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Inst. of Soil, Water, and Environ. Sci., the Volcani Cent., ARO, P.O. Box 6, Bet Dagan 50250, Israel
* Corresponding author (meni{at}agri.gov.il)
Received for publication June 8, 2001.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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2.3 dS m-1 in both treatments. After the rainy season, the average EC in the control was 0.6 dS m-1 in the 0- to 0.6-m depth range and 3.4 dS m-1 in the 0.6- to 1.5-m depth range. Conversely, in the microbasins, the salt was leached to below 1.5-m depth by rainfall. However, further study is needed to determine the salt distribution in the areas between the microbasins.
Abbreviations: CV, coefficients of variation • EC, electrical conductivity • IR, infiltration rate • MSIS, moving sprinkler irrigation system • SAR, sodium adsorption ratio
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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When the water application rate exceeds the soil infiltration rate (IR), runoff occurs. The main factor that decreases the soil IR under sprinkler irrigation and rainfall conditions in arid and semiarid regions is seal formation at the soil surface (Ben-Hur et al., 1987, 1989). A surface seal is thin and is characterized by greater density and lower saturated conductivity than the underlying soil (McIntyre, 1958; Gal et al., 1984). The high application rate of MSISs and the development of a seal enhance the runoff potential in the irrigated field. For example, toward the outer end of a 53-ha center-pivot MSIS operating on a very fine sandy soil, Addink (1975) found runoff values as high as 65% of the applied water. Similarly, Ben-Hur (1994) found that in a silt loam, loess soil irrigated with MSIS (average application rate of 100 mm h-1), the runoff percentage of irrigation water from 3.5-m2 plots in cotton, corn (Zea mays L.), and peanut (Arachis hypogaea L.) fields ranged from 40 to 50% and that in a potato field, it was 60%. Depending on the field characteristics (slope and soil surface roughness), the runoff may flow out of the field, accumulate in small depressions within the field (local runoff), or both. The runoff from a cultivated field is not available for the plants in the field, and it increases soil erosion and fertilizer depletion. In contrast, the local runoff can lead to nonuniformity in water distribution within the field (Ben-Hur et al., 1995).
Soil tillage practices, such as microbasins (pitting) and dikes across the furrows, could increase the surface storage capacity of the field and, in turn, decrease the runoff flow within and out of the field. More details on the microbasin practice were presented by Morin et al. (1984), who observed that this practice decreased the runoff flow from wheat fields under rainfall in arid and semiarid regions. Ben-Hur et al. (1995) found that dikes increased the water uniformity within a peanut field irrigated with fresh water via MSIS. The average peanut pod yield in the dike treatment was 880 kg ha-1 higher than that in the control treatment, in which runoff was allowed to flow downhill. Huang et al. (2000) found that in a corn field irrigated with saline water [electrical conductivity (EC) 4.7 dS m-1] via an MSIS, microbasin practice increased the along-slope uniformity of the water content distribution in the 0- to 1.5-m soil layer and the salt content distribution in the 0- to 0.3-m soil layer. They obtained average canopy corn yields of 21.7 and 30.6 Mg ha-1 after trying at 60°C, in the control and microbasin treatments, respectively.
The effect of MSIS irrigation on surface runoff and the effects of microbasin tillage in prevention of runoff flow along the slope have been studied mainly in loess soils (e.g., Ben-Hur et al., 1989, 1995; Huang et al., 2000), which are sensitive to seal formation and IR reduction. Vertisols also occur commonly in arid and semiarid regions (Yaalon and Kalmar, 1978) and behave differently during irrigation with MSIS. A Vertisol is defined as a deep (>0.5-m depth) soil containing 30% or more clay in all horizons, with montmorillonite as the dominant clay (FAO, 1990). The high clay content in a Vertisol could increase the aggregate stability (Kemper and Koch, 1966) and, consequently, could diminish seal formation and runoff under MSIS irrigation. Moreover, the high content of montmorillonite in a Vertisol increases the swelling of wet soil and the development of cracks in dry soil (Yaalon and Kalmar, 1978), which could in turn increase the IR of the soil.
Water resources in arid and semiarid regions are, in general, scarce. Therefore, in order to maintain sustainable agriculture in these regions, it may be necessary to use unconventional water resources, such as treated sewage water (effluent), for irrigation. However, the effluents contain relatively high levels of salts and have high sodium adsorption ratios (SARs); in Israeli effluents, the EC is 2.0 dS m-1, which is more than twice that of fresh water, and the SAR is 6 (mmol L-1)0.5 compared with 2.5 (mmol L-1)0.5 in fresh water. Consequently, irrigation with effluents could enhance the salinization and sodicity of the irrigated lands to levels at which plant growth is reduced (Mass and Hoffman, 1977) and the soil structure is damaged (Ben-Hur et al., 1998).
To avoid the accumulation of salt in soil under effluent irrigation, salt should be leached from the root zone. The two major methods of salt leaching are (i) addition of a leaching fraction (the fraction of applied water that appears as drainage water) to the irrigation water to hold the salt concentration in the soil below a specific value (Rhoades et al., 1973) and (ii) leaching of salt by rainfall during the winter. The latter method loads less salt on the field and saves water. However, this method has the disadvantage that during the rainy season that follows the season of irrigation with effluent, the combination of high-energy raindrops with very low electrolyte concentration in the rainwater and a relatively high exchangeable sodium percentage in the soil can lead to formation of a dense, low-permeability seal with a consequently low soil IR (Agassi et al., 1981). Under these conditions, water infiltration during the rainy season might be reduced, which would in turn reduce the salt-leaching efficiency of the rainfall. It is hypothesized that tillage of the field with microbasins, in the fall after the season of irrigation with an effluent, would decrease the runoff during the subsequent rainy season and in turn increase the salt leaching by the rainfall.
The objectives of the present study were (i) to determine the effects of microbasins on runoff and soil loss in a Vertisol field under MSIS irrigation with effluent and under rainfall; (ii) to study the effects of surface runoff and microbasins on the distributions of soil water content, soil salt concentration, and cotton yield in a sloping Vertisol field; and (iii) to study the effects of microbasin tillage on salt leaching from the root zone under rainfall. The results of the measurements of runoff rate and water and salt distribution in the Vertisol field were compared with previous results in a loess soil reported by Ben-Hur et al. (1989)(1995) and Huang et al. (2000).
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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450 mm, occurring only in the winter. The average potential transpiration in the summer in this region is 7.7 mm d-1 (Cohen et al., 1995). The soil in the experimental field was a Vertisol, belonging to the subgroup Typic Chromoxererts, according to the U.S. soil taxonomy (Soil Survey Staff, 1975). The soil texture was clay (montmorillonite was dominant), 620; silt, 300; and sand, 80 g kg-1. The soil contained 66 g kg-1 CaCO3 and 39 g kg-1 organic matter and had a cation exchange capacity of 54 cmol kg-1. The soil bulk densities in the field at depths of 0 to 0.3 and 0.3 to 1.5 m were 1.16 and 1.35 Mg m-3, respectively. The gravimetric water contents at field capacity for the soil depths of 0 to 0.3 and 0.3 to 1.5 m were 34 and 33%, respectively. The steady-state IR of the disturbed soil under simulated rainfall (Morin et al., 1967) was 4 mm h-1, and for undisturbed soil in the field measured by mean of double-rings infiltrometer, it was 30 mm h-1. The experiment was divided between two periods.
Irrigation Season
The field was plowed to 0.3-m depth by moldboard and then leveled with a roller to provide a seedbed in spring 1999. Cotton was planted on 5 Apr. 1999 with a row spacing of 1 m and an average population of 10 plants m-1, and the soil was fertilized with 100 kg N ha-1 on 28 Apr. 1999. The field was routinely irrigated using a linear, 200-m-long MSIS with a lateral discharge of 1000 L m-1 h-1. The MSIS was equipped with spray nozzles of spinner type (Nelson, Walla Walla, WA) spaced 2.8 m apart and alternating at heights of 1.6 and 1.8 m above the soil surface. The irrigation was started on 5 June 1999 and continued through 8 Aug. 1999. The total irrigation water applied during the entire growing season was 520 mm, with intervals of 7 d between the applications. The first two irrigation events delivered 60 mm each and the others 50 mm each. This irrigation water amount was to satisfy the crop water demand. The water used for irrigation was effluent after a secondary treatment in aerated oxidation ponds. The chemical properties of the irrigation water at various sampling dates are presented in Table 1. The organic matter in the water was determined with a combustion TOC analyzer (Skalar Analytical B.V., Breda, the Netherlands). The dissolved organic matter in the water was determined after filtration of the water through a 0.45-µm filter. The concentrations of trace elements in the effluent (results not presented) were below the levels permissible in irrigation water (Page and Chang, 1985).
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5%. The MSIS traveled along the longitudinal axis, and the plant rows were parallel to it, which is a standard practice in many commercial fields with MSIS irrigation. Each irrigation event began at the uphill position and moved downhill while the runoff flowed along the longitudinal axis. The experiment included two tillage practice treatments: (i) control (tillage was in accordance with the local practices, as described above) and (ii) microbasin [special machinery (Fig. 1) was used to sink microbasins 0.2 by 0.5 m in area and 0.15 m in depth between each two plant rows, 3 d before the first irrigation event]. In the control treatment, the runoff was allowed to flow freely downhill. In contrast, in the microbasin treatment, the runoff water was trapped in the microbasins during the irrigation and later infiltrated into the soil.
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Soil samples to a depth of 1.5 m at 0.3-m intervals were taken from each sampling site in three of the four plots of each treatment at the beginning of June 1999 (before the start of irrigation) and at the end of October 1999 (after the end of the growing season). The soil sampling site was located midway between two adjacent plant rows (Fig. 1), 2 m downhill from the yield sampling area. The gravimetric water content and the EC of saturated soil paste were determined for each soil sample. No trends of water contents or EC values along the slope or between the treatment plots were observed on the earlier sampling date.
On various dates during the growing season, soil water content was determined at 0.3-m intervals to a depth of 1.5 m by neutron scattering (503 DR Hydroprobe, CPN Co., Martinez, CA). Access tubes were installed in each sampling site in three of the four plots of each treatment; each tube was located adjacent to the plant row (Fig. 1) and 1 m downhill from the soil sampling point. The readings of the neutron probe were calibrated specifically to the soil type of the experimental site using gravimetric water content of soil samples taken to a depth of 1.5 m at the end of the rainy season (wet range) and the growing season (dry range). The calibration curve is presented in Fig. 2 .
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Rainy Season
In the fall of 1999, after the cotton harvest and before rainfall had started, the experimental field was recultivated. The microbasins and the runoff plots were reconstructed to the same design and in the same locations as in the summer, and the field was left fallow during the winter of 1999–2000. In this experiment period, measurements were conducted only in the runoff microplots. The runoff volume and quality and the soil loss were determined after each rainstorm in the same way as during the irrigation season.
Soil samples were taken from a soil sampling site located in the center of each runoff microplot (Fig. 1), at 0.3-m intervals to a depth of 1.5 m, in mid-October 1999 (before rainfall had started) and at the end of April 2000 (after the end of the rainy season). The EC of saturated soil paste was determined for each soil sample.
The results of all of the measured parameters were subjected to analysis of variance as a complete randomized design (Steel and Torrie, 1981). Separation of means was tested according to Tukey's honestly significant difference, with a significant level of 0.05. The nonuniformity of the measured variables along the slope of the field was determined by their coefficients of variation (CV), which were calculated from the average values of the variables, as measured in each site along the slope.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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At the beginning of the irrigation season, the runoff from both soils (Fig. 3; Ben-Hur et al., 1989) increased to a maximum value, mainly because at that time, the cotton canopy was sparse so that the impact energy of the water drops broke down the aggregates at the bare soil surface and formed a seal, which in turn decreased the IR and increased the runoff. The higher runoff percentage in the loess soil (Ben-Hur et al., 1989) than in the Vertisol (Fig. 3) at the beginning of the irrigation season could have been a result of two factors: the higher clay content in a Vertisol than in a loess soil, with consequently greater aggregate stability (Kemper and Koch, 1966) of the Vertisol, and the tendency of a Vertisol to develop cracks when it dries. Qualitative observations made during the irrigation season in the present study indicated that quite extensive cracks in the soil had developed by 3 d after an irrigation event, and just before the next irrigation event, some of the cracks were 2 cm wide (Fig. 4)
. These two factors limited the seal formation and the IR reduction in the Vertisol compared with the loess soil. Ben-Hur et al. (1989) arrived at the same conclusion in fallow plots irrigated with fresh water via an MSIS.
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After 4 July, the rate of soil loss in the control treatment of Vertisol decreased more rapidly than the runoff rate (Fig. 3). Soil erosion involves two major processes: detachment of soil material from the soil surface and transport of the resulting sediment (Watson and Laflen, 1986). The decrease of the runoff after 4 July (Fig. 3) diminished the runoff transport capacity, and the extensive cotton canopy after this date decreased the soil detachment. These two processes were probably responsible for the steep decrease in soil loss during the irrigation season (Fig. 3).
The amounts of runoff and soil loss for each irrigation event were, in general, significantly lower in the microbasin treatment than in the control (Fig. 3). The average runoff and soil loss per irrigation event for the entire irrigation season were 12.5 mm and 64.6 g m-2, respectively, in the control, and 3.3 mm and 13 g m-2, respectively, in the microbasin treatment. In the microbasin treatment, most of the irrigated water that did not infiltrate into the soil during the irrigation events accumulated in the microbasins and infiltrated later, which in turn decreased both the runoff and soil loss in this treatment. The low values of runoff and soil loss in the microbasin treatment in the last irrigation events (Fig. 3) indicate that the capacity of the microbasins to hold runoff water remained high throughout the irrigation season.
The rainfall depth of each rainstorm during the winter of 1999–2000 is presented in Fig. 5
; the total rainfall was 370 mm, which is 82% of the annual average in this region. In this rainy season, no runoff or soil loss were obtained in the first three rainstorms (Fig. 3). For the next two rainstorms, which fell between 3 and 10 January, 2 mm of runoff and 2.1 g m-2 soil loss were measured in the control treatment (Fig. 3). In contrast, in this treatment, runoff and soil loss in the two rainstorms that fell between 18 and 27 January were 20 mm and 123.2 g m-2, respectively. The rainstorms after 27 January caused no runoff or soil loss (Fig. 3). No significant differences in the runoff and soil loss amounts were found between the control and microbasin treatments in the rainstorms that fell between 3 and 10 January (Fig. 3). In contrast, in the rainstorms that fell between 18 and 27 January, the runoff and soil loss were significantly lower in the microbasin treatment than in the control.
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These findings regarding the effects of surface runoff on water and salt distributions in a Vertisol field (Fig. 6 and 7) contrast those obtained in fields with a loess soil (e.g., Ben-Hur et al., 1995; Huang et al., 2000). In the studies on a loess soil, the surface runoff that was allowed to move downhill increased the soil water content in the downhill direction, and similarly, when the irrigation water was saline, the surface runoff in the control treatment increased the soil salinity in the downhill sites. In contrast, microbasin tillage in a loess soil, which decreased the surface runoff, increased the along-slope uniformity of the water and salt contents and the crop yield. It is important to note that the runoff amount and its effects on the distribution of water content and salt concentration along a slope depend also on the size and the gradient of the slope. However, these parameters were similar in the present study and in the studies of Ben-Hur et al. (1995) and Huang et al. (2000).
The lack of any effect of the microbasin treatment and the surface runoff on the measured water and salt distributions and cotton yields in a sloping Vertisol field (Fig. 6 and 7) could be a result of two main factors:
1. The additional infiltrated water in the microbasin plots, compared with the control plots (Fig. 3), flows downward to depths greater than 1.5 m where no measurements were made. However, the distributions of salt in the control and in the microbasin treatments were similar with respect to concentration and soil depth (Fig. 7). This suggests that the amounts of infiltrated water in the control and the microbasin plots in Vertisol were similar. Thus, this factor could not have been the cause of the lack of any effect of the surface runoff on these parameters.
2. The runoff that was measured from 5.5-m2 plots in the present study was local runoff, and its effects on the distributions of water content and salt concentration along a slope did not depend only on its volume, but also on its flow rate and on the down-slope variations in its infiltration. In the studies of Ben-Hur et al. (1995) and Huang et al. (2000), as well as in the present study, each irrigation event began with the MSIS at the uphill position, from which it moved downhill. Under these conditions, in the control plots, the runoff generated in the uphill wetted area flowed downhill to the adjacent dry area. In the case of the Vertisol, the local runoff was relatively low, and it infiltrated into the soil quite quickly, through the cracks in the adjacent dry area (Fig. 4). This, in turn, limited the downhill runoff flow and its accumulation in the downhill direction. In this case, every area along the slope, except those furthest uphill and downhill, in general, contributed similar amounts of runoff to the adjacent downhill area and received similar amounts of runoff from the adjacent uphill area. Consequently, the nonuniformity of the water and salt distributions along the slope in the Vertisol field was low (Fig. 6 and 7). In contrast, a loess soil is more susceptible to formation of a stable seal (Ben-Hur et al., 1989) and develops much less effective cracks when it dries. Consequently, the downhill flow of the surface runoff, which moves relatively large distances, extends the duration of infiltration in the downhill site and decreases the along-slope uniformity of the water and salt distributions (Ben-Hur et al., 1995; Huang et al., 2000).
Average values of the EC of saturated paste of soil samples that were taken from the runoff microplots at the end of the irrigation season (fall) and the end of the subsequent rainy season (spring) in both treatments are presented in Fig. 8
. The ECs at the end of the irrigation season were relatively high, 2.3 dS m-1 down to 1.5-m depth in both treatments. In the control treatment, the EC values in the fall were quite uniform along the soil profile, whereas in the microbasin treatment, some accumulation of salt was observed in the 0- to 0.9-m soil layer. These relatively high EC values in the fall sampling resulted from irrigation with an effluent that contained a relatively high concentration of salt (2.1 dS m-1) (Table 1). In contrast, the rainfall during the winter leached the salt that had accumulated in the soil during the irrigation season downward. In the control treatment, the salt was leached from the 0- to 0.6-m layer, and it accumulated in the 0.6- to 1.5-m soil layer (Fig. 8). In this treatment, the average EC values at the end of the rainy season were 0.6 dS m-1 in the 0- to 0.6-m soil layer and 3.4 dS m-1 in the 0.6- to 1.5-m soil layer. In the microbasin treatment, the salt leaching was below the 1.5-m depth. In this case, at the end of the rainy season, the EC in the 0- to 0.3-m soil layer was 0.4 dS m-1, and an increasing trend of EC with soil depth was observed (Fig. 8).
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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