178 TRAFFIC COMPACTION OF SOIL AND TILLAGE REQUIREMENTS

III. A Model of Soil Water Storage for Tillage Studies

W. ARNDT; C. W. ROSE

A model of soil water storage is presented consisting of a partitioning of rainfall into differentagronomically significant fractions. The model is of assistancein understanding interactions betweentraffic compaction, tillage and the input and storage of water in the soil for later crop use. Thecyclic nature of such inputs is described and the effect of traffic compaction on soil water storage isconsidered, using the model for two contrasting types of rainfall regimes.

The prospects of isolating soil water storage from the other objectives of tillage are discussed.

1. IntroductionOne of the important technological aims of

tillage is to improve water relationships in soilfor plant growth. The optimum tillage require­ments to achieve this will depend on climate,soil, and plant factors. A simple model ispresented which partitions precipitation intodifferent agronomically significant fractions.Because of the complexity of the processesinvolved, such a model is of assistance in under­standing interactions between traffic compaction,tillage, and the input and storage of water in thesoil for subsequent crop use.

2. The modelFor the purpose of this model the bare soil

profile is divided into three layers (Fig. 1), con­sisting of a tillage layer (dt ) , a water storage layer(d2) and a deep drainage layer (da). The physicalproperties ofthe tillage layer are readily modifiedby traffic and tillage and water stored in thislayer is rapidly lost in situations of moderate tohigh potential evaporation.' The upper depthlimit of the water storage layer corresponds tothe depth below which water loss by evaporationis negligible over a reasonable period, i.e. for7-14 d after rain. Water profiles to justify thisapproximate distinction are given in Part IV ofthis series. The lower limit of Layer 2 is definedby the maximum depth of effective rooting by thecrop to be sown. The model is primarilyapplicable to the effectively bare soil conditionsduring land preparation and seedling establish­ment phases when the greatest expenditure ofeffort on soil tillage takes place.

For such a situation precipitation P is parti­tioned as shown in Fig. 1 into run-off Sand

p

E

S

IM d l

- - -- - -- --------,- ----------_.D

r A

dz

-- - - - - - - - - -d

u

Fig. 1. A model for partitioning precipitation in tillagestudies

infiltration I (all expressed as equivalent rainfallquantities), the evaporation during rainfall beingnegligible. Infiltration I results in an increase Min water storage in the tillage layer and theamount involved depends largely upon theextent of the evaporation E from the tillagelayer since the previous rain. Drainage D fromthe tillage layer will represent input to the waterstorage layer. This input contributes to anaccession A of water in the storage layer withsome appearing as deep drainage U to Layer 3.U will not be large until the water content in

W. ARNDT; C. W. ROSE 179

Time

Fig. 2. Variation of volumetric soil water content ofLayer J (top) and of input to Layer 2 through the dryingand wetting phases of soil of Ta and Tw duration

respectively

3. The effect of traffic compaction on soil waterstorage

The water balance equation for Layer 1 is:P = E + S + M + D em ... (1)

From the above, term D is most important inthe wetting phase during which the evaporationwill usually be negligible.

Putting E = 0 in Eqn (1)D = (P - S) - M

= I - Mcm ... (2)For any given soil the effect of traffic compac­

tion on input D to Layer 2 depends upon thecharacter of the rainfall. Two contrasting typesof rainfall are considered below.

3.1. Case of light rainfallIt is assumed that rainfall rate never exceeds

the infiltration rate of compacted areas (I = P).From Eqn (2) any differences in D due to trafficcompaction will depend upon differences in M.Suppose volumetric water content is increasedfrom 61 at the end of the drying phase to 62,

Since rainfall is light in this case, assume()2 ::} 6/.

Then since6 = W (pb/p) ... (3)

where W = gravimetric water content, gigPb = bulk density of soil, g/cm 3

P = water density, g/cm 3

it follows thatM = d1 (62 - ( 1)

= d1 Pb (W2 - W1)/ p cm ... (4)Suppose a fractional area Xl of the tillage layer

is compacted to bulk density Pb\ greater than thebulk density Pbo of uncompacted soil (whichoccupies fractional area X o = 1 - Xl)' Whensuch a tillage layer dries out, field data for alateritic red earth in a tropical savannah climate(Part IV) indicate that changes in gravimetricwater content (W) are practically identical inboth compacted and uncompacted bands. Be­cause of the greater bulk density of compactedlayers, and from Eqn (3), this finding indicatesthat a greater volume of water is lost from com­pacted as compared with uncompacted soil in adrying cycle. In a subsequent wetting cycle morewater will thus be used in recharging the compacttillage layer and therefore less will be available asinput D to the storage layer. A theoreticalexpression will now be derived for this decrease

,-,I I I

I III I II I \ I

!} 'L__----- -----

, I4 ~ ---..r- T.. • ,

I II II II I

Sf •.....................--.-.---.i- -.... --'-'"I I, I

~-~I I

Layer 2 is sufficiently high for water movementunder gravity to be comparable with that due tosuction gradients. The water and nutrientscarried as deep drainage U are largely lost to thecrop and place an upper limit on the amount ofI that is desirable.

Since, in general, P is discontinuous and E iscontinuous the water content of Layer 1 and theinput to Layer 2 are markedly periodic, asillustrated by a complete wetting and dryingcycle in Fig. 2. Both drying and wetting phasesmay vary in duration (Td and Tw respectively)and in the changes in water involved. It is con­venient to commence the cycle with the dryingphase when Layer 1 has drained to field capacity(Of cm3/cm3) . During this drying phase thevolumetric water content in Layer 1 falls to61 cm3/cm3 and water input to Layer 2 is smalland may even be negative. During the followingwetting phase the volumetric water content ofLayer 1 may rise above Of and any infiltration inexcess of that required to raise the water contentto Of will mostly appear as input to Layer 2.

L80 TRAFFIC COMPACTION OF SOIL AND TILLAGE REQUIREMENTS

in input to the storage layer due to traffic com­paction and its magnitude illustrated usingtypical values. It is clear from Eqn (2) that ifI > M, this effect is not of much importance butespecially with light rainfall this will only be thecase occasionally.

Let D = Do when there is no compaction andD = D I with fractional area Xl compacted asmentioned above. Assuming (W2 - WI) to bethe same for both soil conditions, it follows fromEqns (2) and (4) thatDo - DI = dl (W2 - WI) (Xo Pbo + Xl Pbl

- pbO)/p = dl (W2 - WI) Xl(Pbl - pbO)/p em... (5)

Using typical values based on data from PartIV,dl = 15cm,(W2 - WI) =0'08,Pbl = 1·5 andPbo = 1·1 g/cm 3, with the commonly used rowcrop: 2-track: 2-row: fixed traffic system ofcultivation (Part I), Xl = 0·25. Then from Eqn (5)Do - DI = 0'I2cm being therefore possibly of theorder of 1 mm equivalent rainfall per cycle. Theimportance of this effect of compaction on D isgreatest with rainfall consisting of frequent lightfalls in an environment of high evaporationpotential. Since the maximum value of Xl isunity, the maximum value of Xl (Pbl - pbO) couldbe about 0·5 em (Fig. 3).

3.2. Case of intense rainfallIt is assumed that rainfall rate in this case

exceeds for a period T (h) both the infiltration

0'05 0'5g/cm 3

~

E 0'4o<,

'"-~

0'3,,-<,

Q.

Cl- 0·2I

Clo

0·1

Fig. 3. Effect on decrease in input to the storage layerwith magnitude of the gravimetric water content difference

and factor Xl (Pb1 - Pb0)

rates i l and io (em/h) of compacted and uncom­pacted soil respectively. Since compaction ingeneral reduces infiltration rate (Part IV), therewill be a greater run-off and so less water storedin Layer 2 when the tillage layer is compacted.Run-off S over the time interval Tis

So = P - t« T for uncompacted soilSl = P - (xo io + Xl il) Tfor compacted soil

where Xo + Xl = INeglecting evaporation during rainfall as

before and, since rain is now assumed intense,neglecting M as small compared to other terms,it follows from Eqn (1) and the above that

Do - DI = Sl - So= io Xl (1 - rl) T em ... (6)

where relative infiltration r l = ~'0

Quite large differences between infiltrationrates io and il as measured with a double-ringinfiltrometer are reported in Part IV. Underintense rainfall these differences can be reduceddue to the damaging effect of raindrop impact.1

However, to illustrate a not untypical magnitudeof this effect of compaction on the water storageinput, assume rl = 0,25, io = 3 cm/h, T = 0·5 hand Xl = 0·25 as above. Then from Eqn (6)

Do - D I = 0·28 cmIf rain is usually in the form of intense storms

then this effect of compaction in the tillage layeron soil water storage D may be ofgreater import­ance than that discussed in Section 3.1.

The effect of a range of values of relativeinfiltration rates r1 and of Xl on the decrease in Dis illustrated in Fig. 4.

4. DiscussionSince the reduction in soil water storage by

compacted tillage layers depends upon two quitedifferent processes whose relative importancevaries with the nature of the rainfall, the physicalnature of the response in water storage to tillageover a season must be quite variable and com­plex. The loosening of the tillage layer increasesinfiltration rates and decreases the volumetricwater storage at field capacity. For conventionaltillage systems it may not be vital to separatethese two beneficial and complimentary processesbut for minimum tillage purposes their separa­tion seems desirable. For instance the input Ifrom a given precipitationP could be permanently

W. ARNDT ; C. W. ROS E

r,

oX,

Fig. 4. Effect on decrease in input to the storage layer ofthe fraction Xl of total area compacted and the relative

infiltration rate (rI ) of compacted to uncompacted soil

increased by providing banks to increase theretention time T and reduce the run-off S topractically zero. In this way maximum drainageD to Layer 2 could be permanently insured with­out regular expenditure of effort on tillage toreduce the compacted area X l or to increase theinfiltration rate i l . When these factors areeliminated water storage under heavy rainfalIdepends upon Eqn (5) rather than (6). The effectof bulk density on Min Eqn (4) then becomes the

181

dominant reason for tillage for soil water con­servation purposes. It has been shown for atyp ical situation that when the infiltration rate isnot a limiting factor, the compacted traffic bandsmay account for the additional loss by evapora­tion of approximately I mm rainfalI equivalentper cycle. Whilst this amount may be appreciablein the case of small rainfall amounts, it wilI berelatively insignificant if large amounts of rain­fall are retained by banks. In this case very littleexpenditure of tillage effort seems justified forsoil water storage purposes alone. The capitalcost of soil and water conservat ion structures isthus likely to be offset by savings in the annualexpenditure on tillage. With the use of herbicidesthat may largely eliminate the need for mechani­cal weed control, it should eventually be possibleto reduce the aims of tillage to a simple adjust­ment of the bulk density of the soil within waterconservation structures. The effort required forthis operation will depend largely upon the trafficsystem (Part I).

REFERENCES

1 Penman, H. L. Evaporation: an introductory study .Neth. J. agric. Sci., 1956,4 (I) 9

2 Rose, C. W. Some effects ofrainfall, radiant drying, andsoil factors on infiltration under rainfall into soils. J.Soil Sci., 1962, 13 (2) 286