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<br />- <br /> <br />van't Woudt, B.D., 1954, On factors goveming subsurface flow in volcanic ash soils, <br />New Zealand: American Geophysical Union Transactions, v.. 35, no. 1, p. 136- <br />144. <br />This paper presents obser.;'ationi on Lhe hydrology in response to StotID events of soils <br />derived from rhyolitic ash on the North Island of New Zealand. The author identifIes three <br />factors that restricted vertical flow into the soil at various dep hs, resuldng in lateral <br />subsurface flow. One of the factors was a form of water repellency in which the soil was <br />difficult to wet after desiccation, but would eventually become wet ll.ble at higher moisture <br />contents. This condition resulted in a thin layer of lateral flow near :he surface, over a drier, <br />underlying layer. The thickness of the conducting layer inc :eased With continued <br />precipitation. Other factors that led to subsurface flow included a lc ,wer perrneability of the <br />B horizon relative to the A hOri2011, and an increase in particle size witl-_ depth in the soil. <br />The paper also described tl-,e effect of 'tOpography on soil water repellency and subsequent <br />storm flow. Soil on the upper hdf of the slope dried out more quickly between storm <br />events, and consequently maintained its water-repellent qualities, in contrast to soil on the <br />lower portions of the slope, which generally maintained higher moisture contents that <br />reduced the frequency of water repellency. As a result, lateral flo", was observed on the <br />upper portion of the hillside, but not on the lower half. The authc r does not mention the <br />effects of this runoff pattern en erosion, but a previous (1952) paper discusses some <br />implications with respect to soil b~rtility. <br /> <br />Ward, P.R., and Oades, J.M., l'g93, Effect of clay mineralogy and exchangeable <br />cations on water-repellen(:y in clay-amended sandy soils: Australian Journal <br />of Soil Research, v. 31, p. 351-:364. <br />This investigation explored the interaction between clays and w;lter-repellent Australian <br />soils. The introduction cltes previous studies that have observ ~d reductions in water <br />repellency with the addition of fine particles, such as clays, but note; that these studies have <br />proved inconclusive as to the mechanism involved. This research atl empted to resolve some <br />of the discrepancies. A series of tests were performed on a llaturally water-repellent <br />Australian sand, and two wettable s:mds treated with hydrophobic St bstances (one with cetyl <br />alcohol, the other with organic matter extracts). Additions of dry c~ys (both Na+ - and Ca+ - <br />saturated clays) had no effect on water repellency. However, when sand-clay mi..xtures were <br />subjected to a wetting and drying cycle, repellency was signifIcantl y reduced. Researchers <br />concluded that clays reduced repellency by physically "masking" the hydrophobic surfaces of <br />sand grains. The effectiveness of a clay in reducing water repellency was determined, not by <br />its dispersibility, but by its ability to remain dispersed over the surfa:e of sand grains during <br />the drying process. This corresponded to clays that had a "low, but significant charge." <br />Kaolinite was found to be more dfective in reducing water repellency than montmorillonite, <br />and the authors suggested that it could potentially be used to remedy problems with water- <br />repellent field soils. <br /> <br />38 <br />