Neil R. Viney

Performance of conceptual rainfall-runoff models in low-yielding ephemeral catchments

Low-yielding catchments with ephemeral streams involve highly nonlinear relationships between rainfall and runoff, and there is much less documentation and appreciation of the ability to predict streamflow in these veiy difficult cases than in humid catchments. The predictions of three conceptual rainfall-runoff models are assessed in three low-yielding, emphemeral streams over a 10-year period. The models are a simple conceptual model, Generalized Surface inFiltration Baseflow (GSFB; eight parameters), a hybrid metric/conceptual model, Identification of Hydrographs and Components from Rainfall, Evaporation and Streamflow data (IHACRES; six parameters), and a complex conceptual model, the Large Scale Catchment Model (LASCAM; 22 parameters). The Salmon (0.82 km2), Stones (15 km2), and Canning (517 km2) catchments in Western Australia were selected for their range of sizes and low runoff yields (1.6–12.2% of rainfall). Their behavior is representative of a large part of Australia and semiarid regions, where antecedent conditions are critical determinants of streamflow response to rainfall. Such catchments provide a stern test of the capability of conceptual models. Five-year calibration and validation performances were assessed with a range of statistics. The models were run daily but performance was assessed on both a daily and monthly basis by aggregating daily model streamflows and observations up to monthly. The models performed well, particularly in the monthly case where often more than 90% of the variance of observed streamflow was explained in simulation on independent periods. However, while the simple conceptual model is adequate for monthly time periods, the daily simulation results indicate that a slightly more complex model (the hybrid model or the complex conceptual model) is required for daily predictions in these dry catchments. The model simulation results extend the following notion of Jakeman and Hornberger [1993] from humid to semiarid ephemeral catchments: that a model of about six parameters, albeit in an appropriate model structure, is sufficient to characterize the information in rainfall-discharge time series over a wide range of catchment sizes. Models of such modest complexity also predict runoff with good accuracy outside calibration periods, even in ephemeral, low-yielding catchments. The simulation results highlight the critical importance of the deep groundwater and antecedent moisture conditions on stream yields in ephemeral catchments and point to the desirability of accounting for these factors in arid-zone modeling.

Tests of a space-time model of daily rainfall in southwestern Australia based on nonhomogeneous random cascades

A space-time rainfall model is constructed to generate synthetic fields of space-time rainfall, with a daily time step. The model has two components: a temporal model based on a first-order, four-state discrete Markov chain which generates a daily time series of the regionally averaged rainfall and a spatial model based on a nonhomogeneous random cascade process which disaggregates the above regionally averaged rainfall amounts to produce spatial patterns of daily rainfall across the region. The cascade generators used to disaggregate the rainfall spatially are a product of stochastic and deterministic factors; the latter enable the model to capture systematic spatial gradients exhibited by measured data. The model is applied to a 400 km × 400 km region encompassing the Swan-Avon River Basin in the southwest of Western Australia. The model parameters are estimated on the basis of 11 years of observed daily rainfall data from 490 rain gauges located in the region. A detailed testing of the model is undertaken on the basis of a comparison of the statistical characteristics of the spatial and temporal variability of rainfall between the rainfall fields obtained from the rain gauge network and those generated by the simulation model. It was found that the spatial and temporal characteristics of the simulated rainfall field are in very good agreement with those of observed rainfall. In particular, the model preserves the observed systematic spatial variations of daily, monthly, and annual rainfall across the region and the observed seasonal variations. Statistical characteristics of storm durations and interstorm periods at individual gauge locations are not reproduced that well, which suggests generalizations need to be made to include space-time correlations.

Rainfall-runoff Modelling across Southeast Australia: Datasets, Models and Results

Abstract This study describes a daily rainfall, potential evaporation and streamflow data set compiled for the important water resources region of southeast Australia, and the application of six commonly used lumped conceptual rainfall-runoff models to estimate daily runoff across the region. The daily climate data set and the daily modelled runoff are available from 1895 to 2008 at 0.05° grid resolution across the region. The modelling exercise indicates that the rainfall-runoff models can generally be calibrated to reproduce the daily observed streamflow (for 232 catchments in the high runoff generation areas), and the regionalisation results indicate that the use of optimised parameter values from a gauged catchment nearby can model runoff reasonably well in the ungauged areas. There are differences between the six models, but they are relatively small when used to describe aggregated results across large regions.

A conceptual model of sediment transport: application to the Avon River Basin in Western Australia

This paper deals with the coupling of a sediment modelling algorithm to an existing conceptual model of water and salt fluxes (LASCAM). In the model, sediment generation is based on a modified version of the universal soil loss equation. The sediment transport processes of channel deposition and re-entrainment, and bed degradation are coupled with the existing stream-routing algorithm of LASCAM. The model is applied to the Avon River Basin, a large, dry catchment in Western Australia. The paper highlights some of the scientific issues involved in quantifying the sediment discharge processes in this catchment and presents validation results comparing model predictions with observations of sediment transport at different locations in the catchment. The results indicate that despite the strongly non-linear relationship between stream flow and sediment load, both are predicted well by the model. The model also highlights that in the Avon River Basin, disproportionately more stream sediment is generated in the wheat belt region than in the lower catchment. Copyright

Water and salt balance modelling to predict the effects of land-use changes in forested catchments. 3. The large catchment model

This paper presents an application of a long-term, large catchment-scale, water balance model developed to predict the effects of forest clearing in the south-west of Western Australia. The conceptual model simulates the basic daily water balance fluxes in forested catchments before and after clearing. The large catchment is divided into a number of sub-catchments (1–5 km2 in area), which are taken as the fundamental building blocks of the large catchment model. The responses of the individual subcatchments to rainfall and pan evaporation are conceptualized in terms of three inter-dependent subsurface stores A, B and F, which are considered to represent the moisture states of the subcatchments. Details of the subcatchment-scale water balance model have been presented earlier in Part 1 of this series of papers. The response of any subcatchment is a function of its local moisture state, as measured by the local values of the stores. The variations of the initial values of the stores among the subcatchments are described in the large catchment model through simple, linear equations involving a number of similarity indices representing topography, mean annual rainfall and level of forest clearing. The model is applied to the Conjurunup catchment, a medium-sized (39·6 km2) catchment in the south-west of Western Australia. The catchment has been heterogeneously (in space and time) cleared for bauxite mining and subsequently rehabilitated. For this application, the catchment is divided into 11 subcatchments. The model parameters are estimated by calibration, by comparing observed and predicted runoff values, over a 18 year period, for the large catchment and two of the subcatchments. Excellent fits are obtained.

Characterizing and Modeling Temporal and Spatial Trends in Rainfall Extremes

Abstract A hierarchical spatial model for daily rainfall extremes that characterizes their temporal variation due to interannual climatic forcing as well as their spatial pattern is proposed. The model treats the parameters of at-site probability distributions for rainfall extremes as “data” that are likely to be spatially correlated and driven by atmospheric forcing. The method is applied to daily rainfall extremes for summer and winter half years over the Swan–Avon River basin in Western Australia. Two techniques for the characterization of at-site extremes—peaks-over-threshold (POT) analysis and the generalized extreme value (GEV) distribution—and three climatic drivers—the El Nino–Southern Oscillation as measured by the Southern Oscillation index (SOI), the Southern Hemisphere annular mode as measured by an Antarctic Oscillation index (AOI), and solar irradiance (SI)—were considered. The POT analysis of at-site extremes revealed that at-site thresholds lacked spatial coherence, making it difficult to ...

Dominant physical controls on hourly flow predictions and the role of spatial variability: Mahurangi catchment, New Zealand

We present a systematic approach to the development of models for making hourly flow predictions, where each step provides insight into the relative importance of catchment and climatic properties (rainfall, soil properties, vegetation, topography), their spatial variability, and their influence on temporal flow variability at the outlet. Our modelling approach uses a simple conceptual model design requiring minimal calibration and physically meaningful input parameters estimated a priori from landscape data or from analyses of streamflow recession curves. The model structure allows direct measurement of parameter uncertainty and the ability to investigate its propagation through the model to produce bounds of predictive uncertainty via the Monte Carlo method. This method was applied to a number of model designs to assess the tradeoff between model complexity, accuracy and predictive uncertainty, and to identify the most appropriate model design under specific climatic conditions. With the preferred model, sensitivity analysis was used to identify the dominant controls on streamflow variability at the hourly timescale. In summary the aim is not to present a distributed model of universal applicability, but to generate insights into the climate, soil and vegetation controls on streamflow variability at the hourly timescale, and on predictive accuracy and uncertainty, that can be used in future modelling efforts.

Conceptual Rainfall–Runoff Model Performance with Different Spatial Rainfall Inputs

AbstractDifferent methods have been used to obtain the daily rainfall time series required to drive conceptual rainfall–runoff models, depending on data availability, time constraints, and modeling objectives. This paper investigates the implications of different rainfall inputs on the calibration and simulation of 4 rainfall–runoff models using data from 240 catchments across southeast Australia. The first modeling experiment compares results from using a single lumped daily rainfall series for each catchment obtained from three methods: single rainfall station, Thiessen average, and average of interpolated rainfall surface. The results indicate considerable improvements in the modeled daily runoff and mean annual runoff in the model calibration and model simulation over an independent test period with better spatial representation of rainfall. The second experiment compares modeling using a single lumped daily rainfall series and modeling in all grid cells within a catchment using different rainfall inp...

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.

Assessment of existing fine fuel moisture models applied to Eucalyptus litter

Summary The diurnal trends in dead leaf, bark and twig moisture content on the floor of a Eucalyptus forest in south-eastern Australia were examined over a four day period. Substantial differences were found in moisture content among the three fuel types. At night the twigs remained the driest, while the leaves, through condensation of dew and distillation of moisture from the soil, were the wettest. During the daytime the leaves and bark reached lower fuel moisture minima than the twigs. The adequacy of several fine fuel moisture models was assessed. The locally developed Grassland Fire Danger Meter and Forest Fire Danger Meter together with the Fire Behaviour Officers model were the best predictors of diurnal fuel moisture content. The usefulness of the timelag concept in Eucalyptus fuels is discussed and the results of the study suggest that modifications should be made to the Fine Fuel Moisture Code and the BEHAVE model for application to these fuels. The need to incorporate a parameterisation of noc...