Effects of Evaporation and Different Flow Regimes on ... - Springer Link

Transport in Porous Media 56: 245–255, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.

245

Effects of Evaporation and Different Flow Regimes on Solute Distribution in Soil H. S. ÖZTÜRK∗ and ˙I. ÖZKAN Department of Soil Science, Faculty of Agriculture, Ankara University, 06110, Diskapi, Ankara, Turkey (Received: 22 May 2001; in final form: 6 January 2004) Abstract. Water evaporation and solute transport processes were studied in large soil columns filled with a sandy clay loam (SCL) and a clay loam (CL) soils. To create different water flow velocity through the soil column, the 3 cm (Treatment I) and 6 cm (Treatment II) depths of water were ponded at the soil surface during leaching. After leaching, soils were left for evaporation for 10 days. Some salinity parameters were monitored during three leaching and evaporation periods. To achieve the same degree of leaching more water was needed in Treatment II than in Treatment I for both soils. The electrical conductivity (EC) at the soil surface after evaporation increased, to 41–46% of the pre-drying level for the SCL and 28–31% for the CL. Although very low concentrations of Cl− were detected at the soil surface after the first leaching in both soils, high increase was monitored after the evaporation period, due to the high mobility of this anion. The fluctuation of exchangeable sodium percentage (ESP) during the leaching and evaporation periods was attributed to the different transportation rates of Na+ , Ca2+ and Mg2+ . The boron leaching in Treatment I was more effective than that in Treatment II for both soils. Key words: solute transport, leaching, evaporation, upward and downward ion transport, boron leaching.

1. Introduction The transport of both cations and anions and their interactions with the soil surface affect many aspects of soil management. It is therefore important to understand the processes that govern downward and upward solute movement through soil and the influence of flow rate on the concentration of different cations and anions. In irrigated areas water can move upward by capillary force during irrigation intervals, or fallow periods, when there is no downward flow of percolation water. The flux of water flow strongly depends on soil type, depth of the water table and the soil water potential differences. The size of upward water flow determines the amount of solutes being transported (van Hoorn and van Alpen, 1990). Bresler et al. (1982) noted that any practices that would reduce the upward movement of water and salt by reducing evaporation or by increasing infiltration would enhance salt leaching. ∗ Author for correspondence: Fax: +90-312-3178465; e-mail: [email protected]

246

H. S. ÖZTÜRK AND ˙I. ÖZKAN

Soil water and salinity distributions are greatly affected by local meteorological events (e.g. wind radiation and air humidity) and soil conditions (e.g. water content, soil texture, salinity, and tillage practices). Understanding mechanisms of water and chemical movement in soil under evaporative conditions is helpful in managing soil salinity and water content. Salhotra et al. (1985) found that the reduction of evaporation from salinized soil depends on the saturation water pressure and the ionic composition of the salinized water. Fritton et al. (1967) reported that the evaporation zone thickness was a function of the evaporation potential and the duration of evaporation. Whenever changes in the soil water content occur as a result of infiltration, redistribution, and evapotranspiration, the dissolved salts tend to move with the water. This convective transport of solute depends on the macroscopic flow velocity (Bresler et al., 1982). The salt that remains in the soil after irrigation may be transported upwards by capillary water, especially in the areas where high evaporation demands occur. There may be two reasons for the increase of solute concentration in the soil surface: (i) water evaporation decreases water content but solute remains there and (ii) movement of solute toward the evaporating surface as water flows and carries solute (Nassar and Horton, 1999). In the areas where limited water is available for irrigation or leaching, leaching activities would be managed according to the most common ions in soil. Irrigation water leaches the salt below and away from the infiltrating areas but salt accumulates on lateral wetting fronts (Keren, 2000 in Sumner, 2000). It is also important to know that the effects of consecutive leaching–drying periods on the salinity management. The objective of this study was to observe the downward and upward movement behavior of various ions during leaching and evaporation periods under two water regimes.

2. Materials and Methods To determine downward ion transport during leaching and upward ion transport by capillarity during evaporation period, this experiment was conducted in the large soil columns. Each column was leached four times and left for evaporation after each leaching. The first three leaching were performed for salt and boron removal together and the last one done for only boron. Saturated leaching was conducted by ponding 3 cm (Treatment I) and 6 cm (Treatment II) of water on the soil surface using distilled water in a Mariotte flask. To decide the leaching intervals, the EC values of the effluent were measured continuously and, when a desired level of EC was reached during leaching, a siphon system was inactivated to cease the water supply. The soil surface was then covered by a polyethylene sheet for a few days to obtain the equilibrium throughout the soil column. After reaching the water balance the first evaporation period was performed by exposing to artificial radiation for 8 h a day, lasting for 10 days. For this purpose, a lamp was placed above the soil column to provide 25◦ C constant temperature at the soil surface, and a thermometer was also placed to measure the soil surface temperature continuously.

247

EFFECTS OF EVAPORATION AND DIFFERENT FLOW REGIMES

Table I. Selected properties of soils Soil

Clay content (%)

Bulk density (g cm−3 )

Hydraulic conductivity (cm h−1 )

Field capacity (w/w)

Wilting point (w/w)

Organic matter (%)

CEC (cmol kg−1 )

SCL CL

24 32

1.32 1.24

1.62 0.96

24.6 31.3

13.1 17.3

0.2 0.7

13.1 20.2

The soil samples were collected from the research field of Ankara University. The texture of the surface soil was clay loam (CL) and that of the subsurface soil taken from 1.2 m below the surface was sandy clay loam (SCL). Selected properties of the soils are given in Table I. The soils were treated with the mixtures of chlorides, carbonates and sulfates of calcium, magnesium, sodium and potassium and boric acid to establish approximate salinity level of 12 dS m−1 of electrical conductivity (EC). To obtain the uniform salt concentration throughout the whole soil column, these salts were dissolved in 100 L water. The salinization treatment was performed by spraying salt solution on 100 kg soil uniformly spread over a nylon sheet. Although soils were crashed and sieved before the salinization, they were crashed again two times and sieved after the salinization treatment to break the aggregates formed during spraying. Selected chemical characteristics of the salinized soils are given in Table II. The EC and solute concentration were determined in 1:5 (w:w) soil:water extracts. The experiment was carried out using large plexy glass containers, 35 × 40 × 60 cm. The salinized, and air dried soils later moistened stepwise to 0.18 m3 m−3 for SCL and 0.22 m3 m−3 for CL (prewetted) and stored in the polyvinyl containers. The soil columns were uniformly packed with respect to the bulk densities of the original, undisturbed soils. Columns were filled in 5 cm increments in an attempt to provide the uniform compaction along the whole column length. A filter paper, 3–4 mm washed sand, and a nylon texture were inserted in the base of the container. During all leaching and soon after, effluents from the bottom of the containers were collected. Soil samples were taken out of the soil containers by an auger (dia. 30 mm) after each leaching and evaporation periods. Plastic tubes having the same diameter as the auger were inserted tightly into the holes from which the Table II. The chemical characteristics of salinized soil (in 1:5 soil:water extracts) Soil

SCL CL

pH

8.03 7.96

EC25◦C (dS m−1 )

1.26 1.42

me L−1

B (mg kg−1 )

Ca2+ + Mg2+

Na+

Cl−

HCO− 3

6.28 7.32

6.01 6.26

10.4 12.8

1.53 1.21

4.85 4.51


248

soil samples were taken. Each horizontal 5 cm soil layer to the depth of 50 cm was sectioned into 10 samples and all of the ion analyses were done on these samples. Soil water content was determined gravimetrically in every soil layer to calculate the amount of water evaporated during the drying periods. Moisture content of each layer was measured on oven-dry basis. EC of the effluents were measured with YSE 3200 model EC-meter, and pH was with WTWinolab model pH-meter. Ca2+ and Mg2+ were determined by EDTA titration method (Heald, 1965), and Na+ and K+ with a flamephotometer. Cl− was determined by AgNO3 titration method (Richards, 1954). Finally, boron was determined spectrophotometrically after a color reaction with azomethine-H (John et al., 1975).

3. Results and Discussion The amounts of water used for leaching and water lost by evaporation are given in Table III. The amount of water used in Treatment II was always higher than that of Treatment I for the same soil and much more water was used in CL than in SCL to reduce the EC below 2 dS m−1 . The reason of using much water for the leaching of the SCL, comparing to the CL, is mainly due to low clay content and high hydraulic conductivity besides the differences in the initial values of EC (1260 dS m−1 for SCL v.s. 1437 dS m−1 for CL). Miyatoma and Cruz (1986) (in Sumner, 2000) stated that to increase leaching efficiency, the type of the leaching methods should be based on the soil type. Shukla et al. (2000) reported that initial solute concentration in effluent is high when soil texture is finer.

3.1. ELECTRICAL CONDUCTIVITY Changes in the EC of soil extracts with Treatments I and II are presented in Figure 1. The EC for SCL decreased rapidly from 1260 µS cm−1 , the initial concentration, to 280 µS cm−1 at the depth of 30 cm in both treatments after the first leaching and Table III. The amount of water used for leaching and water lost during evaporation period (depth in cm) Soil

Treatment

Water used for leaching (cm)

Water lost by evaporation (cm)

First

Second

Third

Fourth

Total

First

Second

Third

Total

SCL

I II

16.0 20.2

12.3 13.0

15.8 19.4

22.6 26.9

66.7 79.5

3.0 2.4

2.1 2.4

2.2 2.2

7.3 7.0

CL

I II

21.1 22.1

18.4 20.9

20.2 22.4

27.4 31.3

87.1 96.7

2.8 2.2

1.8 1.8

2.2 2.4

7.1 6.4


249

Figure 1. Changes in the EC values from both soils under Treatments I and II. Treatment I represents the leaching by 3 cm ponding water and Treatment II represents 6 cm ponding water on the soil surface.

the effects of following leaching on salt leaching were not high especially at the soil surface. However, a constant decrease was observed in the soil profiles by the end of the experiment. Relatively high EC value at the depths of 45–50 cm was due to interruption of water movement from soil to the filter materials, especially, when water supply was ceased. Although a similar trend was observed for CL, more water was used in the former soil to achieve any specified value of EC. A high decrease in the EC was detected after the first leaching (from 1437 to 285 µS cm−1 ) at the depth of 30 cm in the CL in Treatment I. Selassie et al. (1992) attributed the difference between the amounts of irrigation water for salinity and sodium removal to the texture of the soils. Another factor strongly affecting transport of ions through soil in their reaction with charged colloids via cation exchange (Bond and Phillips, 1990). The EC of the soils decreased with leaching, but increased with evaporation at soil surface. The effects of evaporation were observed from soil surface to the depths of 30–35 cm in columns of the SCL and to 25–30 cm for the CL. An increase in EC was observed generally in the upper 13–15 cm of soil profile depending on the evaporation period in all the treatments for the SCL and in the upper 8–11 cm for the CL. The rate of increase in the EC at the soil surface after evaporation was found to be 41–46% of the pre-drying values of each experiment in the SCL

250


and 28–31% in the CL (Figure 1). Although the amount of water lost during the evaporation period is close to each other in different treatments of the same soils, the effects of drying on upward solute transport in the SCL were more pronounced than with the CL. It is generally accepted that fine textured soils have high capillary water movement due to its capillary tubes. However, in this study the energy provided to the soil surface in the CL column was not enough to dry the deeper parts. Therefore, 10-day evaporation induced dried only of a limited part of the soil surface of the CL due to high water holding capacity of clay. It should be mentioned again here that the water held at the field capacity by the CL was higher than by the SCL soil (31.3% v.s. 24.6%). 3.2. CATIONS Most of the water-soluble Ca2+ and Mg2+ were leached in the first leaching in all treatments. In case of the SCL, after the first leaching the concentration of Ca2+ and Mg2+ decreased from 6.3 to 1.2–1.6 me L−1 with Treatment I and to 1.3–1.7 me L−1 with Treatment II. Small decreases in the concentration of watersoluble Ca2+ and Mg2+ were detected in the following leaching. Relatively high Ca2+ and Mg2+ concentrations were found with both treatments for the CL (1.3– 1.8 me L−1 with Treatment I and 1.6–1.9 me L−1 with Treatment II) after the first leaching. The upward movement of Ca2+ and Mg2+ by capillarity was slow. For the SCL, the concentration of Ca2+ and Mg2+ at the soil surface after drying increased to 9–11% of pre-drying values with Treatment I and 10–13% with Treatment II. The effect of drying on Ca2+ and Mg2+ concentration was observed in the upper 15 cm of the soil profile in case of Treatment I and 18 cm with Treatment II for the SCL and this was observed at the depth of 12–15 cm for the CL. It was found that transport of Na+ was slow but constantly occurred through the end of leaching. It may be due to reason that, Na+ continuously exchanges with Ca2+ that comes from low soluble gypsum as stated by Jurinak (1988). Likewise upward movement of Na+ during evaporation showed a similar trend with downward movement by leaching. Depending on the depth, the initial Na+ concentration (6.0 me L−1 ) for the SCL reduced to 1.2–2.2 me L−1 with Treatment I in the first leaching and to 1.2–2.5 me L−1 with Treatment II. However, for the CL Na+ decreased to 1.5 me L−1 in the upper 30 cm after the first leaching but high Na+ concentration was found below that level. Na+ concentration after first evaporation period increased to 35–39% of the pre-drying Na+ concentration for the SCL and 19–28% for the CL. The ESP is associated with sodium hazard to soil and soil alkalinity (Bresler et al., 1982). The ratio of the exchangeable Na+ to total exchangeable cations is a good indicator for soil structure deterioration. Although, in general, the ESP of 10–15% is accepted as a critical level, an ESP of 25% may have little effect on soil structure in a sandy soil, whereas an ESP of 5% is considered high particularly in soils containing 2:1 clay minerals like montmorillonite (van Hoorn and van Alphen, 1990).


251

The soil analysis showed that the upward Na+ movement during evaporation period is more than that of Ca2+ and Mg2+ . High fluctuations of ESP between leaching and evaporation periods especially in the upper parts of the soil columns may be attributed to the different transport characteristics of Na+ , Ca2+ and Mg2+ . After the first leaching, ESP of the uppermost layer (0–5 cm) was 7.2, but at the end of the drying period this increased to 10.8 for the SCL with Treatment I (Figure 2). This high fluctuation in the upper layers was not found in the CL (being between 6 and 7.2) because the 10-day evaporation did not dry enough the wet soil surface as mentioned before in this text. However, a constant decrease of ESP was observed especially for the SCL with increase in the number of leaching, but this was particularly true for the lower parts of the CL. At the end of leaching, the concentrations of Na+ and Ca2+ –Mg2+ were close each other. Na+ leached continuously as it was

Figure 2. Changes of ESP after leaching and evaporation periods consecutively. Solid lines represents Treatment I and dashed lines are Treatment II.

252


replaced by Ca2+ and Mg2+ and because of the high amount of the internal source of Ca2+ (6.9% CaCO3 for the SCL and 7.7% for the CL) in the soils. As most of the K+ ions leached in the first leaching from both soils, the changes in concentration between the leaching and evaporation periods was not detected. It was found that leaching of K+ from the SCL was relatively faster than that from the CL. The fast leachability of K+ comparing with other cations, that is, Ca2+ , Mg2+ can be explained by the ion selectivity of the clay minerals. Fletcher et al. (1984) reported that the replacement of a monovalent cation by a bivalent cation at the exchanger sites is dependent on the charge fraction of the ion in the solution.

3.3. ANIONS Cl− is accepted as highly soluble soil amendments which are repelled by soil solids and which are not biologically precipitated, degraded or immobilized in the soil. A number of researchers found that Cl− moves downward following the irrigation or rain but moves upward to the soil surface as the soil surface dried out (Cassel, 1971). It is found that a lesser amount of water, including that lost by evaporation, was required to leach Cl− below the given soil depth for Treatment I than for Treatment II. Most of the Cl− was leached during the first leaching. The Cl− concentration reduced from 10.4 me L−1 for the SCL and from 12.8 me L−1 for the CL to less than 1 me L−1 after the first leaching. Although very low concentrations of Cl− were detected after the first leaching for both soils in the uppermost layer of the soil profile, considerable increases were found after the evaporation period due to the high mobility of this anion. The effects of evaporation on upwards Cl− transport advanced to the depth of 35–40 cm for SCL and to 25–30 cm for the CL. The effects of different leaching methods on Cl− leaching for both soils were not observed. The leaching of SO2− 4 was slow and only at the end of the third leaching it reduced to 71% of the initial concentration for the SCL and 76% for the CL. The upward movement of SO2− 4 was not determined during evaporation periods from both soils. The results of B transport after the leaching and the evaporation periods are illustrated in Figure 3. For the SCL, the amount of water used for reducing the B concentration in the leachate from 4.8 mg L−1 to less than 1 mg L−1 was 58 and 69 cm of water for Treatment I and Treatment II, respectively, and for the CL, it was 83 and 90 cm of water to reduce B concentration from 4.5 mg L−1 to less than 1 mg L−1 for Treatment I and Treatment II, respectively. However, the soil analysis showed that after the fourth leaching, approximately 1.5 mg kg−1 B found in the upper 5 cm of the soil layer and 2.5 mg kg−1 at 50 cm depth for the SCL. B was leached easier with Treatment I than with Treatment II for both soils (Figure 3). The effect of evaporation on the upward B movement by capillarity was detected only in the first two layers. The boron concentration in the upper first layer increased after the evaporation periods to 9–18% with Treatment I and 9–15%


253

Figure 3. The boron transport after the leaching and evaporation periods. Treatment I represents the leaching by 3 cm ponding water and Treatment II represents 6 cm ponding water on the soil surface

with Treatment II for the SCL soil and 7–14% and 9–14% with the Treatments I and II for the CL soil, respectively. Tanji (1970) observed that large amounts of water were needed to leach native soil boron to less than 1–2 mg kg−1 , because a portion of the fixed boron was not readily desorbed. Bingham et al. (1979) stated that the volume of water needed to reduce B from toxic to nontoxic levels is twoto three-fold greater than that needed for a comparable reduction in CL− . The effect of evaporation on the downward B movement by capillary was observed only in the first two layers. The boron concentration in the upper first layer increased after the evaporation periods to 9–18% with Treatment I and 9–15% with Treatment II for the SCL and 7–14% and 9–14% with Treatments I and II for the CL, respectively.

4. Conclusions Only experimental data directly taken on during leaching and evaporation periods have been given in this study to provide the better insight into the process of ion transports. Results showed that when sufficient water has been applied for leaching there was a net downward movement of salt. As the soil surface dries, the direction

254


of movement was reverse; towards the soil surface. It was also observed that the magnitude of this transport was mainly determined by the soil texture. Since to reduce B to a desired level much more water was required than for salts, the upward B movement gains an importance for reclamation practices in temperate areas. Finally, it is recommended that the intervals of leaching in intermittent ponding and leaching method must be arranged according to the types of the ions of that soil and the amount of water evaporated should be reduced in soils rich in Cl− and B by some soil management. Our experimental findings, although not tested by the field data, might be a significant step toward development of a model of consecutive leaching and evaporation periods in which different transport behaviors of ions are well represented. Acknowledgements The authors thank the referees for reading and correcting this manuscript and also appreciate Mr Nassar Abbas for his valuable contribution. This work was supported by the Ankara University Project No. 98-25-00-04. References Bingham, F. T., Mahler, R. J. and Sposito, G.: 1979, Effects of irrigation water composition on exchangeable sodium status of a field soil, Soil Sci. 127, 248–252. Bond, W. J. and Phillips I. R.: 1990, Ion transport during unsteady water flow in an unsaturated clay soil, Soil Sci. Soc. Am. J. 54, 636–645. Bresler, E., McNeal, B. L. and Carter, D. L.: 1982, Saline and Sodic Soils. Principles–Dynamics– Modelling, Springer-Verlag, Berlin, p. 236. Cassel, D. K.: 1971, Water and solute movement in Svea Loam for two water management regime, Soil Sci. Soc. Am. J. 35, 859–866. Fletcher, P. F., Sposito, G. and Le Vesque, C. S.: 1984, Sodium–calcium–magnesium exchange reactions on a montmorillonite soil: I. Binary exchange reactions. Soil Sci. Soc. Am. J. 48, 1016–1021. Fritton, D. D., Kirkham, D. and Shaw, R. H.: 1967, Soil water and chloride redistribution under various evaporation potential, Soil Sci. Soc. Am. Proc. 31, 599–603. Heald, W. R.: 1965, Calcium and magnesium, in: Black et al. (eds), Methods of Soil Analysis, Part II, American Society of Agronomy, Inc., Madison, Wisconsin, U.S.A. John, M. K., Chuah, H. H. and Neufeld, J. H.: 1975, Application of improved azomethine-H method to the determination of boron in soil and plants, Anal. Lett. 8, 559–568. Jurinak, J. J.: 1988, Salt-affected soils, Course Notes, Utah State University, Logan, Utah, U.S.A. Keren, R.: 2000, Salinity, in: Sumner (ed), Handbook of Soil Science, Interdisciplinary Aspects of Soil Science, G3-G21, CRS Press, New York, U.S.A. Miyatoma, S. and Cruz, I.: 1986, Spatial variability and soil sampling for salinity and sodicity appraisal in surface irrigated orchards, in: Sumner (ed), Handbook of Soil Science, Interdisciplinary Aspects of Soil Science, G3-G21, CRS Press, New York, U.S.A. Nassar, I. N. and Horton, R.: 1999, Salinity and compaction effects on soil water evaporation and water and solute distributions. Soil Sci. Soc. Am. J. 63, 752–758. Richards, L. A. (ed): 1954, Diagnosis and improvement of saline and alkali soils, USDA Agriculture Handbook No. 60.


255

Salhotra, A. M., Adams, E. E. and Harleman, D. R. F.: 1985, Effects of salinity and ionic composition on evaporation: analysis of Dead Sea evaporation pans. Water Resour. Res. 21, 1336–1334. Selassie, T. G., Jurinak J. J. and Dudly, L. M.: 1992, Saline and sodic saline soil reclamation: first order kinetic model, Soil Sci. 154, 1–7. Shukla, M. K., Kastanek, F. J. and Nielsen, D. R.: 2000, Transport of chloride through water saturated soil columns, Die Bodenkulter 51, 235–414. Tanji, K. K.: 1970, A computer analysis on the leaching of boron from stratified soil columns, Soil Sci. 110, 44–51. van Hoorn, J. W. and van Alpen, J. G.: 1990, Salinity control, salt balance and leaching requirement of irrigated soil – 19, International Course of Land Drainage, Wageningen, the Netherlands.