Tracer-based studies of soil water movement in semi-arid forests of New Mexico
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The related issues of water movement and contaminant transport in arid and semi-arid environments have generated considerable interest and concern in the last few decades. Essential to understanding these issues is knowledge of how water moves through the soils that form the uppermost part of the vadose zone. The use of tracers, both natural and artificially introduced, is proving to be an effective method for gaining such knowledge in dry regions, where investigation by other means is difficult. In this study, natural stable-isotope and chloride tracers were used to investigate water movement in the soils of a pinon-juniper woodland and of a ponderosa pine forest on the Pajarito Plateau in northern New Mexico. The objectives were to (1) estimate and compare near-surface flux rates and evaluate the importance of evaporation in the two communities, and (2) determine to what extent differences in flux rates and evaporation are due to differences in plant cover and/or soil hydraulic properties. The results of this study will aid in evaluating the potential for contaminant mobility in semi-arid systems such as the Pajarito Plateau and, in addition, will increase understanding of nutrient distributions and plant water use in semi-arid environments. The stable-isotope data indicate a similarity between the pinon-juniper and ponderosa communities with respect to evaporation: in both, it is restricted mainly to the upper 10 cm of soil. Chloride profiles from the two communities, on the other hand, show a distinct difference with respect to downward fluxes: in the ponderosa pine forest, these fluxes ( 0.02 cm year-1) are an order of magnitude lower than those in the pinon-juniper woodland ( 0.2 cm year-1), even though total precipitation is about 4 cm year-1 higher in the ponderosa pine forest. This difference, however, appears to be related not to plant cover, but to differences in soil hydraulic properties. The soils of the ponderosa pine forest contain clay-rich B horizons that appear to restrict downward movement of water through the soil matrix, whereas the soils of the pinon-juniper community have B horizons much lower in clay content. The effect of differing soil properties on water movement suggests that contaminant distributions will vary across the Pajarito Plateau. The data on soil water ages support this hypothesis: they indicate that water (and, thus, contaminants) moves through the soil matrix in less than a decade in some areas, whereas in other areas, water takes hundreds of years to pass through the entire soil profile.
