August 2001



Use of automatic weather stations (AWS) and crop growth models (CGM) have been shown to be a successful method for management of efficient irrigation scheduling. Irrigation boards and/or individual farmers are prepared to purchase their own automatic weather stations for this purpose. Apart from scheduling irrigation, the climatic data collected may be used, for example, for climate surveys and decision support for pest control and frost protection. In addition, the weather data can be accessed at any time. Similar networks and decision support systems have been developed overseas and are functioning effectively. This project was aimed at adapting and developing such technology for southern Africa, but housed within a telecommunications network capable of serving farms and irrigation schemes located in various provinces and countries.

In previous WRC research projects conducted by the Department of Agrometeorology, UOFS methods for maximizing irrigation efficiency using AWS and CGM and the Putu computerised procedures in differing climate-soil-crop scenarios had been formulated. This project collated these procedures and demonstrated the technology transfer thereof to irrigation practitioners. Opening markets in the UK and Europe now provide opportunities for the export of high income crops such as potatoes, asparagus, runner beans, tulip bulbs, baby vegetables and fresh flowers etc. In order to be successful in both the existing as well as these new export markets, technology enabling and simplifying irrigation of such crops needed to be developed.

A Successfully operated Irrigation Decision Support System (IDSS) serves as an excellent demonstration of computerised technology transfer. Timeous irrigation decisions based on scientific principles, demonstrate to farming co-operatives, irrigators and/or consultants, who previously had no such information, the advantages of IDSS and create an awareness of the benefits of scheduling and judicious management of limited water resources.

Managers of small to moderate sized farms cannot devote the necessary time to irrigation scheduling without reducing their other farming activities and are in urgent need of time-saving real-time irrigation schedules.

Preliminary feasibility studies had indicated that considerable water savings could be attained in comparison to scheduling based upon Class A pan evaporation. Furthermore, since the introduction of variable ESCOM tariffs could provide opportunities for immense savings in electricity as irrigators are now permitted to choose low tariff periods during which to operate. Such strategies will also assist ESCOM to regulate peak loads and consequently reduce electricity costs to the consumer countrywide.

Computerised scheduling programmes and water management will improve water use and distribution efficiencies which will not only benefit optimum crop production, but will also save water particularly during times of drought and when restrictions are applied.


Taking into consideration the rationale for the study, the overall aims of this research project were the design, development and establishment of an effective automatic weather station network and computerised irrigation decision support system (IDSS). Essentially this entailed establishing a computer aided system for the provision of information upon which to base decisions related to irrigation planning, scheduling and water distribution management.

The specific objectives of the research were:

Network development

Determine which type of telecommunications network system, involving radio links if necessary, is best for the different situations found in southern Africa. Thereafter establish such a network to serve estates/farms, groups of farms and irrigation schemes and test and improve the data retrieval system.

Data management

Develop data management procedures which make possible the efficient use of water on numerous irrigation plots.

Irrigation scheduling and water-use efficiency maximisation

Create a computerised Irrigation Decision Support System (IDSS) resident in research centres at Pietermaritzburg and Bloemfontein, from which scientific irrigation advice may be provided to water managers to enable them to :

Regarding the apportionment of limited water between clients and crops, a theory and a computerised optimisation procedure were developed. Unfortunately, the latter failed to function satisfactorily and hence this aspect and the management of water distribution to numerous farms will not be reported on in this document. Details of the theory however, are included in the complete report.

Information disemination

Devise, by surveys and trial and error how client requests may be received and processed and recommendations may be rapidly disseminated.

Documentation of procedures

Provide a thorough description of the system and detailed instructions on how to operate it.


A survey of selected literature covering the research topics was undertaken. From this the research was planned and guided and the results of the work will now be described.

A telecommunication network for southern Africa

It was shown that efficient scheduling of irrigation can be achieved using the Putu-IDSS based upon automatic weather station data and crop growth models. Water-saving irrigation scheduling advice was transmitted from central computing centres linked by telecom-munication network to distant AW's and farm managers located in other provinces and countries.

Automatic weather stations were connected in a computerised network. Weather data were collected and used in mathematical models to compute crop water-use and recommended irrigation amounts. This information was then disseminated in sufficient time to allow irrigators to act before crop water stress with concomitant yield reductions were induced.

The technology devised, is suited to extensive commercial as well as low-input small-scale operations and the major advantage of the system stens from the use of data from a single weather station and its application to numerous neighbouring farms. This has significant cost and logistic benefits.

The installation and maintenance of AWS's were investigated and applied. The most necessary precautions are detailed in the report. While Putu-IDSS does advocate a standardised format for information dissemination, this was however not always found to be desirable, because of varying client facilities and requirements. Radio, short-haul modem, direct cable and telephonic links all proved successful for the interchange of data and irrigation depth and timing recommendations.

Increasing numbers of clients expressed the desire to schedule irrigation for themselves and several needed only values of reference evaporation. Groups of farmers who purchase an A WS wish also to be able to sell weather data. Furthermore, the use of weather data to calculate disease indexes has become a necessity.

Theory and user instructions

The computational and mathematical procedures and mathematical equations required in the IDSS are outlined in the report. Innovations include algorithms for the numerical solution of the non-linear equation indicating the onset of water stress in plants, the introduction of default values of critical leaf water potential, a method for transforming calendar dates into thermal period from sowing date, and a streamlined software package for creating input data files and irrigation recommendations. Instructions on how to run the computer programs are outlined. The use of thermal period means the crop phenology established in one particular locality may be used elsewhere.

Detailed descriptions on how to estimate certain crop parameters and the soil water characteristic are given. The standard form of the Penman-Monteith equation recommended by the Food and Agriculture Organisation was tested and an appropriate version included in the Putu-IDSS.

An example of a pro-forma contract between irrigation agent and water manager is included.


The irrigation decision support system was comprised of irrigation advisors in Bloemfontein and Pietermaritzburg who were telephonically and/or computer linked to various participants. The telecommunication network grew steadily and included operations in Zimbabwe and Swaziland, with a service planned for Tanzania. Approximately 125 clients and 1900 plots of land were irrigated from centres at the Department of Agrometeorology, UOFS, Bloemfontein and in Pietermaritzburg. Similar operations run by three farming Cooperatives and the Free State Department of Agriculture which also used Putu, but were not part of this project, are included in these numbers. In excess of 34 AWS's, purchased by private entrepreneurs, were involved in the operation.

Apart from pastures, maize, soyabeans, wheat, peas, dry beans, potatoes, runner beans, sugarcane, barley, cotton and vegetable crops; the Putu-IDSS was adapted to handle high income crops such as apples, asparagus,. tulip bulbs, tomatoes, onions, seed maize and mange tout peas. Envisaged expansion includes: the eastern Free State (asparagus, apples, cherries), Zimbabwe (potatoes), Komatipoort, Mpumalanga (plantations), Delmonte, Cameroon (bananas) and Tanzania (diverse).

Two agents set up independent commercial undertakings which proved financially viable. This may be taken as an indication that farmers are prepared to pay for this type of irrigation decision support.

It was found that two levels of decision support made available in the Putu-IDSS are useful in practice. These are:

High level -where the irrigation simulation model simulates field conditions from weather data and computed reference evaporation and enables recommendations regarding the depth and timing of the next irrigation.
Intermediate level -where managers are provided with daily values of reference evaporation and simply irrigate an equal amount of water, or some empirical fraction thereof. Of the abovementioned plots, 295 were serviced at intermediate level.

This approach makes the Putu-IDSS highly suitable for both commercial and small farm enterprises.


Software needs to be developed which will make the Putu-IDSS available on Internet and a Website. This will make it readily accessible to a large number of irrigation farmers.

The sparse canopy and partial cover routines in the irrigation simulation model require refinement and validation.