Introduction to Overhead/Sprinkler Irrigation Design N.L Mufute , LWRM, MSU mufutenl@msu.ac.zw / mufutengoni@gmail.com

Introduction Sprinkler Irrigation systems are systems which usually operate as pressurized piped systems delivering water to the crop by means of sprinklers. Sprinkler irrigation is suited for most crops Sprinkler systems can have unique uses such as seeding germination, Frost protection and fertigation.

Factors to consider when selecting Sprinkler Systems Slopes,- : Sprinkler irrigation is adaptable to any farmable slope,(uniform or undulating). Soils,- : Sprinklers are best suited to sandy soils with high infiltration rates although they are adaptable to most soils. Water,- : Clean supply of water, free of suspended sediments/heavy metals, is required to avoid problems of sprinkler nozzle blockage and corrosion.

Advantages/Disadvantages of sprinkler irrigation High suitability to various slopes Micro-climate modification Low labor input (comparable to surface) Easily automated High uniformity (compared to surface irrigation) DISADVANTAGES High initial cost (compared to surface irrigation) High operation cost (e.g due to pumping) Not suitable for some crops (e.g those do not require repeated wetting of leaves or with flowers that can be dislodged ) Canopy interception Wind effect is considerable

Types of Sprinkler Systems Permanent (Solid-set) system Semi-portable (Hand-moved) system Portable (Movable) system Side-roll system Center pivot system Linear move system Big gun /rain gun system

Components of a sprinkler system

DESIGN PROCEDURE: HAND MOVE SYSTEMS. The procedure for designing sprinkler systems can be divided into two phases: Preliminary design steps Adjustment or final design steps   Preliminary design steps comprise the procedure for synthesizing farm data in order to determine preliminary design parameters, which will be needed in the final design adjustment process. The final design steps reconcile the preliminary design parameters obtained with the irrigation equipment performance characteristics, as well as human, physical and financial factors.

Design Procedure for periodic-move sprinkler systems. Source: FAO, 2002

Steps in Sprinkler Irrigation Design Step 1: Collect basic resource data at farm The data include: A topographic map showing: The proposed irrigated area, with contour lines Farm and field boundaries and water source or sources Power points, such as electricity lines, in relation to water source and area to be irrigated, roads and other relevant general features such as obstacles Data on water resources, quantity and quality over time, on water rights and on cost of water where applicable The climate of the area and its influence on the water requirements of the selected crops The soil characteristics and their compatibility with the crops and irrigation system proposed The types of crops intended to be grown and their compatibility with both the climate in the area, the water availability and the soils; current agricultural practices should be identified

Steps in Sprinkler Irrigation Design Step 2 : Analyze the farm data in order to determine the following preliminary design parameters: peak and total irrigation water requirements infiltration rate of soils to be irrigated maximum net depth of water application per irrigation irrigation frequency and cycle gross depth of water application preliminary system capacity To achieve this designer will; Determine moisture extraction depths (root zone depths=RZD) Calculate Peak Water consumption use (Etc) Define allowable depletion (P) Calculate net irrigation application (dnet) Calculate gross depth of application (dgr) Calculate the irrigation frequency and cycle Calculate preliminary system capacity (Q)

The final design process Involve: identification of irrigation system options with farmer participation preparation of system layout for the field shape and topography the hydraulic design and iterative adjustments irrigation equipment selection taking into consideration economic and financial aspects final irrigation system selection as well as options, taking into consideration farmers' preferences, management capabilities, labour aspects, financial capabilities and constraints The final design process is intended to make the irrigation system selected compatible with the preliminary design factors. Varies between the periodic-move systems and the continuous-move systems. Also vary among the different types of sprinkler systems mentioned in the introductory section of this module.

Steps in Sprinkler Irrigation Design: Final Design Steps To get to the final design, the designer will carry out the following steps; Step 3; Sprinkler selection and spacing Step 4; Determine Hydrant spacing Step 5;Size Laterals and header lines. Step 6 Sub main and Main line design/ sizing Step 7 Size Delivery/supply line Step 8 Determine the Total Dynamic Head (TDH) Step 9 Determine the power requirements Step 10 Select pump and power source Step 11 Select other components and fittings Step 12 Prepare Drawings and Map of Design Step 13 Make a list of materials (Bill of quantities) Step 14 Estimate the capital cost Note: Several design options should be prepared and the final option should be chosen with the participation of the farmer / client; Step 15 Choose final option. Step 16 Prepare operation instructions

Exercise on Design Given a surveyed piece of land (18ha) Fig 1, Design a suitable commercial semi portable system: The following soil and crop data are provided; Soil medium texture loam Crop Wheat peak daily use 5.8mm/day Available moisture 140mm/m and Allowable depletion (P)=50% or 0.5 RZD=0.7 Soil Infiltration rate 6mm/hr: Calc net depth of application i.e.; Determine the gross depth for the system operating at 85% efficiency Calculate Irrigation frequency Calculate the set time Calculate the preliminary system capacity Select appropriate sprinklers for the given conditions Complete the design , layout and produce a Bill of Quantities.

Figure 1 : Farm map