5.5. Field management practices in wastewater irrigation

5.5.1 Water Management

5.5.2 Land and Soil Management

5.5.3 Crop Management and Cultural Practices

Management of water, soil, crop and operational procedures, including precautions to protect farm workers, play an important role in the successful use of sewage effluent for irrigation.

The concept of leaching has already been discussed. The question that arises is how much water should be used for leaching, i.e. what is the leaching requirement? To estimate the leaching requirement, both the salinity of the irrigation water (EC}w) and the crop tolerance to soil salinity (EC}e) must be known. The necessary leaching requirements (LR) can be estimated from Figure 14 for general crop rotations reported by Ayers and Westcot (FAO 1985). A more exact estimate of the leaching requirement for a particular crop can be obtained using the following equation:

             ECw
LR  =  ________________
        5 (ECe) -  ECw 

Figure 14 was developed using EC}e values for the 90% yield potential. For water in the moderate to high salinity range (> 1.5 dS/m), it might be better to use the EC}e value for maximum yield potential (100%) since salinity control is critical in obtaining good yields. Further information on this is contained in Irrigation and Drainage Paper 29. Rev. 1 (FAO 1985). Where water is scarce and expensive, leaching practice should be designed to maximize crop production per unit volume of water applied, to meet both the consumptive use and leaching requirements. Depending on the salinity status, leaching can be carried out at each irrigation, each alternative irrigation or less frequently, such as seasonally or at even longer intervals, as necessary to keep the salinity in the soil below the threshold above which yield might be affected to an unacceptable level. With good quality irrigation water, the irrigation application level will almost always apply sufficient extra water to accomplish leaching. With high salinity irrigation water, meeting the leading requirement is difficult and requires large amounts of water. Rainfull must be considered in estimating the leaching requirement and in choosing the leaching method.

The following practices are suggested for increasing the efficiency of leaching and reducing the amount of water needed:

1. leach during cool seasons instead of during warm periods, to increase the efficiency and ease of leaching, since the total annual crop water demand (ET, mm/year) losses are lower.
2. use more salt-tolerant crops which require a lower leaching requirement (LR) and thus have a lower water demand.
3. use tillage to slow overland water flow and reduce the number of surface cracks which by-pass flow through large pores and decrease leaching efficiency.
4. use sprinkler irrigation at an application rate below the soil infiltration rate as this favours unsaturated flow, which is significantly more efficient for leaching than saturated flow. More irrigation time but less water is required than for continuous ponding.
5. use alternate ponding and drying instead of continuous ponding as this is more efficient for leaching and uses less wwater, although the time required to leach is greater. This may have drawbacks in areas having a high water table, which allows secondary salinization between pondings.
6. where possible, schedule leachings at periods of low crop water use or postpone leachings until after the cropping season.
7. avoid fallow periods, particularly during hot summers, when rapid secondary soil salinization from high water tables can occur.
8. if infiltration rates are low, consider pre-planting irrigations or off-season leaching to avoid excessive water applications during the crop season, and
9. use one irrigation before the start of the rainy season if total rainfall is normally expected to be insufficient for a complete leaching. Rainfall is often the most efficient leaching method because it provides high quality water at relatively low rates of applications.

Drainage

Salinity problems in many irrigation projects in arid and semi-arid areas are associated with the presence of a shallow water table. The role of drainage in this context is to lower the water table to a desirable level, at which it does not contribute to the transport of salts to the root zone and the soil surface by capillarity. What is important is to maintain a downward movement of water through soils, van Schilfgaard (1984) reported that drainage criteria are frequently expressed in terms of critical water table depths; although this is a useful concept, prevention of salinization depends on the establishment, averaged over a period of time, of a downward flux of water. Another important element of the total drainage systems is its ability to transport the desired amount of drained water out of the irrigation scheme and dispose of it safely. Such disposal can pose a serious problem, particularly when the source of irrigation water is treated wastewater, depending on the composition of the drainage effluent.

Timing of irrigation

The timing of irrigation, including irrigation frequency, pre-planting irrigation and irrigation prior to a winter rainy season, can reduce the salinity hazard and avoid water stress between irrigations. Some of these practices are readily applicable to wastewater irrigation.

In terms of meeting the water needs of crops, increasing the frequency of irrigation will be desirable as it eliminates water stress between irrigations. However, from the point of view of overall water management, this may not always produce the desired results. For example, with border, basin and other flood irrigation methods, frequent irrigations may result in an unacceptable increase in the quantity of water applied, decrease in water use efficiency and larger amounts of may not result in decrease in water use efficiency and, indeed, could help to evercome the salinity problem associated with saline irrigation water.

Pre-planting irrigation is practised in many irrigation schemes for two reasons, namely: (i) to leach salts from the sol surface which may have accumulated during the previous cropping period and to provide a salt-free environment to germinating seeds (it should be noted that for most crops, the seed germination and seeding stages are most sensitive to salinity); and (ii) to provide adequate moisture to germinating seeds and young seedings. A common practice among growers of lettuce, tomatoes and other vegetable crops is to pre-irrigate the field before planting, since irrigation soon after planting could create local water stagnation and wet spots that are not desirable. Treated wastewater is a good source for pre-irrigation as it is normally not saline and the health hazards are practically nil.

Blending of wastewater with other water supplies

One of the options that may be available to farmers is the blending of treated sewage with conventional sources of water, canal water or ground water, if multiple sources are available. It is possible that a farmer may have saline ground water and, if he has non-saline treated wastewater, could blend the two sources to obtain a biended water of acceptable salinity level. Further, by blending, the microbial quality of the resulting mixture could be superior to that of the unbiended wastewater.

Alternative treated wastewater with other water sources

Another strategy is to use the treated wastewater alternately with the canal water or groundwater, instead of blending. From the point of view of salinity control, alternate applications of the two sources will be superior to blending. However, an alternating application strategy will require duel conveyance systems and availability of the effluent dictated by the alternate schedule of application.

Figure 15 presents schermatic representations of salt accumulation, planting positions, ridge shapes and watering patterns.