Soil & Tillage Research, 2 (1981) 281--292 281 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

H Y D R A U L I C C H A R A C T E R I S T I C S OF C O N V E N T I O N A L L Y A N D Z E R O - T I L L E D F I E L D P L O T S

S.C. NEGP, G.S.V. RAGHAVAN: and F. TAYLOR:

1School of Engineering, University of Guelph, Guelph, Ont. NIG 2W1 (Canada) ~Department of Agricultural Engineering, Macdonald Campus of McGill University, Ste. Anne de Bellevue, Que. H9X 1C0 (Canada)

(Accepted 12 January 1982)

ABSTRACT

Negi, S.C., Raghavan, G.S.V. and Taylor, F., 1982. Hydraulic characteristics of conven- tionally and zero-tilled field plots. Soil Tillage Res., 2: 281--292.

A silage corn crop was grown on field plots entering their 2rid-year cycle of zero til- lage, and on control plots representing traditional tillage practices. Concurrent measure- ments of soil matric suction and water content were made under transient conditions of a draining profile without evaporation. The results indicated some similarity in the soil moisture characteristics of the untilled soils which produced high crop yields. The time rate of change of soil water content, the root extraction rate and the unsaturated hydraulic conductivity were always smaller in the zero-tilled plots than in the conven- tionally tilled plots.

INTRODUCTION

In s tudy ing the m o v e m e n t and s torage o f wa te r u n d e r field condi t ions , it is essential to d e t e r m i n e the hydrau l ic character is t ics o f the soil profi le . These charac ter i s t ics are best descr ibed by the re la t ionship b e t w e e n soil mat r ic suc t ion and soil wa te r con t en t , the func t iona l d e p e n d e n c e of hydraul- ic c o n d u c t i v i t y u p o n w a t e r c o n t e n t , and the i r t e m p o r a l and spat ial var ia t ion in situ (Hillel e t al., 1972) . In addi t ion , in c r o p p e d fields the r emova l o f moi s tu re by the r o o t zone, r e fe r red to as the r o o t e x t r a c t i o n rate , is o f crit- ical s ignif icance (Van Bavel e t al., 1968) .

This pape r is c o n c e r n e d wi th the hydrau l ic character is t ics o f s o m e t i l led and unt i l led field p lo ts o f a zero-t i l lage e x p e r i m e n t . The specif ic ob jec t ives of the w o r k r e p o r t e d here in were: (a) to measure soil mo i s tu r e suc t ion and moi s tu re c o n t e n t prof i les unde r cond i t i ons of drainage a lone in conven- t ional ly and zero- t i l led plots , and (b) to c o m p u t e the intrinsic hydrau l ic p roper t i e s of the soil and to e x a m i n e the re la t ionship b e t w e e n these prop- ert ies and the p e r f o r m a n c e of a silage corn crop.

0167-1987/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

282

T H E O R E T I C A L CONSIDERATIONS

Although many analytic expressions have been developed to describe the transmission of water in soils (Rose et al., 1965; Molz and Remson, 1970; Kirkham and Powers, 1972; Nielsen et al., 1973), the following method ac- cording to Hillel et al. (1972) was chosen because it involves a minimum of field measurements.

Hydraulic conductivity is the proport ionali ty factor in the Darcy flow equation:

q = - g ( o ) V H (1)

where 0 = volumetric moisture content (%, v/v); q = soil moisture flux (cm/day); K = hydraulic conductivity (cm/day); VH = hydraulic head gra- dient (cm/cm).

The equation of continuity is written as:

~0 aq - ( 2 )

3t az

in which t = time (s); z = vertical coordinate positive downward (era). By combining equation (1) with equation (2), the following differential

equation for the flow of water in a vertical soil profile is obtained;

L[ ~0 K(O) (3) at

where H = hydraulic head, which is the sum of gravitational and matric suc- tion heads (cm).

To determine the unsaturated hydraulic conductivity at any depth, equa- tion (3) is integrated with respect to z from the soil surface to the desired soil depth:

f dz a H j (4)

0

If the soil surface is covered to prevent evaporation, the second term on the right~hand side of equation (4) is zero because there is no flow across the soil surface. The term on the left~hand side of equation (4) is the soil moisture flux, which is denoted by q. Therefore, equation (4) may be written as:

q g ( o ) - (5)

aH/az

In accordance with the equation of continuity, the rate of decrease of the moisture content at any given point is equal to the change in flux with soil depth, provided there are no roots present in the soil and the lateral

283

movement of water is not appreciable. However, in a soil--root system the rate of change of the water content is also influenced by the rate of root extraction and equation (2) is modified (Ogata et al., 1960) to read:

~0 ~q - r z (6)

~t ~z

where r z = root extraction rate (day -1 ).

MATERIALS AND METHODS

The test site used for the experiment is located at the Macdonald College Farm of McGill University and contains uniform soil classified as Ste. Rosalie clay, which contains approximately 65% illitic and chloritic clay size particles. The field is quite level and is subdrained by 7.5-cm poly- ethylene perforated tubes spaced at 25 m. The zero tillage experiment is be- ing carried out on plots which were initially established in the spring of 1978. Silage corn has been grown for three consecutive years (1978--80) on these plots wi thout any intercrop tillage. The results presented in this paper were obtained during the 2nd-year cycle of zero tillage, i.e., in the summer of 1980.

The experiment comprised one conventional and three different zero til- lage treatments (Table I) randomly assigned within two replications. As in- dicated in Table I, in the spring of 1978 the zero-till plots (15XS, 15X0, 000) were subjected to various tillage and traffic treatments. The plots coded 15XS were compacted by 15 passes of a tractor having a ground con- tact pressure of 62 kPa and then subsoiled to a depth of about 0.45 m. The subsoiler consisted of a 0.05 m wide shank raked at 45 ° with a 0.30 m

TABLE I

Details of experimental treatments

Code Treatments spring 1978

Compaction Tillage

No. of Contact passes pressure

(kPa)

Tillage operation 1980

15XS 15 62 Subsoiled Zero tillage 15X0 15 62 None Zero tillage 000 0 0 Ploughed and Zero tillage

rototilled 222 0 0 None Ploughing and disc harrowing a

a Conventional tillage.

284

total width lifting sidewing attached to its base. The plots coded 15X0 were equally compacted but had no post-compaction tillage treatment, and those coded 000 were ploughed and rototilled. The control plots of 1980, coded 222, were conventionally tilled, i.e., mouldboard ploughing in the autumn of 1979 and disc harrowing in the spring of 1980. For clarity, the zero til- lage treatments are referred to as subsoiled (15XS), compacted (15X0) and rototilled (000), and the conventional tillage t reatment as ploughed (222).

Silage corn was hand seeded into the stubble remaining from 1979 in 1.5 × 12 m 2 plots and the crop was manually harvested. Fertilizer and herbicides were applied by driving between the plots. Commencing approx- imately five, nine and 14 weeks after seeding, a 1-m square area was ir- rigated until the entire profile became as wet as possible. Afterwards this area and a surrounding 1 m was covered with a black polyethylene sheet to impede unwanted vegetative growth from weeds and a transparent poly- ethylene sheet was placed on top of it to prevent evaporation. Corn plants in the centre row of each three-row corn plot (row spacing 0.75 m) were al- lowed to grow through slots cut in the plastic sheets. Within the covered area, tensiometers (Jet, fill Model 2725) were installed at 0.15 m, 0.30 m and 0.45 m depth along with a nearby aluminum access tube for the neutron meter (Troxler depth-moisture gauge Model 1322). Soil water con- tent and suction readings were taken at these depths periodically between 1 day and 2 to 3 weeks.

RESULTS AND DISCUSSION

The experimental data on the variation of volumetric moisture content with time were utilized to generate statistical models using an equation of the type (Richards et al., 1956):

0 = a t b (7)

where 0 = water content (%, v / v ) ; t = time (days); a = coefficient, water content on the first day (%. v /v ) ; b = proportionality constant, related to the water-loss rate.

Fig. 1 shows the temporal variation of moisture content for different treatments in the 0--22.5-cm zone. It appears that the water contents ap- proached the final values more rapidly in the zero-tilled plots of the 2nd- year cycle than in the ploughed control plots. This implies that the zero- till plots were better able to retain water than the conventionally tilled plots which allowed water to drain too easily.

The slope of the moisture--time curve ( ~ O / ~ t ) was measured at particular points in time and the soil moisture flux (q) was then calculated through each depth increment by integrating the O-- t curve with respect to depth z (Table II). A typical example of soil moisture flux as a function of time is presented in Fig. 2. I t appears that an initially strong downward flux oc- curred at all depths but diminished considerably with time. The quanti ty of

285

48

o~

( ] ) 4 6

z

z 4 4 Q

a~

P" 4 2

_J

m 4 0

50 t i J i I

DEPTH : O- 225 cm

MODEL : 0 = at b

\!}x~ \ ~ S ROTOTILLED

I , ~ ~. COMPACTED

S

\ \ j ~SUBSOILED

~ y P L O U G H E D

4 8 1~2 116 20

DRAINAGE PERIOD, t (days) F i g . 1 . T e m p o r a l v a r i a t i o n o f s o i l w a t e r c o n t e n t i n t h e 0 - - 2 2 . 5 - c m l a y e r f o r d i f f e r e n t

t r e a t m e n t s d u r i n g t h e t h i r d i r r i g a t i o n c y c l e .

T A B L E II

C a l c u l a t i o n o f soil m o i s t u r e f lux (q) a n d h y d r a u l i c c o n d u c t i v i t y (K) f o r t he subso i l ed t r e a t m e n t

t z aOl~ t d z ( a O / O t ) q = ~. d z ( ~ O l O t ) i~Hlaz K "ff (days ) ( c m ) (day - t ) ( r a m / d a y ) 0 ( c m / c m ) ( m m / d a y ) (%, v / v )

( m m / d a y )

1 0 - - 2 2 . 5 0 . 0 1 9 4 .3 4 .3 1.2 3 .7 50 .6 2 2 . 5 - - 3 7 . 5 0 . 0 3 1 4 .6 8.9 1.2 7 .6 53 .6 3 7 . 5 - - 5 2 . 5 0 . 0 1 8 2.7 11 .6 1 .2 9.9 54 .2

3 0 - - 2 2 . 5 0 . 0 0 6 1 .3 1 .3 1.5 0 .9 4 8 . 5 22 .5 - - -37 .5 0 . 0 1 0 1 .5 2 .8 1 .5 1.9 50 .4 3 7 . 5 - - 5 2 . 5 0 . 0 0 6 0 .9 3.7 1 .5 2 .5 52 .2

5 0 - - 2 2 . 5 0 . 0 0 4 0 .9 0 .9 1 .6 0 .6 47 .5 2 2 . 5 - - 3 7 . 5 0 . 0 0 6 0 .9 1 .8 1 .6 1.1 48 .9 3 7 . 5 - - 5 2 . 5 0 . 0 0 3 0 .4 2 .2 1 .6 1 .4 51 .4

1 3 0 - - 2 2 . 5 0 . 0 0 1 0 .2 0 .2 1.7 0 .1 4 5 . 8 2 2 . 5 - - 3 7 . 5 0 . 0 0 2 0 .3 0 . 5 1.7 0 .3 4 6 . 3 3 7 . 5 - - 5 2 . 5 0 . 0 0 1 0 .1 0 .7 1.7 0 . 4 49 .7

21 0 - - 2 2 . 5 0 . 0 0 1 0 .2 0 .2 2 .0 0 .1 45 .0 2 2 . 5 - - 3 7 . 5 0 . 0 0 1 0 .1 0 . 3 2 .0 0 .2 45 .1 3 7 . 5 - 5 2 . 5 0 . 0 0 1 0 .1 0 .4 2 .0 0 .2 48 .9

water leaving the profile per day remained essentially unchanged after one week. Comparison of treatments indicated significant differences in fluxes in the beginning but after a week there were no discernible differences. In

286

1.2 i i i i i ] I ~ i

SUBSOILED TREATMENT

- o 8 ", i ~ 37.5 - 52.5 cm

x" i '~ I t

. L

~1 ! I I

0.4 1 I ~ / ~ 22.5 - 57,5 cm

= \ " 4

0 . 0 ~ J ] I I I I J I i

0 2 4 6 8 I0 12 14 16 18 20 22

T I M E ( doys )

Fig. 2. S o i l - m o i s t u r e f l u x v e r s u s t i m e fo r t h r e e d e p t h z o n e s d u r i n g t h e f i r s t i r r i ga t i on cycle in t h e s u b s o i l e d t r e a t m e n t .

accordance with the moisture content-- t ime characteristics (Fig. 1), the fluxes operating within the profile were usually lower in the zero-till plots than in the conventionally tilled plots.

The hydraulic head H was computed as the sum of suction and gravita- tional heads, i.e., by adding the tensiometer reading to its depth z. Analyses of H(z) data indicated that straight lines provided reasonable approxima- tions, and accordingly, linear hydraulic head profiles were used for the com- putations. Unsaturated hydraulic conductivity was calculated as a function of depth and water content for each t reatment by dividing flux q by the corresponding hydraulic gradient aH/az (Table II).

To determine the functional dependence of hydraulic conductivity K upon volumetric moisture content 0, values of K and 0 were plotted on semi-log paper, and hydraulic conductivity was found to be an exponential function of water content as depicted in Fig. 3. The grouping of K and 0 values by depth, such as in Fig. 3, indicates that the entire profile cannot be characterized by a single-valued function. With increasing depth, progres- sively higher moisture contents were required for the same hydraulic con- ductivity. Characteristically, these hydraulic conductivity results agree quite closely with those reported by Rose et al. (1965) and Hillel et al. (1972).

Fig. 4 demonstrates that at the same moisture content the hydraulic con- ductivity of conventionally tilled plots was higher than for zero-tilled plots. Within the zero tillage treatments, the conductivity values of the subsoiled

287

I0

5

3

1.0 >..- I.-

0.5 I.-

0.3

z 0

0.1 _I

0.05 ),..

=: 0.03

0.01

I I I I I I

SUBSOILED TREATMENT

K = c e =e

z C

O- 22.5 cm 3.03 x I0 "~ 0.687 / 225-57.5 cm 6.02 x I0"" 0.454 57.5- 52.5cm 4.21 x I0 -~8 0.759

/ / / 1 /

0 - 2 2 5 cm . "

/

" . . .

/ / / / ~ . 3 7 5 ~ " - 52.5 cm

225-37.5 cm.--~ ~ / / /

~ 1 1 / . /

I / I l I I l

43 45 47 49 5i 53 55 57

SOIL WATER CONTENT, 0 ( O/o, v / v ) Fig. 3. Hydraulic conduct iv i ty as a func t ion of depth and water con ten t during the first irrigation cycle in the subsoiled t reatment .

1.0

0.5 0.4

~ 0.3

E ~ 0.2

0.t _

I -

z 0 o 0.05

-- 0.04 _1

0.03

>,- -,r 0.02

i i i r

DEPTH : O- 22.5cm /

/ /

/ SUBSOILED /

PLOUGHED / ~ / " ~ / / / /

/

ROTOTILLED

t/I /' / /1

/ /

ii / t~

,,,'i , o

j /

,/ , i 1 / f

i /

0.01 J ; " I 45 4L7 49 5II 513 55

SOIL WATER CONTENT, e ( % , v / v )

Fig. 4. Hydraul ic conduct iv i ty versus soil water con ten t in the 0--22.5-cm layer for dif- ferent t rea tments after the first irrigation cycle.

288

plots were generally higher than those of the rototilled or compacted soils. However, both of these latter treatments produced the best grain yields. Apparently, the lower (but still reasonable) hydraulic conductivity is made up for by a higher water retention (Fig. 1).

The rate of root extraction was determined by plotting water content 0 versus time t, and flux q versus depth z. The slope ao/at was obtained from the moisture-time curve of the first irrigation cycle and the value of aq/az was found from the f lux--depth curve of the third irrigation cycle. It was as- sumed that early in the growing season (during irrigation 1), i.e., a bare situation, the root water absorption was insignificant; and that the water up- take by roots was normal during irrigation 3 (about 3 months after planting), i.e. a cropped situation. The values of ao/at and aq/az were then compared at each depth and at the corresponding points in time, and the root extrac- tion rates were computed in accordance with equation (6).

An example of the distribution of root extraction rate with depth at four elapsed times after the cessation of irrigation is given in Fig. 5. The numbers in brackets are the integrals under the curves, representing the cumulative root absorption from the soil profile. These numbers show the total amount of water extracted by roots decreased with time rather rapidly in the beginning but quite slowly later. The bulges in the curves indicate

I0

20

50

A 4c

Z 0 I'- O. W 0

I0

, I i i

day I)

120 HOURS

2C

I r I

72 HOURS

( I0 mm day -+ )

312 HOURS /

( 0.5 mm day -I }

5O

( O.6mm doy -] )

+o \ 50 , -- , ,

- I00 -80 -60 -443 -20 o -Jo 40 -4o o RATE OF ROOT EXTRACTION (doy-lx 10 -4 )

Fig. 5. R o o t ex trac t ion rate prof i les for the subso i led treatment .

289

that most of the water uptake originated from the 20--40-cm layer. Comparison of treatments revealed that the water uptake was always smal- ler in the zero-till plots than in the conventionally tilled plots.

An empirical expression relating volumetric water content to soil suction (Campbell, 1974) was used to obtain the soil moisture--characteristic curves:

0 -P

where q~ = soil suetion (m); ~e = soil suction at air entry (m); 0 -- volu- metric moisture content (m a m -a ); 0s = volumetric moisture content at saturation (m 3 m - a ) ; p = slope of the moisture retention curve on log--log paper.

The value of the saturated moisture content 0s for each treatment was calculated from independent field measurements of soil bulk density and gravimetric water content. The coefficients ~e and p were determined by fit- ting the equation to the experimental data, and the results for different treatments at 0.30 m depth are plotted in Pig. 6. At high levels of suction, the control plots had higher water retention than any of the other plots. However, over much of the suction range encountered under field condi- tions (0.5--5 m), the differences in the volume of water retained were not very significant. In fact, the ploughed treatment showed a poorer retention

5 0 0 f , ,

\

w ~ \ " COMPACTEb IO0 ' ~. , ~ / ',

\ \ PLOUGHED

\ \ \

SUBSOILE

~ \ \ \ \ x \

- I0 ROTOTILLED

U 5 \ \ \

,,o MOO ; o : , [ q - ° \

\ 0.5 ~ I

O 0 O.I 0!2 01:5 014 0.5 WATER C O N T E N T , (~ ( mS/m 3)

Fig. 6. Water conten t versus suct ion at 0 .30 m depth for different treatments.

2 9 0

than all other treatments at suctions below 2 m of water. The zero tillage treatments demonstrated analogous behaviour over the entire range of suc- tions of importance for crop production, indicating a similar size distribu- tion of the pores in the untilled soils.

The available water content of the soil (AW), i.e., the amount of water that is available for plant use, was computed from the relation

A W = 6 f - 6 w (9)

w h e r e 6 f = moisture content at field capacity (m 3 m -3); 6 = moisture con- tent at the permanent wilting point (m 3 m-3).

Field capacity was obtained from the temporal variation of volumetric water content (8 vs. t). It was taken as the water content, 6f, at which storage of water in a particular soil layer had reached a practically constant (steady) value. Permanent wilting point was taken as the water content, 6w, corresponding to a suction of 150 m (Russell, ]976) .

Fig. 7 illustrates the relationship between crop performance and water available to plants at 0.30 m depth. The 0.30 m depth was used as an index to the water availability over the soil profile because most of the water up- take originated from the 0.20--0.40-m layer (Fig. 5). It can be seen that higher crop yields are associated with a greater amount of available water. The zero-tilled plots of the second year cycle, whether subsoiled, com- pacted or rototilled, all had 29% or more water available at 0.30 m depth in the soil, and all produced higher yields. The conventionally tilled plots

13

12

>-

" I0

--I I.U ).-

~9 0

hl

.--I

[ I

COMPACTED

~ ROTOT~LED

.J~PLOUGHEO

Y = 0,11 AW + 742 r = 0.78

7 ~ t I0 20 30 40

AVAILABLE WATER, AW (% , v / v )

Fig. 7. Silage corn y ie ld as a funct ion of available water at 0 .30 m depth.

291

had only 15% available water, and yields in these plots were significantly lower.

From the foregoing data and relations it may be concluded that zero til- lage practice changed the soil structure by slightly reducing the coarse pores and strongly increasing the fine pores and, consequently, increased the amount of water held at moderate suctions. This implies that zero tillage would be particularly beneficial under dry conditions but may result in in- sufficient drainage of surplus water in a very wet season. On the other hand, conventionally tilled soil would drain easily amidst plentiful supply of rain but may cause moisture stresses in plants during a dry spell.

CONCLUSIONS

The magnitude of the time rate of change of soil water content was smal- ler in the zero-till plots as compared to the conventionally tilled plots.

The unsaturated hydraulic conductivity was found to be an exponential function of soil-water content. Accordingly, conductivity values were ex- tremely sensitive to changes in soil wetness and decreased rapidly with a de- crease in moisture content. The hydraulic conductivity of conventionally tilled plots was higher than for untilled soils.

The rate of root extraction was smaller in the zero tillage treatments than in the conventional tillage treatment.

For the conditions of the 1980 season, the amount of water available to plants at 0.30 m depth was twice as large in the untilled soils as in the tilled soils. Silage corn yields were proportionally greater in the untilled plots.

The zero-tillage treatments which produced high crop yields exhibited some similarity in their water retention characteristics. The size distribution of pores in these untilled plots was such that a greater proportion of the water was held at moderate suctions (0.5--5 m), i.e. stored in a proper range of void sizes. In other words, water was available at a rate com- mensurate with the amount needed by the plants. Therefore, the ability of the soil to hold a sufficient amount of water at moderate suctions, and yet possess reasonable conductivity, should be the criterion for superior soil physical conditions.

ACKNOWLEDGEMENT

The authors express their thanks to the Engineering and Statistical Research Institute, Agriculture Canada for a research contract which per- mitted this study.

292

REFERENCES

Campbell, G.S., 1974. A simple method for determining unsaturated conductivity from moisture retention data. Soil Sci., 117: 311--314.

Hillel, D., Krentos, V.D. and Stylianou, Y., 1972. Procedure and test of an internal drainage method for measuring soil hydraulic characteristics in situ. Soil Sci., 114: 395--400.

Kirkham, D. and Powers, W.L., 1972. Advanced Soil Physics. Wiley-Interscience, New York, NY, 534 pp.

Molz, F.J. and Remson, I., 1970. Extraction term models of soil moisture use by transpiring plants. Water Resour. Res., 6: 1346--1356.

Nielsen, D.R., Biggar, J.W. and Erb, K.T., 1973. Spatial variability of field-measured soil- water properties. Hilgardia, 42: 215--259.

Ogata, G., Richards, L.A. and Gardner, W.R., 1960. Transpiration of alfalfa determined from soil water content changes. Soil Sci., 89: 179--182.

Richards, L.A., Gardner, W.R. and Ogata, G., 1956. Physical processes determining water loss from soil. Soil Sci. Soc. Am. Proc., 20: 310--314.

Rose, C.W., Stern, W.R. and Drummond, J.E., 1965. Determination of hydraulic conductivity as a function of depth and water content for soil in situ. Aust. J. Soil Res., 3: 1--9.

Russell, E.W., 1976. Soil Conditions and Plant Growth. Longman Inc., New York, NY, 849 pp.

Van Bavel, C.H.M., Stirk, G.B. and Brust, K.J., 1968. Hydraulic properties of a clay loam soil and the field measurement of water uptake by roots: I. Soil Sci. Soc. Am. Proc., 32: 310--317.