Predicting stormflow response of a degraded tropical grassland catchment using a spatially variable infiltration model
<p>Reduced surface infiltration capacity (K<sub>sat</sub>), increased infiltration-excess overland flow (IOF) and soil loss after deforestation and subsequent surface degradation in the humid tropics are well-documented. However, attempts to predict concomitant increases in storm runoff using physically-based approaches or to relate infiltration model parameter values calibrated with observed hyetographs and hydrographs at the small catchment scale to point-based measurements of K<sub>sat</sub> are rare. We used measured rainfall intensity and stormflow rates at 5-min intervals for 37 separate events (receiving 5&#8211;154 mm of rain) from the 3.2 ha degraded fire-climax grassland Basper catchment (Leyte Island, Philippines) to evaluate the performance of a spatially variable infiltration (SVI) model. SVI relates actual infiltration rates to rainfall intensity and a spatially averaged infiltration parameter I<sub>m</sub> after an initial infiltration amount F<sub>0</sub> and has been used successfully to predict IOF at the plot scale at various tropical locations. Quickflow hydrographs were produced using the Hewlett & Hibbert straight-line separation method and actual infiltration rates were derived by subtracting 5-min quickflow rates from corresponding rainfall inputs. SVI-predicted actual infiltration rates were compared with observed rates to derive optimized values of I<sub>m</sub> and F<sub>0</sub> per event. Earlier work at Basper had revealed very low (near-)surface values of K<sub>sat</sub> (implying frequent IOF although there was reason to suspect that K<sub>sat</sub> was underestimated). No explicit measurement was made of hillslope IOF, but stable isotope mass balance computations and a high degree of stream-water dilution during times of rain suggested large contributions of &#8216;new&#8217; water of low electrical conductivity that likely represented OF. Whilst SVI generally replicated individual quickflow hydrographs very well, values of I<sub>m</sub> and F<sub>0</sub> varied markedly between events. Using the median values of I<sub>m</sub> (46 mm h<sup>-1</sup>) and F<sub>0</sub> (6.8 mm) produced reasonable to good results (NSE > 0.6) for a subset of 15 (larger) events only. F<sub>0</sub> was positively related to maximum rainfall intensity over 15 or 30 min while I<sub>m</sub> was not significantly correlated to measured (mid-slope) soil water content or precipitation-based antecedent wetness indicators. However, I<sub>m</sub> exhibited a significant inverse correlation (Spearman r<sub>s</sub>=-0.617) with pre-storm baseflow rate Q<sub>b</sub> (notably for Q<sub>b</sub><0.5 mm d<sup>-1</sup>) suggesting foot-slope wetness status may be important for stormflow generation as well. The spatial distribution of K<sub>sat</sub>-values implied by SVI confirmed the suspected under-estimation of field-based K<sub>sat</sub> across the measured range, presumably reflecting a combination of macropore smearing (near-surface Amoozemeter measurements) and the limited size of the double-ring infiltrometer used for the measurement of surface infiltration rates.</p>