creased forage production and stocking rates of grazing livestock. Concentrated animal agriculture and land ap-The presence of rock fragments in soil layers can have a profound plication of animal manures have increased concerns impact on measured hydraulic properties. Variation of surface soil hydraulic properties influences the amount, distribution, and routing regarding nutrient loads in runoff and accelerated eutroof overland flow. The objective of this study was to assess the effect phication of regional water bodies (Wagner and Woodof rock fragments and soil texture on infiltration, hydraulic conductivruff, 1997). Better understanding of surface hydrology in ity, and related physical properties in soils of a small watershed in the region is needed to develop best management pracnorthwestern Arkansas. Single-ring and tension infiltrometer meatices for animal manure management that incorporate surements at three pressure heads (h ϭ Ϫ0.03, Ϫ0.06, and Ϫ0.12 m) information on the spatial distribution of water flow were completed on the surface soil layer at 42 sites along three tranpatterns. sects crossing the watershed. Upland (Nixa, loamy-skeletal, siliceous, Many soils in the Ozark Highlands were formed from active, mesic Glossic Fragiudults) and side slope (Clarksville, loamylimestone, sandstone, or shale residuum and often conskeletal, siliceous, semiactive, mesic Typic Paleudults) soils had signifitain high amounts of rock fragments. Rock fragments cantly less rock fragments, lower infiltration rates (i), and lower hydraulic conductivities (K) at and near saturation compared with the are defined as coarse fragments Ͼ0.002 m in diameter valley bottom soil (Razort, fine-loamy, mixed, active, mesic Mollic (Miller and Guthrie, 1984). Rock fragments in soils of Hapludalfs). Average infiltration rate at h ϭ Ϫ0.03 m for all soils the Ozark Highlands represent weathering-resistant inwas only 9% of the ponded value suggesting that pores Ͼ1 mm in clusions within the original bedrock (e.g., chert) or pardiameter dominated water flow under saturated conditions. At saturaent material remnants. With natural and culturally acceltion, hydraulic properties tended to increase with rock fragment conerated erosion, rock fragments from higher elevations tent while, at h ϭ Ϫ0.12, the opposite was true. It is hypothesized have accumulated at the base of slopes (colluvium) and in that the source of rock fragments (weathering in place vs. colluvial and floodplains and drainage ways (alluvium). The amount alluvial origin) and contact with the surrounding fine-earth fraction and type of rock fragments in surface soil layers can influence water flow by affecting hydraulic continuity near fragment affect infiltration and water storage, which in turn influsurfaces. These relatively subtle morphological factors may have a disproportionate impact on water flow under near-saturation condi-ence land use and site productivity (Ravina and Magier, tions in these soils. 1984; Brakensiek and Rawls, 1994; Poesen and Lavee, 1994). Rock fragments ...

Tension infiltrometers have become a valuable tool for understanding infiltration in macropores and the soil matrix, but methodology varies. Our objective was to compare tension infiltrometer techniques and calculation procedures for determining unsaturated hydraulic conductivity, K(h), as a function of soil water pressure head (h). Field tension infiltrometer measurements were run to determine K(h) from: (i) steady‐state infiltration into an excavated one‐dimensional column, (ii) calculated sorptivity and measured change in soil water content for steady‐state three‐dimensional infiltration into dry soil, (iii) steady‐state three‐dimensional infiltration with two infiltrometer base sizes, and (iv) steady‐state infiltration for three negative heads at the same location using two different calculation schemes. For one scheme, a nonlinear regression method was used to fit α [a constant relating ln(K) and h] and K(0) from measured infiltration across three negative heads. The fitted α and K(0) were then used to calculate K(h) at each negative pressure head. Calculated K(h) by the nonlinear regression method from three‐dimensional infiltration measurements were 105% of measured one‐dimensional rates (from excavated columns), closer than any other method of calculation. More importantly, this method did not result in calculated K(h) less than zero or larger than three‐dimensional infiltration rates, as some calculation procedures did. The method did not depend on determinations of sorptivity or on initial or final soil water content.

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Numerous field and laboratory studies have documented the occurrence of preferential transport of solutes due to a fraction of the soil water being immobile and not taking part in the transport process. Domain models have been developed that describe these processes, but before we can apply them routinely, we need methods for measuring the required model parameters, particularly the fraction of immobile water to total water θ lm /θ and the exchange coefficient between the mobile and immobile domains, α. We developed a field method for measuring both θ lm /θ and α. The method uses a sequence of conservative anionic tracers consisting of Br − , pentafluorobenzoate, o-trifluoromethylbenzoate, and 2,6-difluorobenzoate infiltrated with time through a tension infiltrometer. Previous studies have confirmed that these tracers have very similar transport properties in a wide range of soils. The method was applied to an undisturbed loam and a greenhouse soil as an initial test of the approach. Calculated θ im /θ fractions averaged 0.69 and ranged from 0.25 to 0.98, while calculated α values averaged 0.0081 h −1 and ranged from 0.0030 to 0.021 h −1. These values compare well with values reported earlier by other investigators. The method is simple and allows routine measurement of transport properties of field soils. The method can also be used to validate the applicability of domain models to specific soils. Disciplines

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Tillage management influences the distribution of macropores (biopores, cracks, interpedal planes, and packing voids) that may provide pathways for rapid infiltration of water. To aid in predicting ranges of saturated hydraulic conductivity (Ksat), macropore distribution in situ was characterized by exposing selected horizontal planes and tracing macropores on clear polyethylene sheets. A methylene blue solution was used to indicate macropore continuity through the pressure pan. Sixteen undisturbed cores were taken in a grid pattern from each Ap horizon and adjacent subsoil for determination of Ksat. Field marking on plastic sheets was superior to photographic slides as a technique to characterize macropores because film noise (false pores) was eliminated, overall analysis time was reduced, and different features could be separated for analysis. Unfortunately, a bias of pore size between observers was possible. Four sites with four different soil series were examined: Nicollet (fine‐loamy, mixed, mesic Aquic Hapludoll), Rozetta (fine‐silty, mixed, mesic Typic Hapludalf), Waukegan (fine‐silty over sandy or sandy‐skeletal, mixed, mesic Typic Hapludoll), and Normania (fine‐loamy, mixed, mesic Aquic Haplustoll). Below the maximum tillage depth, macropores were present at all locations, but tillage disrupted continuity of pores from the surface. No‐till had macropores throughout the upper 70 cm with continuity observed in the 0‐ to 35‐cm range. Numbers of pores (>0.4‐mm diam.) per m2 ranged from 100 to >3000, representing 0.1 to 2% of the total area. When present, horizontal crack length ranged from 1.7 to 19.3 m m−2. Measured Ksat on undisturbed detached cores ranged from 1.1 to 180 µm s−1 with CVs ranging from 44 to 197%. The Ksat could be estimated within a range (out of eight classes) from descriptions of biopore area, cracks, soil structure, and soil texture.

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Soil moisture affects the spatial variation of land–atmosphere interactions through its influence on the balance of latent and sensible heat fluxes. Wetter soils are more prone to flooding because a smaller fraction of rainfall can infiltrate into the soil. The Soil Moisture Ocean Salinity (SMOS) satellite carries a remote sensing instrument able to make estimates of near-surface soil moisture on a global scale. One way to validate satellite observations is by comparing them with observations made with sparse networks of in situ soil moisture sensors that match the extent of satellite footprints. The rate of soil drying after significant rainfall observed by SMOS is found to be higher than the rate observed by a U.S. Department of Agriculture (USDA) soil moisture network in the watershed of the South Fork Iowa River. This leads to the conclusion that SMOS and the network observe different layers of the soil: SMOS observes a layer of soil at the soil surface that is a few centimeters thick, while the network observes a deeper soil layer centered at the depth at which the in situ soil moisture sensors are buried. It is also found that SMOS near-surface soil moisture is drier than the South Fork network soil moisture, on average. The conclusion that SMOS and the network observe different layers of the soil, and therefore different soil moisture dynamics, cannot explain the dry bias. However, it can account for some of the root-mean-square error in the relationship. In addition, SMOS observations are noisier than the network observations.