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After vegetation fires, discharge of streams and rivers is often higher than before. This is usually attributed to decreased canopy interception and evapotranspiration caused by vegetation removal, and to increased overland flow resulting from increased soil water repellency. In this paper we examine whether fire-induced changes in preferential flow can reinforce this postfire streamflow response. We studied five recently burned soils and adjacent unburned soils in Portugal and found that by reducing topsoil moisture and increasing soil moisture variability, fire increased the propensity for preferential flow. This was confirmed by 2-D soil moisture and repellency profiles that showed preferential paths in burned soil that were more distinct, wetter, and slightly narrower than in unburned soil. Since water infiltrating along preferential flow paths bypasses the dry soil matrix, we suggest that narrow flow paths promote deep infiltration– which effect size varies with soil depth, (effective) rainfall, and overland flow. We pose that the resulting increase in infiltration increases drainage and interflow because the excess water cannot stay in the soil, and incorporate fire-induced or -enhanced preferential flow into a conceptual model of flow routing that explains the commonly observed increase in stream flow postfire.

Grass-covered heavy basin clay soils in the Netherlands appeared to be water repellent. Water repellency in the top layers of these soils occurred mainly as a coating on the aggregates. The variation in soil moisture content over short distances was studied by sampling the soil 10 times during the period August 31, 1993, to December 22, 1994. Each time, 35 samples (100 cm3) were taken in close order over a distance of 195 cm at depths of 0–5, 10–15, 20–25, and 30–35 cm. Differences between minimum and maximum soil moisture contents were high in all layers sampled, occasionally as much as 28 vol %. When the clay soil is dry, a major proportion of the water from precipitation or sprinkler irrigation may flow rapidly through shrinkage cracks to the subsoil, bypassing the matrix of the clay peds. However, preferential flow is not limited to macropore flow: irregular wetting patterns are also formed through the small pores of the matrix. The relationship between dry bulk density and volumetric water content was found to be positive when the clay soil was relatively dry and negative when it was relatively wet.

Water and solute input rates on the soil surface may vary considerably from place to place. Distribution flow, i.e., the process of water and solute flow in a lateral direction over and through the very first millimeter or centimeter of the soil profile, is an extremely important process in distributing the rainfall toward places where vertical transport occurs. This study was carried out to quantify the process the distribution flow and its underlying process mechanism. A KBr tracer was applied on two water repellent sandy soils to follow the actual flow paths of water and solutes in the upper part of the profile. On both experimental fields, distribution flow actually displaced the applied bromide laterally through a very thin layer on the top of the soil profile, referred to as the {open_quotes}distribution layer.{close_quotes} Distribution flow was directed to locations within the 0- to 2.5- cm layer, where the soil was the least water repellent. On these relatively wet areas, the highest concentrations of bromide could be found. There was an acceleration in the vertical transport of water and bromide on these sites. Effects of distribution flow on the local concentration of water and solutes can be expected to be evenmore » more pronounced in uneven terrains, where lateral displacements may increase from the millimeter-centimeter scale to tens of meters. Implications for modeling field-scale water and solute flow are outlined. 71 refs., 9 figs., 2 tabs.« less

The dynamics of fingered flow in a water repellent sandy soil was studied in the field by sampling 10 vertical trenches in a 1-year cycle. In dry soil, fingers were formed in those places in the top layer which have the lowest degree of potential water repellency. The finger diameters varied roughly between 10 and 50 cm depending on the sequence of weather conditions. The fingers were wet in the topsoil and increasingly drier with depth. In none of the trenches sampled were there any serious indications of finger merger. The actual water repellent soil volumes between fingers were excluded from the transport of water and solutes for at least several hours. The temporal and spatial variability of these actual water repellent soil volumes is illustrated and evaluated with respect to simulation model development.

Water repellency is an important property of many soils. It causes rainwater to penetrate into the soil as preferential flow paths, and solutes can reach the groundwater more rapidly than in the case of a homogeneous wetting. Water repellency depends on several factors which are principally related to the characteristics of the organic matter of the soil. A distinction between “potential” and “actual” water repellency and the assessment of the “critical soil water content” are introduced and highlighted in this paper. Persistence and degree of potential water repellency of dried samples were examined from 10 trenches in a dune sand with grass cover using the water drop penetration time and the alcohol percentage tests. The spatial variability of water repellency and, therefore, soil wetting was extremely high. The actual water repellency was measured on field-moist samples to obtain critical soil water contents. The soil is wettable above and water repellent below these values. The critical soil water content varies between 4.75 vol % at 5–10 cm and 1.75 vol % at 45–50 cm depth in this sandy soil.

A new stand-alone wireless embedded network system has been developed recently for continuous monitoring of soil water contents at multiple depths. This paper presents information on the technical aspects of the system, including the applied sensor technology, the wireless communication protocols, the gateway station for data collection, and data transfer to an end user Web page for disseminating results to targeted audiences. Results from the first test of the network system are presented and discussed, including lessons learned so far and actions to be undertaken in the near future to improve and enhance the operability of this innovative measurement approach.

In this paper we describe a simple and quick method for determining the presence of water repellency in a soil by using a small core sampler (1.5 cm in diameter, 25 cm long) and applying the water drop penetration time (WDPT) test at different depths on the sandy soil cores. Obtained results provide spatial distribution patterns of water repellency in a soil profile, demonstrating seasonal changes in repellency. An advantage of the method is that the soil is not disturbed by the sampling. For assessment of the persistence of water repellency in strongly to extremely water repellent soils, and for determination of the critical soil water contents, the WDPT test and volumetric water content determinations should preferably be performed in the laboratory.