The disposal of hazardous and radioactive waste in arid regions requires a thorough understanding of the occurrence of soil‐water flux and recharge. Soil‐water chemistry and isotopic data are presented from three deep vadose zone boreholes (>230 m) at the Nevada Test Site, located in the Great Basin geographic province of the southwestern United States, to quantify soil‐water flux and its relation to climate. The low water contents found in the soils significantly reduce the mixing of tracers in the subsurface and provide a unique opportunity to examine the role of climate variation on recharge in arid climates. Tracing techniques and core data are examined in this work to reconstruct the paleohydrologic conditions existing in the vadose zone well beyond the timescales typically investigated. Stable chloride and chlorine 36 profiles indicate that the soil waters deep in the vadose zone range in age from approximately 20,000 to 120,000 years. Secondary chloride bulges that are present in two of the three profiles support the concept of recharge occurring at or near the last two glacial maxima, when the climate of the area was considerably wetter and cooler. The stable isotopic composition of the soil water in the profiles is significantly more depleted in heavy isotopes than is modern precipitation, suggesting that recharge under the current climate is not occurring at this arid site. Past and present recharge appears to have been strongly controlled by surface topography, with increased incidence of recharge where runoff from the surrounding mountains may have been concentrated. The data obtained from this detailed drilling and sampling program shed new light on the behavior of water in thick vadose zones and, in particular, show the sensitivity of arid regions to the extreme variations in climate experienced by the region over the last two glacial maxima.

Computer simulation modelling provides a useful approach for determining the trade-offs between the extent of prescribed burning and the long-term impacts of unplanned fires on management values. In the present study, FIRESCAPE-SWTAS, a process-based fire regime and vegetation dynamics model, was used in the World Heritage Area of south-west Tasmania, Australia, to investigate the implications of different prescribed burning treatments on identified management objectives. Treatments included annual prescribed burning of different proportions of the most flammable vegetation community, buttongrass moorlands. Additionally, a proposed strategic burning treatment for this landscape was simulated for comparison with these treatments. Simulations identified the nature of the relationships between the prescribed burn treatment level and the fire size distributions, the mean incidence, and the mean annual areas burnt by unplanned fires, with all three parameters declining with increases in treatment level. The study also indicated that strategically located treatment units were able to enhance the reduction in the fire risk to vegetation species susceptible to fire (fire-intolerant species).

In many landscapes, an important fire management objective is to reduce the negative impacts from unplanned fires on people, property and ecological values. In Australia, there exists an inherent assumption that high spatial variability in fire ages and hence fuel loads will have negative effects on both the incidence and spread of subsequent fires, and will enhance ecological values. A recent study using the process-based computer simulation model FIRESCAPE-SWTAS predicted several relationships between prescribed burn treatment levels and spatial patterning and management objectives in south-west Tasmania, Australia. The present study extended this investigation to additionally explore the effects of prescribed burning treatment unit size on unplanned fire incidence and area burned both in the general landscape and specifically in fire-intolerant vegetation. Simulation results suggest that treatment level had the greatest influence on modifying fire effects, whereas treatment unit size had the least effect. The model predicted that all three parameters interacted to determine the mean annual area burnt by unplanned fires. In fire-intolerant vegetation, treatment unit size did not influence the incidence of unplanned fires and the area burnt by unplanned fires in these communities. Where significant differences were evident, fire risk was reduced by higher treatment levels, deterministic spatial patterns of burning units, and smaller burning unit sizes.