Seasonal and Interannual Variability of the Effective Flow Cross‐Sectional Area in a Water‐Repellent Soil

Abstract

By limiting infiltration and facilitating preferential flow, water repellency in surface soils produces wetting patterns that differ substantially from those in comparable wettable soils. Seasonal variation in water repellency usually follows a pattern of breakdown during periods of persistent rainfall and reestablishment during dry periods, complicating the prediction of moisture partitioning processes at affected sites. To examine the influence of annual water repellency variation on soil moisture, 20 Simple Soil Moisture Probe (SISOMOP) moisture sensors were installed horizontally at depths of 5 and 15 cm at a highly water‐repellent Banksia woodland reserve in Western Australia, recording soil moisture contents during a 42‐mo period from 2010 to 2013. Stages of water repellency breakdown were quantified in terms of the effective cross‐section (ECS) metric, which indicates the relative area share of flow pathways responsible for 90% of the total flow across a horizontal plane at a specified depth. The ECS data showed a steady spread of flow pathways throughout wet winter months of all years on record, consistent with a gradual breakdown of water repellency, usually reaching a maximum state between July and September. The reestablishment of sufficient water repellency to recreate persistent flow paths of low ECS was found to require a prolonged period of some months of hot, dry weather. Summer rainfall was found to be a significant factor in determining the degree of reestablishment during the warmer months, with a year preceded by an unusually wet summer leading to more rapid breakdown of non‐wetting behavior during the following winter.