ESTIMATING PALEORECHARGE AND PALEOCLIMATE FROM UNSATURATED ZONE PROFILES

Unsaturated zone solute and isotope profiles may be interpreted in terms of past changes in recharge rate in cases where water movement can be approximated by piston flow. A model is presented which describes water movement in terms of piston flow below a surface mixing layer. Below the mixing layer, diffusion and dispersion occur in the liquid phase in the case of solutes, and in both the liquid and gas phases in the case of isotopically labeled water. Climatic events of 4-5 years duration may be preserved in the unsaturated zone for more than 50 years if the recharge rate is greater than approximately 20 mm yr-1. Longer-term fluctuations in climatic conditions (century scale) might be preserved for over 1000 years, provided the recharge mte is greater than approximately 2 mm yr-1, and the unsaturated zone is sufficiently deep. Profiles from Cyprus and northern Senegal appear to record changes in recharge rate over periods up to 400 years. The profiles are well correlated with known variations in rainfall, and fluctuations in lake levels. The model provides limiting conditions to be applied to the interpretation of profiles.