Water yield issues in the jarrah forest of south-western Australia

The jarrah forest of south-western Australia produces little streamflow from moderate rainfall. Water yield from water supply catchments for Perth, Western Australia, are low, averaging 71 mm (7% of annual rainfall). The low water yields are attributed to the large soil water storage available for continuous use by the forest vegetation. A number of water yield studies in south-western Australia have examined the impact on water yield of land use practices including clearing for agricultural development, forest harvesting and regeneration, forest thinning and bauxite mining. A permanent reduction in forest cover by clearing for agriculture led to permanent increases of water yield of approximately 28% of annual rainfall in a high rainfall catchment. Thinning of a high rainfall catchment led to an increase in water yield of 20% of annual rainfall. However, it is not clear for how long the increased water yield will persist. Forest harvesting and regeneration have led to water yield increases of 16% of annual rainfall. The subsequent recovery of vegetation cover has led to water yields returning to pre-disturbance levels after an estimated 12–15 years. Bauxite mining of a high rainfall catchment led to a water yield increase of 8% of annual rainfall, followed by a return to pre-disturbance water yield after 12 years. The magnitude of specific streamflow generation mechanisms in small catchments subject to forest disturbance vary considerably, typically in a number of distinct stages. The presence of a permanent groundwater discharge area was shown to be instrumental in determining the magnitude of the streamflow response after forest disturbance. The long-term prognosis for water yield from areas subject to forest thinning, harvesting and regeneration, and bauxite mining are uncertain, owing to the complex interrelationship between vegetation cover, tree height and age, and catchment evapotranspiration. Management of the forest for water yield needs to acknowledge this complexity and evaluate forest management strategies both at the large catchment scale and at long time-scales. The extensive network of small catchment experiments, regional studies, process studies and catchment modelling at both the small and large scale, which are carried out in the jarrah forest, are all considered as integral components of the research to develop these management strategies to optimise water yield from the jarrah forest, without forfeiting other forest values.     

Photo-sieving: A method for grain-size analysis of coarse-grained,unconsolidated bedding surfaces

The photo-sieving method enables the grain-size analysis of particles > 10 mm from unconsolidated openwork bedding surfaces. The accuracy is equal to the machine-sieving method. The sediment surface is photographed in the field using a camera stand. The enlarged print is digitized for computer processing by tracing the outlines of the pebbles. From the size and shape of the projection area of the individual pebble images the computer determines the ellipsoids of revolution, classifies these ellipsoids, and establishes their grain-size distribution in percentage by weight. Thus photo-sieving differs distinctly from point-counting techniques. With the aid of photo-sieving, the surface grain-size distributions of larger sedimentary systems may be regularly mapped. Alluvial fans, braided rivers, both onshore and offshore coasts or areas of relict sediments and manganese nodules are suitable for this method.     

Instrument intercomparison study on cloud droplet size distribution measurements: Holography vs. laser optical particle counter

During the EUROTRAC Ground Based Cloud Experiment (GCE) 1990, a newly developed HODAR (Holographic Droplet and Acrosol Recording) was operated for the first time to measure cloud droplet size distributions by recording Fraunhofer in-line holograms of small cloud sample volumes in situ and analyzing the holographic images in the laboratory.This technical note compares the resulting size distributions with those obtained from two FSSP-100 laser optical particle counters. For all holograms analyzed during the GCE90 field experiment, the size distributions obtained from the two different methods agree well. Additionally, the liquid water contents (LWC) were measured directly by a Gerber particulate volume monitor PVM-100. The LWC calculated from the measured droplet size distributions deviate from the PVM-100 data.     

Fluxes of latent heat over the oceans: climatological studies and application of satellite observations

A short review is given of the different methods by which latent heat fluxes (or evaporation) over oceans are determined. In more detail, the applicability of the bulk aerodynamical formula is discussed. This formula is mainly used for climatological studies of heat fluxes and for the application of satellite data. As an example, for climatological studies we selected the work of Isemer and Hasse, who did a re-processing of the so-called Bunker data set to determine heat fluxes over the North Atlantic Ocean. In order to check their results, Isemer and Hasse calculated the annual mean heat budget for each latitudinal delt and derived from it the required ocean heat transport. With the aid of inverse modelling, the derived ocean transport was compared with the observed ocean transport and some of the used coefficients (e.g. bulk coefficients for latent and sensible heat flux) were altered. Though the ocean heat transport is changed by a large amount (at the equator 0.3 PW, original Bunker data; 0 PW, Isemer and Hasse; 0.76 PW, after inverse modelling (all northwards)) the overall patterns of the fields of the energy fluxes remain almost unchanged. The bulk coefficient for latent heat flux for example is altered by 5.6%.The geophysical parameters necessary for the bulk aerodynamic method can be determined from satellite observations: SST, q₀, u₀. Studies are described which used data from a microwave radiometer on SEASAT and NIMBUS7 to determine latent heat flux. An error calculation shows that the obtained accuracy is between 26 and 35 W m⁻². This accuracy is adequate enough to allow reasonable estimates to be made of these fluxes. More satellites are planned for launch with microwave radiometers and scatterometers which will increase the possibility of determining geophysical parameters more accurately for use in the bulk aerodynamic formula. They will provide the database from which large-scale fieldsof latent heat flux (for time scales shorter than a month or even for actual situations) can be derived.     

Changes in streamflow components following logging and regeneration in the southern forest of Western Australia

Two small experimental catchments were established in the south-west of Western Australia to study the effects of logging and subsequent regeneration on the mechanism of streamflow generation. Following a six year pre-treatment calibration period (1976–1981), one catchment (March Road) was logged and reforested in 1982 and the other (April Road South) remained as a control. Logging resulted in an increase in groundwater levels and subsequently groundwater discharge area. The deep, permanent groundwater levels in the valley and upslope areas rose until 1986 and then began to decline. The maximum rise was 5 m in the upslope areas. The duration of shallow, intermittent groundwater system, perched on underlying clay, was extended from 2–3 months in winter before logging to 5–6 months after logging. The shallow groundwater level rose in the valley and began to discharge at the ground surface in 1986. Logging resulted in an increase in streamflow. The maximum increase (≈18% of annual rainfall) was in 1983, one year after logging. The increase in streamflow was due to a substantial decrease in interception and evapotranspiration, increased recharge to the shallow groundwater system, decreased soil moisture deficit and consequently an increase in throughflow. The increase in base flow was about twice that of quick flow. The changes in streamflow and its components in the subsequent years were closely related to the groundwater discharge area. Most of the quick flow was generated as saturation excess overland flow from the groundwater discharge area in the valley. The expansion of the groundwater discharge area, increased soil moisture content, higher groundwater level and the presence of the shallow groundwater system for the extended periods were responsible for the process of streamflow generation.     

Water and salt balance modelling to predict the effects of land-use changes in forested catchments. 2. coupled model of water and salt balances

A long-term salt balance model is coupled with the small catchment water balance model presented in Part 1 of this series of papers. The salt balance model was designed as a simple robust, conceptually based model of the fundamental salt fluxes and stores in forested and cleared catchments. The model has four interdependent stores representing salt storage in the unsaturated zone, the deep permanent saturated groundwater system, the near-stream perched groundwater system and in a ‘salt bulge’ just above the permanent water-table. The model has performed well in simulations carried out on Salmon and Wights, two small experimental catchments in south-west Western Australia. When applied to Wights catchment the salt balance model was able to predict the stream salinities prior to clearing of native forests, and the increased salinities after the clearing.     

Hidden information within series of measurements — four examples from atmospheric science

Whether in classical networks such as meteorological networks of in more recent ones of atmospheric chemistry, a wealth of data is at hand. These data have been evaluated in a manner depending on the purpose of the network. However, much more information is hidden in these time series and waits for discovery. Only the imagination of scientists is needed. Four examples are given which lead to new information about the atmospheric aerosol and the behaviour of the atmosphere. These examples are: Atmospheric turbidity from sunshine recordings, Meteorological drainage area from the variance of observations, Location of point sources from air mass trajectories, and Total vertical ozone from turbidity measurements.