School of Agriculture, Food and Ecosystem Sciences - Research Publications

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    Scale-dependency of effective hydraulic conductivity on fire-affected hillslopes
    Langhans, C ; Lane, PNJ ; Nyman, P ; Noske, PJ ; Cawson, JG ; Oono, A ; Sheridan, GJ (AMER GEOPHYSICAL UNION, 2016-07)
    Abstract Effective hydraulic conductivity (Ke) for Hortonian overland flow modeling has been defined as a function of rainfall intensity and runon infiltration assuming a distribution of saturated hydraulic conductivities (Ks). But surface boundary condition during infiltration and its interactions with the distribution of Ks are not well represented in models. As a result, the mean value of the Ks distribution ( ), which is the central parameter for Ke, varies between scales. Here we quantify this discrepancy with a large infiltration data set comprising four different methods and scales from fire‐affected hillslopes in SE Australia using a relatively simple yet widely used conceptual model of Ke. Ponded disk (0.002 m2) and ring infiltrometers (0.07 m2) were used at the small scales and rainfall simulations (3 m2) and small catchments (ca 3000 m2) at the larger scales. We compared between methods measured at the same time and place. Disk and ring infiltrometer measurements had on average 4.8 times higher values of than rainfall simulations and catchment‐scale estimates. Furthermore, the distribution of Ks was not clearly log‐normal and scale‐independent, as supposed in the conceptual model. In our interpretation, water repellency and preferential flow paths increase the variance of the measured distribution of Ks and bias ponding toward areas of very low Ks during rainfall simulations and small catchment runoff events while areas with high preferential flow capacity remain water supply‐limited more than the conceptual model of Ke predicts. The study highlights problems in the current theory of scaling runoff generation.
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    Effects of aridity in controlling the magnitude of runoff and erosion after wildfire
    Noske, PJ ; Nyman, P ; Lane, PNJ ; Sheridan, GJ (AMER GEOPHYSICAL UNION, 2016-06)
    Abstract This study represents a uniquely high‐resolution observation of postwildfire runoff and erosion from dry forested uplands of SE Australia. We monitored runoff and sediment load, and temporal changes in soil surface properties from two (0.2–0.3 ha) dry forested catchments burned during the 2009 Black Saturday wildfire. Event‐based surface runoff to rainfall ratios approached 0.45 during the first year postwildfire, compared to reported values <0.01 for less arid hillslopes. Extremely high runoff ratios in these dry forests were attributed to wildfire‐induced soil water repellency and inherently low hydraulic conductivity. Mean ponded hydraulic conductivity ranged from 3 to 29 mm h−1, much lower than values commonly reported for wetter forest. Annual sediment yields peaked at 10 t ha−1 during the first year before declining dramatically to background levels, suggesting high‐magnitude erosion processes may become limited by sediment availability on hillslopes. Small differences in aridity between equatorial and polar‐facing catchments produced substantial differences in surface runoff and erosion, most likely due to higher infiltration and surface roughness on polar‐facing slopes. In summary, the results show that postwildfire erosion processes in Eucalypt forests in south‐east Australia are highly variable and that distinctive response domains within the region exist between different forest types, therefore regional generalizations are problematic. The large differences in erosion processes with relatively small changes in aridity have large implications for predicting hydrologic‐driven geomorphic changes, land degradation, and water contamination through erosion after wildfire across the landscape.
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    Climate Dictates Magnitude of Asymmetry in Soil Depth and Hillslope Gradient
    Inbar, A ; Nyman, P ; Rengers, FK ; Lane, PNJ ; Sheridan, GJ (AMER GEOPHYSICAL UNION, 2018-07-16)
    Abstract Hillslope asymmetry is often attributed to differential eco‐hydro‐geomorphic processes resulting from aspect‐related differences in insolation. At midlatitudes, polar facing hillslopes are steeper, wetter, have denser vegetation, and deeper soils than their equatorial facing counterparts. We propose that at regional scales, the magnitude in insolation‐driven hillslope asymmetry is sensitive to variations in climate, and investigate the fire‐prone landscapes in southeastern Australia to evaluate this hypothesis. Patterns of asymmetry in soil depth and landform were quantified using soil depth measurements and topographic analysis across a contemporary rainfall gradient. Results show that polar facing hillslopes are steeper, and have greater soil depth, than equatorial facing slopes. Furthermore, we show that the magnitude of this asymmetry varies systematically with aridity index, with a maximum at the transition between water and energy limitation, suggesting a possible long‐term role of climate in hillslope development.
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    Quantifying relations between surface runoff and aridity after wildfire
    Van der Sant, RE ; Nyman, P ; Noske, PJ ; Langhans, C ; Lane, PNJ ; Sheridan, GJ (WILEY, 2018-08)
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