An empirical analysis of local persistency maps for identifying common wetting/drying patterns in non-perennial streams

Non-perennial streams constitute a significant portion of the global river network and play a critical role in catchment hydrology, influencing both the quantity and quality of riverine water resources. The presence of surface flow in a stream reach depends on the imbalance between water inputs from the contributing catchment and the subsurface water transport capacity of the soil prism beneath the riverbed. Since the incoming water flow fluctuates over time due to precipitation events and seasonal cycles, some portions of the stream network may periodically cease to flow, shaping the hydrological behavior of non-perennial streams. The likelihood of a stream reach to maintain surface flow over time is captured by the local persistency index, which indicates the fraction of time during which water is present at the site. Local persistency varies across the river network, reflecting the spatial variability of hydrological and morphological factors that influence the emergence of surface flow. However, this important characteristic of channel networks is often overlooked in existing hydrological models. Understanding how local persistency varies across river networks can provide valuable insights on several hydrological aspects, including: a) identifying scaling laws for estimating the prevalence of non-perennial streams at large scales, b) improving the representation network expansion/retraction and the associated patterns of local saturation in river basins, and c) providing insights on subsurface water fluxes and their spatio-temporal dynamics.

In this study, we analyze local persistency maps from 20 river networks across Europe and the US, spanning a wide range of climatic and geolithological settings and sizes (including a novel dataset for the Biois creek catchment - 20km², Northeastern Italy). In most catchments, the proportion of non-perennial streams remains high even at larger scales (e.g., >50% in nearly all case studies, even in those with the largest contributing areas). The shape of the local persistency distribution varies depending on the underlying climate and morphometric features, revealing distinct hydrological behaviors. Right-skewed distributions, where low persistencies are more common, suggest flashy responses to rainfall events potentially driven by surface or shallow fluxes. In contrast, left-skewed distributions indicate networks with more stable networks where flows dry out only occasionally. Our analysis also reveals distinctive spatial patterns in local persistency. Abrupt changes in persistency are often associated with specific hydrogeologic features, such as localized springs, indicating shifts in surface/subsurface water fluxes usually driven by the underlying geology. In some cases, local persistency increases or decreases gradually as one moves downstream along the network, reflecting the growing of contributing area or the presence of a losing riverbed. A progressive increase in persistency often leads to an upstream expansion of the network, while reductions in persistency correspond with longitudinal disconnections, particularly in regions with local morphological changes (e.g., variations in slope or topographic curvature). By analyzing local persistency patterns within and among catchments, this study provides valuable insights on the common characteristics of non-perennial streams, their relationship with the spatio-temporal variability of water fluxes within a catchment, and their potential role in improving the reliability of hydrological models