FIELD-STUDY OF SPATIAL VARIABILITY IN UNSATURATED FLOW BENEATH AND ADJACENT TO PLAYAS

We quantified unsaturated flow beneath playa and adjacent interplaya s ettings at a site in the Southern High Plains (United States) to resol ve issues related to where and how water moves through the unsaturated zone. This is the first study in which the data density (39 boreholes ) and the variety of techniques used (physical, chemical, and isotopic ) were sufficient to quantify spatial variability in unsaturated flow. Water contents, water potentials, and tritium concentrations were muc h higher and chloride concentrations were much lower beneath playas th an in interplaya settings, which indicated that playas focus recharge. These results refute previous hypotheses that playas act as evaporati on pans or that recharge is restricted to the annular region around pl ayas. Water fluxes estimated from environmental tracers ranged from 60 to 120 mm yr(-1) beneath playas and were less than or equal to 0.1 mm yr(-1) during the past 2000-5000 years beneath natural interplaya are as not subjected to ponding. To evaluate the apparent inconsistency be tween high recharge rates and thick clay layers beneath playas, we app lied bromide and FD&C blue dye to evaluate flow processes. These appli ed tracer experiments showed preferential flow along roots and desicca tion cracks through structured clays in the shallow subsurface in play as.