The role of tephra covers on soil moisture conservation at Haleakala's crater (Maui, Hawai'i)

All soils were coarse, with ∼ 20% gravel and ∼ 94% sand; most fine material (≤ 0.063 mm) was silt, as clay content was negligible. Organic-matter percentage was low (1.65%). Bulk density and porosity were associated with moisture variation both in tephra-insulated and bare soils; 80% of field moisture was statistically (p < 0.001) accounted for by pore space. Air and soil temperatures were recorded at three sites during ∼ one-week periods prior to moisture sampling. Tephra substantially decreased soil maxima and daily thermal amplitude in underlying soils, but did not noticeably affect nightly minima. Thin (5-6 cm) tephra layers were nearly as effective as thicker (9-15 cm) deposits in depressing soil maxima. Possible water-conservation mechanisms under tephra include: decreased evaporation due to ground shielding and lower maxima; reduced capillary flow; greater infiltration depth; nocturnal dew condensation; and fog interception by blocks.