Soil Science Society of America Journal 67:852-858 (2003)
© 2003 Soil Science Society of America
DIVISION S-6—SOIL & WATER MANAGEMENT & CONSERVATION
Effects of Water Applications and Soil Tillage on Water and Salt Distribution in a Vertisol
S. Assouline* and
M. Ben-Hur
Institute of Soil, Water and Environmental Sciences, the Volcani Center, ARO, P.O. Box 6, Bet Dagan 50250, Israel
* Corresponding author (vwshmuel{at}agri.gov.il)
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ABSTRACT
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Under limited water resources, modern irrigation methods tend to save water and improve water and salt regimes within the root zone. This study deals with the combined effects of water application methods and tillage practices on water and salt distributions, runoff production, and soil loss in a field irrigated with moving irrigation systems (MIS). An experiment was conducted in a cotton (Gossypium hirsutum L. cv. Sivon) field at Hazorea, Israel, where the main soil type is vertisol (Typic Chromoxerets). Sprinkling (SP) and flooding (FL) MIS, and conventional (CT) and microbasin (MB) tillage, were compared in terms of runoff and soil loss from runoff microplots (5 m2), soil water content and salinity distribution with depth, yield, and plant height. Under SP conditions, no runoff and soil loss were obtained for either tillage practice. In the FL/CT treatment, the mean runoff and soil loss were about 25% of the irrigation water and 0.59 kg m-2. The FL/MB treatment reduced the runoff and soil loss to 5.8% and 0.02 kg m-2, respectively. The soil water contents in the SP treatments were generally lower than in the FL treatments, especially in the 0.1- to 0.6-m soil layer. No significant differences in the soil salinity, plant height, and seed-cotton yield were observed between the treatments. Microbasins tillage reduces water losses under flooding MIS to a point where they become practically similar to those obtained under sprinkler MIS. It can potentially lead to lower water losses if the microbasins storage capacity is matched to the water application rate, to avoid runoff.
Abbreviations: CEC, cation-exchange capacity • CT, conventional tillage • EC, electrical conductivity • FL, flooding • MB, microbasin tillage • LEPAS, low-energy precision application socks • MIS, moving irrigation systems • SP, sprinkling
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INTRODUCTION
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THE AGRICULTURAL PRODUCTION and the development of arid and semi-arid regions rely mainly on irrigation. However, the water resources in these regions are characterized by two trends: (i) diminishing availability of good-quality water; (ii) increasing use of marginal water (saline water, treated sewage effluent, and floodwater) for irrigation. The soil solution in irrigated fields is frequently more saline than the irrigation water because of evapotranspiration that leaves the dissolved salts from the water in the soil, and the dissolution of soil minerals (Rhoades et al., 1973). Consequently, irrigation with marginal water could enhance the salinization of the irrigated lands, which could eventually reduce plant growth (Maas and Hoffman, 1977) and damage the soil structure (Ben-Hur et al., 1998). Therefore, the goals of modern irrigation are to develop methods that save water and improve both the water and the salt distributions within the root zone.
Moving irrigation systems have become increasingly popular in recent years (Anonymous, 1993). These systems are characterized by a relatively high instantaneous rate of water application (Gilley and Mielke, 1980), which generally exceeds the water infiltration. These water application methods can affect soil structure and lead to the formation of seals at the soil surface (Ben-Hur et al., 1989). Consequently, the use of MIS is often linked to increased runoff and soil loss problems (Addink, 1975; Ben-Hur, 1994, Ben-Hur and Assouline, 2002). Therefore, MIS should be applied along with soil tillage practices that can reduce runoff formation and its flow in the field. Soil tillage practices, such as microbasins (pitting) and dikes across the furrows, can decrease the runoff flow within and out of the field by increasing the surface storage capacity of the field (Morin et al., 1984; Ben-Hur and Assouline, 2002). These practices also increase the water uniformity within the field, which results in higher yields (Ben-Hur et al., 1995; Huang et al., 2000). Huang et al. (2000) have found that the microbasin and dike practices under MIS irrigation with saline water affected the salt distribution with soil depth, the dike practice leading to the maximal salt concentration in the 0- to 0.3-m soil layer.
Two methods of water application can be used in MIS: SP and FL. For the SP method, the spans in the MIS are equipped with sprayers, spaced to ensure uniform wetting of the area. In contrast, for the FL method, the spans in the MIS are equipped with low-energy precision application socks (LEPAS) (Fig. 1)
, which deliver the water directly to the soil surface. Spraying water over the canopy can harm the plants, especially when irrigation water with high salinity and Na concentration is used. Also, water can be lost and wetting uniformity affected because of the wind. Water application directly to the soil surface, below the canopy, can prevent these problems. Huang et al. (2000) found that spraying saline water (electrical conductivity [EC] of 4.7 dS m-1) over a corn (Zea mays L.) canopy from a sprinkler MIS increased the Na concentration within the corn tissue significantly, to approximately 10 g kg-1 compared with 2.7 g kg-1 under irrigation with flooding MIS, with which direct contact between the plant canopy and the irrigation water is avoided.
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Fig. 1. The two water application methods (upper) and the low-energy precision application socks (LEPAS) with the microbasins tillage (lower).
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While the effects of irrigation with sprinkler MIS on runoff formation, water and salt distributions and yields in cropped fields have been quite well documented (Ben-Hur et al., 1989, 1995; Ben-Hur and Assouline, 2002), these effects have been much less studied under irrigation with flooding MIS. Moreover, most of the studies on MIS have been performed on loess soils, characterized by their sensitivity to seal formation that reduces their hydraulic conductivity. However, Vertisols are also common in arid and semiarid regions (Yaalon and Kalmar, 1978). 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 can increase the aggregate stability (Kemper and Koch, 1966) and, consequently, diminish seal formation and related runoff production under MIS irrigation. Moreover, the high montmorillonite content causes a Vertisol to swell when wet and to develop cracks when dry (Yaalon and Kalmar, 1978). The formation of a crack network, in turn, increases the water infiltration. Consequently, a Vertisol could behave differently from a loess soil during irrigation with MIS (Ben-Hur and Assouline, 2002).
The objective of the present study was to determine, in a Vertisol field, the combined effects of MIS water application methods and soil tillage practices, on runoff, soil loss, depth distributions of water content and salt concentration in the soil profile, and cotton yield.
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MATERIALS AND METHODS
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Field Study
The field study was conducted in a commercial cotton field in Kibbutz Hazorea, Yizre'el Valley (Northern Israel). In this region, the average annual rainfall is about 500 mm, occurring only in the winter, and the average potential transpiration in the summer is 7.7 mm d-1 (Cohen et al., 1995). The major soil at the location is a Vertisol, belonging to the subgroup Typic Chromoxererts, according to the U.S. soil taxonomy (Soil Survey Staff, 1975), with montmorillonite being the dominant clay.
The field was plowed, disked, and leveled with a roller to provide a seedbed, in spring 2000. Cotton was planted on 18 Apr. 2000 with two rows, 1 m apart, in each bed and an average population of 10 plants m-1. It was then irrigated with 25 mm for germination. The field was fertilized with 900 L ha-1 of liquid ammonia (168 kg ha-1 N) on 15 May 2000. The field was routinely irrigated with a linear, 200-m long MIS, with a lateral discharge of 1100 L m-1 h-1. The plant rows were parallel to the traveling axis of the MIS. The irrigation period started on 12 June 2000 and ended on 10 Aug. 2000, and included six irrigation events at 10-d intervals. The water depth for each irrigation event ranged from 60 to 70 mm. The total irrigation water applied during the entire growing season was 410 mm.
The irrigation water was a mixture of water from winter floods and secondary sewage effluent. The mean chemical properties of the irrigation water at various sampling dates are presented in Table 1. The organic matter in the water was determined with combustion TOC analyzer (Skalar Analytical B.V., 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 water (results not presented) were low and below the levels permissible in irrigation water (Page and Chang, 1985).
The experimental site was 150 m wide by 50 m long, presenting a slope with a fairly constant gradient of approximately 2% along the longitudinal axis. Three out of the four spans of the MIS irrigated the experimental site. Each irrigation event began at the uphill position and moved downhill, and the runoff flowed along the longitudinal axis. At the upper boundary of the experimental site, a ditch was dug to prevent surface runoff from entering the plots from outside. Likewise, the beds prevented runoff from crossing the plant rows into adjacent areas.
The experiment included two methods of water application: (i) SP where the MIS original water suppliers spaced 2.0 m apart were equipped with appropriate nozzles that regulate the discharge and with sprayers No 1 (Nelson, Walla Walla, WA) alternately at heights of 1.6 and 1.8 above the soil surface (Fig. 1; upper left); (ii) FL where the same MIS original water suppliers as in the SP treatment were split into two delivery tubes below the regulating nozzle and spaced 1 m apart, the ends of these tubes reaching the soil surface and being equipped with a LEPAS (Netafim, Israel) (Fig. 1; upper right). Measurements of the discharge from the water suppliers show that both methods deliver practically the same amount of water, 1050 ± 30 L m-1 h-1 for SP and 1100 ± 20 L m-1 h-1 for FL.
The experiment also included two tillage practices: (i) CT—tillage in accordance with the local practices, as described above; and (ii) MB—pits measuring 0.2 m wide, 0.5 m long, and 0.15 m deep excavated between the plant rows with the aid of special machinery, 3 d before the first irrigation event (Ben-Hur and Assouline, 2002). The microbasins trapped the runoff water and allowed it to infiltrate into the soil later (Fig. 1; lower).
The experiment was run in a factorial manner (four treatments including two tillage practices and two water application methods) with three randomly assigned replicates in each treatment. A layout of the experiment is shown in Fig. 2
. Each plot included three beds, of which the central bed (two plant rows) was used for sampling and the other two beds served as borders. The FL plots were irrigated by seven LEPASs per plot, while three sprayers per plot irrigated the SP plots. In cases where FL plots were adjacent to SP plots, two plots, the closer to the FL treatment being irrigated by LEPASs and the other by sprinklers, were added between the adjacent treatments, to prevent sprayed water from reaching the FL plots.
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Fig. 2. The layout of the field study. The bold frames indicate the experimental plots. The arrow indicates the direction of the travel of the linear MIS.
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Runoff microplots (Ben-Hur et al., 1989), including the central bed (two plant rows) and a furrow on each side, were installed in each treatment plot before the first irrigation event. In each microplot, the area related to runoff production was 5.0 m2, defined by metal borders. Surface runoff from each microplot was collected in a barrel embedded in the soil. The soil loss caused by each irrigation event was determined by evaporating (at 105°C) a 1-L sample of the collected runoff and weighing the eroded materials it contained.
Soil samples to a depth of 1.5 m were taken at 0.3-m intervals from one of the two plant rows in the central bed of each plot on 4 May 2000 (before sowing) and on 14 Nov. 2000 (after harvest). The gravimetric water content and the EC of saturated soil paste were determined for each soil sample. During the growing season, soil water content at the same location was monitored to a depth of 1.5 m, at 0.3-m intervals, by neutron scattering (503 DR Hydroprobe, CPN Co., Martinez, CA) in two replicates. These measurements were conducted before every second irrigation event.
On 22 Oct. 2000, plant heights and cotton yield from a 4-m2 area in the central bed of each plot, were measured.
Rainfall Simulator Study
Disturbed samples were collected from the upper 0.3-m soil layer of the experimental field. The soil from a similar tillage and water application field study conducted at another location in the Yizre'el Valley, Kibbutz Merhavia, was also included in the rain simulation experiment. The physical and chemical properties of the soils are presented in Table 2. Soil texture and cation-exchange capacity (CEC) were determined by standard methods (U.S. Salinity Laboratory Staff, 1954). Organic matter content was determined by the loss-on-ignition procedure (Ben-Dor and Banin, 1989). Soil samples from both locations were air dried, crushed to pass through a 4-mm sieve and mixed thoroughly. The soil samples, in triplicate, were then packed in 0.5 by 0.3 m perforated trays in an 0.02-m-thick layer, to a mean bulk density of 1.29 (± 0.03) Mg m-3, representing the mean bulk density under field conditions. The trays were then placed over a 0.08-m thick layer of coarse sand on a 9% slope in a rotating disk rainfall simulator (Morin et al., 1967). The samples were prewetted from below with tap water (EC = 0.7 dS m-1) and then subjected to a 60-mm rainstorm of the same water. The mean diameter of the raindrops was 1.9 mm, median drop velocity 6.02 m s-1, kinetic energy 18.1 J mm-1 m-2, and rain intensity 42 mm h-1. During the rainstorm, the volumes of rainfall percolating through the soil and of runoff from the soil surface were measured at 5-min intervals, and the corresponding infiltration and runoff rates were calculated. The total runoff was collected and the total soil loss was determined by oven drying the runoff at 105°C and weighing the sediments.
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Table 2. Soils mechanical and chemical characteristics (Mechanical composition; CaCO3 and organic matter, OM, contents; cation-exchange capacity, CEC).
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The results of the two experiments 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 significance level of (P < 0.05).
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RESULTS AND DISCUSSION
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Mean values of runoff, expressed as percentages of the irrigation amount, and of soil loss, for the last three irrigation events and for the various treatments are presented in Fig. 3
. Under SP conditions, no runoff and soil loss were obtained with either tillage practice. Under FL conditions, high runoff and soil loss were measured in the CT treatment. The runoff percentage of the irrigation water on July 17 was 21%, and this increased to approximately 27% in the irrigation events of July 27 and August 10. The soil loss in this treatment was approximately 0.36 kg m-2 on July 17, and it increased sharply to 0.87 kg m-2 in the irrigation event of August 10. The higher runoff and soil loss in the FL/CT treatment than in the SP/CT one resulted from the higher water application rate per wetted area in the flooding application method.
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Fig. 3. Runoff and soil loss amounts collected from runoff microplots after each of the last three irrigation events. Values in parenthesis indicate the irrigation amount in mm. Different letters indicate significant differences (P < 0.05) between the FL/CT and the FL/MB treatments for each irrigation event. Vertical bars represent one standard deviation.
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The MB tillage significantly reduced the runoff and soil losses in the FL treatments (Fig. 3). In the FL/CT treatment, the mean values of runoff and soil loss in the last three irrigation events were 25% of the applied water and 0.59 kg m-2, respectively, while in the FL/MB treatment these values were 5.8% and 0.02 kg m-2, accordingly. The reason is that, in the MB treatment, most of the irrigated water that did not infiltrate into the soil during the irrigation events accumulated in the microbasins and infiltrated later. The low values of runoff and soil loss in the MB treatment in the last irrigation events indicate that the capacity of the microbasins to hold runoff water remained high throughout the irrigation season. However, the results also indicate that the actual capacity of the microbasins could not cope with the water application rate, so that runoff still occurred. More accurate matching of the microbasins storage capacity to the water application rate could further reduce runoff and soil loss and so improve the efficacy of the method.
The results for the SP treatments (Fig. 3) differ from those obtained under similar irrigation and tillage conditions (Ben-Hur and Assouline, 2002) at another experimental site in the same region (Kibbutz Merhavia). There, during the last part of the irrigation season, the mean values of runoff and soil loss per SP irrigation event in the CT treatment were 17.8% of the irrigated water and 48.4 g m-2, respectively. One of the main factors that reduces water infiltration and increases runoff under sprinkler irrigation in arid and semiarid regions is the formation of a seal on the soil surface because of the impact of the water drops (Ben-Hur et al., 1989). To compare the effects of the drop impact on the two experimental soils, disturbed samples of these soils were exposed to a rainfall simulator. The resulting infiltration vs. cumulative rainfall is presented in Fig. 4
. The total soil loss for the entire rainstorm is also indicated in the figure. After the first 5 mm of rainfall, the infiltration rate in Hazorea soil was higher than that in Merhavia soil for all cumulative rainfall amounts. The final infiltration rates—6.9 and 4.8 mm h-1 for Hazorea and Merhavia, respectively—were significantly different. Likewise, the soil loss at Hazorea was significantly lower than that at Merhavia: 0.436 and 0.618 kg m-2, respectively (Fig. 4). It is likely that the higher clay and organic matter contents in Hazorea soil than in Merhavia soil (Table 2) increased the structural stability of the former soil (Kemper and Koch, 1966), and reduced its sensitivity to seal formation, runoff, and soil loss. However, the differences in the infiltration curves of these two soils (Fig. 4) cannot account for the observed large differences between the responses of the runoff microplots to sprinkler MIS, in Hazorea and Merhavia. Another factor that can increase the water infiltration in Vertisols is the formation of cracks during drying (Ben-Hur and Assouline, 2002). It is likely that a denser and more conductive crack network develops in Hazorea than in Merhavia soil because of the higher clay content in the former (Table 2), and that this increases the overall water infiltration and reduces the runoff during SP irrigation.
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Fig. 4. Infiltration curves vs. cumulative simulated rainfall for Merhavia and Hazorea soils. Vertical bars represent two standard deviations.
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The gravimetric soil water contents for the various treatments, soil depths, and sampling dates are presented in Fig. 5
. The water content for each treatment in the 0.1- to 1.2-m soil layer was fairly constant, with some fluctuations, from May 4 until August 8; during this period, the irrigation events kept the water content in the soil relatively high. In contrast, a decrease of the water content in the 0.1- to 1.2-m soil layer was observed in all the treatments after August 8, at a rate that was greatest in the 0.1- to 0.3-m layer and decreased gradually with depth. In the 1.2- to 1.5-m layer, the soil water content on May 4 was relatively low (Fig. 4) but, in all the treatments, the 70-mm irrigation of June 12 increased the water content in this layer to its highest value, from which it decreased gradually with time.
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Fig. 5. The variation of the water content in the soil profile during the irrigation season. Vertical bars represent two standard deviations.
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Differences between the treatments, in the water content of the soil, were observed during the period from June 22 until August 8 (Fig. 4). The water contents in the SP treatments were somewhat lower than those in the FL treatments, these differences being more pronounced in the 0.1- to 0.6-m soil layer. In contrast, after August 8, the water content in the soil in the FL/CT treatment was, in general, the lowest among all the treatments, especially in the 0.6- to 1.5-m layer. These differences between the treatments, in soil water content, could be reflected in their effects on plant water uptake and water losses during the irrigation events. This point is discussed below.
The EC values for the various treatments and sampling dates, before sowing and after harvest, are presented in Fig. 6
as functions of the soil depth. No significant differences between the EC values at the two sampling dates were observed, for each treatment. This was probably because of the relatively low concentration of salt in the irrigation water (EC approximately 1.3 dS m-1) (Table 1), which limited the accumulation of the salt in the soil profile during the irrigation season. The highest EC value, 7.8 dS m-1, was in FL/MB treatment in the 1.2- to 1.5-m soil layer (Fig. 6). However, this value was lower than the salinity threshold for cotton (8.5 dS m-1) (Bresler et al., 1982), therefore, it could be concluded from these results that the cotton yield should not have been affected by the soil salinity.
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Fig. 6. Distribution of soil saturated paste electrical conductivity (EC) with depth before sowing and after harvest. Vertical bars represent two standard deviations.
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The seed-cotton yields and plant heights for the various treatments are presented in Fig. 7
. The yield and the plant height were highly correlated. The higher values were obtained in the FL/MB treatment and the lower ones in the SP/CT one, but these differences were not significant at the (P < 0.05) level.
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Fig. 7. Cotton yield and plant height for the four treatments. Vertical bars represent two standard deviations.
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The plant responses to the treatments did not reflect the differences observed among the runoff percentages (Fig. 3). This apparent inconsistency could be explained as follows. The applied irrigation water amounts were similar in all the treatments. Therefore, since no significant differences were found in yields and plant heights, it can be assumed that evapotranspiration was also the same in all the treatments. The seasonal water balance for a given soil depth is given by Eq. [1]
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where I is the applied irrigation amount (mm), ET is the cumulative evapotranspiration (mm), L is the total water loss (mm), 
is the change in volumetric water content in the soil profile (%), and Z is the soil layer thickness (m). The total amount of water applied during the whole irrigation season, I, was 410 mm. The cumulative ET was approximated as 500 mm (Shalhevet et al., 1981). Therefore, for that approximation, L can be estimated as:
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Based on the water contents in the soil profile on May 4 (before sowing) and on November 14 (after harvest) (Fig. 5), the (10 Z) values for the SP/CT, SP/MB, FL/CT, and FL/MB treatments were -100.5, -100, -135.8, and -104.6 mm, respectively. The corresponding L values (Eq. [2]) were thus, 10.5, 10.0, 45.8, and 14.6 mm, respectively. These results indicate that the values of L in the SP/CT, SP/MB, and FL/MB treatments were practically the same, whereas that in the FL/CT treatment was much higher. Water losses in the SP treatments are mainly because of wind effects, interception by the canopy and deep drainage, whereas in the FL treatments, they are mainly because of runoff and deep drainage. Therefore, in light of the above results, one can say that under the experimental conditions, microbasins tillage reduces water losses from runoff under FL conditions to a point where they become practically the same as those obtained under SP conditions. Since runoff was produced in the FL/MB treatment (Fig. 3), this method can potentially lead to lower water losses than those under sprinkler MIS if the microbasins storage capacity is matched to the water application rate, to avoid runoff.
The yield in the FL/CT treatment was not significantly different from those in the other treatments, because it exploited an extra 30 to 35 mm of the water stored in the soil profile. Therefore, the irrigation water applied in the SP/CT, SP/MB, and FL/MB treatments could have been reduced by 30 to 35 mm, representing seasonal water saving of 7.5 to 8.5%. A higher level of water saving while maintaining the same yield could be expected from the FL/MB method if runoff is completely prevented.
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SUMMARY AND CONCLUSIONS
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The present study has shown that, on the small scale of the runoff microplots, MB tillage significantly reduced runoff and soil loss under flooding MIS application. However, runoff and soil losses were higher than under SP MIS, indicating that the storage capacity of the microbasins has to be designed to match the particular water application rate of the irrigation method applied and the properties of the field soil.
The results for the SP treatments in Hazorea differ from those obtained under similar irrigation and tillage conditions at another experimental site (Merhavia) in the same region where more runoff and soil loss per SP irrigation event were measured in the CT treatment during the last part of the irrigation season (Ben-Hur and Assouline, 2002). This difference can be explained partly by the higher infiltration rates in Hazorea than in Merhavia, when the soils were exposed to simulated rainfall. Another factor can be a denser and more conductive crack network that develops in Hazorea than in Merhavia soil because of the higher clay content in the former.
On the field scale, combining flooding MIS with microbasins led to the highest water content in the root zone during the growing period. Under the experimental conditions, this treatment had practically the same water losses as those obtained under SP MIS, but it showed the potential for lower water losses if the microbasin storage capacity were designed to avoid runoff completely. No significant differences among the treatments, in the cotton yield or plant height, were observed. However, the FL/CT treatment exploited 30 to 35 mm more water from the soil profile than the other treatments, which indicates that seasonal water saving of 7.5 to 8.5% could be achieved with the FL/MB without affecting the yield. A higher level of water saving could be expected if runoff is completely prevented.
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ACKNOWLEDGMENTS
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We thank Mr. H. Tenaw, Mr. M. Rozner, Ms. M. Madar, and Ms. L. Leib for their help in field sampling and laboratory analysis. We thank also Dr. Thanh H. Dao and anonymous referees for their valuable comments and suggestions. This study was supported by the Chief Scientist's Fund of the Ministry of Agricultural and Rural Development of the State of Israel, under research project 304-0248-00. The Chief Scientist's support is gratefully acknowledged.
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NOTES
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Contribution no. 625-01 from the Agricultural Research Organization, the Volcani Center, Bet Dagan, Israel.
Received for publication March 8, 2002.
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REFERENCES
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ABSTRACT
INTRODUCTION
