ALBERT

Description: This paper examines the conservativeness of tracers through the sediment generation process. This is done by comparing a selection of tracer properties of sediment eroded from large plots by simulated rainfall, with the corresponding properties of the source materials within the plots. Sediment was generated using three simulated rainfall events for each of five selected erosion source types in the Tarago catchment, Victoria, Australia. As there were particle size and organic content differences between the source material and the generated sediment, the measured tracer properties of the source material were corrected for these differences. The possible role of analytical errors in this investigation was also addressed. The geochemical property, concentration of Fe 2O 3, was not conservative for any of the process sources investigated. Concentration of Al 2O 3 was not conservative for three of the four process sources investigated, and the sum of molecular proportions of CaO°°, Na 2O, K 2O and Al 2O 3 was not conservative for two of the four process sources investigated. Mineral magnetic properties, IRM 850 and x were also found to be not conservative, although this may be the result of the complex relationship between particle size and mineral magnetic properties not being adequately accommodated in this analysis. The radionuclide tracers, 137Cs and 210Pb ex, were found to be conservative through the sediment generation process. Copyright © 2002 John Wiley & Sons, Ltd.

Description: It is common for runoff and erosion models to be run at coarse time steps (e.g. daily) owing to limited data availability. However, such models are unable to capture adequately the small-scale surface runoff and erosion processes that are dominated by, for example, precipitation characteristics at time-scales of minutes. This dilemma calls for the development of approaches that appropriately capture short time-scale processes when running the models at daily scales. In this paper, we assess two scaling approaches that are simple and are intended to preserve prediction quality as time scales become coarser: (i) use of effective rates of precipitation and runoff, and (ii) use of a rainfall disaggregation scheme. Runoff and erosion are simulated using a six-parameter surface runoff model and a one-parameter erosion model. A downhill simplex optimization algorithm is applied for parameterization at 2 min and daily time steps, using measured 2 min and daily precipitation data respectively, against measured daily runoff and erosion. The 2 min simulation results are aggregated to daily values to make comparisons with the measured plot-scale daily runoff and erosion data for 2 years (1997-98) from six sites in the Jhikhu Khola catchment in Nepal. The runoff and erosion simulation results calibrated from the 2 min precipitation data are good (E = 0.92 for runoff and 0.80 for erosion, all sites collectively), whereas other results, particularly the erosion simulation from the calibrated daily model, are very poor. These results are taken as the reference to compare with the two different scaling approaches. The effective rate approach is poor compared with the 2 min estimates but is a considerable improvement over daily erosion estimates. The rainfall disaggregation scheme is comparable to the 2 min estimates and produces better results than the effective rate approach. This study illustrates that rainfall scaling has significant scope for temporal scaling of hydrologic processes, particularly where continuous simulation is preferred. © 2004 John Wiley and Sons, Ltd.

Description: The topography of a catchment has a major impact on the hydrological, geomorphological, and biological processes active in the landscape. The spatial distribution of topographic attributes can often be used as an indirect measure of the spatial variability of these processes and allows them to be mapped using relatively simple techniques. Many geographic information systems are being developed that store topographic information as the primary data for analysing water resource and biological problems. Furthermore, topography can be used to develop more physically realistic structures for hydrologic and water quality models that directly account for the impact of topography on the hydrology. Digital elevation models are the primary data used in the analysis of catchment topography. We describe elevation data sources, digital elevation model structures, and the analysis of digital elevation data for hydrological, geomorphological, and biological applications. Some hydrologic models that make use of digital representations of topography are also considered. Copyright © 1991 John Wiley & Sons, Ltd

Description: Since the paper of Wood et al. (1988), the idea of a representative elementary area (REA) has captured the imagination of catchment modellers. It promises a spatial scale over which the process representations can remain simple and at which distributed catchment behaviour can be represented without the apparently undefinable complexity of local heterogeneity. This paper further investigates the REA concept and reassesses its utility for distributed parameter rainfall‐runoff modelling. The analysis follows Wood et al. (1988) in using the same topography and the same method of generating parameter values. However, a dynamic model of catchment response is used, allowing the effects of flow routing to be investigated. Also, a ‘nested catchments approach’ is adopted which better enables the detection of a minimum in variability between large‐ and small‐scale processes. This is a prerequisite of the existence of an REA. Results indicate that, for an impervious catchment and spatially invariant precipitation, the size of the REA depends on storm duration. A ‘characteristic velocity’ is defined as the ratio of a characteristic length scale (the size of the REA) to a characteristic time‐scale (storm duration). This ‘characteristic velocity’ appears to remain relatively constant for different storm durations. Spatially variable precipitation is shown to dominate when compared with the effects of infiltration and flow routing. In this instance, the size of the REA is strongly controlled by the correlation length of precipitation. For large correlation lengths of precipitation, a separation of scales in runoff is evident due to small‐scale soil and topographic variability and large‐scale precipitation patterns. In general, both the existence and the size of an REA will be specific to a particular catchment and a particular application. However, it is suggested that a separation of scales (and therefore the existence of an REA), while being an advantage, is not a prerequisite for obtaining simple representations of local heterogeneity. Copyright © 1995 John Wiley & Sons, Ltd