Soil Science Society of America Journal 66:228-235 (2002)
© 2002 Soil Science Society of America
DIVISION S-6 - SOIL & WATER MANAGEMENT & CONSERVATION
Microdrip Irrigation of Field Crops
Effect on Yield, Water Uptake, and Drainage in Sweet Corn
S. Assouline*,
S. Cohen,
D. Meerbach,
T. Harodi and
M. Rosner
Dep. of Environmental Physics and Irrigation, Institute of Soil, Water and Environmental Sciences, Volcani Center, ARO, Bet Dagan 50250, Israel
* Corresponding author (vwshmuel{at}agri.gov.il)
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ABSTRACT
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Microdrip irrigation supplies water at a rate close to that of plant water uptake. It is thus expected to improve yields and reduce water losses from drainage below the root zone. To test this assumption, four drip irrigation treatments were applied on corn (Zea mays L.) at Bet Dagan, Israel: (i) daily irrigation with 2 L h-1 emitters (2-D); (ii) twice-weekly irrigation with 2 L h-1 emitters (2-TW); (iii) weekly irrigation with 2 L h-1 emitters (2-W); (iv) daily microdrip irrigation with 0.25 L h-1 emitters (0.25-D). Total irrigation was similar for all treatments. Soil water content depth distribution was monitored by the neutron scattering method. Total water uptake (sap flow, SF) was measured using the heat pulse method for two consecutive periods of 2 wk. Relative drying of the 0.60- to 0.90-m soil layer was observed only in the microdrip treatment. These might indicate different root structure or water uptake patterns because of the low application rate. Highest estimated drainage fluxes were obtained for the 2-D treatment and lowest for the 0.25-D treatment, especially at the end of the growing period. The SF/ETp ratio was the steadiest for the 0.25-D treatment. The relationship of canopy conductance to vapor pressure difference for 0.25-D was not different from those of 2-D and 2-TW. Lower conductance values were obtained for 2-W at the end of the irrigation cycle. Daily irrigation led to the highest yield, while weekly irrigation led to the lowest. The results indicate that microdrip irrigation might improve yields and reduce water losses from drainage below the root zone.
Abbreviations: 0.25-D, daily microdrip irrigation with 0.25 L h-1 emitters • 2-D, daily irrigation with 2 L h-1 emitters, 2-TW, twice-weekly irrigation with 2 L h-1 emitters • 2-W, weekly irrigation with 2 L h-1 • ETp, potential evapotranpiration • SF, sap flow • VPD, vapor pressure deficit
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INTRODUCTION
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SOCIOECONOMIC PRESSURES for increased agricultural production have led farmers to apply water and fertilizers in excess of crop needs, leading to leaching of water and nutrients below the root zone. An increasing number of countries are concerned by two major problems: the reduction of high-quality water resources allocated to agriculture, and the increasing groundwater contamination, especially by nitrates, which apparently stems from agricultural activities (Addiscott, 1996; Hadas et al., 1999). Therefore, two goals of modern irrigation are increasing the water use efficiency of production systems to save water, and limiting leaching to reduce groundwater pollution hazards. Drip irrigation is an acknowledged technique for achieving high efficiencies in water use of crops by wetting only a limited part of the root zone (Bresler et al., 1982). Several authors have shown that efficiencies might be improved without affecting crop yield by decreasing the amount of water leached from the root zone through more optimal drip irrigation management (Feigin et al., 1982; Hoffman et al., 1984; Darusman et al., 1997).
Minimizing nutrient leaching can be achieved by matching water and fertilizer application rates to plant uptake rates. Soil water potential and water content in the vicinity of active roots generally controls the rate of water and nutrient uptake by plants. In drip irrigation, frequency and emitter discharge determine the variation in soil water potential, and consequently, root distribution and plant water uptake patterns (Coelho and Or, 1996, 1999). Drip irrigation systems generally consist of emitters that have discharge rates varying from 2 to 8 L h-1. In semiarid summer climates crop water use is generally 6 to 8 mm d-1 (Shalhevet et al., 1981), and water is supplied two or three times a week. Therefore the duration of water application is much shorter than the time over which plants take up water. Even if the water is supplied on a daily basis, a water application rate of 2 L h-1 delivers the consumptive needs of plants in a small fraction of the time over which plants photosynthesize and transpire. This means that even for water applications exactly equal to plant water needs, part of the water may not be used by the plant and would most likely drain below the root zone. Lowering emitter discharge rates to as close as possible to plant water uptake rates may reduce these percolation losses.
Recently, microdrip systems have been developed that provide emitter discharge rates lower than 0.5 L h-1. Also, these low rates can be obtained by using low-pressure heads, thus opening the opportunity of low-cost drip irrigation in small holdings (Gilead, 1985). In this regard, some field observations have been made concerning the uniformity of application (Miller, 1990) and irrigation efficiency (Batchelor et al., 1996). However, microdrip systems have been studied most intensively in greenhouses (Koenig, 1997). Preliminary results have shown that they can reduce water consumption of tomato (Lycoperiscon esculentum Mill.) plants by 38%, increase yield by 14 to 26%, and reduce the leaching fraction from 40 to 10% (Ein-Tal Ltd., unpublished data, 1996). To our knowledge, microdrip irrigation has not been studied under field conditions. The objective of this study was to compare the effects on yield, soil water distribution, water uptake, and drainage of microdrip, and conventional drip irrigation of sweet corn.
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MATERIALS AND METHODS
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Corn (cv. Jubilee) was grown in an experimental field at Bet Dagan, Central Israel, from 7 June to 16 Aug. 1999, using recommended agricultural practices for the region (Extension service, 1989). The soil at the site is a sandy loam (Typic Rhodoxeralf). The field was plowed, disked, and leveled with a roller to provide a smooth seedbed. Presowing management included 150 mm applied by sprinkler, application of 100, 20, and 130 kg ha-1 N, P, and K, respectively, and weeds control. Sowing was on 7 June, at 10 seeds m-2, with a distance of 1.0 m between rows. The field had a fairly constant slope of 2% and the rows were planted parallel to the slope direction. To ensure full germination, 65 mm was applied by sprinkler irrigation at sowing and an additional irrigation of 40 mm was applied 9 d later for seedling establishment. The experiment included four drip irrigation treatments, employing one drip line per plant row: (i) daily irrigation with 2 L h-1 emitters (2-D); (ii) twice-weekly irrigation with 2 L h-1 emitters (2-TW) (commonly applied in the region); (iii) weekly irrigation with 2 L h-1 emitters (2-W); (iv) daily microdrip irrigation with 0.25 L h-1 emitters (0.25-D). The microdrip lines were fed at both ends to improve emitter discharge uniformity. Emitter spacing was every 0.5 m for the 2 L h-1 discharge (Tiran 16, Netafim, Israel) and every 0.25 m for the 0.25 L h-1 discharge (MicroTal, Ein-Tal, Ltd, Israel). Drip irrigation began on 21 June.
Ten plots were laid out randomly. Each treatment using the 2 L h-1 emitter was applied to two plots. Since the field size was limited with relatively small plots, the 0.25 L h-1 treatment was allocated 4 plots to increase the total discharge of the treatment and the accuracy of the control flow meters. Each plot was (4.0 by 10 m) and included four plant rows. Only the two central rows were used for sampling. There was no rainfall during the experiment. Total irrigation during the growing season was 555 mm with 300 mm applied through the drip system for all treatments. The irrigation applied through the drip system was scheduled using a combined pan evaporation and growth stage based crop coefficient, according to recommended irrigation practice for the region (Extension Service, 1989). An equal total amount of 110 kg ha-1 N, was applied to all treatments in six weekly applications, as in line fertigation.
During the growing season, the soil water content distribution with depth, 
(Z), was determined to a depth of 1.65 m at 0.30-m intervals by the neutron scattering method (503 DR Hydroprobe, CPN Co., Martinez, CA). Access tubes were installed in one of the two central plant rows of each plot. Measurements of soil water content distribution with depth were made in the morning before water application, to provide a comparison between the different treatments.
Drainage below the root zone at the plant row was evaluated using the water contents, 1 and 2, measured at depths Z1 = -1.05 m and Z2 = -1.35 m. The water retention curve, (
), relating the water content, , to the capillary head, , at each depth, was based on data from Friedman and Meiri (1996), obtained for the same field. The retention curve was expressed using the model of Assouline et al. (1998)(2000):
where Se is the saturation degree, s, the saturated water content, r, the residual water content, r, the capillary head at r, and 
and µ, fitting parameters. The values used in Eq. [1] for s, r, r, , and µ are 0.46 m3 m-3, 0.05 m3 m-3, -149.5 MPa, 0.459 MPa, and 1.482, respectively. Comparisons of measured water content values with those determined from tensiometers readings and Eq. [1] have indicated that this relationship accurately describes the soil water retention curve. The hydraulic conductivity function, K(), relating the unsaturated soil hydraulic conductivity, K, to the water content was evaluated using the model of Assouline (2001):
 | [2] |
where Ks is the saturated soil hydraulic conductivity, and 
, a parameter related to the statistical characteristics of the water retention curve expressed in terms of Eq. [1]. Russo et al. (1997) found that the log-normal distribution represented the spatial variability of Ks in the experimental plots, with a mean of -0.636 and a variance of 1.242 for ln(Ks), Ks in cm h-1. Therefore the mean value of Ks= 0.235 m d-1 was applied in Eq. [2]. The value of corresponding to the () curve is 1.764. Using Eq. [1], the measured water contents 1 and 2 were transformed into their corresponding capillary heads, 1 and 2. Then the drainage flux, qD, was estimated:
 | [3] |
where KM is the harmonic mean of the hydraulic conductivity values K(1) and K(2) within the 
Z interval.
Plant water uptake (SF) was measured during the grain growth period for two consecutive periods of 2 wk each using the heat-pulse method (Cohen et al., 1988) calibrated for corn plants (Cohen and Li, 1996). The heat-pulse sensors were installed at the base of 10 successive plants in one of the central rows. Heat-pulse control, data collections, and processing were carried out using a data logger (CR7, Campbell Sci., Logan, UT) every 15 min. From 17 to 31 July, the measurements were carried out in the 0.25-D and 2-W plots, and from 2 to 13 August, in the 2-D and 2-TW plots. During both periods, an automatic weather station in the experimental field measured solar radiation, air temperature and humidity, and wind speed every 15 min. These were used to calculate hourly ETp, according to the modified Penman equation (Doorenbos and Pruitt, 1975).
Harvest was on 16 August. Crop yield was determined for each plot at the end of the growing season. In each plot, the plants in 1.0-m length from each central row were cut by hand and the dry matter weights of the ears including seeds, cob, and husk, and of the remaining stover, determined after 72 h of drying at 60°C.
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RESULTS AND DISCUSSION
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Yield
The microdrip irrigation had the highest yield, both in terms of ears and stover (Fig. 1)
. The lowest performance was obtained by the 2-W treatment. Statistical analysis using the General Linear Models procedure (GLM) (SAS Inst., 1982) showed that the effect of the treatments on ears yield was highly significant 
. The irrigation effect on the stover yield was much less significant 
, since there was no significant effect on green matter production (stover yield–ears yield). Duncan's Multiple Range Test (DMRT) showed that the ears yield was significantly higher for the two daily irrigation treatments than for the 2-W, while the stover yield for daily microdrip irrigation was higher than for weekly irrigation (Fig. 1). High-frequency drip irrigation has also been found to increase potato (Solanum tuberosum L.) yield (Phene and Sanders, 1976). Therefore, although it is not statistically significant in the specific experimental conditions applied, the trend obtained indicates that daily microdrip irrigation might improve yields, and therefore, water use efficiency. This point should be explored further with respect to soil and crops properties.
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Fig. 1. Corn yield (dry matter) for different irrigation methods. Letters indicate results from Duncan's multiple range test. Bars represent one standard deviation.
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Water Content Distribution with Depth
Water content distributions with depth on 11 July and 8 August, corresponding to the end of the vegetative and grain growth periods, respectively, are expressed relative to the corresponding values at the beginning of the drip irrigation period to eliminate noise because of different local initial conditions and soil properties, and to emphasize the net effect of the different water application rates (Fig. 2)
. At the end of the vegetative growth period (Fig. 2a), after 3 wk of drip irrigation, the differences between the relative (Z) values were still small. The overall trend was that the upper 0.80-m layer was drier than at the beginning of drip irrigation, with increasing dryness towards the soil surface. This might indicate that water uptake extended to that depth. Below that depth, the water content remained practically unchanged. The main noticeable differences between the treatments were that (i) the upper soil layer dried less in the 2-TW treatment; and (ii) the soil below the root zone was wetter in the 2-W irrigation, probably because of excess drainage. By the end of the season (Fig. 2b), two types of relative (Z) distributions had evolved from the irrigation practices. The distributions of the 2-D and the 2-W had a monotonic increase of the relative water content with depth. For the 2-D, relative increased rapidly towards the value of 1.0 at depth 0.75 m, and remained practically constant below that depth. The (Z) distribution of the 2-W also increased rapidly towards the value of 1.0 at depth 0.45 m, remained constant until 1.05-m depth, after which it increased rapidly again. The distributions of the 2-TW and the 0.25-D show a decrease of relative water content to 0.75-m depth and an increase below it. The 0.25-D distribution show significant relative drying of the 0.60 to 0.90-m soil layer, which may indicate different root structure or water uptake pattern resulting from the low application rate. In drip irrigation, root length density, and water uptake distributions with depth are related to the shape of the specific wetted volume generated by the position of the emitter and the irrigation frequency (Phene et al., 1991; Coelho and Or, 1999). By extension, one can assume that they will be also related to the emitter discharge as it also determines the shape of the wetted volume. In fact, visual observations on plants from the experimental plots indicated that the root system developed under the 0.25-D treatment was shallower and denser (higher density of root hairs) than under the 2-W treatment.
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Fig. 2. Relative water content distribution with depth for treatments at the end of the (a) vegetative growth period; (b) grain growth period. Bars represent range between minimum and maximum values.
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As shown by Al-Khalaf et al. (1989), the root system of corn ceases to grow after tasseling. Therefore, we can consider that the results after 18 July correspond to a constant root system in each treatment. At 0.45-m depth (Fig. 3a)
, for all treatments, relative drying of the soil was observed until 25 July, 7 wk after sowing, followed by rewetting to initial values. A similar pattern of lower amplitude was observed at 0.75-m depth for all treatments except 0.25-D (Fig. 3b). For the latter, this layer continued to dry even after rewetting began in the other treatments. This may be related to the differences in water uptake patterns of the shallower and denser root system induced by the low application rate in the microdrip treatment. At 1.35-m depth (Fig. 3c), the relative water content remained practically constant during the whole period except for the 2-W treatment, where it increased both at the beginning and at the end of the irrigation period.
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Fig. 3. Dynamic changes in relative water content during the irrigation period for treatments at depths of (a) 0.45 m; (b) 0.75 m; (c) 1.35 m. Bars represent range between minimum and maximum values.
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Drainage
The 2-D irrigation led to fluxes 50% higher than in the other treatments (Fig. 4)
. The estimated differences in the drainage fluxes are larger than the ones resulting from the 5% error associated with the measurement of using the neutron scattering method (bars). For the remaining three treatments, similar fluxes are obtained, although it seems that at the end of the growing season, the 0.25-D treatment led to 10 to 15% lower drainage fluxes, indicating that some differences in the plant water uptake pattern might have occurred compared with the other treatments.
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Fig. 4. Estimated drainage fluxes below the root zone for treatments during the irrigation period. Bars represent the error resulting from the 5% error in the measurement of using the neutron scattering method.
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These estimates are based on the same hydraulic functions for all the plots, and do not account for the spatial variability that characterizes the experimental field (Russo et al., 1997). In addition, even for a homogeneous soil, drainage fluxes at a given depth under drip irrigation vary both in space and time, according to line and emitter spacing, emitter discharge, amount of applied water, and irrigation frequency. While drainage flux estimates based on monitoring water content distribution at one location on a weekly basis cannot lead to any conclusion regarding the effect of the treatments on the total water lost by drainage over the whole field and during the whole season, the data suggests that microdrip irrigation may have decreased drainage compared with other treatments.
Plant Water Uptake
Water uptake, taken as integrated daily SF measurements were made after the vegetative growth period. Because of equipment constraints, they were carried out during two consecutive intervals of 2 wk for each pair of treatments. The periods were between 17 and 31 July for the 0.25-D and 2-W pair, and between 2 and 13 August for the 2-D and 2-TW pair. To compare the effect of the irrigation methods on plant water uptake, the ratios between the measured SF and reference ETp, during the two periods were calculated (Fig. 5)
. For the 2-TW treatment, the SF/ETp ratio oscillated around 1.0, and the frequency was similar to that of the irrigation. The amplitude of the variations in the 2-D treatment was larger. In both treatments, a peak was observed on Day 10, a relatively cloudy day when the daily ETp was significantly lower than the mean ETp of the whole period (5.8 mm d-1 versus 7.1 mm d-1). In the weekly irrigation treatment (2-W) the ratio was consistently below 1.0, approaching this value during the first 4 d after irrigation, but decreasing sharply 5 d after irrigation and afterwards in both weekly irrigation cycles during the measurement period. In contrast, the ratio for the microdrip irrigation fluctuated least. During the first 4 d of measurement, values were below 1.0, and after that time, SF/ETp exceeded 1.0 and was practically constant for the remainder of the measurement period.
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Fig. 5. Ratio between daily sap flow (SF) and estimated evapotranspiration (ETp) during 2-wk measurement period for (a) the 2-D and 2-TW pair; and (b) the 0.25-D and 2-W pair. Arrows indicate irrigation days for nondaily irrigation treatments.
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The daily course of plant water uptake is shown in Fig. 6
, for two representative days, 4 August for the 2-D and 2-TW pair, and 25 July for the 0.25-D and 2-W pair. For the 2-TW and the 2-W treatments, these days are at the end of an irrigation cycle, and represent the driest conditions measured. Hourly ETp values are given for comparison. For the 2-D and the 2-TW treatments (Fig. 6a), the main differences occurred between 1100 h and 1600 h, where SF for 2-TW was higher than that for 2-D. Compared with the ETp, SF for 2-D was higher until 1100 h, then approximately the same until 1500 h and then lower, while SF for 2-TW was higher until 1500 h and equal to ETp afterwards. Sap flow for 0.25-D (Fig. 6b) was similar to that observed for 2-TW, exceeding ETp for most of the day. Sap flow for 2-W demonstrates the reduction of transpiration that occurs when soil moisture is limiting. Sap flow for 2-W lagged behind that of 0.25-D and ETp in the morning. Peaks were observed late in the morning, followed by a gradual decrease in the afternoon. Late in the day, when SF of 0.25-D and ETp approached zero, SF of 2-W was still significant, indicating that plant water uptake continued after sundown to satisfy the remaining water deficit. These results were expected, since they were made at the end of the longest redistribution and evapotranspiration interval.
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Fig. 6. Representative hourly distributions of sap flow (SF) and evapotranspiration (ETp) for (a) the 2-D and 2-DW pair; (b) the 0.25-D and 2-W) pair. For nondaily irrigation treatments, the day presented precedes water application. The arrow indicates solar noon. Bars indicate one standard error of the mean.
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Canopy conductance expresses stomatal and boundary layer conductance of leaves, together with the aerodynamic conductance of the canopy. It was evaluated, assuming leaf temperature equal to air temperature (Granier et al., 1996), using the expression:
 | [4] |
where G is canopy conductance (m s-1), 
is latent heat of vaporization of water (J kg-1), 
is the psychometric constant (Pa °C-1), 
is the density of dry air (kg m-3), and Cp is the specific heat of dry air at constant pressure (J kg-1 °C-1). Vapor pressure deficit (VPD) is defined as [es(Ta) - e], where es(Ta) is saturation water vapor pressure at air temperature (Ta), and e is actual vapor pressure determined from air temperature and humidity. When boundary layer and aerodynamic conductance are large, the canopy conductance is directly related to leaf conductance. In that case, which can be shown to be true for our conditions, canopy conductance is related to the same environmental parameters as leaf conductance, i.e., radiation, temperature, air to leaf vapor pressure difference, and water status. The relationship of canopy conductance to VPD (taken as an estimate of leaf to air vapor pressure difference) has been shown to explain much of the variance observed in canopy conductance for mid day climate when coupling is high. The relationship between hourly values of canopy conductance, G, for the daytime interval between 1100 h and 1500 h, and VPD, was evaluated using the expression (Jones, 1992):
 | [5] |
Gmax (m h-1) being the maximal conductance, and k (Pa-1) a constant. The linear form of the model,
 | [6] |
was analyzed using linear regression. As expected, G decreased with VPD in all the treatments. Differences between 2-D and 2-TW were very small (Fig. 7)
. The lower regression line for 0.25-D was not significantly different from those of 2-D and 2-TW (Table 1). For the 2-W treatment, G was similar to that of the other treatments for 2 d following irrigation, after which it decreased gradually until the following irrigation, causing relative variance of k for 2-W to be more than twice that for the other treatments. This indicates that at least leaf conductance was able to recover fully from the few days of water stress preceding irrigation.
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Fig. 7. Fitted exponential relationship between canopy conductivity and vapor pressure deficit for daytime interval between 1100 h and 1500 h.
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Table 1. Regression results for the relationship between canopy conductance and water vapor pressure deficit (Eq. [6]). S.E. represents standard error.
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The results indicate that microdrip (0.25-D) did not change the response of canopy conductance to environment, and maintained the plant in conditions as good as those of the 2-D and 2-TW treatments. This is in agreement with the results of ears and stover yields (Fig. 1).
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CONCLUSIONS
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Microdrip irrigation supplies water at a low rate during nearly all the time the plant needs it. When compared with conventional drip irrigation methods using higher dripper discharge, the low application rate mostly affects the water content distribution with depth. The main effects were observed in two domains: (i) in time, during the grain growth period, after the root system was fully developed; and (ii) in depth, at the 0.60- to 0.90-m depths soil layer, which continued to dry while rewetting was observed in all the other treatments. This may have been the result of differences in root distribution or plant water uptake pattern. Visual observations indicate that under microdrip irrigation, the root system was shallower and denser than under weekly irrigation. Root distribution differences should be explored further and quantified.
The highest drainage fluxes below the root zone were estimated for the 2-D irrigation. The fluxes for the remaining treatments were similar and 30% lower than for the 2-D. However, no conclusion could be drawn yet regarding the effect of microdrip on the total water lost by drainage over the field and during the entire season, although it seems that it may decrease drainage compared with other treatments.
Water uptake, taken as integrated daily SF, and the relationship of canopy conductance to VPD (taken as an estimate of leaf to air vapor pressure difference), indicate that microdrip maintained the plants in conditions as good as those of the 2-D and 2-TW treatments. The corresponding SF/ETp ratio presented the steadiest behavior, remaining practically constant and above 1.0 during all of the second week of the measurement interval. This may indicate that microdrip has some beneficial effect on plant water uptake.
The overall result was that daily irrigation led to the highest yield, both in terms of stover and ears, with the lowest performance being obtained from the weekly irrigation. Howell et al. (1997) reported that the yield of corn grown on a Pullman clay loam soil with weekly irrigation did not differ from corn yields obtained with daily irrigation. The lower performance of the weekly irrigation in the current study might be attributed to the relatively low water retention capacity of the sandy loam soil used in the experiment, compared with that of the clay loam soil in the experiment of Howell et al. (1997).
Another important aspect that was not studied here but that might have some positive influence on yield is the effect of microdrip on nutrient availability. Low rate irrigation may affect soil aeration, pH values, and transport rates of ions to root by mass flow and diffusion. This aspect should be the subject of further investigation of the effect of microdrip on plant–soil relationships.
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ACKNOWLEDGMENTS
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We are grateful to M. Shapira (Toki) from Ein-Tal Micro-irrigation Ltd. for providing the microdrip equipment and to Yen Li, J. Sostaric and A. Grava for technical assistance. We thank F.J. Adamsen for his insightful and constructive comments.
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NOTES
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Contribution no. 630/00 from the Agricultural Research Organization.
Received for publication May 22, 2000.
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ABSTRACT
INTRODUCTION
