G. V. Mangano, Consultant, Rome, Italy
Irrigation scheduling is an important tool for optimizing the use of water resources where irrigated agriculture has a long tradition. By contrast, the operation of new irrigation districts in developing countries poses two major challenges:
· the farmer's acceptance of the larger commitments involved in irrigated agriculture (more work, care and discipline) than in rainfed agriculture;
· a suitable organization for operation and maintenance with skilled staff and adequate financing.
In the first stage all efforts should be directed towards increasing the irrigated area with an easy system. The optimization of water use through scheduling procedures should be deferred to a later stage, when the farmers have fully accepted the irrigation practices and are ready to follow more flexible rules.
There is ever greater competition among the different forms of water uses: domestic and public water supply, industry, agriculture, and environment protection.
Because of the increasing difficulty in finding new resources, a strict water saving policy is an obvious must.
Agriculture makes use of three-quarters of the world's fresh water supplies. The main forms of water saving are:
· the increase of overall efficiency (reduction of conveyance, water distribution and application losses);
· the re-use of drainage water (within acceptable salinity limits);
· irrigation scheduling to make available to the biological process the correct amount of water at the appropriate time.
The three forms of water saving, separately or jointly employed, may have alternative goals:
a. to maximize the irrigable area (and thus the number of beneficiaries) for a given annual water volume;
b. to maximize the production obtainable per unit of water;
c. to maximize the average economic benefit (gross marketable product or net income) obtainable in a farm that comprises irrigated and non-irrigated land (dry farming crops, where feasible, or fallow).
The choice of the goal to be reached depends on multiple considerations of the economic, social and financial order; as a long-term policy, hypothesis (b) should be the right option. In all events, the choice will condition the procedures for conceiving and applying the water- saving technologies.
The aim of this paper is to estimate how the irrigation scheduling processes can be applied in developing countries when new irrigation districts are to be implemented; which constraints and limitations hinder the achievement of satisfactory results; by what steps progress should be adopted.
Irrigation scheduling has always been carried out in the planning stage; the recent tendency is to perform it continuously during the operation phase, as a management tool.
The choice of the irrigation volumes and of the relative application times constitutes the starting point for the design of the irrigation network. These parameters are identified in the planning phase on the basis of:
· a hypothesis of climate conditions deduced from available data;
· a hypothesis of the cropping pattern;
· a hypothesis of a sowing and crop development calendar.
Starting from these three hypotheses, the irrigation season is usually subdivided into several periods, each one being characterized by a value of unit water requirement (m3/day or l/s per hectare). The highest (peak) of these values is adopted for sizing the irrigation network.
Taking into account the hydrological characteristics of the soils, a basic constraint, and the water application procedures, the interval between two subsequent waterings is defined, where rotational delivery is adopted. Similar calculations are required for rice fields or localized irrigation.
Comparing the water requirements with the volume of the available resource (natural flow or stored volume), the irrigable area is then estimated.
According to the classical operation method, in each period the management releases into the network the discharge corresponding to the water requirement foreseen for that period.
The water distribution to each user takes place with the predetermined interval and volume, for a time proportional to the acreage. The user only has the option to refuse either totally or partially the volume assigned to him.
Once the system is operating, actual conditions (such as climate, cropping pattern, sowing calendar) differ from the assumed conditions in a more or less considerable and variable way; also the characteristics of the soils can differ somewhat in each single plot from those assumed for the whole district or for each subdistrict.
From all this springs the wish to adapt, day by day, the amount of water supplied to the actual biological requirements. At first this wish gave rise to the setting up of 'on-demand' systems with each user having a greater or lesser degree of liberty to choose the time and volume of his withdrawal from the distribution network. The distribution network must have an operating flexibility so as to adapt itself to the variability of the withdrawal. The substitution of rotation irrigation with on-demand irrigation means that the decisions are transferred from the management to the single farmers; the management should intervene only in the case of emergency.
Past experience has shown, however, that in order to allow each single user sufficient autonomy (especially as regards the choice of the irrigation time), the network has to be oversized in an unsustainable way, with high construction and maintenance costs.
Consequently a new tendency has gradually emerged. The central management will modify, day by day, delivery volumes and times in accordance with the actual conditions and requirements. This means taking into account what is good for each single farm (for instance, with respect to the actual situation of the crops), but in an overall coordinated way for the whole district.
Irrigation scheduling has a higher efficiency when the water is derived from surface or ground storages. In the case of river diversion with pumping the benefit is given by energy-saving. The water saving is greater when irrigation occurs in variable climate conditions, with occasional but not negligible rainfalls. The techniques are more efficient in piped distribution networks and where single-crop farming prevails.
In its schematic lines the problem is as follows: given an irrigation district commanded by a storage structure, how to regulate the offtake from the reservoir and the distribution of water to the users in order to reach the selected goal (see point 3). To this effect it is necessary: (i) to always be informed about the effective condition of the soil moisture; (ii) to foresee on the basis of the actual cropping pattern what the water consumption will be in the near future; and therefore (iii) to determine 'when' and 'to which extent' the soil moisture should be increased.
The concept and application of the above process involve sophisticated technology. In fact, reference is made to the construction and operation of a model in which interact:
· the climate conditions (essentially: rain, temperature, relative humidity, wind);
· the water requirements of each crop during each development stage;
· the water retention characteristics of the soils;
· the conveyance capacity of the network from the main regulation points to the single farms in order to properly regulate the water distribution and utilization.
The following are therefore necessary:
· an accurate collection and transmission (in real time) of the data from the periphery to the centre;
· an intelligent reaction at the centre for processing the data received and determining the appropriate consequences;
· a whole network of telecontrols to convey the required discharges into the various network stretches and to operate the water distribution devices to the single users or groups of users;
· a total readiness by the users to accept what is being decided by the management.
The introduction of an integral scheduling system in an established irrigation district entails a series of economic and organizational problems. However, it is not an impossible undertaking if suitably presented and explained to the farmers.
The users with long irrigation experience are ready to appreciate the advantages derived from lower water consumption: lower charges, reduced energy consumption, prevention of waterlogging, less drainage requirement, less waste of fertilizers, etc.
At the same time, long irrigation practice enables the users to:
· estimate the importance and interdependence of the factors affected by the biological cycle;
· prefer the increase of net income to the achievement of exceptional unit production peaks;
· appreciate the validity of the suggested procedures, organizational patterns and highly sophisticated equipment;
· entrust the management with the control of the irrigation system.
The situation and prospects in the new irrigation districts realized in developing countries appear completely different. The emerging economic and management problems, which are referred to below, are certainly important, but the aspects and the constraints concerning the human factor are prevailing.
The introduction of irrigation constitutes an overwhelming event. First of all, because it is a collective fact that imposes a more or less tight, but nevertheless binding, discipline. Secondly, because it introduces new crops and in a period of the year in which nature 'is sleeping' and man 'is resting'. Finally, as it requires an intensity and continuity of work that rainfed farming usually does not demand.
Faced with this overwhelming situation, the novice is worried and suspicious. Little by little one has to convince him/her that irrigation is a simple practice; that to the increased work there corresponds a more than proportional income; that the network is collective but in the farm everybody is autonomous; that the results will always be better as experience becomes more thorough.
To say that this convincing action is simple and produces rapid results means not telling the truth. However, it is a necessary action that has to proceed, with absolute priority, until a real irrigation conscience has been implanted in the minds of the farmers.
In the above situation the primary goal is to start irrigated farming, i.e., to have a certain number of simple rules accepted and a certain number of operations carried out: preparation of the soil; sowing or transplanting; water application at a given interval and time (for instance Mr. XY will receive water every Monday from 9 a.m. to 4 p.m.); harvesting.
Never tell Mr. XY that because evapotranspiration is lower than usual next Monday water will only be available from 9 a.m. to 2 p.m.; or that water will not be available next Monday but next Wednesday. The farmer who has accepted a rule with difficulty does not tolerate changes to the rule and gives up the undertaking.
To avoid farmers giving up the undertaking and the system remaining partially or totally unused is, therefore, the primary goal. At the beginning other objectives, even though important, must be subordinated to it.
A second issue refers to maintenance problems in developing countries. An efficient irrigation scheduling requires sophisticated equipment for teledetection, telecontrol, and data processing. Generally speaking electro-mechanical equipment frequently falls out of service and its repair often takes a number of months. Spare parts are usually unavailable in a short time.
The installation of such equipment should be avoided as far as possible. We have noticed that in some cases the water distribution scheme has been based on fixed weirs because 'also a sluice gate needs maintenance'.
In addition to the previous considerations it must be borne in mind that an irrigation system suitable for an efficient irrigation scheduling requires:
· higher initial investment;
· trained and efficient management;
· proper maintenance;
· sufficient financial flow to cover the management, operation and maintenance costs.
Many irrigation structures in developing countries have been only partially utilized or even abandoned because of organizational and financial shortcomings, but the simpler the system, the easier it is to operate.
It can thus be suggested that in developing countries new irrigation projects should:
· be implemented in such a way as to allow, in the initial stages, an extremely simple operation, even if higher than optimal unit water consumptions are required;
· be designed so as to allow further introduction (final stage) of all suitable measures and procedures to minimize water consumption through improvement of the irrigation network and full application of the irrigation scheduling procedures;
· have a command area corresponding to the reduced consumptions of the final stage, accepting that at the beginning the actually irrigated area is only a part of the command one.
The opinion can be expressed that irrigation scheduling procedures represent an important tool for optimizing the use of water in countries with a great tradition in the irrigation sector; and that, vice versa, in developing countries where irrigation is a new fact, it is better to postpone irrigation scheduling until a later consolidated phase.
This opinion is in accordance with a conclusion of a well-known Cornell University study (Studies in Irrigation No. 1 - Ithaca: Cornell 1984): in the sector of irrigation projects many failures can be attributed to 'directly transfer technology that is successful in the developed countries to a developing country'.
When I started my professional activity I was told that rigid rules must be imposed on new users from the very beginning, because the modification of acquired bad habits is an impossible task.
After 50 years of experience (mainly in northern and central Africa) I have reached the different opinion expressed in this paper.