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DIVISION S-6—SOIL & WATER MANAGEMENT & CONSERVATION

The Effects of Microdrip and Conventional Drip Irrigation on Water Distribution and Uptake

Dep. of Environmental Physics and Irrigation, Institute of Soil, Water and Environmental Sciences, A.R.O.—Volcani Center, Bet Dagan 50250, Israel

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

	ABSTRACT

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

 
Drip irrigation at a rate close to plant water uptake necessitates low application rates (microdrip), which affect soil water regime and plant response. This study compare the effect of three emitter discharges, 0.25, 2.0, and 8.0 L h^-1, on different aspects of the water regime in daily drip irrigated corn (Zea Mays L.), relying on field observations and numerical simulations. Field observations show that microdrip irrigation (i) tends to increase yield although this was not statistically significant under the experimental conditions; (ii) induces higher relative water content values in the 0- to 0.30-m depth layer, and lower ones in the 0.60- to 0.90-m layer. Numerical simulations, carried out using HYDRUS-2D, show that microdrip irrigation led to the smallest wetted volume with the least extreme water content gradients both in the horizontal and the vertical axes. A saturated zone below the emitter was obtained only for the 8.0 L h^-1 discharge. Comparing water content distributions with depth at the end of the application of the same amount of water close to the emitter, the driest profile is obtained for the lowest application rate. However, when compared at solar noon representing the time of the highest plant water demand, the upper 0.20-m layer of the soil irrigated at the lowest rate was the wettest. Microdrip irrigation has reduced the dynamic changes in the water content during the day in the most active part of the root zone. It also caused the smallest reduction in the simulated water uptake relative to the potential value.

	INTRODUCTION

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

 
DURING THE POSTPONDING PHASE of one-dimensional infiltration, such as under rainfall or sprinkler irrigation conditions, the infiltration rates and their corresponding water content distributions with depth are independent of the water application rate and are unique functions of the infiltrated amount of water (Smith, 1990; Mualem and Assouline, 1996). Under drip irrigation, the ponding zone that develops around the emitter is strongly related to both the application rate and the soil properties, and the uniqueness between the water content distribution in the wetted soil volume and the infiltrated water amount is no longer valid. Consequently, the water application rate is one of the factors, which determine the soil moisture regime around the emitter (Brandt et al., 1971; Bresler, 1978), and the related root distribution and plant water uptake patterns (Phene et al., 1991; Coelho and Or, 1996, 1999).

Drip irrigation systems generally consist of emitters that have discharge varying from 2.0 to 8.0 L h^-1. In semi-arid climates, crop water use during the summer can be 6 to 8 mm d^-1 (Shalhevet et al., 1981), with water supplied two or three times a week. Given the spacings used for both emitters and laterals, the duration of water application is thus much shorter than the time over which the plant takes up water. Even if the water is supplied on a daily basis, a water application rate of 2.0 L h^-1 provides the consumptive need of the plant in a small fraction of the period over which photosynthesis and transpiration occur. This means that even for water application exactly equal to the plant water need, part of the water may not be used by the plant and would most likely drain below the root zone. Therefore, lowering the emitter discharge to as close as possible to the plant water uptake rate may improve irrigation efficiency (Batchelor et al., 1996).

Recently, microdrip irrigation systems have been developed that provide emitter discharges of <0.5 L h^-1. These systems have been studied most intensively in greenhouses (Koenig, 1997), and preliminary results showed that they reduced water consumption of tomato (Lycopersicon esculentum Mill.) plant by 38%, increased yield by 14 to 26%, and reduced leaching fraction by 10 to 40% (Ein-Tal Ltd., internal report, 1998). In a recent application on sweet corn under field conditions, Assouline et al. (2002) have shown that microdrip irrigation may improve yield, reduce drainage flux, and affect the water content distribution within the root zone, especially through an increased drying of the 0.60- to 0.90-m soil layer compared with conventional drip irrigation.

The microdrip technology still raises some problems concerning the uniformity of application (Miller, 1990) and the steadiness of the discharges. However, soil moisture regimes similar to those resulting from continual low water application rates can be achieved by means of pulsed drip irrigation (Zur, 1976). Infiltration experiments on a sandy loam showed that the water content distribution and the rate of wetting front advance under a pulsed water application (at a nominal rate q and a time-averaged rate of q_av) was similar to water applied in a continuous manner at q_av (Zur, 1976), and that temporal fluctuations in flux and in soil water content exponentially damped with depth for periodic pulses applied at the soil surface (Zur and Savaldi, 1977). Consequently, pulsed irrigation using conventional drip emitters could be one way of creating the water regime observed with continual low application rates while bypassing technical problems associated to microdrip emitters.

The relationships between water application rates, soil properties, and the resulting water distribution for conventional emitters (2.0 L h^-1) are well documented (Bresler et al., 1982). The wetting patterns during application generally consist of two zones: (i) a saturated zone close to the emitter, and (ii) a zone where the water content decreases toward the wetting front. Increasing the emission rate generally results in an increase in the wetted soil diameter and a decrease in the wetted depth (Schwartzman and Zur, 1986; Ah Koon et al., 1990). In microdrip irrigation, field observations seem to indicate that there is no saturated zone and that the wetted soil volume is greater compared with that for conventional emitter discharges (Koenig, 1997). The relationship between the water application rate and the resulting water content distribution is complex because it is a three-dimensional outcome related to soil properties and crop uptake characteristics. Therefore, a quantitative representation of the flow processes by means of a simulation model could be beneficial in studying the effects of microdrip irrigation on the water regime of drip irrigated crops. The main goal of this study was to compare the effect of different emitter discharges, including microdrip emitters, on different aspects of the water regime in daily drip irrigated corn.

	MATERIALS AND METHODS

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

 
Field Experiment
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 was 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 of water applied by sprinkler, fertilizer (urea) application of 100, 20, and 130 kg ha^-1 N, P, and K, respectively, and weed control. Sowing was done on 7 June, at 10 seeds m^-2, with a distance of 1.0 m between rows. The field presented a fairly constant slope of 2% and the rows were parallel to the slope direction. To ensure maximum germination, 65 mm of water was applied by sprinkler irrigation at sowing and an additional irrigation of 40 mm was applied 9 d later for seedling establishment. Drip irrigation began on 21 June, 14 d after sowing. The crop was drip irrigated on a daily basis, at 0600 h. The drip lines spacing was 1.0 m (one drip line per plant row). The treatments included three emitter discharges: (i) 8.0 L h^-1; (ii) 2.0 L h^-1 (common dripper used in the region); (iii) 0.25 L h^-1 (microdrip irrigation). Emitter spacing along the drip line was 0.5 m for the 2.0 and the 8.0 L h^-1 discharges (Tiran 16, Netafim, K. Hatzerim, Israel) and 0.25 m for the 0.25 L h^-1 discharge (MicroTal, Ein-Tal, Ltd, Caesarea, Israel). The microdrip lines were fed at both ends to improve emitter discharge uniformity. Eight plots were laid out randomly. Two plots were allocated to the 2.0 and the 8.0 L h^-1 treatments. Four plots were allocated to the 0.25 L h^-1 treatment to increase the total discharge of the treatment and the accuracy of the control flow meters since the plots were relatively small. Each plot had dimension of 4.0 by 10.0 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 based on combined pan evaporation and growth stage crop coefficient, according to recommended irrigation practice for the region (Extension Service, 1989). A total of 110 kg ha^-1 N was applied to all treatments in six weekly applications, as in line fertigation with urea.

During the growing season, the soil water content distribution with depth, (Z), was determined to a depth of 1.65 m at 0.3-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, 0.10 m from the drip line. Measurements of soil water content distribution with depth were made once a week in the morning.

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, husk, and of the remaining stover, determined after 72 h of drying at 60°C. The statistical analysis of the results was carried out using the General Linear Models procedure (GLM) (SAS Institute, 1982).

Numerical Simulation
Water movement in the different treatments was simulated for a period of 28 d using the HYDRUS-2D model (Simunek et al., 1996). The program solves the Richards' equation numerically for saturated-unsaturated flow using a Galerkin type linear finite element scheme. The size of the modeled flow domain was 0.5 m in width and 2.0 m in depth. This is a vertical symmetry plane perpendicular to the drip line, from the emitter to halfway between the drip lines, assuming line source wetting condition. The soil properties in the experimental field varied with depth but the model domain was assumed to have uniform soil physical properties. Russo and Bouton (1992) had characterized the variability with depth of the soil hydraulic properties in the experimental field. The values of the soil parameters for a sandy loam in the HYDRUS-2D database were found to well represent the soil mean parameters characterized by Russo and Bouton (1992). Therefore, the mean values representing the soil profile for _s, the saturated water content, _r, the residual water content, K_s, the saturated hydraulic conductivity, and n, the parameters of van Genuchten's (1980) model, and l, the power in Mualem's (1976) model were 0.41 m³ m^-3, 0.065 m³ m^-3, 1.06 m d^-1, 7.5 m^-1, 1.89, and 0.5, respectively. The root distribution with depth was based on data from Shalhevet et al. (1981) obtained for the Central Coastal Plain of Israel. From the soil surface, and for 0.3-m depth increments, the relative root distribution was 0.3, 0.35, 0.2, 0.1, and 0.05. The root distribution was kept constant with time. The simulation period corresponds to the grain growth period, between 11 July and 8 August, during which the root system was found to be constant (Al-Khalaf et al., 1989). The actual water uptake, R_a, was modeled according to the model of Feddes et al. (1978). The required parameters (P₀, P_opt, P_2H, P_2L, P₃, R_2L) used were (-0.15, -0.3, -3.25, -6.0, and -80.0 m; and 1.0 mm d^-1), as suggested by Wesseling (1991). The potential transpiration rate parameter, R_2H, was set to 7.5 mm d^-1 to fit local conditions (Assouline et al., 2002).

Hourly amounts of water applied and potential transpiration rates, T_p, were used as a time-variable boundary condition for the soil surface of the application area. A daily irrigation amount of 7.0 mm d^-1 was applied in the three cases. The irrigation intervals and the application rates for the respective emitter discharges are shown in Fig. 1 , and compared with the distribution of the hourly potential transpiration rate, T_p, during daytime. Irrigation began in all the cases at 0600 h (time = 0). The drip irrigation boundary condition was implemented through distributing the irrigation flux into a radius of 25 mm from the dripper. Evaporation from the soil surface was neglected as it was fully covered by the canopy during the simulation period. Vertical boundaries were assumed to be no-flow boundaries. The lower boundary was assumed to be a free-drainage boundary, appropriate when the water table is far below the domain of interest (McCord, 1991). Initial water content of 0.15 m³ m^-3, close to the field capacity of the soil, was applied to the whole profile. The effect of this initial condition vanishes after 10 irrigation cycles. Time discretizations were as follows: initial time step, 1 x 10^-3 d; minimum time step, 1 x 10^-5 d; and maximum time step, 0.1 d.

View larger version (18K): [in this window] [in a new window]   Fig. 1. Water application rates corresponding to the three emitters discharge and the potential hourly transpiration rates, Tp, during daytime from the beginning of irrigation (time = 0 corresponds to 0600 h).

	RESULTS AND DISCUSSION

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

View larger version (50K): [in this window] [in a new window]   Fig. 2. Corn yield (dry matter) for the three water application rates. Error bars represent one standard deviation.

View larger version (21K): [in this window] [in a new window]   Fig. 3. Water content distributions with depth, 0.10 m from the emitter, at the end of the grain growth period, relative to the water content distribution at the end of the vegetative period, for the three emitters discharge.

View larger version (71K): [in this window] [in a new window]   Fig. 4. Simulated spatial distribution of the water content in the flow domain for the three water application rates at the end of the application of the same amount of water. The scale represents volumetric water content values.

Compared with conventional emitter discharges, Koenig (1997) has described the water distribution resulting from microdrip irrigation as characterized by the absence of a saturated zone and by a larger wetted zone. The simulated results indicate that the absence of a saturated zone also can be obtained with conventional emitter discharges, depending on the water amount applied, the plant water uptake, and the soil hydraulic properties. The results also showed that, under the simulation conditions, the wetted zone resulting from the low application rate was smaller than for conventional emitter discharges. This should have a direct effect on the structure of the root system and water uptake pattern (Phene et al., 1991; Coelho and Or, 1999).

The simulated water content distributions with depth, 0.10 m away from the emitter, are shown in Fig. 5 for three times of the day: before irrigation; at the end of each respective irrigation interval; at solar noon when the transpiration process is maximal (Fig. 1). Before irrigation, the largest difference in the water content distributions was in the 0.40- to 0.90-m layer, which was dryer for the microdrip irrigation than the other treatments. At the end of the respective irrigation intervals, conventional application rates affected the water content to a depth of 0.30 m, while the microdrip irrigation effect was to a depth of 0.40 m. In the 0- to 0.20-m layer, the water content values were highest for the 8.0 L h^-1 discharge, and lowest for the 0.25 L h^-1 discharge. In the 0.25- to 0.35-m layer, it appeared that microdrip irrigation lead to a slightly wetter zone. The differences in the water content distributions below depth 0.40 m remain practically unchanged and were similar to those before irrigation. At solar noon, when plant transpiration was maximum, redistribution was taking place for hours for the conventional drip discharges while for irrigation at the low water application rate wetting had not yet ended (Fig. 1). As a result, the 0- to 0.20-m layer was wettest for the low application rate. Below that layer, and down to a 1.0-m depth, the soil profile was the driest for the microdrip irrigation and the wettest for the 8.0 L h^-1 discharge, the 2.0 L h^-1 discharge leading to intermediate water content values. These trends were similar to those observed under field conditions ([Fig. 3] and in Assouline et al. [2002]).

View larger version (24K): [in this window] [in a new window]   Fig. 5. Simulated water content distributions with depth, 0.10 m from the emitter, before irrigation, at the end of the irrigation interval, and at solar noon, for the three emitters discharge.

View larger version (26K): [in this window] [in a new window]   Fig. 6. Simulated dynamic changes of the water content during the irrigation cycle at depths 0.20 and 0.50 m for the three application rates. The white strip represents daytime and the black strip, nighttime. The arrow indicate solar noon (time = 0 corresponds to 0600 h, beginning of irrigation).

View larger version (17K): [in this window] [in a new window]   Fig. 7. Simulated dynamic changes of the ratio between actual plant water uptake and potential transpiration during daytime from the beginning of irrigation for the three application rates (time = 0 corresponds to 0600 h).

	CONCLUSIONS

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

	ACKNOWLEDGMENTS

 
I thank Dr. Floyd Adamsen and anonymous reviewers for their constructive comments and suggestions.

	NOTES

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

 
Contribution no. 624/01 from the Agricultural Research Organization.

Received for publication March 12, 2001.

	REFERENCES

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

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (9) Citing Articles via Google Scholar Google Scholar Articles by Assouline, S. Search for Related Content PubMed Articles by Assouline, S. Agricola Articles by Assouline, S. Related Collections Water Use Irrigation