The structure of the critical zone is a product of feedbacks between hydrologic, climatic, biotic, and chemical processes. Ample research within snow-dominated systems has shown that aspect-dependent solar radiation inputs can produce striking differences in vegetation composition, topography, and soil depth between opposing hillslopes. However, more research is needed to understand the role of microclimates on critical zone development within rain-dominated systems, especially below the soil and into weathered bedrock. To address this need, we characterized the critical zone of a north-facing and south-facing slope within a first-order headwater catchment located in central coastal California. We combined terrain analysis of vegetation distribution and topography with field-based soil pit characterization, geophysical surveys and hydrologic measurements between slope-aspects. We observed thicker soil profiles, higher shallow soil moisture, and denser vegetation on north facing slopes, which matched previously documented observations in snow-dominated sites. However, average topographic gradient and saprolite thickness were uniform across our study hillslopes, which did not match common observations from the literature. These results suggest dominant processes for critical zone evolution are not necessarily transferable across regions. Thus, there is a continued need to expand critical zone research, especially in rain-dominated systems. Here, we present four non-exclusive, testable hypotheses of mechanisms that may explain these unexpected similarities in slope and saprolite thickness between hillslopes with opposing aspects. Specifically, we propose both past and present ecohydrologic processes must be taken into account to understand what shaped the present day critical zone.

Over the last several decades, the study of Earth surface processes has progressed from a descriptive science to an increasingly quantitative one due to advances in theoretical, experimental, and computational geosciences. The importance of geomorphic forecasts has never been greater, as technological development and global climate change threaten to reshape the landscapes that support human societies and natural ecosystems. Here we explore best practices for developing socially-relevant forecasts of Earth surface change, a goal we are calling “earthcasting”. We suggest that earthcasts have the following features: they focus on temporal (~1 to ~100 years) and spatial (~1 m to ~10 km) scales relevant to planning; they are designed with direct involvement of stakeholders and public beneficiaries through the evaluation of the socioeconomic impacts of geomorphic processes; and they generate forecasts that are clearly stated, testable, and include quantitative uncertainties. Earthcasts bridge the gap between Earth surface researchers and decision-makers, stakeholders, researchers from other disciplines, and the general public. We investigate the defining features of earthcasts and evaluate some specific examples. This paper builds on previous studies of prediction in geomorphology by recommending a roadmap for (i) generating earthcasts, especially those based on modeling; (ii) transforming a subset of geomorphic research into earthcasts; and (iii) communicating earthcasts beyond the geomorphology research community. Earthcasting exemplifies the social benefit of geomorphology research, and it calls for renewed research efforts toward further understanding the limits of predictability of Earth surface systems and processes, and the uncertainties associated with modeling geomorphic processes and their impacts.