Subsurface water fluxes and their controls in a sloping heterogeneously
layered volcanic soil beneath a supra-wet tropical montane cloud forest
(NW Costa Rica)
Abstract
Tropical montane cloud forests (TMCF) receive additional (‘occult’)
inputs of water from fog and wind-driven rain. Together with the
concomitant reduction in evaporative losses, this typically leads to
high soil moisture levels (often approaching saturation) that are likely
to promote rapid subsurface flow via macropores. Although TMCF make up
an estimated 6.6% of all remaining montane tropical forest and occur
mostly in steep headwater areas that are protected in the expectation of
reduced downstream flooding, TMCF hillslope hydrological functioning has
rarely been studied. To better understand the hydrology of a TMCF on
heterogeneously layered volcanic ash soils under very high net rainfall
conditions (up to 6,740 mm yr-1), we examined temporal and spatial soil
water dynamics and their contribution to subsurface runoff and stormflow
for a year (1 July 2003 – 30 June 2004) in a small headwater catchment
on the Atlantic (windward) slope near Monteverde, NW Costa Rica.
Particular attention was paid to partitioning of water fluxes into
lateral subsurface flow and vertical percolation. The presence of a
gravelly volcanic layer at shallow depth of very high hydraulic
conductivity intercalated between two layers of lower conductivity,
controlled both surface infiltration and delayed vertical water movement
deeper into the soil profile. Soil water fluxes during rainfall were
dominated by rapid lateral flow in the gravelly layer, particularly at
high soil water contents. In turn, this lateral subsurface flow
controlled the magnitude and timing of stormflow from the catchment.
Stormflow amount increased rapidly once topsoil moisture exceeded a
threshold value of ~0.58 cm3 cm-3. Responses were not
affected appreciably by rainfall intensity because soil hydraulic
conductivities across the profile largely exceeded prevailing rainfall
intensities.