Agronomy Journal Grow Your Career With ASA
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (3)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Ben-Hur, M.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Ben-Hur, M.
Agricola
Right arrow Articles by Ben-Hur, M.
Related Collections
Right arrow Flow
Right arrow Structure and Properties
Right arrow Other Soil Management
Right arrow Irrigation
Right arrow Agroclimatology
Right arrow Potato
Right arrow Hydraulic Conductivity
Agronomy Journal 93:1156-1163 (2001)
© 2001 American Society of Agronomy

AGROCLIMATOLOGY

Soil Conditioner Effects on Runoff and Potato Yield under Sprinkler Irrigation

Meni Ben-Hur*

Inst. of Soil, Water, and Environ. Sci., A.R.O., the Volcani Center, Bet Dagan 50250, Israel

* Corresponding author (meni{at}agri.gov.il)

Received for publication March 30, 2000.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Synthetic polymers can be used as soil conditioners, but studies of their effects have addressed laboratory simulations much more than cultivated fields. The objective of this work was to study the effects of a low-molecular-weight, nonionic polymer (P-1O1) on seal formation, infiltration rate (IR), runoff, erosion, and potato (Solanum tuberosum L.) yield. The experiment comprised (i) a laboratory rainfall simulator study and (ii) irrigation of a field in the Negev, Israel with a moving sprinkler irrigation system (MSIS) fitted with various emitters. The emitters were Sprayer No. 1, Spinner, and Super Spray, with discharges of 1254, 2713, and 1777 L h-1, respectively. The studied soil was Calcic Haploxeralf. Under simulated rain, polymer applications of 50, 25, 10, 5, and 0 (control) kg ha-1 resulted in final IR values of 31, 28, 27, 19, and 17 mm h-1, respectively, and erosion rates of 0.9, 0.9, 1.9, 3.5, and 6.1 t ha-1, respectively. In the field, the 40 kg ha-1 polymer treatment, applied with Sprayer No. 1, Spinner, and Super Spray emitters, reduced runoff by 75, 60, and 62%, respectively, compared with the control treatment. The irrigation efficiencies (the ratios between plant yield and total water application) of the MSIS with Sprayer No. 1, Spinner, and Super spray were 56, 31, and 43 kg mm-1, respectively, on untreated soil and 66, 49, and 65 kg mm-1, respectively, under the 40 kg ha-1 polymer treatment.

Abbreviations: IR, infiltration rate • MSIS, moving sprinkler irrigation system


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
IRRIGATION significantly increases food production in arid and semiarid regions. The self-propelled, moving sprinkler irrigation system (MSIS) is a popular system worldwide. For example, >6 million ha in the USA (Anonymous, 1993) and >40000 ha in Israel are irrigated with MSIS. However, the MSIS is characterized by potentially high surface runoff during irrigation. For example, toward the outer end of a 53-ha center pivot MSIS, Addink (1975) found runoff values as high as 65% of the applied water on a very fine sandy soil. Likewise, Ben-Hur et al. (1989b)(1995) found that under irrigation of a silt loam soil with MSIS, runoff of applied water from 3- by 5-m plots in cotton (Gossypium hirsutum L.) and peanut (Arachis hypogaea L.) fields was {approx}40%. Stern et al. (1992) determined that the surface runoff from wheat (Triticum aestivum L.) plots (2 by 5 m) on a silt clay loam soil irrigated with an MSIS was 36.1% of the total irrigation during the growing season.

According to the field characteristics (slope and soil surface roughness), the runoff may flow out of the field, accumulate in small depressions within the field, or both. Runoff from a cultivated field deprives the crop of water and accelerates soil erosion and fertilizer depletion. Local runoff within the field can lead to poor water distribution, and it reduces irrigation efficiency and crop yield (Letey et al., 1984; Ben-Hur et al., 1995).

Ben-Hur et al. (1995) studied the effect of surface runoff on water distribution and peanut yield in a sloping field irrigated with MSIS. They found that in plots where runoff was allowed to flow downhill (control), the uniformity of water distribution along the slope was low. In contrast, in plots where runoff was prevented, the uniformity of water distribution was high, and the average pod yield was 880 kg ha-1 greater than in the control plots.

When the instantaneous rate of water application (application rate) is greater than the soil infiltration rate (IR), runoff may occur. The potential runoff during irrigation is determined mainly by (i) the water application rate of the irrigation system and (ii) the IR of the soil. Ben-Hur et al. (1989b) indicated that seal formation at the soil surface during irrigation with MSIS is the main cause for the IR reduction in semiarid regions; the impact energy of the water drop breaks down soil surface aggregates and forms a seal. This seal is thin (<2 mm) and is characterized by greater density, higher strength, finer pores, and lower saturated hydraulic conductivity than the underlying soil (McIntyre, 1958; Chen et al., 1980).

The emitter type influences the application rate of the irrigation system and the size distribution of the water drops, which in turn, can affect the runoff during irrigation. During recent years, most high-pressure impact MSISs have been replaced with low-pressure impact systems to save energy, which in turn, increases the application rate of the MSIS. Gilley and Mielke (1980) determined maximum application rates of {approx}33, 65, and 310 mm h-1 for high-pressure impact, low-pressure impact, and low-pressure spray nozzle, respectively. The greater maximum application rate of the reduced-pressure system compared with the high-pressure one was due to the smaller radius of the sprayed area under irrigation with the reduced-pressure system when the amounts of water applied by both systems were similar.

One way to increase aggregate stability and prevent seal formation, runoff, and soil erosion is by using synthetic polymers as soil conditioners (Ben-Hur et al., 1989a; Harris et al., 1966; Wood and Oster, 1985). For example, application of about 1 kg ha-1 polyacrylamide, with an 18% negative-charge density, in the irrigation furrow advance water has reduced furrow erosion by as much as 99% (Lentz and Sojka, 1994; Trout et al., 1995; Sojka and Lentz, 1997). This practice has been researched and documented and has become widespread and popular in the USA (USDA-NRCS, 1995).

The use of polymers as soil conditioners has also been found effective under sprinkler conditions. Shaviv et al. (1986) observed that 80 kg ha-1 anionic polymer with low molecular weight (70000–150000 Da) significantly decreased the surface runoff on a silty loam soil under rainfall simulator conditions; the combination of 2.4 Mg ha-1 phosphogypsum with this polymer treatment enhanced the polymer efficiency. Ben-Hur and Letey (1989) determined that a low concentration (<10 g m-3) of cationic polysaccharides in the irrigation water significantly increased the IR of a soil under laboratory sprinkler conditions. Shainberg et al. (1990) found that surface application of 20 kg ha-1 anionic polyacrylamide was sufficient to maintain high IRs in loess and Vertisol. Aase et al. (1998) found that 2 kg ha-1 polyacrylamide applied in 20 mm of irrigation water reduced runoff and soil loss on a Rad silt loam by 70 and 75%, respectively, compared with the control treatment. In contrast, application of this amount of polymer in 8 mm of irrigation water was less effective.

All of the above studies involved rain simulators and were performed under laboratory conditions. The use of polymers as soil conditioners under such conditions has been reviewed by Theng (1982), Levy (1995), and Levy and Ben-Hur (1999). From these reviews, it is concluded that (i) the addition of small amounts (of the order of 10 kg ha-1) of polymers to the soil surface diminishes aggregate breakdown and seal formation; (ii) anionic polymers are more effective as soil conditioners when they are applied together with a source of electrolytes; (iii) polymer effectiveness is enhanced by drying the soil surface after spraying; and (iv) a polymer with lower viscosity is preferable from the application point of view because it is easier to spray than one with high viscosity.

The effects of polymers on seal formation, surface runoff, and crop yield have been much less studied in cultivated fields irrigated with MSIS than under laboratory conditions. Therefore, in contrast to the situation in furrow irrigation, polymers have not been commercially used with sprinkler irrigation. Levy et al. (1991) found that application of 20 kg ha-1 of polyacrylamide significantly reduced runoff from a cotton field with a clay loam soil and from a peanut field with a silt loam soil, both irrigated with MSISs; the runoff level from 3-m2 plots treated with polyacrylamide was 50 to 70% of that from the untreated control plots. Levy et al. (1991) also found that polyacrylamide treatment increased seed cotton yield in 400-m2 plots by 14% compared with control plots.

Stern et al. (1992) studied the effects of applying 20 kg ha-1 polyacrylamide combined with 5 Mg ha-1 phosphosgypsum on surface runoff and spring wheat yield under irrigation with MSIS. They found that the runoff percentage of the applied water was 36.1% in untreated plots and 1.4% in treated plots. Likewise, the total crop biomass production and grain yield in the treated plots were significantly higher than in untreated plots. The average wheat grain yield in the treated plots was 3.02 Mg ha-1 compared with 2.12 Mg ha-1 in untreated plots.

Irrigation with MSIS in Israel is popular mainly in the Negev where the most common soil is loess. Therefore, the objectives of this study were (i) to determine the effects of various amounts of low-molecular-weight, nonionic polymer on seal formation, runoff, and erosion in loess under rainfall simulator conditions and to use this study as a pilot for a field experiment; and (ii) to study the effects of polymer applications on surface runoff and potato yield in a field irrigated by MSIS with various emitter types.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
The experiment comprised two parts: (i) a rain simulator study with disturbed soil samples and (ii) a field experiment in a commercial potato (cv. Chloster) field in the western Negev, Israel. The average annual rain in this region is {approx}230 mm, which falls in the winter. The soil in the experimental field was a homogeneous deep silt loam (Calcic Haploxeralf) containing 19, 33, and 12.4% clay, silt, and calcium carbonate (CaCO3), respectively; it had a cation exchange capacity of 12.2 cmolc and an exchangeable Na percentage of 3.7%. The soil samples for the rain simulator study were collected from the 0.05- to 0.3-m layer in the experimental field. The tested polymer was a nonionic polymer, designated P-101, with molecular weight ranging from 1 x 105 to 2 x 105 Da. More details of this polymer were given by Ben-Hur and Keren (1997). It was chosen for this study because it was previously found more effective in maintaining high IR under water drop impact conditions than anionic polyacrylamide or cationic polysaccharide (Ben-Hur and Keren, 1997). Tap water with electrical conductivity (EC) {approx}1.0 dS m-1 and Na adsorption ratio {approx}2 (mmol L-1)0.5 was used in both parts of the study. Mean separations (P < 0.05) between the treatments were determined with Tukey's multiple comparison procedure (Steel and Torrie, 1960).

Rainfall Simulator Study
A rainfall simulator with rotating disk (Morin et al., 1967) located in the Volcani Center, Israel was used to study the effects of the P-101 polymer on seal formation, IR, and soil loss in the studied soil under high impact water-drop conditions. Typical mechanical parameters of the simulated rain were: rain intensity, 70 mm h-1; raindrop median diameter, 1.9 mm; median drop velocity, 6.2 m s-1; and kinetic energy, 18.1 J mm-1 m-2.

In the summer of 1992, the sampled soil was air-dried, ground, passed through a 4-mm sieve, and packed 2 cm deep over a layer of coarse sand in a perforated tray measuring 30 by 50 cm. The packed soil tray was placed at a 25% slope under the rainfall simulator. This slope is similar to that of the sides of the ridges in potato fields. A 50 g L-1 solution of P-101 in tap water was spread over the soil surface with a hand sprayer at rates of 5, 10, 25, or 50 kg ha-1 polymer. In the control treatment, tap water, at the same volume as in the 50 kg ha-1 polymer treatment, was sprayed over the soil surface. Each treatment was replicated four times (four trays). After the spraying, the soil samples were left to dry for 24 h in the rainfall simulator room and then were saturated slowly from below with tap water and exposed to 68 mm of simulated rain. During the rain period, water percolating through the soil and the surface runoff were both collected and measured. The total soil loss for the entire rainstorm was determined by evaporating (105°C) the runoff water and weighing the eroded materials it contained.

Field Experiment
In the fall of 1992, the field was plowed, disked, and leveled with a roller, and then ridges with {approx}20% side slopes were prepared. Potato tubers were planted in mid-September 1992, at a density of 4.5 tubers m-1 in each row, with the rows spaced {approx}1 m apart. The field had a fairly constant 3% gradient, and the plant rows were oriented down the slope. The field was routinely irrigated with a linear MSIS that was 170 m long and carried emitters mounted alternately at heights of 1.6 and 1.8 m above the soil surface. In each irrigation event, the MSIS traveled downwards along the slope direction. The field was fertilized with N at 256 kg ha-1, applied in the irrigation water during several irrigation events.

Three different emitter types (Table 1) were tested. The MSIS length was divided into three segments, each carrying a different emitter type. Quality observations indicated that (i) the droplet size of the various emitters was in the order Spinner (Nelson, Walla Walla, WA) >> Super Spray (Nelson, Walla Walla, Wa) > Sprayer No. 1 (Senninger, Orlando, FL), and (ii) the radius of the sprayed area was in order Spinner >> Sprayer No. 1 > Super Spray. Different P-101 application rates were studied with each emitter type. Sprayer No. 1 was used at 0 (control), 20, or 40 kg ha-1 P-101; Spinner and Super Spray were used at 0 or 40 kg ha-1. In all of the polymer treatments, a 50 g L-1 solution of P-101 in tap water was sprayed over the soil surface with a hand sprayer after completion of the cultivation operations. In the control treatment, tap water, at the same volume as in the 40 kg h-1 polymer treatment, was sprayed over the soil surface. Irrigation of the field started at least 24 h after the polymer application. The water application rate and irrigation schedule in this experiment conformed with those commonly used in the region, with Sprayer No. 1, to satisfy the crop water demand.


View this table:
[in this window]
[in a new window]
  		Table 1. The emitters' technical characteristics.

 
The treatment plot was 3 m wide by 20 m long and included two main parts, i.e., the upper runoff plot and the lower yield plot (Fig. 1), each with a ditch along its upper boundary to prevent the entry of surface runoff. Likewise, the ridges prevented runoff from crossing the plant rows into adjacent areas. The runoff plot was 3.5 m2 and included a furrow in the middle with a crop row on each side. The runoff area was defined by metal borders so that runoff was not influenced by water moving out the plot. Surface runoff was collected from each plot in a barrel embedded in the soil and was measured after each irrigation event and rainstorm. The yield plot included three plant rows, each 4 m long; the central row was used for yield measurement, and the other two served as borders. At the end of the growing season, the potato tubers were hand-harvested, and their weight was determined. The relative development of the potato canopy in each treatment during the season was determined by measurements of the shadow formed by the canopy at midday (1300 h ± 15 min) with a straightedge (Fuchs and Stanhill, 1980). These measurements were conducted in each runoff plot. Within each emitter type, all of the polymer and control treatments were replicated three times (three plots), and plots were distributed randomly. There were 21 total plots; nine for Sprayer No. 1, six for Spinner, and six for Super Spray.



View larger version (21K):
[in this window]
[in a new window]
  		Fig. 1. Schematic layout of the treatment plot.

 

   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES
 
Rainfall Simulator Study
In order to study the effects of polymer on IR and soil loss and to determine the amounts of polymer to be tested in the field under irrigation with MSIS, various application rates of P-101 were run in a laboratory rainfall simulator. The resulting IR values of loess soil as a function of water application rate are shown in Fig. 2 in the absence of polymer (control) and for various application rates of P-101. In the control treatment, the IR decreased with increasing water application until a steady-state IR of 17 mm h-1 was obtained. The reduction of the IR values was due to breakdown of the aggregates and formation of seal at the soil surface (Morin et al., 1981). Application of >=5 kg ha-1 P-101 to the soil surface before the rain event significantly increased the IR; in general, the greater that the amount of polymer was, the higher the IR (Fig. 2). Application of 50 kg ha-1 polymer ensured the maintenance of a high IR (close to the rainfall intensity) up to 37.5 mm of water application, beyond which the IR decreased gradually to 31 mm h-1. For polymer applications of 25, 10, and 5 kg ha-1, the IR values at the end of the rain event were 28, 27, and 19 mm h-1, respectively. Similar results had been obtained previously for application of P-101 at 25 to 75 kg ha-1 on a Typic Rhodoxeralf (sandy loam) (Ben-Hur and Keren, 1997).



View larger version (28K):
[in this window]
[in a new window]
  		Fig. 2. Average values of infiltration rate (IR) as a function of the water application for various polymer applications under rainfall simulator conditions. (The vertical bars represent standard deviations.)

 
The effects of various polymer applications on seal formation in loess soil under simulated rainfall conditions are shown in Fig. 3. In the untreated soil (control), aggregates at the soil surface were dispersed and formed a smooth crust, whereas the surface of the polymer-treated soil was covered with aggregates. The greater the polymer application, the greater the quantity of aggregates observed on the soil surface.



View larger version (75K):
[in this window]
[in a new window]
  		Fig. 3. Soil surface photograph before and after a rain event for various polymer applications.

 
The average soil loss amounts for the entire 68-mm rain event for the various polymer applications are presented in Table 2. The soil loss in the control treatment was high, 6.1 t ha-1. This amount of soil loss from ridges in a potato field during the growing season could expose the potato tubers and impair their quality. Treating the soil surface with >=5 kg ha-1 polymer significantly decreased the soil loss (Table 2). However, no significant differences were found among the soil loss amounts for the 10, 25, and 50 kg ha-1 P-101 treatments.


View this table:
[in this window]
[in a new window]
  		Table 2. Average soil loss amount for the rain event for the various polymer applications and their standard deviations (SD) under rainfall simulator conditions.

 
The results from Fig. 2, Fig. 3, and Table 2 suggest that the polymer increased the aggregate stability at the soil surface. The polymer molecules were adsorbed on the soil particle surfaces and acted as a cementing material, holding particles together against the destructive forces of water drops (Theng, 1982; Shaviv et al., 1986). Ben-Hur et al. (1989a) hypothesized that when polymer molecules are adsorbed on the external surface of a soil aggregate, once the aggregate breaks, the internal soil surface without polymers would be exposed, and IR would decrease due to seal formation. Ben-Hur and Keren (1997) studied the effects of three different commercial polymers—a low-molecular-weight nonionic (P-101), a medium-molecular-weight cationic (CP-14), and a high-molecular-weight anionic (CG)—on aggregate formation, soil sealing, and IR in a sandy loam (Typic Rhodoxeralf). All three polymers were effective in maintaining higher IR than that of the control; P-101 was the most effective and CG the least. In contrast, in a soil suspension where the soil particles were initially separated and their surfaces exposed to the polymer molecules, the effectiveness of the polymers in aggregate formation was in the order CG > CP-14 > P-101. Ben-Hur and Keren (1997) suggested that the high efficacy of the P-101 in preventing surface seal formation was a result of its capability to penetrate into aggregates and stabilize them because of its small molecular size and the low viscosity of its solution. Increasing the P-101 application up to 50 kg ha-1 (Fig. 2) probably enhanced the penetration of the polymer molecules into the aggregates, which in turn, enlarged the aggregate stability at the soil surface and limited the seal formation (Fig. 3) and IR reduction (Fig. 2).

Interrill soil erosion involves two major processes: (i) detachment of soil material from the surface soil mainly by the raindrop impact and (ii) transport of the resulting sediment by raindrop splash and overland flow (Meyer et al., 1975). Because splash erosion (out of the soil tray) measurements were not taken in this study, the transportation of the sediments in this case occurred by overland flow only. Increasing the polymer application from 0 to 50 kg ha-1 significantly decreased the runoff transportation capacity (decrease of surface runoff; Fig. 2) and the soil detachment (decrease of aggregate breakdown; Fig. 3). However, no further significant increase of soil loss was observed for application of >=10 kg ha-1 P-101 (Table 2). For the 10 kg ha-1 treatment, detachment was considerable compared with the 25 and 50 kg ha-1 P-101 treatments but apparently beyond its runoff transport capacity. In this case, the total soil loss in the 10 kg ha-1 P-101 treatment was limited and similar to the 25 and 50 kg ha-1 P-101 treatments (Table 2). On the basis of the IR and soil loss results obtained in the rainfall simulator study, it was decided to apply two amounts of P-101, 20 and 40 kg ha-1, in the field experiment.

Field Study
Runoff percentages of the irrigation water or rainfall from a 3.5-m2 plot in the potato field during the growing season are presented in Fig. 4 for different emitter types and polymer applications. For the control treatment, in general, the runoff percentage increased with time for all three emitter types. For example, the runoff percentages in the first irrigation event (30 September) were 5% for Sprayer No. 1, 6% for Spinner, and 4% for Super Spray. At the end of the season (18 December), the runoff percentages were 73% for Sprayer No. 1, 78% for Spinner, and 75% for Super Spray. With respect to the irrigation events only, the runoff increased sharply from the beginning of the irrigation season until 21 October and then increased slightly until the end of the irrigation season for Sprayer No. 1 and Super Spray (Fig. 4). However, for Spinner, the runoff from the irrigation events increased fairly constantly throughout the irrigation season.



View larger version (33K):
[in this window]
[in a new window]
  		Fig. 4. Runoff percentages during irrigation and rain events for various emitter types and polymer applications under moving sprinkler irrigation system (MSIS) irrigation. (The vertical bars represent standard deviations, the vertical arrows indicate rain events, and numbers above the symbols indicate the irrigation or rainfall depth.)

 
Ben-Hur et al. (1989b) found that the increase of runoff during irrigation with MSIS was mostly a result of seal formation at the soil surface, i.e., the impact energy of water drops broke down the aggregates at the soil surface and formed a seal. Therefore, one factor that can affect the seal formation and runoff amount during irrigation with MSIS is the percent coverage by the plant canopy, which can cover the soil surface and protect it against the impact energy of the water drops. The variation of the percentage of total surface between the potato plants covered by the plant canopy during the growing season is presented in Fig. 5. Because no statistically significant differences in the covered area were found among the treatments, the average value for all treatments for each sampling date is presented in Fig. 5. The soil surface covered by the potato canopy increased sharply until 6 November when the plant canopy covered 92% of the soil surface. The soil surface was completely covered by the plant canopy from 13 through 20 November and then decreased to 85% on 4 December, probably because of the plant senescence.



View larger version (19K):
[in this window]
[in a new window]
  		Fig. 5. Average percentages of covered soil surface during the potato growing season. (The vertical bars represent standard deviations.)

 
The steep increases in the runoff percentages in the control treatments with all three emitters until 21 October (Fig. 4) were due mainly to the impacts of the water drops on the soil surface. During this time, <40% of the soil surface was covered; therefore, most of it was exposed to the impact of the water drops. However, after 6 November, >85% of the soil surface was covered by the plant canopy and protected against the impact of the water drops (Fig. 5). Under these conditions, the slight increase in runoff after 6 November for irrigation with Sprayer No. 1 and Super Spray (Fig. 4) could have resulted mainly from the impact of the water drops that fell from the plant leaves, slaking of aggregates at the soil surface by their wetting, or both (Le Bissonnais, 1996). In contrast, for Spinner with large droplet size, the continued, constant increase in the runoff after 6 November was probable mainly because more irrigation water drops penetrated the potato canopy compared with the two other emitters.

For all three emitter types, the runoff percentages from the rainstorms (7 and 27 November and 10 and 18 December) in the control treatments were, in general, higher than those in the adjacent irrigation events (Fig. 4). This could be due to the (i) higher intensity of the rainstorms compared with the irrigation events and (ii) lower electrolyte concentration in the rainwater (close to distilled water) compared with the electrical conductivity {approx}1 dS m-1 of the irrigation water. This low electrolyte concentration in the rainwater would cause clay dispersion in the soil surface, which would in turn, enhance the seal formation and decrease the IR (Agassi et al., 1981).

The runoff percentage for each irrigation or rain event in the 20 and 40 kg h-1 polymer treatments was lower than in the control treatment (Fig. 4). This lower runoff percentage in the polymer treatments resulted from the beneficial effect of the polymer. Treating the soil with >20 kg ha-1 P-101 probably increased the aggregate stability on the soil surface and diminished the seal formation and IR reduction. In contrast to the control treatment, in the polymer treatments, the runoff in the rain events was not significantly higher than in the adjacent irrigation events (Fig. 4). This is probably because of the higher stability of the aggregate in the polymer treatments. In this case, the effects of the rain intensity and water quality on clay dispersion and seal formation were less pronounced.

The change in runoff percentage with time in the polymer treatments was different from that in the control treatment (Fig. 4). For the polymer treatments, the runoff percentage increased with time from the beginning of the irrigation season, reached a first maximum value, decreased to a minimum value, and then increased again. While the first maximum value of the runoff percentage occurred on different dates for the various polymer treatments, the subsequent minimum values occurred on the same date, 5 December, for all polymer treatments. The runoff percentage at the first maximum value was statistically significantly higher than that at the minimum value in every polymer treatment. Moreover, the decrease of the runoff percentage after the first maximum value in all of the polymer treatments occurred when the soil surface coverage by the plant canopy was >85%. Ben-Hur et al. (1985) found in untreated soil that drying of crusted soil led to crust breakdown and an increase of the soil IR while subjecting the dried, crusted soil in subsequent rain events renewed the seal formation. Apparently, in the polymer treatments, the beneficial effect of the polymer in increasing the aggregate stability diminished the renewed crust reformation in the subsequent irrigation under 85% coverage by the plant canopy, which in turn, maintained high IR and low runoff.

The increase of the runoff percentage after 5 December in all the polymer treatments was probably a result of decreased beneficial effect of the polymer after this date. Two main factors could cause this decrease: (i) erosion of the polymer-treated topsoil during irrigation and rain and/or (ii) degradation of the polymer in the soil surface with time. Under these conditions, the amount of polymer at the soil surface after 5 December was too low to prevent renewed formation of the seal during the last two rain events. It can be concluded from these results, that the beneficial effect of polymer P-101 applied at 40 kg ha-1 under MSIS irrigation is significant until almost the end of the irrigation season.

In order to show the relationships among the polymer treatments, emitter types, and runoff under MSIS irrigation, total runoff amounts across the entire growing season are presented in Fig. 6 for the control and the different polymer treatments and emitter types. Statistical comparison among the three emitter types for each polymer and control treatment is not possible because of the experimental design. The total runoff was 143, 194, and 193 mm with Sprayer No. 1, Spinner, and Super Spray, respectively (Fig. 6). Because the emitters were all on one boom and their flow rate characteristics were different (Table 1), the total water application of the various emitter types during the entire irrigation season was altered to 278, 395, and 394 mm for Sprayer No. 1, Spinner, and Super Spray, respectively. Under these conditions, the average runoff percentages of the total water application (including rain) for each emitter type was {approx}42%. This pattern of runoff rate for the various emitters was a result of the relationships between the application rate of the emitter and its distribution of droplet kinetic energy, which affects the seal formation and IR of the soil.



View larger version (26K):
[in this window]
[in a new window]
  		Fig. 6. Averages of the total runoff in the entire growing season for various polymer applications and emitter types in the potato field irrigated with moving sprinkler irrigation system (MSIS). (The vertical bars represent standard deviations, and for each emitter type, different letters labeling columns indicate significant differences, P < 0.05, between the treatments.)

 
For each emitter type, the runoff amounts in all polymer treatments were lower than in the control while the runoff reductions in the 40 kg ha-1 polymer treatment were statistically significant (Fig. 6). The runoff in this polymer treatment decreased by 75, 60, and 62% compared with the control treatments for Sprayer No. 1, Spinner, and Super Spray, respectively. These results suggest that under the conditions of the present study, the beneficial effect of the polymer in decreasing runoff is higher for irrigation with Sprayer No. 1 than with the other two emitters. This is probably because of smaller water drops emitted by Sprayer No. 1 than by the other two emitters.

Potato tuber yields in the various polymer treatments and with various emitter types are presented in Fig. 7. The differences in potato yields among the various treatments were not statistically significant, apparently because of the small number of replicates (three) and the wide variation among the replicates in each treatment. This makes discussion of the potato yield quite speculative. In the control treatment, the yield with Sprayer No. 1 was similar to that with Super Spray and higher than that with Spinner, in spite of the total water application with the latter two emitters being higher than that with the first. Irrigation efficiency, with respect to water use, could be defined as the ratio between plant yield and total water application. The irrigation efficiencies of Sprayer No. 1, Super Spray, and Spinner on untreated soil were 56, 43, and 31 kg mm-1, respectively. In contrast, in the 40 kg ha-1 polymer treatment, the irrigation efficiencies of the same emitters were 66.3, 65.0, and 48.5 kg mm-1, respectively.



View larger version (26K):
[in this window]
[in a new window]
  		Fig. 7. Average potato yields for various applications and emitter types in the potato field irrigated with moving sprinkler irrigation system (MSIS). (The vertical bars represent standard deviations.)

 

   SUMMARY AND CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES
 
  1. Under rainfall simulator conditions, it was found that application of >=5 kg ha-1 P-101 on the soil surface increased IR and decreased soil loss. For polymer application of 0, 5, 10, 25, and 50 kg ha-1, the IR values at the end of the rain event were 17, 19, 27, 28, and 31 mm h-1, respectively, and the soil losses were, 6.1, 3.5, 1.9, 0.9, and 0.9 t ha-1, respectively. In the untreated soil, aggregates at the soil surface were dispersed, and the soil surface was covered with a smooth crust, whereas the surface of the polymer-treated soil displayed some aggregates. Probably the cementing effect of the P-101 decreased the breakdown of aggregates, reduced seal formation, maintained high IR values, and decreased the soil erosion.
  2. For the control treatment (no polymer application) under MSIS irrigation, the total runoff in the entire growing season was 143, 194, and 193 mm with Sprayer No. 1, Spinner, and Super Spray, respectively. However, the runoff percentages of the total water application for the three emitters were the same, i.e., 42%.
  3. In the field experiment, with each emitter type, the average runoff percentage in the 40 kg ha-1 polymer treatment was significantly lower than in the control treatment. The total runoff in this polymer treatment decreased by 75, 60, and 62% compared with the control treatment for Sprayer No. 1, Spinner, and Super Spray, respectively.
  4. The beneficial effect of the polymer in runoff reduction under MSIS irrigation was significant until the end the irrigation season for the three emitter types studied. However, this beneficial effect decreased sharply after 5 December, probably because of erosion of the polymer-treated topsoil during the irrigation and rainfall, degradation of the polymer in the soil surface with time, or both.
  5. The potato tuber yields in the various treatments did not differ significantly, which makes conclusions regarding the potato yield quite speculative. In the control treatment, the yield with Sprayer No. 1 was similar to that with Super Spray and higher than that with Spinner. The irrigation efficiencies of Sprayer No. 1, Super Spray, and Spinner on untreated soil were 56, 43, and 31 kg mm-1, respectively. In contrast, in the 40 kg ha-1 polymer treatment, with the same emitters, the irrigation efficiencies were 66, 65, and 49 kg mm-1, respectively.


   ACKNOWLEDGMENTS
 
The author thanks Mr. M. Olicki and Mr. D. Segal for their help in the rainfall simulator and field experiments.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Contribution no. 610/99 from the Agricultural Research Organization, the Volcani Center, POB 6, Bet Dagan 50250, Israel.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 




This article has been cited by other articles:


Home page
 Vadose Zone JHome page
S. Assouline
Rainfall-Induced Soil Surface Sealing: A Critical Review of Observations, Conceptual Models, and Solutions
Vadose Zone J., May 1, 2004; 3(2): 570 - 591.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (3)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Ben-Hur, M.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Ben-Hur, M.
Agricola
Right arrow Articles by Ben-Hur, M.
Related Collections
Right arrow Flow
Right arrow Structure and Properties
Right arrow Other Soil Management
Right arrow Irrigation
Right arrow Agroclimatology
Right arrow Potato
Right arrow Hydraulic Conductivity

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Crop Science Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome