L. Horst, Professor of Irrigation Emeritus, Department of Irrigation and Soil Water Conservation, Wageningen Agricultural University, The Netherlands
Generally, irrigation scheduling is based on soil-water-plant relationships and efficiency considerations. These schedules assume how farmers should cultivate and irrigate their lands and how irrigation agency personnel should operate the irrigation system. Increasingly, evidence shows the existence of a large discrepancy between the irrigation schedules on the one hand and the actual operation and farmers' behaviour on the other. This paper endeavours to analyse this discrepancy and to suggest possible solutions.
This paper deals with irrigation scheduling and water distribution in large-scale smallholder irrigation projects served by open canal systems with manually or mechanically operated structures. Point of departure is the fact that in most irrigation projects the actual distribution of water to the (groups of) farmers deviates strongly from the irrigation schedules determined by the project management. In order to be able to implement irrigation schedules, the water division technology (division and offtake structures) should meet certain requirements with regard to regulation and measurement. In its turn, this technology largely determines the operational options and farmers' acceptance of distribution practices. This paper tries to analyse the interrelationship between irrigation scheduling, the required technology and the operational reality.
Structure of the paper
Changes in concepts on design and operation during the twentieth century are dealt with in Section 2. These changes have led to the important scientific development of the soil-water-plant relationship with its impact on irrigation scheduling and infrastructural design. In Section 3, the assumptions made in design and irrigation scheduling are placed against the operational reality as found in the field. The discrepancy between theory and practice is analysed. In Section 4, three possible solutions are discussed: 1) improvement of the present irrigation scheduling approach; 2) automation and 3) simplification. In Section 5, simplifications of scheduling are discussed by considering Additional Operational Requirements (AOR). The paper ends with some concluding remarks in Section 6.
2. IRRIGATION DESIGN AND OPERATION IN THE TWENTIETH CENTURY
During the first half of the twentieth century, irrigation design and operation were very much empirically determined. Technology and irrigation scheduling were based on previous experiences in other irrigated areas.
FIGURE 1 - Supply and demand curves at the headworks of the irrigation system
Around the middle of this century a period started where design and operation were increasingly determined by scientific methods and techniques. This scientific approach was based on production objectives: optimization of yields and efficiencies. Soil-water-plant relationship became the focus. As a result of extensive research into this relationship, water requirements for crops at various growth stages under different climatic and soil conditions could be determined with great accuracy (see FAO, 1977; 1979). This development enabled designers/planners to better assess required canal capacities, irrigable areas, required reservoir volumes, etc., on the basis of assumed cropping patterns. Agency managers/ operators were able to set up irrigation schedules on the basis of actual cropping patterns (demand-based operation), or previous or assumed cropping patterns (supply-based operation). The possibility to accurately assess crop water requirements during the growing season, together with efficiency considerations, led to irrigation schedules (supply curves) which follow closely the actual or assumed demand curves (Figure 1).
Consequently, these irrigation schedules require water control structures where water flows can be frequently regulated and if necessary measured. These structures generally consist of movable weirs or gates. Although procedures on the regulation, measurement and monitoring of flows are presented in guidelines and operation manuals, in reality, however, the operation of many systems appears to be completely different. This discrepancy will be further discussed in the next section.
3. DESIGN ASSUMPTIONS, IRRIGATION SCHEDULES AND OPERATIONAL REALITY
In order to analyse the complex interrelationship between design assumptions, irrigation schedules and operational realities, it is useful to discern the three major parties involved in irrigation practice: (i) planners/designers (irrigation agency, consultants, donors); (ii) operational office staff (irrigation agency staff in headquarters, provincial and district offices); (iii) operational field staff and farmers (at tertiary and secondary level).
To meet the irrigation scheduling requirements, irrigation systems are generally designed to facilitate regulation and, possibly, measurement of flows at most bifurcating structures (division structures, offtakes, etc.). In other words, the flows of water throughout the system are to be regulated and possibly measured quantitatively in litres per second including the flows through the tertiary offtakes. The way an irrigation system should be operated seldom features in design reports and standards. It is taken for granted that operation by a sufficient number of skilled staff will be satisfactory and all farmers will be satisfied with the water distributed. As a result, in many cases inappropriate water control structures are chosen, leading to hydraulically unstable canal systems that are cumbersome to operate. Furthermore, farmers' perceptions in terms of the understandability of structures are rarely taken into account in the design. Moreover, consideration is seldom given to staff requirements (numbers and skills) in relation to the water division technology chosen.
(ii) Operational office staff
In general, operational staff in district or provincial offices are mainly concerned with water allocation and distribution scheduling. The increasingly refined and accurate assessments of the irrigation requirements has led to increasingly sophisticated and complicated irrigation scheduling. During the few last decades voluminous operation manuals have been compiled in a number of countries (e.g., Indonesia and the Philippines) giving lengthy step-wise procedures to arrive at irrigation schedules. These procedures require an enormous amount of data collection, processing and dissemination. In combination with shortage of staff, little contact with or feedback from the field (especially from the tertiary level), and insufficient water measurement capability, because of malfunctioning of structures, and consequently unreliable data, this often results in a situation in which the administrative activities remain largely paper exercises.
(iii) Operational field staff and farmers
Finally, this third party1 determines how the water is distributed in reality. The actual distribution of water at field level is the product of a number of circumstantial causes.
1 Although field staff and farmers are often adversaries, they are here considered as one party, since the actual distribution of water is mostly the result of interaction between them.
First of all, irrigation scheduling which follows the soil-water balance closely requires varying irrigation intervals and/or varying irrigation applications. In order to accommodate such schedules, coordinated operation of the regulatory and measurement structures are necessary. Combined with hydraulically unstable canal systems with structures cumbersome to operate, the often poorly trained field staff is confronted with an impossible operational task. Furthermore, in many cases the real cropping patterns differ from the ones assumed in the irrigation scheduling, creating uneven distribution. Moreover, the field staff often live in and originate from the area they have to serve. Their loyalty (genuine or bought) lies primarily with the local farmers concerned and less with the office in town. Therefore, when they are confronted with shortages of water, they will distribute the water at their discretion, on the basis of their experience, ignoring official irrigation scheduling instructions. Finally, the field staff also might ignore these instructions when after many years of experience they have learned how to better accommodate the various groups of farmers by taking into account local conditions such as soil differences and topography than by strictly following the official irrigation schedules. Similarly, water might be distributed differently from irrigation scheduling on the basis of negotiations, power relations or traditional rights (see, among others, Van der Zaag, 1992).
Under such circumstances, distribution and measurement of water expressed in litres per second, become irrelevant. Real measures of water flows will be 'too much', 'too little' or 'sufficient'. From studies, research and field observations, it becomes clear that this discrepancy is apparent in a large number of irrigation projects (e.g., IIMI, 1987 and 1989, World Bank 1990, Horst 1996, Van der Zaag 1992, etc.). Clearly, the assumptions on design and irrigation scheduling to deliver water through irrigation systems in predetermined quantified flows in litres per second may have no bearing on operational reality, where water flows are qualified from a different perception. How to address possible solutions of this problem will be dealt with in the next section.
4. POSSIBLE SOLUTIONS
The problem of the incompatibility between design and irrigation scheduling on the one hand and operational reality on the other might be addressed in different ways.
Improvements to present approach
First, one could stick to the premise that irrigation implies the optimization of production. By keeping the soil-water-plant relationship and water use efficiencies as focus points of design and operation, improvements might be looked for in new irrigation scheduling techniques, such as water stress indicators, water yield functions and simulation models. During the last decade or so, the so-called 'crop-based' irrigation has received increasing attention. Its operational success has still to be reported, which is not surprising in view of the enormous amount of data collecting, processing and monitoring involved. Other measures such as training, organizing of water users groups, (re-)calibration of measuring structures, setting up of monitoring programmes, etc., might also be considered.
It is surmised, however, that all these measures will remain cosmetic surgery as long as the fundamental problems are not addressed: the too complicated irrigation schedules requiring too complicated water division structures resulting in cumbersome operation; the shortage of skilled staff; and the perceptions of field operators and farmers on water distribution being different from the official irrigation schedules.
Automation in terms of automatically controlled systems, either hydraulically (by float-operated gates) or electronically, electro-mechanically, by microprocessors or computers, will generally result in fewer persons required to operate the system. Operational and maintenance staff, however, should be very highly skilled. Knowledge of computers, electronics and mechanics is often essential.
Apart from this staffing requirement, this technology can in general only be adopted for projects with reservoir storage providing sufficient water throughout the year. In the event of water shortages this system is vulnerable and easy to tamper with. Also for this reason, this technology is less applicable for projects that are run-of-the-river supplied.1
1 Plusquellec et al., 1994, p. 61: 'in such cases [irrigation schemes that are supplied through river diversions without internal storage] there is indeed little need for precise flow and water level control in the main system...'. 'Modern water control concepts are most valuable in schemes that include upstream reservoirs or substantial buffer storage'.
A third solution comprises a search for simplification in irrigation scheduling and concurrently in irrigation water division technology. Under the device 'You need water to save water', Meijer (1992, p. 95) states:...'apart from crop requirements, water is needed to facilitate a fair and simple water distribution. If these so-called additional operation requirements (AOR), management losses or intentional losses are ignored or not accepted, water distribution schedules tend to be much too complicated and far too rigid for everyday practice. They will preclude any reasonable water use efficiency beforehand'.
FIGURE 2 - AOR for single rice crop (Meijer, 1992)
The AOR can be expressed by the ratio in percentage of the water volume delivered in excess during the period considered to the total water volume supplied during that same period. The principle of AOR can be applied to all distribution levels in the irrigation system above the tertiary outlet. Below the tertiary outlet, the flow is to be divided according to the given cropping patterns and requirements. In order to assess this in terms of AOR, possible simplification of irrigation schedules and concurrent water division technologies, a number of commonly occurring cropping patterns are reviewed in the next section.
5. SIMPLIFICATION BY APPLYING AOR
Rice crop - uniform growing stage
In his paper Meijer discusses a number of examples for a rice area. One example is given in Figure 2. For a constant discharge over the whole growing period, the value for AOR amounts to only 4.5%. In this case fixed proportional distribution of water might render an adequate technological solution. Meijer's AOR concept might also be applied to other cases.
Rice crop - different growing stages
In areas where rice cultivation is staggered in time over the area, the average crop water requirement for that area will follow an approximately constant value, as sketched in Figure 3. Here also, proportional division of water can be practised on the basis of an acceptable figure for AOR.
Non-rice crop - monoculture
On average the water requirements of a non-rice crop area will follow a curve as sketched in Figure 4. In such a case, adoption of one fixed flow (A) through the crop season would generally result in an unacceptably high figure for AOR (30% or more). As a solution, variable flows will result in acceptable AOR values (dotted line B).
FIGURE 3 - Demand and supply for rice - staggered
If the whole area is cultivated by a monocrop with a uniform growing stage, proportional division structures might be adopted. Only at the head end of the system is step-wise regulation of the incoming flow following line B necessary. If low flows become too small to handle at the farm gate, rotation might be considered by means of on/off gates.
Various non-rice crops - different growing stages
FIGURE 4 -Demand and supply for a non-rice crop
In many parts of the world irrigated areas are characterized by large numbers of small plots, with different types of crops in different stages of growth. Here a demand-based or crop-based operation becomes unrealistic. It is physically impossible to follow each water requirement curve for each plot through time. If one considers such an area, however, it is possible to assess the overall composite water requirement curve on the basis of summing individual curves. A constant flow for the whole period (often the dry season) will result in an acceptable figure for AOR (Figure 5). Here also, the constant flow can be distributed by proportional division structures.
From these examples it appears that simplification of irrigation schedules and simplification of concurrent water division structures is possible when additional operation requirements (AOR) are taken into account. Although these AOR might be considered a loss of water, it is surmised that, because of simpler operation and more transparency of water distribution, the overall water efficiencies will increase and unequal distribution will decrease. These simplifications in irrigation scheduling and operation will have a positive effect on the shortages of staff. Contrary to the case of automation, the required skill of the staff may remain low.
6. CONCLUDING REMARKS
Structured irrigation systems
The above proposed solution of simplification of scheduling and technology concurs in broad terms with the principle of structured irrigation systems as developed by the World Bank for the National Water Management Project in India. (The structured level is the level below which the system is proportional.)
Applicability of proportional division
FIGURE 5 - Composite requirements and supply for various non-rice crops
Proportional division can, in principle, be attained by either orifices (pipes, undershot gates) or weirs (broad or sharp crested). The advantages of fixed proportional division are: no need for gate movements, communications or decisions by operators. The disadvantages of orifices, as for example in Punjab, are the required exact design and installation and the changes in flow due to changes in hydraulic conditions (siltation) (Plusquellec et al., 1994, p. 93). The main advantage of the weir type flow divider is its transparency: the division is clear and any tampering is clearly visible; in other structures such as gated orifices or regulating weirs, tampering can be concealed (e.g., Romijn bottom gate). Proportional division throughout the year might be problematic if there are very low flows during the dry season. In such a case, rotation is called for and the openings should be fitted with on/off gates.
An often heard objection to proportional division is its lack of flexibility (in its broadest sense). Numerous case studies, however, have shown that flexible demand systems generally operate at low performance levels, with headstream farmers receiving the major share of the water (Shanan, 1992).
For most of the run-of-the-river projects, it appears impossible to match water demand with water supply. Even in the improbable case of the assumed cropping patterns being adhered to, it remains difficult to translate the water requirements for each individual plot with its own specific crop and specific planting dates into a realistic demand curve. Nevertheless, assuming that this might be achieved, the demand curve will never satisfactorily match the erratic flows in the river. Under these circumstances, the only way to utilize the available water as optimally as possible is to divide the water proportionally to the irrigable areas. The assumption is that the group of farmers in a tertiary unit will use this erratic flow best, because they have field-level knowledge.
In the last few decades, turnover has become an important topic in irrigation. Although much attention has been focused on the handing-over procedures, organizational processes, etc., little thought has been given to the technology to be handed over: the water division structures and their operational requirements. As discussed in this paper, these structures are generally cumbersome to operate and the irrigation staff are in practice unable to handle them in accordance with design assumptions and operation manuals. Measurements are seldom taken below the primary canal level and water is not divided in accordance with irrigation schedules in litres per second, but in accordance with a completely different set of rules. Under these circumstances, turnover of management implies turnover of an inadequate technology.
It is surmised that the simplification of scheduling and operation as discussed in this paper will lead to the handing-over of an irrigation technology which is more compatible with the capabilities and wishes of the farmers.
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