill erosion in a silt loam and a clay soil and (ii) to improve the understanding of the mechanisms that cause interrupted flow to reduce rill erosion and IR in the two soils. It was assumed that the erodibilities of the two different soils could be compared, provided similar runoff rates are maintained.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTION1. . Moisture Redistribution...2. . Consolidation of...3. . Surface Seal...MATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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60%), with kaolinite, illite, and calcite also present (Banin and Amiel, 1970). The fact that the clay content and cation-exchange capacity in the clay soil were twice that in the silt loam indicated that the clay mineralogy in the two soils was similar.
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Air-dried soil was slightly compacted in the flumes to field densities of 1390 kg m-3 for the silt loam and of 1280 kg m-3 for the clay soil. When dry, the volume of the clay soil was intentionally kept smaller than that of the silt loam. However, upon wetting and subsequent swelling, the final volume of the wet clay soil in the flume was similar to that of the silt loam, and the wet bulk density of the silt loam and clay soil was 1390 and 1200 kg m-3, respectively. A V-shaped rill (44 mm wide and 22 mm deep) with a 90° angle between its sides was formed in the soil surface. Water was applied with a peristaltic pump to the upstream metallic rill, and runoff water containing sediment was collected in beakers from the downstream metallic rill. Runoff volume was measured by weighing the beakers and sediment content in the outflow was determined by drying. Inflow and outflow rates were continuously recorded and average IR for each minute of flow time was calculated from the difference. Similarly rill erosion as a function of flow time was calculated.
Each individual experiment was divided into two stages. In the first stage either continuous (control) or interrupted flow was applied. The control treatment consisted of 4 min of flow; the interrupted flow treatment consisted of four cycles of 1 min of flow and 10 min of interruption. Preliminary studies on the effect of off time on rill erosion and IR in the miniflumes indicated that most of the changes in IR and erosion were obtained in off periods of <5 min. Thus it was assumed that an off time of 10 min would be sufficient for the changes in rill erosion and IR caused by flow interruption to be completed. In order to obtain a measurable outflow during the first minute and to obtain similar runoff during the consecutive 3 min, the inflows applied to the clay soil and the silt loam were 320 and 240 mL min-1, respectively. Because the IR in the clay soil was higher than that in the silt loam, it took 57 s for the first surge to reach the end of the rill in the clay soil, and only 17 s in the silt loam. However, during the second, third, and fourth surges, the outflow rates were similar in both soils because of the higher IR in the clay soil. Thus, the shear stress of flowing water on the rill perimeter and stream transport capacity were similar for both soils.
The second stage of the experiment started immediately at the end of the 4-min flow in the control or after completion of the four cycles of interrupted flow in the interrupted flow treatment. At this stage, the inflow was reduced to allow more precise measurements of IR and was applied continuously to simulate field conditions. A continuous inflow of 100 mL min-1 for 10 min was applied to the clay soil, and a continuous inflow of 80 mL min-1 for 10 min was applied to the silt loam. Total inflow, outflow, and soil loss were recorded every minute for both soils. The second stage was terminated when the moisture front reached a depth of 100 mm, and the soil layer at the bottom of the miniflume remained dry. The length of the second stage was estimated from preliminary experiments done on the same miniflumes packed with the same soils. The dry layer of soil at the bottom of the flume assured the presence of the suction needed to consolidate the soil surface. The suction was maintained at the bottom of the soil in the flume to simulated the moisture profile prevailing under field conditions.
Three replicates were performed for each of the soils and the two flow patterns. Each replicate consisted of a miniflume packed with a fresh dry soil sample. For each soil, the Honestly Significant Difference test (Tukey–Kramer, 
= 0.05) was used to compare the means of the IR and rill erosion between the two flow patterns studied. Differences in the IR and rill erosion between the two soils could not be statistically analyzed because inflow rates differed between the soils. However, because the outflow in the two soils were similar, the flow shear force and the stream transport capacity at the down stream end of the rill were similar and rill erodibility of the two soils could be compared and discussed.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTION1. . Moisture Redistribution...2. . Consolidation of...3. . Surface Seal...MATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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In the interrupted flow treatment, the first flow interruption (i.e., off period) was effective in reducing the IR with the effect being similar (in relative terms) in both soils (Fig. 1, second minute). The second off period was effective only in the silt loam in reducing the IR compared with that of continuous flow. The effect of interrupted flow disappeared in the fourth and fifth minute measurements (Fig. 1). With the introduction of continuous low-rate inflow in the second stage of the experiment, the effect of interrupted flow in reducing IR became evident again (Fig. 1). Four cycles of interrupted flow reduced the final cumulative intake of the silt loam by 19% and that of the clay soil by 6% (Fig. 1). The effect of interrupted flow on intake rate was significant in both soils, but its effect was more pronounced in the silt loam.
During the period of flow interruption, compaction and consolidation of the soil surface caused by the soil water tension most likely occurred, and the hydraulic conductivity of the soil surface is thus reduced (Kemper et al., 1988; Samani et al., 1985). More surface consolidation and a decrease in infiltration is expected in soils with weak structure such as the silt loam (Mullins, 1999). This conclusion was verified by a complementary experiment similar to the one described by Samani et al. (1985). In those experiments disturbed dry soil samples (100 g) of the silt loam or the clay soil were placed inside a funnel with a fritted disc (40–60 µm pores) in the bottom. The internal diameter of the funnel was 65 mm and the thickness of the soil samples was 22 mm. The funnel was connected to a plastic tube filled with water. The soil sample was saturated from the bottom by raising the plastic tube. After saturation, the saturated hydraulic conductivity of the soil sample was measured by applying water to the top of the soil sample in the funnel and collecting the outflow from the end of plastic tube. After measuring the saturated hydraulic conductivity, the same soil was drained to a tension of 20 cm by lowering the plastic tube. At the end of the draining process, the soil sample was saturated again by raising the plastic tube and new saturated hydraulic conductivity of the soil samples was measured. Finally, a tension of 50 cm water was applied, the soil sample was saturated, and saturated hydraulic conductivity following 50-cm tension was measured. Under no tension the hydraulic conductivities of the silt loam and clay soil were 11.8 and 55.6 mm h-1, respectively. When a tension of 20 cm H2O was applied, the hydraulic conductivities of the silt loam and the clay soils dropped to 0.69 and 0.94 of the reference values. When the silt loam and clay soils were exposed to 50 cm suction, the hydraulic conductivity dropped to 0.5 and 0.73 of the values at no tension, respectively. The silt loam hydraulic conductivity was more susceptible to the effect of water tension than the clay soil.
The low IR (Fig. 1) and low hydraulic conductivity of the silt loam suggests that its fraction of water-conducting pores was small and a higher soil water tension could develop before air penetrated the soil surface (Kemper et al., 1988). Thus, the more pronounced effects of interrupted flow in the silt loam, compared with the clay soil, is explained by both a greater consolidation of the soil surface and a greater tension that can develop during the off period.
Aggregate disintegration by fast wetting may have also contributed to the beneficial effect of interrupted flow in the silt loam. Rapid advance of the stream front increases aggregate disintegration and seal formation. Conversely, when soils are wetted slowly, entrapment and subsequent explosion of entrapped air is limited, and soil structure is maintained (Kemper et al., 1985, 1988). Fast prewetting predominated in the silt loam, where the 0.5-m-long furrow was wetted in 17 s, compared with 51 s for the clay soil.
Opposing Effects of Interrupted Flow
Applying flow in surges should have two opposing effects on IR: (i) water tension that is developed during the off period consolidates the soil surface and reduces the IR; and (ii) reduced intake leads to an increase in the hydraulic gradient in the soil profile, which in turn increases the IR (Izadi et al., 1990; Izuno et al., 1985). In our study the effects of interrupted flow on reducing the IR decreased with flow time (Fig. 1). Similar observations were made by Izuno et al. (1985), who concluded from field data that the infiltration decrease with surge irrigation occurred in the first cycle only. No further reduction in infiltration rate was observed in subsequent surges of a given irrigation (Izuno et al., 1985). The disappearance of the effect of flow interruption on IR with flow time is explained by the fact that less water infiltrated during subsequent interrupted flow. This is demonstrated in Fig. 2
, where IR is presented as a function of cumulative intake for both continuous and interrupted flow. Comparing IRs of continuous and interrupted flow for both soils at identical cumulative intakes (e.g., during the second minute of water application; Fig. 2) revealed that the intake rate in the interrupted flow treatment was smaller than in the continuous flow treatment. Conversely, in the third minute of water application, less water penetrated the soil in the interrupted flow treatment, and the effect of interrupted flow on intake rate became less pronounced in both soils (Fig. 2). The suction that developed in the interrupted flow treatment (due to the smaller cumulative intake) was high enough to cause an increase in the IR. Consequently, the IR in the interrupted flow treatment increased to a level similar to that in the continuous flow treatment.
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Effects of Soil and Flow Type on the Erosion Process
Effects of continuous and interrupted flow on rill erosion rate for the silt loam and the clay soil are presented in Fig. 3
. For both soils, most of the erosion took place during the first 4 min (Stage 1 of the experiment), when high flow rates (240 and 320 mL min-1 for the silt loam and clay soils, respectively) exerting high shear stresses (Shainberg et al., 1994) were used. In the control treatment (i.e., continuous flow), rill erosion in the clay soil was one to two orders of magnitude greater than that in the silt loam (Fig. 3).
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The differences in runoff between the two soils could not explain in full the differences in rill erodibility of the two soils (Fig. 3). Thus, the higher erodibility of the clay soil was ascribed to the weak cohesive forces that existed between the aggregates (Shainberg et al, 1996). Aggregate stability increases with increase in clay content (Kemper and Koch, 1966). Soils with high clay content, such as the one used in our study (Table 1), have stable aggregates and high interaggregate macroporosity (Rengasamy et al., 1984; Kay and Angers, 1999) leading to greater distance and fewer contacts between adjacent aggregates. The larger distance between aggregates contributes to weak cohesive forces among the aggregates, which in turn makes the aggregates more susceptible to detachment from the soil surface. This may explain the higher erodibility of the clay soil compared with the silt loam. Our results seem not to agree with many observations suggesting that clay soils are less erodible than silt loams (e.g., Laflen et al., 1991; Ben-Hur et al., 1985). Studying the effect of clay content on crusting, runoff, and erosion in soils exposed to simulated rain, Ben-Hur et al. (1985) found that soils with 20% clay were susceptible to crusting and that soils with higher clay content had more stable aggregates and less runoff and erosion. The low erosion in clay soils was because of low runoff. When soil erosion from two soils with similar runoff is compared, as in the conditions of this study, erosion from the clay soil may exceed erosion from the silt loam.
The first flow interruption of 10 min significantly reduced the erosion rate of the clay soil compared with that obtained in continuous flow (second minute, Fig. 3). This decrease in erodibility of the clay soil during the first flow interruption became even more pronounced during the subsequent surge cycles (Minutes 3–5, Fig. 3). Four flow interruptions, each of 10 min, had a lasting effect on the rill erosion of the clay soil during the following 10 min of continuous flow (Fig. 3). In the clay soil interrupted flow reduced cumulative erosion by 84% (Fig. 3).
Rill erosion in the silt loam exposed to inflow of 240 and 80 mL min-1 was too small for accurate measurement, and for evaluation of the effects of interrupted flow on erosion (Fig. 3). Thus, a complementary experiment was performed. Following the continuous and interrupted flow in Stages 1 and 2, the miniflumes with the silt loam were exposed to an additional 3 min of continuous inflow of 320 mL min-1. Amount of erosion obtained in these last 3 min in the silt loam decreased from 42.6 g in the continuous flow to 7.6 g in the interrupted flow treatment. Evidently, the silt loam was less erodible than the clay soil, but when the silt loam was exposed to high flow rate, interrupted flow reduced rill erosion to 18% of the erosion in continuous flow. Interrupted flow in the silt loam was as effective in reducing rill erosion as in the clay soil.
The observed effects of interrupted flow on rill erosion can be attributed to two mechanisms that are active during flow interruption. First, the suction developed at the soil surface during the off period pulled the soil particles closer together and increased the cohesive forces between the surface particles and reduced erosion rate (Kemper and Roseneau, 1984; Shainberg et al., 1996). Second, aging (four periods of 10 min) increased the cohesive forces between soil particles (Kemper and Roseneau, 1984). These authors postulated that slightly soluble components diffusing to and cementing points of contact between particles were responsible for the bonding mechanism of the cohesive forces. Realizing, that net attractive forces acted between clay edges and clay surfaces, and also between clay surfaces with high charge densities, Shainberg et al. (1996) suggested that under conditions of high water content supplemented by an adequate aging period, clay to clay contacts occur, and clay cementing was responsible for the development of a cohesive structure that resisted rill erosion.
Interrupted flow reduced cumulative erosion in the two soils to <20% of the erosion in continuous flow. These results suggested that surge irrigation can be considered as an effective management tool for the control of furrow erosion problems in surface irrigation.
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SUMMARY AND CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTION1. . Moisture Redistribution...2. . Consolidation of...3. . Surface Seal...MATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Rill erosion in the clay soil was higher than rill erosion in the silt loam. However, interrupted flow reduced rill erosion in both soils and to a similar degree. Flow interruption reduced rill erosion to 16 and 18% of the rill erosion in continuous flow for the clay soil and silt loam, respectively.
Our results show that, unlike many studies have shown for interrill erosion, rill erosion is higher in clay soil than in silt loam. However, the results also suggest that interruption of flow might be considered as an effective management tool in surface irrigation to enhance infiltration uniformity and for the control of furrow erosion in the two soil types.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTION1. . Moisture Redistribution...2. . Consolidation of...3. . Surface Seal...MATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Received for publication September 21, 1998.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTION1. . Moisture Redistribution...2. . Consolidation of...3. . Surface Seal...MATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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