Coen J. Ritsema

How water moves in a water repellent sandy soil: 1. Potential and actual water repellency

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.

How water moves in a water repellent sandy soil: 2. Dynamics of fingered flow

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.

Occurrence of soil water repellency in arid and humid climates

Abstract Soil water repellency generally tends to increase during dry weather while it decreases or completely vanishes after heavy precipitation or during extended periods with high soil water contents. These observations lead to the hypothesis that soil water repellency is common in dry climates and rare in humid climates. The study objective is to test this hypothesis by examining the occurrence of soil water repellency in an arid and humid climate. The main conclusion of this study is that the effect of climate on soil water repellency is very limited. Field observations in the arid Middle Rio Grande Basin in New Mexico (USA) and the humid Piedras Blancas Watershed in Colombia show that the main impact of climate seems to be in which manner it affects the production of organic matter. An extremely dry climate will result in low organic matter production rates and, therefore, less potential for the development of soil water repellency. On the other hand, a very humid climate is favorable for organic matter production and, therefore, for the development of water repellency.

Effect of drying temperature on the severity of soil water repellency

Soil water repellency is often recognized in surface layers of soils that dry out frequently. The degree of water repellency of a soil can be measured by using the water drop penetration time (WDPT) test on field-moist or dried samples, referred to as actual and potential water repellency, respectively. A soil layer is actually water repellent below and actually wettable above its critical soil water content. Findings of the present study indicated that the degree of potential water repellency might change with different drying temperatures. For four of the seven sandy soil sites studied in the Netherlands, potential water repellency was greater after drying at 65° C relative to drying at 25°C, whereas it decreased at two sites and remained unchanged at one. The most reliable estimate of water repellency was obtained from undried samples collected during dry periods. Wetting rate measurements illustrated that water repellency increasing as a result of high drying temperatures led to decreasing water absorption by samples. Micromorphological investigations indicated that high drying temperatures resulted in an increase in the

PREFERENTIAL FLOW PATHS IN A WATER REPELLENT CLAY SOIL WITH GRASS COVER

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.

Variation in water content and wetting patterns in Dutch water repellent peaty clay and clayey peat soils

Abstract The variation in water content of grass-covered peaty clay and clayey peat soils was studied at six sites in the Netherlands. The topsoils were water repellent during dry spells. When the topsoils were dry, they could only absorb water with difficulty, which is illustrated by wetting rate measurements. Precipitation could flow rapidly through shrinkage cracks towards the subsoil, bypassing the matrix of the peat. However, the measurements revealed that preferential flow was not limited to macropore flow: irregular, fingerlike wetting patterns were also formed in the soil matrix. Due to these typical wetting patterns, soil water content varied over short distances at all sites at all sampling dates.

Exponential increase of publications related to soil water repellency

Soil water repellency is much more wide-spread than formerly thought. During the last decades, it has been a topic of study for soil scientists and hydrologists in at least 21 States of the USA, in Canada, Australia, New Zealand, Mexico, Colombia, Chile, Congo, Nepal, India, Hong Kong, Taiwan, China, Ecuador, Venezuela, Brazil, Mali, Japan, Israel, Turkey, Egypt, South Africa, Germany, The Netherlands, Spain, Portugal, United Kingdom, Denmark, Sweden, Finland, Poland, Slovakia, Russia, France, Italy, and Greece. Although, water repellent soils already have been indicated at the end of the nineteenth century, they have been discovered and studied in most countries in the last decades. Water repellency is most common in sandy soils with grass cover and in nature reserves, but has also been observed in loam, heavy clay, peat, and volcanic ash soils. From 1940 to 1970 research was focussed on identifying vegetation types responsible for inducing water repellency and on developing techniques to quantify the degree of water repellency. Of special interest has been the effects of wildfire on the development of soil water repellency and its consequences for soil erosion. Due to increasing concern over the threat to surface and groundwater posed by the use of agrichemicals and organic fertilisers, studies on water repellent soils have also been focused on its typical flow behavior with runoff and the existence of preferential flow paths. Since the end of the 1950s, wetting agents and clay amendments have been studied to ameliorate water repellent soils. Since 1883, more than 1200 articles related to soil water repellency have been published in journals, reports, and theses. An exponential increase in number of publications started in 1960, resulting in an average of 200 publications per 5 years.

Uneven moisture patterns in water repellent soils

Abstract In the Netherlands, water repellent soils are widespread and they often show irregular moisture patterns, which lead to accelerated transport of water and solutes to the groundwater and surface water. Under grasscover, spatial variability in soil moisture content is high due to fingered flow, in arable land vegetation and microtopography play a dominant role. Examples are given of uneven soil moisture patterns in water repellent sand, loam, clay and peat soils with grasscover, and in cropped water repellent sandy soils. In addition, the influence of fungi on inducing soil moisture patterns is illustrated as well.

Soil moisture and dry bulk density patterns in bare dune sands

Abstract Recent research has shown that irregular wetting and preferential flow may occur in heterogeneous field soils. Bare dune sands, which at first sight show the greatest homogeneity, were sampled volumetrically at nine locations in temperate and arid areas to determine the spatial distribution of soil moisture content, θ, and dry bulk density. Soil moisture patterns were irregular, even in extremely dry situations. As κ - θ functions for dune sands are very steep in the dry θ range, small differences in water content result in huge differences in hydraulic conductivity, κ. These differences, induced by soil moisture variability, may result in anisotropy of the system, and hysteresis tends to magnify this phenomenon. So, if soil moisture differences exist, they will be consolidated and even amplified during new rain events. At one site on a recently deposited dune sand, infiltrating water was able to transform a one-layer soil system into a two-layer soil system, inducing preferential flow at the layer interface. At another site, preferential flow paths appeared at the soil surface as ‘sand columns’ as a result of the displacement of the dry sand between the flow paths, caused by wind erosion. In all the dunes studied, dry bulk density patterns were irregular and tended to be slightly vertically orientated. Spatial distributions of volumetric and gravimetric soil water content showed great similarity.

Three-dimensional patterns of moisture, water repellency, bromide and pH in a sandy soil

Soil water repellency might lead to preferential flow of water and solutes through the unsaturated zone of soils. To study this process in a water repellent sandy field soil, a bromide tracer had been applied on a 2.2 m long and 0.4 m wide plot. The bromide application rate was 8 g/m2, and the plot was sampled using 100 cm3 steel cylinders after 52 mm of rainfall in 12 days. A total of seven layers were sampled to a depth of 74 cm. Each layer was sampled at 240 locations in a 40 by 6 grid. All samples were used for the determination of soil water content, degree of actual and potential water repellency, bromide concentration and pH. The spatial distribution of these properties was visualized three-dimensionally and compared. The degree of water repellency, bromide concentration, and pH distribution bore close resemblance to the fingered flow induced soil water content distribution. The degree of potential water repellency was relatively low in places with such fingers. Actual water repellency occurred between the fingers at the dry spots. Bromide was not found, or only in very low concentrations, in such places. Bromide depth profiles clearly indicated the occurrence of diverging flow in the wetter subsoil. Most likely, manuring activities during the last decades resulted in relatively high pH values in the topsoil, and in the subsoil along the recurring fingered flow pathways.