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Published online 13 July 2005
Published in Agron J 97:1202-1209 (2005)
DOI: 10.2134/agronj2004.0214
© 2005 American Society of Agronomy
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Production Papers

Irrigation Management of Peanut with a Moving Sprinkler System

Runoff, Yield, and Water Use Efficiency

Z. Plaut and M. Ben-Hur*

Inst. of Soils, Water & Environ. Sci., the Volcani Center, ARO, P.O. Box 6, Bet Dagan 50250, Israel

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

Received for publication August 12, 2004.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Irrigation of peanut (Arachis hypogaea L.) with a moving sprinkler irrigation system (MSIS) could affect runoff, evaporation, and crop yield. The objective was to determine the effects of irrigation management on soil water withdrawal, runoff, peanut yield and quality, and irrigation water use efficiency (WUE). Peanut was grown in a commercial field on a loess soil that is sensitive to seal formation and runoff. Irrigations were applied once every 3 or 7 d during the vegetative stage, and each of these treatments was subdivided into three irrigation-frequency treatments of 3, 7, and 10 d during the flowering and pod-filling stages. The total water applied in the various irrigation treatments, based on the soil moisture deficit up to field capacity, ranged from 575 to 648 mm. The lower the irrigation frequency, the smaller was the amount of water applied. During the vegetative stage, irrigation every 3 d led to faster coverage of the soil surface by the peanut canopy and reduced the amount of runoff by 60 mm compared with irrigation every 7 d. Pod yields in the various treatments ranged from 602 to 651 g m–2, but the differences were statistically insignificant. Decreasing the irrigation frequencies during the vegetative and pod-filling stages increased the WUE. Since peanut yield was insignificantly affected by the irrigation frequency, the enhanced WUE must have been due to reduced water losses during the irrigation season. The water saving through reduced runoff at the high irrigation frequencies may be canceled by higher losses through evaporation from the soil, interception by the canopy, and water removal by wind. One also has to consider the additional labor requirement and nonirrigating movements of the MSIS under high-frequency irrigation management; these are factors that detract from the advantages of the MSIS.

Abbreviations: DAP, days after planting • FC, field capacity • IR, infiltration rate • MSIS, moving sprinkler irrigation system • WUE, water use efficiency


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
PEANUT IS GROWN in many arid and semiarid regions during dry seasons and therefore needs irrigation to produce economic yields. However, the vegetative preflowering growth stage and the late stage of pod maturation have been shown to be insensitive to water stress (Rao et al., 1988; Meisner and Karnok, 1992; Reddy and Reddy, 1993). It was shown by Patel and Golakiya (1988) in India and by Black et al. (1985) and Stirling et al. (1989) under controlled environmental conditions that peanut was most sensitive to water stress during flowering and pod filling. This was also pointed out more recently by Reddy et al. (2003) in a review. The flowering rate was shown to depend on the frequency of water application and was reduced when water was applied less than once in 6 d (Ishag, 1982). Peg elongation depends on turgor and was shown to be delayed by drought (Boote and Ketring, 1990). Adequate soil moisture in the pod elongation zone is critical for peg penetration and the formation of pods (Reddy et al., 2003). Development of pods and kernels is progressively inhibited by drought because of lack of turgor and insufficient assimilates (Boote and Ketring, 1990; Meisner and Karnok, 1992). Other researchers (e.g., Pallas et al., 1979; Metochis, 1993), however, claim that proper soil moisture during the entire growing period is important for optimal production. In semiarid regions, optimal soil moisture content can be maintained by proper irrigation management.

Whereas the response of peanut, in its various growth stages, to soil moisture has been studied in some detail, irrigation management has hardly been examined. Irrigation frequency and the quantities of water to be applied, as well as the irrigation method, determine the depth of soil wetting, the horizontal water distribution, and periods of potential water stress. These are important factors, and their effects on production should be determined.

The self-propelled MSIS is an appropriate means for irrigating peanut as it distributes the applied water fairly uniformly. Additional advantages of the MSIS are high mobility, automation, large-scale coverage, ability to operate on relatively rough terrain, and low labor requirements (Kincaid et al., 1969), all of which has led to this system becoming increasingly popular in recent years. However, the MSIS is designed to apply large amounts of water within relatively short periods, which increases the likelihood of runoff during irrigation, particularly from soils with low infiltration rates (IRs). Ben-Hur et al. (1989) and Ben-Hur and Assouline (2002) indicated that irrigation with an MSIS in semiarid regions caused runoff mainly because of the formation of a seal at the soil surface. The impact energy of the water drops may destroy aggregates at the soil surface due to the splash effect and form a seal that reduces IR (Ben-Hur et al., 1989). Irrigation of a silt loam soil with an MSIS at an average application rate of 100 mm h–1 resulted in runoff of between 40 and 60% of the total water applied (Ben-Hur, 1994). The runoff may flow out of the field or may accumulate in small depressions within the field so that it is not distributed uniformly. The runoff may also increase soil erosion and cause depletion of fertilizers. The average peanut pod yield in a field in which runoff was prevented was 880 kg ha–1 higher than that in a field with unrestricted downhill runoff (Ben-Hur et al., 1995).

Mulching has been found to prevent the impact of drops so that seal formation was prevented, high IR maintained, and runoff reduced (Agassi et al., 1985). However, the canopy of a cotton (Gossypium hirsutum L.) crop, which was intended to serve as coverage of a loess-type soil surface, did not reduce the runoff percentage under weekly irrigations of 50 mm. The runoff was {approx}50% of the amount of water applied, both before and after coverage of the soil surface by the canopy (BenHur et al., 1989). It was suggested that this high runoff might be attributed to the large proportion of soil surface that was exposed to drop impact at an early growth stage before the canopy had developed. Thus, a seal was formed and runoff increased, and by the time the canopy had developed later in the season, it covered an already crusted soil; therefore, runoff remained high even under a complete canopy.

The canopy of peanut might serve as a more adequate mulch than that of cotton since it is much denser, more spread out, and closer to the soil surface. Moreover, to prevent runoff during irrigation with an MSIS, the soil surface should be covered early in the season, a requirement that could be achieved by acceleration of the growth of the plant canopy at the beginning of the irrigation season. It was previously shown that sealing of the soil surface was reduced when the intervals between applications were 2 to 3 d, and the amount of water applied per irrigation event was 15 to 30 mm (Ben-Hur et al., 1989): The runoff was only {approx}20% of the amount of water applied, whereas a runoff of 50% occurred when 50 to 60 mm of water was applied at intervals ≥ 7d (Ben-Hur et al., 1989).

Thus, we hypothesized that irrigation with small amounts of water at short intervals might prevent seal formation and runoff. At the same time, the use of short intervals could enhance the early development of the peanut canopy, which in turn could assist in protecting the integrity of the soil structure. However, such an irrigation regime necessitates high-speed movement of the MSIS, and many nonirrigating movements in the field, which results in inefficient use of the system. Moreover, water losses through evaporation are enhanced, and the small quantities of water per application could result in shallow wetting of the soil, which may, in turn, impair plant development and production. The objectives of the present study were (i) to examine how the runoff amount was affected by the MSIS irrigation management (i.e., quantity of water per application and the irrigation frequency) at the various developmental stages of peanut and (ii) to determine the effects of these irrigation management regimes on peanut production, crop quality, and irrigation WUE.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A field experiment was conducted during 1989 in the northwestern part of the Negev, Israel, where the average annual rainfall of 250 mm is confined to the winter, and there was no rainfall during this experiment. The soil was a silt loam loess (Calcic Haploxeralf), typical to the region. The soil texture is 190 g kg–1 clay, 330 g kg–1 silt, and 480 g kg–1 sand. The soil contained 125 g kg–1 CaCO3, its cation exchange capacity was 12.2 cmolec kg–1, and its exchangeable Na percentage was 3.7. The soil water content at a tension of 33 kPa (considered as field capacity, FC) was 0.255 m3 m–3. This soil is sensitive to seal formation, IR reduction, and runoff (Ben-Hur et al., 1985). The previous crop in the experimental field was rainfed wheat (Triticum aestivum L.), which utilized nearly all the soil-stored water to a depth of over 1.8 m; therefore, a preplanting irrigation was applied that brought the top 0.6 m of the soil to {approx}90% of FC.

The seed bed was prepared by deep plowing, disking, and loosening. The field was fertilized at plowing with superphosphate at an equivalent rate of 90 kg P ha–1, which resulted in a P concentration of {approx}10 mg kg–1 soil. Iron was applied as sequestrine, dissolved in the irrigation water at 0.15 kg ha–1 at planting and 2.5 kg ha–1 on each of three subsequent occasions in the course of the season. Nitrogen was not applied as the soil was inoculated with a rhizobia culture (10 units ha–1). Potassium was not needed as the soluble and exchangeable soil K content (determined by 0.1 M CaCl2 extract) was >70 mg kg–1 soil, which is more than sufficient for peanut.

Peanut (cv. Shulamit), for fresh consumption, was planted on 18 Apr. 1989, at an average in-row density of 8 plants m–1 and with two rows ({approx}0.9 m apart) per 1.92-m-wide bed. Water applications of 50 and 40 mm were applied over the entire field via sprinklers, 31 and 40 d after planting (DAP), respectively, to ensure uniform initial growth, and subsequent irrigations were applied with a linear MSIS, starting at 50 DAP (7 June 1989). The MSIS was equipped with Nelson spray nozzles (Walla Walla, WA, USA) spaced 1.92 m apart (i.e., one nozzle per bed) at alternating heights of 0.6 and 0.7 m above the soil surface. The lateral discharge of the MSIS was 650 L m–1 h–1, and the average application rate was {approx}100 mm h–1. This high application rate, which is very common for MSIS, has been applied to increase the irrigated area per unit of irrigation system. In this case, the irrigation with MSIS becomes feasible. The Christiansen uniformity coefficient of the water application of the MSIS was relatively high, 0.92 (Ben-Hur et al., 1995). The MSIS spans (75 m long) were oriented perpendicular to the plant rows and moved along the rows.

In the present study, the peanut growing season was divided into two main parts: (i) the vegetative stage, from planting until the beginning of flowering (0–82 DAP), and (ii) the pod-filling stage, which included flowering, pod formation, and filling and extended until the end of the growing season (82–140 DAP). Six different treatments (Table 1) were tested in this study: two irrigation frequencies of 3 and 7 d between applications were applied during the vegetative stage; and during the pod filling stage, each of these frequency treatments was subdivided into three treatments with frequencies of 3, 7, and 10 d between water applications. The quantities of irrigation water at each application were based on the soil moisture deficit up to FC to a depth of 1.5 m shortly before irrigation.


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  		Table 1. Treatment designations, intervals between water applications by the moving sprinkler irrigation system (MSIS), total and net (total water applied minus runoff) water applied, and residual water content in the 0- to 1.5-m soil depth at the end of the season. Numbers in parentheses are standard errors of the means.

 
Each experimental plot was nearly 12 m wide (six beds) by 30 m long and was irrigated via six spray nozzles. Only the two central beds were used for plant and soil water monitoring and yield determinations; the remaining rows served as borders. The experiment was designed as randomized blocks with four replicates for each treatment. The blocks were oriented perpendicularly to the plant rows, i.e., parallel to the MSIS spans. Since the various treatments were randomly selected in each block, every treatment in each block had to be irrigated separately, with the water outlets for the other treatments closed. The distance between adjacent blocks was 16 m, and the water outlets were opened and closed while the MSIS was located between the blocks.

The rate of soil surface coverage by the peanut canopy was determined by means of rulers, which were placed between adjacent rows, six rulers per plot in three of the four plots of each treatment. Three of the rulers in each plot were placed within beds and three between beds. The width of the shaded fractions of the rulers was determined once a week at midday, when the sun was at the zenith, and the percentage of the entire space between these rows that was filled with the shaded area was then calculated. The soil moisture content was recorded at two sites in every experimental plot, to a depth of 1.5 m at 0.3-m intervals, by means of a neutron probe device. Leaf water vapor conductances were determined with a LI-COR (Lincoln, NE) porometer. The presented values are averages of 20 measurements on different leaflets, which were the youngest fully expanded ones that were fully exposed to the sun.

Runoff amounts were estimated by means of special runoff subplots (Ben-Hur et al., 1995), which were constructed in three of the four blocks of Treatments 3-3, 3-10, 7-3, and 7-10, at the downhill part of each experimental plot. Each runoff plot was 3 m2 in area (2 by 1.5 m) and included a furrow in the middle with a half-bed on each side. The runoff amounts were measured in all the runoff plots after each irrigation event.

Peanut yields were determined on 15-m sections of the two central beds in each experimental plot. Plants were uprooted at 154 DAP (19 Aug. 1989) and trashed with a hand trash after air drying for 3 d. An additional light irrigation of 24 mm was applied to the entire field shortly before uprooting to improve the soil turnover and to minimize losses. Pods of all the uprooted plants were weighed, and samples were taken to determine pod weight on a basis of 105 g kg–1 moisture content on fresh weight basis. The shoot biomass was determined on oven-dried samples (65°C). The quality of peanut pod yield was evaluated by determining the percentage of the total pod yield that comprised jumbo-size pods, i.e., those larger than 10 mm in diameter and 25 mm in length.

All the treatments were conducted in four replicates. The data were subjected to analysis of variance (ANOVA; SAS Inst., 1995). Separation of means was subjected to Tukey's Honestly Significant Difference Test. A linear regression analysis was conducted to identify relationships between some measured parameters. In these cases, differences among the slopes and the intercepts of the regression lines were determined using Student's t test. All the tests were determined using a single confidence interval value at a significance level of p ≤ 0.05.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Water Application and Depletion
The total amounts of water applied, i.e., the sums of the preliminary sprinkler applications, the brief preharvest irrigation, and the MSIS irrigation events, are presented in Table 1. The lower the irrigation frequency, the smaller was the total amount of water applied: a total of 73 mm more water was applied in Treatment 3-3 than in 7-10, i.e., {approx}13% of the total water applied in the latter treatment. The differences between the treatments, in the amounts of water applied, could be a result of the effects of the irrigation frequencies on water uptake and water losses, as discussed below. Nearly all the soil water was consumed by the crop. In all the irrigation treatments, the residual amount of water in the entire soil profile (0 to 1.5 m) at the end of the growing season was only {approx}100 mm (Table 1), which is equivalent to the soil water content at the permanent wilting point.

The average values of daily water deficit in the soil profile for every {approx}10 d during the irrigation period are presented in Fig. 1 . This water deficit could be attributed mostly to the water use by the crop and evaporation, as we found no excess irrigation or deep percolation of water. At the beginning of the season, during the 10 d before the irrigation with the MSIS and the first 10 d of its operation (40 to 50 and 51 to 61 DAP), the water deficit rates were about 5 mm d–1 in the various plots. In contrast, the water deficit rates were slightly lower in the 7-3, 7-7, and 7-10 d irrigation treatments than in the 3-3, 3-7, and 3-10 d treatments between 62 and 71 DAP. This reduction became more marked during the period between 72 and 82 DAP (Fig. 1). During the pod-filling stage, the water deficit rates were in the descending order: 3-3 > 3-7 > 3-10 and 7-3 > 7-7 > 7-10. These differences became more marked during the later growing periods (123 to 132 DAP) than during 83 to 102 DAP (Fig. 1). The water deficit rate at >103 DAP was {approx}7 mm d–1 in Treatments 3-3 and 7-3 and {approx}6 mm d–1 in 3-10 and 7-10. The higher evaporation rates, expected with the more frequent irrigations, probably contributed to higher soil water deficits.



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  		Fig. 1. Daily averages of soil water deficits for various irrigation treatments and different numbers of days after planting. The water deficits are defined as the differences between soil water content at field capacity and the actual soil water content, as measured shortly before each irrigation event. Values are averages over about 10 d. The standard errors of the means are represented by vertical bars.

 
The relative soil water deficits in the various soil layers, down to a depth of 1.5 m, in the various irrigation treatments are presented in Fig. 2 . The relative water deficit of a given soil layer was calculated by expressing the difference between the water content at FC and the actual soil moisture content of this layer, as a fraction of the water deficit of the entire soil profile (0 to 1.5 m). The water deficits during 20 to 49 DAP, before establishment of the differential treatments with the MSIS, were similar in all treatments; about 90% of the deficits were contributed by the top 0.6-m layer. The top 0.3- and 0.6-m layers contributed {approx}40% and {approx}70%, respectively, of the total soil water withdrawn during the later part (51 to 81 DAP) of the vegetative stage. There was little water withdrawal from the deep 1.2- to 1.5-m soil layer throughout the entire irrigation season. In contrast, the water deficits of the 0.9- to 1.2-m soil layer were significant in all irrigation treatments and were higher during the pod-filling stage than the vegetative stage. From 112 to 140 DAP, these water deficits in the 3-3, 3-7, 3-10, 7-3, 7-7, and 7-10 treatments were 17.5, 19, 21, 21.5, 22.5, and 23.5%, respectively (Fig. 2).



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  		Fig. 2. Relative soil water deficits in different soil layers, irrigation treatments, and numbers of days after planting. The standard errors of the means are represented by vertical bars. DAP, days after planting.

 
Runoff and Soil Coverage by Plant Canopy
The cumulative runoff amounts for the 3-3, 3-10, 7-3, and 7-10 treatments are presented in Fig. 3 as functions of the irrigation date (DAP). No runoff was obtained during the early sprinkler irrigation, probably because of its low intensity. In contrast, starting the MSIS irrigation at a high intensity at 50 DAP caused considerable runoff (Fig. 3). Frequent water applications during the vegetative stage (Treatments 3-3 and 3-10) generated considerably less runoff than the lower-frequency applications (Treatments 7-3 and 7-10). Conversely, changing the irrigation intervals from 3 d to 10 d during the pod-filling stage did not affect the runoff amounts significantly. Therefore, one regression line was calculated for Treatments 3-3 and 3-10 and another for Treatments 7-3 and 7-10, and R2 values > 0.98 were obtained for both regression lines (Fig. 3). The slopes of these two regression lines differed significantly at p ≤ 0.05. Runoff in the intermediate-frequency (7 d) treatments (3-7 and 7-7) during the pod-filling stage was not determined, but it can be assumed that the runoff under Treatment 3-7 would be consistent with that under the Treatments 3-3 and 3-10 and that under 7-7 would be consistent with that under 7-3 and 7-10. The cumulative runoff amount for the entire season was 140 mm in Treatments 7-3 and 7-10 and 80 mm in Treatments 3-3 and 3-10 (Fig. 3). These runoff amounts were 26.8 and 30.4%, respectively, of the total water applied by the MSIS in the 7-3 and 7-10 treatments, and 15.0 and 15.8%, respectively, of that applied in the 3-3 and 3-10 treatments.



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  		Fig. 3. Cumulative amounts of water collected as runoff plotted against days after planting for Irrigation Treatments 3-3, 3-10, 7-3, and 7-10. The linear regression equations were calculated for Treatments 3-3 and 3-10 together and for treatments 7-3 and 7-10 together.

 
The percentages of the entire soil surface area that was covered by the peanut canopy are presented in Fig. 4 as functions of time. The canopy almost completely covered the soil surface in all treatments by 97 DAP. No significant differences in the rates of coverage were found among Treatments 3-3, 3-7, and 3-10 or among 7-3, 7-7, and 7-10. Therefore, differences were statistically tested only between treatments irrigated every 3 or 7 d during the vegetative stage so that each treatment contained 12 replicates. During this stage, the rates of coverage when the interval between water applications was 3 d were greater than those with a 7-d interval and especially so in the period until 77 DAP. It is obvious that hardly any effect of soil surface coverage could be expected during the pod-filling stage since the surface coverage in all treatments had already reached 85 to 90% at this time.



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  		Fig. 4. Changes of relative coverage of the soil surface by the peanut canopy as functions of days after planting. The measurements were conducted after the initiation of irrigation with the moving sprinkler irrigation system. The standard errors of the means are represented by vertical bars. Different letters at each measurement date indicate significant differences between the treatments irrigated every 3 d or every 7 d during the vegetative stage.

 
The runoff during the MSIS irrigation (Fig. 3) could have been generated by two main mechanisms: (i) a decrease in the hydraulic gradient of the soil because of the wetting during the irrigation (Hillel, 1980), which, in turn, would reduce the IR below the MSIS application rate and enhance runoff and (ii) seal formation driven by the water drop impact energy. Ben-Hur et al. (1989) concluded that the main runoff generation mechanism in loess soils during irrigation with an MSIS was seal formation. In the present study, at the beginning of the growing season, the peanut canopy was sparse, and most of the soil surface was bare (Fig. 4) so that water drop impacts were able to form a seal, which, in turn, decreased the IR and increased the runoff. The linear increase in the cumulative runoff during the period when the soil surface was almost fully covered probably occurred because by that time, the soil surface was already crusted so that the protective effect of the canopy was negligible. These results are in agreement with those of Ben-Hur et al. (1989) and Ben-Hur (1994) for various crops growing in loess under semiarid conditions.

The lower runoff amounts during the vegetative stage in the treatments irrigated every 3 d with {approx}25 mm of water per application than in those irrigated every 7 d may be attributed to two factors:

  1. Frequent water application to the top layer, where most of the roots were located during this growth stage, probably ensured a higher rate of canopy development. Early soil surface coverage by the canopy (Fig. 4) retarded the formation of the seal in Treatments 3-3 and 3-10 compared with that in Treatments 7-3 and 7-10, and the less developed seal allowed a higher IR and, consequently, less runoff.
  2. The surface storage capacity of the field was more effective in preventing runoff when the application in each irrigation event was 25 mm rather than 50 mm. However, since no significant changes in runoff occurred when the water application rate was increased from 25 to {approx}50 mm per irrigation event during the pod-filling stage (Fig. 3), it can be concluded that the effect of this factor on runoff was minor.

Crop Yield
The yield parameters of the peanut in the various irrigation treatments are presented in Table 2. The pod yields ranged from 602 g m–2 in Treatment 3-10 to 651 g m–2 in Treatment 7-10 (Table 2), but these differences were not significant. The yields were similar to those previously obtained by Ben-Hur et al. (1995) under similar conditions. Shoot dried biomass ranged from 641 to 756 g m–2, but only in Treatment 7-7 was the yield significantly higher (p ≤ 0.05) than in most of the other treatments.


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  		Table 2. Yields of peanut pods, dry shoot biomass, jumbo-size pods as percentages of total pods, and pod/shoot ratios for various irrigation treatments. Numbers in parentheses are standard errors of the means.

 
The percentages of jumbo-size pods, based on numbers and on weights, are presented in Table 2. Decreases in irrigation frequency, especially during the pod-filling stage, generally led to increases in the percentage of jumbo pods, and this effect was more noticeable when the percentages were based on weights rather than on numbers. It is probable that a deep root system had already developed at this stage so that the maintenance of a wet surface layer did not contribute to pod development and filling. It can be noticed that the highest percentage of jumbo pods was obtained in Treatment 7-7 and not in 7-10, probably because the 10-d interval might have caused slight water stress during the pod-filling stage, as may be concluded from Fig. 5 .



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  		Fig. 5. (A) Leaf water vapor conductance during one irrigation cycle of the treatment irrigated at 11-d intervals during pod filling. The standard errors of the means are represented by vertical bars. (B) Diurnal pattern of leaf water vapor conductance on the 10th day after previous irrigation in Treatment 7-10. Water was applied according to schedule. The standard errors of the means are represented by vertical bars.

 
It seems that pods that developed in the deeper layers might be exposed to less water stress than those in the upper layers and would therefore be larger. The importance of a water supply that is sufficient to prevent water stress during flowering and pod filling was outlined by several investigators (e.g., Meisner and Karnok, 1992; Reddy and Reddy, 1993; Reddy et al., 2003). Deep soil wetting may enhance the availability of nutrients located in the deeper layers; it is also likely that pods that develop in these layers can absorb water directly from the soil and thus grow faster, but no information is available on such direct absorption of water by developing pods. However, it was found that potato (Solanum tuberosum L.) tubers, which also develop within the soil, can absorb water directly from the soil and not only via their conducting elements (Svensson, 1977; Shimshi and Susnoschi, 1985; Weber, 1990; Plaut et al., 1994). It is also interesting that a remarkable increase in peanut pod yield was obtained in response to subsurface drip irrigation (Sorensen et al., 2001), which probably ensured a high soil water content in the vicinity of the developing pods. In several other studies, peanut production was reduced under water stress that was imposed by reducing the quantity of water per application, which must have led to shallower wetting of the soil profile (Metochis, 1993).

Leaf Water Vapor Conductance
Plant water stress during the critical period of pod filling (mid-July through August) was determined by measuring leaf water vapor conductance (Fig. 5). In plants irrigated at 10-d intervals, the conductance did not change during the first 7 d after irrigation and was similar to that of plants that were irrigated at 3-d intervals. A significant drop in conductance was found only after 9 d, and it became very large at 11 d after the previous irrigation (Fig. 5A). This response can also be seen in the diurnal pattern at 1400 h (Fig. 5B), whereas at the same time, the conductance of plants irrigated at the intermediate frequency (Treatment 7-7) was not statistically significantly different from that of those irrigated at the highest frequency (Treatment 7-3). No significant decrease in conductance could be seen earlier in the day even in Treatment 7-10. The decrease in conductance at 1700 h, which was found in all the treatments, was probably due to the decrease in radiation intensity at that time. It thus seems that although plants that were irrigated at intervals of 10 or 12 d during pod filling were subjected to water stress toward the end of the irrigation cycle, this only occurred during part of the day. This decrease in conductance was not severe enough to cause a decrease in pod yield, but it was probably the reason for the decrease in pod size.

Irrigation Water Use Efficiency
Irrigation WUE was defined as the ratio between crop yield and water applied. It was calculated on the basis of both total water applied (WUEt) and net water applied (WUEn) according to Eq. [1] and [2], respectively:

[1]

[2]
where PY is pod yield in g m–2, TWA is total water applied in mm, and NWA is net water applied in mm, as defined in Eq. [3]:

[3]
where RA is runoff amount in mm. The RA values for Treatments 3-7 and 7-7, which were not measured, were calculated by using the corresponding regression equations presented in Fig. 3. The calculated values of NWA for the various irrigation treatments are presented in Table 1.

Water use efficiency values based on total (circle symbols) and on net water applied (triangle symbols) are presented in Fig. 6 , as functions of the intervals in days between irrigation events during the pod-filling stage. Significant positive linear regressions (R2 > 0.97) were found for both, irrigation every 3 or 7 d during the vegetative stage and for WUEt and WUEn. Since pod yield was hardly affected by the irrigation treatments (Table 2), the significant differences between these regression lines must be a result of the differences in water losses between the various irrigation treatments. These water losses could be caused mainly by evaporation from the soil, interception by the canopy, and water removal by wind and runoff.



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  		Fig. 6. Water use efficiency values based on total and on net water applied, as functions of the intervals in days between irrigation events during the pod-filling stage. Different lower- and uppercase letters near the regression lines indicate significant differences between the y-intercepts and the slopes, respectively, of the various regression lines.

 
The plot of WUEn against the intervals between irrigation events during the pod-filling stage eliminated the effect of water lost by runoff on the WUE. Therefore, in this case, the higher WUEn values of the regression line for 7-d intervals than for 3 d during the vegetative stage (Fig. 6) indicates that the lower water loss with 7-d than with 3-d intervals was caused by lower evaporation from the soil and from the plant canopy and lower water removal by wind with the former than with the latter irrigation interval. These differences between the regression lines corresponding to the two respective irrigation intervals were less noticeable when WUEt values were considered. This is because of the greater amount of water lost by runoff with the 7-d than with the 3-d interval during the vegetative stage (Fig. 3). It can be concluded from these results (Fig. 6) that when the water losses during the irrigation are considered, the irrigation WUE increased as the intervals between the water application increased, up to 7 d during the vegetative stage and up to 10 d during the pod-filling stage.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The irrigation management regime applied to peanut growing in seal-forming soils and receiving high-intensity irrigation via an MSIS can lead to significant losses of water through runoff. Use of a high frequency of water applications (intervals of 3 or 4 d), with appropriately small quantities of water (≤25 mm per application) during early vegetative growth, will reduce runoff significantly. This may be either because the storage capacity of the soil surface is limited or because seal formation is mitigated by the enhanced coverage of the soil surface by the crop canopy. Application of larger amounts of water at longer intervals (7 d) will result in higher losses of water through runoff. The amount of runoff was not much affected by changing the irrigation frequency during later development stages, e.g., during pod filling, because by that time, the peanut canopy covered soil that was already sealed, and therefore the protective effect of the crop canopy was negligible.

Water is required from the deeper soil layers mainly during the pod-filling stage, and the larger amounts of water per application when applied at intervals of 7 d than at intervals of 3 d will ensure wetting to a sufficient depth. Although we did not find a statistically significant (p ≤ 0.05) increase in pod yield at 7-d intervals between irrigations, there was an increase in pod size, which is of economic value. A longer interval, e.g., 10 d, may lead to slight water stress and should be avoided.

It is suggested that the water saved through reduced runoff at the high irrigation frequencies may be balanced by the higher losses through evaporation from the soil, interception by the canopy, and water removal by wind. Thus, the WUE increased as the intervals between the water applications increased to 7 d during the vegetative stage and to 10 d during the pod-filling stage. It is also necessary to take account of the additional labor requirement and the nonirrigating movements of the MSIS under high-frequency irrigation management, factors that detract from the advantages of the MSIS.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Contribution from the Agricultural Research Organization, the Volcani Center, 612/04, 2004 series.


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





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