When planners, decision-makers or engineers face the need for a change in drainage water management, the nature of the exact problem is often not clear and the perceived problem depends considerably on the individual's viewpoint. For example, a farmer's perception of a problem related to drainage water management depends on the physical conditions within the farm boundaries and may differ substantially from those of an irrigation district, national water resource authority or environmental pressure group. In such situations, a soft system approach can help define the problem and seek solutions. A key feature of the soft system approach is that it attempts to avoid identifying problems and seeking solutions from only one perspective and excluding others.
The characteristic of a system is that, although it can be divided into subsystems, it functions as a whole to achieve its objectives. The successful functioning of a system depends on how well it satisfies changing external and internal demands. The system itself is part of a broader universe. In all natural or agricultural systems, there exists a hierarchy of levels (Stephens and Hess, 1999). For irrigated crop production this hierarchy might be:
biochemical and physical systems;
plant and cropping system;
irrigation and drainage systems;
regional, river- or drainage-basin system; and
Figure 2 provides a simplified example of a water management system within an irrigation district. It shows some main characteristics of a system as described by the Open University (1997).
1. Defining the boundaries of a system is not a simple task. For example, there could be questions as to whether it would be better to include the regional drainage system within the system boundaries and include pressure from environmental groups, other water users and water quality rules and regulations in the system's environment. The boundaries chosen reflect the perception of the systems analysts and their understanding of the system's behaviour, which may not always coincide with an organizational or departmental unit. System boundaries in drainage water management might more often coincide with hydrological units.
2. The elements included in the system's environment are those which influence the system in an important way but over which the system itself has no control because they are driven by forces external to the system of interest.
3. The elements within the system are functional working parts of the system. The way that the system operates and behaves depends on the interactions between the elements in the system. Elements could be decomposed into smaller subsystems. The level of detail depends on the specific objectives of the systems analysis. For example, in systems analysis for drainage water management, it would be important to show individual farms as subsystems because on-farm water management defines to a great extent the total quantity and quality of the generated drainage water.
4. As the creator of the system image, a systems analyst defines a particular perspective while another analyst of differing disciplinary training may produce a different image.
Figure 2. Example of a water management system within an irrigation district
When applying a systems approach, at first it is necessary to include all elements in the picture whether they relate to physical, technical, economic, legal, political or administrative considerations as well as any subjective considerations based on the understanding, norms, values and beliefs of the stakeholders involved. A premature exclusion of important elements may result in a suboptimal course of action (Open University, 1997). Figure 3 shows an example of a simplified holistic picture of the on-farm water management subsystem. This subsystem contains a mixture of physical, economical, social, legal and subjective considerations that all have some important influence on on-farm water management.
Figure 3. Example of a simplified holistic picture of the on-farm water management sub-system
Where changes in drainage water management are necessary, a simple method to produce predetermined results might not work. Rather, a more open-ended investigative approach is required in which important avenues are explored and considered and in which there is room for iterative processes. The outcome of such an investigative approach is not necessarily the optimal final solution but rather a solution that seems best for those involved, for that particular time and under those particular circumstances. As the external environment and internal demands change constantly, the mix of solutions and needs for change are changing continuously. Moreover, the achieved changes themselves give new insights into processes and enable a continuous learning process.
Figure 4. The seven stages in the soft system methodology
Source: after Bustard et al., 2000.
The soft system approach to identifying causes and seeking solutions takes the aforementioned considerations into account. The soft system methodology is a seven-stage approach (Checkland, 1981) that has been adopted and adjusted for numerous purposes. Figure 4 presents the steps in the soft system method. The soft system methodology was developed for managing changes in the context of human activity systems, i.e. a set of activities undertaken by people linked together in a logical structure to constitute a purposeful whole (Checkland, 1989). As drainage water management is a complex mix of human activity and natural and designed systems, the soft system approach presented in this chapter is not a direct interpretation of the soft systems methodology introduced by Checkland. Rather it is a free interpretation to illustrate the steps that in general need to be taken to identify the actual problems and search for possible courses of action. The following sections explain the different steps in the context of drainage water management in more detail.
Defining the problem and diagnosing the causes is the key to seeking solutions. In the context of drainage water management a problem might be defined as: a need, request or desire for a change in present situation subject to a number of conditions or criteria that must be satisfied simultaneously. This definition implies that a problem situation does not exist independent of the stakeholders who perceive the problem. The solutions to the problem are subject to criteria and conditions based on people's perception of the ideal situation. The conditions and criteria are subjective insofar as they are based on personal or communal objectives influenced by local to national economic, social, cultural, ecological and legal motives, norms and values.
In the field of agricultural drainage water management, the root cause of the problem(s) nearly always stems from human interference in the natural environment.
Step 1. An analysis of the problem and the search for solutions starts in a situation where someone or a group of people perceives that there is a problem. At this stage, it might not be possible to define the problem with precision, as different people involved will have differing perceptions of the problem.
Step 2. The first step in formulating the problem more precisely is to identify all the stakeholders involved and their relation to the problem. To form what is called the rich picture, all the elements must be included whether they relate to physical, technical, economic, legal, political or administrative considerations along with subjective considerations based on understanding, norms, values and beliefs of the stakeholders involved. It is then necessary to extract areas of conflict or disagreement as well as the key tasks that must be undertaken within the problem situation.
Step 3. Once the problem situation has been analysed and expressed from the key issues, the relevant systems and subsystems can be defined. These systems can be formal or informal and are those that carry out purposeful activities that will lead to improvement or elimination of the problem situation. Examples of a relevant system within the context of drainage water management might be an on-farm water management system, a system for development of water quality rules and regulation for drainage water disposal, or a land-use planning system. An analysis of the various elements involved provides valuable insight into different perspectives on and constraints surrounding the situation. For each relevant system, a root definition can be formulated. A root definition is a formulation of the relevant system and the purpose of the system to achieve a situation in which the problem is balanced out or eliminated. Each root definition provides a particular perspective of the system under investigation. In general, a root definition should include the following information: what the purposeful activity carried out by the system is; who the 'owner' of the system is; who the beneficiaries/victims of the purposeful activity are; who will implement the activity; and what the constraints in its environment are that surround the system (Checkland, 1989). A root definition for on-farm water management might be:
In an on-farm water management system the responsible farmer uses irrigation water in such a manner that the drainage water generated is of such quantity and quality as permitted by the drainage disposal act whilst maintaining long-term favourable soil conditions that guarantee the production of valuable crops to ensure the financial sustainability of the farming enterprise.
A root definition for the development of water quality rules and regulation could be:
A governmental system in which water quality rules and regulations for the disposal of agricultural drainage water are promulgated such that they will guarantee water quality for beneficial downstream water uses, including the maintenance of valuable ecosystems, while ensuring the economic sustainability of the agriculture sector.
Step 4. On the basis of the root definitions, conceptual models need to be constructed. These models include all the probable activities and measures that the system needs to implement to achieve the root definition. In other words, alternative scenarios need to be formulated. This involves doing sufficient work on the technical and other details, which need to be defined, in order to enable sound decision-making. Moreover, measurable indicators need to be established to compare the results of the conceptual models with the analysed situation or base case. As the formulation of scenarios is based on a thorough analysis of the systems, it should take into consideration system objectives, possibilities and constraints.
Improvements needed to minimize or correct a particular drainage water management problem may consist of physical structures, non-physical improvements or both. Physical improvements could involve using irrigation water conservatively by on-farm water management practices along with regional drainage practices such as recirculating usable drainage water to meet waste discharge requirements. Non-physical improvements may include implementing tiered water pricing to encourage growers to use water wisely, i.e. charging a penalty for overuse.
Step 5. The next step is to compare the scenarios or conceptual models with the situation analysis. The idea is to test the scenarios and decide whether the implementation of a scenario would resolve the defined key issues.
Step 6. If it would, it needs to be investigated and there needs to be debate as to whether the changes proposed, resulting from implementation of the scenario, are both desirable and feasible. What is desirable and what is feasible might clash as a result of system objectives, possibilities and constraints.
Step 7. The final step is to define the measures and changes to be implemented.
Drainage water problems vary in space and time due mainly to soil heterogeneity and water management practices. The following example illustrates how spatial variability needs to be taken into account in the problem analysis, and also how it influences the options for drainage water management.
Environmental problems related to agricultural drainage water disposal on the western side of the San Joaquin Valley, California, the United States of America, have created a need for improved irrigation water and salt management (SJVDP, 1990). The presence of harmful trace elements, mainly selenium, in the drainage water is of major concern (Tanji et al., 1986) and has led to limitations on drainage water disposal to rivers and impoundments such as agricultural evaporation basins. Where water districts in the problem area fail to meet selenium and salt load targets, they risk monetary penalties and loss of access to disposal sites. Young and Wallender (2000a) raised the question of whether the constraints raised by current or future regulations will reduce drainage to the point where salt accumulation would occur and in which areas this might occur first. Second, they raised the question of the spatial distribution of drainage water disposal costing strategies. To answer these questions, they developed a methodology for the Panoche Water District to calculate the spatial distribution of water-, salt- and selenium-balance components using data collected by the water district. Furthermore, they developed and evaluated spatially distributed drainage water disposal costing strategies. The following is a brief overview of their research findings.
Figure 5. Topography and boundaries of the Panoche Water District
Source: Young and Wallender, 2000a.
The Panoche Water District, situated in the western side of the San Joaquin Valley, is a typical example of a water district that needs to cope with disposal limitation in the form of selenium load targets. Figure 5 shows that the district lies on two alluvial fans and is generally flat with slopes of not more than 1 percent trending in a northeasterly direction.
The groundwater in the Little Panoche Creek alluvial fan contains sodium-chloride type water, relatively low in salt content, and with selenium concentrations ranging from 1 to 27 µg/litre (Young and Wallender, 2000b). In contrast, the groundwater in the Panoche Creek alluvial fan contains sodium-sulphate type water, relatively high in salt content, and with selenium concentrations ranging from 20 to 400 µg/litre. Due to overirrigation since the introduction of surface water delivery systems in the 1950s, the water table rose to within 1-3 m of the ground surface. Subsurface drainage was installed which maintained successfully the water table at an acceptable level for agriculture. However, due to irrigation and drainage practices the naturally occurring salts and selenium in the region's soils are mobilized and enter into the subsurface drainage system as well as into the shallow groundwater.
Figure 6. Annual recharge to groundwater in Panoche Water District
Source: Young and Wallender, 2000a.
Calculation of a spatially distributed water balance revealed that downslope areas with shallow water tables receive groundwater discharge. Drains in these areas intercept lateral and vertical upward flowing groundwater, while the upslope undrained areas recharge to the groundwater (Figure 6). The highest salt load in the collected drainage water occurred in the centre and northwestern part of the district that corresponds with the location of greatest drainage. Salts entered the drainage water via the groundwater with the maximum occurring at the alluvial fan boundaries. Accumulation of salinity occurred largely in the drained regions with the maximum occurring roughly in the regions of maximum groundwater recharge. In the undrained regions, more salt was removed from storage as compared to the drained regions, caused by greater deep percolation in the undrained areas coupled with salt pickup.
Figure 7 shows that the amount of selenium removed by the drains was greatest on the Panoche Creek alluvial fan and the interfan. The drainage system removed selenium through groundwater discharge. Selenium storage in the undrained areas decreased in proportion to the volume of deep percolation, while in drained areas selenium accumulated in areas similar to those where total salt storage increased.
Figure 7. Annual amount of Se removed by the drains
Source: Young and Wallender, 2000b.
Assuming a charge on drainage volume, spatial distribution of a drainage penalty per hectare would unfairly affect growers on the Little Panoche alluvial fan where relatively little selenium originates. In contrast, a charge levied per kilogram of selenium discharged from the drainage systems would result in growers on the Panoche Creek alluvial fan and the interfan paying more for drainage disposal in accordance with the higher selenium loads. Neither of the two methods of assessing drainage penalties addresses the poor water management in the upslope undrained areas that contribute to downslope drainage problems. A more equitable charge on the amount of selenium discharged into the environment from a control volume would account for excessive deep percolation as well as reflect differences in selenium loading caused by geological variations (Young and Wallender, 2000b).
Steps 4, 5 and 6 of the soft system methodology require models to predict changes as a result of a suggested implementation of measures and to enable decision-making.
A major distinction is often made between simple and complicated models in which the former is frequently associated with engineering methods and the latter with scientific methods. The development of these different types of models and the use of the terms stem from the needs of various groups of professionals. Engineers, managers and decision-makers are in general looking for answers and criteria to base their management, decisions or designs on, while scientists are more interested in the underlying processes (Van der Molen, 1996).
The terms simple and complicated in relation to engineering or functional and scientific models are rather subjective. The distinction between scientific and functional refers not only to the purpose of modelling and the intended uses, but also implicitly to the approaches on which the models are based.
The three main groups of modelling approaches are mechanistic, empirical and conceptual approaches. Mechanistic, or as Woolhiser and Brakensiek (1982) define them, physically-based models are based on known fundamental physical processes and elementary laws. In groundwater modelling, this approach is also known as the Darcian approach. As this approach is based on elementary laws it should be, theoretically, valid under any given condition and therefore its transferability is extremely high. On the other hand, empirical approaches are based on relations that are established on an experimental basis and are normally only valid for the conditions under which they have been derived. Finally, conceptual calculation approaches are based on the modeller's understanding of fundamental physical processes and elementary laws, but these are not used as such to solve a problem. Instead, a concept of the reality is used to tackle a problem. The best-known example is the bucket-type approach to describe the flow of water through unsaturated soil.
Scientific models make use of mechanistic calculation approaches whenever possible and avoid the use of empirical and conceptual approaches. In contrast, functional models might include any of the three calculation approaches. Here, mechanistic approaches might be included as long as they do not conflict with other required model characteristics such as simplicity and short calculation time. Empirical and conceptual approaches are used in functional models as the only concern is that the model serves its intended purpose.
The employment of a range of drainage water management options results in certain benefits, interactions and trade-offs not only in the place where the measure is implemented but also in adjacent and downstream areas. To enable decision-makers, managers and engineers to choose from different options and to study the effects of various alternatives, regional simulation models will need to be employed. Regional models normally include three main calculation modules, i.e. water flow and salt transport in the unsaturated or vadose zone, through the groundwater zone and through the irrigation and drainage conduits. Regional models normally require large amounts of data, and model calibration and validation is a time-consuming exercise. It is beyond the scope of this report to introduce the various models that have been developed and the reader may refer to Skaggs and Van Schilfgaarde (1999) and Ghadiri and Rose (1992) for more detail. The following sections introduce only some basic calculation considerations of water flow and salt transport in the unsaturated or vadose zone as these form the basis of several of the calculation methods presented in this publication. Where the water table in agricultural lands is controlled by subsurface drainage or where the water table is close to the rootzone, water and salt transport to and from the groundwater is considered as well. However, this publication introduces no specific groundwater models or calculation procedures for water and salt transport in the saturated zone.
Rootzone hydrosalinity models may range from simple conceptual to complex scientific models. In the more simple models, the spatial component in the control volume (i.e. the crop rootzone) is typically assumed to be homogenous and space averaged (lumped), but water flow pathways are treated as distributed fluxes, e.g. deep percolation and rootwater extraction. The time increments taken may vary from irrigation intervals to crop growing season. Salinity is often treated as a conservative (non-reactive) parameter in simple models. The advantages of simpler functional models include more limited requirements for input data and model coefficients.
In contrast, the more complex, process-based rootzone models simulate water flow based on Richards' equation and treat salinity as a reactive state variable with simplified to comprehensive soil chemistry submodels. Such models provide greater understanding of the complexities in interactive physical and chemical processes. Complex scientific models require extensive input data and model coefficients, and carry out computations over small time and spatial scales.
A word statement of Richards' equation for the rootzone may be given by:
[Rate of change in soil water with respect to time]
[Rate of change in flux with respect to depth]
[Root water extraction sink with respect to depth and time]
In one dimension and taking small soil volume elements, Richards' equation is:
The terms in the parenthesis represent flux taken as the product of hydraulic conductivity (K) and hydraulic head gradient (¶H/¶z). Richards' equation is difficult to solve because there are two dependent variables volumetric water content (q) and H, the relationship is non-linear as K is a function of (q), and the water extraction sink (Se) requires simulation of root growth by soil depth (z) and time (t). Once the soil water flow is simulated, the output data (q, flux) serves as input data for simulating soil chemistry.
Figure 8 describes some of the major chemical reactions involved in simulating changes in soil salinity.
The reactivity and transport of chemical species is obtained from:
k = 1, 2,..., n
The principal solute species (Ck=1..n) modelled are sodium (Na), calcium (Ca), magnesium (Mg), potassium (K), chloride (Cl), sulphate (SO4), bicarbonate (HCO3), carbonate (CO3) and nitrate (NO3). r is bulk density, D is dispersion coefficient, q is soil water flux, is the exchangeable form and is the mineral form of the solute species.
An early hydrosalinity simulation model (Robbins et al., 1980) was later extended to the widely used LEACHM (Wagenet and Huston, 1987). The US Salinity Laboratory has been active in modelling efforts for salt transport and major cations and anions such as UNSATCHEM (Simunek and Suarez, 1993; Simunek et al., 1996) and HYDRUS (Simunek et al., 1998, 1999), both in one and two dimensions. Trace elements of concern such as boron and selenium have yet to be incorporated into these models. Simultaneous water, solute and heat transport modelling of the soil-atmosphere-plant continuum has been developed at Wageningen Agricultural University, the Netherlands, in collaboration with ALTERRA (formerly the DLO Winand Staring Centre). The present version, SWAP 2.0, integrates water flow, solute transport and crop growth according to current modelling concepts and simulation techniques (Van Dam et al., 1997).
Figure 8. Major chemical reactions in salt-affected soils
Source: Tanji, 1990.
The long-term sustainability of irrigated agriculture is heavily dependent on maintaining an adequate salt balance in the crop rootzone. For regions with a high water table, the salt balance needs to be expanded to include the shallow groundwater, too.
Kaddah and Rhoades (1976) examined salt balance in the rootzone subject to high water table with:
Vi = volume of irrigation water (m3);
Vgw = volume of groundwater (m3);
Vdw = volume of drainage water (m3);
Ci = salt concentration of irrigation water (kg m-3);
Cgw = salt concentration of groundwater (kg m-3);
Cdw = salt concentration of drainage water (kg m-3);
Md = mass of salts dissolved from mineral weathering (kg);
Mf = mass of salts derived from fertilizers and amendments (kg);
Mp = mass of salts precipitated in soils (kg);
Mc = mass of salts removed by harvested crops (kg);
DMss = mass of changes in storage of soluble soil salts (kg); and
DMxc = mass of changes in storage of exchangeable cations (kg).
Cation exchange between calcium, magnesium and sodium may modify the balance of these cations in the soil water and affect mineral solubility. Sodium minerals are more soluble than calcium minerals while magnesium minerals may range from highly soluble (sulphate type) to sparingly soluble (carbonate type).
Equation 3 contains some components that are not known or are small in relation to other quantities such as Md, Mf, Mp and Mc (Bower et al., 1969). Moreover, the sources Md and Mf tend to cancel the sinks Mp and Mc. If steady-state conditions are assumed for waterlogged soils, DMss and DMxc may be assumed to be zero so that Equation 3 reduces to:
If the land is not waterlogged, Vgw * Cgw drops out so that salt balance can be viewed simply and leads to such relationships as:
Equation 5 expresses the leaching fraction (LF) and is the simplest form of the salt and water balance for the rootzone where surface runoff is ignored. Where the land is not waterlogged, Vdw consists of deep percolation from the rootzone.
Where the water table in agricultural lands is controlled by subsurface drainage, then the mass of salts in groundwater must be considered in Equation 4. Due to the nature of flow lines to subsurface drainage collector lines, the subsurface drainage collected and discharged is a mix of deep percolation from the rootzone and intercepted shallow groundwater. For example, for the Imperial Irrigation District, California, the United States of America, Kaddah and Rhoades (1976) estimated that deep percolation contributed 61 percent and shallow groundwater 39 percent to the tile drainage effluent based on chloride mass balance. They also estimated that tailwater contributed 10 percent to the total surface and subsurface drainage from the district. The ratio of tailwater plus intercepted deep percolation and shallow groundwater to applied water for the district was 0.36.
In the presence of high water table, shallow groundwater and its salts may move up into the rootzone (recharge) and down out of the rootzone (discharge) depending on the hydraulic head. Deficit irrigation under high water table may induce rootwater extraction of the shallow groundwater. The salinity level of the shallow groundwater is of some concern under such conditions. However, there does not appear to be a simple conceptual model of capillary rise of water and solutes. Chapter 5 and Annex 5 contain a method for computing capillary rise that requires extensive soil hydraulic parameters not normally available for field soils. Hence, the conceptual hydrosalinity models used in this paper are for the more simplified downward steady-state type.
Various versions of the salt balance, Equation 3, have served as the basis for numerous models including SALTMOD (Oosterbaan, 2001), CIRF (Aragüés et al., 1990) and SAHYSMOD, which is under preparation by the International Institute for Land Reclamation and Improvement, Wageningen, the Netherlands. Furthermore, the calculation methods as introduced by Van Hoorn and Van Alphen (1994) are based on the same concepts.