Predicting stormflow response of a degraded tropical grassland catchment using a spatially variable infiltration model
Authors: Cheng Zhuo , Zhang Jun , Yu Bofu , Bruijnzeel L. Adrian 
Published: March 2021

&lt;p&gt;Reduced surface infiltration capacity (K&lt;sub&gt;sat&lt;/sub&gt;), 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&lt;sub&gt;sat&lt;/sub&gt; are rare. We used measured rainfall intensity and stormflow rates at 5-min intervals for 37 separate events (receiving 5&amp;#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&lt;sub&gt;m&lt;/sub&gt; after an initial infiltration amount F&lt;sub&gt;0&lt;/sub&gt; and has been used successfully to predict IOF at the plot scale at various tropical locations. Quickflow hydrographs were produced using the Hewlett &amp; 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&lt;sub&gt;m&lt;/sub&gt; and F&lt;sub&gt;0&lt;/sub&gt; per event. Earlier work at Basper had revealed very low (near-)surface values of K&lt;sub&gt;sat&lt;/sub&gt; (implying frequent IOF although there was reason to suspect that K&lt;sub&gt;sat&lt;/sub&gt; 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 &amp;#8216;new&amp;#8217; water of low electrical conductivity that likely represented OF. Whilst SVI generally replicated individual quickflow hydrographs very well, values of I&lt;sub&gt;m&lt;/sub&gt; and F&lt;sub&gt;0&lt;/sub&gt; varied markedly between events. Using the median values of I&lt;sub&gt;m&lt;/sub&gt; (46 mm h&lt;sup&gt;-1&lt;/sup&gt;) and F&lt;sub&gt;0&lt;/sub&gt; (6.8 mm) produced reasonable to good results (NSE &gt; 0.6) for a subset of 15 (larger) events only. F&lt;sub&gt;0&lt;/sub&gt; was positively related to maximum rainfall intensity over 15 or 30 min while I&lt;sub&gt;m&lt;/sub&gt; was not significantly correlated to measured (mid-slope) soil water content or precipitation-based antecedent wetness indicators. However, I&lt;sub&gt;m&lt;/sub&gt; exhibited a significant inverse correlation (Spearman r&lt;sub&gt;s&lt;/sub&gt;=-0.617) with pre-storm baseflow rate Q&lt;sub&gt;b&lt;/sub&gt; (notably for Q&lt;sub&gt;b&lt;/sub&gt;&lt;0.5 mm d&lt;sup&gt;-1&lt;/sup&gt;) suggesting foot-slope wetness status may be important for stormflow generation as well. The spatial distribution of K&lt;sub&gt;sat&lt;/sub&gt;-values implied by SVI confirmed the suspected under-estimation of field-based K&lt;sub&gt;sat&lt;/sub&gt; 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.&lt;/p&gt;