Diurnal Soil-Water Evaporation: Chloride Movement and Accumulation Near the Soil Surface1
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Diurnal Soil-Water Evaporation:
Chloride Movement and Accumulation Near the Soil Surface1
F. S. NAKAYAMA, R. D. JACKSON, B. A. KIMBALL, AND R. J. REGiNATO2
The movement and accumulation of chloride at shallowdepths (0 to 9 cm) in a bare soil following an irrigation werestudied under field conditions. Chloride accumulation in the0- to 0.5- and 0- to 1-cm depth increments followed a diurnalpattern but out-of-phase from the soil water content during thefirst few days after irrigation. The diurnal amplitude of chloridedecreased with time as the soil progressively dried. At the 1- to2-cm and deeper depths, diurnal cycling of the chloride contentwas not measurable, whereas cycling in the water content wasevident. Most of the total chloride accumulation at the shallow-est depth occurred in the early stages of drying. However, chlor-ide movement was detected as low as 4% volumetric watercontent or approximately 1,000 bars soil water potential.
Additional Index Words: salinity, periodicity.
SALT AND WATER MOVEMENT occur simultaneously in afield soil, but not necessarily in the same direction.Water transfer can proceed in both liquid and vapor phases,but the liquid and vapor may be moving in opposite direc-tions at a given instance. On the other hand, dissolved saltsmust move with the liquid. The net movement from a givenreference plane will depend upon the activity gradient ofthe salt and of the water. Various combinations of gradientsand phases can occur simultaneously in the field. However,a systematic change in the factors affecting water and saltmovement should occur from day to day. Gurr, Marshall,
510 SOIL SCI. SOC. AMER. PROC., VOL. 37, 1973
and Hutton (2) used a salt tracer technique in the labora-tory for separating the contribution of liquid and vaporphase flows in water movement. Jackson's (5) investigationon evaporation occurring in bare field soils showed thatwater movement followed a diurnal pattern. The amplitudeof this pattern gradually damped out as the total water con-tent of the soil decreased with time.
The objectives of our field trial were to study the move-ment of chloride under a naturally occurring condition andto relate the chloride content to the soil water content. Em-phasis was placed on the 0- to 9-cm depth of the soil pro-file where the greatest changes were expected. To avoidcomplications in analysis and physiochemical effects otherthan those occurring naturally, no salt tracer was added tothe soil or water except for that normally present in theirrigation water applied to the soil.
MATERIALS AND METHODSThe field experiment was conducted on the Avondale clay
loam soil (fine-loamy, mixed, hyperthermic, TorrifluventicHaplostoll) formerly classified as the Adelanto loam in Phoe-nix, Arizona, during March and April 1971. The level, bareplot (72 by 90 m) was irrigated with approximately 10 cm ofwell water (12 meq/liter of Cl~) on March 2, 1971, and themeasurements were started on March 5, 1971 as soon as it wasfeasible to walk on the plots. Samples were taken at the 0- to0.5-, 0- to 1-, 1- to 2-, 2- to 3-, 3- to 4-, 4- to 5-, 5- to 7-, and 7-to 9-cm depths. Soils from six separate sites were compositedfor each depth increment. Samples were obtained every 0.5hour continually over a 24-hour period for 2 weeks. Additional0.5-hour sampling was made for two 24-hour periods 1 and 2weeks after this intensive set of measurements. Water content(oven-dried at 105C) was determined and the chloride contentwas measured hourly on all the materials. The chloride analysiswas made on the supernatant solution of a 1:5 soil-to-water
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NAKAYAMA ET AL.: DIURNAL SOIL-WATER EVAPORATION: CHLORIDE MOVEMENT AND ACCUMULATION 511
gradually dried. In general, most of the water was lost dur-ing the 0800-to-1600 hour period for a given day. The startof the water loss at 1 to 2 cm and deeper lagged slightlybehind the shallower depths. The soil regained water evenduring the daylight hours from the deeper depths, as indi-cated by the rise in water content from the minimum point.For this experiment the volumetric water content decreasedfrom 34 to 2% in the shallowest depth through the meas-urement period.
Figures 1-4 show that the chloride content at depths of0 to 0.5 and 0 to 1 cm also have a diurnal pattern with thegreatest changes occurring in the 0- to 0.5-cm depth. Varia-bility in the chloride content measurement was greater thanfor the water content. This was probably caused by the non-homogeneous distribution of salts in the upper part of thesoil profile. With the 1- to 2-cm and deeper depths, consist-ent chloride values were measured. Like the water, the am-plitude of the chloride content gradually damped out withtime. However, the damping out occurred earlier for thechloride content than for the water content. The slowdownin chloride accumulation is expected to occur sooner thanthat of the water loss since the movement of the chargedelectrolyte is restricted to a greater extent than the waterin the presence of the electrostatic charge of the soil mate-rial (6). At the higher water content 3 to 5 days after irri-gation (Fig. 1), a rapid increase in chloride content wasevident on 5 and 7 March 1971 for the 0- to 0.5-cm depthsbetween 0800 and 1800 hours, whereas the greatest decreasein water content for the same days occurred during this in-terval. Any decrease in chloride content indicates net chlo-ride movement downward. On the other hand, a decrease inwater content could be a case of bidirectional flow. The
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Fig. 3Hourly chloride and water contents at the shallowdepth for a bare field soil undergoing evaporation 14, 15,and 16 days (March 16, 17, 18, 1971) after irrigation.
amplitude of the diurnal fluctuation in chloride content andthe net chloride accumulation per day decreased with timeas the water content decreased (Fig. 2).
The chloride-water content changes were less out-of-phase at the lower than at the higher total water contents.With further drying (Fig. 3 and 4) at volumetric water con-tents less than 7%, the diurnal fluctuations of the chloridecontent were difficult to perceive. The gain in chloride atthe 1- to 2-cm depth started to become evident approxi-mately 3 weeks after irrigation as evaporation continuedfrom the bare field plot (Fig. 4). The chloride content inthe 0- to 0.5-cm depth had reached the maximum by aboutMarch 18, 1971.
A clearer picture of the chloride content-depth-time in-terrelationship is shown in Fig. 5, 6, and 7 in three-dimen-sional diagrams. For ease of plotting, only the 2-hour dataare plotted, and the 0- to 0.5-cm depth is deleted. The sharpdemarcation in chloride content between the 0- to 1-cmand the lower depths is more evident than in the precedingfigures. A continuous increase in chloride occurred in the0- to 1-cm depth, whereas the deeper depths remain con-stant at about 1 X 10~3 meq/cm3. Only when the surfacelayer started to reach its maximum chloride content (Figs.6 and 7) did the lower depths begin to show an increase inchloride. If salinity, caused by interaction of evaporationand salt movement, is a problem for seed germination, adescriptive study such as this for a particular soil should aidin determining the placement of the seed. For detailedstudy of salt distribution and movement in the field, it isadvisable to consider small-depth sample increments of 1cm or less.
The chloride contents were averaged over the 12-hourperiods for the 0000- to 1200- and 1200- to 2400-hour in-tervals (Fig. 8). The gain in chloride with time in the ear-lier stages of drying can be seen more readily with thecondensed-time scale than in the preceding graphs. Whenthe water contents were high, the gain in chloride in thedaytime was greater than at night. At least 50% of the chlo-
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512 SOIL SCI. SOC. AMER. PROC., VOL. 37, 1973
Fig. 5Three-dimensional perspective of chloride content vs.depth and time 3, 4, and 5 days after irrigation.
Fig. 6Three-dimensional perspective of chloride content vs.depth and time 14, 15, and 16 days after irrigation.
Fig. 7Three-dimensional perspective of chloride content vs.depth and time 23 and 37 days after irrigation.
Fig. 8Twelve-hour average chloride content for the 0- to0.5-, 0- to I-, and 1- to 2-cm depths as a function of time indays. (Solid circles and triangles and circled x's are the p.m.values.)
ride that would eventually accumulate at the surface 0 to0.5 cm moved in the first 5 days after irrigation, 75% in 8days, and essentially 100% in 14 days. The shape of thechloride accumulation curve could vary under a differentset of environmental and soil conditions.
From the chloride content data, we estimated that chlo-ride accumulation in the 0- to 0.5-cm section was essen-tially zero on March 18. Inspection of the water contentdata for this day indicated that chloride accumulationstopped at approximately 4% volumetric water content orabout 1,000 bars soil-water potential. A similar perusal ofthe 0- to 1-cm and 1- to 2-cm depth increments yielded awater content of approximately 3 to 4% when chloridemovement became negligible. Soil-water diffusivity meas-urements (3, 4) on the same soil (Fig. 9) showed thatliquid water movement is drastically curtailed at water con-tents of approximately 3%, and the reduction in chloridemovement does appear to be directly related to liquid move-ment in this case. Phillips and Quisenberry's (8) study ofthe self-diffusion of radioactive 36C1 in a loam soil as afunction of water content showed that the 36C1 movementstarted to decrease at approximately 20% water content.When their results were extrapolated to zero chloride self-diffusion coefficient, volumetric water contents in the rangeof 3 to 5% were obtained.
Other studies on salt movement where the water contentwas the variable (1, 2, 7, 9, 10) indicated the close inter-relation of salt movement to water content, but some ofthese did not cover the low water content range that wouldbe encountered in the field. Marshall and Gurr (7) foundthat the limiting or threshold water content for chloridemovement was not a specific value, but can range from 2-to 30-% water content. The threshold water content wasalso definitely related to the quantity of the clay-size frac-tion. The preceding interrelation probably indicates thatsurface coverage, i.e., the thickness of water on the soil sur-faces, is the controlling factor in chloride movement. The4% limiting water content is very much less than the 15bars value (15% water content) which is usually desig-nated as the lower limit of water available to plants.
Since salt movement is an integral part of the leachingprocess, it appears that leaching can be accomplished at wa-
GARDNER: EVAPORATION FROM HOMOGENEOUS SOIL 513
K -4Ul 10
O.I 0.2 0.3 0.4
VOLUMETRIC WATER CONTENT(cm3 /cm 3)
Fig. 9Soil-water diffusivity as a function of water content forAvondale clay loam. (Solid symbols represent liquid flowand open symbols represent the sum of vapor and liquidflows.)
ter contents well below the saturation value of the soil aslong as the driving force for the soil solution is downward.Field experiments conducted by other investigators showthe feasibility of leaching out salts without having a contin-uously ponded water over the soil surface. After any leach-ing or irrigation, the salt redistribution pattern near the soil
surface in particular will be governed by the thresholdwater content for soil movement. Furthermore, when theo-retical predictions of salt redistributions are made, thisthreshold value must be considered because salt migrationmay have ceased even though water redistribution may stillbe taking place.