ASSESSING EFFECTS OF NATIVE REFORESTATION ON SOIL MOISTURE DYNAMICS AND POTENTIAL AQUIFER RECHARGE, AUWAHI, MAUI

Understanding the role of soils in regulating water flow through the unsaturated zone is critical in assessing the influence of vegetation on soil-moisture dynamics and aquifer recharge. Because of fire, introduced ungulates, and landscape-level invasion of non-native grasses, less than 10% of original dry forest still exists on leeward Haleakalā, Maui, Hawaiian Islands. Native dry forest restoration at Auwahi has demonstrated the potential for dramatic revegetation, allowing experimental comparison of hydrologic function between tracts of restored forest and adjacent grasslands. We hypothesized that even relatively recent forest restoration can assist in the recovery of impaired hydrologic function, potentially increasing aquifer recharge. The site receives about 730 mm of precipitation annually. To simulate an intense storm event, we experimentally irrigated and measured soil moisture and temperature with subsurface instrumentation at 4 locations within the reforested area and 4 within the grassland, each a 2.5 by 2.5 m plot. Compared to grassland areas, water in reforested sites moved to depth faster with larger magnitude changes in water content. The median first arrival velocity of water was greater by a factor of about 13 in the forested sites compared to the grassland sites. This rapid transport of water to depths of 1 m or greater suggests increased potential aquifer recharge. Improved characterization of how vegetation and soils influence recharge is crucial for understanding the long-term impacts of forest restoration on aquifer recharge and water resources, especially in moisture-limited regions.