Irrigation Manual

6.6.4. Orifices6.6.5. Current meter6.7. Discharge measurement in pipelines6.7.1. Differential pressure flow meters6.7.2. Rotating mechanical flow meters1071081101101107.LAND LEVELLING7.1. Profile method7.2. Contour method7.3. Plane method7.4. The cut : fill ratio7.5. Use of computers1111111111121191198.DESIGN OF THE DRAINAGE SYSTEM8.1. Factors affecting drainage8.1.1. Climate8.1.2. Soil type and profile8.1.3. Water quality8.1.4. Irrigation practice8.2. Determining hydraulic conductivity8.3. Surface drainage8.4. Subsurface drainage8.4.1. Horizontal subsurface drainage8.4.2. Vertical subsurface drainage8.5. Salt problems1231231231231231231241251271281311319.BILL OF QUANTITIES9.1. Bill of quantities for Nabusenga irrigation scheme9.1.1. The construction of a concrete-lined canal9.1.2. The construction of a saddle bridge9.1.3. The construction of a diversion structure9.1.4. The overall bill of quantities for Nabusenga irrigation scheme9.2. Bill of quantities for Mangui irrigation scheme13313313313513813914110. OPERATION AND MAINTENANCE OF SURFACE IRRIGATION SYSTEMS10.1. Operation of the irrigation system10.1.1. Water delivery to the canals10.1.2. Water delivery to the fields10.1.3. Operational success determinants10.2. Maintenance of the irrigation system10.2.1. Special maintenance10.2.2. Deferred maintenance10.2.3. Routine maintenance10.3. Operation and maintenance RENCES149ix

List of 35.36.37.38.39.xTypical components of a surface irrigation systemDefinition sketch showing the surface irrigation phasesBasic infiltration rate and cumulative infiltration curvesCylinder infiltrometersAnalysis of the data of an infiltration test using an infiltrometer on a clay loam soilAnalysis of the data of an infiltration test using actual furrows on a clay loam soilTime-advance graph for various stream flows in a furrowDetermining the headAdvance and recession of water on a borderstripAdvance and recession curves for different borderstrip length needing different total volumes of waterto be appliedAn example of a furrow irrigation system using siphonsFurrow shape depending on soil typeSoil moisture distribution on various soil types as a determinant of furrow spacingExample of a borderstrip irrigation systemCross-section of a borderstripLayout of basin irrigationTypical layout of a surface irrigation scheme on uniform flat topographyThe herringbone irrigation layoutLayout of Nabusenga surface irrigation schemeLayout of Mangui piped surface irrigation schemeCumulative depth of irrigation versus time for different types of soilPlot layout and hydrantsFlowchart for canal design calculationsCanal parametersDifferent canal cross-sectionsHydraulic parameters for different canal shapesChart of Manning formula for trapezoidal canal cross-sectionsLongitudinal profile of a field or tertiary canalLongitudinal profile of a secondary or main canalLongitudinal profile of a conveyance canalExample of a longitudinal profile of a conveyance canalLongitudinal canal profile generated by the Lonsec ProgrammeMethods commonly used to introduce water into the fieldThe longitudinal profile of the conveyance pipeline from Nabusenga dam to the night storage reservoirFriction loss chart for AC pipes (Class 18)Friction loss chart for uPVC pipesSchemes irrigated from different water sourcesHeadwork with offtake structure onlyOfftake possibilities in straight reach of 43464748494953555657616262

3.74.75.76.77.78.79.80.81.82.Possible arrangements for offtakes based on site conditionsAn example of an intake arrangement of a headworkAn example of a diversion structureC1 coefficient for different types of weirs in relation to submergence, based on crest shapeC2 coefficient for different types of weirs in relation to crest shapeTypes of weirsGabion weirTypical parameters used in the design of a stilling basinSchematic view of a weir and apronMasonry weir and apronDam cross-section at NabusengaGravity offtake with diversion damScour sluiceDesign of a typical earthen night storage reservoirCourses in brick wall of a reservoirA simple in-situ concrete proportional flow division structurePrecast concrete block division boxTimber division structuresDuckbill weir photographDuckbill weir designDiagonal weirSome drop structures used in open canalsStandard drop structure without stilling basinA vertical drop structureA chute structureStatic and velocity headsVariation of specific energy with depth of flow for different canal shapesHydraulic jump over a concrete apronThe form of a hydraulic jump postulated in the momentum theoryParameters of a sharp-crested weirTrapezoidal (Cipoletti) weirV-notch weirsBroad-crested weirRomijn broad-crested weir, hydraulic dimensions of weir abutmentsRomijn broad-crested weir, sliding blades and movable weir crestApproach velocity coefficient, Cv, as a function of the total head over the movable weir, HcrtParshall flumeDischarge correction factors for Parshall flumes with different throat widthsHead loss through Parshall flumesTrapezoidal flumeCut-throat flumeCut-throat flume coefficientsExamples of 07xi

Irrigation .98.99.100.101.102.103.104.105.106.107.xiiFree discharging flow through an orificeSluice gate under submerged conditionsOtt C31 propeller instrumentDepth-velocity integration methodVenturi flow meterThe profile method of land levelling: cut and fill and checking gradient levels with profile boardsThe contour method of land levellingGrid map showing land elevation and average profile figuresAverage profile and lines of best fitPart of the completed land levelling map for Nabusenga, assuming GX 0.005Irregular shaped field (elevations 0.0 are located outside the field)Parameters for determining hydraulic conductivityCross-sections of drainsRainfall-duration curveSubsurface drainage systems at field levelSubsurface drainage parametersNomograph for the determination of equivalent sub-stratum depthsNomograph for the solution of the Hooghoudt drain spacing formulaSalt accumulation in the root zone and the accompanying capillary riseCross-section of a concrete lined canal at NabusengaSaddle bridge for NabusengaField canal bank breaching in order to allow the water to flow from the canal onto the fieldPermanent outlet structure used to supply water from the canal onto the fieldAn example of a spile used to supply water from the canal onto the fieldA siphon supplying water from a canal onto a 128130131132133136143144145146

List of 5.36.37.38.39.Typical infiltration rates for different soilsInfiltration rate data from an infiltrometer testInfiltration rate measurement in a 100 m long furrowDischarge for siphons, depending on pipe diameter and headGuidelines to determine when to stop the water supply onto a borderstripMeasurement of water advance and recession distance and time on a borderstripFurrow lengths in metres as related to soil type, slope, stream size and irrigation depthPractical values of maximum furrow lengths in metres depending on soil type, slope, stream size andirrigation depth for small-scale irrigationTypical borderstrip dimensions in metres as related to soil type, slope, irrigation depth and stream sizeSuggested maximum borderstrip widths and lengths for smallholder irrigation schemesCriteria for basin size determinationBasin area in m2 for different stream sizes and soil typesApproximate values for the maximum basin widthSelection of an irrigation method based on soil type and net irrigation depthDesign parameters for Nabusenga and Mangui surface irrigation schemesSummary of the calculated design parameters for Nabusenga and Mangui surface irrigation schemesKm and n values for different types of canal surfaceTypical canal side slopesRecommended b/d ratiosMaximum water velocity ranges for earthen canals on different types of soilCanal capacities for standard Agritex canal sectionsLongitudinal profile for field canal - output from the Lonsec computer programmeSeepage losses for different soil typesHazen-Williams C value for different materialsWeighted-creep ratios for weirs depending on soil typeReinforcement requirements in a clay brick wall of a reservoirCross-sectional areas of reinforcement steel rodsDischarge Q (m3/sec) for contracted rectangular weir, depending on h and bDischarge Q (m3/sec) for Cipoletti weir, depending on h and bDischarge Q (m3/sec x 10) for a 90 V-notch weir, depending on hStandard dimensions of Parshall flumesDischarge characteristics of Parshall flumesLand levelling resultsInput and output data types for computer land levelling programme LEVEL 4EM.EXELand levelling calculations with line of best fit and cut:fill ratio of 1.01Land levelling calculations with 0.5% gradient in the X direction and cut:fill ratio of 1.01Land levelling calculations with line of best fit and cut:fill ratio of 1.21Computer printout of land levelling data for Mangui piped surface irrigation schemeValues for runoff coefficient C in Equation 6779192949899117119120120121121-2126xiii

Irrigation manual40.41.42.43.44.45.46.47.48.49.50.xivConcrete volume for different trapezoidal canal cross-sectionsSummary of the bill of quantities for the construction of the 980 m long lined canal at NabusengaSummary of the bill of quantities for the construction of a saddle bridgeSummary of the bill of quantities for the construction of a diversion structureBill of quantities for Nabusenga scheme, downstream of the night storage reservoirSummary of material requirements for Nabusenga (including 10% contingencies)Bill of quantities for pipes and fittings and pumping plant at Mangui schemeDischarge of permanent wooden field outlet structuresRates of discharge through spiles (l/sec)Discharge of siphons for different head and pipe diameter (l/sec)Weed management and effectiveness134135137138139141142144145146148

List of abbreviationsAACASAECCIJD or SRPPPPWPqQRAreaAsbestos CementAmerican Society of Agricultural EngineersCutCast IronDensity of waterDiameterWater depthGross depth of water applicationNet depth of water applicationEfficiencyElevationFreeboardFillField CapacityFroude NumberAcceleration due to gravityRegression coefficientGalvanized Steelwater depthHeadFriction losses per 100 m of pipeHead LossIrrigation CycleIrrigation FrequencyIrrigation TimeManning roughness coefficientKilopascalkilowattLengthRoughness coefficient ( 1/Km)Night Storage ReservoirAllowable moisture depletionWetted PerimeterPressurePermanent Wilting PointDischarge into one furrow or discharge per m widthDischargeHydraulic radiusxv

Irrigation manualRRZDSTTDHuPVCVV or vzxviCut : Fill ratioEffective Root Zone DepthSlope or gradientIrrigation timeTotal Dynamic Headunplasticized Polyvinyl ChlorideVolumeWater velocityElevation

Unit conversion tableMassLength1 inch (in)0.0254 m1 ounce28.3286 g1 foot (ft)0.3048 m1 pound0.4535 kg1 yard (yd)0.9144 m1 long ton1016.05 kg1 mile1609.344 m1 short ton907.185 kg1 metre (m)39.37 inches (in)1 gram (g)0.0353 ounces (oz)1 metre (m)3.28 feet (ft)1 kilogram (kg)1000 g 2.20462 pounds1 metre (m)1.094 yards (yd)1 ton1 kilometre (km)0.62 miles1000 kg 0.984 long ton 1.102 short tonPressureArea1 square inch(in2)6.4516 x10-2m21 pound force/in26894.76 N/m251.7 mm Hg1 square foot (ft2)0.0929 m21 pound force/in21 square yard (yd2)0.8361 m21 Pascal (PA)1 acre4046.86 m21 N/m2 0.000145 pound force /in21 acre0.4046 ha1 atmosphere760 mm Hg 14.7 pound force/in2(lbf/in2)1 square centimetre (cm2)0.155 square inches (in2)1 square metre (m2)10.76 square feet (ft2)1 atmosphere1 bar1 square metre (m2)1.196 square yard (yd2)1 bar10 metres1 square metre (m2)0.00024 acres1 bar100 kpa1 hectare (ha)2.47 acresEnergyVolume1 B.t.u.1055.966 J1 cubic inch (in3)1.6387 x 10-5 m31 foot pound-force1.3559 J1 cubic foot (ft3)0.0283 m31 B.t.u.0.25188 Kcalorie1 cubic yard (yd3)0.7646 m31 B.t.u.0.0002930 KWh1 cubic centimetre (cm3)0.061 cubic inches (in3)1 Joule (J)0.000947 B.t.u.1 cubic metre (m3)35.315 cubic feet (ft3)1 Joule (J)0.7375 foot pound-force (ft.lbf)1.308 cubic yards (yd3)1 kilocalorie (Kcal)4185.5 J 3.97 B.t.u.1 kilowatte-hour (kWh)3600000 J 3412 B.t.u.1 cubic metre(m3)Capacity1. imperial gallon0.0045 m3Power1. US gallon0.0037 m31 Joule/sec0.7376 foot pound/sec1. imperial barrel0.1639 m31 foot pound/sec1.3557 watt1. US. barrel0.1190 m31 cheval-vapor0.9861 hp1 pint0.5681 l1 Kcal/h0.001162 kW1 US gallon (dry)0.0044 m31 watt (W)1 litre (l)0.22 imp. gallon1 Joule/sec 0.7376 foot pound/sec (ft lbf/s)1 litre (l)0.264 U.S. gallon1 horsepower (hp)745.7 watt 550 ft lbf/s1 litre (l)0.0061 imperial barrel1 horsepower (hp)1.014 cheval-vapor (ch)1 hectolitre (hl)100 litres1 kilowatt (kW)860 Kcal/h 1.34 horsepower 0.61 imperial barrel 0.84 US barrel1 litre (l)1 cubic metre of water (m3)1.760 pints1000 l 227 U.S. gallon (dry)1 imperial barrel164 litresTemperature0C(Celsius or centigrade-degree)0F(Fahrenheit degree)K (Kelvin)0F0C 5/9 x (0F - 32) 1.8 x 0C 0FK 0C 273.15xvii

Irrigation manualxviii

Chapter 1Introduction to surface irrigationSurface irrigation is the oldest and most common methodof applying water to crops. It involves moving water overthe soil in order to wet it completely or partially. The waterflows over or ponds on the soil surface and graduallyinfiltrates to the desired depth. Surface irrigation methodsare best suited to soils with low to moderate infiltrationcapacities and to lands with relatively uniform terrain withslopes less than 2-3% (FAO, 1974).1.1. Components of a surface irrigationsystemFigure 1 presents the components of a surface irrigationsystem and possible structures, which are described inChapter 6. The water delivery system, shown in Figure 1,includes the conveyance system and the field canal systemdescribed below. The water use system refers to the infieldwater use system, showing one field in the block. The tailwater ditch and the water removal system are part of thedrainage system.the conveyance canal itself does not need to be aboveground level all along the canal, but its starting bed levelshould be such that there is sufficient command for thelower order canals. Where possible, it could run quasiparallel to the contour line. Design aspects of canals andpipelines are discussed in Chapter 5.Although an open conveyance canal may be cheaper perunit length than a pipeline, the latter would need to beselected when:YThe water source is at lower elevation than theirrigation area, and thus pumping is requiredYThe topography of the land is very uneven, such thatconstructing an open canal could either be moreexpensive or even impossible (for example whencrossing rivers and gullies)A piped conveyance system also eliminates water lossesthrough evaporation and seepage. An added advantage isthat it does not provide the environment for water-bornedisease vectors along the conveyance.1.1.1. The water sourceThe source of water can be surface water or groundwater.Water can be abstracted from a river, lake, reservoir,borehole, well, spring, etc.1.1.2. The intake facilitiesThe intake is the point where the water enters into theconveyance system of the irrigation scheme. Water mayreach this point by gravity or through pumping. Intakefacilities are dealt with during the design of headworks inChapter 6. Pumping units are discussed in detail inModule 5.1.1.3. The conveyance systemWater can be conveyed from the headworks to the inlet ofa night storage reservoir or a block of fields either by gravity,through open canals or pipes, or through pumping intopipelines. The method of conveyance depends mostly onthe terrain (topography and soil type) and on the differencein elevation between the intake at the headworks and theirrigation scheme. In order to be able to command theintended area, the conveyance system should discharge itswater at the highest point of the scheme. The water level in1.1.4. The water storage facilitiesNight storage reservoirs (NSR) could be built if theirrigation scheme is large enough to warrant suchstructures. They store water during times when there isabstraction from the water source, but no irrigation. InSouthern Africa it is common practice to have continuousflow in the conveyance system combined with a NSRlocated at the highest point of a block or the scheme.Irrigation would then be practiced during daytime using thecombined flow from the conveyance system and the NSR.Depending on the size of the scheme one could constructeither one reservoir located at the highest part of thescheme or a number of reservoirs, each located at theentrance of a block of fields. The conveyance system endsat the point where the water enters the reservoir.1.1.5. The field canal and/or pipe systemCanals or pipelines are needed to carry the water fromthe conveyance canal or the NSR to a block of fields.They are called the main canal or pipeline. Secondarycanals or pipelines supply water from the main canal orpipeline to the tertiary or field canals or pipelines, which1

Irrigation manualFigure 1Typical components of a surface irrigation system (Source: Walker and Skogerboe, 1987)Parshall flumeWater supplyCheckWaterdeliverysystemGated pipeField canalDropHead ditchWater usesystemDivision boxTail water ditchWater removal system (drain)are located next to the field. Sometimes no distinction ismade between main and secondary and the canal or pipesystem from the reservoir to the tertiary canal is calledmain canal or pipeline. The tertiary canals or thepipelines with hydrants are used to supply water to thefurrows or borderstrips or basins. Where canals are usedto deliver irrigation water, they should be constructedabove ground level, as the water level in canals should be2above field level for siphoning to ta

5.1.1. Calculation of the cross-section, perimeter and hydraulic radius of a canal 38 5.1.2. Factors affecting the canal discharge 38 5.1.3. Hydraulic design of canal networks using the chart of Manning formula 42 5.1.4. Canal section sizes used by Agritex in Zimbabwe 43 5.1.5. Longitudinal canal sections 45 5.1.6. Field canals for small .