R O C ZN IK I G L E B O Z N A W C Z E T. X X X V , NR 2, W A R S Z A W A 1984
A N D R Z E J G R E L E W IC Z
W A T E R M O V E M E N T I N T H E S O IL S U R F A C E L A Y E R S IN D U C E D B Y D A I L Y T E M P E R A T U R E W A V E (M O D E L S T U D IE S )
Department o f Soil Science, Institute o f B iology, Copernicus University o f Toruń
IN T R O D U C T IO N
Considering the com plexity o f water movement in a porous medium, such as soil, it is generally accepted both in mathematical description and in practical fore casting o f water conditions that this movem ent takes place under isothermal con ditions. In real fact, under natural conditions, the temperatures o f the soil surface layers are subject to continuous daily and annual variations, which lead to a variable in time differentiation o f temperatures in the soil profile.
The effect o f the temperature gradients in the upper layers o f the soil profile on water transport is unquestioned. M any students o f this phenomenon point out that water thermotransport can play an important part in the form ation o f water conditions in soil [1, 2, 11, 12, 13, 14]. H ow ever, in spite o f the considerable amount o f research on the subject, the quantitative effect o f thermally transported water content distribution is still un-known. This is due to the difficulty in assessing the percentage share o f each individual constituent o f the total flux o f m ovin g water under non-isothermal conditions, particulary when temperature gradients, like those in natural soil, are variable in time [1, 8, 14].
The aim o f the present w ork is the assessment through comparative m odel studies o f the effect o f cyclic daily temperature variations simulated on the top surface o f a m odel profile o f sem ihydromorphic soil on the vertical distribution o f water content.
M A T E R I A L A N D M E T H O D
The m odel profiles fo r study were 40 cm high and consisted o f homogeneous m onofractional sand, 0.25-0.50 mm in grain diameter. The lowerm ost layers o f the material were in direct contact with the ascending water at a steady temperature o f 20°C. Retention curve o f the material has been shown in Fig. 1.
Fig. 1. Retention curve o f soil material
1 — determined in the low-pressure chamber o f Richards, 2 — determined in the high-pressure chamber o f Richards* ' - 3— determined by the water vapour desorption method
Fig. 2. Programmed daily temperature wave o f the top T 0 and bottom T 40 surface o f the m odel profile
W ater movement in the soil... 5
The equipment set used fo r the experiment was described in an earlier paper [6]. It makes possible to do comparative studies o f water movement in m odel pro files. The set includes tw o vertical columns sealed at the top. In the first one water movem ent proceeds under isothermal conditions at a constant temperature o f 20°C. In the other one water movement proceeds under non-isothermal conditions similar to those prevailing in natural profile. On the top surface o f the material in that column, a programmed temperature story is generated, while the temperature o f 1 its bottom surface is constant, 20°C.
The results presented in this paper concern tw o thermal systems in the model profile. T o obtain those, one o f the tw o sinusoidal daily temperature stories o f an amplitude o f 20 К was generated on the top surface o f the material in the non- -isothermal column (Fig. 2). The first o f them in the temperature range from 0°C to 20°C, repeated fo r 28 days, corresponds with the thermal situation in the profile during the cooling down o f soil, and can occur under natural conditions mainly in autumn (Fig. 2a). The other story was generated fo r 14 days running in the tempera ture range from 20° to 40°C as typical o f periods o f heating, occurring in spring and summer (Fig. 2b).
The experiments were done fo r different variants o f initial moisture content o f the soil material. The water content distributions in columns were determined by the gravimetric method at the beginning o f each experiment (before supplying the ascending water) and at the end.
R E S U LTS O F E X P E R IM E N T S A N D D IS C U S S IO N
The distributions o f water content in the isothermal and non-isothermal column at the beginning end at the and o f the experiments have been presented in Figs 3 and 4. As seen from the drawings, the corresponding distributions fo r consecutive variants o f the experiment differ significantly from each other.
The differences in temperatures in the m odel profile o f semihydromorphic soil, characteristic o f periods o f soil cooling, induces a considerable increase in water content in the surface layers and a relative decrease in water content in the capillary ascent zone (F ig . 3). In the upper part o f the non-isothermal column can be dis tinguished two zones, S ! and S 29 in which a particularly great increase in water content was observed. The temperature story in the upper part o f the profile in dicates that the layers with the greatest increase in water content also show the lowest temperatures in successive daily cycles. In the lowest temperature con tinues fo r about 19 hours, in S 2 fo r about 5 hours. Hence these zones can be con sidered to be the condensation zones o f m oving water vapour. Further diagrams point out that the final distribution o f water content in the non-isothermal column largely depends on the initial water content o f the material. The greatest increase in water content in the uppei pa i l o f the m odel profile, above the zone o f capillary ascent, occurred when the initial water content was W psr= l A 5 % , whereas the smallest differences in water content distribution were observed in variant W psr= = 3.82%.
Water movement in the soil... 7
Fig. 3. Water content distributions in non- isothermal column A and isothermal column В at the start o f the experiment (lines WpAi WpB) and at the end (curves WkA and WkB) after 28 days
A comparison o f the final distributions o f water content in the isothermal and the non-isothermal column in the case o f the temperature story characteristic o f the periods o f soil heating shows that this particular thermal pattern in the profile induces water movement from the upper layers towards the lower part o f the pro file (Fig. 4). Considerable desiccation was found in the surface layers after 14 days
o f the experiment, while in the capillary ascent zone was observed an increase in water content in relation to the isothermal column. Further diagrams show that as the initial water content o f the material increases the desiccation zone decreases in thickness.
The differences in the distributions o f water content between the isothermal and the non-isothermal column for the particular variants o f the experiment result from the dilferent conditions o f soil water movement. W ater movement in the isothermal column is induced by the force o f gravity and the water content gradients. A t low water contents water can also move in the form o f vapour owing to the gradient o f its tension. In the isothermal column the main part in water transport is played by the liquid phase, whereas water movement through vapour, subject to the laws o f gas diffusion in porous medium, is o f little significance.
W ater movement under non-isothermal conditions is much more complex. The occurrence o f temperature gradients modifies the “ isothermal” water movement in the non-isothermal column induced by the above mentioned factors and elicits a number o f mechanisms o f its movement besides those existing under isothermal conditions. It follow s from theoretical considerations that about ten mechanisms can contribute to soil water thermotransport [2]. This movement can proceed through the intermediary o f water vapour by thermal diffusion o f vapour, by convectional movement o f particles in the gaseous area o f pores (m icroconvection) and between individual pores or as a result o f the movement o f the gaseous phase o f the soil medium effected by pressure gradients [1, 6]. A m o n g the mechanisms o f the liquid phase movement induced by temperature gradients are distinguished such as the meniscus-thermocap’llary, the film -therm ocap;llary, the thermomechanical and the thermoautodiffusive mechanism. The share o f the particular mechanisms in water thermotransport is not yet known well enough. There are differences o f opinion regarding the share o f the liquid phase in water transport [3, 4, 5, 10]. The part o f water vapour in the thermically transported water flux, on the other hand, is beyond doubt. [3, 4, 5, 7, 9, 11, 16].
The greatest role is ascribed to the non-classical mechanism o f soil water mo- movement in which take part water vapour (active thermotransport) and the liquid phase by whose means proceeds the capillary transport o f water from one pore to the next one [2, 12, 15, 16]. It has been found that by this mechanism considerable amounts o f water can be transported at a speed several times that at which proceeds classical therm odiffusion o f vapour. This is due to the occurrence in the soil pores area o f temperature microgradients, exceeding several times the mean value o f the temperature gradient, and linked with the differences in value amoung the coefficients o f thermal conductivity o f the gaseous, liquid and solid phase o f the soil medium [12].
Fig. 4. Water content distributions in non-isothermal column A and isothermal column В at the start o f the experiment (lines W pA and WpB) and at the end (curves WkA and WkR) after 14 days
Water movement in the soil.., 11
This mechanism operates most effectively at moderate water contents o f the soil material. It is believed that the great differences in water content distribution in the upper layers o f the m odel profile within the range o f moderate initial water contents is caused by the operation o f this mechanism [Fig. 3].
C O N C L U S IO N S
The results o f the experiments point to the essential part that can be played by water thermotransport in the development o f the water conditions in the upper layers o f soil. During the time o f soil cooling, which under natural conditions takes place in autumn, this phenomenon results in increasing the water supply in the upper soil layers. A t the time o f soil heating during spring and summer the thermo- transport o f water by quickly rem oving it from the upper layers deeper down the profile prevents loss o f water store from the profile due to surface evaporation.
The fact that in the first variant o f temperature story condensation zones occur alternately with layers o f lower water content, while in the second variant there occurs a zone o f strong dessication indicates that the speed at which proceeds water therunotransport in send material o f low and moderate water content is higher than the speed o f the “ isothermal” water movement induced by water content gradients.
R E FE R E N C E S
[1] C a r y J. W .: Soil moisture transport due to thermal gradients: practical aspects. Soil Sei. Soc. Amer. Proc. 1966, 30, 428-433.
[2] G lo b u s A . M . : Eksperimientalnaja gidrofizika poczw. Leningrad 1969, 279-336.
[3] G lo b u s A . M .: Eksperimentalnoje issledowanije fazowo sostawa włagi poczw i gruntów pieriedwigajuszczesja pod wlijanijem gradienta tempieratury. Dokłady A . N . SSSR 1960, 160, 918-920.
[4] G lo b u s A . M .: Srawnitielnoje issledowanija pierienosa vieszczestw razlicznoj prirody w za krytych kapilarno-poristych sistiemach. Dokłady A . N . SSSR 1966, 171, 1317-1320. [5] G lo b u s A . M .: Fazowyj sostaw potoka pri tiermopierienose wlagi w poristoj sriedie po
dannym mietoda razlicznych gazowych dawlenii. Dokłady A . N . SSSR 1972, 207, 394-396. [6] G r e le w ic z A .: Laboratory method o f study o f soil water thermotransport induced by the
daily temperature wave. Rocz, Glebozn., 1981, 32, 3.
[7] G u rr C .G ., M a r s c h a ll T. J., H u t t o n J. T .: Movement o f water in soil due to a tempe rature gradient. Soil Sei. 1952, 74, 335-345.
[8] J a c k s o n R. D., R e g in a t o R. J., K im b a ll B. A ., N a k a y a m a F. S.: Diurnal soil-water evaporation: comparison o f measured and calculated soil-water fluxes. Soil Sei. Amer. Proc. 1974, 38, 861-866.
[9] D u z m a k J. M ., S e r e d a P. J.: The mechanism by which water moves through a porous material subjected to a temperature gradient, 1. Introduction o f a vapor gap into a saturated system. Soil Sei. 1957, 84, 291-299.
[10] K u z m a k J. M ., S e r e d a P. J.: The mechanism by which water moves through a porous material subjected to a temperature gradient. 2. Salt tracer and streaming potential to detect flow in liquid phase. Soil Sei. 1957, 84, 419-422.
[11] O n c z u k o w D . N .: Sutocznyje zakonomiernosti pierienosa tiepła i włagi w poczwie. Po- czwowiedienie 1956, 5, 25-30.
[12] P h ilip J. R., de V r ie s D . A .: Moisture movement in porous materials under temperature gradients. Trans. Amer. Geophys. Union 1957, 38, 222-232.
[13] R o s e C. W .: Water transport in soil with a daily temperature wave. I. Theory and ekspe- riment. Autral. Jour, o f Soil Res. 1968, 6, 31-44.
[14] R ose C. W .: Water transport in soil with a daily temperature wave. If. Analysis. Austral. Jour, o f Soil Res. 1968, 6, 45-57.
[15] S m ith W .O .: Thermal transfer o f moisture in soils. Trans. Amcr. Geophys. Union 1943, 2, 511-523.
[16] de V r ie s D. A ,: Simultaneous transfer o f heat and moisture in porous media. Trans. Amer. Geophys. Union 1958, 39. 909-916. А. ГР Е Л Е В И Ч Д В И Ж Е Н И Е ВО ДЫ В П О В Е Р Х Н О С Т Н Ы Х С Л О Я Х П О Ч В Ы П О Д В Л И Я Н И Е М Т Е Р М И Ч Е С К О Й В О Л Н Ы (М О Д Е Л Ь Н Ы Е И С С Л Е Д О В А Н И Я ) Отдел почвоведения Инстигуга биологии Университета им. Н. Копердика в г. Тору не Р е з ю м е В статье рассматриваются результаты модельных исследований по влиянию суточных синусоидальных колебаний температуры верхней поверхности модельного профиля полу- гидроморфой песчаной почвы на движение воды в поверхностных слоях профиля. И сследо вания проводились по сравнительному методу в двух идентичных полузакрытых вертикально установленных почвенных колоннах высотой 40 см наполненных почвенным материалом. На верхней поверхности почвенного материала в первой колонне генерировали один из двух синусоидальных ходов температуры соответственно запрограммированных по отно шению к постоянной температуре нижней поверхности профиля. Движение воды в этой колонне происходило в неизотермических условиях. Во второй колонне температура в общ ем объеме почвенного материала была постоянной, составляя 20°С. Движение воды в этой колонне происходило в изотермических условиях. В исследованиях учитывали два варианта хода температуры на верхной поверхности материала в неизотермической колонне (рис. 2). Первый ход температуры в интервале 0°-20°С повторяемый через каждых 28 дней отвечал положению во время охлаждения почвы, а второй ход — в интервале 20°-40°С генерирован ный через каждых 14 дней отвечал положению во время огревания почвы. Исследования учитывали разные начальные влажности почвенного материала при участии подпитывающейся воды. Результаты исследований указывают на существенную роль играемую термотранспо ртом воды вызванным суточными колебаниями температуры верхней поверхности почвенного материала в образовании распределения влаги в верхних слоях модельного профиля. Во время охлаждения почв в природных условиях осени происходит в связи с указанным явле нием обогащение водой верхних слоев почвы. В период обогрева почв весной и летом благодаря быстрому перемещению воды в направлении глубже лежащих слоев, термотран спорт воды противодействует убытками воды в профиле вызванным поверхностным ис парением. Эффективный термотранспорт воды в верхних слоях профиля зависит от начального распределения влаги. Наличие зон конденсации попеременно со слоями с низкой влажностью для первого варианта хода температуры и зон сильного переосушения для второго варианта
Water movement in the soil.., 13 опыта показывает, что скорость термотранспорта воды в песчаном материале с низкой или умеренной влажностью превышает скорость „изотермического” движения воды под вли янием градиентов влажности. A . G R E L E W IC Z R U C H W O D Y W P O W IE R Z C H N IO W Y C H W A R S T W A C H G L E B Y P O D W P Ł Y W E M DOBOW EJ F A L I T E R M IC Z N E J (B A D A N IA M O D E L O W E )
Zakład Gleboznawstwa Instytutu Biologii U M K w Toruniu
S t r e s z c z e n i e
W pracy przedstawiono wyniki badań modelowych nad wpływem dobowych sinusoidalnych wahań temperatury górnej powierzchni modelowego profilu piaskowej gleby semihydromorficznej na przemieszczanie wody w powierzchniowych warstwach profilu. Badania prowadzono metodą porównawczą w dwóch identycznych, półzamkniętych pionowo ustawionych kolumnach glebo wych o wysokości 40 cm, wypełnionych materiałem glebowym. N a górnej powierzchni materiału glebowego w pierwszej kolumnie generowano jeden z dwóch sinusoidalnych przebiegów tempera tury odpowiednio zaprogramowany względem stałej temperatury dolnej powierzchni profilu. Ruch wody w tej kolumnie zachodził w warunkach nieizotermicznych. W drugiej kolumnie temperatura w całej objętości materiału glebowego była stała i wynosiła 20°C. Ruch wody w tej kolumnie odbywał się w warunkach izotermicznych. Badania wykonano dla dwóch wariantów przebiegu temperatury na górnej powierzchni materiału w kolumnie nieizotermicznej (ryc. 2). Pierwszy z nich w zakresie temperatur od 0°C do 20°C powtarzany przez 28 dni odpowiada sytuacji podczas ochładzania gleby, natomiast drugi przebieg w zakresie temperatur od 20°C do 40°C generowany przez 14 dni odpowia da sytuacji podczas nagrzewania gleby. Badania wykonano dla różnych początkowych wilgotności materiału glebowego przy udziale podsiąkającej wody.
Wyniki badań wskazują na istotną rolę, jaką odgrywa termotransport wody wywołany dobo wymi wahaniami temperatury górnej powierzchni materiału glebowego w kształtowaniu rozkładów wilgotności w górnych warstwach modelowego profilu. W czasie ochładzania gleby występującego w naturalnych warunkach w okresie jesiennym, w wyniku tego zjawiska następuje wzbogacenie w wodę górnych warstw gleby. W okresach nagrzewania gleby, występujących w okresach wiosen nym i letnim, dzięki szybkiemu przemieszczaniu wody w kierunku głębiej położonych warstw, termiczny transport wody przeciwdziała ubytkom zapasów wody w profilu wskutek parowania powierzchniowego.
Efektywny termotransport wody w górnych warstwach profilu zależy od początkowego roz kładu wilgotności. Występowanie stref kondensacji na przemian z warstwami o niskich wilgot- nościach dla pierwszego wariantu przebiegu temperatury oraz stref silnego przesuszenia w drugim wariancie doświadczenia wskazują, że szybkość z jaką zachodzi termotransport wody w materiale piaskowym o niskich i umiarkowanych wilgotnościach przewyższa szybkość „izotermicznego” ruchu wody pod wpływem gradientów wilgotności.
D r A n d r z e■' Gre lew ie* In s ty tu t Biologii U M K Toruń, ul. Gagarina 9
