Tony Wells

Goulburn River experimental catchment data set

(651 km 2 ) and Krui (562 km 2 ) subcatchments in the northern half of this experimental catchment with a few monitoring sites located in the south. The data set comprises soil temperature and moisture profile measurements from 26 locations; meteorological data from two automated weather stations (data from a further three stations are available from other sources) including precipitation, atmospheric pressure, air temperature and relative humidity, wind speed and direction, soil heat flux, and up- and down-welling shortand long-wave radiation; streamflow observations at five nested locations (data from a further three locations are available from other sources); a total of three surface soil moisture maps across a 40 km � 50 km region in the north from � 200 measurement locations during intensive field campaigns; and a high-resolution digital elevation model (DEM) of a 175-ha microcatchment in the Krui catchment. These data are available on the World Wide Web at http://www.sasmas.unimelb.edu.au.

An assessment of digital elevation models and their ability to capture geomorphic and hydrologic properties at the catchment scale

Digital elevation model (DEM) data quality is paramount for accurate representation of the land surface and drainage network. This issue was investigated within a small agricultural catchment in the Upper Hunter Valley region of New South Wales, Australia for a DEM created by use of a Differential Global Positioning System (DGPS) and a 25 m DEM from the New South Wales Governments land mapping authority, Land and Property Information (LPI). A hierarchical scaling approach was used to investigate the effect of increasing DEM grid size on a number of geomorphic and hydrologic descriptors (i.e. area–slope relationship, cumulative area distribution, hypsometric curve, width function, Strahler stream order and stream network statistics), as well as addressing the issue of source data accuracy. Results of qualitative and quantitative assessments indicate that as DEM grid size increased, average slope gradients decreased and the drainage network became increasingly simplified. Geomorphic descriptors such as the width function, cumulative area distribution and hypsometric curve appear largely insensitive to DEM scale. The area–slope relationship loses definition in the diffusive region of the curve at large grid scales; however, the fluvial region appears largely insensitive to changes in DEM resolution. A comparison of long-term field soil moisture data with wetness indices derived from DEMs clearly demonstrates that high resolution DEM data are needed to model soil moisture distribution. A 5 m DEM was found to have the minimum resolution required for the current study site in order to accurately capture catchment geomorphology and hydrology and to model the spatial distribution of soil moisture.

The indirect estimation of saturated hydraulic conductivity of soils, using measurements of gas permeability. I. Laboratory testing with dry granular soils

A comprehensive knowledge of soil hydraulic conductivity is essential when modelling the distribution of soil moisture within soil profiles and across catchments. The high spatial variability of soil hydraulic conductivity, however, necessitates the taking of many in situ measurements, which are costly, time-consuming, and labour-intensive. This paper presents an improved method for indirectly determining the saturated hydraulic conductivity of granular materials via an in situ gas flow technique. The apparatus employed consists of a cylindrical tube which is embedded in the soil to a prescribed depth. Nitrogen at a range of pressures was supplied to the tube and allowed to escape by permeating through the soil. A 3-dimensional, axisymmetric, steady-state, finite element flow model was then used to determine the value of the soil intrinsic gas permeability which produces the best fit to the pressure–air flow data. Saturated hydraulic conductivities estimated from the application of the gas flow technique to 5 granular soils covering a wide range of permeabilities were in close agreement with values determined using a conventional permeameter. The results of this preliminary study demonstrate the potential of this approach to the indirect determination of saturated hydraulic conductivity based on measurement of gas flow rates in granular and structured soils.

Water Content Measurement in Expansive Soils Using the Neutron Probe

Capacitance-type methods for measuring soil water content are known to be unreliable in expansive soils, as cracking disrupts the intimate contact between the soil and the measuring device. The neutron probe, which infers water content from the thermalisation of a cloud of neutrons, is potentially less affected by cracking. The effect of cracking on neutron probe measurements was investigated by a series of numerical simulations using an axisymmetric finite element model based on seven-group neutron-diffusion theory. The simulations employed a consistent soil cracking model based on Maryland clay, in which crack volumes are determined from the changes in void ratio in the shrinking bulk soil. The results show that the presence of cracks in a clay soil affects the inferred water content and that measurements affected by air-filled cracking under-predict not only the water content in the uncracked soil peds but also the average water content in the larger cracked soil mass. The reason for this under-prediction is understood by considering the spatial distribution of the thermalised neutrons in the cracked and uncracked soils. The fast neutrons emitted from the source are seen to diffuse preferentially along air-filled cracks, traveling a large distance from the detector before they become thermalised, thus reducing their likelihood of being back-scattered to the detector where they can be counted. The proximity of the first crack to the probe in the ground also affects the measurement. Water-filled cracks are seen to have the opposite (but lesser) effect to air-filled cracks. A comparison of a simple uniform width crack model to a more realistic model in which crack width varies with changing water content shows that the model is sensitive to crack distribution and that the linear calibration expressions that are typically employed for neutron probes are likely to be unreliable in cracked clay soils.

Neutron soil moisture probe operation in saline environments

Abstract Neutron soil moisture probes enable soil moisture profiles to be measured in a rapid and nondestructive manner. This study examined experimentally and analytically the impact of salinity on neutron probe measurements. Experimentally, a decrease in the neutron count was associated with all common saline species (by as much as 50% when exposed to 10 wt.% aqueous solutions), with the decrease resulting from the absorption of lower energy neutrons by chloride nuclei. The experimental results also showed that the heterogeneity of the boundary region at the saline soil surface affected the neutron probe response. A seven-group finite element model of the neutron probe operation successfully predicted the response of the probe to saline conditions, with only a slight overprediction of the rate of decline in the neutron count as salinity increased. The finite element model simulation of the neutron probe operation in more realistic agricultural scenarios showed that the impact of salinity on the neutron count versus soil moisture curve is likely to be minor except in extreme cases. Extreme (electrical conductivity = 15 dSm-1) levels of salinity, however, are likely to cause significant deviations from the (nonsaline) calibration curve, particularly at higher soil moisture contents. The impact on the neutron count of nonuniform salinity distribution throughout the soil column was predicted to be highly dependent on the relative positioning of the areas of salinity and the neutron probe source and detector.