A Simple Technique for Measuring Wetting Front Depths for Selected Soils

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The depth of the wetting front within a soil sample in infiltration measurements, especially in soils that develop cracks on drying, is difficult to ascertain simply and nondestructively. A technique was developed to determine wetting front locations on prepared soil beds, with a miniature penetrometer probe of the needle type, immediately following a simulated rainfall event. The method involves placing a 0.5-kg weight atop a miniature penetrometer probe and measuring the penetration depth of the probe relative to a known datum. Five texturally different soils were tested under similar laboratory conditions to evaluate this method. The penetrometer-based method provided accurate estimates of the wetting front position in laboratory simulated rainfall infiltration studies for clay, silty clay, and sandy clay soils that differed from visually observed depths by <1 mm. For the silt loam soils, however, this method underestimated mean wetting front depths by as much as 4 mm, with a standard deviation of 1.6 mm and 95% confidence limits of ±2.5 mm. The penetrometer method was especially useful for detailed characterization of wetting front depths in soils where wetting was highly variable or irregular (e.g., cracking clay soils).