Irrigation with poor quality water

~

:

E LS EV l E R

Agricultural water management Agricultural Water Management 25 ( 1994) 271-297

Review Article

Irrigation with poor quality water J.D. Oster Department of Soil and Environmental Sciences, Universityof California, Riverside, CA 92521, USA Accepted 21 April 1994

Abstract Supplies of good quality irrigation water are expected to decrease in the future because the development of new water supplies will not keep pace with the increasing water needs of industries and municipalities. Thus, irrigated agriculture faces the challenge of using less water, in many cases of poorer quality, to provide food and fiber for an expanding population. Some of these future water needs can be met by using available water supplies more efficiently, but in many cases it will prove necessary to make increased use of municipal wastewaters and irrigation drainage waters. Aside from increased levels of nitrogen, phosphorus, and potassium, the salinity (total salt content) and sodicity (sodium content) of these waters will be higher than that of the original source water because of the direct addition of salts to the water and the evapoconcentration that occurs as water is used. While the use of these waters may require only minor modifications of existing irrigation and agronomic strategies in most cases, there will be some situations that will require major changes in the crops grown, the method of water application, and the use of soil amendments. Use of poor quality waters requires three changes from standard irrigation practices: ( 1) selection of appropriately salt-tolerant crops; (2) improvements in water management, and in some cases, the adoption of advanced irrigation technology; and (3) maintenance of soil-physical properties to assure soil tilth and adequate soil permeability to meet crop water and leaching requirements (LR). This paper looks at farmers' experiences, research, and computer modelling in these areas, and concludes with a discussion of examples of farm experiences with waters that caused problems with infiltration rates and soil tilth and the practices used to mitigate these problems. Keywords: Irrigation regime; Irrigation water quality; Salinity, crop effect, soil effect; Salty water, production function

1. Leaching, a key factor The key to salinity control and to irrigation sustainability is leaching, a net downward movement of soil water and salt through the root zone. Leaching interacts closely with crop 0378-3774/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved

SSDIO378-3774(94)OOO26-V

272

,LD Oster/Agricultural Water Management 25 (1994) 27l -2!)7

growth, irrigation methods, and soil-physical properties. The net downward movement of water and salt controls salt accumulation in the soil, generates drainage water and influences drainage requirements and drainage water quality. The greater the salinity of the irrigation water, the greater the leaching, or drainage, required to maintain salinity in the soil at levels which are not toxic to crops. However, leaching need not occur with every irrigation. Starting with a non-saline soil in the upper portion of the root zone, as may occur after a rainy season or after preirrigation and/or early crop irrigations with low salinity irrigation waters, it makes little sense to start leaching until salinity levels in the soil have reached hazardous levels. In fact, as pointed out by Shalhevet (1991), the rate of salinization depends on the amount of saline water applied, which should thus be kept to a desirable minimum. In other words, before soil salinities reach hazardous levels, water applications should not exceed the amount of water used by the crop. One implication of the increased need for leaching as the salinity of the irrigation water increases is that soil-physical properties must be maintained, and in some instances improved, so that the additional water required for leaching will infiltrate and move through the soil. Since the increased levels of salinity in municipal wastewaters and agricultural drainage waters are usually associated with increased levels of sodium, there is also a need to be aware of the sodicity hazards associated with water infiltration, hydraulic conductivity, and soil tilth (Sumner, 1993) and of management options or strategies available to mitigate them (Oster et ai., 1995).

2. Crops, water, and salt 2.1. Crop response to water and salinity Determination of appropriate crops for irrigation is done on the bases of the salt tolerance of the crop and the salinity of the irrigation water (Ayers and Westcot, 1985; Maas, 1990; Pratt and Suarez, 1990). The objective of the selection process is to identify crops for which achievable levels of leaching will result in soil salinities that do not reduce crop yields. The optimal leaching is usually thought to depend only on the salinity of the irrigation water, the salt tolerance of the crop, and the amount of water required to maximize yields. However, maximum yields may not correspond to maximum profits because of the costs of reusing or disposing of the resulting drainage water. Annualized disposal costs, where natural sites are not available, can exceed $150/ha (Knapp et al., 1986). Where disposal into surface streams is possible, disposal costs are far less, but farmers downstream must contend with more saline waters (van Schilfgaarde, 1982). It follows that selection of the optimal crops and irrigation management strategies depends on how crop yields and drainage volumes are affected by the amount of applied water (AW). It becomes appropriate in some situations to ask, how much can crop yields and profits be expected to decline if less water is applied to reduce leaching and drainage? Using mathematical relationships which describe how crops respond to water and salinity, Letey et al. (1985) and Solomon (1985) have developed computer programs which generate crop-water production functions that can answer such questions.

273

J.D. Oster/Agricultural WaterManagement 25 (1994) 271-297

Crop response to water and salinity is a continuous function. Crop growth and evapotranspiration (ET) are linked: ET increases with crop growth (deWit, 1958). Hanks et al. (1977) and Stewart et al. (1977) demonstrated a linear relationship between crop yield and ET when water stress was caused either by high salinity in the water or by limited water supplies. Since salinity reduces crop growth, it also reduces ET. This, in turn, tends to increase leaching. Solomon (1985) wrote the following about the dynamic aspects of the possible relationships between crop yields and leaching when saline waters are used for irrigation: "Irrigating with saline water will cause some degree of salinization of the soil. This, in turn, will cause a decrease in crop yield relative to yield under non-saline conditions. This reduced yield ought to be associated with a decrease in plant size and a decrease in seasonal ET. But, as ET goes down, effective leaching will increase, mitigating the initial effect of the saline irrigation water. For any given amount and salinity of irrigation water, there will be some point at which values for yield, ET, leaching, and soil salinity all are consistent with one another. The yield at this point is the yield to be associated with a given irrigation water quantity and salinity." These concepts are illustrated by the production functions (Fig. 1) for alfalfa (Letey and Dinar, 1986) for irrigation waters of different salinities (ECw, in dS/m). In Fig. 1, yields are given on a relative basis with a value of one representing maximum yield, and AW is scaled to pan evaporation (Ep). In the mathematical model used to calculate production functions for different crops (Letey et al., 1985), the effects of salinity on crop growth are based on the relationship between relative yield and average root zone salinity [Maas and Hoffman, 1977; see Eq. (1)]. Also, an exponential water uptake function (Hoffman and

ALFALFA

A

1.0

B

/ ~ ~~ 2~ ~ ~ - ' - -

3 4

0.8

,,,

L

~10.6> 0.4

0.2 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

AW/Ep Fig. 1. Computedrelative yields of alfalfa for variousquantifiesof appliedwater, AW, whichare scaledto pan evaporation,Ep,Eachcurveis for a givenirrigationwatersalinity (dS/m ) as indicatedby the numbersassociated withthe lines.

.I.1). Oster/Agrlcultura[ Water Management 2.5 (1994) 271-297

274

van Genuchten, 1983) is used to relate the average root zone salinity to the salinity of the irrigation water and to the leaching fraction. The upper line in Fig. 1 is the production function for non-saline irrigation water (ECw = 0). Relative yields for non-saline water are projected to increase linearly with increasing AW/Ep up to a value of about 0.76. At this value, the AW equals the maximum ET attainable by alfalfa for a given Ep or climatic condition. At lower values of AW/Ep, deficit irrigation occurs and water limits relative yield. At higher values of AW/Ep, water is not limiting and the excess water results in drainage. Relative yields decrease with increasing salinity of the irrigation water (Fig. 1). For a given ECw, relative yields increase with increasing AW/Ep, with the maximum relative yields of 1.0 achievable with irrigation water salinities of I and 2 dS/m provided sufficient water is applied. However, maximum yields are not projected to be possible with more saline irrigation waters. The drainage relationships for alfalfa given in Fig. 2 (Letey and Dinar, 1986) correspond to the production functions in Fig. 1. Drainage water depth increases as the salinity of the irrigation water increases. Note that drainage is projected to occur at all values of AW/Ep when saline water is used; in other words, it is projected to occur even when crops are underirrigated with saline waters. Maximum yields using irrigation waters with t and 2 dS/ m are projected to occur at AW/Ep values of 0.93 and 1.25 (locations labelled A and B in Fig. l ). According to Fig. 2, the corresponding drainage water depths are about 18 and 53 cm, respectively (locations labelled A' and B' in Fig. 2). Assuming an Ep value of 106 cm, the projected AW values for the I and 2 dS/m waters would be 99 and 132 cm. The corresponding leaching fractions - the ratios of drainage water to AW - would be 0.18 and 0.40. What if, as a consequence of drainage disposal costs, one were to consider reducing yield

100

ALFALFA

~

468

8O E

L.~ 60

,< z

a 40

D'

20 0 0.0

0.2

0.4

0,6

0.8 AW/Ep

1.0

1.2

1.4

1.6

Fig. 2. Computed values of drainage when alfalfa is irrigated with various quantifies of AW which are scaled to Ew The coordinates A', B', C' and D' correspond to those labelled with the same letters in Fig. I.

J.D. Oster/Agricultural WaterManagement25 (1994) 271-297

275

of alfalfa to increase profitability? Using Figs. 1 and 2 to evaluate a case where the salinity of the irrigation water is 2 dS/m, relative yield is 0.8, and Ep is 106 cm, the corresponding AW/Ep ratio (Fig. 1) would be about 0.72 (labelled C in Fig. 1); and for an El, of 106 cm, AW would be 76 cm and the deep percolation would be about 13 cm (labelled as C' in Fig. 2). How do these numbers compare for non-saline waters for relative yields of 1.0? AW has been reduced from 132 to 76 cm, deep percolation has been reduced from 53 to 13 cm, and the leaching fraction has been reduced from 0.40 to 0.17. If the salinity of the irrigation water were 1 dS/m rather than 2 dS/m, AW would be reduced from 99 to 68 cm, deep percolation from 18 to 5 cm, and leaching fraction from 0.18 to 0.07. Whether the savings in AW and drainage costs would result in greater profitability will depend on local conditions, but lowering the yield expectation should result in considerably fewer problems with managing drainage water. For example, it might forestall the need to install artificial drainage systems, which can have significant impacts on the optimum irrigation strategies (Knapp et al., 1990), and it would delay the onset of groundwater degradation in closed basins (Suarez and van Genuchten, 1981). Another result of reducing the yield expectation is to increase the salinity of the irrigation water which may be considered acceptable. For example, a relative alfalfa yield of 0.8 is projected to be possible with an irrigation water with a salinity of 4 dS/m. This salinity is twice the average root zone salinity above which the relative yield of alfalfa is expected to decline (Maas, 1990). Irrigation waters with salinities of 3--4 dS/m water have been used successfully to sprinkle irrigate alfalfa in the Arava Valley of Israel, where rainfall is nil and maximum summer temperatures are about 45°C. In a visit in 1994 to this valley, alfalfa fields were observed that had been sprinkle irrigated for several years with these saline waters (Fisher and Oster, travel notes). The reported yields were comparable to those obtained under similar climatic conditions in the Palo Verde Valley of California, where the irrigation water has a salinity of 1.2 dS/m. Since sprinkler irrigation with these saline waters causes leaf damage and possibly a yield reduction, the Arava Valley farmers are now experimenting with subsurface drip irrigation (Greenberg, Director, Arava Desert Agricultural Research Station, private communication, 1994). The drip tubing is placed 45 cm deep at spacing of 90 cm spacing with an emitter spacing of 40 cm. 2.2. Water quality assessment and leaching requirement

How are assessments of water quality and leaching requirements (LR) usually made? There are several methods available for estimating LR, based on different perspectives on how to estimate the average root zone salinity (Ayers and Westcot, 1985; Hoffman, 1990; Pratt and Suarez, 1990). Differences among methods can be significant, particularly if the root zone salinity is weighted for the amount of water uptake as Rhoades and Merrill (1976) proposed for high frequency irrigation. Leaching requirements obtained using this approach are considerably smaller than for the other methods, indicating that increasing irrigation frequency should be beneficial when irrigating with saline water. Shalhevet (1991), however, concluded that although guayule benefits from high frequency irrigation, data for a broad spectrum of vegetable and field crops indicate there is no general advantage to increasing irrigation frequency, and in some cases it can have negative impacts. In this paper, the LR based on a water uptake distribution of 40:30:20:10% for the first through

276

J.D. Oster /Agricultu¢al Water Management 25 (1994) 271 -297

[ourth quarters of the root zone will be calculated (Rhoades and Merrill, 1976; Ayers and Westcot, 1985; Pratt and Suarez, 1990) and will be refered to as the tabular method. Requirements will also be calculated using a simple equation proposed by Rhoades (1974). Both sets of results will be compared to those obtained from production functions. Average root zone salinities of a saturated soil extract taken from the root zone, EC~, for a 40:30:20:10 water uptake distribution can be calculated for different leaching fractions using the F values given in Table 1. EQ, equals the product ECw-F. The LR for 100% yield of a given crop corresponds to the leaching traction at which EC~ equals the threshold salinity, ECt, for the crop. Alternately, one can divide EC, for a given crop by ECw to obtain an F value which can then be used in conjunction with Table 1 to estimate the leaching and relative crop water requirements. The same procedures can be used to estimate LR for relative yields, K less than 100% by using the Maas-Hoffman equation [Eq. (I)1 to calculate the corresponding EC~.: Y= 1 0 0 - ( E Q - EC,)S; for EC,, >/EC~

(1)

where S represents the rate yield declines with increasing salinity. The equation proposed by Rhoades (1974) LR = ECw/(5ECt

-

(2)

ECw)

is simpler to use than the tabular method. For relative yields less than 100%, EC, is replaced by the EC~ corresponding to the desired yield as calculated from Eq. ( 1). How do the LR values calculated by the tabular method and by Eq. (2) compare to those obtained from the production functions for the crops reported by Letey and Dinar (1986) ? They all tend to give similar results (Table 2a), particularly considering that differences in LR of 5-10% are small compared to the uncertainty in crop water requirements (Shalhevet, 1991). Leaching requirements for crops with low threshold salinities based on production functions tend to be greater than for the tabular method (Table 2a). This is because the production function method uses an exponential water uptake function which results in a Table 1 Concentration factors (F) for predicting leaching requirement (LR) and crop water requirement Concentration factor (F)

Leaching fraction (LR)

Applied crop water requirement (% of ET)

3.2 2.1 1.6 1.3 1.2 1.0 0.9 0.8 0.7 0.6 0.6

0.05 0.10 0.15 0.20 0.25 0.30 0.40 0.50 0.60 0.70 0.80

105.3 lll.l 117.6 125.0 133.3 142.9 166.6 200.0 250.0 333.3 500.0

277

.I.D. Oster /Agricultural Water Management 25 (1994) 271-297

Table 2 Calculated leaching requirements for selected crops and for relative yields of 1.0 and 0.8 assuming an irrigation water salinity of 2 dS/m. Calculation procedures used are as follows: ( I ) Letey and Dinar production functions; (2) Tabular method based on a water uptake distribution of 40:30:20:10 and (3) Rhoades equation Crop

Salt tolerance coefficients*

leaching requirements

ECt dS/m

Letey and Dinar production function (%)

S % per dS/m

Tabular method ( %)

Rhoades equation ( %)

(A) Relative yield of 1.0 Lettuce 1.3 Corn 1.7 Alfalfa 2.0 Tomato 2.5 Celery 1.8 Wheat 6.1 Cotton 7.7

13 12 7.3 9.9 6.2 3.2 5.2

67 49 38 31 33 7 <5

65 45 30 27 27 7 <5

45 30 25 20 20 7 5

(B) Relative yield of 0.8 Lettuce 1.3 Corn 1.7 Alfalfa 2.0 Tomato 2.5 Celery 1.8 Wheat 6.1 Cotton 7.7

13 12 7.3 9.9 6.2 3.2 5.2

17 -

18 13 20 10 8 <5 <5

7 12 17 10 9 3 4

*Maas (1990). higher EC~ value for a given ECw and leaching fraction than the 40:30:20:10 uptake distribution used in the tabular method. Finally, the leaching requirements obtained using Eq. ( 2 ) tend to be low for crops with low threshold salinities. However, Eq. (2) is quite useful for quick estimates o f LR. Both the tabular method and Eq. (2) are currently more useful than the production function approach, primarily because functions are available for only 11 crops (Letey and Dinar, 1986) whereas threshold salinities are available for 69 crops (Maas, 1990). As mentioned above, yield reductions due to salinity can be taken into account by both the tabular method and Eq. ( 2 ) in assessing irrigation water suitability for different crops and the associated LR. Reducing the relative yield reduces the LR, as can be seen by comparing the results in Table 2a with those in Table 2b. There is closer agreement between the LR obtained by the tabular method and Eq. (2) for a relative yield o f 0.8 (Table 2b) than for a relative yield of 1.0 (Table 2a). The LR o f 17% obtained using both methods for alfalfa agrees with the 17% figure obtained using information provided by a production function. For the case o f reduced yields, applying LR to the determination of the water requirement (Table 1 ) involves an uncertainty in how much the crop ET will be reduced by salinity. If

278

.I.D. Oster/Agricuhural WaterManagement25 (1994) 271 297

crop ET values for maximum yields are used, the calculated water requirements, AWr, will be high: AWr - E T / ( 1 - L R )

(3)

Correcting ET is relatively straightforward for crops like alfalfa, where the linear relationship between relative yield and ET intersects the origin: the reduction in ET is directly proportional to the reduction in yield. However, for crops where evaporation from the soil surface is a significant component of the water requirement, such as annual, short-season row crops like lettuce and tomato, the relative yield-ET relationship extrapolates to a finite ET value at zero relative yield (Letey and Dinar, 1986). For these crops, it follows that estimating the effect of salinity on ET requires knowledge about evaporation, particularly during the germination and crop establishment phases of crop growth, and how it is affected by different methods of irrigation. For example, soil evaporation from a partially wetted soil surface resulting from drip irrigation will be less than from a fully wetted soil surface from sprinkler or furrow irrigation. 2.3. Use of saline waters for irrigation Farmer experience. Farmers have successfully used waters that are conventionally classified as having moderate to severe restrictions to irrigate a broad spectrum of crops (Ayers and Westcot, 1985; Rhoades et al., 1992) in Bahrain, Egypt, Ethiopia, India, Iraq, Israel, Pakistan, Somalia, Tunisia, United Arab Emirates, and the United States. FAO Publications 29 (Ayers and Westcot, 1985) and 48 (Rhoades et al., 1992) and ASCE Manuals and Reports on Engineering Practice no. 71 (Tanji, 1990) provide comprehensive information on management practices for agricultural water and salinity problems. In this section, farmer and researcher experiences in the United States and Israel will be highlighted with the use of saline irrigation waters. This will be followed by a discussion of how these experiences have been incorporated into computer models of transient soil water and salinity conditions and their potential use for planning on-farm management strategies. In the Pecos Valley of Texas, groundwaters with salinities averaging about 3.5 dS/m but ranging as high as 8.0 dS/m have been used successfully to irrigate chile pepper, cotton, small grains, sorghum and alfalfa (Miyamoto, 1984). The threshold salinities for these crops range from 2 to 8 d S / m . Examples of special irrigation practices used to mitigate the effects of salinity include alternate furrow irrigation to move salts to the dry side of the bed, planting seeds on the edges of flat beds where salt accumulation is minimal, replanting following rainfall if the resulting crusting limits seedling establishment, and single-row plantings on narrow beds followed by removal of the peaks of the beds prior to seedling emergence to remove soil and/or salt crusts. In Arizona (Dutt et al., 1984), farmers use well waters with salinities ranging from 3 to 4 dS/m together with alternate furrow irrigation to establish cotton. Saline well waters as high as 11 dS/m are used after the crop is established. In southwestern Colorado, rainfall before and during the crop season facilitates the use of river waters with salinities ranging from 2 to 5 dS/m for the irrigation of alfalfa, sorghum, winter wheat, barley, and sugarbeets (Miles, 1977). By selecting an appropriate crop rotation, a grower can often maintain productivity. Alfalfa irrigation in the Imperial Valley of California is often just sufficient to meet the

J.D. Oster /Agricultural Water Management 25 (1994) 271-297

279

crop's ET needs, because no more water will infiltrate. The result is inadequate leaching and increased soil salinity. Rotation of alfalfa (EC t = 2.0) with winter crops that have a low ET requirement such as lettuce (ECt= 1,3 dS/m) provides an opportunity to apply the additional irrigation water need for leaching. The salinity of the irrigation water from the Colorado River water used in the Imperial Valley ranges from 1.2 to 1.5 dS/m. This is relatively saline when compared to the threshold salinity of lettuce. However, because lettuce is shallow rooted, soil salinities in the root zone can be reduced to levels which are not hazardous for lettuce by sprinkler irrigation during the seedling and germination phases of crop growth. Continued application of more water than needed by lettuce can satisfy leaching needs left unmet during the time alfalfa was grown. This is an example of how farmers can take advantage of climate and the range in salt tolerance and rooting depths among crops to achieve the LR through the overall crop rotation system. In the Arava Valley of Israel, where rainfall is generally less than 25 mm, peppers, melons, tomatoes, potatoes, onions, sweet corn, and alfalfa are grown commercially with surface drip and sprinkler irrigation techniques, using moderately saline groundwaters ranging in salinity from 2 to 4 dS/m The threshold salinities for these crops range from 1.2 dS/m for onion to 2.5 dS/m for tomato. Where two waters with different salinities are available, the lower salinity water is used for irrigation during germination and seedling establishment. Sprinkler irrigation is commonly used for 2 to 3 weeks during the seedling and early stages of crop growth to leach the seed bed and obtain uniform plant stands. Thereafter, surface drip irrigation is used. Drip irrigation simplifies the use of saline waters for irrigation: low soil salinities are maintainable in the major portion of the root zone provided the crop and the drip line are located along the same line, and soil-water contents can be constantly maintained at high levels. "The drip irrigation method provides the best possible conditions of total soil-water potential for a given quality of irrigation water" (Shalhevet, 1991 ). However, farmers must be aware that salts accumulate at the perimeters of the wetted area and that tillage practices to incorporate crop residues and form new seed beds can also incorporate these salts into the seed zone. Consequently, extra irrigation for leaching during seedling germination and plant establishment may be necessary to re-establish satisfactory soil salinity levels in the root zone. Research experiences. Since the salt tolerance of some crops increases as the plant matures (Pasternak et al., 1986; Maas and Poss, 1989) and crops benefit from a shorter exposure to salinity (Shalhevet, 1991), the availability of some good quality irrigation water, particularly before and after planting, facilitates the use of moderately saline irrigation waters. A cyclic strategy of using waters of different salinities as proposed by Rhoades (1987) has been tested and demonstrated to be sustainable in maintaining crop rotations which include both salt-sensitive and salt-tolerant crops. Moderately salt-sensitive crops can be included in the rotation because the distribution of soil salinity with depth will change with time. The non-saline water is used for pre-plant and early crop irrigations of the moderately salttolerant crop and for all irrigations of the moderately salt-sensitive crop. Salt-tolerant crops are irrigated with saline water after they have reached a salt-tolerant stage of growth, since both the moderately salt-tolerant and the salt-tolerant crops benefit from a shortened exposure to saline water (Shalhevet, 1991 ). After the salt-tolerant crop is grown, a pre-irrigation with low-salinity water was proposed to reclaim the upper portion of the soil profile in order

280

,I. l), Oster/ Agricultural Water Management 25 (I 994) 271-297

to establish the salt-sensitive crop. In a successful test of the cyclic strategy conducted in the San Joaquin Valley of California, a 0.5 dS/m water was used to irrigate cotton (EC, = 7.7 dS/m) during germination and seedling establishment, and 7.9 dS/m water was used thereafter (Rhoades, 1987). Wheat ( ECt = 6.1 dS / m) was subsequently irrigated with the 0.5 dS/m water, followed by 2 years of sugar beets (ECt=7.0 dS/m) with the cyclic strategy used again for irrigation. Rhoades et al. (1989) reported the results from a second study conducted in the Imperial Valley of California. In a rotation of wheat, sugar beets, and melons, Colorado River water ( 1.5 dS/m) was used to irrigate the melons and for the pre-plant and early irrigations of wheat and sugar beets. Alamo River drainage water (4.6 dS/m) was used for all other irrigations. Sugar beet and wheat yields were not reduced, and crop qualities were often improved from the use of saline drainage water. Ayars ( 1986 a,b) used drip irrigation tor three consecutive years to apply a 8.0 dS/m water to cotton after it was established with 0.5 dS/m water. A wheat crop irrigated with the 0.5 d S / m water followed cotton; sugar beets followed wheat and were irrigated with the 8.0 d S / m water alter stand establishment. Yields under these conditions were the same as from continuous irrigation with the good quality water. Shennan et al. (1987) and Grattan et al. (1987) tested a modified cyclic strategy on a crop rotation that consisted of 2 years of cotton followed by processing tomatoes (EC t = 2.5 dS/m). Saline drainage water ( E C = 7 . 9 d S / m ) was applied to the tomato crop at a relatively tolerant growth stage to improve fruit quality. High-quality water was used for reclamation on the subsequent two cotton crops. To reclaim the soil, high-quality water (EC = 0.4 dS/m) was used to irrigate the two subsequent cotton crops. Yields of tomatoes were sustained and soluble salts were increased when drainage water, applied after first flower, supplied over 65% of the irrigation water requirements. Pasternak et al. (1986) also reported higher soluble solids in tomatoes increased irrigated with a 7.5 dS/m water after the fourth or eleventh leaf stage, but yields were reduced by 30%.

2.4. Computer models Several research groups have developed soil-water-plant models which are useful for the evaluation of different management strategies, particularly where two or more waters with different qualities are available for irrigation (Childs and Hanks, 1975; Wagenet and Hutson, 1987; Cardon and Letey, 1992). Data generated by these models also indicate crop yields are increased if soil salinities are low during the early portion of the growing season (Bradford and Letey, 1993a). These authors used a transient state salinity model to simulate the effects of various cyclic and blending strategies using non-saline and saline waters for irrigation of alfalfa. They concluded that no significant difference in yield occurred whether the waters were mixed prior to application, or intermittently applied for different lengths of time later in the growing season. This is consistent with the conclusion reported by Meiri et al. (1986) that potato and peanut crops responded to the weighted mean water salinity regardless of whether different salinity waters were blended before application to the soil or were applied intermittently and allowed to blend within the soil. However, lower yields were obtained with an early season irrigation using saline water and late season irrigation with non-saline water. Bradford and Letey (1993a) obtained similar results for alfalfa. When saline and non-saline waters were applied in alternate years, total annual yields were

J.D. Oster/Agricultural WaterManagement 25 (1994) 271-297

281

out of phase with the salinity of the irrigation water, but in phase with the soil salinity during the initial part of the year. Soil salinities generated during one year of irrigation with a given water carried over to the beginning of the next year when a different water was used. Computer models provide a way to answer the question, how saline can the water be? Dinar et al. (1986) used a seasonal model to generate production functions from which they were able to compute equal yield curves for combinations of waters of two qualities. They concluded that waters of ECw = 4, 6, or 11 dS/m cannot be substituted for ECw = 0 water for lettuce ( E C t = 1.3 dS/m). With oats (ECt=2.2 dS/m), a broad range of substitution was possible with 4 dS/m water, but only a limited substitution was possible with a 6 dS/m water. With c o t t o n ( E C t = 7.7 dS/m), substitution was possible for all three salinity waters considered. Substantial contributions to the ET of cotton have been shown to occur from shallow saline groundwaters (Wallender et al., 1974). Bradford and Letey (1993b) simulate the effects of a high water table and irrigation scheduling on cotton production. One of the findings from the simulation for water table conditions was that higher simulated yields were achieved by applying less irrigation during the crop season and more during the preirrigation for salt leaching purposes. The authors concluded that "high cotton yields could be achieved for several years even if the water table was saline and no drainage occurred if the irrigation water was low in salinity". In the future, computer programs could play a key role in the development of alternative irrigation plans for on-farm use. This will be particularly true where there are two or more sources of irrigation water with different qualities. For example, along the west side of the San Joaquin Valley in California, there are four sources of water in some regions: ( 1 ) nonsaline surface water (EC =0.4 dS/m), (2) shallow water tables with salinities ranging from 8 to 12 dS/m, (3) groundwaters with salinities ranging from 1 to 4 dS/m, and (4) rainfall. Rainfall during the winter season is highly variable from one year to the next, with overall annual averages ranging from 15 cm in the (southern end of the valley to 300 mm in the north). Irrigated agriculture is being encouraged to reduce its use of non-saline surface water and to increase the use of groundwater. Increased use of groundwater will, in time, increase its salinity because agricultural drainage water is one of the water sources for aquifer recharge (Quinn, 1991; Letey & Oster, 1993). One potential strategy will be to increase water use efficiency through improved irrigation practices, which would lower shallow water tables and decrease the volume of drainage waters returned to underlying aquifers thereby reducing their rate of salinization. A broad spectrum of vegetable, grain, and fiber crops are grown in the region, with a corresponding broad diversity of salt tolerances, growing seasons and rooting depths. Computer models would be useful for simulating the effects of proposed management schemes. As discussed above, computer programs exist which account for transient water and salt movement, salt tolerance of the crop, seasonally-variable potential ET or precipitation, irrigation water salinity, rooting depth increases with time, and the presence or absence of a water table, which are capable of multi-seasonal simulations with crop rotation possibilities (Cardon and Letey, 1992). Run times for individual simulations on 486 computers require less than 10 rain. However, further work on the computer code is needed to increase its friendliness and program manuals need to be written before consultants, farm advisors, specialists, and farmers will find it easy to use them.

,I.D. Oster /Agricultural Water Management 25 (1994) 271 297

282

3. Water quality and soil-physical properties An underlying assumption made in assessing LR is that the AW can readily infiltrate and move through the soil to produce the computed drainage necessary for leaching. In general, infiltration rates (IR) and hydraulic conductivities (K) decrease with increasing SAR* and decreasing EC of the irrigation water. This suggests that saline waters can be applied without generating adverse soil-physical properties because high EC waters offset the effects of the SAR of the water. In practice, however, this is not entirely true. After saline water is applied to the soil, the exchangeable sodium percentage (ESP) of the surface soil equilibrates with the SAR of the irrigation water. As the soil water is concentrated by evaporation or ET, the EC and SAR of the soil water and the ESP of the soil increase. Here it is important to note that the numerical values of SAR and ESP, after equilibrium conditions are established, are about the same in the range of 0 to 40. Irrigation with a non-saline water or rainfall tends to rapidly reduce the EC of the soil water near the soil surface. The ESP, on the other hand, will not be reduced as much because in a given volume of soil the number of exchangeable ions is generally 50 to 500 times greater than the number of ions in the soil water. Consequently, the number of calcium and magnesium ions available in the soil solution are much smaller than the number needed to replace exchangeable sodium. In calcareous soils, dissolution of calcite (CaCO3) is another source of calcium. However, the rate calcite dissolves can be too slow to reduce exchangeable sodium as fast as the EC of the soil water is reduced. If EC becomes too low to counteract the effects of exchangeable sodium, clay swelling and dispersion occurs resulting in reduced infiltration rates and hydraulic conductivities and in dense, hard crusts at the soil surface when the soil dries.

3.1. Clay swelling and dispersion Clay swelling and dispersion are the two mechanisms which account for changes in hydraulic properties and soil structure (Quirk, 1986). Swelling that occurs within a fixed soil volume reduces pore radii, thereby reducing both saturated and unsaturated K (McNeal et al., 1966; Rengasamy et al., 1984). Swelling results in aggregate breakdown, or slaking (Abu-Sharar et al., 1987), and clay particle movement, which in turn leads to blockage of conducting pores (Frenkel et al., 1978). Clay swelling occurs because clay particles imbibe water to lower the exchangeable cation concentration near the negatively charged surfaces of the clay. Divalent calcium ions are more strongly absorbed to clay surfaces than monovalent sodium. Consequently, calcium clays swell less than sodium clays. The salinity of the soil water also affects swelling. Swelling of both calcium and sodium clays increases as the salinity decreases. *SAR has units of (mrnolJl) I/2 because SAR =

cN,

where ion concentrations C are in mmolJl. Following conventional usage, SAR values in the text will not include these units.

J.D. Oster/Agricultural WaterManagement 25 (1994) 271-297

283

Other factors that also impact swelling include pH (Suarez et al., 1984), iron and aluminum oxides (Goldberg and Glaubig, 1987), organic matter (Martens and Frankenburger, 1992), and interactions among the negative and positively charged surfaces of the different clay minerals in soil (Ben-Hur et al., 1992). The effects of these factors on swelling and dispersion are beyond the scope of this review and are described in detail in a reviews written by Sumner (1993) and Shainberg and Letey (1984).

3.2. Hydraulic conductivity (K) Typical effects of salt concentration, i.e. salinity, and exchangeable sodium on the K of soils from the western United States are shown in Fig. 3. Each soil responds differently to the same combination of salinity and SAR because of differences in clay content and mineralogy, iron and aluminum oxide content, and organic matter content. Quirk and Schofield (1955) introduced the concept of"threshold concentration", or salt concentration 120 A

Gila

._1 O

E E 100

V

¢-

._o ¢,.) "O

80

n"

60 O

40 O tO

O 20

0ffvi° =0 100

80

60

40

20

\

0

Sodium-Adsorption-Ratio(SAR) Fig. 3 Combinations of salt concentration and SAR required to produce a 25% reduction in hydraulic conductivity for selected soils from the western U.S. (adapted from McNeal and Coleman, 1966).

284

.I.D. Oster/ Agricultural Water Management 25 (1994) 271-297

at which a 10-15% decrease in K may occur for a given SAR. A plot of threshold concentration against SAR for a British soil (threshold concentration, Quirk and Schofield, Fig. 1) resulted in an approximately linear line for 0 < SAR < 60. Salt concentrations above and to the right of the line for the British soil were greater than the threshold for a given SAR and K was stable; in other words decreases in K were less than 10%. Hydraulic conductivities were not stable at salt concentrations below and to the left of the line, because these concentrations were inadequate to prevent swelling and/or dispersion. With the exceptions of the Vale and Aiken soils (McNeal and Coleman, 1966; Aiken not shown on graph), 25% reductions in K were associated with higher salinities than the "threshold concentration" associated with 10-15% reductions for the British soil. In other words, the Oasis, Grangeville, Pachappa, and Waukena soils were somewhat more sensitive to salinity than the British soil and Gila was much more sensitive. In summary, swelling and dispersion increase with increasing SAR and decreasing salinity, thereby influencing the physical properties of each soil in a unique manner. Significant reductions (10-25%) in saturated K for soils with ESP values of 15 can be expected if salt concentrations are less than 5 to 50 m m o l J l (0.5 to 5 dS/m). Based on research conducted since 1966 (Frenkel et al., 1978), similar reductions can be expected for soils with ESP values as low as 3 if soil salinity is less than 0.2 to 1 dS/m.

3.3. Infiltration rate (IR) When water is applied to the soil surface, whether by rainfall or irrigation, the rate of water entry into the soil, or infiltration rate (IR), is high during the initial stages of infiltration but decreases exponentially with time to approach a constant rate. Two main factors cause this drop in IR: ( 1 ) a decrease in the matric potential gradient which occurs as infiltration proceeds, and (2) the formation of a seal or crust at the soil surface. In cultivated soils from semiarid regions, the organic matter content is low, soil structure is unstable, and sealing is a major determinant affecting the steady-state IR (Duley, 1939; Morin and Benyamini, 1977). Seal formation at the soil surface is in turn due to two processes: (1) physical disintegration of soil aggregates and their compaction caused by the impact of water, especially water drops; (2) chemical dispersion and movement of clay particles and the resultant plugging of conducting pores. Both of these processes act simultaneously, with the first enhancing the second (Agassi et al., 1981). Infiltration rates are especially sensitive to the effects of SAR and EC of the irrigation water, because of the mechanical and stirring action of falling water drops and the relative freedom of particle movement at the soil surface (Oster and Schroer, 1979). Where irrigation waters with different qualities were applied for 19 months to cropped, undisturbed columns of a loam soil, the authors reported considerably better correlation of the final IR to the SAR and EC of the AW than to the SAR and EC of the soil solution of the surface soil layer, or to average the values for the entire soil column The effect of soil ESP on the IR of a sandy loam subjected to rainfall is presented in Figs. 4a,b (Kazman et al., 1983). Increasing the ESP of the sandy loam soil (Fig. 4a) from 1.0 to 2.2 dropped the final IR from 7.5 to 2.3 mm/h; increasing the ESP to 4.6 dropped the final IR to 0.6 mm/h. Similar results were obtained with the loessial silt loam (Fig. 4b). For both soils, spreading phosphogypsum at the soil surface was effective in reducing seal formation and the associated

J.D. Oster / Agricultural Water Management 25 (1994) 271-297 i

26

24 o

~

- ~'\~.X

/\

, •

" x ',

ESP 2.2

Rain Intensity 2 6 m r n / h r

~

"~ ~ ESP 1.0 and 2.2 _Gypsum~ ~ ,,,5 ton/ha ~ '~-~ ~ ~ ESP 11.6 ~ ~ -- ~ -- --

24 •

15.0 t

I

i

i

~

,

C u m u l a t i v e Rain, m m

I

60

Rain Intensity 26mm/hr

' ~ '~2. S 1oNh a

~8 ~

40

Aggregatesize 0-4mm

,,

rr' 12 t'O

~

2O

r

16

•

/

i

LOESS,NAHAL-OZ

x

~

8 ~SP4~ ESP

i

x xx ~

n-

°0

~" t~

Aggregate size 0-4ram

x

4~-

i

HAMRA-NETANYA

"~'-

~

0

i

285

ESP 1.8

~ ~ - -- _ "

ESP 6 4

"

ESP 6.4 . . . . .

4 I

0

8O

~ ~

0

i

20

i

4'0

Cumulative

~

I

~,

Rain, rnm

Fig. 4. Effects of ESP and phosphogypsum on the infiltration rate of a Hamra-Netanya sandy loam (left) and a loessial-Nahal-Oz soil (right) (Kazman et al., 1983).

drop in IR (Figs. 4a,b). Phosphogypsum dissolves and increases the EC and decreases the SAR of the soil solution thereby reducing clay dispersion and seal formation. Magnesium is not as effective as calcium in enhancing IR. At a given ESP, replacing exchangeable calcium with magnesium enhanced the rate of IR decline and lowered the final IR for the soils used to obtain the data in Figs. 4a, b (Keren, 1991). This specific effect of magnesium was attributed to the difference in size between hydrated magnesium and calcium ions, with resulting differences in strength of attraction to cation exchange sites. Hydrated magnesium, which is larger than hydrated calcium, enhances swelling and dispersion which, in turn decreases the amount of raindrop energy needed to cause seal formation.

3.4. Water quality guidelines The lines in Fig. 5 labelled "Q&S, 1955" and "O&S, 1979" provide quick comparisons of threshold salinities for K and IR. EC/SAR combinations to the right of the "Q&S, 1955" line are likely to result in less than 25% reductions in K. Similarly, EC/SAR combinations to the right of the "O&S, 1979" line are likely to result in less than 25% reductions in steady-state IR. These generalized differences in impacts of EC and SAR on K and IR are consistent with the final figure in the review written by Shainberg and Letey (1984) as well as the EC/SAR guidelines in Table 3 proposed by Ayers and Westcot (1987). These guidelines were also based on the experience of farm advisors and specialists in California obtained as a result of advising growers about management alternatives to deal with slow water infiltration and water movement through soils. When using the guidelines in Table 3, keep in mind that unexpected impacts of EC and SAR can occur. If so, water infiltration or hydraulic conductivities are likely to improve if amendments are used to increase the EC and decrease the SAR of the soil water. Laboratory tests using soil columns, treated and untreated with gypsum can be conducted to determine if gypsum will improve water infiltration or hydraulic conductivities (Rhoades and Loveday, 1990). For water infiltration problems, the EC~ and SAR of saturation extracts of the surface

286

J.D. Oster / Agricultural Water Management 25 (1994) 271-297 60

--- I

50

KI -~----~i ~"f---i I --~-' l [- I

I

c040

0

0

2

4

6

8

10

Soil Salinity, dS/m

Fig. 5. Effect of SAR and salinity on hydraulic conductivity, K, and on infiltration rate, IR. The line labelled K is based on a 10-15% reduction in K of a British soil (Quirk and Schofield, 1955); the line labelled IR represents the combinationof SAR and salinity which resulted in a 25% reduction in steady state IR of a North Dakota loam soil (Oster and Schroer, 1979). Table 3 Combined effects of sodium adsorption ratio and electrical conductivity of either the irrigation water or of a saturation extract on the likelihood of problems with low infiltrationrates or hydraulic conductivities When sodium adsorption ratio of the irrigation water or soil water is

0-3 3.1~ 6.1-12 12.1-20 20.140

Potential water problem Infiltrationunlikely if ECe or ECw is

Hydraulic conductivity likely if ECe or ECw is

>0.7 >1.0 >2.0 >3.0 >5.0

<0.3 <0.4 <0.5
10 cm of soil are good indicators o f a problem as they will be greater than the corresponding values for the irrigation water. In such a case, injecting gypsum into the irrigation water or applying and incorporating g y p s u m into the soil surface is likely to improve infiltration. G y p s u m requirements, G R ( T a b l e 4) depend on the cation exchange capacity ( C E C ) and initial and final levels of ESP (or S A R ) according to Eq. (4): GR = 0 . 0 0 8 6 ( F ) (Ds) (Pb) ( C E C ) (ESPi - ESPf)

(4)

where F (unitless) represents the Ca-Na exchange efficiency factor, Ds is the soil depth ( m ) , Pb is the soil-bulk density ( M g / m 3 ) , CEC is the cation exchange capacity (mmolc/ kg), and ESPi and ESPf represent the initial and final exchangeable sodium percentages. The efficiency factor ranges from 1.1 for an ESPf o f 15 to 1.3 for an ESPf of 5 (Oster and Frenkel, 1980). S A R can be substituted for ESP in the range 0 < SAR < 40. The cation exchange capacities in Table 4 range from 100 to 500 m m o l J k g which represent soil

J.D. Oster/Agricultural WaterManagement 25 (1994) 271-297

287

Table4 Impactof cationexchangecapacityand initialsodiumadsorptionratio (SARI) of a saturationextract,or an initial •exchangeablesodium percent (ESPI), on gypsumrequirements* for a soil depth intervalof 10 cm and a final SAR or ESP of 5 SARI or ESP~

Cation exchange capacity (mmolc/kg) 100.0

200.0

300.0

400.0

500.0

2.5 7.4 12.4 17.4

3.3 9.9 16.5 23.2

4.1 12.4 20.7 29.0

Gypsum requirements (Mg/ha) 10.0 20.0 30.0 40.0

0.8 2.5 4.1 5.8

1.7 5.0 8.3 11.6

*Gypsum requirements were calculated using Eq. (4) assuming F and Pb were 1.3 and 1.48 Mg/m 3, respectively.

textures ranging from sandy loams to clays. The gypsum requirements given in Table 4 assume a depth interval of 10 cm and an SARf (or ESPf) of 5. This final SAR would be a desirable goal for several situations: (1) prevention or amelioration of infiltration problems related to rainfall, (2) irrigation with dilute irrigation waters (EC, < 0.7), and 3) cyclic irrigation strategies using sodic irrigation waters (SAR> 10) and non-saline irrigation waters (ECi < 0.7 dS/m). In assessing potential gypsum requirements stemming from the use of a sodic irrigation water in conjunction with non-sodic water, one would use the SAR of the irrigation water for SARi. This recommendation assumes that the SAR of the soil surface will equilibrate with the SAR of irrigation water. Equilibrium conditions may require more than one irrigation season to establish. This is particularly true where the salinity of the sodic irrigation water is low. The amount of irrigation water required to achieve equilibrium conditions depends primarily on the salinity of the irrigation water, CEC, and initial SAR (or ESP) of the soil. Computer models exist which could be used to evaluate changes in SAR (or ESP) of the soil as a function of the amounts and compositions of the irrigation waters used (Wagenet and Hutson, 1987; Simunek and Suarez, 1994). For calculating gypsum requirements to improve K, a final SAR of 10 would likely be more appropriate than 5 for calculating gypsum requirements. The numbers in Table 4 can still be used. The values in the table would be reduced by the gypsum requirements given for SARi = 10; in other words, for a given cation exchange capacity subtract the number in the first row from the other numbers in the same column to obtain a gypsum requirement corresponding to an SARf of 10.

4. Soil-physical properties: maintenance and improvement Problems with reduced infiltration rates, poor soil tilth and crusting can occur in most soils, with the possible exceptions of sands and loamy sands. In California, where infiltration problems are usually related to the use of non-saline waters for irrigation (Oster and Singer, 1984), problems occur with soil textures that range from sandy loams to clay loams even

288

J./). Oster/ A~ricultural Water Management 25 ( I q94) 271-297

when the ESP percent is low ( < 5). Where infiltration is adequate, crop growth will likely maintain salinities at levels which will maintain K. For sodic soils, cropping can increase K (Robbins, 1986) and it is the common method used to reclaim sodic soils (Oster et al., 1995). Where infiltration is inadequate, the management options used to mitigate the problem will maintain, and possibly improve K. Infiltration of low salinity irrigation waters and/or rainfall is essential to the success of a cyclic strategy where one of the irrigation waters is saline. The foremost objectives of the farming practices under these circumstances will be the following: ( 1 ) to assure adequate water infiltration to leach the salt from the upper portion of the root zone and to bring the water content of the root zone to field capacity, (2) to assure adequate soil tilth so that seedbeds can be prepared where seeds germinate and seedlings emerge, and (3) to assure adequate aeration of the root zone, particularly during seedling establishment. Farmers effect soil-physical properties through various combinations of tillage, amendment, and cropping practices (Oster et al., 1992). Tillage destroys surface crusts and increases water penetration for at least one irrigation. The water composition can be changed with chemical amendments. Sulfuric acid or sulfur dioxide could be added the to water to lower the bicarbonate concentration, thereby reducing the potential SAR of the infiltrating water and the ESP of the surface soil. Gypsum could be applied to the soil surface or added to the water to both increase the EC and reduce the SAR of the infiltrating water. Sulfuric acid could be applied to calcareous soil to reduce the ESP of the surface soil. Another example of the use of acid, practiced in the Salinas Valley of California is band application of phosphoric acid over the seed row of lettuce, which temporarily reduces soil-crust development and helps plant emergence. The use of gypsum to increase water infiltration is an old practice. Field trials, conducted in Australia between 1921 and 1933 (Simms and Rooney, 1965) on soils which contained little exchangeable sodium demonstrated that surface application of gypsum reduced soil crusting thereby increasing water infiltration and, in turn, crop yield. Between 1963 and 1965, an estimated 445 000 ha of fallow soil was treated with gypsum to improve dryland wheat yields in the Wimmera and Southern Malle Districts of Victoria, Australia. Doneen (1948) reported that 270000 Mg of gypsum were applied to the soil in 1945 by farmers in the San Joaquin Valley of California to improve infiltration. The addition of gypsum to the dilute Friant-Kern irrigation water (EC = 0.05 dS/m; SAR = 0.5), or to the non-saline, nonsodic soils - irrigated with this water, for the purpose of improving infiltration water a common practice in the 1950s on the east side of the San Joaquin Valley of California between Fresno and Bakersfield (personal communication, Robert Ayers, 1980). Ayers and Westcot (1985) report that for a potato crop, gypsum applied and disked into the soil at rates as high as 10 M g / ( h a / y r ) resulted in greatly improved infiltration. Adding gypsum to the water at rates sufficient to increase the calcium concentration by 2 to 3 m m o l J l was also effective. Machines are now available which make it relatively easy to apply the gypsum to the irrigation water (Oster et al.~ 1992). Another alternative used in the Bakersfield region was to use the limited quantity of higher salinity well water on the potato crop and low salinity Friant-Kern water on the deeper rooted, less water sensitive crops like cotton, grapes and tree crops. The author has been directly involved in three field situation where gypsum amendments were effective in improving infiltration and one situation where it likely would have been

J.D. Oster /Agricultural WaterManagement 25 (1994) 271-297

289

effective. Three occurred in California and one in Israel. A brief description of the field setting, observations, recommendations, and farmer adopted practices follow for these four cases. 4.1. Case 1: Non-saline, sodic well water; ECw = 0.6 dS/m, SAR = 13

This well water is used to irrigate citrus near the town of Porterville in Tulare County in southeastern San Joaquin Valley, California. The climate during the summer is hot and dry. Rainfall between December and March varies, ranging from 50 to 300 mm. The topography is hilly and sprinklers are used to apply the irrigation water. Table 5 provides soil-solution compositions at the soil surface and through the root zone calculate by WATSUIT (Oster and Rhoades, 1990) from the composition of the well water. Also given are the calculated Ca/Mg and SAR ratios, the EC, and the amount of calcite dissolution. Dissolution is indicated by the negative numbers for calcite. The latter reduces the SAR to levels less than that of the irrigation water for the soil surface and depths 1 and 2. Also note that total ion concentrations, EC, and SAR increase with depth due to decreasing leaching through the root zone. The decrease in pH with depth reflects the assumed increase in pCO2 with depth. These data indicate several potential problems. Excessive surface SAR relative to the surface EC, and high SARs relative to ECs in the root zone may result in poor IR and reduced K. Rainfall would increase this problem. Problems with salinity for citrus were not expected. The ECs in Table 5 are for the soil solution at field water contents. Consequently, they are about two times greater than the Table 5 Soil-water composition resulting from irrigation with Porterville well water at a leaching fraction of 0.10 as predicted by WATSUIT, a computer model (Oster and Rhoades, 1990) Depth

LF

1/LF

CA

MG

NA + K

CL

CO3

HCO3

SO4

2.67 4.95 8.01 12.60 21.23

1.25 1.95 3.38 6.58 12.50

mmolJl 0 ! 2 3 4

1.00 0.64 0.37 0.19 0.10

1.00 1.56 2.70 5.26 10.00

Depth

pH

CA/MG

1.14 2.26 3.03 2.53 1.78

0.05 0.08 0.14 0.26 0.50

SUM CAT.

4.95 7.73 13.38 26.05 49.50

1.75 2.73 4.73 9.21 17.50

0.47 0.43 0.43 0.45 0.54

Calcite

Gyp

EC (dS/m)

SAR ( m m o l J l ) °'5

0.00 0.00 0.00 0.00 0.00

0.61 0.94 1.5 2.69 4.90

6.34 7.08 10.53 21.81 45.94

mmolc/1 0 1 2 3 4

8.37 7.77 7.48 7.48 7.56

22.87 28.92 22.41 9.62 3.55

6.14 10.07 16.54 28.85 51.78

- 1.03 -2.10 -2.76 -2.01 - 0.78

290

,I,D. Oster / Agricultural Water Management 25 (1994) 271--297

corresponding salinities for soil-water extracts obtained from saturated soil pastes. The average EC for the values given in Table 5, 2.1 dS/m, is less than twice the EC, of 1.7 dS/m for citrus. Although salinity problems were not expected, based on the composition of the irrigation water at a leaching percentage of 10, the tree foliage was not as dense as expected for healthy trees. The average ECc of soil samples obtained beneath the trees to a depth of 90 cm was 2.9 d S / m which is greater than the threshold salinity. The recommendation given to the farmer (Pehrson et at., 1985) was to apply 7 Mg/ha of gypsum to a lightly tilled soil surface every 2 to 3 years to increase water infiltration and to continue irrigation into the start of the rainy season to assure adequate leaching. Since the soil was calcareous, other alternatives include the injection of sulfuric acid or sulfur dioxide gas, using a sulfur burner, into the irrigation water to neutralize 90% of the bicarbonate in the irrigation water. This would increase soil-calcite dissolution and reduce the SAR of soil solution. 4.2. Case 2: Non-saline, moderately sodic well water: EC~,=0.8 dS/m, SAR = 7

This well water is used to irrigate a vineyard near Paso Robles, in the south central portion of the coastal mountain range of California. The summer climate is dry, with warm afternoons and cool nights. Rainfall between November and April ranges from 150 to 500 mm. The vineyard is irrigated at a rate of 3.3 m m / h with a sprinkler irrigation system. The topography is hilly with sandy loam soils (CEC = 80 mmolc/kg) along the hill tops and upper slopes. Tillage with a cultivator was required before each irrigation to assure sufficient infiltration to wet the upper 15 cm of soil. However, tillage did not stop runoff which typically started about four hours after irrigation began. In 1981, the SAR in the upper 23 cm of soil was 13, about two times that of the irrigation water. The gypsum requirement was 1.0 Mg/ha to achieve a final SAR of 5 in the surface 10 cm of soil. A replicated field trial was conducted over a period of 3 months, and six irrigations, beginning in May 1981 (Oster et al., 1982). Runoff from 22X 3 m plots was measured along with soil-water contents to a depth of 0.6 m. All the plots were tilled only once, before the first irrigation of the trial. Runoff from control plots, where no amendments were applied after tillage, ranged from 30% of the AW for the first irrigation to 50% by the sixth irrigation. For the gypsum-treated plots, runoff was less than 10% for the first five irrigations; it increased to about 30% for the sixth irrigation. Fig. 6 shows the impact of gypsum application of 3 Mg/ha immediately following tillage: gypsum was applied to the soil surface in the foreground but not in the background where surface ponding occurred. The final SAR of the surface soil in the gypsum treated plots was 6 as compared to the initial value of 13. The operators of the vineyard adopted a practice of biannually applying 5 Mg/ha of gypsum. This effectively improved infiltration, controlled problems with runoff, and reduced the need for tillage particularly during the first year. They also achieved similar results by growing an intercrop of blando brome, a self seeding winter annual. However, this practice was discontinued because it aggravated hay fever problems with employees. 4.3. Case 3: Saline-sodic drainage water (0.9 < EC < 11.6 dS/m, 3 < SAR < 32)

This shallow groundwater is intercepted by an extensive artificial drainage system in the Tulare Basin located in southern portion of the San Joaquin Valley of California. The

J.D. Oster/Agricultural WaterManagement 25 (1994) 271-297

291

Fig. 6. Impact of gypsum applied to a freshly tilled soil on infiltration in the field project conducted near Paso Robles, CA (Oster et al., 1982). Gypsum was not applied in the backgroundwhere pondingoccurred. climate is the very similar to that given for Case 1. The Tulare Basin is hydrologically closed, except in extraordinarily wet years, and feed by three major streams that flow from the Sierra Nevada mountains to the east of the basin. The surface soils (CEC = 315 mmolc/kg) have a high clay content because of clay deposition which occurred from lakes formed in the basin during wet years. A 9-year (1984-1992) experiment was conducted on an 8-ha site in the Tulare Basin to

292

L D. Oster / Agricultural Water Management 25 (1994) 27I-.297

evaluate the effect of irrigation with drainage waters of varying salinity levels on the growth and yield of cotton and safflower (Rolston et al., 1988; Goyal et al., 1992). The experimental treatments consisted of irrigating with water of six salinity levels, each replicated four times. The EC levels were 0.9, 2.5, 5.0, 7.0, 9.3, and 11.6 dS/m; the corresponding SAR levels were 3, 10, 16, 20, 28, and 31, respectively. During the winter months, all plots received annually a pre-plant irrigation with the lowest salinity water in addition to any rainfall that occurred.. The crop rotation consisted of 2 years of cotton followed by one year of safflower. Cotton yields were not affected by salinity treatments for the first 2 years. Effects of exchangeable sodium on soil tilth was identified as a major cause of declining cotton yields beginning the fourth year of the project. The photograph in Fig. 7 shows the effects of salinity treatments on the fourth cotton crop that were evident in July of 1988. Tillage to prepare the seedbed occurred during late March or early April. Beds were not formed. Instead the ground was left fiat and cotton was planted about 3 cm deep into wet soil. However, in the higher salinity treatments it was not possible to prepare a seedbed where the smallest aggregates were smaller than the cotton seed. Gypsum amendments were not used to reduce exchangeable sodium levels in the surface soil during the pre-plant irrigation. Consequently, the pre-plant irrigation caused particle dispersion and hard, large, soil clods. Based on the SAR of soil samples (0-15 cm) obtained in November 1988 following fifth irrigation season, the gypsum requirements for the upper 10 cm of soil for a

Fig. 7. Effects of irrigation water salinity on cotton in the Tulare Lake Basin project (Goyal et al., 1992). The salinity treatments were not randomized: the order of salinity treatments within the four replicates was the same. Starting from the left side of the picture, the order of salinity treatments for the first six irrigation plots, or first replicate, is 12, 7, 2.5, 1.0, 5, and 9 dS/m. This order is repeated four times across the 8-ha site.

J.D. Oster/Agricultural WaterManagement 25 (1994) 271-297

293

final SAR of 5 ranged from 5 Mg/ha to 25 Mg/ha for the 2.5 and 11.6 dS/m salinity treatments, respectively. This suggests the annual gypsum requirement ranged from 1 to 5 Mg/ha. The problem with soil tilth increased in severity with time and with increasing salinity of the irrigation water. By the sixth cotton crop in 1992, yield decline occurred (Goyal, et al., 1992) in all salinity treatments where ECw exceeded 0.9 dS/m. 4.4. Case 4: saline sodic well water: 2.5 < ECw < 8.5 dS/m, 15 < SAR < 26

Portions of the western Negev region of Israel are underlain with aquifers that contain saline sodic water. The dominant soils are silty loams and the climate is Mediterranean, with winter rainfall ranging between 250 and 400 mm. Cotton is the dominant crop. Sixteen years of irrigation with water from a well at Kibbutz Nahal-Oz (ECw of 4.6 dS/ m and SAR of 26) demonstrates that irrigation with such a poor quality water can be sustained (Keren et al., 1990). Irrigation during the summer (450 mm) results in ESP values ranging from 20 to 26 in the upper 60 cm of soil. There is no deterioration in soilhydraulic properties during the summer because the ECw is sufficient to counteract the deleterious impacts of exchangeable sodium. However, deterioration does occur during the rainy season due to the low salt concentrations of the rainwater. To offset this, phosphogypsum is spread annually on the soil surface, following tillage in the fall, at a rate of 5 Mg/ha. This prevents the formation of surface seals and crusts and maintains high infiltration rates which, in turn, provide sufficient infiltration of rainfall to leach salts from the root zone. Fall application of phosphogypsum and leaching during the rainy season, coupled with adequate irrigation with the saline-sodic water to meet crop needs during the summer months, has resulted in seed cotton yields averaging 5 Mg/ha between 1979 and 1988. These yields were similar to those obtained when only non-saline sodic water was used for irrigation.

5. Concluding comments The number and capability of computer models that describe impacts of water quality on crop yields, and on soil water and exchange composition will increase greatly in the near future. Development of computer friendly models for use by farm advisors and consultants is the next step, and with it can come further model development that reflects existing farmer experience with the use of poor quality water. The subject of amendment requirements to control the impacts of salinity and sodicity on infiltration and soil tilth needs to be examined in the future. Computer models exist that describe the impacts of irrigation water composition and of calcium amendments on the chemical reactions that occur during reclamation of a sodic soil. The replacement of exchangeable sodium with calcium involves an efficient chemical reaction. As leaching occurs during reclamation, almost all the soluble calcium that becomes available replaces exchangeable sodium. The same models also are capable to describing the reverse process, namely, the chemical reactions that occur as the exchangeable sodium increases during irrigation ofa non-sodic soil with a sodic irrigation water. In this case, the exchange reaction is not efficient; only a fraction of the available sodium will replace exchangeable calcium.

294

.I.D. Oster/Agrictdtural Water Management 25 (I994) 271 297

The increase in exchangeable sodium will occur slowly. This, in turn, significantly effects the a m o u n t of calcium a m e n d m e n t s required as well as the o p t i m u m timing and mode of application. The latter include choices such as adding the a m e n d m e n t to the irrigation water, surface application to the soil surface, or incorporation into the soil after application. Finally, the effects of salinity and sodicity on K and IR are not predictable for specific s o i l / c r o p / t i l l a g e situations. Prediction of salinity and sodicity impacts on K continues to be the subject of research (Jayawardane, 1992). Prediction of impacts on IR will be more difficult because soil particles of all sizes at the soil surface are subject to rearrangement by the applied irrigation water and by tillage. The best predictive tool currently available is the water quality guidelines in Table 3. However, since these guidelines do not always apply, farmers and their advisors will need to be aware of the types of problems that can occur, the diagnostic procedures that are available to identify their source, and the appropriate m a n a g e m e n t techniques available for their mitigation.

References Abu-Sharar, T.M., Bingham, F.T, and Rhoades, J.D., 1987. Stability of soil aggregates as affected by electrolyte concentration and composition. Soil Sci. Soc. Am. J., m51:309-314. Agassi, M., Morin, J. and Shainberg, 1.. 1981. Effect of electrolyte concentratio and soil sodicity on infiltration rate and crust formation. Soil Sci. Soc. Am. J., 45: 848-851. Ayars, J.E., Hutmacher, R.B., Schoneman, R.A. Vail, S.S. and Felleke, D., 1986a. Drip irrigation of cotton with saline drainage water. Trans. ASAE, 29: 1668-1673. Ayars, J.E., Hutmacher, R.B., Schoneman, R,A. and Vail, S.S., 1986b. Trickle irrigation of sugar beets with saline drainage water. Annual Report of the Water Management Research Laboratory USDA-ARS, Fresco, California, pp. 5-6. Ayers, R,S. and Westcot, D.W., 1985. Water quality for agriculture. Irrig. Drain. Paper No. 29, Rev. 1, FAO Rome, Italy. Ben-Hur, M., Stern, R., van der Merwe, A.J. and Shainberg, I. 1992. Slope and gypsum effects on infiltration and erodibility of dispersive and nondispersive soils. Soil Sci. Soc. Am. J., 56: 1571-1576. Bradford, S. and Letey, J., 1993a. Cyclic and blending strategies for using saline and non-saline waters for irrigation. Irrig. Sci., 13: 123-128. Bradford, S. and Letey, J., 1993b. Water table and irrigation scheduling on cotton: Simulation effects. Irrig. Sci., 13: 101-107. Cardon, G.E. and Letey, J., 1992. A soil-based model for irrigation and soil salinity management. I. Tests of plant water uptake calculations. Soil Sci. Soc. Am. Proc., 56: 1881-1887. Childs, S.W. and Hanks, R.J., 1975. Model of soil salinity effects on crop growth. Soil Sci. Soc. Am. Proc., 39: 617-622. deWit, C.T., 1958. Transpiration and crop yield: Verslad, van Lanbouch, Onderzoek No. 64.6, 88 pp. Dinar, A., Letey, J. and Vaux Jr., H.J., 1986. Optimal ratios of saline and non-saline irrigation waters for corp production. Soil Sci. Soc. Am. J., 50: 440-443. Doneen, L.D., 1948. The quality of irrigation water and soil permeability. Soil Sci. Soc. Am. Proc., 13: 523-526. Duley, F.L., 1939. Surface factors affecting the rate of intake of water by soils. Soil Sci. Soc. Am. Proc., 13: 523526. Dutt, G.R., Pennington, D.A. and Turner, F., 1984. Irrigation as a solution to salinity problems in fiver basins. In: R.H. Frenc (Editor), Salinity in Watercourses and Reservoirs. Ann Arbor Science, Michigan, pp. 465-472. Frenkel, H.O., Goertzen, J.O. and Rhoades, J.D., 1978. Effects of clay type and content, exchangeable sodium percentage, and electrolyte concentration on clay dispersion and soil hydraulic conductivity. Soil Sci. Soc. Am. J., 42: 32-39.

J.D. Oster/Agricultural WaterManagement 25 (1994) 271-297

295

Goldberg, S. and Glaubig, R.A., 1987. Effect of saturating cation, pH, and aluminum and iron oxides on the flocculation of kaolinite and montmorillonite. Clays and Clay Min., 35: 220-227. Goyal, S.S., Rains, D.W., Qualset, C.O., Dvorak, J., Biggar, J,W., Lauehli, A., Nielsen, D.R., Rolston, D. and Oster, J.D., 1992. Saline drainage water reuse for crop irrigation in the San Joaquin Valley. Technical Progress Report, Department of Agronomy and Range Science, Div. of Agr. and Nat. Resour., University of California, California. Grattan, S.R., Sherman, C., May, D.M., Mitchell, J.P. and Burau, R.G., 1987. Use of drainage water for irrigation of melons and tomatoes. Calif. Agric., 41 (9/10): 27-28. Hanks, R.J., Sullivan, T.E. and Hunsaker, V.E., 1977. Corn and alfalfa production as influenced by irrigation and salinity. Soil Sci. Soc. Am. J., 41: 606-610. Hoffman, G.J., 1990. Leaching fraction and root zone salinity control. In: K.K. Tanji (Editor), Agricultural Salinity Assessment and Management. ASCE Manuals and Reports on Engineering Practice No. 71. ASCE, New York, pp. 237-261. Hoffman, G.J. and van Genuchten, M.Th., 1983. Soil properties and efficient water use: water management for salinity control. In: H.M. Taylor, W.R. Jordan and T.R. Sinclair (Editors), Limitations to Efficient Water Use in Crop Production. American Soc. of Agron, Madison, Wisconsin, pp. 393-417. Jayawardane, N.S., 1992. Prediction of unsaturated hydraulic conductivity changes of a loamy soil in different salt solutions by using the equivalent salt solution concept. Aust. J. Soil Res., 30: 565-571. Kazman, Z., Shainberg,I. and Gal, M., 1983. Effect of low levels of exchangeable Na and applied phosphogypsum on the infiltration rate of various soils. Soil Sci., 135: 184-192. Keren, R., 1991. Specific effect of magnesium on soil erosion and water infiltration. Soil Sci. Soc. Am. J., 55: 783-787. Keren, R., Sadan, D., Frenkel, H., Shainberg, I. and Mieri, A., 1990. Effect of saline water on soil properties and crop yield (in Hebrew). Hassadeh, 70: 1822-1829. Knapp, K.C., Dinar, A. and Letey, L., 1986. On-farm management of agricultural drainage water: an economic analysis. Hilgardia, 54: 1-31. Knapp, K.C., Stevens, B.K., Letey, J. and Oster, J.D., 1990. A dynamic optimization model for irrigation investment and management under limited drainage conditions. Water Res. Res., 26: 1335-1343. Letey, J. and Dinar, A., 1986. Simulated crop-production functions for several crops when irrigated with saline waters. Hilgardia, 54: 1-32. Letey, J. and Oster, J.D., 1993. Subterranean disposal of irrigation drainage waters in Western San Joaquin Valley. In: R.G. Allen and C.M.U. Neale (Editors), Management of Irrigation and Drainage Systems: Integrated Perspectives. Proc. 1993 National Conference on Irrigation and Drainage Engineering, Park City, Utah, July 1993. Irrig. Drain. Div. ASCE, 691-197. Letey, J., Dinar, A. and Knapp, K.C., 1985. Crop-water production function model for saline irrigation waters. Soil Sci. Soc. Am. J., 49: 1005-1009. Maas, E.V., 1990. Crop salt tolerance. In: K.K. Tanji (Editor), Agricultural Salinity Assessment and Management. ASCE Manuals and Reports on Engineering Practice No. 71. Am. Soc. Civil Engineers, New York, pp. 262304. Maas, E.V. and Hoffman, G.J., 1977. Crop salt tolerance - current assessment. J. h'rig. Drain. Div. ASCE, 103(1R2): 115-134. Maas, E.V. and Poss, J.A., 1989. Salt sensitivity of wheat at various growth stages. Irrig. Sci., 10: 29-40. Martens, D.A. and Frankenburger, Jr., W.T., 1992. Effects of organic amendments on water infiltration and soil properties of an irrigated soil. Agron. J., 84: 707-717. McNeal, B.L. and Coleman, N.T., 1966. Effect of solution composition on hydraulic conductivity. Soil Sci. Soc. Am. Proc., 30: 308-312. McNeal, B.L., Norvell, W.A. and Coleman, N.T., 1966. Effect of solution composition or, soil hydraulic conductivity and on the swelling of extractod clay soils. Soil Sci. Soc. Am. J., 30: 313-317. Meiri, A., Shalhevet, J., Stolzy, L.H., Sinai, G. and Steinhardt, R., 1986. Managing multi-source irrigation water of different salinities for optimum crop production. BARD Tech. Report No. !--402-8 I. Volcani Center, BetDagan, Israel. Miles, D.L., 1977. Salinity in the Arkansas Valley of Colorado. Interagency Agreement Report EPA-IAG-D40544. Environmental Protection Agency, Denver, Colorado. Miyamoto, S., Moore, J. and Stichler, C., 1984. Overview of saline water irrigation in Far West Texas. In: J.A.

296

.I. l). Oster / Agricultural Water Management 25 (1994) 2 7 1 ~ 9 7

Replogle and K.G. Renard (Editors), Waler Today and Tomorrow. Proc. Speciality Conf. Irrigation and Drainage Division of ASCE, Flagstaff, Arizona, 24-26 July 1984. ASCE, New York, pp. 222-230. Morin, J. and Benyamini, Y., 1977. Rainfall infiltration into bare soils. Water Resour. Res.. 13:813-817 Oster, J.D, 1982. Gypsum usage in irrigated agriculture; a review. Fertil. Res., 3: 73-89. Oster, J.D. and H. Frenkel. 1980. The chemistry of the reclamation of sodic soils with gypsum and lime. Soil Sci. Soc. Am. J., 44: 4 1 4 5 . Oster, J.D. and Rhodes, J.D., 1990. In: K.K. -fanji (Editor), Agricultural Salinity Assessment and Management. ASCE Manuals and Reports on Engineering Practice No 71. Am. Soc. Civil Engineers, New York, pp. 469481. Oster, J.D. and Schroer, F.W,, 1979. Infiltration as influenced by irrigation water quality. Soil Sci. Soc. Am. 3, 43: 444-447. Oster, J.D. and Singer, M.J., 1984. Water penetration problems in California soils. LAWR Tech. Report 10011. Div. Agr. & Natl. Res., University of California, Davis, California. Oster, J.D., Hautala, E., Wildman, W.E., Wood, J.D. and Fuller, R., 1982, Effect of xanthate, phosphogypsum and straw on infiltration and runoff of a sprinkle irrigated soil. Proceedings of the California Plant & Soil Conference, 1982. Oster, J.D., Singer, M.J., Fulton, A., Richardson, W. and Pilchard, T., 1992. Water penetration problems in California soils: prevention, diagnosis and solutions. In: K. McKibben and B, Gable (Editors), Kearney Foundation of Soil Science, Div. of Agr. and Nat. Resources, University of California, Davis, California, 166 PP. Oster, J.D., 1. Shainberg and I.P. Abrol, 1995. Reclamation of salt-affected soils. Am. Soc. of Agronomy Drainage Monograph (in press). Pasternak, D., De Malach, Y, and Borovic, J., 1986. Irrigation with brackish water under desert conditions. VII. Effect of time of application of brackish water on production of processing tomatoes (Lycopersecon escuelentum Mill.). Agric. Water Manage., 12: 149-158. Pehrson, J., O'Connell, N. and Oster, J.D., 1985. Salinity management for San Joaquin Valley citrus. Citrograph, November, 15-17. Pratt, P.F. and Suarez, D.L., 1990. Irrigation water quality assessments. In: K.K. Tanji (Editor), Agricultural Salinity Assessment and Management. ASCE Manuals and Reports on Engineering Practice No 71. Am. Soc. Civil Engineers, New York, pp. 220-236. Quinn, N.W.T., 1991. Ground-water pumping for water table management and drainage control in the western San Joaquin Valley. In: A. Dinar and D. Zilberman (Editors), The Economics and Management of Water and Drainage in Agriculture. Kluwer Academic Publishers, Boston, pp. 71-97. Quirk, J.P., 1986. Soil permeability in relation to sodicity and salinity. Philos. Trans. R. Soc. Lond. A 316: 297317. Quirk, J.P. and Schofield, R.K., 1955. The effect of electrolyte concentration on soil permeability. J. Soil Sci., 6: 163-178. Rengasamy, P., Greene, R.S.B., Ford, G.W. and Mehanni, A.H., 1984. Identification of dispersive behavior and the management of red brown earths. Aust. J. Soil Res., 22: 413-432. Rhoades, J.D., 1974. Drainage for salinity contorl. In: J. van Schilfgaarde (Editor), Drainage for Agriculture. Am. Soc. Agron, Monograph No. 17, pp. 433-462. Rhoades, J.D., 1987. Use of saline water for irrigation. Water Qual. Bull., 12: 14-20. Rhoades, J.D. and Loveday, J., 1990. Salinity in irrigated agriculture. In: B.A. Stewart and D.R. Nielsen (Editors), Irrigation of Agricultural Crops. Agron, Monograph No. 30, pp. 1089-1142. Rhoades, J.D., Bingham, F.T., Letey, J., Hoffman, G.J., Dedrick, A.R., Pinter, P.J. and Replogle, J.A., 1989. Use of saline drainage water for irrigation: Imperial Valley study. Agric. Water Manage., 16: 25-36. Rhoades, J.D., Kandiah, A. and Mashali, A.M., 1992. The use of saline waters for crop production. FAO Irrigation and Drainage Paper No. 48, Rome, Italy. Rhoades, J.D. and Merrill, S.D., 1976. Assessing the suitability of water for irrigation: theoretical and empirical approaches. In: Prognosis of Salinity and Alkalinity. Soils Bull. 31, FAO, Rome, Italy, pp. 69-109. Robbins, C.W., 1986. Sodic calcareous soil reclamation as affected by different amendments and crops. Agron. J., 78: 916-920. Rolston, D.E., Rains, D.W., Biggar, J.W. and Lauchli, A., 1988. Reuse of saline drain water for irrigation, Paper presented at the UCD/1NIFAP Conference, Guadalajara, Mexico, March 1988. Shainberg, I. and Letey, J., 1984. Response of soils to sodic and saline conditions. Hilgardia, 52: 1-57. Shalhevet, J., 1991. Using water of marginal quality for crop production: major issues. In: J. Shalhevet, L.

J.D. Oster /Agricultural Water Management 25 (1994) 271-297

297

Changmimg and X. Yuexian (Editors), Water Use Efficiency in Agriculture. Proceedings of the binational China-Israel Workshop, Beijing, China, Preil Publ., Rehovot, Israel, pp. 17-53 (and this issue). Shennan, C., Grattan, S.R., May, D.M., Burau, R. and Hanson, B., 1987. Potential for the long-term cyclic use of saline drainage water for the production of vegetable crops. Technical Progress Report, U.C. Salinity/Drainage Task Force, Div. of Agric. and Natural Resour., University of California, Davis, California, pp. 142-146. Simms, H.J. and Rooney, D.R., 1965. Gypsum for difficult clays wheat growing soils. J. Dept. Agric. Victoria, 63: 401-409. Simunek, J. and Suarez, D.L., 1994. Two-dimensional transport model for variable saturated porous media with major ion chemistry. Water Res. Res. (in press). Solomon, K.H., 1985. Water-salinity-production functions. Trans ASAE, 28: 1975-1980. Stewart, J.I., Danielson, R.E., Hanks, R.J., Jackson, E.B., Hagan, R.M., Pruitt, W.O., Franklin, W.T. and Riley, J.P., 1977. Otimizing crop production through control of water and salinity levels in the soil. Utah Water Lab. PRWG 151-1, Logan, Utah. Suarez, D.L. and Genuchten, M.Th., 1981. Leaching and water-type effects on ground-water quality. J. Irrig. Drain. Div. ASCE, 107(IR1): 35-52. Suarez, D.L., Rhoades, J.D., Savado, R. and Grieve, C.M., 1984. Effect of pH on saturated hydraulic conductivity and soil dispersion. Soil Sci. Soc. Am. J., 48: 50--55. Sumner, M.E. 1993. Sodic soils: new perspectives. Aust. J. Soil Res., 31: 683-750. Tanji, K.K. (Editor), 1990. Agricultural Salinity Assessment and Management. ASCE Manuals and Reports on Engineering Practice No. 71. Am. Soc. Civil Engineers, New York. van Schilfgaarde, J., 1982. The Welton-Mohawk dilemma. Water Manage. Supply, 6:115-127. Wagenet, R.J. and Hutson, J.L., 1987. LEACHM: Leaching estimation and chemistry model: A process based model of water and solute movement, transformations, plant uptake and chemical reactions in the unsaturated zone. Continuum, Vol. 2. Water Resour. Inst., Cornell University, Ithaca, New York. Wallender, W.W., Grimes, D.W., Henderson, D.W. and Stromberg, L.K., 1979. Estimating the contribution of a perched water table to the seasonal evapotranspiration of cotton. Agron. J., 71: 1056--1060.