The distribution of deuterium and oxygen-18 during unsteady evaporation from a dry soil
1989
Barnes, C.J. | Walker, G.R.
Evaporation from a dry soil is studied by modelling the movement and distribution of the stable isotopic species of water under transient isothermal conditions. This model extends an earlier steady-state model to account for unsteady evaporation. It is shown that with appropriate initial and boundary conditions, isotope profiles in space and time can be described in the same way as the water content profiles in terms of a similarity variable, proportional to depth and (time)-1/2. The analysis provides a means for experimental examination of the detailed processes of evaporation, allowing quantitative partitioning of the evaporative flux between vapour and liquid movement at all points within a one-dimensional column. Isotope and chloride distributions are analysed numerically for a step-function water content distribution, and compared with steady-state profiles calculated for similar conditions. It is shown that whereas for steady state the peak isotope ratios are determined by atmospheric and deep soil isotope ratios, for these transient conditions the peak value depends also on water content, and may be higher or lower than for steady state. For very dry soils, decreasing water content leads to deeper, broader but lower isotope maxima.
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