Subsurface flow-driven hydrology of semi-arid coastal lagoons

Coastal water bodies—lagoons, estuaries, and associated wetlands are dependent on and thus vulnerable to changes in both ocean and watershed dynamics. In semi-arid and Mediterranean climates, estuaries and coastal lagoons persist despite ephemeral riverine discharge (on seasonal or interannual timescales) and intermittent connection via tidal inlets to the ocean. The persistence of coastal surface water bodies in the absence of riverine or tidal inflow suggests subsurface flow as the main driver of coastal hydrology in these systems.This work explores the coastal water bodies of three watersheds in Central Chile. The Huaquén watershed is a 151 km² coastal basin with an ephemeral river but perennial coastal wetland and lagoon. Except for immediately after large storms, the lagoon does not have a tidal connection to the ocean.  The much larger Petorca (1989 km²) and La Ligua (1979 km²) watersheds drain into the Pacific Ocean through a shared estuary. The confluence of the two rivers is located 1 km upstream from the intermittently open inlet. These watersheds with origin in the Andean foothills, despite their large size, have very low riverine discharge due to climate, drought, and water-intensive agricultural development.Here we present results spanning two years of in-situ measurements of water level in the Pichicuy lagoon at the outlet of the Huaquén watershed, and the Ligua–Petorca estuary and nearby groundwater wells, combined with satellite remote sensing of surface water bodies using 10m resolution Sentinel-2 data and longer-term monitoring of groundwater and surface water by the Chilean water agency.Results highlight the dominance of groundwater exchange in the dynamics of coastal lagoons without an open tidal inlet. Measurements in the small Pichicuy lagoon show hydrology dominated by ocean-driven exchange via flow through the sandbar. This flow depends on the hydraulic gradient driven by wave setup and modulated by the tide, which is attenuated through the sandbar. In the much larger Ligua-Petorca watershed, little ocean influence is observed within the closed lagoon, but the surface area and water levels are shown to vary seasonally with watershed groundwater level fluctuations and on longer timescales with groundwater depletion by drought and water over-exploitation. This work highlights the importance in considering subsurface exchange flows between the ocean, coastal estuaries and lagoons, and the watershed, especially as climate change alters conditions in both the coastal ocean and in semi-arid and Mediterranean watersheds worldwide.