The need to improve the on-farm performance of irrigation systems to apply upgraded irrigation scheduling

P. Augier, D. Baudequin and C. Isbérie, Cemagref (Division ouvrages hydrauliques et équipements pour l'irrigation), Le Tholonet, France

SUMMARY

Several international studies, as well as recent surveys in France, have shown that the performance of irrigation practices and equipment, especially in the uniformity of water application, is still too low. This is due to farmers lacking the management skills to manage their irrigation systems properly. Consequences include reductions in crop yields and a waste of water resources.

To improve irrigation performance, it is necessary not only to promote the implementation of irrigation scheduling methods, but concurrently to improve system design and performance and to enhance farmers' skills to control and manage their irrigation system more efficiently during its operation.

For the three major irrigation techniques (surface irrigation, sprinkler irrigation, and microirrigation), the difficulties which have a significant negative effect on irrigation performance are listed. Solutions include, on the one hand, selecting the most appropriate technique, matching the local context, and using certified equipment which has a performance meeting relevant standards and which can ensure a minimum quality for on-field irrigation and, on the other hand, implementing upgraded management methods for such equipment and techniques, and developing skills to avoid those errors currently occurring during the ordinary operation of the system.

The search for an improved utilization of agricultural irrigation water, which is critical in periods of scarcity, has led to a method to quantify its apportionment, in date and in volume, according to the water requirements of plants. These requirements are estimated from various scheduling techniques.

Irrigation scheduling defines 'when' it is necessary to irrigate a crop and 'how much' water application depth shall be delivered. Once the appropriate irrigation water application depth has been chosen, using scheduling methods, a farmer then has to convey the water to every plant at the required date and with the required precision and uniformity.

However, irrigation scheduling does not take into account the actual performance capability of the on-farm irrigation system as operated by farmers, which is often poorly known. This can induce severe variances between the amounts of irrigation water determined by the irrigation scheduling technique, and the higher amounts of water which are effectively required when taking into account the performance of on-farm irrigation equipment. Imprecise and non-uniform water application patterns most often result in yield heterogeneity at field level and in water wastage.

The management of on-field water application systems constitutes a complex problem which farmers are faced with daily. It is often an important bottleneck for efficient implementation and large-scale development of advanced irrigation scheduling practices.

To obtain sustainable water savings, it will be mandatory to require a minimum degree of technical quality for systems and to transfer to users a true mastery in managing water in on-farm irrigation systems, so enabling the water losses resulting from heterogeneities in the distribution of water to be controlled.

CONSTRAINTS TO IRRIGATION SCHEDULING

Constraints due to water supply to the field

The implementation of irrigation scheduling techniques, based on the field soil water balance, requires that farmers take an appropriate amount of water from the supply system at the proper time. This practice matches poorly with the most traditional water distribution technique, 'the rotational irrigation', which pre-sets the flow rate, the dates and duration of water availability to the field. This technique frequently results in excessive water depths being applied when the water is available. On the other hand, water stress periods occur during the gaps between successive water applications when these gaps are too large.

In the case of pressurized collective network systems operating on free demand, this constraint appears to be less important, because farmers can usually count on the water being available from their hydrants at any moment. However, the operational and organizational constraints at farm level, linked to the availability of equipment or manpower, frequently encourage farmers to use an irrigation scheduling calendar (low flexibility).

The technical quality of water delivery from a free surface canal network supply system can vary greatly between the farmers located upstream and those located at the far ends of the system.

Constraints due to on-farm irrigation techniques and equipment

The performance of on-farm irrigation depends on the management skills of farmers, on the irrigation technique and on the initial quality of the equipment and of the system design. But the actual performance can significantly deteriorate under field conditions, for usually no provision is made for adapting systems to the local operating conditions or to the farmers' technical skill for managing their systems.

In these conditions, we can often find a poor knowledge of the actual irrigation water depths applied and a significant heterogeneity in the distribution patterns, whatever the irrigation technique used may be. This can have two negative consequences:

· On the one hand, farmers may not perform uniformly and precisely the irrigation water application that they have decided on, possibly using scheduling tools. They will develop a general tendency to apply an excess of water over the whole field in order to match the requirements of the zones which receive the poorest amounts.

· On the other hand, the real existence of non-uniform water application, the variability of which is often not fully appreciated by farmers, is a strong limitation to field installation of scheduling sensors, since methodologies for the selection of their location in the field for appropriate representativeness, have not been fully developed. As a consequence, more difficulties are found by farmers while using water stress criteria and scheduling techniques.

ON-FARM IRRIGATION TECHNIQUES AS CONSTRAINTS TO THE IMPLEMENTATION OF IRRIGATION SCHEDULING METHODS: SURFACE IRRIGATION

This irrigation technique is both the most ancient and the most widespread in the world (about 90% of the irrigated area). But its average performance is still low. The irrigation efficiency must be considered in terms of uniformity of water application and losses and also in terms of the ease of scheduling an timing irrigations. Table 1 gives water application efficiency values (Keller, 1992), which are considered to be seasonal values attainable with good design and management levels.

TABLE 1 - Efficiency of different surface irrigation techniques

Surface irrigation technique	Efficiency (%)
Basin	70-90
Border	70-85
Furrow	65-85

The improvement of surface irrigation performance at field level is currently facing several problems (Berthomé 1984, 1991; Tron et al., 1988; Pereira, 1996):

· The variability, in time and in space, of infiltration characteristics is often very important. This induces water runoff and deep percolation in some parts of the field while others are underirrigated. It has been shown (Childs et al., 1993) that this variability can play a more important role in the variability of infiltrated water than the factors governing the intake opportunity time.

· The control of field levelling is difficult. The preparation of the soil at the beginning of the irrigation season is particularly important because it conditions the homogeneity of the water distribution over the irrigated field, as well as the soil characteristics.

· The control of field intake discharges and runoff, which is a common problem for farmers, is essential to effectively control the depths of the water to be applied.

Possible improvements to surface irrigation techniques

Numerous proposals for improving traditional practices in surface irrigation have been suggested (Tron et al., 1988; Pereira, 1996):

· Land levelling: laser planing devices enable performance improvements in infiltration systems for level basins and level furrows. It has been observed (Sousa et al., 1994) that the impact of levelling accuracy on distribution uniformity and on yield is significant (Table 2). The need for appropriate maintenance and precision of land levelling is evident as it facilitates irrigation scheduling, induces higher yield and reduces water wastage.

· Mechanization of surface irrigation systems and automation of the distribution of water by placing distributors at the upstream end of the field: gated pipes (Renault, 1993), soft-hose distributors, low pressure supply systems, automated valves (Duke et al., 1990; Renault, 1993), cablegation (Kemper et al., 1987), surge flow (Humpherys, 1989a, b).

· Flow management: irrigation with constant flow, dual flow, decreasing flow, surge flow, blocked furrows, etc.

· Control of irrigation performance, namely using computer methods for optimizing irrigation flow rates and duration including real time data logging. This enables, for example, while monitoring the displacement of the front of water during the advancement phase, the comprehensive characteristics of the current irrigation to be derived and, if needed, the settings during the accumulation phase to be adjusted (Berthomé, 1991).

· On-field estimation and measurement techniques for the irrigation parameters (water flows and infiltration rates). The knowledge of these parameters is indeed essential to optimize surface irrigation performance and to efficiently manage the duration and amount of irrigation flow, according to the irrigation objectives that the farmer has set.

TABLE 2 - Yield increase for grain corn for three scenarios of laser controlled levelling (Source: Pereira 1996)

Variable	Scenario 1		Scenario 2		Scenario 3
	Before finishing	After finishing	Before finishing	After finishing	Before finishing	After finishing
Standard deviation (cm)	2,5	1.2	3.0	1.7	4.0	1.7
Relative distribution uniformity DU/DU max	0.63	0,85	0.56	0.75	0.43	0.75
Yield/Yield max	0.82	0.94	0.77	0.90	0.68	0.90
Yield increase (kg/ha)		1320		1430		2420
Water saving (mm)		164		180		396
Benefit/cost		3.9		1.4		1.15

All these improvements help optimize water application and opportunity of irrigation and enable upgraded irrigation scheduling techniques to be applied efficiently.

ON-FARM IRRIGATION TECHNIQUES AS CONSTRAINTS TO THE IMPLEMENTATION OF IRRIGATION SCHEDULING METHODS: SPRINKLER IRRIGATION

Sprinkler irrigation is often considered as being very effective compared to surface irrigation because it enables better control of water application. However, this control is dependent on a good quality level in the irrigation system design and in the selection of equipment, and also requires that the farmers develop appropriate skills for managing their irrigation system (knowledge and control of the pressures and flows that enable the system to distribute water uniformly over the field).

TABLE 3 - Irrigation efficiency for various sprinkler irrigation techniques

Irrigation technique		Efficiency (%)
Lateral hand move		65-80
Travelling gun		55-70
Centre-pivot
	Standard (400m)	75-80
	With corner	65-85
	Long	65-85
Linear Moving		65-85
Solid set		65-75

Sprinkler irrigation systems are generally designed using standard single sprinkler performance data, available from laboratory tests, manufacturers or independent organizations. The tests are performed in fully controlled no-wind conditions according to the specifications described in the standards.

Estimates of irrigation efficiency for sprinkler systems are presented in Table 3 (Keller, 1992). The lower values reflect significant wind distortions or evaporation. The higher values indicates excellent management and favourable conditions.

Actual on-field performance of sprinkler systems is often much lower than potential performance:

· on the one hand, because several disturbance factors that farmers cannot control (wind, variations of pressure on systems, etc.), were not taken into account in the initial design. Experimental and research data related to the on-field effects of such disturbances on the system can require complex processing, and approaches to use them for improving system design have been developed only very recently (Seginer, 1991; Al Naeem, 1993; Augier et al., 1995);

· on the other hand, because of farmers' lack of skill in managing the system itself (poor knowledge of system operating pressure and flow, of the actual operating conditions, of the influence of the settings and the mode of operation of the equipment on the system performance).

TABLE 4 - Summary of field performance evaluations of sprinkler irrigation systems in the south-west of France (Source: Dubalen, 1993)

Irrigation technique			Gun and reel machine	Solid set sprinklers
Number of systems controlled			382	78
PRECISION OF WATER APPLICATION
Deviation between the declared and the measured average water application depth
	. below 10%		24%	41%
	. between 10% and 20%		30%	25%
	. over 20%		46%	34%
CAUSES OF LOW UNIFORMITY OF WATER APPLICATION
Reel machines
	. Wrong lane spacings		65%
	. Non-symmetrical setting of gun sector angle		59%
	. Deviations in gun travelling speed along the line
		. below 10%	24%
		. between 10% and 20%	37%
		. over 20%	39%
Solid set sprinklers
. Spacing too wide				70%
. Pressure deviation between distant sprinklers of a line exceed 20%				56%
UNIFORMITY OF SPRINKLER DISCHARGES
Average pressure measured at sprinkler or gun nozzle
	. correct		52%	58%
	. too low		38%	20%
	. too high		10%	22%

For example, several surveys were conducted, in various French regions, on a large number of sprinkler irrigation systems between 1988 and 1991. The results (Table 4) highlight some of the problems farmers are still having in efficiently controlling their systems; this in a country that was one of the first in the world to develop this technique.

The following facts concerning the on-field performance of the three major sprinkler irrigation systems can be stated:

Solid set sprinkler systems

Precision of the average water application depth: only 41% of farmers know with an acceptable precision (< 10%) the average water application depth that they actually achieve on their field. This is essentially due to:

· ignorance of real sprinkler flow,

· a selection procedure for irrigation durations that gives more priority to convenience and labour organization than to the water application depth defined by scheduling methods,

· poor methods to estimate the on-field water application depth (0, 1 or 2 catch cans only, without taking into account the 'design' heterogeneity of the irrigation system).

Uniformity of irrigation water application depth: The main causes that deteriorate water application heterogeneity are:

· irrigation system design methods do not take accurately into account the effects of wind, nor the variations in system supply pressure. So, the actual water application uniformity will be below the design uniformity,

· pressure deviation observed between extremal sprinklers on an irrigated plot is often too high.

Gun and reel machine irrigation systems

Precision of average water application depth: results (Table 4) highlight the fact that only 24% of the farmers control the average water application depth with an acceptable precision (< 10%). The average water application depth is calculated with the following equation:

D: average water application depth (mm),
Q: flow discharge (m³/h),
V: gun travelling speed (m/h),
E: lane spacing (m).

It has been observed (Dubalen, 1993) that the main causes that deteriorate water application uniformity are (Table 4):

· intervals between two gun lanes not adapted to the local conditions (wind effect);

· bad adjustment of the gun sector angle;

· poor control of the gun travelling speed with mechanically controlled reel machines;

· bad control of the pressure at the gun nozzle, that can easily be too low or too high and that can change during the irrigation.

Centre pivot or moving lateral irrigation systems

This type of system appears to be one of the systems that enable the highest levels of precision of the average application depth and of the uniformity of distribution over the field. Provided that the nozzle chart of the machine has been correctly calculated and implemented, and that the end gun is correctly set, the tunings and adjustments initially implemented are not likely to deteriorate with time and they usually require no extra action from the farmer, except maintenance.

The distribution of water from continuously moving sprinklers enables some integrating of the pattern along the travel path. This integration reduces much of the non-uniformity of application observed with stationary sprinklers. Under a well-designed centre pivot irrigation system, the uniformity, using the standard Heermann and Hem (1968) Coefficient of Uniformity (CU hh) is typically over 90%. The values used for application efficiencies are generally lower because they represent an average value over the irrigation season, taking into account all the changes in the operating conditions (Table 3).

Possible improvements to sprinkler irrigation techniques

The improvements apply to equipment and to on-field systems:

· Selection of quality equipment according to its performance (increasing the availability of data on equipment performance, promoting and disseminating procedures for equipment certification).

· Information and training for farmers to improve skills in controlling and managing irrigation systems at field level (setting equipment, operation of systems) using sensors to check and monitor pressure and/or water volumes. Improvement of on-field practices so as to increase the uniformity of water application and irrigation performance.

· Popularization of methods for evaluating, at field level, the actual system performances, including various local disturbance factors:

. wind,
. variations of pressure from supply systems,
. topography,
. equipment tuning.

· Upgrading the system design methods to accommodate the consequences on performance of on-field local disturbances into the design procedure, and popularize the most effective practices for operating equipment. It has been observed (Augier et al., 1995), that in the south-west of France, farms irrigated by travelling gun can reduce wasted water by 10% to 20% and increase corn crop yields by 10% to 15%, with proper management of their irrigation systems. A methodology has been proposed using the simulation of scenarios during several irrigation seasons, to elaborate guidelines to adapt the use of equipment (selection of appropriate tuning, lane spacing, lane direction/wind, water application depth) to the regional agronomic, pedological and meteorological conditions (windy conditions). This methodology takes into account the regional irrigation scheduling practices.

· Automation of the system enables high performance. As an example, centre pivot irrigated farms can benefit from improved pivot control capabilities which enable the farmer to match water applications more closely to the crop water requirements of various sub-areas within a field. Software is available to facilitate irrigation scheduling, taking into account the effects of soil and crop spatial variability. A study on two adjacent farms growing corn in northern Colorado indicates a seasonal pumping depth of 710 mm for non-scheduled fields and a seasonal pumping depth of 555 mm for scheduled fields, i.e., a reduction of about 22% in the amount of water pumped (Buchleiter et al., 1993).

ON-FARM IRRIGATION TECHNIQUES AS CONSTRAINTS TO THE IMPLEMENTATION OF IRRIGATION SCHEDULING METHODS: MICROIRRIGATION

'Microirrigation' typically applies to several systems operating at low pressure including drip and trickle, miniature distributors, bubblers, nozzles, and tapes. It is characterized by the localized application of irrigation water using low flow and high frequency applications, either to the surface of the ground, or underground (sub-irrigation).

This technique features many advantages:

· the agronomic efficiency is improved because of a higher satisfaction of crop water requirements through frequent applications;

· it enables water amounts to be reduced, maybe by 10 to 30% (Hoffman and Martin, 1993), with the identical application efficiency of other systems. This is because it reduces evaporation and consumptive use by carrying water only to a small portion of the soil area and profile;

· it enables energy consumption to be reduced.

TABLE 5 - Irrigation efficiency for various microirrigation techniques

Irrigation technique		Efficiency (%)
Orchard
	Drip/spray	75-90
	Bubbler	60-85
	Hose-pull	65-90
	Hose-basin	55-80
Row-crop
	Reusable	65-90
	Disposable	60-80

Estimations of irrigation efficiency are given (Keller, 1992) in Table 5.

However, microirrigation systems also feature constraints and risks that farmers have to know and take into account:

· the amounts of water applied are more difficult to check visually, especially in the case of sub-irrigation, and the large number of water emitting points in the system cause control procedures to be more complicated and more time consuming;

· it is necessary to select carefully the equipment to best match agronomic, climatic and pedological conditions. Equipment specifications have to be far more precise than for other irrigation techniques;

· this technique requires the most extensive knowledge of system performance to enable correct operation by the users, avoiding water excesses or deficits and heterogeneity in water application;

· a preliminary treatment of the water (filtration, injection of agricultural chemicals) is often necessary, and controlling these additional devices will require extended skills from farmers;

· maintenance, including cleaning of filters, control of emitter flows, operation of line flushing, is a critical process, but indispensable for it conditions the life span of such systems, which usually involve superior investment ratios per hectare.

Possible improvements to microirrigation techniques

More than for other irrigation techniques, production yields are dependent on the reliability of microirrigation systems and on the conservation of system performance over time.

· the quality of irrigation equipment should be considered as a major point (certification by approved bodies). The problem is even more critical whenever fertigation is performed. As a matter of fact, this technique requires a very good uniformity of distribution and precise irrigation scheduling techniques, but would not be implemented without risk for crop production if the system performance evolves negatively over time;

· system monitoring, preventive and curative maintenance including the implementation of methods for evaluating and controlling the actual system performance on the field;

· the possible automation of this technique saves labour and facilitates the implementation of precise irrigation scheduling techniques.

· use of upgraded design techniques;

· control and prevention of soil salinization.

Several control systems have been developed for microirrigation systems to apply scheduling techniques. Recent progress concern both computation procedures, electronic engineering and soil water sensors. This enables the soil water contribution to be more accurately taken into account. As an example, the method called 'Humicro 2000' (Peyremorte and Tron, 1995) uses a microcomputer and electronic tensiometers to schedule drip irrigation or micro-sprinkler systems. Several experiments with this technique enable water consumption to be cut by about 50%. It has been observed (Isbérie and Peyremorte, 1995) that a water application depth of 182 mm gave the same crop yield for melons than a traditional scheduling technique requiring 358 mm.

CONCLUSION

The implementation by the farmers of recent results of the research into improving scheduling tools and techniques is obviously a necessary condition for improving agronomic and economic irrigation performance. However, on its own it is not sufficient and the concurrent improvement of on-field irrigation equipment and of farmers' management skills is indispensable.

Procedures for obtaining long-term improved results from irrigation systems can be approached with a classical 'global system quality' analysis method.

The quality of irrigation systems shows critical improvement possibilities and emphasizes the need for considering, promoting and checking:

· the quality control of equipment, based on standards and certification,

· the quality control of design, inclusive of operational adaptation to local field conditions, and

· the quality control of procedures for disseminating information to users.

Advances should focus on developing operational methods enabling the mastering of on-field irrigation equipment, on insisting on the minimum performance standards required from irrigation equipment, on upgrading existing on-field design procedures, and on promoting farmers' capability for improved on-field system control and operation. Doing so will enable farmers to derive higher benefits from using modern scheduling techniques and thus to improve overall irrigation performance (agronomic and economic performance, waste water reduction, etc.).

To improve current on-field irrigation system performance, these operational methods will have to point out the importance:

· on the one hand, of selecting the most appropriate technique, of matching the local context, and of using certified equipment with performance meeting relevant standards, and which can ensure a minimum quality for on-field irrigation,

· on the other hand, of implementing upgraded management methods for such equipment and techniques, and of developing skills to avoid those errors currently occurring during ordinary system operation. These methods have to take into account the constraints which farmers are facing on the field, and promote the proper use of equipment and techniques to cope with them.

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