ROCZNIKI GLEBOZNAW CZE, T. X X X II, NR 3. W ARSZAW A 1981
A N D R Z E J GRELEW ICZ
LABORATORY METHOD OF STUDY OF SOIL WATER THERMOTRANSPORT INDUCED BY THE DAILY TEMPERATURE
WAVE
D ep artm en t of S o il S cien ce, In stitu te of B iology, C opernicus U n iv e r sity of T oruń
IN TRO DU CTIO N
There are two basic concepts in the methodical approach to the study of the p art of the therm ally transported w ater in the variation in w ater content in the surface soil layers. W ithin the fram ew ork of the first one, initiated by L e b e d e v {14, 15] experim ental studies of therm al m ovement of soil w ater are conducted in the field conditions [1, 15, 17, 24]. The other concept, initiated by B o u y o u c o s [2], consist in studying under laboratory conditions w ater therm otransport in soil samples induced by constant tem perature gradients [4, 6, 7, 8, 10, 12, 13, 21, 22]. C a r y [3], R o s e [19, 20] and J a c k s o n et al. [11] have presented attem pts at using experim ental data obtained by this m ethod for assessment of the p art of therm ically transported w ater in the surface soil layers subject to daily variations of tem perature. In the calculations, equations of w ater movement in porous medium were applied [4, 18, 23]. In using this method no account is taken of hysteresis of soil w ater potential, and the therm al diffusion of soil w ater is assumed to be a stationary process. For this reason the results obtained in this way give only approxim ate inform ation on the p art of therm ically transported w ater in the total w ater flux [5].
Considering the difficulties just discussed in the assessment of the p a rt of therm ically transported w ater in the distribution of w ater content in the surface layers, it seems expedient at this stage of research to study the problem under laboratory conditions by method which will now be described.
It is a laboratory m ethod of studying in a model soil profiles w ater movement effected by the daily tem perature wave on specially construc ted equipment. The tem perature histories sim ulated in the model profile are sim ilar to those occurring under n atural conditions. The results
26 A. G relew icz
obtained in this way provide direct inform ation on the p art of therm ic- ally transported w ater in the distribution of w ater content. Specimens of results obtained by this method are presented in the final of the paper.
The proposed m ethod and equipm ent make it possible to study w ater movement inducted by the daily tem peratury wave under laboratory conditions. In order to assess the w ater content induced in the model profile by the daily tem perature wave, com parative studies are conduced in two vertically set columns filled w ith identical soil m a terial (Fig. 2). On the top surface of the soil m aterial in column (A) is
Fig. 1. W ater m o v em en t con d ition s in n o n -iso th erm a l colum n A, and in isoth erm al colu m n В
<9 — w at er content, T — temper atu re, p — pres sure g a se o us phase, q — w a te r f l u x e s
generated the daily sinusoidal tem perature history program m ed adequa- tly to the tem perature of the bottom portion of the profile. Due to the variation in time of the therm al differentiation of the particular soil layers the w ater m ovement proceeds under non-isotherm al coditions, sim ilar to those in natu ral soil. In the control column (B) the tem pe ratu res of top and bottom layer of the soil m aterial are therm ally stabilized. In this profile there is no therm al differentiation of the particular layers of soil m aterial. The soil w ater m ovem ent proceeds then under isotherm al conditions.
The columns are sealed so as to elim inate the effect of evaporation and condensation of w ater vapour from the am bient on the distribution of w ater content.
A graphic representation of w ater m ovem ent conditions in column (A) and column (B) at time “t ” is found in Fig. 1. W ater movement
L aboratory stu d y of so il w ater therm otran sp ort 27
in the non-isotherm al column (A) is induced by gradients of w ater content, of tem perature, of the gaseous phase pressure and gravity. In the isotherm al column (B) th a t movement is induced only by the w ater content gradients and gravity. For the m athem atical description of w ater m ovement under non-isotherm al conditions P h i l i p and .de1 V r i e s equation [18, 23] is usually applied. In the equation no account is taken of the w ater vapour diffusion effected by the differences in pressure resulting from the changes in volume of th e gaseous phase consequent on the periodically varying tem peratures of the particular layers of soil m aterial. If we introduce a term expresing the pressure diffusion of w ater vapour, the tem porary density of w ater fluexs QAь
Qa2 moving through layers x x and x 2 in column (A) can be expressed
as follows :
In column (B) w ater movement proceeds under isotherm al conditions.
The tem perary density of w ater fluxes QBъ Qb2 moving through layers
Xi and x 2 in column (B) can be put down as :
where :
4ai, 4a2 — w ater fluexs moving respectively through layers Xj and
x 2 in column (A) (g • cm-2 • s“ 1),
c1bu 4b2 — w ater fluxes moving respectively through layers x 1 and x 2 in column (B) (g • cm'"2 • s-1),
— coefficients of isotherm al diffusion of w ater vapour and of the liquid phase of soil w ater (cm2 • s-1),
DTl., Dtl — coefficients of therm al diffusion of w ater vapour and of the liquid phase of soil w ater (cm2 • s-1 • K “ 1),
Dvp — coefficient of pressure diffusion of w ater vapour (cm2X
&L T
s - 1 • h P a “ 1),
— liquid phase content (cm3 • cm“ 3), — tem perature (K),
28 A. G relew icz P <? К — pressure (hPa), — w ater density (g • cm“ 3),
— hydraulic conductivity of u n saturated soil (cm • s“ 1), In accordance w ith the continuity equation :
Э 0 3Q
Э t Эх (5)
in the case under study the speed changes in the m ean w ater content of the soil m aterial in layer {x2 — x^) is :
in column (A)
Inform ation on the dynamics of w ater content is obtained directly from the m asurem ents taken in both columns. From it is possible to assess the resultants of density of w ater fluexs moving through the particular layers. The resultants of density of w ater flux moving through layer (x2 — x 1) are :
in column (A)
Having the resultant values of flux densities (8), (9), we can assess the effective therm ally transported w ater flux qKT induced by the action of program m ed tem perature wave through layer (x2 — x t) on the soil w ater movement. The effective w ater flux moving through layer (X2-X i)
in column (B)
in column (B)
Q.et~ [(Qa2 Qa i) (Qb2 Qbi)] • Q (10)
Laboratory study of soil water thermotransport 29 in columns (A) and (B). Using expressions (8), (9) and (10) the effective therm ally transported w ater flux is :
/10 А Л&В
Чет~ ^ (*^2 ^i) • £? ( 4 )
As seen from (11) value q£T can be assessed in a simple way on the ground of the dynamics of the w ater content in the soil m aterial in columns (A) and (B).
D E SC R IPT IO N O F EQ U IPM EN T
Figure 2 shows the equipm ent used for laboratory studies of w ater therm otransport induced by daily tem perature wave in the model profile of semihydrom orphic sandy soil. The set is made up of the following subunits :
— soil columns (A) and (B),
— tem perature stabilizing system for ascending w ater tem perature and for the soil m aterial top surface tem perature in column (B) (ultra therm ostat U, m em brane pump P b flow containers 5, tem perature stabilizing container 16),
— tem perature wave generator (steering unit S, freezing aggregate
H, heat exchanger C, m em brane pump P 3, electrom agnetic valve z,
therm ally active elem ent 13, joining leads 15, tem perature probes 9), — tem perature m easuring and recording system (tem perature m e asuring device T, б-channel pointing recorders R lt R 2, tem perature probes 8),
— system of m easurem ent of w ater content dynamics (alternating cu rrent bridge M, resistance m easuring probes 8, contact plates 10).
The equipm ent set includet two glass cylindrical columns (A) and (B), 20 cm in inner diam eter and 100 cm in height. The columns can be filled w ith soil m aterial 1 up to the required height. A t the bottom the columns are sealed w ith nylon net 3. They are placed on grooved plates 4 ensuring free assess of ascending w ater ; these are placed in flow containers 5 connected by rubber tube 7. The walls of the columns are therm ally isolated from outside w ith a 10 cm thinck foamed poly styrene coat 6. In each column there is a glass tube 2 25 mm in diam eter close to the column wall w ith lateral openings through which come out the leads of tem perature and m oisture m easuring probes 8. During each successive day program m ed daily sinusoidal tem perature wave are generated on surface of the soil m aterial 1 in column (A) by means of the therm ally active elem ent 13 of the tem perature wave generator. The tem perature wave generator is a device for reproduction of the program m ed tem perature stories on the soil m aterial surface and was
30 A. Grelewicz
F ig. 2. D iagram of eq u ip m en t for lab oratory stu d y of w a te r th erm otran sp ort in d u ced by d a ily tem p eratu re w a v e
A — n o n -iso th erm a l colu m n, В — iso th erm a l colu m n , S — steer in g u n it, H — freezn ig ag g reg a te, С — h eat ex ch a n g er, р ь P 2, P3 — m em brane pum ps, z — electrom agn etic v alve, U — u tra- therm ostat, T — tem p era tu re m easu rin g d ev ice, R h R 2 — p o in tin g recorders, M — a ltern a tin g cu rren t bridge, 1 — so il m aterial, 2 — g la ss tub e, 3 — n y lo n n et, 4 — gro o v ed p lates, 5 — flo w con tainers, 6' — iso la tin g coat, 7 — rubber tube, 8 — tem p eratu re and m oisture probes, 9 — tem p er a tu re probes of th e r m a lly a ctiv e e lem en t, 10 — co n ta ct p la te s, 11, 12 — p o ly e th y le n e sh ee tin g , 13 — term a lly a ctiv e elem en t, 14 — rubber ring, 15 — jo in in g lead s, 16 — tem p er a tu re
sta b ilizin g con tain er
described in the previous paper 9. In column (B) the soil m aterial is kept under isotherm al conditions by using a tem perature stabilizer 16 and the flow containers 5 joined in the stabilized tem perature w ater circuit. The therm ally active elem ent 13 and the tem perature stabilizer
16 are provided w ith rubber rings 14 sealing columns (A) and (B) at
the top.
Throughout the experim ent, at certain times of the day m easurem ents of w ater content distribution are taken in both columns. In the equip m ent set for m easuring w ater content dynamics represented in the picture, the correlation between electric resistance of the soil m aterial and w ater content was used as a supplem entary method. It is possible to apply other non-destruction methods of w ater content m easurem ent, e.g. the m ethod of gamma radiation absorption. The experim ents are generally carried on until the w ater content distribution in both columns is found out, or for a certain num ber of days. Besides w ater content m easurem ents, recording of tem perature stories are taken in column
(A) and control m easurem ents are taken in column (B).
L aboratory stu d y of soil w ater therm otransport 31
is possible to study w ater therm otransport in the surface layers of soil profiles w ith the exclusion of ascending water.
R E SU L TS OF E X PE R IM E N T S
Speciments of the experim ental results have been presented in Figs 3 and 4. In both cases presented columns (A) and (B) (Fig. 2) were filled up to 40 cm high w ith soil m aterial (fraction 0.25-0.50 mm of quartz sand) of uniform w eight w ater content equal to 1.15°/o. The w ater Content distributions in column (A) and (B) at the beginning of the experim ent and at the final of the experim ent were made by the gravi m etric method. The bottom layers of the soil m aterial were in direct contact w ith the ascending w ater of a constant tem perature 20 °C.
In the first experim ent l(Fig. 3), on the upper surface of the soil
Fig. 3. W ater con ten t d istrib u tio n s in n o n -iso th erm a l colu m n A and iso th erm a l colu m n В at th e start of th e ex p e r im e n t (lin es and Wi2JB) and at the end
of the 28-day e x p erim en t (cu rves W/cA and W kD)
m aterial in column (A) was genrated a sinusoidal daily tem perature wa've of 20 К am plitude in tem perature range from 0°C to 20°C for a tim e of 28 days. The tem perature distribution sim ulated in the profile is typical of late sum m er and early autum n, when soil becomes gradually cooler.
In column (B) a constant tem perature 20°C was m aintained throughout the experim ent. The results obtained show th a t the therm al differentiation in the model profile of “semihydrom orphic soil,”
chara-32 A. G relew icz
cteristic of periods of soil cooling effects a considerable increase in w ater content of the capillary ascent zone. The final distribution of w ater content in the upper portion of profile (A) points to the occurrence of two zones of condensation of w ater vapour and whose movement is induced by tem perature gradients (Fig. 3). The saturation degree of the 0“ 18 cm layer expressed as the ratio of w ater store in th a t layer to the store at the so-calles “field capacity’' a t the start of the experim ent was 0.40 b ut at the end 1.15. This means th a t during the experim ent the w ater store in the 0-18 cm layer increased nearly threefold. The am ount of w ater contained on th a t layer after 28 days of experim ent exceeds the w ater store corresponding to the fild w ater capacity of the upper portion of the profile. Excess w ater formed as a result of w ater vapour condensation will later move down the profile. Consequently, the so-called “field w ater content distribution” will be reached. The result of the experim ent indicates th a t in soil cooling seasons the therm al soil w ater movem ent plays an im portant p art in supplying the upper profile layers w ith w ater.
In the second experim ent (Fig. 4) on the upper surface on the soil m aterial in column (A) a sinusoidal daily tem perature wave of 20 К am plitude in the tem perature range from 20°C to 40°C was generated for a time of 14 days. The tem perature distribution sim ulated in this case is characteristic of spring and early summer, when the soil becomes gradually w armer. From a comparison of the final w ater content
F ig. 4. W ater con ten t d istrib u tio n s in n o n -iso th erm a l colu m n A and isoth erm al colu m n В at th e start of th e e x p e r im e n t (lin es and and at th e end
Laboratory study of soil water thermotransport 33
distributions in the non-isotherm al column (A) and the isotherm al column (B) it follows th a t the therm al differentiation in the model profile in this case induces w ater vapour m ovem ent from the surface layers down the profile. A fter 14 days of the experim ent the 0-8 cm layer of the soil m aterial in column (A) was found to be considerably desiccated, while in the lower portion of the profile a com parative increase in w ater content was found (Fig. 4). The fact th a t as a result of w ater vapour therm otransport down the profile (under natu ral conditions also of surface evaporation) a surface layer of low w ater content is formed is of great im portance from the point of view of the w ater budget of the soil profile. Firstly, only p art of the w ater contained in this layer evaporates into the atmosphere, while a considerable .portion of it in the form of w ater vapour moves down profile. This means th a t therm otransport of w ater vapour in soil heating seasons counteracts loss of w ater store by evaporation from the surface layers. Secondly, the desiccated surface soil layer constitutes an isolating layers largely inhibiting fu rth er evaporation of w ater from the deeper layers of the profile.
A part from the briefly described results of experim ent, certain sug gestions follow from the data concerning the w ater budget in the model profile. In the first experim ent (Fig. 3) the increase in w ater store in the upper portion of profile (A) in relation to the store in the same layer in column (B) does not m uch exceed the relative loss of w ater store from the lower portion of the profile. The w ater loss from the lower p art of the profile is made up after some time by ascending w ater. In second experim ent (Fig. 4), on the other hand, the w ater loss from the upper portion of the profile is half the increase in w ater store in the lower portion. The significant differenes between the loss and gain in w ater store in this case m ay be linked w ith the mechanisms of w ater m ovement under non-isotherm al conditions. Under these conditions there is simultaneous “isotherm al” movem ent of the liquid phase (capil lary and film water) induced by m oisture gradients from the lower tow ards the surface layers and therm al diffusion of w ater vapour in the opposite direction. In the surface layers of the profile then soil w ater cycling goes on w ith the gaseous and the liquid phase of soil w ater taking part. Considering the occurrence of hysteresis of soil media w ater cycling m ay cause an increase in w ater content in the layer underlying the desiccated layer. As a result, in spite of considerable w ater loss from the thin soil surface layer, the underlying layers of sem ihydro- morphic soil m ay become enriched in w ater. In both experim ents descri bed, there is then “therm al w ater pum ping” tow ards the upper layers of the profile. The examples presented point out th a t the soil w ater movement induced by the daily tem perature wave significantly affects the variation in w ater content in the surface layers of semihydrom orphic
34 A. Grelewicz
soil. Complete results of the experim ents and their interpretation will be presented in a separate publication.
C O N C LU SIO N S
The proposed m ethod of study of w ater therm otransport perm its to assess the effect of program m ed daily tem perature wave on the d istri bution of m oisture in the soil model profile under strictly controlled laboratory conditions. By m easuring the dynamics of w ater content distribution in a non-isotherm al and isotherm al column, it is possible to assess the effective w ater fluxes therm ally transported through the particular layers distinguished. The results obtained in this way provide inform ation on the actual p art of w ater therm otransport in the model soil profile effected by daily tem perature wave. The experim ental data presented in the paper and obtained by the proposed m ethod point to the essential role of therm ally transported w ater in the variation in w ater content in the surface layers of sem ihydrom orphic soil. The observed facts point out th at during the periods of soil cooling the w ater .content in the surface soil layers becomes greatly increased due to therm otransport of soil w ater, w hereas during soil heating the surface layers become desiccated as a result of m ovem ent of w ater vapour down the profile. In both cases a resu ltan t upw ard m ovement of soil w ater was found in the model profile of sem ihydrm orphic soil induced by the daily tem perature wave.
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Laboratory study of soil water thermotransport 35
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A. GRELEWIGZ
L A B O R A T O R Y JN A M ETO DA B A D A N IA R U C H U W ODY G LEBOW EJ W YW OŁANEGO D OBOW Y M I W A H A N IA M I TEM PE R A T U R Y
In sty tu t B io lo g ii U M K w T oruniu
S t r e s z c z e n i e
W p racy p rzed sta w io n o lab oratoryjn ą m etod ę b ad an ia ruchu w o d y w m o d elo w y m p rofilu g leb o w y m pod w p ły w e m d ob ow ej fa li term iczn ej. P rop on ow an a m etoda i aparatura u m o żliw ia ją o k reślen ie e fe k tó w w ilg o tn o śc io w y c h sp ow od
o-36 A. Grelewicz
w a n y c h w y stę p o w a n ie m d ob ow ej fa li term iczn ej w p ro filu drogą badań p o ró w n a w czych, p ro w a d zo n y ch w d w óch p ó łza m k n ięty ch p io n o w o u sta w io n y c h k olu m n ach , w y p e łn io n y c h w id en ty czn y sposób m a teria łem g le b o w y m (rys. 2). N a górn ej p o w ierzch n i m a teria łu g leb o w eg o w k o lu m n ie A jest g en ero w a n y d ob ow y sin u so i d a ln y p rzeb ieg tem p eratu ry, od p ow ied n io za p rogram ow an y w z g lę d e m sta łej te m p era tu ry d oln ej części p ro filu . R uch w o d y w tej k o lu m n ie zach od zi w w a ru n k a ch n ieizo term iczn y ch , zb liżo n y ch do w y stę p u ją c y c h w n a tu ra ln ej gleb ie. W d ru giej k o lu m n ie В tem p era tu ry górnej i d oln ej p o w ierzch n i m a teria łu g leb o w eg o są sobie rów n e. W ty m p ro filu n ie w y stę p u je zró żn ic o w a n ie tem p eratu r p o szczeg ó l n ych w a rstw . W arunki ru chu w o d y w k o lu m n a ch n ieizo term iczn ej A i iz o term icz- nej В p rzed sta w io n o na rys. 1.
N a p o d sta w ie d y n a m ik i w ilg o tn o śc i o d p o w ia d a ją cy ch sob ie w a r stw w k o lu m n ach A i B, dla ok reślo n eg o p rzeb ieg u tem p era tu ry na p o w ierzch n i m o d elo w eg o p ro filu w z g lę d e m tem p era tu ry d oln ej części p rofilu , m ożna o k reślić e fe k t y w n e stru m ien ie qET w o d y tra n sp o rto w a n ej term iczn ie. Z esta w ap aratu ry do p ro w a dzen ia badań p rzed sta w io n o na rys. 2. Isto tn y m e le m e n te m ze sta w u jest gen erator fa li term iczn ej (patent — urząd P a te n to w y PR L P-212-155). U rząd zen ie to u m o ż li w ia o d tw a rza n ie na p o w ierzch n i m o d elo w eg o p ro filu w k o lu m n ie A d ow oln ego, zaprogram ow an ego, dob ow ego p rzeb ieg u tem p era tu ry w za k resie od ~ 1 0 ° C do 60°C.
P rzy k ła d y e k sp ery m en ta ln y ch w y n ik ó w b ad ań u zy sk a n y ch tą m etodą p o k a zano na rys. 3 i 4. P r z e d sta w io n e w y n ik i w sk a z u ją na isto tn ą rolę w o d y tran sp or to w a n ej term iczn ie w k szta łto w a n iu rozk ład ów w ilg o tn o śc i p o w ierzch n io w y ch w a r stw g le b y sem ih y d ro m o rficzn ej. W obu b a d a n y ch p rzyp ad k ach stw ierd zon o, że w y p a d k o w y ruch w o d y g leb o w ej pod w p ły w e m d ob ow ej fa li term iczn ej w m o d e lo w y m p ro filu g leb y sem ih y d ro m o rficzn ej od b yw a się z d o ln y ch w a rstw p ro filu w k ieru n k u w y żej p ołożonych.
Dr A n d r z e j G r e l e w i c z I n s t y t u t B io lo gii U M K T o r u ń , ul. S i e n k i e w i c z a 30
