Technical Note: Real-time updating procedure for flood forecasting with conceptual HBV-type models

International audienceFlood forecasting is of increasing importance as it comes to an increasing variability in global and local climates. But rainfall-runoff models are far from being perfect. In order to achieve a better prediction for emerging flood events, the model outputs have to be continuously updated. This contribution introduces a rather simple, yet effective updating procedure for the conceptual distributed rainfall-runoff model PREVAH, whose runoff generation module relies on similar algorithms as the HBV-Model. The current conditions of the system, i.e. the contents of the upper soil reservoirs, are updated by the proposed method. The testing of the updating procedure on data from two mountainous catchments in Switzerland reveals a significant increase in prediction accuracy with regards to peak flow

Multiobjective inverse parameter estimation for modelling vadose zone water movement

Inverse modelling techniques for estimating unsaturated soil hydraulic parameters have become increasingly common in the past two decades. In contrast to single-objective parameter estimation which yields a single set of "best fit" parameters, multiobjective parameter estimation results in a number of Pareto optimal solutions which allow the analysis of the trade-off between different, sometimes conflicting, model objectives. In this study, modelling tools for identification of Pareto optimal sets of vadose zone water transport parameters are presented utilizing the numerical water and solute transport model HYDRUS-1D. Root-mean-square error (RMSE) values are calculated to measure the fit of the simulated and observed pressure head data at three different depths at a vadose zone of volcanic origin in New Zealand

Technical Note: Updating procedure for flood forecasting with conceptual HBV-type models

International audienceFlood forecasting is of increasing importance as it comes to an increasing variability in global and local climates. But rainfall-runoff models are far from being perfect. In order to achieve a better prediction for emerging flood events, the model outputs have to be continuously updated. This contribution introduces a rather simple, yet effective updating procedure for the conceptual semi-distributed rainfall-runoff model PREVAH, whose runoff generation module relies on similar algorithms as the HBV-Model. The current conditions of the system, i.e. the contents of the upper soil reservoirs, are updated by the proposed method. The testing of the updating procedure on data from two mountainous catchments in Switzerland reveals a significant increase in prediction accuracy with regards to peak flow

Automated equilibrium tension lysimeters for measuring water fluxes through a layered, volcanic vadose profile in New Zealand

In this technical note we present the design, installation, and evaluation of a field monitoring system to directly measure water fluxes through a vadose zone. The system is based on use of relatively new measurement technology-automated equilibrium tension lysimeters (AETLs). An AETL uses a porous sintered stainless-steel plate to provide a comparatively large sampling area (0.20 m(2)) with a continuously controlled vacuum applied under the plate. This vacuum is in "equilibrium" with the surrounding vadose zone tension to ensure measured fluxes represent those under undisturbed conditions. Fifteen of these AETLs have been installed at five depths through a layered volcanic vadose zone to study the impact of land use changes on water quality in Lake Taupo, New Zealand. We describe the development and testing of the AETLs, the methods used for installing these devices, a condensed data set of the measured physical properties of the vadose zone, and the initial results from the in situ operation of the AETLs, including the preliminary results from a bromide tracer test. For an AETL installed at the 0.4-m depth, where soil pressure heads are most dynamic, the average deviation between the target reference pressure head, as measured in the undisturbed vadose zone and the pressure head measured above the sampling plate was only 5.4 hPa over a 180-d period. The bromide recovered in an AETL at the same depth was equivalent to 96% of the bromide pulse applied onto the surface area directly above the AETL. We conclude that this measurement technique provides an accurate and robust method of measuring vadose zone fluxes. These measurements can ultimately contribute to better understanding of the water transport and contaminant transformation processes through vadose zones

Does vadose zone flow forecasting depend on the type of calibration data?

Unsaturated subsurface water flow is often described by a flow model which is calibrated on either observed soil water content or tensiometric pressure head measurements. For a given model structure the calibration on one data type may lead to significant errors in predictions of the other data type. These errors are difficult to quantify since simultaneous measurements of pressure head and water content are generally not available. Independent vadose zone data of both types were recorded at an intensively investigated experimental field site in the Lake Taupo catchment, New Zealand. A numerical flow model was set up and calibrated (i) using tensiometric pressure head observations, (ii) using soil water content (TDR) observations, and (iii) using both tensiometric and TDR data. The global multi-method search algorithm AMALGAM was used to estimate five soil hydraulic parameters in five model layers, totaling 25 optimized parameters. In the cases (i) and (ii), a single aggregated objective function was defined to fit measurements from four different depths in the vadose zone profile. The third model calibration was placed in a multi-objective context to include the two different data types simultaneously. The trade-off pattern between the fit to the water content and pressure head observations was investigated. Parameter sets from the three calibrations were then used for predicting pressure heads and water content in the vadose zone for independent data, not previously used in the calibration process. The results suggest that predictions of tensiometric pressure head and volumetric water content significantly depend on the type of data used for model calibration. Large differences in the model predictions occur when calibrating to one data type and predicting the other. This demonstrates the need to inform the model about the required prediction data type in the calibration process. This is a prerequisite to make reliable forecasts of vadose zone water flow and to determine realistic uncertainty bounds in vadose zone flow modeling

Optimisation of monitoring data for increased predictive reliability of regional water allocation models

This paper discusses the optimization of monitoring data for the increased reliability of regional groundwater models and the predictions that depend on them. The significant costs of commissioning and maintaining groundwater monitoring networks are such that there is great benefit in being able to assess where data gathering has the greatest impact on improving predictive reliability. This optimization assessment can be made on the basis of existing networks or prior to any data acquisition efforts. Various data acquisition strategies, for quite disparate data types, can be compared in terms of their ability to increase the reliability of model based predictions; data collection strategies which provide the greatest return for investment can then be selected for implementation. Similarly the relative merits of making measurements at different locations and times can be assessed. Using the Lockyer Valley ground water model (RPS 2010) we demonstrate how predictive uncertainty analysis can provide a powerful foundation for optimizing both existing monitoring networks and future data acquisition strategies to support model based environmental management. Such analyses are efficient yet robust. The particular characterization of model predictive variance in the problem formulation employed (Moore and Doherty, 2005), ensures that the contributions to predictive uncertainty by both measurement errors and environmental heterogeneity that cannot be captured by the calibration process is accounted for in the analysis. Efficiency is gained via a linearity assumption in the equation used in the analysis, which allows the calculation to be made sufficiently rapidly, so that it can be repeated at many alternative existing or proposed monitoring sites and times. Furthermore, this analysis has no cost barriers, as the software for such analyses is in the public domain (Doherty, 2011a and b). These are particularly important benefits in the large scale regional model context, where monitoring is typically a significant effort and is subject to public scrutiny in terms of both cost and rigour

Fate of urine nitrogen through a volcanic vadose zone

© CSIRO 2014. To investigate the fate of nitrogen (N) from urine, dairy cow urine was amended with bromide (Br) and chloride (Cl), and applied onto a loamy sand soil with an underlying vadose zone of gritty coarse sands and pumice fragments with groundwater at ∼5.5m depth. Textural changes and hydrophobicity resulted in heterogeneous flow and high variability in the Cl, Br and N masses captured. Three forms of N derived from the urine, organic-N (org-N), ammonium-N (NH4-N) and nitrate-N (NO3-N), were measured at 0.4m depth. At 1.0m depth, effectively all measured N was NO3-N. At 4.2m, the mass of recovered N (average 33% of applied N, s.d. 21%), although solely speciated as NO3-N, was not significantly different from that at 0.4m (average 24.5% of applied N, s.d. 0.1%), suggesting that no substantial assimilation of NO3-N had occurred in this vadose zone. Below the interface of the Taupo Ignimbrite and the Palaeosol at 4.2m depth, recoveries of the Cl and Br tracers were negligible. In addition, the isotopic signatures (δ18O and δ15N) of the nitrate were different and the NO3-N concentrations were higher than in the upper vadose zone. These results all suggest that the Palaeosol was acting as a hydraulically limiting layer resulting in lateral unsaturated flow occurring at this interface. The fact that no nitrate assimilation was observed in this field study, despite previous laboratory studies showing substantial assimilative capacity, underlines that that the nitrate assimilative capacity in the vadose zone is a function of both hydrological and biogeochemical factors

Dual-domain mixing cell modelling and uncertainty analysis for unsaturated bromide and chloride transport

Land use intensification is considered the main reason for early signs of deterioration in the water quality of Lake Taupo, New Zealand. Little is understood, however about the origin, and governing flow paths of the contaminants and their respective transformation processes that affect the water quality of Lake Taupo. In this study we investigate contaminant transport and its small-scale variability in the volcanic vadose zone surrounding the Lake. Lateral and preferential solute transport is analysed to better understand the risks of diffuse groundwater pollution from contaminant sources at the land surface. As part of the investigations into this problem the Spydia experimental facility has been installed under a pastoral agriculture land use in the Lake Taupo region, New Zealand (Barkle et al. 2011). A multiple tracer experiment was conducted at the site and vadose zone drainage volumes were measured using Automated Equilibrium Tension Plate Lysimeters (Figure 1). The chemical composition of the drainage samples was analysed in the laboratory. A dual-domain mixing cell model was set up to simulate the unsaturated advective-dispersive tracer transport at selected monitoring sites for two different bromide-chloride (Br⁻, Cl⁻) tracers that were applied at the land surface at two different regions (Figure 1). Some model parameters were constrained by mixing calculations of the measured total Br⁻ and Cl⁻ load, whereas others were calibrated using the measured Br⁻ and Cl⁻ breakthrough curves and drainage volumes. Multi-objective inverse modelling using the AMALGAM evolutionary search method (Vrugt & Robinson, 2007) showed a significant trade-off between simulated transient Br⁻ and Cl⁻ breakthrough curves and corresponding drainage volumes, but also a compromise solution that fits both objective functions reasonably well. Estimates of parameter and model predictive uncertainty were subsequently derived using the differential evolution adaptive metropolis, DREAMZS adaptive Markov chain Monte Carlo algorithm (Vrugt et al., 2011) with a formal Bayesian likelihood function (Wöhling & Vrugt, 2011). Uncertainty bounds derived by this MCMC method simultaneously capture the observed Br⁻ and Cl⁻ breakthrough curves and corresponding drainage volumes. Our results demonstrate that (1) flow and transport in the vadose zone is highly variable, and (2) contaminants at the land surface can travel rapidly through the soil to larger depths and this cannot be described with the classical advection-dispersion equation