THE APPLICATION OF THE SAPWAT WEBSITE   CT Crosby 133 Analees street, Meyerspark, 0184   INTRODUCTION   The Sapwat computer program for estimating crop irrigation requirements in South Africa has been released as Water Research Commission Report No.  624/1/99.  The methodology employed is based on atmospheric demand utilising the Penman-Monteith calculated evapotranspiration from a sward of short grass as the reference evaporation and is similar in principle to the well-known procedure based on the use of the A-pan.  The Penman-Monteith value is calculated from weather data utilising internationally agreed procedures and has the major advantage that climatic factors influencing plant growth are taken into account.   A consequence of changing the reference evaporation is the that the A-pan crop factors are replaced by a modified range based on what has come to be known as the FA0 procedure.  The importance of this is that crop factors can be developed to cater for regional variations, varieties, management practices and irrigation methods.  It is even possible for a user to develop valid factors for a crop that is unfamiliar and to explain the rationale followed to others.   Sapwat is an interactive procedure   Sapwat is not a black box providing instant estimates of crop irrigation requirements, essentially it is a procedure combined with crop and climatic data bases that enable the planner, water manager and designer to develop realistic estimates that reflect the complex factors that determine crop water requirements.  The program contains extensive default information on crops, climate, soil, irrigation methods and management practices that expedite the development of the estimates but it is realised that not all the conditions met in the field are adequately catered for or reflect the most recent research findings or field practices.    Users as they any gain experience can contribute to improving and up-dating the data bases and to developing new techniques for approaching local and specialised situations.  It is anticipated that Sapwat will always be a dynamic program with users contributing to development.  At the same time there will be those that have neither the interest or the time to develop customised solutions and require pre-packaged spreadsheets, tables and graphs directly applicable to local climatic conditions and cropping patterns.   It was decided that one way of approaching this situation was to create a dedicated website that would promote two-way communication between the Sapwat authors and the diverse users of the program as well as between the users themselves and irrigation scientists.  In addition it would then be possible to develop specific instruction sheets on applications that could be updated periodically when the need arose.   A website has been built but is still under construction.  It can be accessed at Sapwat.org.za and the program downloaded.  There is no charge but a registration form is completed as part of the downloading procedure.  This is to enable the managers of the website to communicate with users on a regular basis.  It is appreciated that not all users or potential users will have access to the website and ways and means of overcoming this difficulty are discussed.   The Sapwat database currently includes long term weather data for approximately 400 stations located throughout the country and 70 crop files.  Each of these files contains on average three options applicable to seven climatic zones into which the country is divided.  This amounts to a surprising 1000 default crop factor files that can be individually edited.  Provision is also made for three levels of target yield and the full range of irrigation methods.  These statistics may appear intimidating, but in practice estimating crop irrigation requirements is no more complex than it was with the well-known A-pan method.  Because default information is extensive and can be directly accessed it is, in fact, quicker and easier.    “How to tackle the task”   Sapwat has a built-in help function that covers running and using the program but experience has shown that when the program is applied to specific tasks additional guidance is required.  It is the intention to develop “how to tackle the task” notes that will be available on the website and that these will be periodically updated.  For example Sapwat has been designated by the Department of Water Affairs and Forestry as the procedure to be used for estimating irrigation requirements.  The Pricing Strategy of the Department of Water Affairs and Forestry (DWAF) requires that Sapwat be used as the procedure to calculate crop water requirements and irrigation requirements where new benchmarks are required.  The Water Resource Planning Directorate also uses Sapwat for basin studies and in the Water Balance Model.  Furthermore the Directorate of Water Utilisation is using Sapwat as a tool in the registration of water use.  It is therefore logical that Sapwat should be adopted as the primary tool for the calculation of benchmarks in the implementation of the Water Conservation/Demand Management strategy in agriculture.  It is important that Sapwat is applied appropriately with due attention being given to administrative and legal procedures as well as the practical operational circumstances.   Each of these aspects has been, or will be, covered by “how to tackle the task” notes.  These and several other aspects will be considered in greater detail in the section dealing with the proposed content of the website.   THE WEBSITE      The website comprises a number of pages:   Executive Summary of the Water Research Commission report Sapwat status that outlines the progress to date of program and website development. ·         Download program:  Requests that the user registers and then makes the download procedure for the latest version of the program available.  The file is 4.5 megabytes and downloading can take from 5 to 30 minutes depending on line condition.  The file is saved to disc and subsequent installation only takes a few seconds. ·         Discussion forum:  This is an on-line discussion forum to support Sapwat and to facilitate sharing of ideas and information for later incorporation in the program. ·         Feed back:  This page provides the opportunity to comment on the website itself. ·         Update crop files:  Individual crop files can be accessed and the latest default version downloaded. ·         How to tackle the task:  Notes on individual subjects can be accessed and downloaded see “Applications of the website”.   APPLICATIONS OF THE WEBSITE   What follows is a listing of "How to tackle the task" notes that can be downloaded from the website.  It is anticipated that users will come to contribute to the further development of these notes in the future not only by improving what is currently available but also by suggesting new applications and methods       Conventional calculation of long-term average crop irrigation requirements   An appropriate weather station is identified on the map and the program loads long term weather data including the Penman-Monteith reference evaporation and rainfall.  The crop is identified on a drop down menu and the crop factor curve comes up on the screen.  The planting month and date is then selected.  The program then does the necessary calculations and monthly and seasonal values for crop evapotranspiration, effective rainfall and irrigation requirements come up on the screen.  This procedure is suitable for “standard” design and planning purposes.  While the approach is familiar the introduction of the “short grass” reference evaporation and FAO type crop factors may result in significantly different irrigation requirement values.  This is particularly applicable to the summer rainfall areas.   Approach to estimating crop irrigation requirements under non-standard circumstances   The conventional approach to estimating crop irrigation requirements is based on an idealistic scheduling approach developed when flood irrigation and conventional sprinkler irrigation were the norm. These were long cycle irrigation regimes that have now been largely replaced by high frequency micro, drip and centre pivot irrigation.  A further difficulty is that no account was taken of the practical realities of management so that in many cases the estimates were not a realistic reflection of the situation on the ground.  Sapwat makes it possible to simulate most of the situations found in practice. The program separates transpiration from evaporation and this makes it possible to reflect the influences of irrigation frequency, wetted area, stand density and crop characteristics. The procedures followed are not complex but do require knowledge of irrigation practices.   Application in irrigation system design   The primary concern of system designers is to ensure that peak demand requirements can be met while still retaining reliability and a reasonable cost structure. They are understandably less concerned with estimating annual water requirements that they normally prefer to leave in the hands of planners and water providers, or with the details as to how the farmer will manage and schedule irrigation.  Sapwat does, however, provide a common ground for discussions with a farmer client. Present farming practices can be analysed and alternatives assessed.  Irrigation strategies can be weighed up and the economic advantages and feasibility of making use of off peak electricity tariffs considered.  At a very practical level it becomes possible to weigh-up rainfall and risk and to deal with this in the design process.    Dealing with tree crops   Tree crops present particular problems in the estimation of irrigation requirements. Their growth habit differs from that of conventional crops in that they do not completely cover the orchard area even when mature and air is free to circulate below the tree canopies. The pattern of evaporation from the soil surface is consequently different.  Initially consideration was given to developing a separate Sapwat module to cater for fruit trees but this has not proved necessary.  Adequate estimates are being developed with the standard program but additional judgment is required.  Allowance must be made for the canopy cover of trees at various stages of development, the degree to which the space between rows is subject to irrigation of cover crops as well as the irrigation methods and strategies that are being utilised.   Planning irrigation systems for emergent farmers   Studies of emergent farmers have indicated that water requirements for irrigation are significantly lower in many cases than the values that were estimated using conventional planning techniques.  One of the motivations for developing Sapwat was the need to cater for this situation.  In many cases emergent of farmers adopt strategies that differ from those applied by well-established commercial farmers.  Normally little account was taken in the past of the production methods of the emergent farmers and irrigation requirements were over estimated.  It can be argued that in time emergent farmers will have developed to the level where water requirements will be at the same level as that of well established farmers utilising capital intensive equipment and methods.  What, however, is required is a planned progression bearing in mind profitability and sustainability.  It is important to cater for this progressive development in technology and the irrigation requirement projections developed applying Sapwat are in line both with research results and practical experience.   The development and updating of programmed irrigation schedules   Increasingly use is being made internationally and in South Africa of pre-programmed irrigation as an adjunct to the more intensive modern instrument based scheduling procedures. The method relies on atmospheric demand, long-term weather records and regular checks of the water content of the soil profile.  It aims at maintaining a satisfactory soil water balance from one week to the next and requires limited input on the part of the farmer.  Developing and updating programmes of this nature is not the prime purpose of Sapwat but is a useful support function. In any area and for any crop and management practice a normal water requirement for each week of the coming season can be generated. This water requirement is provided by the combination of irrigation and rain.  The estimated actual water use for each week is calculated and circulated to the farming community. It in the case of conventional crops this method can be management and cost effective provided the water content of be profile is checked on a regular basis. It must be considered an entry-level introduction to scheduling and scheduling consultants aimed at the bulk of irrigation farmers.   The development of default irrigation requirement spreadsheets   Sapwat is designed to be flexible enough to deal with almost any irrigation situation requiring realistic estimates that can be motivated and defended should disputes arise.  Such situations are the exception rather than the rule and in the past a limited number of standardised crop factors were considered adequate.  Admittedly localised weather data was utilised providing for climatic differences and variations but the lack of flexibility in the crop factors was a serious deficiency in the procedure.  Experience gained early on in the DWAF water use registration process showed that the process was facilitated if monthly and annual irrigation requirement values were made available to officials interviewing farmers.  These values provided a starting point for establishing valid water use figures.  Normally this information was developed in the form of a computer spread sheet and targeted all the crops grown in the specific area served by a representative weather station.  Development was undertaken by local officials and consultants competent in applying Sapwat and familiar with the varieties produced in the area and the farming and irrigation practices.  These values were then discussed with farmers in individual interviews and modified where justified.  A spin-off of the process has been the development of spreadsheets covering the entire country.  This will amount to between four and five spreadsheets per province and as they will be of great use to designers and planners arrangements are being made to include them on the website and to organise periodic updating.   Evaluating the effectiveness of alternative irrigation practices and strategies in an area.   It is normal to find that certain irrigation practices and methods become generally accepted in an area. There is a tendency to presume that these practices are correct and that the chances of major improvements being possible are not good. It is seldom that a critical analysis has been made of the current situation and Sapwat is effective as a catalyst when reviewing current practices and their validity. Opinions and perceptions can be quantified and subjected to “what-iffing” that leads to the realisation that there are possibly other avenues that should be explored.  This is not a difficult process but guidance is necessary if the full potential of the facility is to be realised.   Estimating irrigation efficiency   The methodology for estimating crop evapotranspiration has advanced significantly over the past decade making it possible to develop improved estimates of net irrigation.  It is only recently, however, that attention has focused on the efficiency factor applied to convert net irrigation to gross irrigation. In South Africa default irrigation systems efficiencies have been standardised for use in planning and design.  Sapwat includes these values in a drop down menu and they are one of the factors applied to develop irrigation requirements.  As a temporary measure an additional factor has been added that takes into account the effect of uniformity of distribution, variations in the hydraulic properties of the soils and the target yield level.  It is common knowledge that designs are based on the worst soils in the block and that up to a certain limit high yields require more water than average yields   Estimating default crop irrigation requirements for water use registration, allocation and licensing   It is anticipated that Sapwat estimates will continue to play a role in these processes so that it is important that users have a sound understanding of the procedures applied and are kept up to date with developments.   Demand Management and Water User Association water management plans   These are new concepts in South Africa and a series of pilot projects have been launched to ensure that procedures and methods are thoroughly evaluated before general introduction.  Sapwat obviously has an important part to play in the various phases of this process and once again the intention is to promote development and co-ordination through the medium of the website.   Incorporating Sapwat in the irrigation modules of hydrological models.   Estimates of crop irrigation requirements are important in planning simply because irrigation accounts for more than half of the water used in South Africa.  In the past A-pan based procedures were used but the decision has been taken to convert to Penman-Monteith/FAO crop factor based procedures utilising Sapwat.  At this stage this has been limited to the conversion of the modules concerned applying lumped crop factors but it is anticipated that there will be more far reaching implications in the future and it is hoped that researchers, consultants and planners will make use of the website in order to facilitate co-ordination.   Micro-climates and “own” weather stations   Microclimates and “holes” in weather station distribution are an important factor in determining irrigation requirements and despite the increasing availability of automatic weather stations and improved electronic databases users may still want to develop “modified ”weather stations to meet their needs.  Sapwat includes the facility to calculate ETo values and also to add personalised weather stations but guidance may be required on the procedures to be followed to meet specific circumstances.