Richard Silberstein

The sensitivity of a catchment model to soil hydraulic properties obtained by using different measurement techniques

Most studies on the use of physically based hydrological models have identified saturated hydraulic conductivity (Ksat) as one of the most sensitive input parameters. However, Ksat is also one of the most difficult landscape properties to measure accurately, casting doubt on the ability of modellers to estimate this parameter a priori for catchment simulations. Several studies have shown that Ksat estimates are greatly influenced by the measurement method used, primarily because of scale effects. In this paper, we evaluate the effect of Ksat measurement method on catchment simulations aimed at predicting water yield from forested catchments. A series of simulations are conducted using the Topog_Dynamic catchment model, with Ksat estimated by means of the constant head well permeameter, small core (6·3 cm×7·3 cm) and large core (22·3 cm×30 cm) methods. These were applied in a deep, permeable forest soil in which macropore flow has been noted to occur. The three measurement methods yielded very different Ksat estimates and these had a large effect on model results. The model predictions based on small core and well permeameter measurements were extremely poor, as these methods did not adequately account for preferential flow through the soil. The large core estimates of Ksat , which were one to three orders of magnitude higher than the values obtained by the other two techniques, produced good predictions of catchment discharge and known spatial patterns of water table depth. Our results highlight the need for caution when applying soil hydraulic measurements to catchment-scale models. Copyright

Predicting and controlling water logging and groundwater flow in sloping duplex soils in western Australia

Water logging and groundwater recharge were studied at a site in southwestern Australia characterised by sloping duplex soils in a Mediterranean environment. The specific objectives of the study were: (a) to determine the effectiveness of land management systems involving trees, shallow interceptor drains, and perennial pasture in reducing water logging and recharge risk; and (b) to predict water logging risk at the plot and catchment scale. We found that properties inherent in the site (soil hydraulics, topography, surface dams) had a larger control over seasonal water logging than differences in vegetation cover, with large variations in water logging and recharge over a relatively small area. Such variability would be difficult to capture in any detail using a process model of water logging. The tree/drain systems had local effects on water logging control, but this was mostly due to the direct effects of the trees, which provided localised discharge from deeper groundwater systems.

On the validation of a coupled water and energy balance model at small catchment scales

Abstract Catchment runoff is the most widely used catchment scale measurement in modelling studies, and we have a reasonable degree of confidence in its accuracy. The advent of satellites gives access to a new suite of measurements taken over a defined spatial range. These measurements, principally reflected or emitted radiation, provide hydrologists with new possibilities for quantifying the state of a catchment. Surface temperatures can be readily measured by a satellite on a scale comparable to the size of a small catchment. In this paper we show that satellite sensed temperatures can provide an important measure of catchment status, which can complement runoff measurements in water balance studies. A one-dimensional model, which couples the land surface energy balance with the soil and surface water balance is tested by comparison with runoff and with remotely sensed surface temperature measurements. Simulations have been run over four years for two small catchments which have a fairly homogeneous vegetation, one being forest and its neighbour pasture. Satellite “surface” temperatures have been interpreted in terms of the energy balance, and used as a test of modelling accuracy. An “effective” surface temperature is calculated as a weighted mean of temperatures of the separate soil and leaf surfaces. This modelled “effective” temperature correlates well with Landsat TM surface temperatures. When pasture replaces forest, the model predicts a reduction in evapotranspiration of around 30%, a three-fold increase in runoff, and an increase in mean soil moisture status. The change to pasture also results in a rise in mean effective surface temperature of about 4°C, and an increase in summer diurnal temperature range from 10 to 22°C. The winter diurnal temperature range is similar for both vegetation systems. Inclusion of soil moisture variability in thermal properties results in an increase in mean daily maximum temperature of about 2°C in summer and winter, without much change in daily minima. The daily mean temperature is not significantly affected.

Field testing of the universal calibration function for determination of soil moisture with cosmic‐ray neutrons

The semitheoretical universal calibration function (UCF) for estimating soil moisture using cosmic-ray neutron sensors was tested by comparing to field measurements made with the same neutron detector across a range of climates, soil, latitude, altitude, and biomass. There was a strong correlation between neutron intensity and the total amount of hydrogen at each site; however, the relationship differed from that predicted by the UCF. A linear fit to field measurements explained 99% of the observed variation and provides a robust empirical means to estimate soil moisture at other sites. It was concluded that measurement errors, neutron count corrections, and scaling to remove altitudinal and geomagnetic differences were unlikely to explain differences between observations and the UCF. The differences may be attributable to the representation of organic carbon, biomass or detector geometry in the neutron particle code, or to differences in the neutron energy levels being measured by the cosmic-ray sensor and modeled using the particle code. The UCF was derived using simulations of epithermal neutrons; however, lower energy thermal neutrons may also be important. Using neutron transport code, we show the differences in response of thermal and epithermal neutrons to the relative size of the hydrogen pool. Including a thermal neutron component in addition to epithermal neutrons in a modified UCF provided a better match to field measurements; however, thermal neutron measurements are needed to confirm these results. A simpler generalized relationship for estimating soil moisture from neutron counts was also tested with encouraging results for low biomass sites.

Measuring and monitoring the effects of agroforestry and drainage in the ‘Ucarro’ sub-catchment

A small sub-catchment in the western Australian wheatbelt was intensively monitored for approximately 5 years to investigate the effect of an established surface water management system, and associated tree belts, on the flow of water in the landscape. This change in approach to land management attempts to address the loss of farm productivity due to water logging and secondary salinity. The paper describes the geomorphological setting and the installation of various instrumentation used to monitor the flow of water at both field and catchment scale.

Overstorey evapotranspiration in a seasonally dry Mediterranean eucalypt forest: Response to groundwater and mining: Evaporation from Mediterranean forest: response to groundwater and mining

CSIRO, 147 Underwood Avenue, Floreat, Western Australia 6014, Australia Environmental Department, Alcoa of Australia, PO Box 172, Pinjarra, Western Australia 6208, Australia Centre of Ecosystem Management, School of Science, Edith Cowan University, Joondalup Campus, 270 Joondalup Drive, Joondalup, Western Australia 6027, Australia Correspondence Craig Macfarlane, CSIRO, 147 Underwood Avenue, Floreat, Western Australia 6014, Australia. Email: craig.macfarlane@csiro.au