K.R.J. Smettem

Three-dimensional analysis of infiltration from the disc infiltrometer: 2. Physically based infiltration equation

In situ measurement of soil hydraulic properties may be achieved by analyzing the unconfined efflux from disc tension infiltrometers, once consistent infiltration equations can be derived. In this paper an analytical, three-dimensional infiltration equation is developed, based on the use of parameters with sound physical meaning and adjustable for varying initial and boundary conditions. The equation is valid over the entire time range. For practical purposes, a simplified solution is also derived. The full and simplified equations give excellent agreement with published experimental results and are particularly useful for determining soil hydraulic properties through application of inverse procedures.

A Reactive Soil Moisture Sensor Network: Design and Field Evaluation

Wireless sensor network technology has the potential to reveal finegrained, dynamic changes in monitored variables of an outdoor landscape. But there are significant problems to be overcome in order to realize this vision in working systems. This paper describes the design and implementation of a reactive, event driven network for environmental monitoring of soil moisture and evaluates its effectiveness. A novel feature of our solution is its reactivity to the environment: when rain falls and soil moisture is changing rapidly, measurements are collected frequently, whereas during dry periods, between rainfall, measurements are collected less often. Field trials demonstrating the reactivity, robustness, and longevity of the network are presented and evaluated, and future improvements proposed.

Infiltration Theory for Hydrologic Applications

Here is your state-of-the-art guidebook through soil infiltration theory in response to hydrologic problems. By focusing on the theoretical basis of physically based infiltration functions and their application, Infiltration Theory for Hydrologic Applications presents an in-depth review of current issues and concerns. For scientists wishing concise and robust equations that can be applied in models for a variety of objectives.

Field testing a wireless sensor network for reactive environmental monitoring [soil moisture measurement]

Wireless sensor network technology promises to reveal fine-grained, dynamic changes in monitored variables of an outdoor landscape. But there are significant problems to be overcome in translating this vision to working systems. This paper describes the design and implementation of a reactive, event driven network for environmental monitoring of soil moisture and evaluates the effectiveness of this solution. A novel feature of our solution is its reactivity to the environment: when rain falls and soil moisture is changing rapidly, measurements are collected frequently, whereas during dry periods, between rainfall events, measurements are collected much less often. Reactivity allows us to focus on dynamic responses and limit the amount of useless data gathered, as well as improving robustness and network lifetime. The main contribution of the paper is to demonstrate that a reactive sensor network can deliver useful data on soil moisture responses to rainfall. Field trial results on the reactivity, robustness and longevity of the network are evaluated, and future improvements proposed.

Three‐dimensional analysis of infiltration from the disc infiltrometer: 1. A capillary‐based theory

The hydraulic properties of an unsaturated homogenous and isotropic soil can be obtained from the unconfined flux out of a disc infiltrometer into the soil over the depth of wetting. The disc infiltrometer is becoming increasingly popular, but methods of analysis have generally relied on the restrictive assumptions of one-dimensional flow at early times or quasi-steady state flow at large times. We provide an approximate analytical expression for three-dimensional unsteady, unconfined flow out of a disc infiltrometer, and this includes the geometric effect of the circular source but ignores gravity. This physically based solution is tested against data obtained from laboratory experiments on repacked material. The results illustrate that the difference between three-dimensional and one-dimensional flow is linear with time.

Estimating the contribution of preferential flow to subsurface runoff from a hillslope using deuterium and chloride

The concentrations of deuterium and chloride in rainfall, soil water and throughflow are reported for flow components through a mid-level gently sloping hillside podzolic soil with a well developed network of macropores. The deuterium and chloride signatures of the throughflow are shown to resemble that of rainfall rather than soil water for all rainfall events. Flow through macropores is considered to be the major mechanism for infiltration and throughflow. During the initial stages of rainfall, infiltrating water bypasses much of the soil matrix and a transient water table is established. Storage associated with this water table dampens much of the noise from the deuterium and chloride signature of the rainfall while maintaining the mean rainfall signature. The possible implications on the use of stable isotopes and conservative tracers such as chloride for streamflow partitioning are discussed.

Measuring the hydraulic properties of a stable aggregated soil.

Reliable modelling of soil-water flow requires accurate and convenient measurement of the appropriate soil hydraulic properties, which can be particularly problematic in structured soils with a distinct macropore-matrix dichotomy in the flow regime. In this paper the hydraulic properties of an aggregated soil at water supply potentials, Ψ0, near saturation are obtained using the disc permeameter. The sorptivity, S, the hydraulic conductivity, K(θ{symbol}), and the wetting soil-water characteristic, Ψ(θ{symbol}), are measured directly. The soil-water diffusivity, D(θ{symbol}), is derived from measurements of K(θ{symbol}) and Ψ(θ{symbol}) and is also shown to be well described by deconvoluting the sorptivity using the Parlange approximation. This description of D(θ{symbol}) conveniently supplements estimates obtained over a wider range of water contents using the absorption method of Bruce and Klute, with water content profiles obtained by gamma scanning. Finally, the measured K(θ{symbol}) function is accurately described by introduction of a method for scaling Ψ(θ{symbol}) over the inter- and intra-aggregate flow regions and matching to the measured hydraulic conductivity at two points.

The effect of farm dams and constructed banks on hydrologic connectivity and runoff estimation in agricultural landscapes

System coupling and landscape connectivity control the flow of water and sediment through landscapes. Although coupling is well known to control long-term landscape development and shorter-term sensitivity to disturbance, the anthropogenic influences on coupling are seldom considered in hydrologic investigations. In particular, the building of small-scale water diversion (earth banks) and collection (farm dams) infrastructure on hillslopes in dryland agricultural areas may significantly alter hillslope-channel coupling. Twelve sub-catchment basins in a dryland agricultural region were investigated under their natural (ignoring infrastructure) and modified (including infrastructure) conditions to investigate the influence of water collection infrastructure on hydrologic connectivity, and whether manual modification of a Digital Elevation Model (DEM) could account for the impact of these factors in hydrologic simulation of hydrologic and geomorphic processes. Dam numbers and density have both increased over the period of available aerial photography (1965-1999), resulting in an average 39.5% reduction (range 4.3-86.7%) in the area retaining hydrologic connectivity with the basin outlet. Analysis of basins dominated by either banks or dams, and with combinations of both was performed using the Cumulative Area Distribution (CAD), Hypsometric Curve (HC), Simplified Width Function (SWF) and Instantaneous Unit Hydrograph (IUH). The geomorphic descriptors (CAD and HC) showed little change in basin structure as a result of farm dam and bank construction, but hydrologic descriptors (SWF and IUH) indicate that hillslope processes are significantly altered by farm dams and banks. Because runoff models are sensitive to catchment area, incorporating hillslope water capture and diversion infrastructure into the base data sets may offer a solution to improved parameterisation of spatial models of hydrology, particularly in dryland agricultural regions.

Soil-water residence time and solute uptake. 1. Dye tracing and rainfall events

Fluorescent dyes have been used to label rainfall infiltrating into a well-structured calcareous woodland soil in situ. Soil-water outflow was monitored using throughflow troughs. The occurrence of dye output from the soil could not be totally predicted by the use of a simple storage model but was related to the occurrence of intense-rainfall events equal to or greater than 3 mm hr.−1 lasting for at least 2 hr.

The influence of macropores on runoff generation from a hillslope soil with a contrasting textural class

The hydrological response of a well-structured hillslope soil under pasture is described. In this soil, during the winter recharge period, macropores provide pathways for water movement which effectively by-pass the soil matrix. In consequence, a perched water table does not develop within the soil profile. New input water is able to penetrate rapidly through the macropore system within the clayey B horizon to the soil/bedrock interface. A highly transient ‘water table’ is then established within the macropore system and overland flow commences only when the capacity of this system is exceeded. During the summer, this flow pattern changes and, although the soil might be expected to absorb water more readily than during winter, overland flow is the dominant runoff mechanism. The macropore-matrix dichotomy of the soil hydraulic properties is characterized in situ using auger hole and disc permeaters. The measured soil hydraulic properties are used in a finite element model to predict independently the subsurface runoff response during the winter recharge period and to check the adequacy of a throughflow collection system. The model predictions are in reasonable agreement with measured water tables during drainage.