REDEFINING THE BLACK BOX - PROFESSOR FRED PHILLIPS’ CONTRIBUTIONS TO DESERT VADOSE-ZONE SCIENCE

Over the past several decades, desert vadose zones have transitioned from being treated as black box buckets to black box recorders containing valuable information about past and present fluxes of water and solutes. Scientific advances led by Fred Phillips through interpreting environmental-tracer and hydraulic data from vadose zones of the American Southwest have contributed substantially to this shift in thinking and practice. Fred’s seminal work published in the mid-1990’s described the use of environmental tracers generated by nuclear-weapon fallout and natural meteoric chloride deposition. Comparisons among a suite of environmental tracers from sites spanning the American Southwest provided early insight into: (1) the preservation of potential paleohydrologic information archived in desert vadose zones, (2) the long-term control by desert vegetation of deep hydraulic regimes, (3) the nearly ubiquitous retention of solutes as evidenced by chloride “bulges”, and (4) the importance of vapor transport in desert vadose zones. A substantial body of work has stemmed from and built on these pioneering concepts. Examples highlighted here include studies from the Basin and Range, Chihuahuan, Mojave, and Sonoran Deserts. Results from non-isothermal multiphase flow and transport modeling of environmental tracer and water-potential profiles demonstrate that desert shrubs sustain multi-millennial drying of deep vadose zones and reduce groundwater recharge. The modeling framework provides a means of quantifying water, vapor, and solute-flux histories in desert vadose zones in response to the Pleistocene-Holocene climatic and vegetational transition. The framework also serves as a basis for paleorecharge reconstruction, with far-reaching implications for basin floor recharge and aquifer replenishment. Fred’s chloride bulge research led to the discovery of multi-millennial accumulation and sequestration of nitrate and perchlorate beneath the root zones of desert vegetation. This work has broadly impacted research on nitrogen cycling and contaminant transport in arid and semiarid regions. Current research is still extending Fred’s pioneering studies to other chemical constituents and other arid regions around the world.