Using annual water balance analyses may mask intra-annual variability in runoff generation, which could limit our understanding of the similarities and differences between water- and energy-limited catchments. This may be especially limiting in comparisons between catchments close to the threshold between water- and energy-limitation. For this study, we examined runoff generation as a function of catchment storage in four watersheds, with focus on two that exist close to these thresholds to identify how year-to-year variability in storage resulted in intra-annual variations of runoff generation efficiency. Specifically, we focused on one energy-limited catchment in the humid subtropics and one water-limited in a Mediterranean climate. We used measured and calculated daily water balance components to calculate variations in the relative magnitude of daily storage. We isolated precipitation events to draw connections between storage and runoff generation at intra-annual scales and compared our findings to the same metrics in two intensely energy-limited landscapes. We observed distinct stages in daily storage across water years in watersheds at the threshold, where systems experienced wet-up, plateau, and dry-down stages. During the wet-up, precipitation was partitioned to storage, and runoff ratios ( RR) were low. In the plateau, storage was filled, precipitation was partitioned to runoff, causing high RRs. During the dry-down, storage decreased as precipitation was partitioned to evapotranspiration and runoff, causing low RRs. The critical role of evapotranspiration during the growing season resulted in relatively higher RRs during the wet-up than during the dry-down for a given storage value. Thus the same storage amount was partitioned to evapotranspiration or runoff differently throughout the year, depending on the storage stage. Despite their different positions on opposite sides of the threshold, the similarity between the two focus catchments suggests a potential characteristic behavior of systems at the threshold common to both humid and semi-arid landscapes.
Salt marshes are hotspots of nutrient processing en route to sensitive coastal environments. While our understanding of these systems has improved over the years, we still have limited knowledge of the spatiotemporal variability of critical biogeochemical processes within salt marshes. Sea-level rise will continue to force change on salt marsh functioning, highlighting the urgency of filling this knowledge gap. Our study was conducted in a central California estuary experiencing extensive marsh drowning and relative sea-level rise, making it a model system for such an investigation. Here we instrumented three marsh positions with different degrees of inundation (6.7%, 8.9%, and 11.2% of the time for the upper, middle, and lower marsh positions, respectively), providing locations with varied geochemical characteristics and hydrological interaction at the site. We continuously monitored redox potential (Eh) at depths of 0.1, 0.3, and 0.5 m, subsurface water levels (WL), and temperature at each marsh position to understand how drivers of subsurface biogeochemical processes fluctuate across tidal cycles, using wavelet analyses to explain the interactions between Eh and WL. We found that tidal forcing significantly affects biogeochemical processes by imparting controls on Eh variability, likely driving subsurface hydro-biogeochemistry of the salt marsh. Wavelet coherence showed that the Eh-WL relationship is non-linear, and their lead-lag relationship is variable. We found that precipitation events perturb Eh at depth over timescales of hours, even though WL show relatively minimal change during events. This work highlights the importance of high-frequency measurements, such as Eh, to help explain factors that govern subsurface geochemistry and hydrological processes in salt marshes.