In the context of this publication, drainage water management refers to the management and control over the quantity and quality of the drainage water generated in an agricultural drainage basin in arid and semi-arid areas and its final safe disposal. This is achieved through irrigation water conservation measures and the reuse, disposal and treatment of drainage water. Managing drainage water at the field, irrigation-scheme and river-basin levels entails a number of activities including:
regulating water table levels in the drainage system to ensure the maintenance of favourable soil moisture conditions for optimal crop growth and salinity control;
developing irrigation and drainage water management strategies to ensure that disposal regulations and water quality standards are met and dealing with issues, problems and conflicts that might occur;
setting distribution priorities and criteria for reuse in water scarce areas; and
establishing cost sharing imposed on stakeholders for the use of poor quality water and required treatment to meet the water quality standards for drain discharge.
The main reasons for developing a drainage water management strategy are: (i) prevention of economic and agricultural losses from waterlogging, salinization and water quality degradation; (ii) concern for quality degradation of shared water resources; and (iii) the need to conserve water for different water users under conditions of actual or projected water scarcity. In addition, the need to comply with drainage water policies and regulations can provide a strong incentive for improved drainage water management.
Many countries and states, e.g. Australia, India, Egypt and California, the United States of America, have a drainage water disposal policy and drainage effluent disposal regulations. In California, the United States of America, and in Australia, the drainage policy guidelines consist of difficult but achievable targets with an active enforcement of regulations. India and Egypt have policy guidelines of a general nature but have not reached the maturity of Californian and Australian laws. Law enforcement often fails in these countries, mainly as a result of administrative shortcomings and unrealistic quality guidelines for their conditions and resources. In these countries and in countries where clear laws and regulations are absent, the prevention of economic and agricultural losses and the conservation of water for other beneficial uses will normally be the main driving factors behind the development of a drainage water management strategy.
Figure 9 identifies the physical drainage water management options that are available to planners, decision-makers and engineers and how they relate to one another. The measures have been grouped into four categories: water conservation, drainage water reuse, drainage water treatment and drainage water disposal measures.
Figure 9. Physical drainage water management options and how they relate to one another
A major goal of conservation measures is to reduce the volume of drainage effluent generated and the mass discharge of salts and other constituents of concern while at the same time saving water for other beneficial uses. Conservation measures can directly affect the need for and extent of reuse as well as the quantity and quality of drainage effluent requiring disposal and/or treatment. Where competition for water quantity and quality among different groups of users is a major issue and where drainage effluent disposal is constrained (as in a closed drainage basin) or threatens ecologically sensitive areas, conservation measures are among the first to be considered. The various conservation measures include: source reduction (SJVDIP, 1999e); shallow groundwater table management (SJVDIP, 1999a and 1999f); groundwater management (SJVDIP, 1999f); and land retirement (SJVDIP, 1999c). Table 2 provides a short description of the four conservation measures and some points for consideration.
Table 2. Conservation measures, practices and points for consideration
|
Option |
Practices |
Points for consideration |
|
Source reduction |
Reduce the volume of deep percolation through: improving irrigation performance by surface irrigation, changing from surface to precision irrigation methods, modifying irrigation schedules, upgrading irrigation infrastructure, etc. |
Farm- and system-level costs of system improvements have to be considered against the regional benefits. Salinization and concentration of toxic elements in the rootzone need to be prevented by guaranteeing minimum leaching. |
|
Shallow groundwater table management |
Encourage the use of shallow groundwater to meet crop evaporation through maintaining sufficiently high groundwater tables and practising deficit irrigation. |
There is a danger of salinization and concentration of toxic elements in the upper soil layers due to induced capillary rise. Moreover, the risk of insufficient aeration during rainfall in the rootzone needs to be addressed. |
|
Groundwater management |
Pumping from vertical wells could control water tables. Pumped water of adequate quality could serve as a substitute for surface water. |
Prevent overexploitation of groundwater resources. There is a danger of intrusion and upwelling of saline groundwater. As groundwater often contains elevated concentrations of salts and trace elements, their buildup to toxic levels and soil degradation needs to be prevented. |
|
Land retirement |
Retire or fallow irrigated lands that are heavily affected by waterlogging and salinity or those lands that generate drainage effluent with extremely high concentrations of salts and/or other trace elements. |
Retired lands can become excessively salinized and high concentration of toxic elements can build up in the topsoil preventing natural vegetation from establishing itself. Contaminated bare lands can affect productivity and be a health risk to humans in the vicinity due to wind erosion. |
Source: SJVDIP, 1999a, 1999c, 1999e, 1999f.
The first line of action in on-farm water conservation is source reduction or reducing deep percolation. Where the goal is not achievable by source reduction, it may be necessary to implement other conservation measures in combination with reuse measures. As different crops require different amounts of water for optimal crop growth and as rooting patterns differ among crops, source reduction and water table management could be combined with a change in cropping pattern to optimize the desired effect of the measures. Changes in cropping pattern should only be considered where compatible with the broader development objectives for the area under consideration.
The major aim of reuse measures is to reduce the amount of drainage effluent while at the same time making additional water available for irrigation and other purposes. Reuse measures comprise: reuse in conventional agriculture; reuse in saline agriculture; Integrated Farm Drainage Management (IFDM) systems; reuse in wildlife habitats, wetlands and pastures; and reuse for initial reclamation of salt-affected lands (SJVDIP, 1999a). Table 3 provides a brief description of the alternative reuse measures.
Table 3. Reuse measures, practices and points for consideration
|
Option |
Practices |
Points for consideration |
|
Reuse in conventional agriculture |
Agricultural drainage water is collected and redistributed among farmers. Reuse can be direct or in conjunction with other sources of irrigation water. Conjunctive use can be through blending or by cyclic use of drainage water and other sources of irrigation water. |
The extent of reuse depends on the quality of the drainage effluent, time of availability, crop tolerance, etc. Soil quality degradation and production losses need to be prevented through mitigation measures. Residuals of reused waters, which are often highly concentrated with a reduced volume, need to be managed. |
|
Reuse in saline agriculture |
Moderately and highly saline drainage water is collected and used to cultivate salt tolerant shrubs, trees and halophytes. |
Sustainability of the saline agricultural systems needs to be safeguarded while the highly concentrated drainage effluent needs to be managed. |
|
IFDM System |
On-farm sequential reuse of agricultural drainage water on crops, trees and halophytes with increasing salt tolerance. In every reuse cycle the volume of drainage water decreases while the salt concentration increases. The final brine is disposed in a solar evaporator. Salt utilization might be feasible. |
To maintain adequate control of soil salinity and sodicity and to prevent a buildup of toxic elements, the leaching fraction must be sufficient. The solar evaporator should be designed in such a manner that it will not sustain aquatic life and attract birds. The salts have to be disposed of in a safe and sustainable manner. |
|
Reuse in wildlife habitats and wetlands |
Where of suitable quality, drainage water may be utilized to support wildlife, including waterbirds, fish, mammals and aquatic vegetation that serves as food and cover for wildlife. |
Of primary concern is the possible presence of trace elements that may be toxic to wildlife through bioaccumulation in the food chain e.g. selenium, molybdenum and mercury. |
|
Reuse for reclamation of salt-affected soils |
Use of (moderately) saline drainage water for initial reclamation of saline, saline sodic or sodic soils. On sodic soils the saline water may help prevent soil dispersion and degradation of soil structure. |
Once initial reclamation is obtained, water of sufficient quality and quantity needs to be available for desired land use. Initially, the drainage effluent will be highly saline and/or sodic and needs to be disposed of in a safe manner. |
Source: SJVDIP, 1999a.
Reuse measures can be implemented in combination with conservation measures. Where the drainage water is of relatively good quality, its reuse potential in conventional agriculture is high. Where it is moderately to highly saline, its reuse may be limited to salt tolerant plants. In California, the United States of America, a new IFDM system reuses drainage water sequentially in high water table lands with no opportunities for disposal of subsurface drainage (SJVDIP, 1999a). Freshwater is used to grow salt sensitive crops, and subsurface drainage water from it is reused to grow salt tolerant crops. Drainage water from the salt tolerant cropland is used to irrigate salt tolerant grasses and halophytes. When the drainage water is no longer usable, it is disposed into solar evaporators for salt harvest. In this system and others, it is always necessary to generate a minimum volume of drainage effluent to prevent the rootzone from becoming too contaminated for any beneficial water use activity. Similar systems are being developed and tested in Australia.
Drainage water treatment in a drainage water management plan normally takes place only under severe constraints, such as stringent regulations on disposal of saline drainage waters into streams, or severe water shortage. The drainage water treatment options are based on physical, chemical and/or biological processes (SJVDIP, 1999b). Many of these processes are borrowed from water treatment processes for drinking-water, sewage and industrial wastewater. A few are new processes to remove selenium. The water quality requirements of the treated water need to be well understood prior to the selection of any treatment measure. Table 4 provides the array of drainage water treatment options.
Table 4. Array of drainage water treatment options
|
Description |
Processes |
Constituents removed or treated |
|
Physical/ chemical |
Sedimentation/coagulation |
Remove sediments and associated nutrients, pesticides and trace elements in sedimentation ponds with or without coagulants. |
|
Adsorption |
Remove soluble constituents onto surfaces of adsorbents. |
|
|
Ion exchange |
Exchange constituents with another using ion exchanger resins or columns. |
|
|
Reverse osmosis |
Under pressure, separate out dissolved mineral salts through semi-permeable membranes. |
|
|
Coagulation/precipitation |
Use chemicals such as alum to coagulate or precipitate constituents of concern. |
|
|
Biological |
Reduction/oxidation |
Reduce oxidized mobile forms such as selenate to reduced immobile forms such as elemental selenium through microbially mediated processes. |
|
Volatilization |
Some plants and microbes are capable of taking up constituents such as selenium and volatilizing the methylated forms into the atmosphere. |
|
|
Plant/algal uptake |
Certain terrestrial plants and algae are capable of extracting large amounts of constituents such as selenium, nitrate and molybdenum. |
|
|
Constructed flow-through wetlands |
Constituents such as selenium and heavy metals are removed from drainage water. For selenium, the principal removal mechanism is reduction to elemental selenium and organic forms in the detrital matter. For heavy metals, the principal sink mechanism is sorption or fixation on the sediments. |
Source: SJVDIP, 1999b.
The high costs of many of the treatment processes make them unsuitable for agricultural reuse. Treatment measures such as reverse osmosis are normally only considered for high-value purposes such as drinking-water supply. One option is to partially treat saline waters to a level that could be used for agriculture (e.g. desalt to about 1 dS/m instead of less than 0.1 dS/m). However, the cost of brine management and disposal remains a major problem.
An exception to the typical high cost is the treatment of drainage water through constructed wetlands. Investigations are underway in California, the United States of America, in which wetland cells are planted with cattails, saltmarsh bulrush, tule, baltic rush, saltgrass, smooth cord grass and rabbitsfoot grass. The cells remove about 60-80 percent of the mass of selenium with hydraulic residence times ranging from 7 to 21 days. This treated water is then disposed into agricultural evaporation ponds with reduced toxic selenium effects on waterbirds. Constructed wetlands appear to have some potential to protect aquatic ecosystems and fisheries either downstream or in closed basins.
Even after the successful implementation of conservation and reuse measures, there will always be a residual volume of drainage effluent requiring disposal. Disposal options depend mainly on the situation of the drainage outlet in relation to natural disposal sites such as rivers, streams, lakes and oceans. Disposal options to surface water bodies comprise discharge into rivers and streams, river mouths and constructed outfall drains for direct disposal to oceans or saline lakes, and evaporation ponds (Figure 10).
Figure 10. Options for disposal to surface water bodies
Another potential disposal option is deep-well injection. However, in California, this approach failed due to the slow permeability of the geologic strata from the plugging of conducting pores (Johnston et al., 1997). Besides the natural drainage conditions, the suitability of each of these measures depends on: the quality and quantity of drainage water requiring disposal; environmental and health risks; available technology and resources; and economic considerations. Table 5 provides a short description of the disposal measures.
Drainage water disposal into natural surface water bodies should entail minimal deleterious impacts on other downstream water uses (agricultural, industrial and municipal) and wildlife. Disposal on land in closed basins and agricultural evaporation basins should avoid undue harm to the ecology, particularly aquatic biota including fish and waterbirds.
Table 5. Disposal measures, practices and points for consideration
|
Option |
Practices |
Points for consideration |
|
River discharge |
The drainage effluent is released into rivers, streams, etc. from where it finds its way to an ultimate salt sink, i.e. oceans and salt lakes. This option is especially appropriate during high discharge periods. |
Disposal in rivers and lakes should not unduly impair other downstream uses including water required for sustaining fragile aquatic ecosystems. River disposal is often limited by disposal regulations. |
|
Evaporation ponds |
Disposal of drainage water in natural depressions or specially designed unlined basins. The impounded water dissipates through evaporation and inadvertent seepage losses, and deposits salts and trace elements. |
Concentration of trace elements could adversely affect birds and wildlife through bioconcentration in the aquatic food chain to toxic levels. Adverse toxic impacts should be mitigated through special measures. Excessive seepage losses may pose a serious contamination risk to groundwater resources. |
|
Outfall to saline lakes or oceans |
Constructed main drain or disposal into river mouths to discharge effluents into the ocean or saline lakes. |
Construction of outfall drains over long distances is normally an expensive undertaking and should only be considered where other alternatives are not feasible or the stream water quality is fragile. Disposal into oceans, bays and estuaries may be restricted if toxic trace elements are present. Disposal into tidal rivers needs tidal gates to prevent saltwater intrusion during high tide. |
|
Deep-well injection |
Treated drainage water is often injected into deep underlying permeable substratum in confined aquifers. |
Formation of microbial slimes and colloidal particles may affect permeability in the stratum. There is a risk of seepage of poor quality water into fresh groundwater bodies. |
Source: SJVDIP, 1999d, 1999g.
In order to be fully effective, non-physical management measures, i.e. policy and legislation, should accompany physical drainage water management options. Different countries have implemented several policies, and these are often part of a general pollution policy.
One of the most frequently mentioned principles with respect to pollution is that of the polluter pays. This means that economic policy instruments are applied to change the behaviour of farmers in such a way that pollution is minimized or that at least the polluter pays for its effects. Weersink and Livernois (1996) provide an overview of such economic policy instruments for resolving water quality problems from agriculture. This overview was compiled for pollution by nutrients in humid temperate areas, such as Canada. However, some of these policies have relevance to pollution from the drainage of irrigated lands. The main problem of policy instruments in drainage water management is that water quality degradation from agriculture is non-point source pollution. Direct links between agricultural practices and water quality degradation are often difficult to determine and quantify.
Table 6. Economic policy instruments
|
Base |
Target |
Form |
|
Performance of agricultural systems |
Emission |
Charges |
|
Ambient |
Charges |
|
| |
Salinity permits |
|
|
Agricultural practices |
Inputs |
Charges |
| |
Subsidies |
|
|
Outputs |
Charges |
|
| |
Subsidies |
Source: based on Weersink and Livernois (1996).
Weersink and Livernois divided the policy instruments into two groups: (i) those based on the performance of an agricultural system, i.e. on the actual amount of pollution that caused; and (ii) those based on agricultural practices such as fertilizer and agrochemical uses (Table 6). In the latter case, the underlying assumption is that certain agricultural practices will lead to more or less pollution. These policy instruments try to influence the practices and, indirectly, the pollution resulting from these practices.
Fees are levied on the discharge of the polluter into the water. The aim is to stimulate the polluter to adopt practices that minimize pollution or to make the polluter pay for some of the damage caused. This requires measurement of individual discharges, which is often costly and not always practical to implement. Measurements at the individual farm level are probably only feasible with modern and large farms, such as in the United States of America and Spain. However, the case of the Panoche Fan (Chapter 2) shows that it remains difficult to establish equitable fines. Discharges from upstream farms with natural internal drainage cannot be measured while they may contribute significantly to the pollution in downstream regions. An alternative to measuring and levying fees on individual farms is to implement it at district level or at that of a water users association. This has the disadvantage that farmers applying conservation measures may still pay for the pollution caused by badly managed farms in the same district.
When basing fines on emission levels, there should be a decision as to whether to base them on concentrations of polluting elements or on the total load (i.e. concentration multiplied by flow). Concentration is the water quality parameter used in drinking-water standards or for the health of aquatic animals and plants. Placing only concentration limits on discharge might encourage dilution or inefficient water use. Where both concentration and load limits are enforced, they tend to promote efficient water use.
Basing charges on the pollution in the downstream receiving water body can sometimes be a feasible alternative to basing them on the emission level, i.e. where the source of pollution is extremely difficult to measure. For example, in the case of pollution of groundwater, individual contributions to the pollution are difficult to measure and thus difficult to quantify. A disadvantage of using ambient levels under these conditions is that all farmers, including those applying sound water management practices, are charged equally.
Another case where this policy measure may be an attractive option is where the carrying capacity of the receiving water body changes continuously. For example, in India during the monsoon, river discharge is high and effluent disposals during this period increase downstream concentrations only slightly. On the other hand, during the dry season when river flow is low, emissions affect water quality substantially. In this case, using the ambient criterion reflects the major concern: maintaining river water quality for downstream users. This method stimulates corrective measures during critical periods.
An interesting solution for salinity management on a regional level is that adopted in the Murray-Darling basin in Australia. States within the basin have to meet electrical conductivity (EC) levels at the end of their river valleys, this in order to maintain a favourable water quality in the entire downstream river. In order to reach this goal, a system of salt credits and debits is used. Credits are obtained for the implementation of any works that reduce the salinity in shared rivers. Debits are incurred based on the estimated shortfall in protecting shared rivers. The balance of credits and debits is registered for each state, and as a general principle each state must be in credit. The credits and debits are converted to EC impact at a location in the downstream area of the basin. This method allows states and catchment management authorities to decide on the most cost-effective options for their area whilst contributing to the overall basin-wide river salinity management plan (Murray-Darling Basin Ministerial Council, 2001).
Instead of charging for pollution outflow from irrigation systems, economic incentives can be directed at reducing the inflow, which will normally lead to a reduced pollution outflow. Thus, as a way of reducing pollution, input charges seek to conserve water and minimize the use of agrochemicals and fertilizers. Volumetric water pricing is the main way of providing an economic incentive to save water. However, research from Iran (Perry, 2001) has highlighted some limitations of this approach. In the Iranian case, to serve as a proper economic incentive to change to pressurized irrigation, the total costs of water would need to equate to 60 percent of the revenues of a wheat crop, while at the moment they stand at 5 percent. Such an increase is probably politically impossible and would reduce farm incomes drastically. Surface irrigation improvement is less costly than changing to pressurized irrigation. Therefore, volumetric water pricing may be an economic incentive to improve surface irrigation water management.
Charges on inputs should also consider the way in which farmers can reduce water consumption. The main changes farmers can make are: grow less-water-demanding crops; apply water saving technologies; and change over to rainfed farming (generally not practised in arid and semi-arid areas). All these responses lead to less water consumption, but sometimes also to significant drops in farm income level.
On the basis of computer simulations, Varela-Ortega et al. (1998) analysed the responses of farmers in Spain to increasing water prices. Results showed that in modern irrigation districts, where irrigation efficiency is already quite high, no changes in water demand would occur unless the prices became restrictively high and farmers needed to change to rainfed farming. In contrast, in areas with the older irrigation systems, farmers would have ample scope for improving their water use efficiency.
An alternative to volumetric pricing is tiered water pricing. In this system, farmers pay a lower volumetric price for a reasonably necessary amount of water to grow a certain crop. A higher volumetric price is charged for additional amounts of water. Such a system is explicitly designed to reduce the amounts of water that generate most of the drainage water. The reasonably necessary amount should include the minimal leaching requirement in order to maintain a favourable salt balance in the rootzone unless sufficient rainfall occurs in the area. A main problem would be to define the 'reasonable necessary amount of water'. In Broadview Water District, California, the United States of America, tiering levels differ only between crops and not between soils, because farmers consider the latter approach to be inequitable. Tiering levels were established at 90 percent of the average depths applied in the years before the system was implemented. This system of tiered water pricing led to a decrease in water applied to five of the seven main crops. More importantly, the drained volume in 20 of the 25 subsurface systems in the water district decreased by 23 percent and the salt load declined by 25 percent (Wichelns, 1991).
Volumetric pricing policies are typically only applicable in on-demand systems. Other systems deliver fixed amounts of water to farms, and they fall outside the responsibility of the individual farmers. In such systems, more gains could be obtained by matching supply and demand more closely to each other in a technical/operational way.
A changeover to water saving or pollution prevention measures might not always be the objective of individual farmers nor might it always be to their advantage. In the case of Iran, it was calculated that the investments in sprinkler irrigation are of the same order of magnitude as the value of the water that can be saved, if this is used to irrigate a larger area or high-value crops (Perry, 2001). However, if the water saved is diverted to irrigate land in other areas, upstream farmers will be investing while benefits accrue to other areas. Another example is the case of farmers located in the higher areas with free drainage in the Panoche Fan. Farmers in these areas contribute much to the pollution, but there is no direct incentive for them to invest in water saving or pollution prevention measures. In both cases, subsidies might stimulate farmers to invest in water saving technologies or pollution prevention measures.
A final instrument proposed by Weersink and Livernois (1996) is a charge on the crops. In the case of nitrogen leaching, the charge focuses on crops that need large amounts of nitrogen, and thus have a higher risk of causing pollution. In irrigated agriculture, one could think of policies that discourage growing crops with large water requirements (such as alfalfa) by charging the crops or by promoting deep-rooting crops or less-water-demanding crops through subsidies. A common example is where farmers pay a price per area depending on the crops grown.
A single economic or policy instrument will probably not resolve water pollution problems originating from irrigated agriculture. Combinations of policies can help to enforce their effect. Especially in situations where farmers are inequitably charged, a combination of measures might help to charge farmers more equitably. For example, emissions from an irrigation district could be measured and fines applied where limits are surpassed. These fines could then be distributed amongst the farmers, applying a ranking based on how farmers have used their inputs or what measures they implement. The most inefficient users should then pay comparatively more than the most efficient ones.
Normally, more than one drainage water management option will need to be considered and implemented to attain the desired objectives. In this case, interactions and trade-offs occur. Because of the complexity of the processes and interactions, computer models are best suited to selecting an optimal combination of drainage measures. As was explained in Chapter 2, the selection of a set of drainage water management measures requires more considerations than solely technical feasibility. Figure 11 shows a flowchart for selecting a set of drainage water management measures (AHCC, 2000).
Figure 11. Flowchart of process for selecting an optimal set of drainage water management options
Source: based on AHCC, 2000.
Step 1 is to gather technical, environmental, social, economic and institutional information about the site and the available drainage water management options. Step 2 is to evaluate the desirability of the options from a technical and environmental perspective. Typically, this step requires the use of computer models. Figure 9 might also help in the evaluation process. Step 3 involves ranking the site's options, based on the technical and environmental desirability. Step 4 involves an analysis of the relative economic efficiency of the various options. Marginal cost curves may be useful in determining which options are more efficient than others. When evaluating the economic efficiency of a highly complex, multi-parameter system, it may be advantageous to turn to a computerized economic optimization model. The course of action ultimately decided upon also depends on the social, organizational and institutional suitability of the proposed options and may run counter to what is economically efficient. Step 5 evaluates the social and institutional issues. Step 6 is perhaps the most difficult as it always involves intuitive judgement and a certain measure of creativity. Collating all the information and developing a stepwise path for implementing the favoured options takes time and considerable thought. The preparing of a report describing the recommended options and their order of implementation (Step 7) follows directly from Step 6.
The Australian National Committee on Irrigation and Drainage is exploring another approach called benchmarking as part of its efforts to improve the performance of irrigated agriculture. The principal goal of benchmarking is to "find and implement best practice for the organization in question" by "learning about their own organization through comparison with their historical performance and with practices and outcomes of others" (Alexander, 2001). In 1998/1999, this committee evaluated 47 parameters in 46 water providers to gauge system operation, environmental issues, business processes and financial performance. New parameters to be considered with specific regard to on-farm water use include: security of water supply, water savings from operational and seepage remediation, metering of supplies, water trading, salt balance, water quality monitoring, and operating costs per unit length of channel or pipeline. The benchmarking reports and compiled data have already led the water industry to consider and/or adopt improved technologies to enhance economic, environmental and social performance. This approach has received positive responses from the water industry, and the plan is to now evaluate additional comparative parameters, such as groundwater quality, absolute changes in water table, extent of water reuse and recycling, average water use on major crops, and financial reports on economic value added.
