L.S. Pereira, Chairman ICID Working Group on Sustainable Crops and Water Use Professor, Department of Agricultural Engineering, Instituto Superior de Agronomia Universidade Técnica de Lisboa, Lisbon, Portugal
This paper reviews the main aspects relating to mutual influences of irrigation scheduling methods and on-farm irrigation systems. Surface, sprinkler and trickle irrigation systems are briefly described with identification of main constraints on the practical application of irrigation scheduling. For these three methods, factors influencing irrigation performances are identified. This paper shows that irrigation efficiencies definitely depend on both the quality of the irrigation method, which provides for uniformity of water application, and the irrigation scheduling. Finally, economic and environmental impacts of irrigation scheduling are analysed.
Irrigation scheduling concerns the farmers' decision process concerning 'when' to irrigate and 'how much' water to apply in order to maximize profit. This requires knowledge on crop water requirements and yield responses to water, the constraints specific to each irrigation method and irrigation equipment, the limitations relative to the water supply system and the financial and economic implications of the irrigation practice. Thus, the consideration of all these aspects makes irrigation scheduling a very complex decision making process, one which only very few farmers can understand and therefore adopt.
Research has made available a large number of tools (e.g., Hoffman et al., 1990) including procedures to compute crop water requirements, to simulate soil water balance, to estimate the impact of water deficits on yields and to estimate the economic returns of irrigation. Despite this vast number of tools of varied nature, irrigation scheduling is not yet utilized by the majority of farmers. Furthermore, only limited irrigation scheduling information is utilized worldwide by irrigation system managers, extensionists or farmer advisers. It is recognized, however, that the adoption of appropriate irrigation scheduling practices could lead to increased yields and greater profit for farmers, significant water savings, reduced environmental impact of irrigation and improved sustainability of irrigated agriculture.
Consequently, there is a need to better identify the factors that could enhance the adoption of appropriate irrigation scheduling practices, favour the transfer of technology from research to practice, and give new orientation to researchers. These aspects are particularly important in that they concern the inter-relationship between on-farm irrigation systems and irrigation scheduling, and involve two related disciplines: irrigation engineering and agronomy.
Few papers have been submitted to this workshop under the scope of Theme 2. Therefore, this report is largely based on the author's experience and work, and makes use of other publications relating to the sub-themes intended to be covered as indicated in the call for papers. Furthermore, this paper raises several issues for discussion on subjects not covered or insufficiently dealt with, while at the same time providing an opportunity for referring to relevant papers or reports from other themes.
SCHEDULING IRRIGATION UNDER DIFFERENT IRRIGATION METHODS
Surface irrigation systems
Surface irrigation is the most common irrigation method worldwide. There are several types of surface systems, namely:
· Basin irrigation: Water is applied to levelled surface units (basins) which have complete perimeter dikes to prevent runoff and to allow infiltration after cut-off. The best performance is obtained when advance time is minimized by using large non-erosive discharges and the basin surface is precision levelled. This method is the most commonly practised worldwide both for rice and other field crops including orchard tree crops. In general, basins are small and water application is manually controlled. Basin irrigation using large laser-levelled units with automated or semi-automated control is practised in a few areas in developed countries.
· Furrow irrigation: Water is applied to furrows using small discharges to favour water infiltration while advancing down the field. Fields must have a mild slope, and inflow discharges must be such that the advance is not too fast, producing excessive runoff losses, nor too slow, which induces excessive infiltration in the upper part of the field. Short blocked furrows with manually controlled water applications are practised by traditional irrigators. Long and precisely levelled furrows with automated or semi-automated control have become increasingly popular in developed countries.
· Border irrigation: Water is applied to short or long strips of land diked on both sides and open at the downstream end. Irrigation takes place by allowing the flow to advance and infiltrate along the border. Larger inflow rates are utilized when the field slope is very small with management becoming similar to that of basins. Similarly, basin irrigation automated water control is often applied.
There are other surface methods like contour ditch, wild flooding and water spreading, but these are of less significance. In general, these types of surface irrigation systems are manually operated and often the modernization of irrigation practices leads to their conversion to other types of systems.
Two types of systems can be considered under the perspective of irrigation scheduling practices:
· Traditional systems: The water control is carried out manually according to the ability of the irrigator. In small basins or borders and in short furrows, a common practice is where the irrigator cuts off the supply when the advance is completed. Because the fields are often uneven and the discharges are variable and largely unknown, this practice induces large variations in the volumes of water applied in each irrigation. It is therefore extremely difficult to control 'how much' water is applied.
· Modernized systems: In these systems, some form of control of discharge such as syphons, gated pipes, lay-flat tubes or gates, and/or some form of automation including surge flow valves, cablegation or automated canal gates is used. In this case the fields are often levelled, while the advance and supply times and the inflow rate can be measured or estimated. Thus, in these systems, it is possible to control 'how much' water should be applied at a given irrigation.
Relationship between irrigation performance and irrigation scheduling
The two main parameters used to characterize surface irrigation performance are the distribution uniformity and the application efficiency.
The distribution uniformity DU (%) is defined as the ratio of the average infiltrated depth in the low quarter of the field (Zlq, mm) to the average infiltrated depth. (Zav, mm) as follows:
The distribution uniformity results from several design and management variables that characterize an irrigation event. Functionally, it can be described as:
DU = fl (qin, L, n, So, Ic, Fa, tco), (2)
qin inflow rate to the furrow (or per unit width of the border or basin)
L length of the furrow (or border, or basin)
n roughness coefficient
S0 longitudinal slope of the field
Ic intake characteristics of the soil
Fa furrow (or border, or basin) form
tco time of cut-off
The application efficiency ea (%) is defined as the ratio of the average depth added to the root zone storage (Zr, mm) and the average depth applied to the field (D, mm), as follows:
The application efficiency also depends on several design and management variables. This can be expressed functionally as follows:
ea = f2 (qÎ , L, n, S0Ic, Fa, tco, SMD), (4)
where SMD is the soil moisture deficit at the time of irrigation. All other variables are as defined above. It can be seen from Equations 2 and 4 that the application efficiency depends on both the distribution uniformity and the irrigation scheduling. The latter controls the value of SMD.
These relationships have recently been discussed by Pereira (1996) and Pereira et al. (1996) and are the object of numerous other studies. A very modern approach to improve both surface irrigation performance and scheduling using real time control is given in the paper by Malano et al. (1996). In it, the importance of the relation between irrigation scheduling and field application performance is described as an integral part of a surface irrigation scheduling system.
Field evaluations of irrigation systems play a fundamental role in improving surface irrigation management. They provide the information required for design, model validation and for advising irrigators on how to improve their management practices. The field observations required for conducting a system evaluation include: inflow and outflow volumes; irrigation times/phases, particularly advance and recession; soil moisture before and after irrigation; the slope/topography of the field being irrigated; and the management rules currently utilized by the irrigator.
Field data from evaluation can also be utilized to derive the infiltration parameters for a Kostiakov type infiltration model. The estimation of infiltration parameters from the advance phase using the procedure termed 'two-point' has been presented by Walker and Skogerboe (1987). The estimation of the infiltration parameters and the roughness coefficient 'n' for surface flow has also been achieved through the solution of the inverse surface irrigation problem. Using field data on advance, recession, inflow rates, field length and slope, the infiltration and roughness parameters are calculated iteratively by coupling a hydraulic model and optimization procedures (Malano et al., 1996). Once infiltration and roughness parameters become available, they can be utilized to design improved systems and practices.
Field implementation of irrigation scheduling
Most surface irrigated areas are supplied from collective irrigation canal systems. Farm irrigation scheduling depends upon the delivery schedule, e.g., rate, duration and frequency of irrigation are dictated by the system's operational policies (Goussard, 1996). Discharge and duration impose constraints on the volume of application and frequency determines the irrigation timing. In general, surface irrigation delivery systems are rigid and the time interval between successive deliveries is often too long. Irrigators tend to compensate for this by applying all the water they are entitled to use. For deep rooted crops and soils without excessive permeability this strategy may be appropriate. However, for shallow rooted crops and/or soils with small water holding capacity, crops may become highly stressed and percolation losses can be substantial.
The realization of the benefits of modernization of surface irrigation are constrained by rigid delivery schedules. Irrigation scheduling can only be applied if farmers are in control of supply, or when the delivery schedule is sufficiently flexible to adequately select 'when' to irrigate. This applies to both traditional and modern irrigation. In the case of modernized surface systems, the adoption of improved irrigation scheduling methods should go, as it often does, together with the improved operation of the delivery system to allow more flexibility in selecting the appropriate inflow rates and supply time (Mailhol, 1992).
Substantial improvement can be achieved if delivery is made by an arranged delivery schedule. This normally requires a certain notice time between the demand for water by the farmer and the time of supply. Often, this can be achieved more easily when users are in control of the distribution units (Tollefson, 1996; van Hofwegen, 1996). Farm surface irrigation systems that rely on large irrigation depths and large irrigation intervals have enough flexibility to accommodate a demand lag time of two or three days without inducing severe water stress for most crops. This could favour the adoption of arranged delivery schedules. An illustration of the application of irrigation scheduling to a supply system with arranged demand is illustrated in the paper presented by Malano et al. (1996)
In general, it is difficult to apply small irrigation depths by surface irrigation. Smaller depths can be applied in basin irrigation provided that the land is precision levelled and the discharge rate is sufficiently large to obtain a fast advance. In furrow irrigation the time of cut-off should be larger than the advance time to provide for sufficient intake opportunity time; while in border irrigation, inflow is normally cut off before the advance front has reached the end of the field. This implies that surface irrigation applications are usually large which makes this method more suitable for situations where the SMD and the management allowable deficit (MAD) are relatively large. Under these circumstances scheduling can be simplified as proposed by Hill and Allen (1996). The adoption of simple irrigation scheduling calendars is advantageous for collective system management (Horst, 1996) and for introducing improved tools for controlling the discharge rates at farm level. It is obvious that, to achieve this, it is necessary that volume delivered be known (Burt, 1996), e.g., irrigators should have control on the discharge rate and duration.
Another irrigation scheduling option is to use irrigation scheduling simulation models to perform the soil water balance using soil, crop and meteorological data and to provide information on the soil moisture status. Numerous examples of such models are available in the literature, namely through the ICID workshops (Pereira et al., 1992, 1995). However, the use of models present serious difficulties for application in real time since they require not only validation but also a support advisory service and training of irrigation extensionists, managers and/or farmers.
The use of soil water monitoring devices (Itier et al., 1996) is generally more appropriate for large fields and crops that may economically justify the costs of purchase and operation. Normally, the use of these devices requires the selection of an irrigation threshold corresponding to the MAD. Observations of the SMD have to be regularly performed in time and space to enable the prediction of the irrigation date when SMD = MAD (Pereira et al., 1996).
A further degree of sophistication can be achieved by combining soil water balance simulation models with soil moisture observations. This can be done in two ways: (a) soil moisture information is used to validate the model predictions, and (b) continuous soil moisture and meteorological monitoring coupled with a model for forward prediction of the soil water status. An example of the latter approach is presented by Malano et al. (1996).
Sprinkler irrigation systems are normally used under more favourable operational conditions than surface systems as farmers can better control the discharge rates, duration and frequency. Many sprinkler systems have independent water supply or are connected to networks which may be operated on demand. However, the pressure head from the hydrants is often not appropriate resulting in lower (or higher) discharges than those envisaged during the design phase. Pressure head (and discharge) variations at the hydrant should be identified by the user if appropriate equipment is available.
Different types of sprinkler systems can be considered in relation to irrigation scheduling:
· Set systems: Sprinklers irrigate in a fixed position. There are no limitations on the duration of the set time. Thus, they can be utilized to apply small depths and frequent waterings (not suitable for the movable systems because of operational constraints), or large depths and large irrigation intervals (less appropriate for systems with very low application rates).
· Travelling guns, hose-reel or hose-pull: These systems in general apply irrigation depths ranging from 15 to 40 mm. These, systems are not suitable for applying very small or large depths, mainly because of limitations in system capacity;
· Continuous move laterals: These systems are well adapted to apply small and frequent irrigations.
The hydraulic design of sprinkler irrigation is generally well understood. The overall sprinkler design process is, however, more complicated since it requires the selection and the best combination of the following: pressure head (H), nozzle diameter (Dn), sprinkler discharge (qs) and wetted diameter (Dw); spacings between sprinklers and/or rows which depend on the application rate (is), wind conditions and distance between rows of tree crops; and sprinkler characteristics including the size of drops and the distribution pattern, in relation to soil and climate conditions.
The most common design criteria for sprinkler laterals is that sprinkler discharge (qs) should not vary by more than 10% between the points of highest and lowest pressure in the system. However, this condition is often not achieved in practice. Augier et al. (1996) report that in France 56% of solid set systems evaluated showed a pressure variation higher than 20%.
Many sprinkler systems have not been properly designed, are not operated according to design rules, or their operation is hampered by poor maintenance. This results in inadequate pressures and discharges along the system. Unfortunately, these problems are much more frequent than expected even in developed countries: in France, as reported by Augier et al. (1996), the actual application depths deviate by more than 20% from the design ones in 46% of travelling guns and 34% of solid set systems. This would indicate that the main limitation to appropriate scheduling is that most farmers do not know the discharges delivered by their systems and are therefore only in control of the duration.
In the case of direct supply by a farm pumping system, the pressure head can also be lower (or higher) than expected due both to design shortcomings or lack of maintenance. When appropriate measurement devices are installed, these problems can be identified and either the set time or the travelling speed can be corrected according to the actual discharge.
Relationship between irrigation performance and irrigation scheduling
Christiansen's coefficient of uniformity CU (%) is the most commonly used performance indicator to index application uniformity. It is defined as:
X absolute deviations of application depths observations from the mean (millimetres)
m mean of observed depths (millimetres)
n number of observations.
The coefficient of uniformity1 is determined by several design variables and can be functionally expressed by the following relation:
CU relates with DU (Eq. 1) through the relation (Keller and Bliesner, 1990): DU = 100 -1.59 (100 - CU).
CU = f1_ (H, D H, S, Dn, DP, W), (6)
H pressure head available at the sprinkler
D H variation of the pressure in the operating set, or along the moving lateral
S spacing of the sprinklers along the lateral (and between laterals) or spacings between travellers
Dn nozzle diameter, which influences the discharge qs and wetted diameter Dw for a given pressure head
DP the distribution pattern of the sprinkler
W wind speed.
All the above variables are set during at the design stage. However, spacings are often oversized. Augier et al. (1996) report that excessive spacing occurs in 65% of travelling guns and 70% of solid set systems evaluated in France.
The application efficiency ea (Equation 2) depends not only on the design but on management variables and can be functionally described by:
ea = f2_ (H, D H, S, Dn, DP, W, Ic, is, ti, SMD), (7)
Ic intake characteristics of the soil
is application rate of the sprinkler
ti duration of the irrigation event
SMD soil moisture deficit before the irrigation event.
Most of the parameters in Equation 7 are controlled by the designer, while the duration of the application and SMD are management variables controlled by the irrigator. Hence, as with surface irrigation, the application efficiency depends upon the application uniformity and the irrigation scheduling. Field evaluations of systems under operation are very useful for improving management, including irrigation scheduling.
Field implementation of irrigation scheduling
Overall, in sprinkler irrigation, the management of the system is easier than in surface irrigation and performance is largely dependent on the system design. Therefore quality control of services, equipment and design are very important in supporting farmers' operations.
For set systems and travelling guns, the MAD is commonly smaller than for surface irrigation. The methods for irrigation scheduling that can be used are the same as for surface irrigation: (a) soil water balance simulation models; (b) monitoring the soil water status; and (c) a combination of both. The observation of the SMD requires more accuracy and to be more frequent when irrigation depths are small. In areas with a large number of small farmers the use of simple irrigation scheduling calendars may be more appropriate.
Irrigation scheduling models for sprinkler irrigation to serve several farms may provide not only information on when and how much to irrigate but also the operation plan for the individual irrigation machines. The example reported by Przybyla (1996) shows positive results for large collective farms in Poland. However, this approach requires major modifications if it is to be applied to a large number of small farms. The successful use of a simple simulation model through a videotel system for advising farmers within a large irrigation project has been recently reported by Giannerini (1995).
When frequent irrigations are applied the error in estimating the SMD may be larger than the depths to be applied. This is the case for daily irrigations with centre pivot laterals. Thus, other scheduling strategies based on the replacement of the volume of water consumed during the precedent irrigation interval must be applied. For centre pivots, special irrigation scheduling models have been developed which associate modules for water application, fertigation, chemigation and energy management. An example of such models is SCHED (Buchleiter, 1995).
Trickle irrigation systems are set systems that use a variety of low discharge emitters to apply water to the field. Systems for tree crops often have one or more emitters per tree depending on the wetted area, hydraulic properties of the soil, emitters discharge (qe) and emitters wetting pattern.
There are several types of emitters, including drippers, sprayers and micro sprinklers, microjets, and bubblers.
For row crops, a lateral supplies water for every one or two crop rows when drippers are utilized. Underground perforated pipes have also become popular. When the soil is highly permeable sprayers and micro sprinklers are preferable.
Emitters discharge qe (litres per hour) relates to the pressure head H (metres) through the following relation:
qe = K Hx, (8)
x emitter discharge exponent
K discharge coefficient of the emitter.
The parameter x varies from near x = 0, for self compensating emitters, up to x = 1, for laminar flow emitters. Most commonly, x values vary from 0.4 to 0.55. Self-compensating emitters are used to minimize discharge variations along very long laterals or in sloping laterals.
The design of trickle systems involves the selection of the appropriate combination of: pressure H, discharge qe and emitter discharge exponent x; percent area wetted; emitter type and spacings; emitter susceptibility to clogging, cleaning and ability to undergo variations in temperature; type of filters, their location and cleaning capability; requirements for application of fertilizers and chemicals, control of pressure and/or flow, type of devices and their location in the system; and automation facilities.
To achieve this, the designer must know not only the irrigation requirement of the crop but also the duration and frequency of irrigations (including fertigation/chemigation requirements). Hence, design and irrigation scheduling processes are intertwined. Performance is highly dependent on the quality of design and equipment selected.
Relationship between irrigation performance and irrigation scheduling
The most useful system performance indicator for trickle systems is the emission uniformity EU (%) which, in the case of field evaluation, is defined as:
EU = 100 qlq/qa, (9)
qlq average discharge rate of low quarter of emitter discharge observations (litres/hour)
qa average discharge rate of all observations (litres/hour).
For design purposes, EU is defined as:
Cv coefficient of manufacturing variation for the emitter
N number of emitters per tree (N = 1 for row crops)
qn minimum emission rate corresponding to minimum pressure in the system (litres/hour)
qa average (or design) emission rate (litres per hour).
The emission uniformity is determined by a combination of design parameters which can be functionally expressed as follows:
EU = f1'' (H, D H, x, Ec, Cv, FI), (11)
H pressure head at emitters
D H variation in pressure along the system
x emitter discharge exponent
Ec emitter characteristics related to variations in discharge
Cv oefficient of manufacturing variation for the emitter
FI filtering capabilities of the system
The above-mentioned parameters mostly depend on design decisions. Thus, as for sprinkler systems, the irrigator has very little control on the uniformity performance of the system except for the maintenance of the system.
As with surface and sprinkler irrigation systems, the application efficiency ea is also determined by the combination of design and management variables as indicated in the following functional relation:
ea = f2'' (H, D H, x, Ec, Cv, FI, Ks, SW, ti,,D ti), (12)
Ks hydraulic conductivity of the soil
SW soil water conditions before irrigation
ti duration of the irrigation
D ti time intervals between irrigations
The irrigator's control is basically reduced to the timing of irrigation and the volumes and frequency of applications. Thus to achieve good performance in trickle irrigation good support to farmers in terms of quality control of equipment, services and design is required (Augier et al., 1996). Field evaluations of systems under operation can play a fundamental role in improving performance.
Field implementation of irrigation scheduling
Farmers adopting trickle systems are in control of flow rate, duration and frequency of irrigations. However, as with sprinkler systems, they may not be aware of the discharge when pressure head variations occur in the supply system or when systems are not properly designed (these problems can be reduced when self-compensating emitters are utilized).
Most of the problems derive from inappropriate design and selection of equipment. It is often said that a localized irrigation system is selected or should be selected to save water when in fact such a system should be selected to maximize yields and profits. For these systems, management is tied up with design. The decisions made during the design phase may affect to a large degree the operation and management of the system (Keller and Bliesner, 1990).
Trickle irrigation systems are normally used for frequent or very frequent applications. Under intensive horticultural production more than one application per day may be necessary. Larger irrigation intervals of a few days are appropriate for deep rooted orchard trees.
Irrigation scheduling can be based on crop indicators or soil water status. Videotel or Irritel systems, as well as other forms of water balance models are also appropriate (Specty and Isbérie, 1996). For very frequent waterings the irrigation and fertilization strategies must be integrated.
ENVIRONMENTAL AND ECONOMIC ASPECTS
Environmental impact and control
The most common environmental impacts of on-farm irrigation are:
· soil salinization due to use of saline water or to the rise of saline water table;
· waterlogging due to excess water application;
· nitrate and pesticide contamination of the groundwater (and surface waters) due to excessive use of chemicals in intensive agricultural production and/or to overirrigation;
· soil erosion due to surface runoff from surface and sprinkler irrigation systems;
· soil degradation due to modifications of the soil profile from inappropriate land grading;
· deterioration of the soil structure from both surface flow and sprinkler raindrops;
· degradation of water bodies receiving saline irrigation return flows.
These impacts can be reduced when appropriate water application techniques coupled with irrigation scheduling are applied. In this regard, it is, however, necessary to stress the following aspects:
· Irrigation scheduling techniques cannot be properly implemented nor produce the desirable consequences unless proper water application practices are implemented. This may be one reason why innovations in irrigation scheduling techniques are not widely adopted in practice;
· Application efficiency depends not only on system design criteria but also on the volume and timeliness of water applications, e.g., scheduling decisions. Thus, the adoption of improved hardware is not by itself enough to attain high levels of performance.
High application uniformity minimizes the differences in water application over a field, e.g., areas receiving too much or too little water. Application uniformity affects both yields and the extent of environmental impact from irrigation.
Application efficiency, ea, has a lesser effect on crop yield but low ea due to excessive water application has the potential to create waterlogging and various environmental impacts listed above.
Only one paper presented in this Workshop deals with the environmental benefits of irrigation scheduling (Mao, 1996). This paper is concerned with 'water-saving rice irrigation' which substitutes the traditional permanent flooding of paddy rice cultivation by a technique that requires the soil moisture to be maintained near saturation (minimum values at 60 to 80% of soil saturation according to the growth stages). The benefits identified by Mao (1996) are as follows:
· reduced irrigation water demand by about two-thirds of the paddy rice requirement. Water saving is obtained mainly from reduced percolation and seepage losses;
· reduced losses of fertilizers resulting from a decrease in seepage, which is particularly relevant for nitrates;
· increase in the soil redox potential of the soil and in oxygen in the soil atmosphere;
· improvement of the soil fauna and flora resulting in more favourable conditions for the crop to assimilate the nutrients, particularly organic fertilizers;
· the relative humidity tends to decrease and air temperature to increase. These changes in microclimate are considered to be favourable for the reduction of diseases and insect pests in rice.
The analysis of environmental benefits derived from improved irrigation scheduling practices requires further research, mainly from the implementation of irrigation scheduling services or programmes at project or regional level.
There is a vast body of research showing that the timely application of water especially during the more sensitive growth stages produces higher yields and/or higher economic profits. However, there is little information on similar benefits from irrigation scheduling services or programmes when applied in practice.
The paper by de Jager and Kennedy (1996) reports three levels of irrigation scheduling technology applied to both surface irrigation and centre pivot systems. It shows clear benefits (20% yield increase) for surface irrigation, less obvious ones in the case of daily sprinkling with the centre pivots. Neutral impacts with centre pivot irrigation may be due to the fact that, for frequent irrigations, positive results are commonly associated with both water and fertigation by reducing fertilizer leaching, improving fertilizer use and reducing the environmental cost of fertilizer losses (Heermann and Duke, 1992). Thus, when an irrigation system is used to apply very frequent irrigations, consideration must be given to water and fertilizer application (and losses), e.g., to irrigation and fertigation management; in addition to irrigation scheduling.
Another paper from South Africa, by Van der Westhuizen et al. (1996) discusses the costs and benefits of irrigation scheduling and the reasons why farmers do not easily recognize the benefits of irrigation scheduling. However, no evidence of a favourable benefit-cost relation is given nor is an improved benefit-cost analysis methodology suggested.
In the paper by Przybyla (1996), on irrigation scheduling with sprinkling machines, positive impacts are shown including yield increases of about 60% on pastures, 15% on small grains and 20 to 35% on sugar beet. However, the overall details provided are rather scant.
A study by Cavazza et al. (1996) analyses how farmers adopted the irrigation scheduling recommendations for tree crops and horticultural field crops. It shows that farmers largely deviated from the advice for tree crops adopting much smaller irrigation volumes and number of irrigations. For horticultural crops such as tomato the actual practice was closer to that advised. Estimated reduction in yields resulting from the deviations between practice and advice are given. In particular, it became evident that farmers did not use drip irrigation systems as expected, e.g., they preferred to apply less frequent but larger waterings. However, the study does not include any economic evaluation of the benefits of scheduling according to the advice or a discussion about the reasons why farmer behaviour deviated from that advised. Thus, the four studies cited above show the need to develop appropriate studies to evaluate the economic benefits of irrigation scheduling in practice.
CONCLUSIONS AND RECOMMENDATIONS
Suitability, adaptability and constraints of irrigation scheduling methods in relation to the irrigation methods
Surface irrigation systems
· In surface irrigation systems supplied through collective irrigation networks, the first step for irrigation scheduling is the delivery on a volume basis, in such a way that farmers can control irrigation discharge rates and duration. A step further is the adoption of arranged delivery systems that allow farmers to control the frequency of irrigation.
· Modifications in the delivery system should go together with improvements of the on-farm system in such a way that farmers control both the discharge rates applied to basins, furrows or borders and the supply time. This requires simple equipment to control inflow rates and, thus, the applied depths.
· It could be advisable that simple irrigation scheduling calendars and/or simplified forms of exploring irrigation scheduling simulation models be developed and installed to support information for farmers and system managers.
· The tools referred to above should constitute simple technological packages that could help transfer management responsibilities to farmers at sector or distribution level (turnover).
· When modernization of on-farm surface irrigation systems is already developed, the improved use of surface irrigation equipments and automation devices could be achieved if management decisions concerning the system state variables (inflow rate, advance time, infiltration characteristics) can be coupled with irrigation scheduling decisions.
· For modernized on-farm surface irrigation systems, it could be suitable to progressively adopt simulation models and/or soil moisture monitoring to support irrigation scheduling decisions and, in the case of collective supply systems, to provide enough demand lag time for the arranged deliveries.
· In the case of more advanced surface irrigation systems when real time or feedback control is utilized, it is advisable to couple the corresponding surface irrigation models with real-time scheduling methods.
Sprinkler irrigation systems
· In sprinkler irrigation systems, unlike in surface irrigation, farmers are in control of discharge rates and duration of irrigation. However, in most case, farmers do not know the discharge rates being applied, namely when variations in pressure head induce variations in flow rates. The use of simple discharge measuring devices upstream of the system could be the first step for farmers to appropriately define the application depths.
· For set systems and travelling gun systems, several irrigation scheduling methods could be applied based on the estimation of SMD: simulation models using soil, crop and meteorological information, monitoring of soil moisture or soil water potential; or a combination of both, as well as videotel systems in the case of more advanced farming conditions. The use of simple irrigation scheduling calendars could be of interest for scheduling a large number of small farms.
· For lateral moving systems applying very frequent irrigations and small application depths, specific approaches combining irrigation, fertigation, chemigation and energy management are suitable.
Trickle irrigation systems
· In the case of very frequent irrigations, the scheduling is dictated by the replacement of the water consumed and includes fertigation and chemigation scheduling. Specific approaches and models could be applied.
· In the case of less frequent irrigations, several scheduling methods can be utilized such as simulation models including videotel systems, soil water potential monitoring, or crop water stress indicators.
· The effectiveness of scheduling requires that farmers know both the flow rate they are utilizing as well as the full characteristics of the system and equipment they are using.
Irrigation performances and irrigation scheduling
· In surface irrigation, the irrigation scheduling variables are intimately related to the performances. The distribution uniformity depends on the applied depth through the couple inflow rate and time for cut-off. Beside these variables, the application efficiency, also depends on the timeliness of irrigation. Consequently, research and development for improving on-farm surface irrigation or the irrigation scheduling in gravity systems have to take a multidisciplinary approach.
· For sprinkler irrigation, uniformities mostly depend on the quality of design and of the equipment selected. Application efficiencies are influenced both by the uniformity and by the depth and timeliness of irrigations. Therefore, increased attention has to be paid to the design of sprinkler systems and to relations between design and operation, including scheduling.
· The performance of trickle irrigation systems essentially depends on the quality of design and equipment and consequent rules for operation. Irrigation scheduling should be carefully oriented to maximize profit and combine water applications with fertigation/chemigation.
· Field evaluations of on-farm irrigation systems under operation should be extensively performed, aiming at improving both systems operation and irrigation scheduling.
· The control of quality of systems design and equipment is relevant for improving the performances of on-farm irrigation systems.
· Multidisciplinary research, combining agronomy and irrigation engineering disciplines should look for new improvements in coupling irrigation methods and irrigation scheduling.
Environmental and economic aspects
· It is recognized that improved irrigation scheduling, and irrigation management in general, contribute to controlling the environmental impacts of irrigation. However, research and appropriate methodologies are required to expand the analysis and assessment of the environmental benefits of irrigation scheduling.
· Similarly, research and appropriate methodologies for evaluating the economic impacts of irrigation scheduling are also required.
· Policy-makers and decision-makers need evidence of benefits of irrigation scheduling. Thus, the expansion of related environmental and economic impact assessment could help the adoption and funding of irrigation scheduling programmes and services.
Augier, P., Baudequin D. and Isbérie, C. 1996. The need to improve the on-farm performance of irrigation systems to apply upgraded irrigation scheduling. In: Irrigation Scheduling: From Theory to Practice, Proceedings ICID/FAO Workshop, Sept. 1995, Rome. Water Report No. 8, FAO, Rome.
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