Distribution of water and salt in soil under trickle and pot irrigation regimes

Agricultural Water Management, 3 (1980/1981) 195--203

195

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

DISTRIBUTION OF WATER AND SALT IN SOIL UNDER TRICKLE AND POT IRRIGATION REGIMES

M.H. ALEMI

Irrigation Engineering Department, University of Tehran, Karaj (Iran) Contribution from Technical University of Berlin and Tehran University (Accepted 17 September 1980)

ABSTRACT Alemi, M.H., 1981. Distribution of water and salt in soil under trickle and pot irrigation regimes. Agric. Water Manage., 3: 195--203. An ellipitical clay pot was buried vertically in the centre o f a lysimeter as a means of supplying water to the soil. The distribution o f water and salt in the soil emerging from the pot source was compared with that under trickle irrigation. Five hundred milliequivalents of calcium chloride was applied to the soil by both methods. Calcium chloride was subsequently leached by applying 50 1 of tap water. The soil solution was sampled periodically using suction cups. Soil samples were also taken for measurements of water content and chloride ion concentration. Water applied at the rate of 130 ml/h by the pot moved the salt to a radial distance of 41.5 cm in 390 h, but applying water by trickle at the rate o f one l/h moved the salt 42 cm in 52.5 h. F o r an equal amount of water applied, salt moved deeper in the profile at the lower application rate. More salt spreading was observed from the trickle source with higher application rate. After 72 h of redistribution, the wetted volumes were approximately equal for trickle and pot irrigation regimes.

INTRODUCTION

Among the well known methods of irrigation such as sprinkling, flooding, furrow, trickle, etc., underground irrigation is the oldest. Today, irrigation by sprinkler has proved especially useful for gardening in regions with a temperate climate, but not in hot zones where water loss by evaporation is high. Due to heavy loss of water by evaporation, percolation, and surface runoff, a large a m o u n t of water is required by flooding or furrow irrigation. On the other hand, the advent of trickle irrigation created widespread interest among irrigators in dry parts of the world due to its higher irrigation efficiency. Underground irrigation also has its advantages. By underground irrigation water can be directed to the root zone with minimal loss. Another advantage is that the nutrients can be added to the irrigation water, and only the root zone will be irrigated. Up until 10 years ago local farmers in dry parts of Iran

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196 buried clay pots near trees and periodically added water to the pots to irrigate the plants. This m e t h o d of irrigation is called p o t irrigation. An understanding of the displacement of salts by water during irrigation is important for increasing the efficiency of applied fertilizers. There have been extensive experimental and theoretical studies on the movement of water and salts in saturated and unsaturated soils (Biggar and Nielsen, 1962, 1967; Bresler and Hanks, 1969; Warrick et al., 1971; Kirda, 1972; Parlange, 1972, 1973; Shaaf, 1973; Bresler, 1973, 1975). Infiltration of water and salt from a point source has also been investigated, both theoretically and experimentally, b y many investigators (Killingmo, 1967; Brandt et al., 1971; Raats, 1971; Bresler et al., 1971; Warrick, 1974; Bresler, 1975; Bar-Yosef and Sheikholslami, 1976). Distribution of water and salt emerging from an elliptical source into soil has not been studied so far. Recently Javaheri (1976) studied the possibility of using clay pots for irrigation on a large scale and made some recommendations. The objectives of this study were: (i) to investigate distribution of water and salt in an air-dried soil from a vertical, buried elliptical pot source; (ii) to study the effect of redistribution time on the distribution of salt and water in the soft; (iii) to compare the findings from the p o t source with those obtained from a trickle source for equal amounts of water, salt, and redistribution time in a sandy loam soft. EXPERIMENTS

Two round lysimeters of 1.12 m inside diameter (ID) and 1.5 m deep were filled uniformly with air-dried soil, 4% water content. A sandy loam top soil passed through a 2 mm sieve was used in the experiment. To obtain a uniform compaction, the lysimeters were filled 0.05 m at a time. Predetermined amounts of soil were used for each layer to reach a bulk density of 1.5 g cm -3. Four ceramic cups (one bar) were installed along a transect at depths of 0.10, 0.40, 0.70, and 1.0 m on the right side and 0.25, 0.55, and 0.85 m on the left side of the lysimeter axis. The suction cups were placed with slight inclination along the radius at various locations at 0.15 m intervals laterally to 0.45 m from the centre of the lysimeter. With this arrangement it was possible to sample the soft solution in a vertical plane passing through and extending to b o t h sides of the lysimeter axis. The cups were connected to collection bottles with nylon tubing (2 mm ID) and a vacuum pump was used to collect samples of soil solution. An elliptical clay pot with a volume of 950 cm 3, 0.17 m along the major axis and 0.10 m along the minor axis, was placed in the centre of one lysimeter, with the neck of the pot above the soil surface. A nylon tube connected to a Marriot-burrette passed through the stopper to maintain the water level in the pot at the same level as the soil surface. In the second lysimeter the trickle system was designed to supply water at

197 th e rate o f one liter per h o u r t o the soil surface. The e m i t t e r consisted of a n y l o n t ube of 2 m m ID, fed f r o m a water reservoir at a const ant head. The e m i t t e r was placed 2 cm above t he soil surface in t he centre of the lysimeter (similar set up as in t he p o t case, b u t with a suction cup below the em i t t er at a d e p t h o f 0.10 m). T w o and a half litres o f water containing 500 milliequivalents of calcium chloride was applied t o the soil, by b o t h p o t and trickle sources, and subsequently leached with 50 1 o f tap water. The p o t was initially filled with salt solution. After 20 h when t he slug o f salt had infiltrated t he soil, the p o t was emptied using the vacuum system, and immediately filled with tap water. T he water level was kept const ant and the flow rate o u t of t he p o t was measured during water application. A ft er termination, the p o t was emptied and redistribution started. Soil solution samples were t aken from t he w e t t e d zone in b o t h lysimeters after 10, 20, 30, and 50 1 o f tap water had infiltrated into the soil. Samples o f soil solution were also collected after 24 and 72 h o f redistribution. A ft er 50 1 of water had infiltrated, soil Samples were taken, at radial distance (0.15, 0.30, 0.375 m) and depth, using a 2 cm (ID) t ube auger on one side of the source, perpendicular t o t he plane of the suction cups. T o minimize the disturbance caused b y sampling, the holes were filled with moist soil, compacted t o its original status. After 72 h of redistribution, o t h e r soil samples TABLEI Summary of experiments Stage

Hours from start of experiment Pot

Treatment

Trickle

Sampling Soil solution

I

20

2.5

II

105.5

12.5

10 I tap water infiltrated

X

III

190.5

32.5

20 1tap water infiltrated

X

IV

266

42.5

30 I tap water infiltrated

X

V

390

52.5

50 l tap water infiltrated

X

VI

414

76.5

24 h of redistribution

X

VII

462

72hof redistribution

X

124

Soil

2.5 1salt solution infiltrated

X

X

198 were taken on the opposite side of the source. The moisture c o n t e n t was determined by oven drying at 40°C (to prevent loss of chloride) and chloride ion concentrations in the soil samples were measured in 1:2 soil/water extract using a Buchler-Cotlove Chloridemeter. The lysimeters were covered by plastic sheets t o prevent evaporation during irrigation and redistribution. A summary of the experiments is presented in Table I. EXPERIMENTAL RESULTS

Pot irrigation Chloride ion concentrations in soil solution samples are presented in Table II. After infiltration of 10 1 o f tap water the wetting front at the soil surface was 0.23 m from the centre of the source and most salt had accumulated outside radius 0.15 m. Apparently, the salt f r o n t passed the radial distance of 0.15 m and de pt h o f 0.25 m (IIP, Table II). When 20 1 of water had infiltrated, the salt f r ont was observed at a radial distance of 0.30 m, passed depth of 0.40 m, but did n o t reach depth 0.55 m (IIIP). When 30 1 of water had infiltrated, most of the salt had passed a radial distance of 0.30 m and depth of 0.55 m with accumulation on the b o u n d a r y o f the w et t ed volume. At this stage the wetting front was 0.375 m from the centre of the source (IVP). At the termination of infiltration the salt was leached further, but did n o t reach b e y o n d 0.45 m radial distance. After 24 h of redistribution the salt f r ont was observed at the radial distance of 0.45 m (VIP). At 72 h of redistribution, chloride concent rat i on decreased at this distance (VIIP). Results of soil analysis are presented in Table IV. In general, chloride ion co nc e nt r at i on measured in soil water extract was greater than t hat in soil solution obtained by suction cups.

Trickle irrigation Table III shows chloride ion concent rat i on in soil solution at various stages of irrigation and redistribution. The wetting and salt fronts do not always coincide with the position o f suction cups at the time of sampling, but the highest concentrations measured were always observed at the b o u n d a r y of volume sampled. The chloride ion concent rat i on continued to decrease during redistribution on the b o u n d a r y of the volume sampled. Table IV indicates t hat chloride ion concent rat i on at 0.375 m undergoes a slight decrease during redistribution. At the end of 72 h of redistribution the concentration of chloride at 0.375 m decreased, which indicates further displacement of salt. Immediately after infiltration, water c o n t e n t was higher on the inner part of the w e t t e d volume. Water c o n t e n t decreased on the inner part of the volume during redistribution.

3.82 . . . .

VP -0.99 0.99 1.03 3.11 .

0.40 0.55

0.70

0.85

Depth (m)

0.10

0.25

0.40

0.55 0.70

0.85

.

.

.

. . .

1.08 18.20

1.01

1.02

.

.

.

1.01

.

2.46

4.78

.

.

.

3.50 --

1.45

0.99

1.20

.

.

--

--

0.30

. .

.

.

---

--

--

--

0.40

.

--

--

0.45

0.231

.

.

.

.

1.00 1.89

0.99

0.98

--

VIP

.

.

1.78 .

1.59

--

0.0

.

1.02 14.75

1.02

1.00

0.98

1.86

1.74

1.52

0.15

.

R a d i a l distance ( m )

IIIP

1Radial distance o f w e t t i n g f r o n t f r o m t h e c e n t r e o f t h e s o u r c e (m).

.

--

0.15

R a d i a l distance ( m ) 0.0

0.25

(m)

0.I0

Depth

IIP

C h l o r i d e ion c o n c e n t r a t i o n in soil s o l u t i o n f o r p o t irrigation ( m e q / l )

T A B L E II

.

2.17 --

1.20

1.00

1.22

--

21.34

39.13

0.30

.

---

--

31.48

37.60

0.41

0.45

0.315

.

-

1.13 1.35 3.07

1.02 1.30

1.03

1.01

--

VIIP

-

0.0

.

1.06 7.82

1.06

1.03

1.04 --

1.16

1.01

1.30

5.22 2.59 3.00 --

1.16 1.16 1.96 9.61

1.05

0.30

0.15

R a d i a l distance ( m )

IVP

---

--

26.63

23.90

0.415

0.45

0.375

¢0 ¢0

1.57 1.94 1.81 2.26 21.04 .

.

.

. .

.

.

. .

1.64 1.72 1.80 2.73 --

.

15 77

0.15

.

.

.

.

.

.

7.60 5.62 6.11 24.05 -.

.

--. . 0.41

.

.

14.72 ----.

.

---

C ~5

0.301

.

.

.

.

(meq/1)

.

1.66 1.56 1.74 2.02 4.03

VID

.

2.75 -17.85

0.0

.

. . .

1.66 1.66 1.66 2.00 15.44 .

2.72 22.00 40.27

0.15

Radial distance (m)

IIID

for trickle irrigation

1Radial d i s t a n c e o f w e t t i n g f r o n t f r o m t h e c e n t e r o f t h e s o u r c e ( m ) .

0.10 0.25 0.40 0.55 0.70 0.85

VD

(m)

Depth

0.0

0.30

in soil solution

Radial distance (m)

36.00 10.60 . . . .

(m)

0.10 0.25 0.40 0.55 0.70 0.85

Depth

ion concentration

Chloride IID

III

TABLE

3.52 2.83 3.59 11.32 -.

10.40 10.30 --

0.30

.

7.94 ----.

0.415

0.45

0.38

2.00 1.56 1.64 1.71 2.45 .

VIID

2.10 2.14 2.61

0.0

.

1.62 1.56 2.00 1.70 4.65

2.04 2.19 3.58

0.15

Radial distance (m)

IVD

1.92 2.42 2.51 4.54 --

8.08 7.70 --

0.30

5.32 -----

0.42

0.45

0.395

b~

13.12 . . 0.65

0.52-~.60 0.67-0.75

5.33 .

1.59 6.17 3.62 .

11.35 .

12.24 13.46 12.72

VD

10.41 11.07 11.02 7.89 .

W

0.55

. 0.63

0.30

10.57 . .

5.58 5.20 6.28

11.78 4.76 6.00 18.06 .

C

.

--

9.42 10.05 7.80

8.03 9.08 8.57 7.06

W

1Radial d i s t a n c e o f w e t t i n g f r o n t f r o m t h e s o u r c e (m). 2Wetting f r o n t vertical d i s t a n c e f r o m surface o f t h e soil (m). 3Soil w a t e r c o n t e n t . 4Chloride c o n c e n t r a t i o n .

18.86 17.80 16.58

0.07- 0 . 1 5 0.22--0.30 0.37-0.45

C4 5.59 4.26 6.32 7.07 20.96 0.722

13.26 14.01 13.08 8.68 6.8

W3

0.15

0.07-0.15 0.22 -0.30 0.37-0.45 0.52-0.60 0.67-0.75

Depth (m)

Radial d i s t a n c e ( m )

VP

--

14.80 27.80 15.38

0.425

0.41

0.525

9.41 21.38 29.11 19.79

C

0.375

0.401

12.14 8.45

11.74 12.29 12.41

11.00 10.82 11.15 10.48 7.02

W

0.75

0.74

0.15

4.94 8.04

4.42 3.22 3.38

VIID

4.00 6.19 5.73 3.05 14.24

C

Radial distance (m)

VIIP

G r a v i m e t r i c w a t e r c o n t e n t (%) a n d c h l o r i d e i o n c o n c e n t r a t i o n ( m e q / l ) in soil w a t e r e x t r a c t

T A B L E IV

9.35 6.96

10.47 10.86 11.34

9.29 9.34 9.41 7.87 5.36

W

C

0.64

6.20 12.06

4.58 4.23 2.55

8.80 4.28 12.96 13.18 27.30 0.635

0.30

8.93 9.36 9.46 7.07

8.37 8.63 7.97 5.58

W

0.54

0.42 15.67 14.74 14.37 16.69

0.555

15.89 17.14 15.18 14.15

C

0.375

0.415

t~

202 DISCUSSION AND CONCLUSIONS Comparing the results of pot and trickle irrigation at each stage indicates that more complete transport of salt was observed where salt and water were applied by pot. Comparing IIIP with IIID, the salt accumulated outside the soil volume bounded by radius 0.15 m, and at stage IV salt accumulation was observed outside radius 0.30 m with better transport in the case of the pot. At the end of irrigation by p o t the salt slug was completely leached from the soil volume b o u n d e d by radius 0.30 m (Table II). Complete leaching from the same volume occurred 72 h after termination of irrigation by trickle {Table III). The maximum salt concentration on the boundary of the volume sampled was always less for trickle, which indicates more mixing in trickle irrigation due to higher flow rate. Comparing the salt concentration in the soil in trickle and pot irrigations shows that the same amount of water applied by pot, having a lower application rate, leached the salt further in the profile. The total amount of salt added to the soil was not found in the volume sampled because the wetting front was between the suction cup locations. Table IV shows that similar conclusions can be derived from soil analysis. At the end of infiltration, salt and water moved to an almost identical distance laterally but the vertical distance of the wetting front was deeper for pot irrigation. This difference is due to the fact that immediately after infiltration, the water content at each location was less for pot irrigation, and the elliptical infiltration surface extended 0.17 m below the soil surface. At the end of the redistribution period the wetted volumes were equal for both methods of irrigation. Some previous reports for one-dimensional salt and water (Biggar and Nielsen, 1962; Kirda, 1972, Warrick et al., 1971) indicated that, after infiltration, salt is displaced deeper b y the same amount of water added at a slower than at a faster rate, and concluded that the effect of hydrodynamic dispersion on mixing was less at the slower rate. In the present study similar results were obtained. Immediately after irrigation chloride did move further for the same amount of water added at the slower rate. As a result, chloride was transported more efficiently to a greater distance with relatively less chloride left in the inner part of the wetted volume by p o t irrigation (slower application rate). Less transport was observed for trickle irrigation. But when the infiltration times are compared, trickle irrigation, with the higher flow rate, is more efficient for salt transport. ACKNOWLEDGEMENTS I am grateful for the generous cooperation of the staff of the Technical University of Berlin, in particular Professor G. Krzych and H. Wolkewitz. I am also grateful for the support provided b y the German Academic Exchange Service.

203 REFERENCES Bar-Yosef, B. and Sheikholslami, M.R., 1976. Distribution o f water and ions in soils irrigated and fertilized from a trickle source. Soil Sci. Soc. Am. Proc., 40: 575--582. Biggar, J.W. and Nielsen, D.R., 1962. Miscible displacement. III. Theoretical considerations. Soil Sci. Soc. Am. Proc., 26: 216--221. Biggar, J.W. and Nielsen, D.R., 1967. Miscible displacement and leaching phenomenon. In: R.M. Hagan, H.R. Haise and T.W. Edminster (Editors), Irrigation of Agricultural Lands. Agronomy, 11: 254--274. Am. Soc. Agron., Madison, WI. Brandt, A., Bresler, E., Diner, N., Ben-Asher, I., Heller, J. and Goldberg, D., 1971. Infiltration from a trickle source. I. Mathematical models. Soil Sci. Soc. Am. Proc., 3 5 : 6 7 5 682. Bresler, E., 1973. Simultaneous transport of solute and water under transient unsaturated flow condition. Water Resour. Res., 9: 975-- 986. Bresler, E., 1975. Two-dimensional transport o f solutes during nonsteady infiltration from a trickle source. Soil Sci. Soc. Am. Proc., 39: 604--612. Bresler, E. and Hanks, R.J., 1969. Numerical m e t h o d for estimating simultaneous flow of water and salt in unsaturated soils. Soil Sci. Soc. Am. Proc., 33: 827--832. Bresler, E., Heller, J., Diner, N., Ben-Asher, I., Brandt, A. and Goldberg, D., 1971. Infiltration from a trickle source. II. Experimental data and theoretical predictions. Soil Sci. Soc. Am. Proc., 35: 683--689. Javaheri, P., 1976. Pot irrigation. Studies on the possibility o f using cephalin pots for irrigation. Ministry of Agriculture and Natural Resources of Iran. Soil Institute Publication, 54 pp. (in Persian). Killingmo, O.H., 1967. Studies on plant nutrients in soils. 1. Distribution of some nutrition elements in a soil fertilized by a drip irrigation equipment. Acta Agric. Scand., 16: 155--162. Kirda, C., 1972. Simultaneous transport of chloride and water during infiltration and redistribution, P h . D . Thesis, University of California, Davis, 223 pp. Parlange, J., 1972. Theory of water movement in soils. 5. Unsteady infiltration from spherical cavities. Soil Sci., 113: 156--161. Parlange, J., 1973. Movement o f salt and water in relatively dry soils. Soil Sci., 116: 249--255. Raats, P.A.C., 1971. Steady infiltration from point sources, cavities, and basins. Soil Sci. Soc. Am. Proc., 35: 689--694. Shaaf, J.R., 1973. Unsteady, unsaturated flow from a horizontal, buried sylindrical source into a porous medium. Ph.D. Thesis, Stanford University, CA, 166 pp. Warrick, A.W., 1974. Time dependent linearized infiltration. I. Point sources. Soil Sci. Soc. Am. Proc., 38: 383--386. Warrick, A.W., Biggar, J.W. and Nielsen, D.R., 1971. Simultaneous solute and water transfer for an unsaturated soil. Water Resour. Res., 7: 1216--1225.