a sufficient rate and quantity to be ofimportance in the growth of the turf.

Under normal soil conditions themovement of water from a water tableby capillary flow at a rate to be sig-nificant in growing grass is limited toless than two feet. Furthermore, becauseof the relationship between soil air andwater, it is preferable in sports fieldsconstructed on natural soil materials tonot have a water table within two feet ofthe surface. Tile drainage is recom-mended to prevent the water table fromcoming closer to the surface.

of the glass). Of course the capillarymovement of water also acts in adownward direction to increase the rateof flow due to gravity.

Capillary movement of water is ofgreat importance in supplying grassroots with water because it allows waterto be replenished at the surface of a rootas the zone within a millimetre or two ofthe root dries out due to the absorptionof water by the grass.

The amount of water that will moveto the root by this process and the speedat which it moves is dependent on thesize, number and continuity of the micropores. Large numbers of relatively smallmicro pores are to be found inclay soils,therefore; capillary movement is ofgreatest significance in fine texturedsoils.

The smaller the micro pores, the fur-ther the water can move by capillaryforces. On the other hand, the slower itwill move. In a sand-based rooting zonewith relatively large micro pores, watercan move relatively rapidly over a shortdistance to a root surface. In the sand thedistance over which the water willtravel, however, will be measured incentimetres regardless of the time al-lowed. In a clay soil the water may moveseveral feet, however, it will take weeksfor this to occur.

The principle of capillary flow isemployed in the design of sand-basedsports fields. In this design a rootingzone of 30 cm. of sand overlies gravelwhich creates a temporary, 'perchedwater table' or zone of saturation of afew centimetres depth at the base of thesand Water may move upward from thissaturated zone to replenish the watersurrounding the roots near the surface at

Understanding Turf ManagementThe fifth in a series by

R.W. Sheard, PhD., P.Ag.

Gravitational FlowFlow of water by gravity is important

in the rapid removal of excess water andthe return of air to the system. Gravita-tional flow, often referred to assaturated flow, occurs in the macropores of the soil and only occurs whenthe moisture content of the soil risesabove field capacity.

When gravitational flow is restrictedit is necessary to install artificialdrainage systems. Removal of gravita-tional water, however, does not removeany water of value in the production ofgrass.

Infiltration RateAn important measurement of soil

water movement is the rate at whichwater enters the soil surface - the in-filtration rate. The value is an indicationof the potential for erosion or waterrunoff, an event which seldom occurswith a turf covered surface. Undersports field conditions localized pond-ing may occur after heavy rains if theinfiltration rate is low. It must be

MOVEMENT OF WATER IN SOIL

Inthe last article of this series an ex-planation was given of the physical

principles by which water is held in thesoil. Water, however, does not remainstationary in the soil, but is continuallymoving. Water always moves fromplaces where there is a high amount toplaces where there is a lesser amount.Primarily the water is movingdownward due to the pull or forces ofgravity. This type of water movement iscalled gravitational flow. Nevertheless,at the same time water may be migratingsideways, or even upward due to thecapillary forces generated by the occur-rence of the small micro pores in thesoil.

Capillary FlowWhile it is easy to understand that

water will move down due to gravity theconcept of capillary flow is less obvious.As the flow by capillary forces isthrough the micro pore it is oftenreferred to as unsaturated flow. Capil-lary flow is the movement of water atmoisture contents of field capacity orless.

A simple illustration of capillarymovement is to fill a glass to the brimwith water and place a dry sponge overone half of the glass. The water in con-tact with the sponge will be immediatelymove upward into the sponge. Ifwateris slowly added to the glass to maintaincontact between the sponge and thewater, it will soon be noticed that thesponge is being wetted sideways fromthe edge of the glass as well as upward.The water is moving from an area ofhigh concentration (in the glass) to anarea of low concentration (in the drysponge, both above and beyond the edge

1 hour after Irrigation 5 days after Irrigation

~I II 1/10 bar water

i I 5 bar water15 I i I-, 15 i I

I ~I II I

1 bar water30I 2 bar water 30 I- I E I

E

i I I

t r r I~ 45 ...J ~ 45 i t -Is: I s: Ia. a.Q) IQ) I

0 I 0 I 1/3 bar water60 I 60 t i I

I 1/3 bar water II I

Water table I I75 75 o Tile I--,

~II 0 bar water JJ, Water table i o bar waterI90 90

~ or f represents capillary flow of water ~ represents gravitational flow of water

Fig. 1: The direction and type of movement of water in a soil profileunder turf one hour and five days after irrigation or rain. The

remembered that where the infiltrationrate is low the rate at which irrigationwater may be applied must be restricted.

Generally the infiltration rate in soilsgrowing turf is related to the clay con-tent, the degree of compaction and theoccurrence of thatch. Soils high in clayhave a lower infiltration rate than sandsdue to the lower percentage of macropores. Similarly compaction, whichtends to reduce macro porosity, restrictsthe infiltration rate. Dry spots occurwhere there is thatch build up, par-ticularly if the thatch is allowed to be-come air dry. Due to the resistance torewetting, the water tends to run to areaswhere the thatch is thinner or has notdried to the same degree, causing un-even wetting of the soil.

Generally an infiltration rate slightlygreater than the expected intensity ofstorm rains is desired. Under Ontarioconditions the intensity is seldomgreater than 7.5 ern. per hour. Sand-based systems which have properlyselected sands will have an infiltrationrate meeting this standard.

approximate tensions on the water at different depths are given onthe right side of each diagram.

Textural BarrierPlaying field construction using an

imported material of significantly dif- -ferent texture from the underlyingmaterial, creating a 'texturaldiscontinuity' or 'textural barrier', mayresult in a 'perched water table' . A 'per-ched water table' is a temporary zone ofsaturation because gravitational flow ofwater is restricted.

Textural barriers occur in sportsfields under two widely different condi-tions. The first condition is where acoarse material, such as sand, is placedover clay. The percolation rate in theclay may be 1000 times or more slowerthan in the sand, resulting in a temporarysaturated zone of a few centimetres atthe surface of the clay layer. Eventuallyas the soil dries the sand will, in reality,dry faster due to the increased suctionplaced on the sand layer from the finemicro pores in the underlying clay.

The second condition is found wherea fine sand material is placed over smallstone. Again, due to the marked dif-ference in pore size, water will not move

from the sand into the stone layer untila zone of saturation builds up at the baseof the finer material. The water must beat zero tension before it will 'drip' intothe stone layer.

This second condition of a 'texturalbarrier' is an advantage in sand-basedsports field construction because itprovides a reservoir of water which maymove upward to the active root growtharea by capillary flow. By using the'perched water table' principle greaterwater use efficiency is achieved and thefrequency of irrigation is reduced.

It is interesting to note that at thesurface of a drain tile a similarphenomenon occurs. Gravitationalwater does not enter the tile line until thetension on the water reaches zero.Therefore there will be a thin 'perchedwater table' at the surface of the tileuntil the soil moisture content drops tonear field capacity. Placing stonearound the tile only moves the 'perchedwater table' back to the interface be-tween the stone and the soil material anddoes not speed up the flow of water intothe tile.