Stormflow response and ‘effective’ hydraulic conductivity of a degraded
tropical Imperata grassland catchment as evaluated with two infiltration
models
Abstract
Predicting catchment stormflow responses after tropical deforestation
remains difficult. We used five-minute rainfall and storm runoff data
for 30 events to calibrate the Green–Ampt (GA) and the Spatially
Variable Infiltration (SVI) model and predict runoff responses for a
small, degraded grassland catchment on Leyte Island (the Philippines),
where infiltration-excess overland flow is considered the dominant storm
runoff generating process. SVI replicated individual stormflow
hydrographs better than GA, particularly for events with a small runoff
response or multiple peaks. Calibrated parameter values of the SVI model
(i.e., spatially averaged maximum infiltration capacity, Im and initial
abstraction, F0) varied markedly between events, but exhibited
significant negative linear correlations with (mid-slope) soil water
content at 10 cm (SWC10) – as did the ‘catchment effective’ hydraulic
conductivity (Ke) of the GA model. SWC10-based values of F0 and Im in
SVI resulted in satisfactory to good predictions (NSE >
0.50) for 18 out of 26 storms for which data on SWC10 were available,
but failed to reproduce the hydrographs for six events (23%) with
mostly small runoff responses. Median values of field-measured
near-surface Ksat (~2–3 mm h-1, depending on method)
were distinctly lower than the median Im (32 mm h-1) and, to a lesser
extent, Ke (~8 mm h-1), confirming previously suspected
under-estimation of field-measured Ksat. Using pre-storm topsoil
moisture content and 5-min rainfall intensities as the driving variables
to model infiltration with SVI gave more realistic results than the
classic GA approach or the comparison of rainfall intensities with
field-measured Ksat.