The influence of antecedent moisture content (AMC) on infiltration into water repellent soil: Laboratory experiments and model calculations

The sensitivity of infiltration rate to antecedent moisture content (AMC) in wettable soils is well-established with a low AMC promoting a higher initial infiltration rate. For water repellent soils, such as those found on fire-affected landscapes, we know little about how AMC may affect infiltration. Here we seek to understand how AMC affects infiltration for sub-critically water repellent soils (soils for which water forms a contact angle <90°). We conducted laboratory experiments using uniform #40-70 quartz sand with different degrees of water repellency from which we development a process-based model for simulating sorptivity and infiltration rate as a function of AMC. The experiments exhibited a highly non-linear relationship between contact angle and initial saturation degree (as a direct measure for AMC). We found the observed contact angle of water repellent sand was highest for air-dry conditions (as expected) but decreased rapidly with increasing initial saturation degree (AMC). Sorptivity of water repellent sand (which integrates wettability, pore sizes and AMC), exhibited a local minimum at the air-dry condition; a maximum for initial saturation degrees between 3% and 6%; then again a local minimum for initial saturation degree near 40%. Using the developed model along with measured contact angles and associated sorptivity values, maximum infiltrates were associated with an initial saturation degree around 5%. Thus, for water repellent soils, the maximum infiltration rates are associated with slightly moist rather than air-dry AMC. Model simulations also agreed well, qualitatively, with field-measured sorptivity data collected from a fire-affected, water repellent loam in Wyoming, USA. This research was supported by the U.S. National Science Foundation under Grant Nos EAR‐1324894 and OIA-2148788 as well by the US Army Corps of Engineers under Grant Numbers DACW42-03-2-0000 and W912HZ17C0037.