heat and water movement under surface rocks in a field soil: ii. moisture effects1

5
Heat and Water Movement Under Surface Rocks in a Field Soil: II. Moisture Effects 1 W. A. JURY AND B. BELLANTUONi 2 ABSTRACT Results qualitatively confirmed the observations of the sealed labora- tory column experiment. Results are presented from a field experiment conducted to deter- mine the effect of surface rocks on soil water content changes under Additional Index Words: thermal conductivity probe, water vapor bare soil. First, stones were placed at intervals over an initially-dry movement, mulch field (gravimetric water content = 0.021 g/g) and left for 6 weeks. Subsequent sampling showed a small but detectable excess of water stored under the rock compared to adjacent bare soil. Following an ir- A p REVIO us PAPER (Jury and Bellantuoni, 1976a), the rigation, buried thermal conductivity probes were used to monitor J_ authors reported heat flow measurements made in a field water content changes under and adjacent to surface rocks. After 24 . . , ,, .. MIT days, the soil under the rock contained significantly more water than experiment under and around rocks covering bare soil Hav- did the soil region adjacent to the rock, a finding confirmed by gravi- ln g consistently observed a net lateral movement of heat metric sampling. Following this, the stones and probes were relocated toward the soil under the rocks on a 24-hour basis in the for a further 24 days of observation, with similar results obtained. summer months, we reasoned that accompanying move- In a separate laboratory experiment using a large sealed soil column ment of water due to thermal gradients, primarily in the with a rock covering part of the surface it was demonstrated that a sig- vapor phase, could result in a net deposit of moisture under nificant amount of water moved to the cylinder of soil under the rock the rocks which would then be insulated from evaporative from the soil region under the bare surface due to horizontal tempera- [ oss ture gradients induced by the rock covering part of the surface. Tesdng Mg hypothesis in a field environment, however, A two-dimensional computer program calculating water vapor j s , difficult fm sevefal reaSQns A nondestmctive moni . movement under thermal gradients was used with measured field tern- . , .. .. , . , , . .. 7 . ^ , .. . , tonne of soil water content, normally achieved by using perature boundary conditions to estimate the amount of water vapor . ., , . ,. , J , , movement expected to occur due to a rock cover on the soil surface. tensiometers or soil psychrometers, is complicated by the large daily change in soil temperatures experienced at the ————— locations of interest. Gravimetric soil sampling is achieved 'Contribution of the Dep. of Soil Science & Agricultural Engineering, , . rrnliratitKJ arrn<;>; trip field and results in a one time University of California, Riverside, CA92502. Research was supported in Onl y ^ replicating across ttie Held ana results in 3 One time part by funds from the National Science Foundation under IBP Desert only comparison between soil under and adjacent to the Biome, Utah State Univ., Ecology Center, Subcontract No. 539. Received r ock. Also, comparison between rocks sampled at different 8 Dec. 1975. Approved 12 April 1976. . ., , , , , . . ..... . c ,, "Assistant Professor of Soil Physics andLaboratory Technician, respec- tlmes ls g reatl y hampered by lateral variability in field lively. properties. Further, the highly dynamic and three-dimen-

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Page 1: Heat and Water Movement Under Surface Rocks in a Field Soil: II. Moisture Effects1

Heat and Water Movement Under Surface Rocks in a Field Soil: II. Moisture Effects1

W. A. JURY AND B. BELLANTUONi2

ABSTRACT Results qualitatively confirmed the observations of the sealed labora-tory column experiment.

Results are presented from a field experiment conducted to deter-mine the effect of surface rocks on soil water content changes under Additional Index Words: thermal conductivity probe, water vaporbare soil. First, stones were placed at intervals over an initially-dry movement, mulchfield (gravimetric water content = 0.021 g/g) and left for 6 weeks.Subsequent sampling showed a small but detectable excess of waterstored under the rock compared to adjacent bare soil. Following an ir- A pREVIOus PAPER (Jury and Bellantuoni, 1976a), therigation, buried thermal conductivity probes were used to monitor J_authors reported heat flow measurements made in a fieldwater content changes under and adjacent to surface rocks. After 24 . . , , , . . MITdays, the soil under the rock contained significantly more water than experiment under and around rocks covering bare soil Hav-did the soil region adjacent to the rock, a finding confirmed by gravi- lng consistently observed a net lateral movement of heatmetric sampling. Following this, the stones and probes were relocated toward the soil under the rocks on a 24-hour basis in thefor a further 24 days of observation, with similar results obtained. summer months, we reasoned that accompanying move-

In a separate laboratory experiment using a large sealed soil column ment of water due to thermal gradients, primarily in thewith a rock covering part of the surface it was demonstrated that a sig- vapor phase, could result in a net deposit of moisture undernificant amount of water moved to the cylinder of soil under the rock the rocks which would then be insulated from evaporativefrom the soil region under the bare surface due to horizontal tempera- [oss

ture gradients induced by the rock covering part of the surface. Tesdng Mg hypothesis in a field environment, however,A two-dimensional computer program calculating water vapor js , difficult fm sevefal reaSQns A nondestmctive moni.

movement under thermal gradients was used with measured field tern- . , .. .. , . , ,. .. 7 . ^ , .. . , tonne of soil water content, normally achieved by usingperature boundary conditions to estimate the amount of water vapor . ., , . ,. , J , ,

movement expected to occur due to a rock cover on the soil surface. tensiometers or soil psychrometers, is complicated by thelarge daily change in soil temperatures experienced at the

————— locations of interest. Gravimetric soil sampling is achieved'Contribution of the Dep. of Soil Science & Agricultural Engineering, , . rrnliratitKJ arrn<;>; trip field and results in a one timeUniversity of California, Riverside, CA 92502. Research was supported in Only ̂ replicating across ttie Held ana results in 3 One time

part by funds from the National Science Foundation under IBP Desert only comparison between soil under and adjacent to theBiome, Utah State Univ., Ecology Center, Subcontract No. 539. Received rock. Also, comparison between rocks sampled at different8 Dec. 1975. Approved 12 April 1976. . ., , , , , . . ..... . c ,,

"Assistant Professor of Soil Physics and Laboratory Technician, respec- tlmes ls greatly hampered by lateral variability in fieldlively. properties. Further, the highly dynamic and three-dimen-

Page 2: Heat and Water Movement Under Surface Rocks in a Field Soil: II. Moisture Effects1

510 SOIL SCI. SOC. AM. J., VOL. 40, 1976

sional nature of the heat flow patterns suggests that largerwater content measuring instruments (i.e., gamma raytransmission) buried in the soil might perturb the thermalenvironment sufficiently to cause unwanted influences onwater movement. Because of these limitations, it was de-cided to use buried thermal conductivity probes (deVriesand Peck, 1958), supplemented by gravimetric sampling, tostudy the patterns of water accumulation and movement inrock-covered soil.

EXPERIMENTAL DESCRIPTIONThe field used in the water movement studies is the same 3.5 m

by 3.5 m subplot described in Jury and Bellantuoni (1976a). In thefirst experiment 6 May to 24 June 1975, the field was initiallysampled for water content at four locations and three depth inter-vals (0-7.5 cm, 7.5-15 cm; 15-22.5 cm), and then four medium-sized rocks (flat, 25-cm diameter, 10-cm thickness) were placedon the surface at random locations across the field. Occasionalsampling under and around the rocks was accomplished by liftingup the rock, taking out the plug of soil and replacing it with a coretaken from another part of the field, and then putting the rock backin place. Subsequent samples under the rock were taken at dif-ferent locations in the covered area. A second experiment (1-24July 1975) was initiated by removing all rocks, irrigating the field,and then replacing the rocks in new locations. Separate testsshowed that the soil under the rocks wet up thoroughly after an ir-rigation, so the rocks were placed on the field after the conclusionof the water input on 1 July. In addition, three thermal conduc-tivity probes (Fritton et al., 1974) were planted horizontally at the5-cm depth under the center and 5 and 10 cm adjacent to one of therocks. Thermal conductivity A. was measured occasionally andrelated to water content 6 from a laboratory-determined curve of\(0) (Jury and Bellantuoni, 1976a). Final soil samples were takenunder and around the other rocks on 24 July. The probes and rockswere then moved to other parts of the field and a third observationperiod initiated without an irrigation (25 July to 22 Aug. 1975).

One experiment was conducted under controlled laboratory con-ditions to study the water movement under and around a rock inthe absence of surface evaporation. A sample of fine desert sandfrom the Playa in Jornada, New Mexico was sieved with a 2-mmscreen, mixed to a volumetric water content of 0.10 (cm3/cm3) andpacked as uniformly as possible to a bulk density of 1.4 (g/cm3) ina large (56-cm diameter, 12.7 cm deep) plastic circular tub with asealed bottom. A flat marble slab (25 cm by 10 cm by 2 cm) wascentered on the surface with thermocouples located on the top ofthe slab, on the soil surface under and 10 cm adjacent to the rock,and at 2.5 and 5.0 cm under and 10 cm adjacent to the rock. In ad-dition two thermal conductivity probes were buried at the 5-cmdepth, one extending under the rock and one 10 cm adjacent to therock. A thin plastic sheet was laid over the soil surface except atthe rock location and a thin layer of dry soil was spread over thetop of it in order to prevent water loss through the surface. Thebottom of the tub was laid on a surface of circulating water kept at35 ± 1°C (the average surface temperature) by cooling coils con-nected to a constant-temperature reservoir (Forma-Temp). The soilsurface was exposed to four heat lamps run from a variable voltageinput to permit variation of surface radiation exposure. A cylindri-cal reflector surrounded the lamps to help in transmitting a uniformenergy distribution across the surface. The setup of this experi-ment is shown in Fig. 1.

THEORETICAL ANALYSISTo supplement the field measurements of water content changes

in the vicinity of surface rocks, a simulation model was con-structed to estimate the amount of water vapor movement expectedto occur under the changing thermal profiles measured in the field(Jury and Bellantuoni, 1976a).

Simulation of water vapor movement in soil is very difficult

1HEAT LAMPS

e e o o30cm

SIDE REFLECTOR

~~Z cm

• ' 1

12,5 cm

1 -1

^RQ£K^1 _ _ ^j

,/** ^z-/^~*. ——— ̂ —0°^*-

THERMOCOUPLES ^--PROBF

TO HV — ̂ = — s=^CONSTANT P^-riPriM ATIMr

TEMPERATUREP-C1RCULATING

SIDE

SOIL

O cm- 2.5cm— 5.0cm

^ __ _ ^ ___ .

WATER BATH

VIEW

TOP VIEWFig. 1 —Laboratory soil columns used to study water movement under

surface rocks. A buried plastic sheet on the soil surface preventsevaporative loss.

owing to phase changes and complicated interaction within liquidand vapor regions (Philip and deVries, 1957). Transient flow ofwater in the vapor phase may be described by a species continuityequation (deVries, 1958)

(d6v/dt)+V-Ja = [1]

whereJ» = rate of flow of water vapor/area/time (cm Iiq/cm3/sec)Os = volumetric liquid equivalent of vapor (cm3/cm3), andE = rate of change of liquid to vapor/soil volume

(cm3/cm3/sec).The evaporation term E is a function of the vapor pressure of the

liquid phase in the medium, which is in turn determined by tem-perature and the relative humdity of the surrounding vapor. Theflux term Jv will in general be driven by gradients of vapor density,which for fairly wet soil may be represented by a general equation

v = ~/3L(T)VT [2]

where L(T) is a vapor transport coefficient and ft is a geometricfactor.

Letey (1968) applied Eq. [2] to a large variety of laboratory datafrom the literature and found the measurements could be repre-sented by a single function L(T) -0.007 exp(O.OSr) (cm2 day-1

°C~l) with /3 ranging between 1.0 and 2.0.As an estimate of the net amount of water vapor transport ex-

pected to occur according to Eq. [1] and [2], one could ignorephase changes and calculate changes in water storage according to

36v/dt = V-h/3L(T)VT [3]

where h = 1 for 0V > 0 and h = 0 for 0V = 0.This is tantamount to assuming that the medium consists of

vapor and solid matrix only and that the vapor will redistribute ac-cording to changes in density induced by temperature gradients.

Equation [3] was coupled to the output of the two-dimensional

Page 3: Heat and Water Movement Under Surface Rocks in a Field Soil: II. Moisture Effects1

JURY & BELLANTUONI: HEAT AND WATER MOVEMENT UNDER SURFACE ROCKS: II. MOISTURE 511

Table 1—Differences (rock minus soil) in gravimetric water content ofthe 0- to 7.5-cm depth between rock-covered and adjacent bare

surface soil measured on an initially-dry field (average0, = 0.021 (g/g)).

Location of rockDate

16 May23 May2 June9 June

24 June

1

0.0070.0020.0030.0090.003

2

0.0100.0120.0100.0080.000

3

tt

0.0050.0030.002

4

tt

0.0050.0030.001

t No samples taken.

Table 2—Gravimetric water content profiles under and adjacent tosurface rock cover (20 cm diameter), 24 days after an irrigation

of the bare field.Areal

Depth

0 to 7.57.5 to 15

15 to 22.5

Rock

0.0610.0610.070

Soil

0.0350.0560.071

Area 2Rock

0.0480.0760.078

Soil

0.0440.0600.068

temperature simulation in rock-covered soil described in Jury andBellantuoni (1976a) together with the boundary conditions:

1) No vertical flow of vapor across soil surface,2) No horizontal flow of vapor across rock midpoint plane and

soil plane midway between rocks,3) No vertical flow of vapor at Z = —25 cm.Conditions (1) and (3) correspond to a sealed surface and bot-

tom while (2) follows from symmetry arguments. Evaporation wassuppressed in order to isolate the influence of temperature gra-dients on soil water content changes.

RESULTS AND DISCUSSIONWater content changes during the first experimental

period (6 May to 24 June) were monitored by gravimetricsampling only which was conducted on a very dry field thatreceived no external water input at any time. The initialgravimetric water content 6g averages for the four samplestaken on 11 May were: 0.021 ± 0.004; 0.035 ± 0.005;0.036 ± 0.005 (g/g) at the 0- to 7.5-, 7.5- to 15-, 15- to22.5-cm depths, respectively. The amount of variation inwater content across the field was dee'med sufficient to re-strict comparisons in water accumulation to samples takenof soil under and immediately adjacent to a given rock.

Table 1 shows the difference in gravimetric water contentbetween soil under and adjacent to rocks recorded at the 0-to 7.5-cm depth as a function of time. The results, althoughconsistently showing more water underneath the rocks, donot show any pattern of water accumulation. In view of theinherent errors of sampling, profile replacement, and fieldvariability, they must be regarded as inconclusive.

The second experiment was designed to test the responseof a rock-strewn system to an external water application.Figure 2 shows the readings of the thermal conductivityprobes as a function of time after irrigation, adjusted fortemperature fluctuations by the method of Jury and Bellan-tuoni (197 6b) and related to water content using the curve inJury and Bellantuoni (1976a). The probe 1 was locatedalong the center line of a 25-cm diameter rock, probe 2 was5 cm from the south edge, and probe 3 was 10 cm from thenorth edge, all probes being buried at the 5-cm depth.Measurements were made in the morning before the soilheated up, and several readings at each location were taken.

PROBE MEASUREMENTS AAT 5 cm DEPTH

—— UNDER ROCK- - 5 cm SOUTH OF ROCK••••- 10 cm NORTH OF ROCK

CD

0.220 ur-

O0.195 °

IU

0.170

O0.145 f£

eo

UJ

0.120

5N JULY T,ME (days)

Fig. 2—Thermal conductivity readings at the 5-cm depth as a functionof time after irrigation during 1 July to 24 July 1975 experiment.Surface rock of 25 cm diameter.

=- l.2r

PROBE MEASUREMENTSAT 5cm DEPTH

— AT EDGE OF ROCK— 10 cm FROM EDGE

20

TIME (days)Fig. 3—Thermal conductivity readings at the 5-cm depth as a function

of time during 25 July to 15 August experiment. Surface rock of 25cm diameter.

These tended to agree within 10% or better if the firstmeasurement was omitted. This first reading deviated sig-nificantly from the others and was considered to be inaccu-rate, possibly due to contamination from dew formation onthe probe.

Figure 2 suggests that all regions were heavily depletedby drainage during the first day following irrigation, that theexposed regions subsequently lost water by evaporation,and illustrates the ability of rock cover to conserve watercollected after a precipitation. This is also shown in Table2, which summarizes the final analysis (24 July) of watercontent under and around the two other rocks on the field.

Following this experiment the probes were checked andmoved along with the rocks to another part of the field andobserved for another 24 days, a period marked by intense

Page 4: Heat and Water Movement Under Surface Rocks in a Field Soil: II. Moisture Effects1

512 SOIL SCI. SOC. AM. J., VOL. 40, 1976

Table 3—Gravimetric water content profiles under and adjacent tosurface rock cover (20 cm diameter), 28 days after rock place-

ment on previously bare soil.Area 3 Area 4

Depth

0 to 7.57.5 to 15

15 to 22.5

Rock

0.0250.0370.048

Soil

0.0190.0330.048

Rock

0.0220.043

Soil

0.0200.0360.052

radiation. (The average maximum air temperature for theperiod was 35°C.) Figure 3 shows the output of probe 1,planted right at the edge of the rock, and probe 2 located 10cm away. The pattern of drying again is different in theregion near the rock, which is losing water less rapidly thanthe bare soil surface region. The final soil sampling acrossthe field confirmed that bare soil and rock-covered areaswere both losing water. Table 3 shows the profile for theseareas, which were bare from 1 July to 24 July and rock-covered thereafter.

Although these results demonstrate the effectiveness ofrocks in water retention, they do not shed any light on theinfluence of thermal gradients in concentrating water underrocks. The column experiment (Fig. 1) was devised to lookat changes in water content caused by temperature gradientsin the absence of evaporation. The heat lamps were turnedon and thermal conductivity probe measurements taken withbackground temperature compensation (Jury and Bellan-tuoni, 1976b) for 5 hours, after which the lamps wereturned off. Figure 4 is a plot of the probe readings under therock and adjacent to the rock, along with a plot of the tem-

UJo

UJ

0.6

0.4

TS0 0.2

-.2

1.40

1.20ITg

§ g 1.00

llE~°-80x

0.60

PROBE LOCATION AT 5cmO UNDER ROCKA BESIDE ROCK

oA *

50

40

30

20

10

0.170

§

a:uiQ_5

o2a:

<$

0 2K0830

4 6TIME (hrs.)

8

0.145 O _ro

Ui ||— \0.120 g ̂

oO ~(t

0.095 £S

0.070 §

Fig. 4—Thermal conductivity readings as a function of time at the 5-cm depth under the rock (O) and 10-cm adjacent to the rock edge(A), along with horizontal temperature gradient at the 2.5-cm depthand surface temperature for the sealed column experiment.

perature gradient at the 2.5-cm depth ((TSOIL -cm).

This figure definitely shows substantial movement ofwater over a short period of time in the same direction as thelateral component of heat flux. Further, the final ther-mocouple probe readings indicate that the region under therock accumulated water until the horizontal temperaturegradient reversed, and then began to lose water.

SIMULATION RESULTSFigure 5 is a plot of the volumetric water content, as-

sumed initially to be 0.10, as a function of time at threedepths for a 3-day simulation. This was obtained by usingthe field surface temperature data for 25 June 1974 in thetwo-dimensional heat flow calculation (Jury and Bellan-tuoni, 1976a) and calculating the water content changeswith Eq. [3] assuming no loss through the surface. The rockand soil readings are taken 1.25 cm inside and outside of therock edge, respectively. The simulation is a vast over-simplification, particularly as it ignores phase changes andliquid flow, but it does point out the size of changes in waterstorage exclusive of evaporation to be expected from purevapor movement due to temperature gradients in a rock-covered soil environment. The most dramatic changes occurnear the surface where the temperature gradients are largest,but there is in addition a net lateral transfer of moisture tothe rock induced by lateral temperature gradients, similar tothat observed in the sealed column experiment. Since thethermal gradients would still be present in dry soil withevaporation occurring, one effect of the stone cover wouldbe to cause accumulation of water underneath the rockwhere it would be shielded from loss to the atmosphere.

CONCLUSIONSThe series of field and laboratory measurements taken in

this study illustrate the effectiveness of soil temperaturegradients caused by surface rocks in helping accumulate andretain water in the soil profile underneath. Evidence for the

SIMULATED WATER CONTENT PROFILES—— 1.25cm INSIDE ROCK EDGE-- 1.25cm OUTSIDE ROCK EDGE

40

TIME (hrs.)Fig. 5 — Simulated water content profiles caused by vapor movement

induced by temperature gradients.

Page 5: Heat and Water Movement Under Surface Rocks in a Field Soil: II. Moisture Effects1

ELUOTT ET AL.: SOLUBLE CATIONS BENEATH FEEDLOT AND ADJACENT FIELD 513

ability of the rocks to create thermal patterns which help to ACKNOWLEDGMENTconcentrate water from the adjacent soil underneath the , . , . , , . , * f ,.-. ... , . , ,, The authors would like to thank an anonymous referee for his extremelyrocks was indirectly observed through field temperature thorough and helpful review.measurements which consistently showed a net horizontalmovement toward the soil, directly observed in a laboratoryexperiment, and calculated in a simulation of water vapormovement.

The effectiveness of the rock cover is expected to be mostpronounced in dry soil with high soil surface temperatures.Although the absolute amount of moisture transferred to therock was not large in any of the above experiments, it wascertainly sufficient to make a significant contribution to thewater balance of a drought-tolerant desert plant.

Further, the determining characteristic of the surfacecover is that it be discontinuous, permitting part of the sur-face to be heated by incoming radiation while the coveredregions are partially insulated. Thus, an isolated creosotebush in the desert, by shading the region of soil housing themajority of its roots, is causing lateral movement of energyand moisture toward the shaded region. This effect, if sig-nificant, would represent a previously overlooked contribu-tion to the water budget of arid zone vegetation.