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Humans are recognized as having a major role in influencing environmental variability and change, including their influence on the climate system. To advance scientists' understanding of the role of humans within the climate system, there remains a need to resolve which of the following three hypotheses is correct: Hypothesis 1: Human influence on climate variability and change is of minimal importance, and natural causes dominate climate variations and changes on all time scales. In coming decades, the human influence will continue to be minimal.

at various scales, it may be possible to provide independent constraints on the components of coupled models used to address field-scale reactive transport. Both laboratory "sand box" and in situ field experiments are recommended. This approach will provide insight into the lim itations of conceptual models and parameterizations developed at the laboratory bench scale in accurately depicting a field-scale situation. The need for calibration of field-scale reac tion parameters in RTM should not be under estimated. Current limitations in characterizing multiscale variability in subsurface properties and observing in situ behavior result in knowledge gaps that will only be effectively addressed through process model calibration against field observations. While this may some what limit the robustness of model predictions, decisions based on RTM predictions can be justified if the conditions modeled are within the range of calibration conditions and the predicted outcomes can be bracketed to the extent that they clearly differentiate alternatives. The WG concludes that addressing the research issues noted above would greatly improve the application of RTM to many public health issues where better predictions are needed, and ultimately result in cost savings to federal agencies.

The relationship between stream water mean transit time (MTT), catchment geology, and landscape structure is still poorly characterized. Here, we present a new simple index that builds on the Jackson, Bitew, and Du (2014) index that focuses specifically on permeability contrasts at the soil-bedrock interface and digital elevation modelbased physical flow path measurements to identify broad landscape trends of moisture redistribution in the subsurface of steep wet headwater catchments. We use this index to explore the relationship between geology, landscape structure, and water transit time through the lens of landscape anisotropy. We hypothesize that catchments with a greater tendency to shed water laterally will correlate with younger stream water MTT and catchments with a greater tendency to infiltrate water vertically will correlate with older stream water MTT. We tested the new index at eight geologically diverse Pacific Rim catchments in Oregon, Japan, and New Zealand. The new index explained 77% of the variability in measured stream water MTT across these varied sites. These findings suggest that critical zone anisotropy and catchment form are first-order controls on the time scales over which catchments store and release their water and that a simple index may usefully capture this relationship.anisotropy, bedrock permeability, critical zone, headwaters, hydrology, landscape structure, mean transit time, turnover time