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DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Polyacrylamide, Sediments, and Interrupted Flow Effects on Rill Erosion and Intake Rate

Institute of Soils, Water and Environmental Sciences, Agricultural Research Organization (ARO), the Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel

vwguy{at}volcani.agri.gov.il

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
The reduction in the intake rate (IR) during interrupted irrigation is difficult to predict. Sediments in irrigation water decrease the effect of interrupted irrigation on IR. Polyacrylamide (PAM) reduces rill erosion, but its effect on IR is controversial. The effects of water quality (tap water, tap water containing sediments, and 10 g m^-3 PAM solution) and interrupted flow on IR and rill erosion in an Alfisol (Calcic Haploxeralf) and a Vertisol (Typic Chromoxerert) were studied using laboratory miniflumes. Rill erosion in both soils was eliminated by the PAM treatment in both continuous and interrupted flow. The PAM application reduced IR in the Alfisol and increased it in the Vertisol. In the Alfisol, interrupted flow reduced IR of the PAM solution by 37% compared with only 18% for tap water. In the Vertisol, interrupted flow reduced IR only slightly and the decrease was not affected by the polymer. When the water contained sediments, cumulative infiltration was reduced by 22% for the Vertisol and 59% for the Alfisol in comparison with tap water. These reductions were attributed to depositional seal formation. The IR of the Alfisol was more susceptible to depositional seal formation than the Vertisol. The presence of sediments in water was effective in reducing rill erosion. The effects of interrupted flow with PAM on reducing IR were explained by partial blocking of the conducting pores leading to greater suction and compaction of the soil surface. For sediment-laden irrigation water, interrupted flow had no advantage over continuous flow in reducing IR because of depositional seal formation associated with the sediments in the water.

Abbreviations: HC, hydraulic conductivity • IR, intake rate • PAM, polyacrylamide

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
SURFACE IRRIGATION is the most used irrigation practice worldwide, but its water use efficiency is low (Wolters, 1992). Interrupted irrigation, which is the intermittent application of irrigation water during the advancement stage of furrow irrigation (Stringham, 1988), has the potential to reduce IR and improve the efficiency of surface irrigation by increasing field water application uniformity. In spite of much research (Izuno et al., 1985; Jalali-Farahani et al., 1993; Kemper et al., 1988; Samani et al., 1985; Trout, 1991), the process is still not fully understood and its effects on IR are difficult to predict.

Two basic phenomena have been identified during interruption of flow: (i) moisture redistribution in the soil profile and (ii) consolidation of the soil near the rill surface. During the interruption of water application, water drainage into the underlying dry soil and moisture redistribution result in the development of negative pressure suction near the soil surface. This negative pressure increases the forces that pull water into the soil during the next flow period (Samani et al., 1985), and should increase the IR. However, the development of negative pressure in the soil surface during flow interruptions consolidates the soil near the rill surface, increases surface bulk density, and reduces the hydraulic conductivity (HC) of this surface layer. Thus, this thin layer can have a significant effect of reducing water infiltration in succeeding irrigation events (Izuno et al., 1985; Jalali-Farahani et al., 1993; Samani et al., 1985).

An additional important mechanism controlling IR in furrow irrigation, which is not necessarily related to interrupted irrigation, is the formation of a depositional seal at the furrow perimeter. The HC of depositional seals has been reported to be two to three orders of magnitude lower than that of the underlying soil (Shainberg and Singer, 1985). Trout (1991) observed a 50% reduction in infiltration in the Portneuf (coarse-silty, mixed, superactive, mesic Durinodic Xeric Haplocalcid) silt loam during interrupted irrigation, and ascribed it to surface seal formation. The HC of the depositional seal depends on the size and mineralogy of the sediment particles, and on the electrolyte concentration of the water (Shainberg and Singer, 1985). Thus, the effect of sediment concentration on the IR varies from one irrigation scheme to another.

Soil erosion can be prevented by amending the soil with organic polymers, such as PAM, with high molecular weight and moderate negative charge density (e.g., Lentz et al., 1992; Shainberg et al., 1990; Sojka et al., 1998a, 1998b). If rill erosion is prevented, no depositional seal is formed and the rill IR increases (Lentz et al., 1992; Sojka et al., 1998b). Thus, an indirect effect of the PAM treatment is the increase in IR. However, Malik and Letey (1992) and Letey (1996) found that the addition of 10 g m^-3 of PAM to water decreased the HC of fine porous media to 50% of that obtained when salt solutions were used. They suggested that the effective viscosity of polymer solutions in porous media was higher than would be anticipated according to standard viscosity measurements, and that the relative viscosity depended on the pore-size distribution of the soil. The effect of PAM in reducing the HC of porous media could also be explained in terms of partial blocking of conducting pores by the tails of the macromolecules that were adsorbed on soil particles. This partial blocking would probably become more pronounced in soils with narrow pores. Letey (1996) proposed that in furrow irrigation PAM treatment will reduce IR and increase the advancement rate of water in the furrows. It is possible therefore that the effect of PAM on IR in furrow irrigation depends on soil properties, a topic to be clarified in this study.

The effects of interrupted irrigation in reducing IR depend on sediment concentration (Trout, 1991). As sediment concentration increases, IR decreases and the beneficial effect of interrupted irrigation for reducing the IR is reduced (Trout, 1991). It could be argued therefore, that PAM treatments will magnify interruption-induced effects on IR reduction. On the other hand, since PAM also stabilizes the structure at the soil surface (Sojka et al., 1998b), PAM may prevent the consolidation of the surface by the interrupted flow and the net effect of interrupted flow on IR in PAM treatments will be negligible. Hence it is difficult to assess the effects of PAM on the IR in interrupted flow.

Miniflumes have been used to evaluate the interaction between flow characteristics, soil properties, and water quality on rill erosion in the laboratory (Shainberg et al., 1994, 1996). The rill erodibility data obtained with the miniflumes agreed well with field data (Shainberg et al., 1994). Miniflume studies were also found to simulate well the effect of PAM on rill erosion in the field (Lentz et al., 1992; Shainberg et al., 1994). Using miniflumes, Shainberg et al. (1996) studied rill erosion in an Alfisol and a Vertisol and found that rill erosion decreased with aging of several hours and that it depended on water content in the soil. These researchers postulated that aging and water tension enhanced clay to clay contacts, increased the cohesive forces between soil particles, and led to reduction in rill erosion. We hypothesized that miniflumes may also be used to study the processes that operate in interrupted irrigation.

Interrupted flow reduced erosion in irrigated furrows (Yonts et al., 1998). In miniflumes, the same effect has been observed and was attributed to consolidation of the soil surface (Sirjacobs, 1999, unpublished data). If rill erosion and depositional seal formation are reduced, a high IR should be maintained. Interrupted flow may therefore have two opposing effects on IR; that is, it may reduce IR by consolidating the soil surface or increase IR by reducing rill erosion and seal formation. The net effect of interrupted flow on IR may be evaluated either by preventing rill erosion (e.g., by irrigation with water containing PAM) or by increasing the sediment content of the irrigation water. When rill erosion is prevented, no depositional seal is formed and the effect of interrupted flow in consolidating the rill surface and reducing IR is predominant. Conversely, sediment deposition and seal formation may be enhanced by the use of sediment-laden water (Shainberg and Singer, 1985). Applying water containing sediments will, therefore, decrease IR in both continuous and interrupted flow applications. However, the relative effects of water containing sediments on the IR in continuous and interrupted flow effect are not clear and will be studied.

The objective of our study was to investigate the effects of PAM- and sediment-containing inflow on IR and rill erosion under continuous and interrupted flow conditions in two soil types. The interaction between sediments, PAM, and soil properties on the effect of interrupted flow on IR and rill erosion were evaluated by comparing the results obtained with clear tap water with those obtained with PAM solutions and sediment-laden inflow water.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
Two arable soils of differing texture were chosen for this study: a silty loam Alfisol (Calcic Haploxeralf) from Nevatim, northern Negev, and a clay Vertisol (Typic Chromoxerert) from Hafetz-Haim, Pleshet Plains, Israel. Some basic physical and chemical properties of the soils are given in Table 1 .

Air-dried soils, crushed to pass through a 4.0-mm sieve, were slightly compacted in the flume to densities of 1390 kg m^-3 for the Alfisol and 1200 kg m^-3 for the Vertisol. The dry volume of the Vertisol was slightly smaller than that of the Alfisol. However, upon wetting and subsequent swelling, the final volume of the wet Vertisol in the flume was similar to that of the Alfisol. 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 the runoff containing sediments was collected from the downstream metallic rill in beakers.

Three water types were studied in the experiments: (i) laboratory tap water (electrical conductivity = 0.95 dS m^-1; Na adsorption ratio = 2.5 [mmol_c L^-1]^0.5; Ca + Mg = 5 mmol_c L^-1; Na = 4 mmol_c L^-1; Cl = 6.2 mmol_c L^-1); (ii) tap water containing 10 g m^-3 PAM; and (iii) tap water containing 7.5 g L^-1 of suspended sediments. The PAM solution was prepared from a concentrated polymer solution that contained 1 g L^-1 high molecular weight (2 x 10⁷ Da) anionic PAM with a moderate negative charge (20% hydrolysis). Suspensions of each soil were prepared by shaking 300 g of soil with 3 L of tap water for 1 h. After shaking, the coarse particles were allowed to settle out of the suspension for 3 min. Sediment content of the suspensions was 7.5 g L^-1 for each soil. During each miniflume run, the suspension was stirred continuously in order to ensure its homogeneity. Samples of the suspension were taken periodically during the run and the sediment content of the suspension was recorded.

Each individual experiment was divided into two stages. In the first stage either continuous (control) or interrupted flow was applied, and the three water qualities were used. 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 interruption time on rill erosion and IR indicated that for an interruption time of <5 min, changes in interruption time affected IR and rill erosion, but choosing interruption times longer than 5 min did not affect the IR and rill erosion. Inflows applied to the Vertisol (320 mL min^-1) and the Alfisol (240 mL min^-1) were chosen in order to obtain a measurable outflow during the first minute and to obtain a similar flow rate and runoff during the consecutive 3 min. Because the IR in the Vertisol was higher than the IR in the Alfisol, it took 51 s for the clear water to wet the soil and reach the end of the rill in the Vertisol and 17 s in the Alfisol. However, during the second, third, and fourth pulses of flow, the average outflow rates in the two soils were similar (Fig. 3 and 5) . Total inflow, outflow, and soil loss were recorded for every minute of flow.

View larger version (41K): [in this window] [in a new window]   Fig. 3 Intake rate as a function of cumulative flow time for the Vertisol during (a) Stage 1 and (b) Stage 2 of the experiment. Significant differences between water types for a given cumulative flow time and flow type are indicated by upper-case letters (P < 0.05). Significant differences between flow types for a given cumulative flow time and water type are indicated by lower-case letters (P < 0.05). TW is tap water, PAM is polyacrylamide-containing water, and SED is sediment-containing water. (C) and (I) denote continuous and interrupted flow, respectively

View larger version (36K): [in this window] [in a new window]   Fig. 5 Intake rate as a function of cumulative flow time for the Alfisol during (a) Stage 1 and (b) Stage 2 of the experiment. Significant differences between water types for a given cumulative flow time and flow type are indicated by upper-case letters (P < 0.05). Significant differences between flow types for a given cumulative flow time and water type are indicated by lower-case letters (P < 0.05). TW is tap water, PAM is polyacrylamide-containing water, and SED is sediment-containing water. (C) and (I) denote continuous and interrupted flow, respectively

Three replicates were performed for each of the twelve combinations tested (two soils, interrupted and continuous flow, and three types of irrigation waters). The effect of water type on interrupted flow was analyzed separately for the Alfisol and for the Vertisol. For each soil, the effects of two factors (water type and flow type) on rill erosion and IR were considered. Our experiments involved three levels of water type (tap water, tap water with PAM, and tap water with sediments) and two levels of flow type (continuous and interrupted flow). For each minute and for each variable measured, a full factorial analysis of variance, based on the Standard Least Squares test , was applied. When an interaction between the two factors was found, the different levels of water type were compared within each level of flow type and vice versa. When no interaction was detected, each factor was studied individually, without distinction between the levels of the other factor.

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
Because the effects of interrupted flow on IR depend on rill erosion and depositional seal formation (Kemper et al., 1988; Trout, 1991), the effects of water type on rill erosion in continuous and interrupted flow are discussed first.

Water Type Effects on Rill Erosion
Polyacrylamide
Rill erosion in the Vertisol exposed to a continuous flow of tap water was high (Fig. 1) , and that in the Alfisol was low (Fig. 2) . Application of interrupted flow caused a significant reduction in rill erosion only in the Vertisol. Most of the rill erosion in the two soils occurred during the first 4 min, when the flow rate was high (320 and 240 mL min^-1 for the Vertisol and Alfisol, respectively) and the flow shear force was high. In the second stage of the experiment, when a continuous low-rate flow was used, erosion was low (Fig. 1 and 2). In the Vertisol, intraaggregate stability is greater than that in the Alfisol (Shainberg et al., 1992). However, interaggregate cohesive forces in the Vertisol are weaker than in the Alfisol (Shainberg et al., 1996), thus detachment of aggregates by the flowing water could possibly be easier in the Vertisol than in the Alfisol. At the same time, it is expected that detached particles in the Vertisol are larger than those in the Alfisol, because of the better aggregation of the Vertisol, and would thus be less available for transportation by the flowing water. Our results showed more erosion in the Vertisol, suggesting that under our experimental conditions the size of the detached particles in the Vertisol did not affect their transportability. Therefore, Vertisols, owing to their high clay content (Table 1), have a stable aggregated structure with weak cohesive forces among soil particles, which in turn, made them more susceptible to detachment and subsequently to transportation than the aggregates of the Alfisol.

View larger version (26K): [in this window] [in a new window]   Fig. 1 Cumulative soil loss as a function of cumulative flow time for the Vertisol during (a) Stage 1 and (b) Stage 2 of the experiment. Significant differences between water types for a given cumulative flow time and flow type are indicated by upper-case letters (P < 0.05). Significant differences between flow types for a given cumulative flow time and water type are indicated by lower-case letters (P < 0.05). TW is tap water, PAM is polyacrylamide-containing water, and SED is sediment-containing water. (C) and (I) denote continuous and interrupted flow, respectively

View larger version (25K): [in this window] [in a new window]   Fig. 2 Cumulative soil loss as a function of cumulative flow time for the Alfisol during (a) Stage 1, and (b) Stage 2 of the experiment. Significant differences between water types for a given cumulative flow time and flow type are indicated by upper-case letters (P < 0.05). Significant differences between flow types for a given cumulative flow time and water type are indicated by lower-case letters (P < 0.05). TW is tap water, PAM is polyacrylamide-containing water, and SED is sediment-containing water. (C) and (I) denote continuous and interrupted flow, respectively

In the PAM treatment, erosion in both soils was very low under continuous flow, and no further decrease in erosion because of interrupted flow was possible (Fig. 1 and 2). As the binding between soil particles in the presence of PAM was already strong enough to prevent erosion, further strengthening of interparticle cohesive forces by aging and compaction during flow interruptions caused no further reduction of rill erosion.

Sediments
In the Vertisol, the use of sediment-loaded water led to the formation of a visible and homogeneous seal at the rill perimeter. This depositional seal dramatically reduced rill erosion compared with inflow of tap water (Fig. 1). For continuous flow, the presence of sediments in the irrigation water was as effective as the presence of PAM in preventing rill erosion. When interrupted flow was applied, PAM was more effective than sediments in preventing rill erosion (Fig. 1). The difference in cumulative erosion between interrupted and continuous flow appeared from the second minute (Fig. 1). The higher erosion under interrupted flow was attributed to lower intake rate in the interrupted flow (see below), which resulted in higher flow velocity and greater shear force (Kemper et al., 1988). In our study, rill erosion was high only in the second minute of inflow. Thereafter, hardly any additional difference in soil loss between the interrupted and continuous flow treatments was observed. Apparently, aging and compaction mechanisms that acted during the flow interruption created a sufficient increase in soil cohesive forces that could resist the high shear force and thus limit soil loss.

Unlike the case of the Vertisol, a net deposition of sediments was observed in the Alfisol under both interrupted and continuous flow during the 4 min of application of water containing sediments. This is represented in Fig. 2 by the negative values of cumulative soil loss, which demonstrate that 20% of the sediments flowing into the flume were deposited on the rill perimeter. In the second stage of the experiment when a continuous low flow rate of tap water was used for 20 min (as opposed to only 10 min in the Vertisol), a small amount of soil erosion was observed in the continuous flow treatment. Conversely, in the interrupted flow treatment, no sediments were measured in the tap water runoff and no increase in cumulative erosion was demonstrated (Fig. 2).

Deposition of sediments at the soil surface filled the pores and created a depositional seal with a smooth surface on the rill perimeter. The fine particles within the pores acted as a cementing material between soil particles, and the sealed rill surface became more resistant to erosion then the original soil surface. Similar findings were reported by Brown et al. (1988), who studied the effects of sediment-laden water on IR and furrow erosion in the field. These researchers concluded that deposition of the fine sediments on the perimeter decreased IR and thus increased soil water tension, consequently leading to an increase in the forces that hold the sediments at the perimeter, and to a decrease in erosion. The presence of sediments in the inflow water was effective in preventing net soil loss erosion in both soils. The presence of sediments in the water in continuous flow was more effective in decreasing rill erosion than interrupted flow or the addition of PAM to the inflow water. The beneficial effect of sediments in the inflow water suggest that sediments in irrigation water should not be removed prior to irrigation. This conclusion corroborates the findings of Brown et al. (1988) obtained in short furrows in the field.

Water Type Effects on Intake Rate
Polyacrylamide
In the first stage of the experiment (i.e., first 4 min) for both flow types in the Vertisol, addition of PAM to the inflow water resulted in IRs that were higher than, or similar to, those for tap water. Considering the effect of PAM on advancement time, a similar conclusion is derived. Whereas the time needed for the first pulse of tap water to reach the end of the 0.5-m rill was 51 ± 2 s, it took 57 ± 3s with the PAM solutions. Prevention of rill erosion and depositional seal formation prevented the decrease in IR during the first minute, and the advance time for the PAM solution was longer than that for tap water. Similar findings were reported by Lentz at al. (1992) and Lentz and Sojka (1994) who observed, in field experiments, that PAM reduced furrow erosion and increased furrow IR.

The effects of PAM on the cumulative intake of the Vertisol exposed to continuous and interrupted flow are presented in Fig. 3. The PAM treatment increased the cumulative intakes under both continuous and interrupted flow by 6%. This increase in IR and cumulative intake was contrary to the predictions of Letey (1996) and Malik and Letey (1992), who suggested that PAM increased the apparent viscosity of the solution within the soil pores, and therefore, that soil IR should decrease. The PAM-related IR increase observed in the Vertisol was attributed to two possible mechanisms: (i) PAM prevented rill erosion and the formation of depositional seal (Lentz et al., 1992; Sojka et al., 1998a) and (ii) PAM stabilized the soil structure and prevented deterioration of the soil surface HC (Shainberg et al., 1990). The effects of interrupted flow on IR in the PAM and the tap water treatments were similar (Fig. 3); interrupted inflow in the Vertisol decreased both cumulative intakes by 6% (Fig. 4) . Interrupted flow was as effective in decreasing IR in tap water and the accompanying high erosion as it was when PAM was used and only a small amount of sediments was present in the water. The similarity in the decrease in IR for the tap water and PAM treatments is suggested to be related to the structure of the seal formed. When sediments were deposited from a solution that had an electrolyte concentration exceeding the flocculation value of the soil clay (i.e., tap water), the seal formed had an open structure (Shainberg and Singer, 1985) that was susceptible to compaction and consolidation when exposed to suction, in a way similar to that of an unsealed soil surface.

View larger version (26K): [in this window] [in a new window]   Fig. 4 Cumulative intake as a function of cumulative flow time for the Vertisol during (a) Stage 1 and (b) Stage 2 of the experiment. Significant differences between water types for a given cumulative flow time and flow type are indicated by upper-case letters (P < 0.05). Significant differences between flow types for a given cumulative flow time and water type are indicated by lower-case letters (P < 0.05). TW is tap water, PAM is polyacrylamide-containing water, and SED is sediment-containing water. (C) and (I) denote continuous and interrupted flow, respectively

View larger version (26K): [in this window] [in a new window]   Fig. 6 Cumulative intake as a function of cumulative flow time for the Alfisol during (a) Stage 1 and (b) Stage 2 of the experiment. Significant differences between water types for a given cumulative flow time and flow type are indicated by upper-case letters (P < 0.05). Significant differences between flow types for a given cumulative flow time and water type are indicated by lower-case letters (P < 0.05). TW is tap water, PAM is polyacrylamide-containing water, and SED is sediment-containing water. (C) and (I) denote continuous and interrupted flow, respectively

Addition of PAM to the inflow water did not alter the interrupted flow effect on IR in the Vertisol. Interrupted flow decreased the final cumulative intake by 6% in both the PAM and tap water treatments; however, in the Alfisol, interrupted flow was more effective in decreasing the IR with PAM than with tap water. In the latter case, it reduced cumulative intake by 18% compared with continuous flow, whereas in the PAM solution it reduced cumulative intake by 38% (Fig. 6). The beneficial effect of interrupted flow with PAM solutions in the Alfisol can be explained as follows: PAM decreased the HC of the soil surface by partial blocking of the conducting pores, thus also reducing air penetration into the soil surface. Consequently PAM increased the soil water tension that developed during the flow interruptions (Kemper et al., 1988) and in turn caused enhanced compaction and consolidation of the wetted perimeter, thus reducing the infiltration rate. It is hypothesized that a similar phenomenon occurs in surge irrigation and that the surge effect on irrigation efficiency is improved in PAM treatments in silty loam soils like the Alfisol.

Sediments
The effects of sediments in inflow water on IR in the Vertisol and the Alfisol are presented in Fig. 3 and 5, respectively. In both soils, IR was significantly lower from the very first minute of flow with water containing sediments than with tap water (Fig. 3 and 5). This decrease in IR led to total reductions in cumulative intake of 22% in the Vertisol (Fig. 4) and 59% in the Alfisol (Fig. 6), compared with that obtained when tap water was used. The reduction in IR was related to sediment deposition and the formation of a seal at the rill perimeter (Trout, 1991). The reduction in IR was more pronounced in the Alfisol than in the Vertisol because the Alfisol was more susceptible to seal formation (Ben-Hur et al., 1985). The Alfisol, with its poor structure, was more easily clogged with suspended clay particles than the Vertisol, with its developed structure and large water conducting pores. As a result of the low IR, this treatment also increased the advancement rates on both soils; the advancement times for irrigation with water containing sediments and with tap water were 10 and 17 s, respectively, on the Alfisol and 36 and 51 s, respectively, on the Vertisol. Sediments decreased the advancement times in the Alfisol by 41% and in the Vertisol by 29%. The effect of sediments in irrigation water in increasing the advance rate in furrow irrigation may be included in consideration of the efficiency of surface irrigation.

In the Vertisol, the use of interrupted flow caused a significant decrease in cumulative intake for both tap water and sediment-laden water (Fig. 4). However, the use of sediment-laden water did not have a greater beneficial effect (in relative terms) in decreasing cumulative intake than interrupted flow with tap water. Interrupted flow decreased the final cumulative intake in the Vertisol by 7% with water containing sediments and by 6% with tap water (Fig. 4). In the Alfisol too, the use of interrupted flow caused a significant decrease in cumulative intake for both tap water and sediment-laden water (Fig. 6). However, in the Alfisol a trend was noted whereby interrupted flow was more effective in decreasing cumulative intake with tap water (18%) compared with sediment-containing water (15%) (Fig. 6). This trend may be explained by the greater IR reduction by the formation of a depositional seal in the Alfisol than in the Vertisol (Fig. 3 and 5). When a seal with a low HC is formed, the seal controls the IR and the effects on IR of interrupted flow and the suction that develops during the off time are negligible. Similar observations were made by Trout (1991), who observed that infiltration was reduced by 50% (because of surface seal formation) when sediments were present in irrigation water and that the interrupted flow effect was less pronounced when a depositional seal of low HC was present.

	Summary and conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
The effects of PAM and sediment concentration in inflow water on the effect of interrupted flow on infiltration rate and rill erosion was studied in a silty loam Alfisol and a clay Vertisol, using miniflumes. In both soils, rill erosion was dramatically reduced by the PAM treatment. Polyacrylamide increased the IR in the Vertisol and decreased IR in the Alfisol. The contradictory effect of PAM on rill IR was explained by two opposing mechanisms: (i) enhancement of IR by prevention of erosion and of a depositional seal formation (Lentz et al., 1992; Trout, 1991) and (ii) reduction of IR because of increased apparent viscosity of the solution in the soil pores or by clogging of the conducting pores by the tails of adsorbed polymer molecules (Letey, 1996). The second mechanism dominates in the Alfisol with little rill erosion and no depositional seal formation. The polymer did not influence the interrupted flow effect on IR in the Vertisol. In the Alfisol, interrupted flow reduced the final cumulative intake by 37% in the PAM treatment and by 18% with tap water. The effect of interrupted flow with PAM solutions in the Alfisol was explained by the partial blocking of the conducting pores, which increased the suction and compaction of the soil surface, and so reduced the IR.

When applying sediment-containing water, depositional seal formation markedly decreased the IR of both soils. The reduction of final cumulative infiltration was more pronounced in the Alfisol (59%) than in the Vertisol (22%). The Alfisol, with its unstable structure and narrow conducting pores, was more easily clogged by sediments than the Vertisol, with its stable structure and large pores. When the inflows contained sediments, the interrupted flow effect on IR was limited. Continuous irrigation with water containing sediments increased the advancement rate and reduced rill erosion more effectively than interrupted flow or PAM treatment. This effect should be considered when water containing sediments is used in surface irrigation.

Our results indicate that the potential benefits of interrupted flow as a means of improving surface irrigation efficiency and controlling rill erosion will be gained mainly in weakly structured soils. Addition of polymer to the irrigation water may enhance interrupted flow effects in these soils. When irrigation water contain sediments, interrupted flow has no advantage over continuous flow.

	ACKNOWLEDGMENTS

 
D. Sirjacobs is grateful to the Agricultural Research Organization, Bet Dagan, Israel, for providing him with a grant that made the realization of this work possible. This study was supported by grants TA-MOU-96-CA16-016 and TA-MOU-97-CA17-008, from the U.S.-Israel Cooperative Development Research (CDR) Program, Office of Science Advisor, U.S. Agency for International Development (AID). The support of the CDR is gratefully acknowledged.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
Contribution from the Agricultural Research Organization, the Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel, no. 629/98 series.

Received for publication August 24, 1999.

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TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 

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