MOISTURE FLOW INDUCED BY THERMAL GRADIENTS WITHIN UNSATURATED SOILS

RESULTS ARE PRESENTED OF AN EXPERIMENTAL INVESTIGATION ON THREE SOILS OF DIFFERENT TEXTURE UNDER CONDITIONS OF PHASE COMPOSITION THAT FAVOR MOISTURE TRANSMISSION IN THE VAPOR PHASE. A BRIEF REVIEW IS PRESENTED OF THE OBSERVATIONS AND INVESTIGATIONS OF THE INFLUENCE OF TEMPERATURE GRADIENTS, AND VARIATIONS IN TEMPERATURE, ON THE RETENTION AND MOVEMENT OF WATER IN SOIL. SOIL COLUMNS 30 CM. LONG WERE ENCLOSED IN SPLIT LUCITE TUBES WITH AN INTERNAL DIAMETER OF 10 CM, AND A WALL THICKNESS OF 0.6 CM. VARIOUS HOT AND COLD FACE TEMPERATURES WERE USED WITHIN THE RANGE FROM -4 TO 25 C WITH TEMPERATURE GRADIENTS RANGING FROM 0.5 TO 0.8 C/CM. DETAILS OF THE CONSTRUCTION OF THE THERMAL UNIT ARE PRESENTED. DATA ARE PRESENTED TO ILLUSTRATE THE CHARACTER AND MAGNITUDE OF THERMALLY INDUCED MOISTURE FLOW WITHIN UNSATURATED SOILS, WITH PARTICULAR REFERENCE TO SOIL MOISTURE CONDITIONS CHARACTERISTICS OF MOISTURE TENSIONS OF EXCESS OD 0.1 ATMOSPHERES. THE COMPARATIVE DATA OBTAINED FROM THE EXPOSURE OF RELATIVELY LARGE SOIL COLUMNS TO TEMPERATURE GRADIENTS WHICH WERE CALCULATED TO GIVE SIMILAR CONDITIONS OF VAPOR PRESSURE DISTRIBUTION, BUT WHICH PRODUCED BOTH FREEZING AND NON-FREEZING CONDITIONS ADJACENT TO THE COLD PLATE, WOULD INDICATE THE FOLLOWING CONCLUSIONS WITH REGARD TO THE CHARACTER AND MAGNITUDE OF THERMALLY INDUCED MOISTURE FLOW WITHIN THE SOIL: (1) A TEMPERATURE GRADIENT IMPOSED ON A SOIL COLUMN, INITIALLY OF UNIFORM MOISTURE CONTENT AND TEMPERATURE, WILL CREATE A RELATIVELY LARGE GRADIENT OF VAPOR PRESSURE FROM WARM TO COLD REGIONS, BUT ONLY A VERY SLIGHT GRADIENT OF MOISTURE TENSION WITHIN THE LIQUID PHASE IN THE SAME DIRECTION, (2) IF SOIL MOISTURE CONDITIONS ARE BELOW THE PERMANENT WILTING PERCENTAGE, SMALL CHANGES IN MOISTURE CONTENT WILL PRODUCE A LARGE CHANGE IN THE VAPOR PRESSURE OF SOIL WATER, (3) SOIL MOISTURE CONTENTS ABOVE ARE THOSE CHARACTERISTICS OF THE PERMANENT WILTING PERCENTAGE, THE INDUCED CONDITIONS OF VAPOR PRESSURE GRADIENT WILL BE SUCH AS TO CREATE AN UNSTEADY DIFFUSION OF WATER VAPOR FROM WARM TO COLD REGIONS, AND SINCE THE SOIL-WATER VAPOR PRESSURES ARE CLOSE TO THE SATURATION VAPOR PRESSURES AT SIMILAR TEMPERATURES, THE CONDENSATION OF VAPOR WILL PRODUCE NO CHANGE IN VAPOR PRESSURE CONDITIONS, (4) SINCE THE TEMPERATURE-INDUCED GRADIENTS OF PRESSURE WITHIN THE LIQUID PHASE WILL BE EXTREMELY SMALL, THE NET CONDENSATION AND NET EVAPORATION WILL INDUCE A PRESSURE GRADIENT WITHIN THE LIQUID PHASE AND WILL CREATE A FLOW OF LIQUID IN OPPOSITION TO THE VAPOR DIFFUSION WITHIN BOTH THE WARMEST AND COLDEST REGIONS, (5) UNDER SOIL MOISTURE CONDITIONS IN THE VICINITY OF, AND SLIGHTLY ABOVE THE PERMANENT WILTING PERCENTAGE, A LARGE GRADIENT OF PRESSURE WITHIN THE LIQUID PHASE WILL BE REQUIRED TO INITIATE LIQUID FLOW FROM COOL TO WARMER REGIONS, SINCE THE CAPILLARY PERMEABILITY WILL BE VERY LOW, (6) THE CONDITIONS DESCRIBED ABOVE WILL BE RESPONSIBLE FOR THE PARABOLIC RELATIONSHIP OBSERVED BETWEEN NET MOISTURE MOVEMENT UNDER THE INFLUENCE OF A THERMAL GRADIENT AND THE MEAN MOISTURE CONTENT OF THE SOIL, (7) FOR MOISTURE CONDITIONS ABOVE THE PERMANENT WILTING PERCENTAGE, THE OBSERVED COEFFICIENTS OF VAPOR DIFFUSION WERE FROM SIX TO EIGHT TIMES GREATER THAN THOSE CALCULATED FROM KNOWN RELATIONSHIPS FOR ISOTHERMAL CONDITIONS, AND (8) FROST-INDUCED PRESSURE GRADIENTS WITHIN THE LIQUID PHASE WILL NOT CAUSE ANY APPRECIABLE FLOW OF WATER WHEN SOIL MOISTURE CONDITIONS ARE DRIER THAN THOSE CHARACTERISTIC OF MOISTURE TENSIONS IN THE VICINITY OF ONE ATMOSPHERE.

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  • Accession Number: 00235712
  • Record Type: Publication
  • Files: TRIS, TRB
  • Created Date: Aug 17 1970 12:00AM