Storage variability controls seasonal runoff generation in catchments at
the threshold between energy and water limitation
Abstract
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.