Runoff generation in ephemeral streams of the Virgin Islands: The case of Salt River, St. Croix

The island of St. Croix in the U.S. Virgin Islands, Lesser Antilles. Small islands worldwide have limited water resources and an increased need to understand the mechanisms underlying island water budgets, but they also usually lack adequate data. Our study focused on the Salt River watershed on the island of St. Croix in the Virgin Islands, which is an ephemeral stream network comprising both volcanic and carbonate hydrogeology. We used hydrometeorological and soil moisture data at upstream and downstream sites (chosen by geospatial topographic and soil information) to characterize thresholds for runoff generation and to analyze the disrupted connectivity of the ephemeral stream network. We found evidence of runoff thresholds, which align with work in similar catchments elsewhere. Connectivity from headwaters to lowland areas was uncommon, and saturation-excess overland flows and subsurface stormflow appeared to dominate runoff events. The upstream subcatchment (with volcanic hydrogeology) was much more responsive (runoff coefficient = 0.237) than the entire catchment (with partly carbonate hydrogeology; runoff coefficient = 0.086). The transition between these hydrogeologic zones may disrupt connectivity and play an important role in aquifer recharge, while buffering the marine environment from water quality problems. For resource managers, this study offers the possibility of identifying sensitive groundwater recharge zones, and provides insight into the frequency of ridge-to-reef flow events. • Upstream subcatchment had higher runoff coefficients than the whole catchment. • Downstream lowlands flowed less, acting as a losing stream. • Ridge-to-reef connectivity was directly related to deep soil water content. • Impoundments in the stream may play an important role in hydrologic connectivity.