Salinity

Irrigation water quality in terms of salinity is assessed by determining the amount and types of salts available in the water (Westcot & Ayers, 1985). Crops are significantly different in their tolerance of salinity conditions (FAO, 2003). Increasing the salinity of

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treated wastewater will reduce the possibility of irrigation reuse due to the damaging impacts on soil and crops (Maas & Grattan, 1999).

However, there are several approaches suggested by FAO (2003) to overcome the salinity problem when irrigating with treated wastewater:

 Selection of crops tolerant to the wastewater salinity and still commercially viable.

Generally, most crops can be successfully grown under salinity of less than 3 dS/m with good management of wastewater. However, with increasing salinity, selection of suitable crops will be difficult and the choice for fodder crops will be highly restricted. Table (2.11) lists the tolerance of some crops based on specific ranges of salinity.

 Selection of crops of high absorbency of salts without toxicity impact such as salt harvesting crops. Sudax, Bermuda grass, sorghum and barely are examples of salt harvested crops.

 Selection of uniformly applied irrigation system with high efficiency and the ability for frequent irrigation. Moreover, using a modern irrigation system with suitable management can result in better crop yield.

 Irrigation scheduling is an essential factor to control salinity as it can be directly affected by the amount and frequency of irrigation water application. For example, using micro-irrigation systems requires high frequency of water application and in this case the salinity of irrigated soil should be conserved at minimum levels.

 Leaching is another possible approach to control salinity but will not be appropriate in the case of a shallow water table, insufficient drainage and water shortage. When irrigating soil for a long time with wastewater, then the total applied salt (salt in) should be equal to that up-taken by plants and taken through leaching (salt out). This approach is very important to select suitable crops and better management of wastewater for irrigation reuse. Moreover, using of salt harvest crops will obtain good results and the cultivation of such crops periodically is highly recommended.

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 Soil conditioners such as polymers can be used under certain conditions and for a specific duration. However, these conditions are not recommended for open field crops due to their short half-life and high cost.

 Drainage facilities should be available to avoid waterlogging and salinisation in arid and semi-arid areas. However, the combination of drainage and sufficient scheduled irrigation will allow salts in plant root zones to be accessed via leaching processes. Furthermore, Westcot & Ayers (1985) reported that the only practical solution to manage the salinity problem is to ensure there is a downward flux for water and salt through the root zone. This will provide good drainage to allow the driving of water and salt under the root zone. Without adequate drainage, irrigation with treated wastewater will not be possible for long-term conditions. However, when drainage water salinity exceeds the thresholds for crops, the blending of treated wastewater with fresh water either before or during irrigation will be a possible solution to reduce the salinity level in irrigation water and extend the volume of water available for irrigation (Rhoades, 1999; Oster & Grattan, 2002). A salinity problem in irrigation water is usually indicated by measuring the electrical conductivity (Ec). However, FAO (2003) recommended that irrigation water salinity should not exceed 3000 µS/cm for vegetable production (Table 2.5).

Regarding wetlands efficiency in terms of salinity removal, Lymbery et al. (2006), performed a study on pilot-scale subsurface flow wetlands incorporating Juncus kraussii constructed to investigate the system efficiency in terms of salt and nutrients removal at several concentrations from inland saline aquaculture effluent of Western Australia for a duration of 38 days. The authors’ results showed that 44 to 53% of the total sodium chloride (NaCl) was removed by the system indicating the capability of wetlands to remove the salinity from ground water discharged by aquaculture processes. However, the authors reported these results during their short-term experiment and they

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suggested that for long-term operation, the accumulation of salts in the soil and plants will affect the system efficiency in terms of salt removal. Because of this, using salt- tolerant plants (halophytes) in wetland systems is a suitable alternative for treating water of high salinity as they can remove up to 35,000 mg/l of effluent salinity (Brown et al., 1999).

Table 2.11: Some cultivated crop salinity tolerances (adapted and updated from FAO (2003)).

Irrigation water salinity (dS/m or mg/l) < 2 or < 1280 2–3 or 1280– 1920 3–4 or 1920– 2560 4–5 or2560– 3200 5–7 or 3200– 4480 > 7 or > 4480

Citrus Fig Sorghum Soybean Safflower Cotton

Apple Oliver** Groundnut Date palm*** Wheat Barely Peach Broccoli Rice Harding grass Sugar beet Wheat grass

Grapes Tomato Beets Trefoil Rye grass

Strawberry Cucumber Tall fescue Artichokes Barley grass

Potato Cantaloupe Bermuda

grass

Pepper Watermelon Sudax

Carrot Spinach Onion Vetch Beans Sudan grass Corn Alfalfa

Note: 1 dS/m=640 mg/l; ** Much higher Ec levels were reported (up to 6 dS/m) for olive in Tunisia; and *** _{Similar higher Ec levels were reported for date palm trees in Algeria (up to 7–8 dS/m).}