Chapter 6. Drainage water reuse

Introduction

In areas where irrigation water is scarce, the use of drainage water is an important strategy for supplementing water resources. Furthermore, reuse may help alleviate drainage disposal problems by reducing the volume of drainage water involved. The reuse of drainage water for irrigation can reduce the overall problems of water pollution. Reuse measures consist of: reuse in conventional agriculture; reuse to grow salt tolerant crops; IFDM systems; reuse in wildlife habitats and wetlands; and reuse for initial reclamation of salt-affected lands. This chapter deals with only those drainage water reuse measures that relate to agricultural production.

Drainage water is normally of inferior quality compared to the original irrigation water. Adequate attention needs to be paid to management measures to minimize long-term and short-term harmful effects on crop production, soil productivity and water quality at project or basin scale. The drainage water quality determines which crops can be irrigated. Highly saline drainage water cannot be used to irrigate salt sensitive crops, but it can be used on salt tolerant crops, trees, bushes and fodder crops. A major concern in reuse measures is that drainage water from reused waters is often highly concentrated, requiring careful management.

Relevant factors

Drainage water quality is the major concern in reuse possibilities as it defines which crops can be irrigated and whether long-term degradation of soil productivity is a major issue. On the other hand, the soil type, drainage conditions of the land and the crop salt tolerance define what quality drainage water can be used for irrigation in combination with the availability of other freshwater resources.

Pollutants from surface runoff, i.e. sediments, pesticides and nutrients, play a minor role in reuse for crop production. However, for sustainable agricultural practices and to prevent environmental degradation, nutrients supplied with reused drainage water should be deducted from the fertilizer requirements in order to prevent imbalanced and excessive fertilizer application.

Subsurface drainage water generally shows increased concentrations of salts and sometimes certain trace elements and soluble nutrients. Salts and trace elements play a major role in the reuse of drainage water. Above a certain threshold value, high total concentrations of salts are harmful to crop growth, while individual salts can disturb nutrient uptake or be toxic to plants. A high sodium to calcium plus magnesium concentration ratio may cause unstable soil structure. Soils with unstable structure are subject to crusting and compaction, degrading soil conditions for optimal crop growth. Toxic trace elements such as boron can interfere with optimal crop growth and others such as selenium and arsenic can enter the food chain when crops are irrigated with water containing high concentrations of these trace elements. This is of major concern for human and animal health.

Table 13. Quality indicators for some main drains

Source: DRI, 1997a.

Considerations on the extent of reuse

Municipalities and industries often use agricultural main drains to dispose of their wastewater. In many areas around the world, municipal and industrial wastewater is either insufficiently treated, or not treated at all. Bacteriological and organic and inorganic compounds seriously pollute main drains, posing an environmental hazard to both human and wildlife and restricting reuse from main drains. For example, in the Nile Delta, Egypt, the discharge of untreated wastewater is a major concern for the sustainability of an irrigated agriculture that depends heavily on the large-scale reuse of agricultural drainage water to meet water shortfalls. Table 13 lists several water quality indicators of some of the main drains and the official limits for the quality of drainage water that is allowed to be mixed with freshwater resources. The biochemical oxygen demand (BOD) is defined as the amount of oxygen consumed by microbes in decomposing carbonaceous organic matter. The chemical oxygen demand (COD) is the amount of oxygen required to oxidize the organic matter and other reduced compounds. The high chemical versus biological oxygen demand (COD/BOD) ratios imply significant industrial pollution (EPIQ Water Policy Team, 1998). Total coliform is the most probable number of faecal coliform (MPN) in 100 ml and a high value indicates severe pollution from municipal sewage water.

The degraded water quality threatens the expansion and even the continuation of the reuse of drainage water from the main drains (official reuse) in the Nile Delta. Since the 1990s, many reuse mixing stations have been under increasing pressure of water quality deterioration. Indeed, since 1992, 7 of the 23 main reuse mixing stations have been entirely or periodically closed (DRI, 1995). Due to the increasing deterioration of water quality, new opportunities for reuse of drainage water are being explored. Between the centralized official reuse and the localized unofficial reuse (where farmers pump drainage water from the collector drains directly onto fields), there is the option to capture drainage water from branch drains and pump it into the branch canals at their intersections. This level of reuse, termed intermediate reuse, captures relatively good quality drainage water before it is discharged into the main drains where it is lost for irrigation because of pollution from other sectors. This problem highlights the need for catchment level planning to protect and use all water resources sustainably.

Maintaining favourable salt and ion balances and soil conditions

Maintaining a favourable salt balance

The major concern in the reuse of agricultural drainage water is the buildup of salts and other trace elements in the rootzone to such an extent that it interferes with optimal crop growth and degradation of the aquifers. Applying more water than necessary during the growing season for evapotranspiration can leach the salts. In areas with insufficient natural drainage, leaching water will need to be removed through artificial drainage. As safe disposal of agricultural drainage water is often the hindrance to sustainable drainage water management, the solution in reuse lies in applying just enough water to maintain a favourable salt balance. This was termed in Chapter 5 as beneficial non-consumptive use.

Table 14. Flow-weighted concentration of salinity and boron concentrations and mass transfer of salts in Broadview Water District, 1976

* Water quality was measured weekly and the flows of fresh canal water and mixed supply water were measured daily while that of captured drainwater was estimated by monthly electrical charges on the pump and assumed pump efficiency. Thus, the mass of salts in captured drainwater appears to be underestimated.

Source: Tanji, et al.1977.

The following example from the Broadview Water District in the San Joaquin Valley illustrates that drainage water reuse is a viable management option but that upper limits need to be established. Moreover, a minimum amount of leaching is required to avoid deleterious impacts on the more salt sensitive crops. The Broadview Water District is a landlocked tract of 4 100 ha of irrigated land without a surface drainage outlet since 1956. Surface irrigation return flow, both tailwater and subsurface drainage, was recycled back completely into the irrigation supply ditch. Table 14 contains a summary of water quality and mass flow of salts for this type of reuse. The annual 18 324 million m³ of imported canal water was a low-salt, low-boron water while the captured annual 16 363 million m³ of drainage water was a moderately saline, 2 mg/litre boron water. The blended water serving as the supply water to the Broadview Water District was considerably degraded. In terms of mass of salts, the captured drainage water contributed about 66 percent of the salts applied in the water district.

Figure 34 shows the average concentration of soil salinity and boron in seven soil profiles in the Broadview Water District in 1976. The subsurface drains are installed about 2.4 m deep and spaced about 90 m apart. The crops grown in the district were cotton, tomatoes, barley, wheat, sugar beet, and alfalfa seeds. Over time, the 800 ha of tomato plantings dropped to 0 ha in 1987 when the seeds progressively failed to germinate. In 1989, the district gained access to discharge part of the saline drainage waters out of the district and reduce the blending of drainage water into the supply water. Within a few years, the tomato plantings were re-established at former levels.

Maintaining favourable soil structure

The sodium hazards of irrigation water are related to the ability of excessive sodium or extremely low salinity concentrations to destabilize soil structure. The primary processes responsible for soil degradation are swelling and clay dispersion. Provided that the salt concentration in the soil water is below a critical flocculation concentration, clays will disperse spontaneously at high exchangeable sodium percentage (ESP) values, whereas at low ESP levels inputs of energy are required for dispersion. The salt concentration in the soil water is crucial to determining soil physical behaviour because of its effects in promoting clay flocculation (Sumner, 1993). However, the boundary (ESP/salt concentration of the soil water) between stable and unstable conditions varies from one soil to the next and changes with the clay mineralogy, pH, soil texture, and clay, organic matter and oxide content (FAO, 1992b).

Figure 34. Average concentration of soil salinity and boron in seven soil profiles in Broadview Water District in 1976

The short-term effects of irrigating with water having excess sodium or very low salt concentrations relate mainly to infiltration problems. The reduction of infiltration can be attributed to the dispersion and migration of clay minerals into soil pores, the swelling of expandable clays and crust formation. The potential infiltration problems in relation to the quality of the irrigation water are normally evaluated on the basis of the salinity and SAR^[3] of the irrigation water (Box 6).

Various authors have developed stability lines related to the total salinity concentration and the SAR. The actual line that represents the division between stable and unstable soil conditions is unique for each soil type and varies with soil conditions. Published guidelines (Figure 35) on infiltration problems in relation to the SAR and the salinity of the applied water can therefore provide only approximate guidance. Where large-scale reuse of drainage water is planned and sodium hazards might be expected, stability lines need to be established for local conditions.

Infiltration problems caused by the sodicity of irrigation water also depend on irrigation and soil management. Especially at lower SAR levels when chemical bonding is weakened, but no spontaneous dispersion takes place, inputs of energy are required for actual dispersion. For example, sprinkler irrigation increases the likelihood of surface crusting due to the high physical disruption as the drops hit the soil surface aggregates. In soil management, incorporation of organic matter increases the stability of the soil aggregates and reduces the hazards of structural degradation as a result of sodicity.

Figure 35. Relative rate of water infiltration as affected by salinity and SAR

Source: FAO, 1985b.

Reduction of the hydraulic conductivity in the soil profile is normally a long-term effect resulting from the use of sodic water. In particular, the presence of carbonates and bicarbonates in the water could result in soil degradation in the long term because precipitation of calcium carbonate increases soil SAR. Calcium in the form of calcite is one of the first salts to precipitate. Upon further concentration magnesium salts will also precipitate.

There is little need to undertake action to increase infiltration and or hydraulic conductivity unless crop water or leaching requirements cannot be met or if secondary problems reduce crop yields or impede seedling emergence. Secondary problems include crusting of seed beds, excessive weed growth and surface water ponding that can cause root rot, diseases, nutritional disorders, poor aeration and poor germination (FAO, 1985b). Management options to mitigate and reduce these problems can be chemical, biological and physical. Chemical management options entail adding chemical amendments to soil or water and thereby changing the soil or water chemistry. The aim of biological methods is to improve soil structure or to influence the soil chemistry through the decomposition of organic materials. Physical methods include cultural practices to increase infiltration rates during irrigation and rainfall or to prevent direct contact between ponding water and plant stems, roots and seeds.

Chemical soil and water amendments to prevent infiltration problems

FAO (1985b) has analysed the management of infiltration problems by means of soil and water amendments in considerable detail. This section highlights the major issues. The aim of applying chemical amendments to soil or water is to improve poor infiltration caused by either a low salinity or by excessive sodium. The problem is most severe at low electrolyte concentration and high SAR. Improvements can be expected if the soluble calcium content is increased or a significant increase in salinity is achieved. Most soil and water amendments supply calcium directly or indirectly through acid that reacts with soil calcium carbonates. Acid is not effective where calcium carbonate is not present in the soil profile. However, calcium carbonate is often present in arid soils. Water amendments are most effective when infiltration problems are caused by low to moderate saline water (EC < 0.5 dS/m) with a high SAR. Where moderate to high saline water (EC > 1.0 dS/m) causes problems, soil amendments are more effective.

Amendments need to react quickly if they are to solve actively infiltration problems caused by irrigation water. Many of the amendments for improving or reclaiming sodic soils react only after oxidation. Thus, they are slow and less suitable for solving infiltration problems caused by irrigation water. Slow reacting amendments include acid forming substances such as sulphur, pyrite, certain fertilizers and pressmud from sugar-cane factories (in India and Pakistan).

Gypsum (CaSO₄.2H₂O) is the most commonly used amendment. It can either be applied to the soil or irrigation water. Infiltration problems normally occur primarily in the upper few centimetres of the soil. Hence, it is normally more effective to apply small frequent doses of gypsum left on the soil surface or mixed with the upper few centimetres of the topsoil than higher doses incorporated deeper in the soil profile.

The application of gypsum to irrigation water to prevent infiltration problems usually requires less gypsum per hectare than when it is used as soil application. Gypsum application to water is particularly effective when added to low salinity water (< 0.5 dS/m) and less effective for higher salinity water because of the difficulty of obtaining sufficient calcium in solution. In practice, it is not possible to obtain more than 1-4 meq/litre of dissolved calcium, or 0.86-3.44 g/litre of pure gypsum (Box 7), in fast-flowing irrigation streams. In low salinity water, these small amounts of dissolved calcium ions may increase the infiltration rate by as much as 300 percent. To use gypsum as a water amendment, finely ground gypsum (< 0.25 mm in diameter) is preferred as it has a higher solubility. This is also normally the purer grade of gypsum. A drawback is that finely ground purer grades of gypsum are more expensive, which often prevents small farmers from using it. The coarser grinds and lower grades of gypsum are more satisfactory for soil application.

Experiments have been conducted with the placement of gypsum rocks in the watercourse. The problem is that the amount of calcium dissolving from the gypsum rocks is low so the effectiveness depends on the stream velocity and volume. Experiments from Pakistan have shown that sufficient head and length of the supply canal from the tubewell to the watercourse (which is also used for fresh canal water) are required to dissolve enough calcium in tubewell water. Where technically feasible, the use of gypsum stones has proved to be financially attractive for farmers (Chaudry et al., 1984). In large-scale applications, a drawback might be the cost of canal maintenance, as the gypsum blocks need to be removed before mechanical cleaning.

Sulphuric acid is an extensively used amendment for addressing infiltration problems. It is effective only where lime is present in the soil surface. This highly corrosive acid can be applied to the soil directly, where it reacts with lime making the naturally present calcium available for exchange with adsorbed sodium. Sulphuric acid reacts rapidly with soil lime making it a useful amendment to combat infiltration problems. When added to the irrigation water, it is neutralized by the carbonates and bicarbonates in the water and any excess would contribute towards dissolving the soil lime.

Biological soil amendments

Crop residues or other organic matter left in or added to the field improve water penetration. The more fibrous and less easily decomposable crop residues are more suitable for mitigating minor infiltration problems. Fibrous organic materials keep the soil porous by maintaining open voids and channels. The use of crop residues forming polysaccharides as a cementing agent is most effective where they are incorporated only in the upper few centimetres. Incorporation of easily decomposable organic matter and incorporation deeper into the soil profile do not generally help reduce infiltration problems although they do improve soil structure by producing polysaccharides that promote soil aggregation and enhance soil fertility. Another important aspect of the decomposition of organic matter is the production of carbon dioxide, which in turn increases the solubility of lime. Box 8 provides an example of a green manure from Pakistan and India.

Cultural practices

Cultivation is usually done for weed control or soil aeration purposes rather than to improve infiltration. However, where infiltration problems are severe, cultivation or tillage are helpful as they roughen the soil surface, which slows down the flow of water, so increasing the time during which the water can infiltrate. Cultivation is only a temporary solution. After one or two irrigations, another cultivation may be needed. Moreover, the construction of (broad) beds may help mitigate the ill effects of standing water as it prevents direct contact of the plants with the water. Research from Pakistan has shown that cotton in particular benefits from planting on broad beds.

Maintaining favourable levels of ions and trace elements

High concentrations of trace elements in soil, ground and drainage water can occur in association with high salinity and can be affected by the same processes. However, in some places they may also occur independently of salinity. In examining ways to control levels of ions and trace elements in the rootzone, it is necessary to understand the processes that affect their mobility. Deverel and Fujii (1990) provide a framework for evaluating concentrations of trace elements in soil and shallow groundwater. The two processes that largely control the mobility of trace elements in the soil water are: i) adsorption and desorption reactions; and ii) solid-phase precipitation and dissolution processes. These processes are influenced by changes in pH, redox state and reactions, chemical composition and solid-phase structural changes at the atomic level (Hinkle and Polette, 1999).

There are no well-tested and simple models for estimating changes in trace element concentration as a result of irrigation and drainage water management. Nor have irrigation water quality criteria for trace elements been established. However, guidelines have been developed for trace elements based on results from sand, solution and pot trials, field trials with chemicals and laboratory studies of chemical reactions. Pratt and Suarez (1990) list recommended maximum concentrations for 15 trace elements. These guidelines are designed to protect most sensitive crops and animals from toxicity where the most vulnerable soils are irrigated.

Selenium

Albasel et al. (1989) reviewed the quality criteria for trace elements in irrigation water compiled by the National Academy of Engineering in 1973. The recommendation for selenium was that the concentrations in irrigation water should not exceed 20 mg/litre. The guideline was recommended for all irrigation water on any land without consideration for soil texture, pH, plant species, climate and other water characteristics such as sulphate concentration. In the San Joaquin Valley, specific conditions indicated that a review of this guideline was required: i) selenium in the drainage water is in the form of selenate, which is not readily adsorbed onto soil particles and is thus readily leached; ii) water containing high selenium concentrations also has a high total salinity content and thus requires high leaching fractions to prevent salinity build up in the soil profile; and iii) water containing selenium has high concentrations of SO₄^- that greatly inhibit plant uptake of selenium. Albasel et al. (1989) used the concentration fractions for various LFs to convert the concentration of selenium in the irrigation water (Se_I) into selenium concentration in the soil solution (Se_ss). To derive the relationships shown in Figure 36, they assumed a water uptake pattern of 40-30-20-10 percent from the first to the fourth quarter of the rootzone. The concentration factors under this water uptake pattern are 5.56, 3.76, 2.58, 2.05, 1.74 and 1.53, respectively, for LF 0.05, 0.10, 0.20, 0.30, 0.40 and 0.50. To establish guidelines on the basis of these relationships, it is necessary to know the maximum selenium concentration in the soil solution (Se_ssm) at which the maximum concentration of selenium in the harvested product (Se_hp) will not be exceeded. Moreover, it is necessary that the LF be achievable.

Figure 36. Relationship between leaching fraction of the soil solution, selenium concentration of the irrigation water and the selenium concentration in the soil solution

Source: Albasel et al., 1989.

Research with alfalfa, which is the most sensitive crop to selenium accumulation in relation to the use of the harvested product, showed that Se_ssm is 250 mg/litre without exceeding the Se_hp of 4-5 mg/kg if the saline irrigation water is dominated by sulphate. Assuming that the LF would be 0.2 or more, the Se_I could be 100 mg/litre (Albasel et al., 1989). Where other crops are grown on soils different from those in the San Joaquin Valley, new guidelines will be necessary. These should be based on soil and water properties and the maximum Se_hp specific for those crops and the use of the harvested product.

Boron

Boron is an essential micronutrient for plants but it is toxic at concentrations only slightly above deficiency. The range of boron tolerance varies widely among crop plants. Salt sensitive crops such as citrus, fruit and nut trees are sensitive to boron while salt tolerant crops such as cotton, sugar beet and Sudan grass tolerate higher levels of boron (FAO, 1985b). Leaching of soil boron is more difficult than soluble salts such as chloride. This is because of the slow dissolution of boron minerals and desorption of boron adsorbed to oxides of iron and aluminium in the soil. Hoffman (1980) established a relationship to calculate the relative decrease of soluble boron in soils during reclamation:

(20)

where:

B_sst = desired boron concentration in the soil solution (mg/litre);
B_ss0 = initial boron concentration in the soil solution (mg/litre);
D_L = depth of leaching water (mm); and
D_s = depth of soil to be reclaimed (mm).

Keren et al. (1990) reported that native soil boron is more difficult to leach than boron accumulated from irrigation with boron-rich water.

Where steady-state conditions exist between boron adsorbed and boron in the soil solution (B_ss), then the input and output of boron from the rootzone, and thus B_ss, is related to the boron concentration in the irrigation water (BI) and the LF (Pratt and Suarez, 1990). To establish the relationship between BI and B_ss (Figure 37), a water uptake pattern of 40-30-20-10 percent from the first to the fourth quarter of the rootzone is assumed. The concentration factors under this water uptake pattern are as the same as for selenium above. Lysimeter experiments have shown that Figure 37 can be used to assess the use of boron-containing water for irrigation (Pratt and Suarez, 1990) where near steady-state conditions exist.

Figure 37. Relationship between mean Bss in the rootzone and between BI for several leaching fractions

Source: Pratt and Suarez, 1990.

Reuse in conventional crop production

Drainage water of sufficiently good quality might be used directly for crop production. Otherwise, drainage water can be reused in conjunction with freshwater resources (Figure 38). Conjunctive use involves blending drainage water with freshwater. Alternatively, drainage water can be used cyclically with freshwater being applied separately. In cyclic use, the two water sources can be rotated within the cropping season (intraseasonal cyclic use), or the two water resources can be used separately over the seasons for different crops (interseasonal cyclic use). The choice of a certain reuse option depends largely on: drainage water quality; crop tolerance to salinity; and availability of freshwater resources. The quantity and time of availability of drainage water is of major importance. For example, where reuse takes place in an irrigation system in which water is distributed on a rotational basis, the probable mode of reuse is either direct or cyclic.

Figure 38. Use of drainage water for crop production

Direct use

The direct use of drainage water is implemented mainly at farm level, whereby the drainage water is not mixed with freshwater resources. Research results from India, Pakistan, Central Asia and Egypt (Part II), where surface irrigation methods are applied, show that drainage water can be used directly for irrigation purposes without severe crop yield reductions where the salinity of the drainage water does not exceed the threshold salinity value for the crops grown and good drainage conditions exist. As crops are often more sensitive to salinity during the initial growth stages, research in India has revealed the importance of pre-irrigation with good quality irrigation water. Higher crop yields were attained when freshwater pre-irrigation was applied with only drainage water being applied thereafter. Under these conditions, drainage water with salinity levels exceeding the threshold value could be used whilst maintaining acceptable crop yields.

In Pakistan, the major crops are wheat, cotton, sugar cane, rice and a variety of fodder crops. The water quality guidelines state that the maximum salinity of water used directly for irrigation is 2.4 dS/m (Pakistan case study in Part II). Assuming a concentration factor of 1.5^[4] between irrigation water and saturated paste, this limit is rather high for sugar cane, which has a threshold value of 1.7 dS/m. Based on Maas and Grattan (1999), sugar cane irrigated with 2.4 dS/m water would yield 89 percent of the maximum potential yield. The long-term sustainability of direct use of drainage water depends on maintaining a favourable salt balance and preventing soil degradation due to sodicity problems. Box 9 describes direct reuse practices in Egypt and Pakistan.

Conjunctive use - blending

Where drainage water salinity exceeds the threshold values for optimal crop production, it can be mixed with other water resources to create a mixture of acceptable quality for the prevailing cropping patterns.

Where reuse takes place by mixing drainage water from main drains with surface water in main irrigation canals, the most salt sensitive crop determines the final water quality. For example, according to the regulations for the Nile Delta, the maximum salinity of the blended water is rather low to ensure optimal production of the major crops grown, i.e. cotton, maize, wheat, rice and berseem. Maize and berseem are the most salt sensitive crops with an EC_e threshold of 1.7 and 1.5 dS/m, respectively (Maas and Grattan, 1999). To ensure potential maximum yield production of these two crops and assuming a concentration factor of two between the EC of the irrigation water and the EC_e, the maximum allowable salinity of the irrigation water is about 0.8-0.9 dS/m (Table 15).

Table 15. Drainage water quality criteria for irrigation purposes in the Nile Delta, Egypt

Source: Abu-Zeid, 1988.

Where mixing takes place at the farm level, the salinity of the blended water can be adjusted towards the salt tolerance of individual crops. Table 16 shows an experiment from India, where blended water with different levels of salinity was used to cultivate wheat.

Table 16. Effect of diluted drainage water on wheat yield

Source: Sharma et al., 2000.

This experiment shows that for the case of all irrigations applied with a blended water mixture having a maximum salinity of 3 dS/m, a potential crop yield of 90 percent of the maximum yield was obtained. According to the crop tolerance data published by Maas and Grattan (1999) and assuming a concentration factor of 1.5 between water salinity and soil salinity, the maximum potential yield could be attained. The difference between theory and the actual field situation might be caused by a discrepancy in the actual concentration factor, which is the inverse of the LF. Moreover, the actual yields depend on many factors including physical, climate and farm management practices. In general, where all irrigations use the blended water, the maximum allowable salinity depends on the threshold value of the crops. Where the pre-irrigation uses freshwater, the blended water could have a salinity level exceeding the threshold value without affecting yields. The experiment from India shows that water used in post-plant irrigations with a salinity level of three times (EC_I = 9 dS/m) the maximum allowable salinity level when only blended water is used (EC_I = 3 dS/m) results in 90 percent yields.

The leaching of salts is necessary to maintain a favourable salinity balance in the rootzone. The procedures presented above might be used for this purpose. On the other hand, if higher salinity levels are tolerated towards the end of the growing period or if a more salt sensitive crop is grown after a more salt tolerant crop, the salinity levels have to be reduced sufficiently low in order not to interfere with the growth of the next crop. Equation 19 could be used to calculate the required amount of leaching.

Conjunctive use - cyclic use

Cyclic use, also known as sequential application or rotational mode, is a technique that facilitates the conjunctive use of freshwater and saline drainage effluent. In this mode, saline drainage water replaces canal water in a predetermined sequence or cycle. Cyclic use is an option for where the salinity of the drainage water exceeds the salinity threshold value of the desired crop. A condition for cyclic use is that two different water sources can be applied to the field separately. Therefore, it is not normally applied at irrigation-scheme level but at a tertiary or farm level. In India and Pakistan, the canal irrigation water is delivered on a rotational basis to the watercourses (tertiary canal) and individual farms. This offers considerable potential for cyclic use on tertiary or farm level. Modelling and field studies have demonstrated the feasibility of the cyclic reuse strategy (Rhoades, 1987; Rhoades et al., 1988a and b; Rhoades 1989; and Rhoades et al., 1989).

Table 17. Effect of cyclic irrigation with canal and drainage waters on yield of wheat and succeeding summer crops (t/ha)

¹ Canal water application, ² drainage water application, ³ during the growing period, ⁴ as green forage

Source: Sharma et al., 1994.

The cyclic use of drainage water can be either intraseasonal or interseasonal. The latter mode of cyclic use follows the same principles for each cropping season as the direct use of drainage water.

In intraseasonal cyclic use, the strategy implies that non-saline water is used for salt sensitive cropping stages and saline water when the salt tolerance of the plant increases. Such experiments have been carried out in India (India case study in Part II). Saline drainage water (EC_I of 10.5-15.0 dS/m) was combined with canal water for use in a pearl millet/sorghum-wheat rotation. In the experiment, canal water was used for pre-plant irrigations and thereafter four irrigations each of 50 mm depth were applied as per the planned modes to irrigate wheat. Pearl millet and sorghum were given pre-plant irrigation and thereafter did not receive further irrigation except by monsoon rains during the growth period. Table 17 shows that the results of the experiment support the cyclic use strategy. No significant yield losses occurred in wheat when saline drainage water was substituted in alternate sequences (canal water - drainage water, or drainage water - canal water), or when the first two irrigations were with canal water and the remaining two irrigations were with drainage water, or when the first two irrigations were with saline drainage water and the remaining two irrigations were with canal water.

Intraseasonal cyclic use offers considerable potential as not all crops tolerate salinity equally well at different stages of their growth. Most crops are sensitive to salinity during emergence and early development. Moreover, the flowering stage is also critical. Tolerance to salinity generally increases with the age of the crop (Table 18). An exception is the salinity tolerance of mustard during the flowering to reproductive development stage.

The long-term sustainability of this drainage water management option entails devoting sufficient to maintaining a favourable salt balance and to preventing a buildup of trace elements in the rootzone to levels toxic to plant growth.

Cyclic use also requires attention to soil degradation as a result of using sodic water. A high exchangeable sodium percentage on the soil exchange complex does not normally lead to soil degradation if it is compensated by a high soil moisture salinity to suppress the extent of the so-called diffuse double layer. However, upon irrigation with low saline irrigation water or rainfall, the diffuse double layer swells resulting in soil dispersion. Adding sufficient soil or preferably water amendments to compensate for the high sodium to calcium and magnesium content in the saline irrigation water can prevent these problems.

Table 18. Crop response to salinity for three crops at various growth stages

¹ EC₀ = EC_e at which zero yield is obtained
² EC₅₀ = EC_e at which yields are reduced to 50 percent
Source: Minhas, 1998.

Crop substitution and reuse for irrigation of salt tolerant crops

Crop substitution

Crops differ significantly in their tolerance to concentrations of soluble salts in the rootzone. The difference between the tolerance of the least and the most sensitive crops may be tenfold. A number of salt tolerant crop plants are available for greater use of saline drainage effluent (Annex 1). Raising the extent of the salinity limits through selecting more salt tolerant crops enables greater use of saline drainage effluent and reduces the need for leaching and drainage. Table 19 shows promising cultivars from India and Pakistan. In other parts of the world, other salt tolerant varieties have been developed.

Reuse for irrigation of salt tolerant plants and halophytes

Where the irrigation water is too saline to grow conventional agricultural crops, irrigation of halophytes might be considered. The maximum amount and kind of salt that salt tolerant plants and halophytes can tolerate vary among species and varieties. Halophytes have a special feature as their growth is improved at low to moderate salinity levels (Goodin et al., 1999). In contrast, salt tolerant crops have maximum growths up to a threshold salinity level after which growth is reduced (Figure 39). Salt tolerant plants and halophytes have been grown successfully in many places in the world to produce fuel, fodder and to a lesser extent food. Institutes in Australia have gathered useful salt tolerance data on a large number of trees and shrubs species (Annex 6). Growing salt tolerant plants and halophytes under saline conditions requires management of the salt balance in the rootzone. Where natural drainage is insufficient, artificial drainage is required to remove the leaching water.

Table 19. Promising cultivars for saline and alkaline environments in India and Pakistan

Source: AICRP Saline Water, 1998; and Qureshi, 1996.

Fuel

Many people in developing countries rely on wood for cooking and heating. As agricultural land is required to feed growing populations, it is unlikely that good quality agricultural land will be used for fuel production (Goodin et al., 1999). Salt tolerant trees and shrubs can be grown for fuel production and building materials using saline water and marginal lands. Among the promising tree species for these purposes are Prosopis, Eucalyptus, Casuarina, Rizophora, Melaleunca, Tamarix and Acacia.

Figure 39. Relative growth response to salinity of conventional versus halophytes

Source: Goodin et al., 1989

Fodder

Pasture improvement programmes in salt-affected regions throughout the world have used halophytes and salt tolerant shrubs and grass species. Trees and shrubs can be valuable complement to grasslands. They can serve as a nutrient pump and lower saline shallow groundwater tables. They are less susceptible to moisture deficits and temperature changes than grasses. They might also provide valuable complementary animal food or fuelwood. Salt tolerant grasses, shrubs and trees with potential for fodder use include:

Other products

Salt tolerant plants can also produce other economically important materials, e.g. essential oils, gums, oils, resins, pulp and fibre. Moreover, salt tolerant plants can be used for landscape and ornamental purposes and irrigated with saline water, thereby conserving freshwater for other purposes (Goodin et al., 1999).

Reuse in IFDM systems

The IFDM system aims to utilize drainage water as a resource to produce marketable crops and to reduce the volume of drainage water to be discharged (SJVDIP, 1999d; and Cervinka et al., 2001). Figure 40 depicts the principles of a typical IFDM system. Under IFDM, drainage water is used sequentially to irrigate crops, trees and halophytes with progressively increasing salt tolerance. Each time the drainage water is reused, the volume of effluent is reduced and the salinity concentration increased. A typical IFDM system consists of four zones. In Zone 1, traditional salt sensitive crops are grown, e.g. vegetables, fruits, beans and corn. In Zone 2, traditional salt tolerant crops are grown, e.g. cotton, sorghum and wheat. In Zone 3, salt tolerant trees and shrubs are grown. In Zone 4, only halophytes can be planted. The final non-re-usable drainage water is discharged in a solar evaporator.

Figure 40. Principles of an IFDM system

Source: SJVDIP, 1999d.

Figure 41. Layout of sequential reuse of subsurface drainage waters and salt harvest at Red Rock Ranch

Source: SJVDIP, 1999a.

The solar evaporator consists of a levelled area lined with plastic on which the brine is disposed and the crystallized salts are collected. The daily discharge of drainage water corresponds to the daily evaporation, this to prevent water ponding that attracts waterbirds. This is only important where high concentrations of toxic trace elements are present in the drainage water, otherwise a normal evaporation basin can be used.

Figure 41 shows an example of IFDM from California and depicts the current layout of the 260-ha Red Rock Ranch. The ranch was waterlogged and salt affected to such an extent that the farmer was unable to obtain economic crop yields. Therefore, the farmer transformed it into an agroforestry reuse system with final disposal in a solar evaporator.

The shallow groundwater flows northeast, and in some areas the water table was within less than 30 cm of the land surface. The farmer planted rows of Eucalyptus camendulensis in the upslope western boundary to intercept some of the lateral groundwater, and then tile drained the four parcels of land progressively, starting from the southwest parcel, then the northwest and southeast parcels, and finally the northeast parcel. Within a few years of subsurface drainage, the land was reclaimed to the point where alfalfa and broccoli were successfully grown in the non-saline parcels with imported irrigation water of 0.5 dS/m EC_I and 0.2 ppm boron. Cotton, sugar beets and salt tolerant grasses were grown successfully in the low-saline parcel using tile drainage water (EC_I 6-8 dS/m) from the three non-saline parcels. Because there are no opportunities for off-farm disposal of drainage waters in this subarea, the residual drainage water from this ranch is sequentially reused until no longer usable. A portion of the northeast parcel has been set aside to irrigate saltgrass (EC_I about 10-20 dS/m) in the moderately saline zone. The subsurface drainage water from saltgrass is used to irrigate Salicornia, and then its drainage water (EC > 30 dS/m) is disposed into a solar evaporator to harvest salts. A market for the salts is currently being sought.

Table 20. Changes in salinity and boron by depth at locations in the tile drained Red Rock Ranch

Source: Cervinka et al., 2001.

Table 20 documents the extent of reclamation of the salt-affected soils on Red Rock Ranch principally due to the installation of subsurface drainage and growing salt tolerant cotton and sugar beets initially. Today, about 75 percent of the land has been reclaimed sufficiently to grow salt sensitive, high-cash-value vegetable crops. The remaining 25 percent of the land is devoted to drainage water reuse in salt tolerant plants and salt harvest.

Table 21 details the average quality of the supply water and sequentially reused drainage water from salt sensitive crops, salt tolerant crops and halophytes. The quality of irrigation water for salt tolerant crops has improved and the salinity of the irrigation water is now about 6 dS/m in contrast to the reported 9.4 dS/m. Some difficulties have been encountered in managing the collected drainage water, especially for reuse on moderately saline and halophyte plots due to their comparatively small area. Drainage water disposed into the solar evaporator is controlled closely to prevent ponding by adjusting sprinkler irrigation rates and timing to daily ETo values. The extremely high selenium concentration in the drainage water is of concern to wildlife biologists. This system was initially established about 10 years ago but the present configuration has been operating for about 3 years and thus its sustainability is not yet known.

Table 21. Water quality of supply and reused drainage waters on Red Rock Ranch

Source: Cervinka et al., 2001.

Reclamation of salt-affected land

Sodic soils often have low hydraulic conductivity as a result of the high sodium percentage on the soil exchange complex. The reclamation of sodic soils requires that a divalent solute (mainly calcium) pass through the soil profile, replacing exchangeable sodium and leaching the desorbed sodium ions from the rootzone. Therefore, the rate at which sodic soils can be reclaimed depends on the water flow through the soil and the calcium concentration of the soil solution.

The application of leaching water with a high electrolyte concentration promotes flocculation of the soils and thus improves soil permeability. This expedites the reclamation process. Amendments need to be added in order to replace sodium with calcium ions on the soil exchange complex. Over time, less-saline water needs to replace the saline leaching water in order to lower the salinity levels sufficiently to establish a crop.

The use of saline drainage water to reclaim salt-affected soils is not a permanent solution for reducing drainage effluent disposal volumes. It is only a substitute for the use of good quality irrigation water for reclamation purposes. FAO (1988) has compiled a list of procedures and measures for the reclamation and management of saline and sodic soils.