Stormflow Response and "Effective" Hydraulic Conductivity of a Degraded Tropical Imperata Grassland Catchment as Evaluated With Two Infiltration Models.

Predicting catchment stormflow responses after tropical deforestation remains difficult. We used 5‐min rainfall and storm runoff data for 30 events to calibrate the Green–Ampt (GA) and the Spatially Variable Infiltration (SVI) models and predict runoff responses for a small, degraded grassland catchment on Leyte Island (the Philippines), where infiltration‐excess overland flow (IOF) is considered the dominant runoff process. SVI replicated individual stormflow hydrographs better than GA, particularly for events with small runoff responses 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 were statistically significant and negatively correlated with (mid‐slope) soil moisture content at 10 cm (SWC10)—as did the "catchment‐wide effective" hydraulic conductivity (Ke) of the GA model. Using SWC10‐based estimates of F0 and Im in SVI yielded satisfactory to good simulations for 11 out of 17 events with runoff coefficients ≥15%, but failed to reproduce the hydrographs for events with very small runoff amounts (0.25–1 mm) and low runoff coefficients (3%–6%). The median field‐measured near‐surface Ksat (2 mm hr−1) was distinctly lower than the median Im (32 mm hr−1) and, to a lesser extent, Ke (∼8 mm hr−1), suggesting an underestimation of the spatially averaged Ksat by the field measurements. Application of SVI is expected to give the most realistic results for situations where IOF is dominant, that is, where surface conditions are degraded and rainfall intensities high. Plain Language Summary: It is important to be able to predict the streamflow volume and peak flow rate during intense rainfall events. We used rainfall and streamflow data for a small, degraded tropical grassland catchment on Leyte Island (the Philippines) to calibrate two rainfall infiltration models of comparable complexity: the Green–Ampt model (GA), that describes a decrease in infiltration capacity with time, and the Spatially Variable Infiltration (SVI) model for which the infiltration capacity is a function of the rainfall intensity. SVI generally performed better than GA in simulating observed streamflow responses to rainfall, especially for events with multiple rainfall peaks. Values for the two main parameters of SVI (the amount of rainfall required to initiate stormflow, and the maximum infiltration capacity of the soil) were related to the amount of moisture in the top 10 cm of the soil prior to the rainfall event. Using the measured topsoil moisture contents for 26 rainfall events to estimate the SVI parameter values and predict the stormflow response from the measured rainfall intensity produced satisfactory to good results for most of the larger events. However, it failed to reproduce the stormflow patterns for six events with mostly small to very small runoff responses. Key Points: The Spatially Variable Infiltration (SVI) model outperformed the Green–Ampt model, especially when simulating multi‐peaked storm hydrographsSVI‐model parameter values correlated negatively with antecedent topsoil moisture contentModel‐derived catchment‐scale infiltration capacities were higher than field‐measured Ksat, regardless of the model or field method used [ABSTRACT FROM AUTHOR]