Seasonal and Interannual Dynamics of Non-Rainfall Water Inputs and Evapotranspiration in a Coastal Mediterranean Dune Ecosystem

Evapotranspiration (ET) dominates the water balance of Mediterranean dune ecosystems, yet it is tightly coupled to small but frequent non-rainfall water inputs (NRWI) that modulate near-surface moisture availability during prolonged dry periods. Here, we present a three-year (2021–2024) lysimeter-based assessment of the interplay between ET and NRWI in a coastal Mediterranean dune system in Doñana National Park (SW Spain), using two large, high-precision weighing lysimeters installed under contrasting surface conditions: bare sand and shrub vegetation.

Hourly lysimeter mass changes were analysed under rain-free conditions to quantify ET and to detect and partition NRWI into dew, fog, frost, and water-vapour adsorption (WVA) using a physically based meteorological classification. Cumulative ET strongly exceeded NRWI at both sites, reaching 844.5 mm for bare soil and 931.9 mm for shrub-covered soil over the monitoring period, confirming vegetation as a major amplifier of evaporative losses. In contrast, cumulative NRWI amounted to 174.0 mm (bare soil) and 112.0 mm (shrub), highlighting a persistent but secondary moisture input.

Despite its smaller magnitude, NRWI occurred frequently, often on more than half of all rain-free nights, and directly offset early-morning evaporative losses. Bare soil consistently accumulated more NRWI due to stronger nocturnal cooling and tighter coupling to atmospheric humidity, whereas shrub cover reduced NRWI while enhancing daytime ET through transpiration and increased turbulent exchange. Among NRWI components, WVA dominated both in frequency and cumulative contribution across all seasons and years, while dew showed strong interannual variability linked to nighttime temperature and humidity, and fog inputs were negligible.

Our results demonstrate that while ET governs the annual water balance, NRWI, particularly vapor adsorption, plays a critical buffering role by repeatedly replenishing surface moisture prior to daytime evaporation. This interaction is highly sensitive to vegetation structure and climate variability and is considered to be highly relevant for ecohydrological models for Mediterranean drylands.