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<br />Krammes, J.S., and DeBano, L.F., 1965, Soil wettability: a neglected factor in
<br />watershed management: Water Resources Research, v 1, no. 2, p. 283-286.
<br />TIus represents one of the first studies to investigate the formaticn of water-repellent soils
<br />and their effects on runoff pwcesses. Researchers used the method of water drop
<br />penetration time (rather than contact angle) to rapidly assess wat"r repellency in the fIeld.
<br />TIle authors distinguish betwee:J lhe water repellency of extremel~ - dry soils (caused by the
<br />fomlation of impenetrable air l'ilms at the water-soil interface) w:1ich transmit water after
<br />being moistened, and soil, that maim3ll water-repellent properties at fairly high moisture
<br />contents (caused by hydro:?hobic ,mbstances on the mineral grains) Water-repellency of the
<br />fomler can be overcome by application of water to the soil under a vacuum, but in the latter,
<br />such treatment will not signifIcantly affect water uptake. Water-repellent so~:s of the second
<br />type were identifIed in the San Dimas Experimental Forest 0 f Southern California, a
<br />chaparral environment, by water drop tests in the fIeld and laboraory. The condition was
<br />more prevalent in coarse-textured soils, and appeared to be relal ed to temperature. The
<br />author also noted that the water-repellent soils were covered by saturated layers, and had
<br />underlying layers that were wet where roots or other channels were present. This is a
<br />frequently cited paper that forms the foundation for later research.
<br />
<br />Letey, John, 1969, Measurement of contact angle, water drop penetration time, and
<br />critical surface tension, in D.eBano, L.F., and Letey, Jo m, eds., 'Water-
<br />repellent soils: University of California, Riverside, Ma~' 6-10, 1968,
<br />Proceedings, p. 43-47.
<br />This paper describes various techniques of characterizing wa :er repellency, including
<br />measurement of contact angles, water drop penetration time, and t le potential use of critical
<br />surface tension. The author re\'iews the concepts involved in his 1962 publication on
<br />measurement of liquid-solid com:act angles, and explains that since the contact angle is
<br />diffIcult to measure directly on rol!gh surfaces, the apparent conta:t angle can be calculated
<br />instead by measuring the rate of water flow through a soil column, or the height of capillary
<br />rise. This paper is often cited for the method of using water drop :,enetration time (WDPT)
<br />to rapidly assess water repellency, although it has been used in prior research as well. Letey
<br />emphasizes that WDPT does not necessarily indicate contac: angle, hut rather, the
<br />persistence of water repellency At best, one can assume that a: some point the contact
<br />angle is greater than 90" if water beads up on the surface. This paper presents the
<br />preliminary stages of using cr:.tical s.rrface tension to characteri:e water repellency. The
<br />method is further developed i~ later research (Watson and Letey, 1970), and then slightly
<br />modifIed by King (1981).
<br />
<br />Letey, John, Osborn, J., and l'elishd<, R.E., 1962a., Measurerr ent of liquid-solid
<br />contact angles in soil and sand: Soil Science, v. 93, no, 3, p. 149..153.
<br />This paper is often cited for its explanation of contact mgle measurement and
<br />characterization of water repellen::y. The model applies Poiseuille's approximation of flow
<br />in capillary tubes. Since ethanol bas a zero contact angle and wets all solids, inf1ltration tests
<br />with ethanol were used to calculate the effective pore radius and to turn, the contact angle
<br />for other solutions. However, the model did not work well for r ure sand, possibly due to
<br />the retention of trapped air withir. the pores. The authors also SUi gest that water drop tests
<br />have limitations in deftning wate:: repellency, especially when comparing soils of different
<br />textures or aggregations.
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