CONTENTS Planning. 1 Introduction

Transcription

1 CONTENTS Planning 1 Introduction Soil survey and analysis Identify potentially arable soil Investigation Select irrigable land Soil conservation Norms Climate Crop study Select crop Consultant approach Instruction approach Crop water requirement Crop factor Net irrigation requirement Planning models BEWAB Donkerhoek data irrigation scheduling program SWB VINET Water source survey and hydrological study Survey Source discharge Source share Available pressure Location Quality Water sources Perennial stream or river Stored natural run-off Water supply authority Borehole water Management aspects Selection of irrigation system Survey Irrigation requirement Net irrigation requirement System efficiency Gross irrigation requirement Water balance

2 11 Scheduling planning Planning diagram: Discussion of model and equations Net irrigation requirement Readily available water Cycle length Net irrigation requirement per cycle Gross irrigation requirement per cycle Standing time Emitter discharge Net application rate Gross application rate Planning diagram: Example Theoretical size of irrigation group System flow Number of emitters per group Area of group References All rights reserved Copyright 2003 ARC-Institute for Agricultural Engineering (ARC-ILI) ISBN

3 Planning Introduction Normally the grower approaches the designer with the problem: crops (often more than one) are to be planted (either simultaneously or as alternatives) on possibly arable soils, and must be irrigated from an existing or a potential water source. The planning stage of project design therefore actually commences before anything else has been done. In this process the project should be evaluated in its entirety. A survey of all relevant inputs must be done at first contact, including (especially!) managing aspects related to the final product. If the system is to be designed for small-scale or emerging farmers, a particular approach is required during the planning process. The reason therefore is that emerging farmers manage their production inputs and irrigation practices from a viewpoint where risk is limited to an acceptable minimum. They therefore often plant at lower densities than the commercial farmer and this can lead to lower peak water requirements. If the assumptions and relations used for designing economic systems for commercial farmers are applied at random on small-scale farmers, it can cause the planning to have an unfavourable outcome. It can also mean that the final system can result in extremely high repayment requirements for the farmer. It is therefore recommended that the WRC publication A review of planning and design procedures applicable to small-scale farmer irrigation projects (WRC report no. 578/2/0) is consulted if systems for small-scale farmers must be designed. The first step therefore is to conduct a survey of all factors which could affect the peak and seasonal water requirements of the crop(s), and whether the water source (taking all features into account) will be able to cope with all aspects of demand. At the same time it is vital that the management features of the operator be evaluated inconspicuously but effectively. Figure 11.1 : Planning diagram

4 11.2 Irrigation Design Manual Figure 11.2 : Plan of irrigation site

5 Planning 11.3 It is often possible (and necessary!) at this stage to roughly determine the water balance (refer section 10) before going about it in any detail. In the event of conspicuous problems, this step can eliminate a lot of unnecessary work. Even estimated data (areas, crop water requirement, water source discharge) can be utilised quite effectively for this purpose. If, however, there is any doubt at this stage, it will be necessary to proceed to compile all the physical details before making any decisions. A practical example will be shown on most pages as illustration. Example: A grower has approached the designer to supply a design for four different irrigation systems as shown in Figure 11.2: Planted pastures on that part of the land where the largest possible centre pivot can be operated. Melons, preferably under drip irrigation. Due to the fact that marketing can take place gradually over a fairly long period of time, and also due to crop rotation practices, provision must be made for four blocks of 1 ha each. These blocks must be irrigated either individually or in succession (all above ground equipment must therefore be identical and interchangeable). Citrus on an area of approximately 5 to 6 ha. The rest of the identified area will consist of pastures under a drag-line sprinkler irrigation system. The following five steps do not have to be executed in this particular order. Due to the interacting nature of the decision making process (refer Figure 11.1) execution can even take place simultaneously: Soil survey/analysis Climatic study Crop study Water source survey/hydrological study Management aspects 2 Soil survey and analysis (Consult Chapter 3 : Soil for more detail) 2.1 Identify potentially arable soil On farms with an abundance of available land, it is good practice to provisionally identify areas fit for crop cultivation under irrigation on a visual basis, especially with regard to the following considerations: Conspicuous quality aspects, especially e.g.: soil type, texture, etc. Positioning of new system, especially from logistic and economical view points Positioning in relation to existing irrigation or agricultural activities Topographical position with regard to water source (pump or gravity feed) Land gradient with regard to requirements and limitations of the type of irrigation system Natural and other obstructions and interruptions 2.2 Investigation The next step is to thoroughly investigate areas provisionally identified: Profile pits must be dug and a complete soil description done (note restrictive layers!). Samples taken must be analysed with respect to physical and chemical properties from an irrigation point of view.

6 11.4 Irrigation Design Manual 2.3 Select irrigable land Priorities must now be drawn up with respect to soil, water and crop properties in selecting the final irrigation area. 2.4 Soil conservation The Act on Conservation of Agricultural Resources (Act No 43 of 1983) requires certain control measures to be taken on cultivated land to protect such land against water and wind erosion and/or prevent or control waterlogging or brackishness. Furthermore the cultivation of new lands is subject to written permission by the Executive Official before any work commences. Designers must be aware that soil conservation forms an integral part of irrigation design. Soil conservation technicians from the Department of Agriculture s offices in the relevant area are usually available by appointment to lend assistance and advice in this regard. 2.5 Norms Potentially irrigable lands must conform to the SABI norms for soil choice in the particular area in which the system falls. Example (continued) In this particular case the area within the surrounding dotted line on the plan was identified as potentially irrigable land. Note: From 2.1: The serious limitations created by the network of high voltage power lines especially with respect to the centre pivot. (Two positions for the centre pivot were initially considered, but the full circle, as shown, proved to be both larger and less costly than the alternative semi-circle, further to the right.) The same powerline, however, offers pump-power points at both balancing dams. Roads, obviously poor and/or shallow soils, dongas and other obstructions determined the positions of the remaining boundaries. From 2.2: Soil analysis of 11 samples, originally taken from 13 profile pits, was done. Due to similarities, numbers 8 and 11 were combined with others. No physical or chemical restrictions were found. Water-holding capacity between -10 and -100 kpa (WHC 100 ) 120 mm/m. Infiltration rate 10 mm/h.

7 Figure 11.3 : Profile description Planning 11.5

8 11.6 Irrigation Design Manual From 2.3: The provisional suggested location of the different systems was based on: The unavoidable centre pivot position The expediency to operate the drag line system as a homogeneous unit The soil analysis showed that the best soils for the drip and micro sprinkler systems were in the region of especially profile pits 2 and 3. From 2.4: The farm in question is situated in the Eastern Cape Province and resorts under the Eastern Cape branch of SABI. The selected areas must therefore meet the "SABI norms for the choice of land for irrigation in the Eastern Cape region". 3 Climate (Consult Chapter 4: Crop-water relationships and climate for more detail) Weather data from the closest weather station to the particular irrigation system is now required. Careful selection will ensure that micro climatic differences are taken into account in making reasonably accurate data available. Example (continued) Due to the fact that no direct data for Cookhouse is readily available, data for Somerset East will be adapted for this purpose. Table 11.1: Adapted evaporation and rainfall records for Cookhouse Month Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Total A-pan evaporation (E o ) [mm] Average rainfall (R) [mm] Crop study (Consult Chapter 4: Crop-water relationships and climate for more detail) 4.1 Select crop There are two different approaches to crop selection Consultation approach The grower requests advice from a designer (possibly in conjunction with a crop expert). This method is seldom used.

9 Planning Instruction approach Normally, at this stage, the grower would already have determined the following: Marketability, profitability and logistics Compatibility to existing crops Availability and/or reliability of labour Existing cultivation and spraying equipment Local conditions like theft potential Personal preferences and/or limitations Therefore all that now remains is to determine if the particular crop is compatible to the: Soil (physically, chemically, limitations) Water (some crops are for example more tolerant to salt than others) Climate (frost resistance, heat, humidity, etc.) Example (continued) In this example, the grower has made the crop selection and merely informed the designer thereof: Citrus (approximately 5 to 6 ha) spaced 6 m 3 m in groups of approximately 1 to 1½ ha Tramway planted melons ( 4 1 ha blocks ), at 2,76 m row spacing (plants 0,3 m in row) Planted pastures (remaining area) 4.2 Crop water requirement Crop water requirement (CWR) [mm/period] is the depth of water required by the specific crop for evapotranspiration (ET) during a specific period. (This excludes system operating losses and is determined by the sum of the net irrigation requirement (NIR) and the effective rainfall (Re). 4.3 Crop factor The crop factor (f) is the relationship between crop evapotranspiration (ET c ) and pan evaporation (E o ) for a crop of a certain age, in a specific growth stage, with a certain amount of leaf cover, which is found in a specific climatic region. ET f E0 c (11.1) where ET c crop evapotranspiration [mm/period] E o pan evaporation [mm/period] f crop factor [fraction] Obtain the crop factors for the selected crops and/or cultivars from existing literature or other scientific sources. Alternatively, the crop evapotranspiration can be determined by means of SAPWAT (See Chapter 4: Crop water relationships and climate).

10 11.8 Irrigation Design Manual Example (continued) Table 11.2: Crop factors for pastures (Green book) Period Crop factor 1 July - 31 August 1 September - 30 September 1 October - 31 October 1 November - 31 March 1 April - 30 April 1 May - 31 May 1 June - 30 June 0,5 0,6 0,7 0,8 0,7 0,6 0,5 Table 11.3: Crop factors for citrus (Eastern and Western Cape) Period Crop factor July-November December January February March April May June 0,45 0,50 0,55 0,60 0,65 0,70 0,65 0,60 Crop factors for melons were less readily available. The growing season is approximately from October to December/January. Western Cape data is adapted for cultivar and plant density: Table 11.4: Adapted crop factors for melons Period Crop factor October November December January 0,3 0,5 0,7 0,6

11 Planning Net irrigation requirement Net irrigation requirement (NIR) [mm/period] is the depth of irrigation water required for the plants' evapotranspiration (ET) [mm/period] in its specific growth phase and during the specific period. The determination of ET may be done according to various methods as shown in Chapter 4: Crop water relationships and climate. As the A-pan evaporation and adapted crop factor method is presently the best known and most generally used, it will be used in the following example: Example (continued) The seasonal water requirement of the relevant crops will now be determined from the adapted climatic data and the applicable crop factors, as shown in Table 11.5 : Table 11.5: Calculation of net irrigation requirement Month Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Tota l 1 E o [mm] R [mm] f [frac] Pastures Citrus Melons ET [mm] Pastures Citrus Melons R e [mm] NIR[mm] Pastures Citrus Melons where All data monthly averages Row 1: E o A-pan evaporation Row 2: R rainfall Row 3: f crop factor Row 4: ET c evapotranspiration ( ET c f E o ) Row 5: R e effective rainfall ( R e ( R - 20 ) 2 ) Row 6: NIR nett irrigation requirement ( NIR ET - R e )

12 11.10 Irrigation Design Manual From the above the NIR of the relevant crops will be: Pastures: 998 mm/year Citrus: 712 mm/year Melons: 350 mm/year Normally, as a relatively conservative approach, ET figures are used to determine system capacity. 4.5 Planning models As described in the previous section, there is a worldwide trend to move away from the use of A-pan evaporation for determining irrigation requirements. In South Africa, the Department of Water Affairs and Forestry has identified the SAPWAT program as the official method for irrigation requirement planning. SAPWAT makes use of historical weather station data obtained from the South African Weather Service to calculate reference evapotranspiration by means of the Penman-Monteith equation and then calculating the irrigation requirement by means of adjustable crop coefficients. This method can be used as alternative in the planning diagram, (see Section 11.1 Planning diagram: Discussion of model and equations) for determining the crop evapotranspiration (ET c ) before consideration of rainfall. For a complete description of SAPWAT, as well as a series of tables with calculated irrigation requirements, see Chapter 4: Crop water relations and climate. There are other proven models to be used for planning and/or scheduling. A few of the models are discussed briefly and more information is available from the report An investigation into software for irrigation scheduling of the ARC-ILI (Jordaan, 2000) BEWAB BEWAB (Irrigation Water Management program), is a water balance model which uses research data to make irrigation recommendations. The water use of the crop is projected according to dayto-day irrigation requirement curves determined by historical measurements. Prof. A.T.P Bennie developed the BEWAB program on account of the Water Research Commission report, 'n Waterbalansmodel vir besproeiing gebaseer op profielwatervoorsieningstempo en gewasbehoeftes, (Bennie et al, 1988) Donkerhoek data irrigation scheduling program The Donderhoek Data Irrigation Scheduling Program makes use of up to date weather data, to make an irrigation recommendation on a daily basis, so that optimal irrigation can be done. The program contains a function that can control the opening and closing of valves in the field. The program was developed by Donkerhoek Data (Pty) Ltd and Mr. T. du Preez. The total development of the program was funded by Donkerhoek Data (Pty) Ltd. The program is used mainly by commercial farmers and consultants in the Western Cape and along the Orange River. The program contains an option for calculating a water budget for a season or part thereof. Historical data can be used for this purpose.

13 Planning SWB SWB (Soil Water Balance) is an irrigation scheduling model which uses current climatic data to simulate the salt balance and soil water balance of generic crops. With sufficient weather, soil and crop data, it gives a complete description of the soil-plant-atmosphere continuum. The model contains sufficient data and equations to simulate plant growth mathematically. SWB is based on the improved general crop reproduction of NEWSWB. The program was developed by the University of Pretoria s Plant Production and Soil Science Department and dr. N Benadé of NB Systems. The program was funded by the Water Research commission, the University of Pretoria, Chamber of Mines, Agricultural Research Council s Institute for Vegetable and Ornamental Plants, Potatoes, South Africa and Langeberg Foods. SWB is mainly used for actual current irrigation scheduling. Researchers, commercial farmers, irrigation officials and consultants are the main users of the program VINET 1.1 VINET 1.1 (Estimating Vineyard Evapotranspiration for Irrigation System Design and Scheduling) is designed to assist the producer with the decision-making process on when, how much and how long irrigation must be applied. In the past, decision-making was hampered due to the variation between vineyards because there are differences between foliage, soil and climatic factors. VINET 1.1 was developed by Dr. P.A. Myburgh and Mr. C. Beukes of the ARC-Nietvoorbij. The research was partly funded by Dried Fruit Technical Services, Deciduous Fruit Producer s Trust and Winetech. VINET 1.1 is currently used by commercial farmers, consultants, engineers and small farmers. 5 Water source survey and hydrological study (Consult Chapter 5: Water for more detail) 5.1 Survey A survey, regarding all aspects of the water source to be used for a specific system must be done. It broadly entails a study of: Source discharge Determine the nature and strength of the source discharge either in terms of volume, flow rate, stability, minimum delivery or continual availability Source share Is the grower the sole consumer of the source (river, stream, fountain, etc.) or are there others with shared rights to the same source? What is the grower's share? How is the sharing managed? Is the source shared between different irrigation systems on the same property? If so, in what proportion? How much water is available for this system?

14 11.12 Irrigation Design Manual Available pressure Does the source supply water under pressure? If so, what are the normal and minimum assured pressures? Location Is the source far away? Does this influence costs and/or practical operating aspects? Do transport losses occur, and to what extent (e.g. earth channels)? Are evaporation losses (e.g. dams) a factor, and to what extent? Does the topography influence the supply system and what are the practical implications? Quality (physical and chemical composition) Water quality must comply to requirements as stipulated in Chapter 5: Water. 5.2 Water sources This study is required for the following general sources: Perennial stream or river Water rights: Legal share of the source and limitations on its use Source stability: Tested delivery during normal dry cycle as well as during peak crop demand and especially long-term consistency Stored natural run-off Hydrological investigation of catchment area with the emphasis on long-term consistency Water yield (net i.e. after allowing for normal evaporation) and balancing ability of the storage facilities Water supply authority Allocation/listing: Maximum yearly volume (or volume/ha) available Delivery method: Constant withdrawal or in turn Discharge: Guaranteed flow rate Discharge pressure: Minimum pressure at which water is supplied from a pipe network Borehole water The discharge must be tested according to the norms as stipulated by the Borehole Association of Southern Africa in their publication "Minimum code of practice for borehole construction and pump installation" Balancing facilities should be supplied where limited pumping hours are used in the irrigation system, while withdrawing continuously from the borehole(s) Example (continued) In this case, the following data applies: Source: Orange/Fish River Scheme, Hougham-Abrahamson Irrigation Board. Delivery: Existing canal system Water rights: 420 ha

15 Existing systems: 310 ha Allocation: m 3 /ha per year (gross, measured on site) Discharge: On demand Balancing facilities: Two earth dams as shown on figure 11.2 ( m 3 each) Water quality: Tested and meets required standards Planning This indicates that: The balancing capacity, demand and discharge situation is endlessly adaptable for normal 5 or 6 or 7 days per week operating procedures due to the fact that discharge takes place on demand and that existing sufficient balancing capacity is likely. Balancing capacity should, however, be checked in the water balance Provision must be made in the water balance (section 10) for evaporation from the balancing dams 6 Management aspects No recommendation on an irrigation system can be made in the planning phase without determining the grower's management potential. While a designer may not consider this to be his function, it will be disastrous if a system is recommended (or even demanded by the grower), which requires management skills beyond the grower's ability. Although it cannot be expected of a designer to be an expert in social sciences as well as his field of expertise, a series of guided observations can be advantageous in the process of final system decision making. During a recent scientific study on this subject, an attempt was made (using the PC programme SAPFACT) to prioritise the important factors in a specific decision-making format. Information is discreetly obtained during the initial visit and conversation with the grower and even his family, later to be entered into an extremely user-friendly PC package. Entries may be edited at will, producing immediate results. Each of the six following identified aspects consists of eight factors as indicated. Irrigation management Use and maintenance of equipment, design and installation of equipment, annual water supply, regularity of water supply, understanding of irrigation, attitude towards water management, applicability of irrigation methods, scheduling practices. Crop profit potential Soil suitability, climate suitability, alternative crop possibilities, crop yield, production costs, gross margin potential, market and rate risks, production risks. General management Supervisory support, personal supervision, seasonal planning, recording system, receiving help and advice, management training and experience, management structure, long-term planning. Labour management Labour organisation, labour situation, attitude towards legislation, remuneration, development actions, training inputs, labour efficiency and supervisory contribution. Grower's personal aspects Life style, career stage, property plans, decision making, tension, family aspects, spouse support on farm, community involvement. Financial situation Accounting services, credit sources, accessibility to income, effect of inflation, marketability of farm, bond status, size of venture, income expectations. Each of these aspects are evaluated and the findings, although at times subjective, can form a good guideline for the process, if correctly combined. A study hereof, together with the PC programme SAPFACT, can make a valuable contribution to sensible decision making.

16 11.14 Irrigation Design Manual 7 Selection of irrigation system (Consult Chapter 10: Irrigation systems for more detail) The diagram as shown in Chapter 10 may be utilised to obtain an indication of how the selection process ought to take place. There are, however, practical considerations which may move the emphasis to a different choice from the initially logical one. Naturally the grower's priorities are to be considered in the process. Example (continued) Consideration: Pastures: Labour is scarce and unreliable mechanise therefore limiting drag-lines to a minimum centre pivot for the largest possible area Consideration: Melons (and some similar runner crops) are very dependent on pest and plague control substances which should not unnecessarily be washed from the leaves. Therefore drip irrigation seems appropriate (also from a labour point of view) Consideration: Citrus mostly performs as well under micro sprayers as with drip irrigation. However, due to the larger soil water reservoirs created by micro sprayers, they can be operated with a lower level of management inputs. Consideration: Consolidate control and filter installation in one place therefore drip and micro close together 8 Survey A contour plan of the potentially irrigable soil is required at this stage to provide the planner with an initial accurate account of especially relevant boundaries, restrictions, areas, slopes as well as location of water sources. As the same plan will later be used for detail design, the scale, contour intervals, detail and accuracy should be combined in such a way that the particular irrigation system required or recommended can easily and accurately be designed and that survey and design data can readily be recorded. Naturally a centre pivot survey may be considerably less accurate than one for a drip or micro sprayer system. For the latter system it is even required to be able to indicate each planting row (and also the number of plants per row) as equipment is supplied per plant unit. The design method for micro irrigation also requires high topographical accuracy. The following scale and contour interval combinations are generally used: Contour interval Smallest scale Micro irrigation : 0,5 m 1: 500 (narrow row spacing: 3 m ) 1,0 m 1: (wide row spacing: > 3 m ) Sprinkler irrigation : 1-2 m 1: Centre pivots : 2-5 m 1 : Flood irrigation : 0,5 m 1 : Example (continued) In this case the survey had to be done for approximately 10 ha of drip and micro systems for which the location had not yet been fixed. The whole area has a symmetrical shape and topography thereby making it possible (and necessary) to survey it to a level of accuracy which would make 0,5 m contour intervals possible.

17 Planning Irrigation requirement Subsequently the crop irrigation requirement must be determined. 9.1 Net irrigation requirement Net irrigation requirement (NIR) [mm/period] is determined as explained in Section 4.4. Example (continued) According to Table 11.5 in Section 4.4, the NIR of the relevant crops will be: Pastures : 998 mm / year Citrus : 712 mm / year Melons : 350 mm / year 9.2 System efficiency System efficiency ( s ) [% or fraction] is the efficiency with which water is delivered from the irrigation dam or supply point on the farm boundary through the irrigation system to the soil. Example (continued) According to the accepted SABI norms of the East-Cape, s [fraction] is: Centre pivot : 0,85 Sprinkler system : 0,80 (drag-line) Micro system : 0,85 Drip system : 0, Gross irrigation requirement Gross irrigation requirement (GIR) [mm/period] is the net irrigation requirement (NIR) of the crop plus the operating losses of the system (this does not include effective rainfall (R e )). NIR GIR s (11.2) Example (continued) The GIR of the relevant crops can now be calculated according to equation 11.2, with NIR's as determined in Section 9.1, and with system efficiencies according to Section 9.2: Pastures (centre pivot) 998/0, mm/year Pastures (drag-line) 998/0, mm/year Citrus (Micro) 712/0, mm/year Melons (Drip) 350/0, mm/year

18 11.16 Irrigation Design Manual 10 Water balance Subsequently it is necessary to check that the supply of irrigation water from the source is sufficient to satisfy the crop's total gross irrigation requirement. Consult Chapter 5: Water for more information. Example (continued) The GIR of the individual crops is now determined and converted to a volumetric basis. Thereafter, the sum of these results is added to the balancing dam evaporation (also volumetric) to obtain the total amount of water required per annum. This total is then compared to the total amount of water available for the irrigation system. Evaporation from the balancing dams which have a constant water level throughout the year is determined as follows: Evaporation A-pan evaporation [mm] dam area [ha] 10 0, ,5 10 0, m 3 per annum The amount of water available is determined as follows: Available water Allocation [m 3 /ha per annum] area [ha] , m 3 per annum The water balance is shown in Table Table 11.6 : Water balance Crop / system Area [ha] Total GIR [mm/ha per year] Total GIR [m 3 /ha per year] Total GIR [m 3 per year] Pastures / centre pivot Pastures / drag-line Citrus / micro Melons / drip Total GIR water required [m 3 /year] Evaporation : balancing dams [m 3 / y] Total volume of water required [m 3 / y] Total volume of water available [m 3 /y] Conclusion: With sound planning, high efficiency (systems) and good management and scheduling, this grower has sufficient water to meet his requirements. In addition, it appears that the soil, crops and management aspects satisfy the grower's instruction to the designer and that the scheduling planning can now begin.

19 Planning Scheduling planning The purpose of this study is to determine the quantity of water required by crops per cycle during peak demand periods and how often it is to be applied, taking practical operating practices into account. It is important to note that long-term data is used in providing average peak conditions, especially seeing that these calculations influence the determination of system capacity. In addition to a study of water sources, the four most important components of the scheduling function were considered, namely: Soil (refer to Section 2) Climate (refer to Section 3) Crop (refer to Section 4) Irrigation system (refer to Section 7) To this the management aspects of each of these components (as discussed in Section 6) are added. Inputs and aspects of all these components are integrated in compiling practical values for: Cycle length: The time lapse, during peak demand, between the start of consecutive irrigations, is based on the available water in the soil, the plants' requirement and its ability to utilise the available water. Gross irrigation requirement per cycle: This describes the amount of water to be applied by the system per cycle, to replace that used by the plant. Standing time: The irrigation time per cycle required to meet the GIR. It is very important to make the necessary adjustments so that practical irrigation activities will tie in with established farm practices. Emitter discharge: Emitter choice is an important part of the whole process. The discharge (at acceptable economical operating pressures) as well as distribution properties as regards crop and soil type, must inevitably meet all the above-mentioned requirements. All the inputs and aspects of the five main scheduling components consist of variables, some of which are "not negotiable", while others may be considered reasonably "variable". Careful and sensible application of variables can, however, ensure practically acceptable scheduling in virtually all cases. The composition of the respective components, the inputs regarding each one as well as the development of the required results are shown in Figure 11.3 which follows. The equations used in the integrated calculation process are shown in Figure 11.4 thereafter and a complete calculation is shown at the end of the chapter in Figure 11.5 as a practical example. The diagrams contained in the above figures are self explanatory. They are illustrated with a set of notes aimed at highlighting certain important concepts as well as supplying practical hints. Inputs as required from the main components (climate, crop, soil and irrigation system) are covered in detail in the specific chapters and a study thereof will contribute to better understanding of the diagrams.

20 11.18 Irrigation Design Manual Figure 11.4: Scheduling: The planning function (model)

21 Figure 11.5: Scheduling: The planning function (equations) Planning 11.19

22 11.20 Irrigation Design Manual 11.1 Planning diagram: Discussion of model and equations (refer Figures 11.3 and 11.4) All the main components, and the necessary inputs they supply, are arranged so that all inputs relating to each other for a specific function are placed adjacent to each other Net irrigation requirement (NIR) The inputs required for determining the crop water requirement (and their composition) are shown between the left border and the first vertical dotted line on Figure The only inputs required for this determination are all climatic factors and one crop input, namely crop coefficient. As described in Chapter 4: Crop water relationships and climate, NIR may be determined by a number of methods. In this case A-pan evaporation and crop factors are used. As an input, rainfall (R) is only used in determining effective rainfall (R e ). R e ( R - 20 ) 2 (11.3) where R e effective rainfall [mm/mo.] R measured rainfall [mm/mo.] Effective rainfall is only of importance in areas where its consistence justifies it as a factor. In by far the most cases rainfall is so unreliable that it is regarded as negligible in this example. Class A-pan evaporation (E o ) consists of long-term statistical data, based on average monthly values. Crop factors (f), obtained from reliable scientific sources, for the corresponding month of the E o data, are used. Note that crop factors do not necessarily follow the same pattern as evaporation and that data for both E o and f should be used for the specific month where the product of these two inputs is the highest. ET c is determined by these inputs. ET c f E o (11.1a) where ET c crop evapotranspiration [mm/mo.] f crop factor [fraction] E o class A-pan evaporation [mm/mo.] Alternatively the crop evapotranspiration can be calculated by means of the Penman-Monteith method, as described in Chapter 4: Crop water relationships and Climate. ET c k c ET o (11.1b) where ET c crop evapotranspiration [mm/period] k c crop coefficient [fraction] ET o reference evapotranspiration [mm/period]

23 Planning Net irrigation requirement per month (NIR m ) can therefore be determined from these inputs as well as net irrigation requirement per day (NIR d ). NIR m ET - R e (11.4) NIRm NIRd (11.5) n where NIR d net irrigation requirement per day [mm] NIR m net irrigation requirement per month [mm] n number of calender days in relevant month [d] The fact that the NIR d as calculated here, can be less than the actual maximum NIR d, must be taken note of. This can result in the system capacity being insufficient during a certain hot period and may lead to irreclaimable losses. The designer must therefore always compare the average NIR d with the reported daily values (based on historical weather data, if possible) and make the necessary adaptations. Equations 11.1 to 11.5 can therefore be combined as follows: Readily available water ETc - R e NIRd (11.6) n The inputs used to determine the quantity of ground water "readily" available to the crop, before replacement is necessitated, appear between the two dotted-line boundaries on the diagram. While most of these inputs are covered in detail in the relevant chapters, a few warrant further attention. Effective root depth is considered as the shallower of natural root depth (NRD) and effective soil depth (ESD) according to the following equation: ERD min ( NRD and ESD ) (11.7 where ERD effective root depth [m] NRD natural root depth [m] ESD effective soil depth [m] The combination of allowable water depletion () and the soil's water-holding capacity (WHC) is approached in practice according to two different methods. On the one hand (probably the popular method), use is made of laboratory determined water-holding capacity between -10 kpa and -100 kpa ( WHC 100 ) and a definite, adapted set of values for a specific crop. On the other hand the water-holding capacity between -10 kpa and kpa (WHC 1500 ) is used with a different set of values for the same crop. Note that, while both methods produce approximately the same results, care must be taken in using the relevant and WHC values so that those used are applicable to the particular calculation.

24 11.22 Irrigation Design Manual Percentage wetted area (W) is equally important. The soil water reservoir is often limited by properties of the specific irrigation method (especially features of emitters and their positioning), therefore the plant only has this limited supply to draw from. The limitation per plant is therefore determined by expressing the wetted area per plant as a percentage of the soil area occupied by the plant. This wetted area may vary from single wetted spots (drip system on citrus or single micro sprayers on large nut trees or avocados) or even wetted strips (drip systems on vineyards or e.g. strip wetting by means of micro sprayers on citrus). Available water (AW 100 ), and the applicable readily available water (RAW), can therefore be determined from the inputs in this section by means of the following equations : AW SWC ERD (11.8) where AW 100 available water in soil [mm] SWC 100 water capacity of soil between -10 kpa and -100 kpa [mm/m] ERD effective root depth [m] RAW 100 AW 100 (11.9) where RAW readily available water [mm] 100 allowable water depletion [%] Summarising: Equations 11.7 to 11.9 can therefore be combined as follows: 100 RAW SWC100 ERD 100 (11.10) Cycle length The cycle length (calendar days) is determined by dividing the plant's daily water requirement (NIR d ) into the total amount of readily available water (RAW) per cycle. The theoretical figure is therefore obtained from the inputs set out in and RAW W tc (11.11) NIR 100 d where t c cycle length [ calender days] RAW readily available water [mm] NIR d net irrigation requirement per day [mm/d] W wetted area as percentage of total soil area per emitter [%] Further considerations to produce a practical cycle are as follows: In practice most growers prefer to schedule their irrigation on a week-based system once per week (every 7 calendar days), twice per week (every 3½ calendar days), thrice per week (every 2,3 calendar days) and even daily irrigation (i.e. every calendar day).

25 Planning The shorter the cycle, the more difficult it becomes to manage sensibly when only a limited number of working days per week are used. It can only be run efficiently when use is made of a computerised irrigation control system. This aspect must be cleared up with the grower before a practical cycle length is chosen. The selected practical cycle length must then be thoroughly tested to the norms and limitations of all inputs possibly influenced by this alteration, especially where the practical cycle is longer than the theoretical figure. (The latter situation may cause serious problems in cases where water shortages, which may arise from such a decision, cannot be supplemented in time.) A process of multiple iteration may be used to re-assess some or all inputs Net irrigation requirement per cycle Net irrigation requirement per cycle (NIR c ) is purely the product of net irrigation requirement per day ( NIR d ) and cycle length ( t c ) as determined in and above. NIRc NIRd tc (11.12) where NIR c net irrigation requirement per cycle [mm] NIR d net irrigation requirement per day [mm/d] t c cycle length in calendar days [d] Gross irrigation requirement per cycle This figure is determined by incorporating system efficiency ( s ) with net irrigation requirement per cycle (NIR c ). 100 GIR c NIRc (11.13) s where GIR c gross irrigation requirement per cycle [mm] NIR c net irrigation requirement per cycle [mm] s system efficiency [%] At this stage it has been determined that a certain amount of water (GIR c ) must be applied every specific number of calendar days (t c ) Standing time The standing time per cycle (t s ), i.e. the number of hours during which irrigation must take place on every block during a cycle, must now be selected. This determination takes place in close consultation with the grower and the following facts must be thoroughly considered: The type of irrigation system (fixed lay-out or moveable) The availability of labour for moveable systems The minimum amount of daylight hours for moveable systems Stage of automation and/or computerisation Management aspects A few alternatives may be determined, of which the most acceptable will be used as an initial input in the next step of the process.

26 11.24 Irrigation Design Manual Emitter discharge Emitter discharge ( q e ) can therefore now be calculated according to the already known factors GIR c and t s. GIRc qe A (11.14) ts where q e emitter discharge [/h] GIR c gross irrigation requirement per cycle [mm] t s total standing time (or irrigation time) of emitter per cycle length [h] A total theoretical soil area served by one emitter [m 2 ] L d L e [m²] where L d lateral spacing [m] L e emitter spacing [m] Note that GIR c is calculated in terms of application over the entire soil area These results (theoretical) must be compared to practical aspects associated with available emitters (especially preferences regarding quality, service, material properties and composition, required or available operating pressure, distribution pattern, etc.) and altered if necessary. Alterations may be done both in the form of operating pressure (the same emitter) or reselection of alternative emitters available. Note, however, that any alteration to q e at this stage may require alterations in terms of emitter spacing (L e ) which can have a rippling effect on percentage wetted area (A) and eventually may possibly even influence cycle length (t c ). Smaller alterations, however, can possibly be absorbed in the process, in the parameters set for other inputs like allowable water depletion () and even water holding capacity (WHC) both of which should be considered with great caution! Net application rate The net application rate is now determined by comparing the final emitter discharge to the effective wetted area of the specific emitter, taking application efficiency into account. NAR q A b e a 100 (11.15) where NAR net application rate [mm/h] q e emitter discharge [/h] A b effective wetted area of emitter [m 2 ] a application efficiency [%] (system efficiency, excluding all transport losses) This result must now be checked with the infiltration rate of the soil.

27 Figure 11.6 : Scheduling : The planning function (example) Planning 11.25

28 11.26 Irrigation Design Manual Gross application rate The gross application rate is now determined by comparing the final emitter discharge to the effective wetted area of the specific emitter: GAR q A e b (11.16) where GAR gross application rate [mm/h] q e emitter discharge [/h] A b effective wetted area of emitter [m 2 ] This result must comply with the norms for minimum emitter application rate Planning diagram (refer to Figure 11.5) Example (continued) Figure 11.5 shows an example of scheduling planning, based on the citrus component of the continuous example used as practical illustration in this chapter. The inputs, as shown below, were continually accumulated during the planning process and are now combined to form a complete image. Information Climate: (refer to Table 11.1) Although rainfall is recorded for this month, it will not be taken into consideration for system capacity purposes From the data in this table, the average class A-pan evaporation for January is 213 mm Crop: (refer to Section 4.3) From the information in Table 11.3, the crop factor for January is 0.55 General crop knowledge determines that citrus can possibly withdraw 50% of WHC 100 without damage to plant or crop yield and that NRD is in the region of 0,6 m. A row spacing of 6 m, with 3 m between trees, is generally used in this area Soil: (refer to Section 2.2) No restrictive layer in the soil was encountered, therefore a nominal value of 1,0 m (deeper than NRD) will be entered for this purpose The soil analysis shows a WHC 100 of 120 mm/m System : (refer to Section 7) During the system selection process it was decided, due to a variety of relevant reasons, to make use of micro sprayers with the following limitations: Scarce and costly labour forces the system towards a cycle with approximately a week interval A reasonably large reservoir will therefore be strived after therefore aim at strip wetting of approximately 80% of the row width Computerisation is not being considered at this stage, therefore a maximum of three shifts per day can be made. Initially a standing time of 8 hours will be maintained, with three sets per day Emitter spacing can comfortably coincide with plant spacing as indicated above and following a product study it has provisionally been decided to use an emitter which will probably meet all the requirements in terms of spacing (6 m 3 m), gross application rate ( 3 mm/h), distribution pattern and radius (80% wetted strip width) According to Section 9.2 the system efficiency is 85%.

29 Planning Solution : Although all the individual calculation steps are shown in logical order on the diagram in Figure 11.5, only the summarising equations will be used in the solution. Net irrigation requirement per day According to equation 11.6 : NIR d f E o - Re n ( 0, ) ,8 mm/d Readily available water According to equation : SWC ERD ,6 100 RAW ,0 mm Cycle length According to equation : t c RAW W NIRd ,0 80 3, ,6 calender days This cycle length, however, is theoretically determined and must now be adapted to a more practical figure. The grower initially indicated that a weekly cycle would suit his operating system well. The practical solution would therefore now be to choose a cycle length of seven calendar days and then, by means of continuous iteration, determine firstly how inputs will be influenced by this decision and secondly if all requirements, norms and limitations are still met. In this particular case merely one calculation proved that only AWD () needs to be altered to a level of 46% a figure which is quite acceptable. Note two important aspects: Any of the more negotiable values (or even a combination thereof) may be tested and/or altered, within limits, for this purpose Potential danger lurks in the lengthening of practical cycle length above the theoretical figure. If it must be considered at all, it must be done with great caution, and management (skills) play a dominating role.

30 11.28 Irrigation Design Manual Gross irrigation requirement per cycle Substitute equation into equation 11.13, then: GIR c NIR d t c 3, mm(practical) s Note that the practical cycle length is incorporated in this equation. The GIR of 31 mm is therefore lower than the theoretical figure of 34 mm. Emitter discharge According to equation : q e GIRc A t s 31 6,0 3, /h This result was compared to the original emitter selection and found to be acceptable. The specific emitter meets all the requirements in terms of discharge, working pressure and wetted diameter (approximately 80% of the lateral spacing) and is also popular in the industry due to quality, cost and service aspects. At this stage, any alteration to selection will inevitably have to be checked against all other requirements by continuous iteration. For example, a different wetted diameter will influence the percentage wetted area (A), with the subsequent repercussions. Net application rate Net application rate is now calculated by means of equation : q e a NAR Ab ,8 3, ,1 mm/h Gross application rate Gross application rate is now calculated by means of equation : GAR q e Ab 70 4,8 3,0 4,9 mm/h This conforms to the general norm which requires that GAR 3 mm/h for micro sprayers.

31 Planning This illustrates the firm connection of all the inputs and an acceptable solution for each situation which may arise may be obtained by means of continuous iteration. Both the parameters in which inputs are limited and the spectrum that available equipment covers, allow sufficient room to accommodate the few fixed inputs, even if computerising the operation system is considered a "way out". 12 Theoretical size of irrigation group The ideal group size in any system is that area in which total emitter discharge is equal to the flow in the system System flow System flow (Q) is calculated as follows: Q GIRc t A T 10 (11.17) where Q total system flow per crop [m³/h] GIR c gross irrigation requirement per cycle [mm/cycle] A T total system area [ha] t operating hours per cycle [h] GIR c is determined by equation A T is the area of the total zone allocated to a crop or group of crops with identical physical layout and requirements. t is determined by taking the total number of working hours into account, which in turn depend on practical circumstances (grower, system, labour, etc.). t should therefore be in multiples of the standing time (t s ) Number of emitters per group The number of emitters per group (n e ) is calculated as follows: where n e number of emitters Q system flow [m³/h] q e emitter flow [/h] 12.3 Area of group Q ne 1000 (11.18) q The area of a group (A g ) is calculated as follows: e A g ne Ld L e (11.19) where A g area of group [ha] n e number of emitters L d lateral spacing [m] emitter spacing [m] L e