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J. Trop. Agric. and Fd. Sc. 26(2) (1998): 195–201

Initial irrigation requirements of dry tilth paddy soil(Keperluan pengairan permulaan untuk tanah padi putar kering)

C. S. Chan*

Key words: dry-seeded rice, irrigation, water advance rate, land soaking requirement

AbstrakKajian ini memberikan maklumat tentang keperluan pengairan kali pertama untukpengairan sawah bagi penanaman padi tabur kering. Keperluan pengairan ialahjumlah air untuk memenuhi keperluan meresap tanah serta kehilangan melaluiproses peresapan dan penelusan. Kajian ini termasuklah hubungan kemampatantanah dengan masa air mara, kadar aliran air dengan masa berlaluan, kadar aliranair dengan keperluan meresap tanah, masa berlaluan dengan keperluan meresaptanah. Selain ini, masa air mara yang diperhatikan dan yang dikira juga dibanding.Kebanyakan parameter ini didapati berkait rapat satu sama lain seperti yangditunjukkan oleh pekali regresi yang agak tinggi. Misalnya apabila kadar aliranair ialah 1.0 L/saat/m, masa purata air mara di permukaan tanah ialah 110 minituntuk sawah yang sepanjang 80 m dengan kedalaman purata pembajakan 120mm. Dalam keadaan ini, keperluan meresap tanah ialah 80 mm kedalaman air.

AbstractThis study provides information related to the first irrigation requirement inirrigating a dry direct-seeded rice field. Irrigation requirement is the total amountof water applied to meet land soaking requirement, seepage and deep percolationlosses. Soil compaction and time of water advance, water flow rate and elapsedtime, water flow rate and land soaking requirement, elapsed time and landsoaking requirement relationship were studied. The observed and calculatedvalues of the water advance time were compared. Most of these parameters areclosely related to each other as expressed by its relatively high coefficient ofregression. For instance, when the water flow rate is 1.0 L/s/m, the average timeof advance over soil surface is 110 min for an 80-m field run of an averagetillage depth of 120 mm. The land soaking requirement is about 80 mm depth ofwater under such conditions.

*MARDI Research Station, Seberang Perai, Locked Bag 203, 13200 Seberang Perai, Pulau Pinang, MalaysiaAuthor’s full name: Chan Chee Sheng©Malaysian Agricultural Research and Development Institute 1999

IntroductionWith the limitation in irrigation wateravailability for paddy production in the dryseason (March-April), dry direct-seeded riceis adopted to save water. This wouldeliminate the practice of soil presaturationand wet soil puddling. As such, theseedlings will be raised initially under dry

conditions while inundation would follow ata later stage. Thus, the irrigation practice,particularly the initial irrigation to soak adry direct-seeded paddy field, is a totallydifferent scenario compared with theconventional presaturation irrigationpractice. The dry direct-seeding practice isonly practical for the off-season planting. An

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accurate estimate of the net amount of waterapplied for the initial irrigation is a basicrequirement for optimal irrigationmanagement. Applying the correct amountof water is important both for efficientwater use and to reduce groundwatercontamination resulting from deeppercolation (Zur et al. 1994).

Basically, the principles of surfaceirrigation are comparatively simple(Podmore and Eynon 1983). Water isapplied to one end of the field at a rate thatwill provide coverage of the entire field in ashort period. The water is then ponded untilmost of it infiltrates into the soil. In standardsurface irrigation design manual (Anon.1974), the formulae developed describe thefield with intake family depending on soiltype. For lowland direct-seeded rice field,the topsoil is pulverized by rotavation andthe layer below the topsoil is compacted(hardpan). This soil characteristic does notfit well into any intake family.

Paddy soils, generally characterized byheavy texture, poor structure, low porosityand permeability as well as shallowhardpans, are different from other uplandagricultural soils. Thus, the irrigationrequirements for the dry tilth paddy soil willalso be different from the conventionalupland irrigation practices. The design andplanning of an efficient surface irrigationsystem requires knowledge of the time ittakes for the advancing water front to reacheach point in the field. This will determinethe total time available for infiltration(opportunity time) at any specific point. Thefirst irrigation always takes the longest timeof advance compared with subsequentirrigation, and hence the first irrigationparameters become the determining factorsfor irrigation system design. This study wasconducted to determine the initial irrigationparameters for establishing irrigation designand management of dry direct-seeded rice.The study took into considerations thedifferent amounts of water applied andlevels of soil compaction on a dry tilledpaddy field which is believed to improve

seed germination according to localpractices.

Materials and methodsThe experiment was conducted in MARDIResearch Station, Seberang Perai, during theDecember-to-February drought period from1993 to 1996. The area was previously arubber estate before it was converted intopaddy cultivation. The paddy has beenplanted in the area for the last 14 years.Generally, the soil textures can becategorized as clay soil. The particle-sizedistribution for sand, silt and clay is 9, 25and 66% respectively. The experimentalfield was graded to 0.1% and levelled withthe aid of a laser levelling device attached toa tractor-mounted back-bucket. The threetreatments were arranged in a randomizedblock design with three replicates asfollows:

• T0 – the field was prepared by threerounds of rotavation to 120 mm oftillage depth without compaction(control),

• T1 – the field was prepared similar to T0but with one round of roller compaction,and

• T2 – the field was prepared similar to T0but with two rounds of roller compaction.

The experimental blocks run parallel toeach other with dimensions of 5.6 m by 80 mand separated by wood-reinforced ridges tominimize overflow, lateral seepage andleakage. The roller used for compactionmeasured 0.39 m in diameter, 2.52 m inlength and weighed 101 kg. A constant anduniform stream of water was discharged intoeach block by using an elevated constant-head design fibreglass tank. The flow ratewas calibrated for every water applicationand the total amount of water supplied wasrecorded. The water advance rate wasrecorded at 10-m intervals by visualobservation of the water advance front onthe soil surface. To compare time of advanceover the soil surface under different levels ofsoil compaction, a constant flow rate was used.

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The experiment assumed that priorconditions for studying various relationshipsof flow rate and time of advance of a dryseeded field are as follows:

• accurate land levelling and maintenanceof a level surface,

• volume of water delivered to the field isadequate to cover the field to an averagedepth that is equal to the gross irrigationapplication (80–120 mm), and

• depth of flow is not greater than the fieldridges.

Basic soil physical data were recordedbefore each water application. Theseincluded soil bulk density, soil moisturecontent and soil clod size distribution.Infiltration parameters for the Kostiakov-Lewis equation were taken using cylinderinfiltrometer from test area and best fitted asa straight line after a logarithmictransformation (Table 1). The relation(Walker and Skogerboe 1987) is:Z = Kτa + foτwhere Z = cumulative infiltration (m3/m)

K = empirical constant (m3/mina/m)τ = intake opportunity time (min)a = empirical exponent (dimensionless)fo = basic intake rate (m3/min/m)

There is an implied width of 1 m for borderand basin system.

The computer simulation programSIRMOD (Anon. 1989) was used to simulatethe time of water advance over the soil

surface at the field level for comparisonwith visual observation. SIRMOD is asoftware for simulating the hydraulics ofsurface irrigation systems at the field level.This involves the numerical solution at theSaint-Venant Equations. The softwareprovides analytical capability with respect toall of the variables affecting surfaceirrigation design and management. Itsimulates the system response to inputvalues of each variable.

Results and discussionSoil compaction and time of advanceAt a unit flow rate of 1.0 L/s/m, the resultsshowed that time of advance between thethree levels of soil compaction (at the soilmoisture content of less than 10% on dryweight basis) was not significantly affectedby compaction effects. This may be due tothe high coefficient of variation (30%).Nevertheless, treatment 2 (T2) resulted inthe fastest average advance flow rate ofwater (Figure 1) by 20% over treatment 1(T1). At this soil moisture content (10%), thesoil cannot be further compacted asmeasurements of soil bulk density indicated(Table 1). For the advancing front toproceed forward, the tilled soil above thehardpan has to be saturated first whichsubsequently will develop an inflow depthand push the water further. Since there is nosignificant difference in compaction effects,the rate of soaking is also not influenced by

Table 1. Some soil physical characteristics of experimental plot

Treatment Bulk Soil Distribution (%) of Empirical Empirical Basicdensity moisture 4 clod sizes (mm) constant exponent intake rate(g/cm3) (% dry (m3/mina/m) (a) (m3/min/m)

weight) >30 20–30 10–20 <10

T0 1.19 7.8 3.3 6.9 19.3 71.6 0.0040 0.60 0.00002T1 1.17 4.7 4.4 8.2 24.7 62.7 0.0045 0.56 0.00002T2 1.23 4.7 3.6 7.5 24.4 64.6 0.0041 0.66 0.00002

C.V. (%) 5.56 12.65 65.4 46.3 12.4 12.2

T0 – the field was prepared by three rounds of rotavation to 120 mm of tillage depth without compaction(control)

T1 – the field was prepared similar to T0 but with one round of roller compactionT2 – the field was prepared similar to T0 but with two rounds of roller compaction

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the compaction treatments as expected.Therefore, a pooled equation can be used toestimate the time of water advance under thesimilar field condition given the unit flowrate. The relation is T = 0.016x2 + 0.6x +5.9 where T is elapsed time (min) and x isthe distance from head ditch (m). Value ofthe Manning Roughness Coefficient, 0.1 forthe freshly tilled soil condition, was used assoil clod on the field surface obstruct thewater movement (Walker and Skogerboe1987). The average time of advance to coveran 80-m field run ranged from 90 min to110 min, when the flow rate was 1.0 L/s/m.

Flow rate and elapsed timeResults of the regression analysis betweenflow rate and elapsed time for an 80-m fieldrun showed that the elapsed time decreasedlinearly with an increase in flow rate(Figure 2). The relationship can beexpressed as T = –91.0q + 225.8 where

T is elapsed time (min) and q is flow rate(L/s/m). The flow rate should not exceed themaximum erosive velocity, that is 31.4 L/s/mfor basin irrigation (Walker and Skogerboe1987) which generally depends on the soiltype. On the other hand, the flow velocitymust be large enough to attain high waterapplication efficiency which is dependent onsoil infiltration characteristics and the fieldslope. In normal practice, the flow rateshould range from 1.4 to 2.4 L/s/m and theelapsed time should not be more than 3 hfor a 100-m field run.

Flow rate and land soaking requirementLand soaking requirement is the totalamount of water applied to a depthequivalent to the depletion of soil moisturebelow field saturation at the time ofapplication over the entire field. The termland soaking requirement is used hereinstead of presaturation requirement or

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Figure 1. Advance of water (at1.0 L/s/m) over soil surface attwo levels of soil compaction

Figure 2. Relationship betweenflow rate and elapsed time to coveran 80-m field

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application depth because of two reasons.Firstly only the tilled soil is saturated soonafter contact with the water front andsecondly low vertical percolation occurs dueto low permeability of the hardpan.Presaturation requirement is the total amountof water required to saturate a dry paddyfield and maintain 50–100 mm depth ofstanding water before land preparation.Normally, the presaturation period lastsmore than a week. Under the presentirrigation practice, the designedpresaturation requirement is 150 mm depthof water. The studies showed that the landsoaking requirement declined linearly withan increase in flow rate (Figure 3).However, the relationship is not wellcorrelated where r value is only 0.61. If theflow rate values were separated into twosections ranging from 0.7 to 1.4 L/s/m, and1.4 to 2.1 L/s/m, the land soakingrequirement values for the first section wasalways higher than 100 mm compared withthe second section. As the flow rateincreases, an inflow depth will be developedat the advancing front in a shorter time,which gives the driving force or a hydraulicgradient. This causes the advancing front totravel faster on the soil surface, whichsubsequently will reduce the verticalpercolation and lateral seepage losses for agiven length of field due to the fastercompletion of advance. When the flow rateis 0.8 L/s/m, the average depth of water

required to cover an 80-m field is 120 mmcompared with the flow rate of 2.0 L/s/mwhere it requires only 80 mm depth ofwater. Thus, a 30% reduction of waterrequirement can be achieved with a greaterflow rate. For field practice, flow rateshigher than 1.4 L/s/m should berecommended. The current irrigation systemwas designed for transplanting. Thedesigned water duty for presaturation periodwas 41.5 mm/day (4.8 L/s/ha), andsupplementary irrigation of 7.6 mm/day (0.9L/s/ha) (Kitamura 1987). This supply rate isfar too low to meet the irrigationrequirement compared with the flow rateused (125 L/s/ha) in the experiment.

Elapsed time and land soaking requirementThe relationship of elapsed time and landsoaking requirement can be established(Figure 4) from data shown in Figure 2 andFigure 3, and can be expressed asD = 0.38T + 57.1 by regression analysiswhere D is the land soaking requirement(mm) and T is elapsed time (min). The landsoaking requirement increases linearly withthe increase of elapsed time, indicating thatthe longer it takes to soak the field, thehigher the land soaking requirement. Thus,to save water, the duration for the water tocover the field should be reduced as muchas possible, but the flow rate should bewithin the maximum allowable flow toprevent erosion. For instance in this case,

Figure 3. Relationshipbetween flow rate and landsoaking requirement underdry tillage environment

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when the elapsed time to cover an 80-mfield run is less than 100 min, thecorresponding land soaking requirement isalways within 100 mm depth of water.

Observed versus simulated time of advanceThe simulated time of advance values fordifferent flow rates were predicted based onthe Saint-Venant Equation which is thecombination of mass conservation andmomentum conservation principles (Anon.1989). Since the computation involved anumerical solution, the computer programSIRMOD was used. The input data requiredto run the program include method of waterapplication, flow rate, Manning Roughness

Coefficient, infiltration parameters and fieldlength. When compared with the visualobservation values, values of simulated timeof advance were close to the calculatedvalues when the time of advance is small.When the time of advance becomes longer,the calculated values were lower than theobserved values which appear as data pointabove 1:1 line (Figure 5). This could be dueto the large opportunity time, whichsubsequently caused more seepage losses,that was not taken into account in the Saint-Venant Equation. Thus, this could indicatethat the lateral seepage component is themost important factor that must be

Figure 5. Observed versussimulated time of advance

Figure 4. Relationshipbetween elapsed time andland soaking requirement tocover an 80-m field

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considered when this equation is used,particularly if the field is relatively narrow.

ConclusionThe initial irrigation requirement can becalculated based on the stream sizeavailable, plot configuration and the averagetillage depth. For a dry tilled paddy field,initial irrigation requirement decreases withan increase in flow rate, a decrease inadvance time and consequently a betterapplication efficiency. At an average tillagedepth of 120 mm and a flow rate of 1.0L/s/m, the average time of advance over thesoil surface is 110 min. While the averageapplication depth is 80 mm depth of waterto cover an 80-m field. The recommendedflow rate for initial irrigation on dry tilthpaddy soil should range from 1.4 to 2.4 L/s/mfor a 100-m field.

Experiments also showed thatcompacting dry soil at less than 10% of soilmoisture content on dry weight basis usingroller compactor has no significant effect onadvance rate. The SIRMOD model can beemployed to calculate the advance rate orinitial irrigation requirement as long as theeffect of lateral seepage is given dueconsideration for a narrow basin fieldlayout.

AcknowledgementsThe author wishes to express his sincereappreciation to Mr Abu Hassan Daud for hisassistance in the design and manufacture ofthe soil compactor. Thanks are also accordedto Mr Mohd. Rasad Matsom for histechnical assistance.

ReferenceAnon. (1974). Border irrigation. SCS National

Engineering Handbook Washington D.C.:USDA

–––– (1989). Surface irrigation simulation software(SIRMOD) Irrigation Software EngineeringDivision, Department of Agricultural andIrrigation Engineering. Utah State University,Logan Utah

George, J. M. and Manuel, A. (1994). High-frequency basin irrigation design for uplandcrops in rice land. J. Irri. and Drain. Engg.118 (4): 564–83

Kitamura, S. (1987). Irrigation engineering researchon system water management in the MudaScheme. A cooperative study by MADA andTARC (mimeo.)

Podmore, C. A. and Eynon, D. G. (1983).Diagnostic analysis of irrigation systems. Vol.2: Evaluation Techniques. Water managementsynthesis project. University Services Center,Colorado State University Fort Collins. Co.

Walker, W. R. and Skogerboe, G. V. (1987). Surfaceirrigation, theory and practice p. 104–78.Englewood Ciffs, New Jersey: Prentice-hallInc.

Zur, B., Ben-Hanan, U., Rimmer, A. and Yardeni,A. (1994). Control of irrigation amountsusing velocity and position of wetting front.Irrig. Sci. 14: 207–12

Accepted for publication on 30 December 1998