| Abstract: | The response of water in the unsaturated zone to seasonal changes of temperature (T) is determined analytically using the theory of nonisothermal water transport in porous media, and the solutions are tested against field observations of moisture potential and bomb fallout isotopic (36Cl and3H) concentrations. Seasonally varying land surface temperatures and the resulting subsurface temperature gradients induce thermal vapor diffusion. The annual mean vertical temperature gradient is close to zero: however, the annual mean thermal vapor flux is downward, because the temperature-dependent vapor diffusion coefficient is larger, on average, during downward diffusion (occurring at high T) than during upward diffusion (low T). The annual mean thermal vapor flux is shown to decay exponentially with depth; the depth (about 1 m) at which it decays to ??-1 of its surface value is one half of the corresponding decay depth for the amplitude of seasonal temperature changes. This depth-dependent annual mean flux is effectively a source of water, which must be balanced by a flux divergence associated with other transport processes. In a relatively humid environment the liquid fluxes greatly exceed the thermal vapor fluxes, so such a balance is readily achieved without measurable effect on the dynamics of water in the unsaturated zone. However, if the mean vertical water flux through the unsaturated zone is very small (<1 mm y-1), as it may be at many locations in a desert landscape, the thermal vapor flux must be balanced mostly by a matric-potential-induced upward flux of water. This return flux may include both vapor and liquid components. Below any near-surface zone of weather- related fluctuations of matric potential, maintenance of this upward flux requires an increase with depth in the annual mean matric potential; this theoretical prediction is supported by long-term field measurements in the Chihuahuan Desert. The analysis also makes predictions, confirmed by the field observations, regarding the seasonal variations of matric potential at a given depth. The conceptual model of unsaturated zone water transport developed here implies the possibility of near-surface trapping of any aqueous constituent introduced at the surface. |
| Genre: | Article |
| ProdID: | 70018510 |
| Citation Author: | Milly, P. C. D. |
| Citation Contributing Office: | |
| Citation Datum: | |
| Citation Day: | |
| Citation Edition: | |
| Citation Editor: | |
| Citation End Page: | 518 |
| Citation Issue: | 3 |
| Citation Keywords: | |
| Citation Language: | English |
| Citation Larger Work Title: | Water Resources Research |
| Citation LatN: | |
| Citation LatS: | |
| Citation LonE: | |
| Citation LonW: | |
| Citation Month: | |
| Citation No Pagination: | |
| Citation Number Of Pages: | 10 |
| Citation Online Only Flag: | |
| Citation Phsyical Description: | |
| Citation Projection: | |
| Citation Public Comments: | |
| Citation Publisher: | |
| Citation Series: | |
| Citation Series Code: | |
| Citation Series Number: | |
| Citation Search Results Text: | Effects of thermal vapor diffusion on seasonal dynamics of water in the unsaturated zone; 1996; Article; Journal; Water Resources Research; Milly, P. C. D. |
| Citation Start Page: | 509 |
| Citation Volume: | 32 |
| Citation Year: | 1996 |
| Type: | citation/reference |
| Text: | Effects of thermal vapor diffusion on seasonal dynamics of water in the unsaturated zone; 1996; Article; Journal; Water Resources Research; Milly, P. C. D. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/outside_thumb.jpg |
| URL (DIGITAL OBJECT IDENTIFIER): | http://dx.doi.org/10.1029/95WR03489 |
| Date Other: | Mon, 1 Jan 1996 00:00 -0600 |
| Publisher: | |
