water Technical Note
Optimized Subsurface Irrigation System (OPSIS): Beyond Traditional Subsurface Irrigation M. H. J. P. Gunarathna 1,2, * ID , Kazuhito Sakai 3, *, Tamotsu Nakandakari 3 , Momii Kazuro 4 , Tsuneo Onodera 5 , Hiroyuki Kaneshiro 6 , Hiroshi Uehara 6 and Kousuke Wakasugi 7 1 2 3 4 5 6 7
*
United Graduate School of Agricultural Sciences, Kagoshima University, 1-21-24 Korimoto, Kagoshima-shi, Kagoshima 890-0065, Japan Faculty of Agriculture, Rajarata University of Sri Lanka, Puliyankulama, Anuradhapura 50000, Sri Lanka Faculty of Agriculture, University of the Ryukyus, 1 Senbaru, Nishihara-cho, Okinawa 903-0213, Japan;
[email protected] Faculty of Agriculture, Kagoshima University, 1-21-24 Korimoto, Kagoshima-shi, Kagoshima 890-8580, Japan;
[email protected] Paddy Research Co., Ltd., 9 Haraoka, Minamikawa-machi, Tome-shi, Miyagi 987-0432, Japan;
[email protected] Midori Net Okinawa, 453-3 Motobu, Haebaru-cho, Okinawa 901-1112, Japan;
[email protected] (H.K.);
[email protected] (H.U.) Institute for Rural Engineering, National Agriculture and Food Research Organization, 3-1-1 Kannondai, Tsukuba-shi, Ibaraki 305-8517, Japan;
[email protected] Correspondence:
[email protected] (M.H.J.P.G.);
[email protected] (K.S.); Tel.: +81-70-4419-3534 (M.H.J.P.G.); +81-89-895-8783 (K.S.); Fax: +81-89-895-8734 (K.S.)
Received: 25 May 2017; Accepted: 17 July 2017; Published: 12 August 2017
Abstract: Technologies that ensure the availability of water for crops need to be developed in order for agriculture to be sustainable in the face of climate change. Irrigation is costly, so technologies need to be improved or newly developed, not only with the aim of the sustainable use of precious water resources, but also with the aim of reducing associated labor and energy costs, which lead to higher production costs. OPSIS (optimized subsurface irrigation system) is a super water-saving subsurface irrigation system developed to irrigate upland crops by soil capillarity. It is an environmentally-friendly, solar-powered automatic irrigation method with minimum energy consumption and operational costs. In soils vulnerable to drought damage, OPSIS can outperform other irrigation methods. This technical note introduces OPSIS. Keywords: automatic; environment friendly; upland crops; climate change; water saving
1. Introduction Rainfed agriculture occupies 80% of the world’s agricultural lands and currently contributes 60% of the world’s food production [1]. However, the sustainability of rainfed agriculture in some regions is in peril as it is gravely threatened by climate change [2], as a result of which not only food security [3] but also the social structure [4] in many countries is in danger. Since water availability directly influences the efficient use of all other inputs, better water availability in turn ensures optimum yields from a given combination of inputs [5]. Therefore, emerging irrigation technologies ideally should be developed to enhance crop water availability in order to make agricultural practices sustainable in the long run. Although shifting from rainfed to irrigated agriculture or from low-efficiency to high-efficiency irrigation methods offers important remedial measures against a changing climate, adaptation could be inhibited by the reluctance of farmers to adopt practices that elevate operational costs, such as high-efficiency irrigation methods. Therefore, new irrigation technologies would be
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more likely to be adopted if they were designed to save precious water resources and at the same time keep associated labor and energy costs as low as possible. Various irrigation methods, such as surface, subsurface, sprinkler, and drip irrigation, can be used to irrigate upland crops. The method selected would depend on physical, economic, and social factors, and in turn determines the efficiency of resource use, economic viability, and sustainability of upland farming systems [6]. In the past, surface irrigation methods such as basins, borders, and furrows have been used to irrigate upland crops in many regions of the world, owing to their simplicity and low cost. In surface irrigation methods, water flows over the entire field or along furrows by gravity. When flowing, water infiltrates the soil and provides irrigation water to the root zone of crops. The uniformity of distribution and application efficiency basically depends on the degree of land leveling; therefore, it consumes high labor costs for land preparation [7,8]. With increasing energy and labor costs, however, and with increasing demand for diminishing water resources, surface irrigation has been replaced to some extent by subsurface, sprinkler, or drip irrigation methods. However, surface irrigation still is the major irrigation method used to irrigate upland crops worldwide. Drip and sprinkler irrigation methods were developed for high-frequency irrigation of crops by means of a systematically installed pipe network and emitting devices [6,9]. In drip and sprinkler systems, water is supplied under pressure and water often passes through various types of filters depending on the type of irrigation system and water source [9]. Sprinkler irrigation—including solid sets, periodic move or continuous move systems, traveling guns, and boom sprinkler systems—has advantages over surface irrigation in terms of its high efficiency of water application, ease of fertilizer application, and high resultant crop yields [10]. However, it also has some drawbacks such as high setup costs, high operational costs due to its high energy and maintenance requirements, and its tendency to be adversely affected by wind conditions [7]. Drip irrigation (irrigation systems that are designed to slowly apply water to individual points), on the other hand, overcomes some of these drawbacks by way of low energy requirements and not being affected by wind. It has distinctive water and energy saving features while supporting the agronomy of crops in addressing the challenges faced in irrigated agriculture [11]. However, it may perform poorly with crops that have high water requirements. The major drawback of drip irrigation systems is the clogging of emitters, which leads to poor performance and calls for frequent maintenance. Further, damage by weathering and farm machinery partly explains why such an appealing technology remains unpopular among farmers [6,7,10]. Although subsurface drip irrigation (application of water below the soil surface by drip emitters) systems have been developed to overcome the prevailing practical issues of drip irrigation, they have not performed as expected, since they further aggravate the problem of poor water distribution efficiency due to emitter clogging [12–14]. Therefore, new irrigation methods are being developed to irrigate upland crops that aim to use water more efficiently and effectively while minimizing costs so as to improve profitability and sustainability. In this regard, it is essential to minimize major water losses through evaporation, surface runoff, and percolation in order to economize on the limited availability of water, while also driving down the labor and energy requirements and keeping operational costs to a minimum. Therefore, any new design should be able to ensure (i) high application efficiency with uniform distribution; (ii) low capital investment; (iii) low energy and labor requirements to minimize operational costs; (iv) automated operation with minimum supervision; (v) minimum influence from weather, topography, or soil type; (vi) minimum disturbance to other management practices; and (vii) environmentally-friendly technology. Optimized Subsurface Irrigation System (OPSIS) Capillarity, upward water movement in tubes due to cohesion, adhesion and surface tension forces, can also be happen in soil. In soils, water can move upwards through soil pore spaces between soil particles. The height of capillary rise is dependent on pore size as smaller soil pores show higher capillary rise. Our newly developed “optimized subsurface irrigation system” (OPSIS) is designed to irrigate upland crops by means of the capillarity of soil. Water is released by perforated pipes just
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perforated pipes just below the root zone and water moves upward due to the capillarity of soil in belowtothe root zone and water movesirrigation upward due to theperform capillarity of soil in order to irrigate the order irrigate the crops. Subsurface methods better in soils that are vulnerable crops. Subsurface methods perform betterand in soils are vulnerable toresults drought damage to drought damageirrigation (soils with low available water), hencethat OPSIS shows better than other (soils withmethods low available water), and hence OPSIS shows better results than other irrigation methods irrigation in such environmental contexts. OPSIS shows super water-saving capability as in it suchminimize environmental OPSIS super water-saving capability as it can minimize runoff, can runoff,contexts. evaporation andshows percolation. Since only a small solar-powered pump is used to evaporation and percolation. Since only small any solar-powered pumpcosts, is used tosince makeitthe make the elevation head, OPSIS does nota incur ongoing energy and canelevation operate head,minimal OPSIS does not any ongoing energy costs, and since it can operate minimal labor as with labor asincur an automated system, it should drastically bring downwith the operational costs anirrigation. automated system, it should drastically bring down the operational costs of irrigation. of 2. Technical Technical Details 2. Details of of OPSIS OPSIS In OPSIS, OPSIS, water water is is elevated elevated using using aa solar solar powered powered submersible submersible pump pump to to create create an an elevation elevation In head to a higher level, it then flows along the gravity. Subsurface perforated pipes leak water while head to a higher level, it then flows along the gravity. Subsurface perforated pipes leak water while flowing by by gravity, gravity, then then soil soil capillarity capillarityprovides providesthe theirrigation irrigationwater watertotothe thecrops. crops.OPSIS OPSISconsists consists flowing ofof a a main water control unit and a water distribution system (Figure 1). After series of laboratory and main water control unit and a water distribution system (Figure 1). After series of laboratory and field experiments experiments on on the the irrigation irrigation amount, amount, water water distribution distribution and and cost cost effectiveness effectiveness under under local local soil soil field conditions in in Okinawa, Okinawa, the the dimensions dimensions and and materials materials of of construction construction for for each each part part were were determined. determined. conditions
Figure 1. Schematic Schematic diagram diagram of of Optimized Optimized subsurface irrigation system (OPSIS). Figure
2.1. Main Main Water Water Control System The main water control control system systemincludes includesaawater watertank tanktotostore storewater watertemporarily, temporarily, a solar pump a solar pump to to elevate water, a water supply column to control the flow, waterand flow, and a fertilizer tank to elevate thethe water, a water supply column to control the water a fertilizer tank to facilitate facilitate fertigation. It regulates the quantity and the of pressure of coming all waterinto coming into the fertigation. It regulates both the both quantity and the pressure all water the irrigation irrigation system andcontrolled provides water controlled water and flow outdistribution to the water distribution system and provides and fertilizer flowfertilizer out to the water system. system. 2.1.1. Water Tank 2.1.1.The Water sizeTank of the water tank (Figure 2) varies according to the source of water and the requirements of theThe farmer. supply of water, such as from an irrigation canal groundwater, is regulated by size The of the water tank (Figure 2) varies according to or the source of water and the a ball tap. Because of the head difference between the water tank and the field, excess water from requirements of the farmer. The supply of water, such as from an irrigation canal or groundwater, is the OPSISby lines andtap. drainage water field flows back to the water tank. solar-powered regulated a ball Because of thefrom headthe difference between the water tank andAthe field, excess submersible pump is used to pump the water out from the tank and into the water supply water from the OPSIS lines and drainage water from the field flows back to the watercolumn. tank. A solar-powered submersible pump is used to pump the water out from the tank and into the water supply column.
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Figure 2. Schematic diagram of water tank. Figure 2. Schematic diagram of water tank. Figure 2. Schematic diagram of water tank.
2.1.2. Water Supply Column 2.1.2. Water Supply Column 2.1.2. The Water Supply Column water supply column (Figure 3) provides a controlled and constant flow of water and The water supply column (Figure system. 3) provides a controlled constant flow of water and fertilizer fertilizer into the water distribution A constant waterand level is maintained in the column by The water supply column (Figure 3) provides a controlled and constant flow of water and intomeans the water distribution system. A constant water level is maintained in the column by means of a a drainage tube attached tosystem. it. The A pressure volume discharged the water fertilizerofinto the water distribution constantand water levelofis water maintained in thetocolumn by drainage tube attached to it. The pressure and volume of water discharged to the water distribution distribution system istube controlled bytoa it. micro mechanism in the column. A thin tube wrapped means of a drainage attached The tubing pressure and volume of water discharged to the water system is controlled by asends microthe tubing mechanism in slowly the column. Aoutlet thin tube wrapped around around the center pipe water smoothly and into the of the water column. Thethe distribution system is controlled by a micro tubing mechanism in the column. A thin tube wrapped center pipeofsends the water smoothly and slowly into thecolumn, outlet ofand thecan water column. The volume volume discharge depends on the water height in the therefore be controlled by of around the center pipe sends the water smoothly and slowly into the outlet of the water column. The discharge depends on the water height in the column, and can therefore be controlled by adjusting adjusting the height of the drainage tube. The water column also connects to the fertilizer tank and volume of discharge depends on the water height in the column, and can therefore be controlled by provides controlled of water to the column water distribution column through pipe. theadjusting height ofathe the drainage tube. The water also connects the fertilizer tank and tank provides height offlow the drainage tube. The water column alsoto connects to an theunderground fertilizer and a controlled flow of water to the water distribution column through an underground pipe. provides a controlled flow of water to the water distribution column through an underground pipe.
Figure 3. Water supply column (a) Schematic diagram of water supply column (b) Use of micro tubing mechanism. Figure 3. Water supply column (a) Schematic diagram of water supply column (b) Use of micro Figure 3. Water supply column (a) Schematic diagram of water supply column (b) Use of micro tubing mechanism. tubing mechanism. 2.1.3. Fertilizer Tank
2.1.3. Fertilizer Fertilizer dissolved Tank in water is added to a compressible bag inside the fertilizer tank. The water 2.1.3. Fertilizer Tank fed by the water columnincreates pressure inside the fertilizer which compresses bag and Fertilizer dissolved water is added to a compressible bagtank, inside the fertilizer tank.the The water thus releases the fertilizer at very low rates into the irrigation system (Figure 4). Fertilizer dissolved in water is added to a compressible bag inside the fertilizer tank. The water fed by the water column creates pressure inside the fertilizer tank, which compresses the bag and fedthus by the water column creates pressure inside the fertilizer tank, which compresses the bag and thus releases the fertilizer at very low rates into the irrigation system (Figure 4). releases the fertilizer at very low rates into the irrigation system (Figure 4).
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Figure 4. Schematic diagram of fertilizer tank.
Figure 4. Schematic diagram of fertilizer tank. Figure 4. Schematic diagram of fertilizer tank.
2.2. Water Distribution System Figure 4. Schematic diagram of fertilizer tank.
2.2. Water Distribution System 2.2. Water Distribution System
The water distribution system is the part responsible for distributing the irrigation water 2.2. Water Distribution System The The water distribution is athe part responsible for distributing the end irrigation equally equally over thedistribution field. It system includes water column the head the water fieldwater that water system is the distribution part responsible for atdistributing the of irrigation The water distribution system is the part responsible for distributing the irrigation water distributes the water equally among the OPSIS lines, polyvinyl chloride (PVC) or metal sheet to the overequally the field. It includes distribution column at the headatend the field that over the field. aItwater includes a water distribution column theofhead end of thedistributes field that equally over the field. It includes a water distribution column at the head end of the field that control percolation, and perforated pipes buried horizontally under the field surface to irrigate water equally among theequally OPSIS lines, (PVC) orchloride metal sheet control distributes the water amongpolyvinyl the OPSISchloride lines, polyvinyl (PVC)toor metal percolation, sheet to distributes the waterand equally amongpipes the OPSIS lines, polyvinylunder chloride or metal to field.percolation, control buried the (PVC) field surface to sheet irrigate and the perforated pipes buriedperforated horizontally under the horizontally field surface to irrigate the field. control percolation, and perforated pipes buried horizontally under the field surface to irrigate the field. the field. Water Distribution Column 2.2.1.2.2.1. Water Distribution Column 2.2.1.The Water Distribution Column water distribution column(Figure (Figure5) 5) distributes distributes water perforated pipes (OPSIS lines) The water column watertoto5–7 5–7 perforated pipes (OPSIS lines) 2.2.1. Water distribution Distribution Column buried below the soil surface. The mechanism in the water distribution column allows water to be The water distribution column (Figure 5) distributes water to 5–7 perforated pipes (OPSIS lines) buried below the soil surface. The mechanism in the water distribution column allows water to be The water distribution column (Figure 5) distributes water toin 5–7the perforated pipes (OPSIS lines) distributed equally to surface. all OPSIS lines despite any irregularities land. Equal distribution of buried below the soil The mechanism in the water distribution column allows water to be distributed equally all surface. OPSIS lines despite anyinirregularities in the land. Equalallows distribution of be water buried thetosoil Thelines mechanism the water column waterWhen to water tobelow all OPSIS lines is ensured by having same heightdistribution of water the discharge tubes. distributed equally to all OPSIS despitetheany irregularities in theinland. Equal distribution of to allseveral OPSIS lines is ensured by having the same height of water in the discharge tubes. When several distributed equally to all OPSIS lines despite any irregularities in theinland. Equal distribution of distribution are used onthe slopes, the equal water can be ensured water towater all OPSIS lines iscolumns ensured by having same height ofdistribution water theofdischarge tubes. When water distribution columns are used on slopes, the equal distribution of water cantubes. be ensured by water to all OPSIS lines is ensured by having the same height of water in the discharge When by adjusting the discharge tubes in all water distribution columns to be at the same level (Figure 6). several water distribution columns are used on slopes, the equal distribution of water can be ensured adjusting the discharge tubes in all water distribution columns to be at the same level (Figure 6). several water distribution columns are used on slopes, the equal distribution of water can be ensured by adjusting the discharge tubes in all water distribution columns to be at the same level (Figure 6). by adjusting the discharge tubes in all water distribution columns to be at the same level (Figure 6).
Figure 5. Schematic diagram of water distribution column of OPSIS. Figure 5. Schematic diagram of water distribution column of OPSIS.
Figure 5. 5. Schematic distributioncolumn column OPSIS. Figure Schematicdiagram diagramof of water water distribution ofof OPSIS.
Figure 6. OPSIS can ensure equal discharge on sloping land. Figure 6. OPSIS can ensure equal discharge on sloping land. Figure 6. OPSIS can ensure equal discharge on sloping land.
Figure 6. OPSIS can ensure equal discharge on sloping land.
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2.2.2. Perforated OPSIS Lines Water 2017, 9, 599
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Perforated 50-mm pipes release the water. When OPSIS is operating, water flows under gravity 2.2.2. Lines advances, it can move to the outside soil, depending on the water along thePerforated pipes. AsOPSIS the water potential.Perforated As water50-mm movespipes outside, thethe soil becomes Thereafter, theflows waterunder potential of the release water. Whensaturated. OPSIS is operating, water gravity alongsoil theand pipes. As the water advances, can move to This the outside soil, depending on the water and outside inside of the pipe reachesitequilibrium. equilibrium controls the amount As water movesthe outside, soil to becomes Thereafter, the waterlayer potential of the ratepotential. of irrigation. Further, waterthe starts move saturated. upward from the saturated owing to the outside soil and inside of the pipe reaches equilibrium. This equilibrium controls the amount and water potential created by matrix effects such as capillary action created via surface tension. As water rateupward, of irrigation. Further, the waterof starts move upward fromand theprovides saturatedirrigation layer owing to the moves the moisture content roottozone soil increases water to the water potential created by matrix effects such as capillary action created via surface tension. As crop. For crops planted in rows, the spacing of the OPSIS lines could be maintained to match the row water moves upward, the moisture content of root zone soil increases and provides irrigation water spacing. In sandy soils, closer OPSIS lines would be preferred, whereas in clayey soils, a much wider to the crop. For crops planted in rows, the spacing of the OPSIS lines could be maintained to match spacing may be used. However, field and laboratory experiments on water distribution confirmed that the row spacing. In sandy soils, closer OPSIS lines would be preferred, whereas in clayey soils, a it is much advisable maintain distance of aboutfield 1–3 and m between two OPSIS lines according to the soil widertospacing mayabe used. However, laboratory experiments on water distribution typeconfirmed and crop spacing to ensure an even water supply for all crops while minimizing costs and water that it is advisable to maintain a distance of about 1–3 m between two OPSIS lines losses. Field and laboratory experiments on water distribution, workability condition and possible according to the soil type and crop spacing to ensure an even water supply for all crops while damage by tillage equipment confirmed that the of the lines could vary between 30 and 60 cm minimizing costs and water losses. Field anddepth laboratory experiments on water distribution, workability by tillage equipment confirmed that the depth of the according to thecondition soil typeand andpossible the rootdamage zone depth. lines could vary PVC between 30 andsheet 60 cmcan according toto thecontrol soil type and the root zone depth. A waterproof or metal be used percolation losses. The sheet is buried A waterproof PVC or metal sheet can be used to control percolation losses. The sheeton is buried in an inverted trapezoidal shape (Figure 7). Ability to control percolation losses, effect crop yield in an inverted trapezoidal shape (Figure 7). Ability to control percolation losses, effect on crop and the availability of materials were considered when determining the size and shape of theyield PVC or and the availability of materials were considered when determining the size and shape of the PVC or metal sheet to control percolation losses. After a series of field and laboratory experiments, optimum metal sheet to control percolation losses. After a series of field and laboratory experiments, optimum minimum height to minimize percolation was identified as 15 cm. Therefore, the optimum size of minimum height to minimize percolation was identified as 15 cm. Therefore, the optimum size of the shape has been determined to be 12 cm wide at the base, 30 cm wide at the top, and 15 cm high the shape has been determined to be 12 cm wide at the base, 30 cm wide at the top, and 15 cm high considering the prices of materials available in the market. The perforated pipe is positioned in the considering the prices of materials available in the market. The perforated pipe is positioned in the center of the shape. center of the shape.
Figure 7. Use of polyvinyl chloride or metal sheet to control percolation losses.
Figure 7. Use of polyvinyl chloride or metal sheet to control percolation losses.
2.3. OPSIS can Act as a Drainage System
2.3. OPSIS can Act as a Drainage System
During the rainy season (when OPSIS might not operate, owing to low solar radiation) the During thesaturated rainy season (when OPSIS might notpipes operate, owing towater low solar radiation) theAs water water in the soil can enter the perforated following the potential gradient. in the saturated soil can enter the perforated pipes following the water potential gradient. As water water is circulating, the tail-end water collects in the drainage pipe and is diverted to the water tank. is Thus, OPSIS could actwater as a subsurface drainage system pipe during rainy periods. to the water tank. Thus, circulating, the tail-end collects in the drainage and is diverted Figure shows the daily drainage rainfall, irrigation or drainage OPSIS, and level of soil saturation OPSIS could act8 as a subsurface system during rainybyperiods. (percent water-filled volume to the total porosity of theby soil) in the root zone of the Figure 8ofshows the daily rainfall, irrigation or drainage OPSIS, and level of area soil saturation experiment field in Itoman, Okinawa, Japan during July 2014. The irrigation or drainage axis shows (percent of water-filled volume to the total porosity of the soil) in the root zone area of the experiment the net amount of water that goes out of (positive values) or comes into (negative values) the water field in Itoman, Okinawa, Japan during July 2014. The irrigation or drainage axis shows the net tank. Positive values represent the irrigation while negative values show the amount of drainage. amount of water that goes out of (positive values) or comes into (negative values) the water tank. The level of soil saturation (%) in the root zone area every day was calculated using daily soil Positive values represent the irrigation while negative values show the amount of drainage. The level moisture (volume basis) and soil porosity. It shows that OPSIS can manage soil moisture content at
of soil saturation (%) in the root zone area every day was calculated using daily soil moisture (volume basis) and soil porosity. It shows that OPSIS can manage soil moisture content at desirable levels even
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desirable levels even in very high rainfall seasons, such as that shown in Figure 8, where rainfall was in desirable very rainfall seasons, as that seasons, shown insuch Figure 8, where was 832 mm per month, even in very such high rainfall asofthat shown in Figure 8, where rainfall was 832 mmhigh perlevels month, including a maximum daily rainfall 350 mm. rainfall including maximum daily rainfall of 350 mm. 832 mmaper month, including a maximum daily rainfall of 350 mm.
Figure Variationininrainfall, rainfall,irrigation/drainage, irrigation/drainage, and saturation. Figure 8. 8. Variation andpercentage percentageofofsoil soilmoisture moisture saturation. Figure 8. Variation in rainfall, irrigation/drainage, and percentage of soil moisture saturation.
2.4. System Installation 2.4. System System Installation Installation 2.4. The major component of OPSIS installation is the laying out of the perforated OPSIS lines. The major component ofof OPSIS installation is the out ofout the of perforated OPSIS lines. Initially The major component OPSIS installation is laying theexcavator laying perforated OPSIS lines. Initially this was done by digging with a conventional with athe bucket. Since this is time this was done by digging with a conventional excavator with a bucket. Since this is time consuming Initially this was done by digging with a conventional excavator with a bucket. Since this is time consuming and expensive, an attachment was developed to lay out the system more efficiently and and expensive, an attachment was developed to lay out the system more efficiently and effectively. consuming and expensive, an is attachment to layand outthe thesheet system more efficiently and effectively. The attachment able to laywas outdeveloped both the pipe simultaneously, thus The attachment isattachment abledown to lay the outable bothtoand the sheet simultaneously, drastically drastically cutting cost time of the installation whilethe significantly improvingcutting the effectively. The is laypipe outand both the pipe and sheetthus simultaneously, thus down the cost andaccuracy time of installation while significantly improving thedeveloped workability and accuracy of workability and of system layout. The attachment iswhile being to further reducethe drastically cutting down thethecost and time of installation significantly improving the system layout. The attachment is being developed to further reduce the initial establishment cost the initial establishment cost and accuracy of the system layout (Figure 9). workability and accuracy of the system layout. The attachment is being developed to further reduce andinitial accuracy of the system (Figure 9). the establishment costlayout and accuracy of the system layout (Figure 9).
Figure 9. Installation of OPSIS lines (a) Manually laying out lines, (b) Use of machine to lay out lines.
Figure 9. Installation of OPSIS lines (a) Manually laying out lines, (b) Use of machine to lay out lines. Figure 9. Installation of OPSIS lines (a) Manually laying out lines, (b) Use of machine to lay out lines.
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Field were set to examine the optimum depth of installation, optimum 8number of Waterexperiments 2017, 9, 599 of 11 OPSIS lines per water distribution column, possible length of OPSIS lines. Further, the growth and 2.5. of OPSIS OPSIS 2.5. Field Field Testing Testingof of yield performances sugarcane, maize and soybean with OPSIS were tested in different parts of Field were to examine examine the optimum optimum depth of installation, optimum number Japan. Some keyexperiments results showed that maize and soybeandepth with of OPSIS reported 40% and 50%of Field experiments were set set to the installation, optimum number ofhigher lines per compared water distribution distribution column, possible length Further, growth andduring yields, OPSIS respectively, to surface irrigation Fieldlines. experiments conducted OPSIS lines per water column, possible methods. length of of OPSIS OPSIS lines. Further, the the growth and yield performances of sugarcane, maize and soybean with OPSIS were tested in different parts of yieldto performances sugarcane, maize and soybean withthat OPSIS were tested in different parts cane of Japan. 2012–2014 study the of performance of OPSIS showed OPSIS increased the fresh yield by Japan. Some key results showed that maize and soybean with OPSIS reported 40% and 50% higher Some key results showed that maize and soybean with OPSIS reported 40% and 50% higher yields, 79–116% compared to the rainfed conditions, as attributed by higher plant height, cane diameter and yields, respectively, compared toirrigation surface irrigation Field experiments during respectively, compared to surface methods. methods. Field experiments conductedconducted during 2012–2014 a higher numberto of millable stalks perof unit area. Figures 10 and 11 shows the cane growth and 2012–2014 study the performance OPSIS showed that OPSIS increased the fresh yield by yield to study the performance of OPSIS showed that OPSIS increased the fresh cane yield by 79–116% performances oftosugarcane with OPSIS compared to rainfed conditions. 79–116% to the rainfed conditions, as attributed higher plantcane height, cane diameter and comparedcompared the rainfed conditions, as attributed by higherby plant height, diameter and a higher anumber higherofnumber millable stalks perFigures unit area. Figures 10 and showsand theyield growth and yield millableofstalks per unit area. 10 and 11 shows the11 growth performances performances of sugarcane with OPSIS compared to rainfed conditions. of sugarcane with OPSIS compared to rainfed conditions.
Figure 10. Growth of sugarcane with OPSIS and rainfed conditions (a) Plant height, (b) Canopy Figure 10. Growth of sugarcane with OPSIS and rainfed conditions (a) Plant height, (b) Canopy Figure 10. Growth of sugarcane with OPSIS and rainfed conditions (a) Plant height, (b) Canopy cover. cover. cover.
Figure 11. Fresh cane yield of sugarcane with OPSIS and rainfed condition.
Figure 11. Fresh cane yield ofofsugarcane withOPSIS OPSIS and rainfed condition. Figure 11. Fresh cane yield sugarcane with and rainfed condition. 3. Discussion
3. Discussion 3.1. Special Features of OPSIS 3. Discussion A series of field and laboratory experiments were carried out to examine the performances of
3.1. Special Features of OPSIS 3.1. Special Features of OPSIS OPSIS. Based on the results, observations and experiences, the following special features of OPSIS A series of field and laboratory experiments were carried out to examine the performances of
identified. A were series of field and laboratory experiments were carried out to examine the performances of OPSIS. Based on the results, observations and experiences, the following special features of OPSIS OPSIS.were Based on the results, observations and experiences, the following special features of OPSIS identified. 3.1.1. Water-Saving Irrigation Method were identified. shows improved capabilities compared with other irrigation methods as it 3.1.1.OPSIS Water-Saving Irrigationwater-saving Method is able to function with minimum percolation, evaporation, and surface runoff. 3.1.1. Water-Saving Irrigation Method OPSIS shows improved water-saving capabilities compared with other irrigation methods as it is able toEnsures function with minimum percolation, evaporation, and surface runoff. 3.1.2. Uniform Water Distribution
OPSIS shows improved water-saving capabilities compared with other irrigation methods as it is able to function with minimum percolation, evaporation, andsuitable. surfaceItrunoff. OPSIS can be used on slopes, where surface irrigation is not requires less attention to land leveling than surface irrigation methods, and it is better than other irrigation methods in achieving the equalWater distribution of irrigation water on slopes. 3.1.2. Ensures Uniform Distribution
OPSIS can be used on slopes, where surface irrigation is not suitable. It requires less attention to land leveling than surface irrigation methods, and it is better than other irrigation methods in
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3.1.2. Ensures Uniform Water Distribution OPSIS can be used on slopes, where surface irrigation is not suitable. It requires less attention to land leveling than surface irrigation methods, and it is better than other irrigation methods in achieving the equal distribution of irrigation water on slopes. 3.1.3. Ensures Good Crop Yields Field experiments conducted in different places in Japan using sugarcane, maize, and soy beans as test crops confirmed the high yields obtained with OPSIS compared with other irrigation methods. 3.1.4. Ensures Sanitary Field Conditions Since the surface layer remains dry, OPSIS provides optimum workability conditions and allows the maintenance of sanitation in the field. The dry state of the topsoil helps to maintain good workability and creates low humidity, especially in protected greenhouses. No topsoil splattering or erosion hazards occur, as there are no surface water droplets or flowing water with OPSIS. 3.1.5. Enables Fertigation Water-soluble fertilizers can be effectively used with the irrigation water with OPSIS. The slow-release nature ensures higher fertilizer use efficiency than can be achieved under other fertigation or fertilizer application methods. 3.1.6. Minimal Operational Costs OPSIS is powered by solar energy, and therefore it has no real energy costs. As an automatic system, it also requires minimum human supervision for irrigation during the cropping season. As a subsurface system, it causes minimal disturbance to other field operations. Further, it does not require comprehensive land leveling. Therefore, OPSIS has the lowest operational costs of all irrigation methods. 3.1.7. No Clogging No large debris can enter to water distribution system as water has to pass the micro tube in the water supply column. As an open-ended system, OPSIS do not experience negative pressures inside the lines nor soil ingestion when stopping the system, which usually happens in subsurface drip irrigation systems. 3.1.8. Long Durability Once OPSIS is installed, it can be used for years without any need for re-installation because there is no damage by sunlight, animals or farm machinery. 3.1.9. Drainage System OPSIS can act as a subsurface drainage system. Therefore, no separate drainage system is required for water management in fields where OPSIS is installed. 3.1.10. Environmentally-Friendly Technology Being a solar-powered irrigation system, OPSIS does not consume any fossil fuels. As a subsurface fertigation system, it emits less greenhouse gases than the surface application of fertilizers or fertigation. Further, owing to minimum percolation losses and the slow release of fertilizer, OPSIS helps to minimize the contamination of groundwater with fertilizer.
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3.2. Limitations of OPSIS The installation of OPSIS requires considerable initial cost. However, factors such as no need for a separate drainage system, high crop yields, minimum operational costs, and long durability can help recover the high initial cost of installation within few years. As some percolation losses are inevitable, losses of fertilizer can happen with OPSIS. After the installation of the system, deep plowing will be impossible as it can damage the system. Therefore, to the extent possible, care should be taken to avoid the development of a hardpan, and suitable machinery that can break up the hardpan should be used only with utmost care. 4. Recommendations Although OPSIS is commercially available for sugarcane farmers in Okinawa, Japan, it needs validation of its performance and further improvement to make it a more highly efficient and low-cost irrigation method. Further, to help farmers operate OPSIS with minimal problems and to make irrigation more profitable and sustainable, guidelines for best management practices need to be developed and introduced. Installation of OPSIS requires a considerable initial investment; therefore, new strategies and technologies should be developed to minimize the initial cost. OPSIS provides irrigation water to the field automatically whenever there is solar radiation available to operate the pump. This can lead to some percolation losses, especially during dry periods and in fields with low groundwater levels or sandy soil. Changing the automatic operational mechanism to one that adjusts the pump according to soil moisture by incorporating soil moisture sensors could be helpful to further reduce percolation losses and maximize the lifespan of the pump. The optimum depth at which to install the perforated OPSIS pipes may vary depending on the soil type, crop to be grown, and depth to the groundwater table. Since OPSIS has been tested with only a limited number of crops and soil types, further studies should be carried out under different soil and climate conditions with different crops. Patterns of root distribution that develop with subsurface irrigation systems might differ from those that develop with other irrigation systems. Therefore, studies focused on root distribution and soil moisture extraction patterns might be helpful for the further development of OPSIS. It might be difficult to break the hardpan after installing the OPSIS. Therefore, new technologies or strategies might have to be developed to minimize the creation of hardpans and to break them without damaging OPSIS lines. OPSIS has been tested only for small- and medium-scale fields in Japan. The maximum length of OPSIS lines tested was less than 100 m. Therefore, its applicability for large-scale fields has yet to be examined. Author Contributions: T.O. and K.W. designed the OPSIS; M.H.J.P.G., K.S., T.N., M.K., T.O., H.K., H.U. and K.W. conceived, designed and performed the experiments; M.H.J.P.G., K.S., T.O., T.N. and M.K. analyzed and interpreted the data; M.H.J.P.G. and K.S. wrote the paper; all authors read and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.
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