Design of Subsurface Drip Irrigation Systems in Humid Areas

1Design of Subsurface Drip Irrigation Systems in Humid Areas 3Garry L Grabow, Assistant Professor 4Department of Biological and Agricultural Engineering 5North Carolina State University 6Campus Box 7625 7Raleigh, NC 27695-7625 8Garry_Grabow@ncsu.edu 10Kerry Harrison, Senior Public Service Associate 11Department of Biological & Agricultural Engineering CES 12P.O Box 1209 13University of Georgia 14Tifton, GA 31793-1209 15kharriso@uga.edu 16 17W Bryan Smith, Area Extension Agent - Agricultural Engineer 18Clemson Extension Service 19P.O Box 160, Newberry, SC 29108 20wsmth@clemson.edu 21 22Earl Vories, Agricultural Engineer 23USDA-ARS-Cropping Systems and Water Quality Research Unit 24Delta Center 25147 State Hwy T, Box 160 26Portageville, MO 63873 27VoriesE@Missouri.edu 28 29Heping Zhu, Agricultural Engineer 30USDA-ARS-Application Technology Research Unit 311680 Madison Ave 32Wooster, OH 44691 33Zhu.16@osu.edu 34 35Ahmad Khalilian, Professor 36Department of Agricultural and Biological Engineering 37Clemson University 3864 Research Road 39Blackville, SC 29817 40akhlln@clemson.edu 41 i TABLE OF CONTENTS 2Table of Contents ii 3Introduction 4Design Criteria .2 Water Requirements Soils Management and Operation Considerations Water Quality .8 9Water Sources For Subsurface Drip irrigation .12 10 Surface Water 12 11 Ground Water 13 12 Alternative Water Supplies 13 13Pumps and Power Sources for Subsurface Drip Irrigation 14 14Filtration requirements for Subsurface Drip Irrigation 15 15 Media Filters 16 16 Screen Filters 17 17 Disk Filters .18 18Chemical Injection for Subsurface Drip Irrigation 19 19 Venturi injector 20 20 Metering Pump 21 21 General Design Considerations for Chemical Injection Systems 22 22Valves for SDI systems 24 23 Control Valves 25 24 Vacuum/Air Relief Valves 26 25 Backflow Prevention 27 26 Pressure regulating valves .27 27Main and Submain Design 28 28 Slope 31 29 Lateral Length 31 30 Field Shape .31 31 Mainline / Submain Flushing 32 32Dripline design .33 33 Dripline Depth 35 34 Dripline Spacing 35 35 Dripline length 35 36Dripline Flushing Manifold Design .37 37 Pipe Size 38 38 Field Shape .39 39Instrumentation and controls 39 40 Irrigation control 39 41 Backflushing 43 42 Chemigation .43 43Summary 44 44References 44 ii INTRODUCTION Subsurface drip irrigation (SDI) is similar to surface drip irrigation, but has drip lines that 3are buried beneath the surface Although many surface drip systems have lines buried up to a 4few inches deep, SDI is normally defined as a system that is “permanent”, that is, the drip lines 5are not taken up every year Before the design of an SDI system is done, it must be determined that the intended site is 7suitable for SDI These considerations include factors important to all irrigation systems, but 8some factors are particularly important to the success of an SDI system These factors include 9adequate water supply, acceptable water quality, and appropriate topography Another 10consideration is management which is important to all drip irrigation systems, and especially 11important to SDI systems in which drip lines are out of sight 12 This publication is one in a series of publications that deal particularly with SDI in humid 13areas These areas, such as the southeastern United States, have particular climate, topography, 14soils, cropping systems, and water sources that require special consideration when considering 15and implementing an SDI system The other publications in this series deal with site selection, 16installation, management, and chemical treatment of water related to SDI systems with particular 17consideration given to humid areas 18 The design of an SDI system is similar to the design of other drip irrigation systems, with 19additional consideration giving to system flushing and traffic In humid regions, topography and 20field layout will normally demand extra attention Proper design of a SDI system will ensure 21uniformity of water application, as well as reduced operation and maintenance cost 1 DESIGN CRITERIA The successful design of a subsurface drip irrigation (SDI) system requires that pertinent 3information be collected and incorporated into the design This information is oftentimes 4referred to as “design criteria” For an SDI system, these criteria will include information on 5climate, crops, soils, water quality, and system management and operational considerations 6Water Requirements The SDI system must be sized to deliver the required amount of water to the crop at the time 8it needs it The water requirement should be thought of as both a flow rate and a total amount of 9water The SDI system must be designed to deliver the required flow rate, and the water supply 10must be adequate to deliver the amount of water required over the growing season The amount 11of water required will depend on many things including, climate, crop, and soils 12 Climate affects the water requirements of an irrigation system by dictating, along with the 13growth stage of the crop, how much water a crop will need at any time of the growing season In 14humid areas, the water requirement is normally reduced over more arid regions This is due to 15higher humidity, which reduces the vapor pressure gradient, or driving force, for 16evapotranspiration, the sum of the water that passes through a plant (transpiration) and the water 17that directly evaporates from the soil surface Other factors such as temperature, sunshine and 18wind influence evapotranspiration With an SDI system, evaporation from irrigation is reduced 19to a negligible amount in most cases, since the soil surface is not normally wetted 20 The crop to be irrigated with an SDI system will also influence its design Different crops 21use different amounts of water, and also may be grown during different times of the year with 22different environmental demands The SDI system may be intended to irrigate more than one 23crop (rotation) in which case the crop with the highest water demand should be satisfied The most important aspect of crop water use for SDI design is the “peak” water requirement 2or the amount of water that a crop uses during its highest water use period This is because it is 3during this period that the SDI system must deliver the greatest amount of water The peak use 4rate that is used to design an irrigation system is normally derived from an average use rate of the 5peak use month (NRCS, 1970) Since drip systems are designed to apply small amounts of 6water frequently, the average peak use rate for the peak week should be used While rain may 7be factored in to reduce the irrigation requirement for a season, it should not be factored in when 8calculating a peak use rate This is because even in humid regions, the probability of receiving 9appreciable rain in a few-day period with high dependability (80% of the time), is low In 10addition, moisture in soil storage is normally not considered when designing for peak demand 11 Peak use rates may be calculated using a number of methods (Allen et al., 1997; USDA- 12SCS, 993) or be obtained through a Small Vegetable Water Use Piedmont 13local University extension office For 0.3 14most humid areas, a typical peak use 16day See Figure for an example of 17crop water use rates This depth is 18converted to a flow rate gpm (l/m), Early 0.2 ET, inches/day 15design rate is 25 inches (6 mm) per 0.25 Aug 0.15 Late 0.1 March 15 0.05 0 10 20 30 40 50 60 70 80 90 100 Days after Planting 19which for 25 inches per day is 4.7 20gpm per acre irrigated This flow rate 21is normally increased by considering Figure Crop Water Use for Small Vegetables in the Piedmont Region of North Carolina Note that the peak rate for early planting is about 0.25 in/day while that for late plating is about 0.18 in/day 22system application efficiency and by factoring in operating times less than 24 hours per day If 23designing a system to pulse (irrigate multiple times per day), the off-cycle times need to be 1included when estimating system daily operating time System inefficiency is derived from 2multiple sources; manufacturer variation in emitters, pressure variation within the system, and 3system operation and management The design target for emission uniformity along a lateral is 4normally 90% This is controlled in the design process by limiting drip line length Considering 5the other sources of inefficiency, the application efficiency for SDI design purposes can normally 6be set at 85% Assuming an application efficiency of 85% and an operating time of 12 hours per 7day, the system flow rate to satisfy a crop water requirement of 0.25 inches per day is 11 gpm per 8acre irrigated The total system flow rate is obtained by simply multiplying the per acre flow rate 9by the total number of acres to be irrigated by the system Flow rates larger than those arrived at 10in the design process may be used, but larger pumps and mainlines will be required, resulting in 11greater costs 12 The design flow rate calculated for crop water needs must be matched to the manufacturer 13specified drip line flow rates at the recommended pressure This requirement normally leads to 14“zoning” a field, since the flow rate required to satisfy crop water requirements will likely be 15lower than the SDI system flow rate if operating the whole field at once (or conversely the 16application rate of the system exceeds that required for crop needs) For certain situations, such 17as small fields, it may be preferable to size the pump and mainline to deliver to the whole field 18simultaneously If media filters are used in smaller fields, back flushing flow requirements may 19exceed normal operation flow rates and may dictate pump capacity 20Soils 21 The soil type into which an SDI system will be installed can impact system design Soil 22characteristics such as texture, structure and layering can affect soil hydraulic characteristics 1such as infiltration rate and hydraulic conductivity These hydraulic characteristics, in turn, play 2a part in system performance Drip lines will need to be more closely spaced in a sandy soil since the lateral spread of 4water from the drip lines will be less pronounced than in a finer texture soil Of course, crop 5rotation and cultivation will also dictate drip line spacing, especially for row crops when spacing 6will usually be a multiple of the row spacing Slow emitter emission rates may be required on heavy textured soils, such as clay, so that 8the emission rate does not exceed the hydraulic conductivity of the soil When this happens 9“chimneying” may occur in recently installed systems as water takes the path of least resistance 10to the surface via void spaces This can also occur in older systems, in which repeated irrigation 11cycles tend to wash out the fines above the drip line 12 Although soils in humid areas often lead to a restriction in the depth of the root zone, drip 13lines will need to be installed deep enough to avoid damage from tillage equipment In general, 14drip line depths should be shallower in coarser textured soils and deeper in finer textured soils 15 While design considerations can address many soil-related issues, operation and 16management will also insure optimal performance of the SDI system in various soils 1Management and Operation Considerations An essential step in the design of a subsurface drip irrigation (SDI) system is to consider 3how the use of the area will vary It is not enough to design only for next year’s crop A well4designed system should be in operation for at least ten to fifteen years, so some attempt must be 5made to plan for the future Important questions that should be asked include: 6• Will the same crop be grown each year or will there be a rotation of multiple crops? As previously mentioned, the water requirements and thus the system capacity will vary depending on the crop Other factors such as traffic pattern and types of equipment could also be affected 10• Will the entire field be planted to one crop or will it be divided into smaller areas of different 11 crops? The field will most likely be divided into zones and knowledge of the cropping 12 pattern will allow for the most practical layout of the zones, allowing for more efficient 13 management 14• Will the different crops in a rotation employ different cropping systems? In humid regions, 15 row crops and field crops are often rotated For example, corn and cotton may both be 16 grown on 30-, 36-, or 38-inch beds without much difference in the production systems 17 However, winter wheat, soybeans or rice will likely have different traffic patterns than the 18 row crops, a fact that should be considered even if they will not use the SDI system for 19 irrigation Another good example is peanuts Even though peanuts may be produced as a 20 row crop, the fact that the fruit is produced underground could affect the placement of the 21 drip tubing In general, row crops will dictate drip line spacing with spacing a multiple of 22 the row spacing If field crops such as winter wheat or soybeans are to be irrigated in addition to row crops, drip line spacing may be limited to one or two rows, although economic considerations might override efficiency gained from the narrower spacings 3• Is subsoiling a part of the production system? If so, is it necessary because of soil hardpans or is it done “because everybody else does it”? If it is an essential part of the soil management plan, then the placement of the drip tubing must be planned around subsoiling One way to achieve this is to place the drip tubing deep enough to avoid contact with the subsoil plows This is usually not a good alternative since it would mean placing the drip tubing relatively deep in the soil profile and both the placement of the drip tubing and the depth of the subsoil plows would tend to undulate somewhat A better 10 system would probably be to coordinate the horizontal placement of the drip tubing and the 11 subsoil plows However, for this system to work it is essential to know the location of the 12 drip tubing After several years of normal field operations it is quite easy to lose track of 13 the location of the drip tubing by a few inches, even if the field has been bedded the entire 14 time If the cropping pattern is such that the beds have been periodically destroyed and 15 rebuilt, the location of the beds relative to the drip tubing may have shifted by several 16 inches Just a small error in locating the drip tubing before subsoiling could lead to major 17 repairs of the SDI system 18• A final consideration is harder to quantify Is there time and willingness to learn a new 19 system and manage it correctly? With the recent state of the farm economy, many 20 producers are spread over such a large area that it is all that they can to keep up with 21 what they already have SDI will most likely be different from anything else the producer 22 is doing As such, there will be a learning curve associated with it In addition, there will 23 be system maintenance operations (e.g., acid injection, iron settling, flushing, etc.) not required or less extensive than for other systems Ignoring the maintenance to save time will most likely lead to a significantly shorter life for the system SDI offers opportunities for more efficient use of irrigation water in many situations 4However, the system must be well planned and maintained to achieve the potential benefits A 5poorly planned system will not function properly and will require time-consuming repairs 6Water Quality When designing an irrigation system, water quality concerns may include two sources of 8design criteria, one for the system and one for the crop Water quality criteria for crops normally 9focus on leaching requirements or application concerns (foliar burning, etc.) In humid regions, 10where salts not build up in the root zone, a leaching requirement is not required Also, since 11SDI systems don’t wet the plant, problems resulting from the contact of irrigation water with the 12plant are not an issue As a result, water quality criteria for the design of SDI systems in humid 13areas focuses on irrigation system concerns Emitter clogging is the primary concern with SDI 14systems as with all drip systems 15 Groundwater is generally of higher quality than surface water and therefore presents less of 16a concern of emitter clogging However, many existing and potential water supply sources for 17SDI systems in humid areas are from surface water 18 Water quality will dictate filtration requirements, chemical injection requirements, and 19management of SDI systems to prevent emitter clogging Causes of emitter clogging in SDI 20systems may be chemical (precipitates or scale), physical (grit or particulates such as sand and 21sediment), biological (such as algae or bacteria), and rarely water temperature (affects solubility 22of precipitates, or tendency of lime to precipitate) Water temperature can also affect the pressure 23rating of PVC and polyethylene pipe (ASAE, 1989) 1Slope Topography and elevations will have the same impact on SDI systems as on any other 3irrigation system As water is pumped to a higher elevation pressure is lost due to the weight of 4the water This pressure loss (or increase if water is pumped downhill) can be managed 5relatively easily if a pressure-compensated dripline and pressure regulators are used in the 6system Turbulent-flow (non-pressure compensating) dripline will require a more careful design 7approach In either case, the mainline and submain must be designed within pre-determined 8friction loss constraints If these constraints cannot be met – or can only be met by using piping 9of a size that is economically infeasible – the section or zone being designed should be separated 10into two or more segments This may or may not require an additional control valve, but will in 11many instances require at least an additional pressure regulating valve 12Lateral Length 13 Lateral length will determine the zoning and therefore the layout and design of submains 14New developments in dripline production allow designers to extend lateral lengths to distances 15that were previously considered hydraulically impossible Larger diameter driplines and lower 16flow rate emitters combine to allow lateral lengths of up to 1,320 feet (0.4 kilometer) in some 17cases While these distances are now possible, they may not be advisable when dripline flushing 18is considered This is because it will take longer to flush the longer laterals, and also because the 19pressure required at the head of the lateral to sustain an adequate flushing flow rate at the distal 20end of the lateral may not be attainable (see Figure 17) 21Field Shape 22 In the humid Southeastern region, irregular field shapes are more typical that rectangular or 23square fields due to topography and property boundaries 31 Care should be taken to properly size submains where field shape varies In these instances, 2each dripline lateral may have a different length and a different total flow rate for that lateral 3Submains should be designed based on actual flow rates of the laterals and not on an “average” 4flow rate per lateral for irregularly shaped fields 5Mainline / Submain Flushing Mainlines and submains should be flushed at least once per year and preferably several 7times per year Design objectives for flushing may result in different pipe diameters being 8selected than those selected in the design process for normal operation This is because the 9flushing flow rate required to achieve a desired flushing velocity in any section of a main or 10submain may be different than the design flowrate (normal operation) 11 There are two basic flushing design procedures; one that sizes mainlines and submains for 12objectives and constraints under normal operation (irrigation only); and one that sizes pipe based 13upon a required flushing flow and velocity When designing for the former, required flushing 14flow and velocities in the mainlines are achieved by adjusting the number of zones flushed at one 15time In a buried drip system, the number of driplines flushed within a zone cannot be altered 16Using this design process, only the last section or two of pipe, where operating flows are low, 17may have insufficient flushing velocities However, sizing the piping system to allow flushing of 18the entire main or submain will allow a more thorough cleaning if the need to flush the entire 19main or submain should arise 32 When designing the system specifically for flushing, a flow rate should be calculated that 2will achieve a flow velocity of at least 1.5 feet per second in the largest section of the main or 3submain Valves sized to allow the respective flushing flow rates should be placed on the end of 4the main and submains Telescoping the pipe size down to a very small size relative to the initial 5pipe size of the main or submain may prevent the flow rate required to flush the largest diameter 6pipe in the main or submain from being achieved A general rule of thumb is that the smallest 7pipe diameter used in a main or submain should be no smaller than one-half the diameter of the 8largest pipe in the main or submain Friction losses incurred to the flushing discharge point at 9the required flushing flow rate can be used with the pump curve to see if the desired flushing 10flow rate can be achieved under the calculated total dynamic head requirements in flushing 11mode Designing the mainline and submains based upon flushing requirements may result in 12selection of larger pipe sections than those selected for normal operation in order to achieve the 13required flushing velocities 14 DRIPLINE DESIGN 15 With any irrigation system, the design process starts at the plant and works “upstream” 16Hydraulically speaking this means that the first part of the design process in an SDI system is 17dripline design The design of dripline for SDI systems consists of dripline selection, and 18specification of dripline depth and spacing Consideration must also be given to connections 19between the dripline and the supply and flushing manifolds Also, dripline length needs to be 20specified, which depends on desired emission uniformity and flushing considerations 21 Dripline selection will depend upon plant spacing, soil characteristics, and dripline 22durability and hydraulic characteristics Driplines come in a variety of discharge rates and 23emitter spacings While rigid tubing can be use in SDI systems, drip tape is normally chosen due 33 1to its relatively low cost A minimum drip tape wall thickness of 15mil is normally specified 2Discharge rates are normally expressed in gpm per 100 feet (l/hr per 100 meters) There are a 3variety of discharge rates and emitter spacings to choose from (Hanson et al., 2000) Different 4emitter types respond differently to pressure variation in the line Some are pressure 5compensating, and will discharge at nearly the same rate within a range of pressure Another consideration in dripline selection is clogging potential Clogging can occur from 7either soil particles in the tubing or from root intrusion Some dripline products have either a 8physical or chemical barrier to root intrusion In general, higher flow rate emitters tend to clog 9less due to larger flow passages (Hanson et al., 1997) 34 1Dripline Depth Dripline depth should be specified in any SDI system design Dripline depth will depend on 3soil characteristics, rooting depth, and cultivation practices SDI is normally defined as a drip 4system placed more than a few inches below the surface Most systems installed to date in 5agronomic, turf and forest crops have placed the dripline between and 18 inches below the soil 6surface (Camp, 1998) In general, SDI systems are too deep to aid in germination, but in 7medium to heavy textured soils with a higher potential of horizontal and upward movement due 8to capillarity, it may be a consideration Driplines should be installed below tillage depth If 9deep tillage is required (at or deeper than the dripline), it must avoid the driplines In this case, 10precise knowledge of dripline location is required Other considerations when selecting dripline 11depth may be weed germination and disease control Generally the deeper the drip line, the less 12the system will promote weed germination 13Dripline Spacing 14 Dripline spacing, like depth depends on soil characteristics as well as the crops to be grown 15In general, coarser textured soils will require a narrower dripline spacing than a finer textured 16soil, since lateral water movement is less in coarse soils Oftentimes there will be a critical crop 17in rotation that will dictate spacing In rotations that include a row crop, dripline spacing is most 18often a multiple of row spacing 19Dripline length 20 Dripline length is determined by field length and layout, allowable pressure variation within 21a zone, and flushing considerations Uniformity is normally expressed as emission uniformity or 22EU The general rule of thumb is to design for a dripline lateral EU of 90% System EU will 23also account for flow variation within a zone and between zones Most manufactures publish 35 1allowable dripline lengths to attain a desired uniformity along the dripline Software is also 2available at no charge from manufacturers that will calculate uniformity for their various drip 3tapes at various lengths and slopes, as well as provide estimates of flushing times A longer 4length of dripline will result in more pressure loss and therefore more flow variation Longer 5run lengths can be achieved by selecting a dripline with a larger diameter and/or lower flow rate 6drip line Diameters range from 5/8 inch (10 mm) to 3/8 inch (35 mm), and flow rates range 7from 0.17 to 1.00 gpm per 100 feet of drip line Dripline with pressure compensating emitters 8can be an alternative in cases of steep slopes, but is more expensive than regular, non-pressure 9compensation drip line 10 Driplines need to be periodically flushed, and the length of dripline run will impact 11required flushing times Longer lines need longer flushing times both to flush any soil particles 12or organic material out of the line, and to purge any chemicals that may have been introduced 13During flushing, the flushing manifold is opened and flow is discharged through the end of the 14driplines connected to the manifold as well as out of the emitters A 1.0 foot per second (.305 15meters per second) velocity should be maintained at the end of the lateral Velocities upstream of 16this point will be higher since there is more flow In order to create a foot per second discharge 17in a 5/8 inch (16 mm) drip line, a gallon per minute (0.06 liters per second) flow must exit the 18end of the line In order for the required discharge at the end of the line to occur, a certain 19pressure is required at the head of the dripline In some cases, this may not be achievable, 20especially with pressure regulated zones Required inlet pressures for various typical 5/8 inch 21diameter drip tape is shown in Figure 17 22 36 80 Required Flushing pressure, psi 70 24-0.28 12-0.4 8-0.67 60 50 40 30 20 10 0 200 400 600 800 1000 1200 Lateral Length, ft Figure 17 Required Inlet Pressure to Achieve a gpm (0.06 lps) discharge at the end of a 5/8 inch (16mm) dripline open to the atmosphere A flushing manifold would create some backpressure, necessitating slightly higher inlet pressures Legend numbers indicate emitter spacing (inches) and discharge (gpm/100 ft) DRIPLINE FLUSHING MANIFOLD DESIGN Subsurface drip irrigation systems require regular maintenance in order to prevent emitter 3clogging and related problems One of the design requirements for this regular maintenance is a 4dripline flushing manifold or submain connected to the end of the laterals on each zone This 5flushing manifold allows the laterals to be flushed with a water flow velocity higher than the 6normal operational flow velocity when a flushing valve attached to the manifold is opened The 7higher flow velocity allows some small amount of scouring to help remove settled solids and 8precipitants from the system to help prevent emitter plugging 37 In the humid Southeast, irregularly-shaped fields may require the use of many different 2lateral lengths within the same irrigation zone The shorter laterals in the irrigation zone will 3offer less restriction and allow more water to flow through to the closed flushing manifold during 4normal irrigation This excess flow may pressurize the flushing manifold to the extent that water 5in the manifold flows into the end of the longer laterals connected to the flushing submain 6(similar to the flows found in a looped piping system) This may mean that any sediment or 7precipitants allowed to remain in the flushing manifold will be re-introduced into the longer 8laterals Removal of sediment and precipitants from the flushing manifold will help prevent 9emitter clogging from this sediment re-introduction 10Pipe Size 11 Flushing manifold pipe sizes should be determined by considering the actual flow through 12the end of the laterals or driplines during flushing Allowing the entire irrigation zone or a 13portion of a zone to flush will in effect increase the flow requirements of the system temporarily, 14which in turn will decrease the system pressure The flushing manifold should be sized for a 15flow velocity of 1.5 feet per second (0.46 meters per second) through the laterals to ensure 16sediment removal from the laterals Assuming that the lateral inlet pressure requirements are 17met, end of lateral flushing flow rates can be summed along the manifold and the flushing 18manifold can be telescoped accordingly Water flow will move in the flushing manifold from the 19smallest pipe to the largest pipe in the manifold, which is the reverse of the normal pressurized 20supply manifold piping system With this concept in mind there is no need to limit the smallest 21size of piping used in the flushing manifold The flush valve on the flushing manifold will be the 22same size as the largest pipe size and will not introduce appreciable restriction on the flushing 23flow rate 38 1Field Shape Irregular field shapes found in the humid Southeastern region may in some instances require 3large amounts of piping to connect the ends of all the laterals in a particular section or zone 4When zones are relatively large, the pumping system may not be able to supply the flushing flow 5rates required to achieve the desired velocity at the ends of the laterals If this condition occurs 6the irrigation zone should be separated into two or possibly three separate flushing manifolds 7This manifold separation will allow a proper flushing pressure to be maintained INSTRUMENTATION AND CONTROLS Automation of irrigation has increased in the past couple of decades Automation can pay 10for itself by reducing labor requirements and by enabling more precise irrigation Since SDI is a 11relatively permanent system, it lends itself to automation Basic instrumentation starts with 12meters that help monitor system performance and that help diagnose potential problems It is a 13good idea to provide flow meters at strategic locations such as at submains for system 14monitoring as well at to potentially provide feedback for irrigation control Pressure gages are 15also vital in an SDI system to monitor pressure and to help diagnose problems Low pressure 16and/or increased flow rates during normal operation may indicate a leak 17Irrigation control 18 Irrigation control can be achieved by two general types of systems: open control loop 19systems and closed control loop systems Open loop systems not incorporate feedback and 20amounts and timing of irrigation are pre-determined by the operator This type of system is 21usually a simple irrigation controller operated with a clock A number of irrigation controllers 22are commercially available In general, these controllers initiate irrigations at preset times and 23control the duration of irrigation by activating solenoid control valves that serve zones The 39 1controllers vary in the number of valves that can be controlled, the number of valves that can be 2simultaneously held open, the number of separate irrigation programs available, and the number 3of start times available for each program These controllers are not normally set to operate with 4feedback, although most offer a rain switch that terminates irrigation during precipitation events Since humid areas by definition have appreciable rainfall, soil moisture may change 6unpredictably, and therefore make it difficult to schedule irrigations As such, a closed looped 7system offers many advantages Automation of irrigation using feedback can prevent leaching of 8chemicals, and reduce pumping costs, by only allowing irrigation when the crop needs it Many 9different systems for automating irrigation scheduling are available These systems can broadly 10be divided into two groups: systems that infer crop water stress using soil-water content 11information and those that estimate crop evapotranspiration (ETc) or crop water use 40 Scheduling irrigation based on soil moisture stress requires the soil to be monitored 2periodically for the measurement of soil-water content Feedback can be incorporated into the 3system using instrumentation for measuring soil moisture Simple systems can disable an 4irrigation controller by making or breaking a common wire in the circuit between the controller 5and solenoid valves based upon soil-water status Switching tensiometers can be used to 6complete circuits and are very inexpensive They are regular tensiometers that complete the 7circuit when a preset tension is exceeded, allowing for irrigation to occur They can be directly 8connected to a solenoid valve or connected to an irrigation controller Recently, adjustable 9controllers with soil moisture sensors have entered the market The devices query the sensor at 10the irrigation controller’s start time and either latch or de-latch the irrigation controller to either 11allow or disable an irrigation cycle respectively Different types of soil moisture sensors are 12used with these new generations of controllers Examples are time domain reflectometry (TDR) 13sensors and time domain transmissometry sensors TDR sensors work by sending an 14electromagnetic pulse through a transmission line, buried in the soil The signal reflected back 15from the end of the transmission line and analyzed to estimate moisture content in the soil Time 16domain transmissometry (TDT) is similar to TDR except in TDT, an electromagnetic signal is 17sent from one end and is received and analyzed at the other end Figure 18 shows a picture of a 18TDT probe Some of these types of moisture sensors are soil texture sensitive and should be 19selected based on the soil type of the field For example, manufactures of the TDT sensors offer 20different sensors for clay and sandy soils 41 Other systems use weather information rather than soil 2water status to schedule irrigations Typically these systems use 3calculations of evapotranspiration based on local weather 4information to aid in irrigation scheduling Pan Figure 18 A TDT moisture sensor, made by E.S.I Environmental Sensors Inc 5evaporation is one simple method for scheduling 6irrigations that can be fully automated to measure evaporation continuously, and to transfer the 7real-time data to the controller Larger or more sophisticated systems may have computer-based control systems for 9everything from irrigation to chemigation These systems are either interactive, and allow 10control of the system from a remote location by an operator, or fully automatic in which a 11monitoring system is used to automatically control pumps and valves Appropriate software and 12hardware is required in these applications Figure 19 shows an example of computerized control 13system for a subsurface drip irrigation system Damage from lightning is the biggest concern for 14fully automated irrigation systems in humid regions 15 42 A B C D Figure 19 The controller and example screen of “Aqualink Intelligent Irrigation System” where A) is the location of TDT sensors, B) represents the irrigation valve, C) represents the fertigation pump, and D) represents the sand media filter 2Backflushing Backflushing of media filters can also be automated A backflush cycle is triggered when a 4preset pressure differential exits between the inlet and outlet of the media filters These 5automated systems are normally provided by the dealer and are engineered by the manufacturer 6Alternately, backflushing can be initiated by an irrigation controller and can be set to operate at 7the end of an irrigation cycle or program, or at predetermined intervals on a program dedicated to 8backflushing 9Chemigation 10 Controls are critical for chemical injection systems Most of the automation and control for 11chemical systems are for safety purposes, and are oftentimes used to meet regulations 12 Other details on instrumentation and controls for all aspects of drip irrigation can be found 13in Boman, et al (2002) 43 SUMMARY The proper design of any irrigation system, and especially an SDI system, is critical In 3humid regions, design criteria and design considerations may be different than in arid regions 4This publication is not intended as a step-by-step design manual, but rather is intended to aid in 5the design process of an SDI system, particularly in humid areas For specific designs, it is best 6to work with an industry representative and dealer or to consult with your local university 7extension REFERENCES 10Boman, B S Smith and B Tullos 2002 Control and Automation in Citrus Microirrigation 11 Systems Circular 1413, Agricultural and Biological Engineering Department, Florida 12 Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of 13 Florida 14Burt, C 1999 Guide to the Use of Air Release/Vacuum Relief and Continuous Acting Air Vents 15 Agricultural Products, Inc Technical Manual 16Evans, R.O., J.H Hunt and R.E Sneed 1991 Pumping Plant Performance Evaluation North 17 Carolina Cooperative Extension Service, publication number AG-425-6 18Allen, R.G., L.S Pereira, D Raes, and M Smith 1998 Crop evapotranspiration: Guidelines for 19 computing crop water requirement FAO Irrigation and Drainage Paper 56 Rome 20Haman, D Z., A.G Smajstrla, and F.S Zazueta 1994 Media Filters for Trickle Irrigation in 21 Florida Fact Sheet AE-57, Agricultural Engineering Department, Florida Cooperative 22 Extension Service, Institute of Food and Agricultural Sciences, University of Florida 23Hanson 1997 Irrigation Pumping Plants Water Management series publication number 3377 24 University of California, Davis 25Hanson, B., L Schwankl, S Grattan, and T Priihard 1997 Drip Irrigation for Row Crops 26 Publication 3376, University of California Irrigation Program, University of California, Davis 44 1Hanson, B.R G Phipps, and E.C Martin 2000 Drip Irrigation of Row Crops: What is the State of the Art? Proceedings of the 4th Decennial Irrigation Symposium ASAE 3Pitts, D.J., D.Z Haman and A.G Smajstrla 1990 Causes and Prevention of Emitter Plugging in Microirrigation Systems Bulletin 258, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida 6Schwankl, L B Hanson, and T Prichard 1997 Filtration Equipment: An Overview, from Drip Irrigation for Row Crops, Publication 3376, University of California Irrigation Program, University of California, Davis 9USDA, SCS 1993 Irrigation Water Requirements National Irrigation Handbook, Part 623, 10 Chapter 45 ... the design process in an SDI system is 17dripline design The design of dripline for SDI systems consists of dripline selection, and 18specification of dripline depth and spacing Consideration... minimum of 15 psi higher than desired outlet pressure to obtain accurate 19regulation 20 21 MAIN AND SUBMAIN DESIGN Proper design guidelines of mainline and submain piping for Subsurface Drip Irrigation. .. between the design of “normal” irrigation systems and SDI is the 8increased importance of proper pipe flushing, including mainlines, submains, and laterals The 9piping system must be designed not