bluuebunny/crossref_metadata_2025 huggingface dataset:
Gauging the ungauged basin: how many discharge measurements are needed?
Authors: Seibert J. , Beven K.
Published: April 2010

Abstract. Runoff estimation in ungauged catchments is probably one of the most basic and oldest tasks of hydrologists. This long-standing issue has received increased attention recently due to the PUB (Prediction in Ungauged Basins) initiative. Given the challenges of predicting runoff for ungauged catchments one might argue that the best course of action is to take a few runoff measurements. In this study we explored how implementing such a procedure might support predictions in an ungauged basin. We used a number of monitored Swedish catchments as hypothetical ungauged basins where we pretended to start with no runoff data and then added different sub-sets of the available data to constrain a simple catchment model. These sub-sets consisted of a limited number of single runoff measurements; in other words these data represent what could be measured with limited efforts in an ungauged basin. We used a Monte Carlo approach and predicted runoff as a weighted ensemble mean of simulations using acceptable parameter sets. We found that the ensemble prediction clearly outperformed the predictions using single parameter sets and that surprisingly little runoff data was necessary to identify model parameterizations that provided good results for the "ungauged" test periods. These results indicated that a few runoff measurements can contain much of the information content of continuous runoff time series. However, results also indicated that results may differ significantly between catchments and also depend on the days chosen for taking the measurements.
A Bayesian approach to estimate sensible and latent heat over vegetation
Authors: van der Tol C. , van der Tol S. , Verhoef A. , Su B. , Timmermans J. , Houldcroft C. , Gieske A.
Published: April 2010

Abstract. Sensible and latent heat fluxes are often calculated from bulk transfer equations combined with the energy balance. For spatial estimates of these fluxes, a combination of remotely sensed and standard meteorological data from weather stations on the ground is used. The success of this approach depends on the accuracy of the input data and on the accuracy of two variables in particular: aerodynamic and surface conductance. This paper presents a Bayesian approach to improve estimates of sensible and latent heat fluxes by using a priori estimates of aerodynamic and surface conductance alongside remote measurements of surface temperature. The method is validated for time series of half-hourly measurements in a fully grown maize field, a vineyard and a forest. It is shown that the Bayesian approach yields more accurate estimates of sensible and latent heat flux than traditional methods.
Multi-objective calibration of a distributed hydrological model (WetSpa) using a genetic algorithm
Authors: Shafii M. , De Smedt F.
Published: April 2010

Abstract. A multi-objective genetic algorithm, NSGA-II, is applied to calibrate a distributed hydrological model (WetSpa) for predicting river discharge. The evaluation criteria considered are the model bias (mass balance), the model efficiency (Nash-Sutcliffe efficiency), and a logarithmic transformed model efficiency (to emphasize low-flow values). The concept of Pareto dominance is used to solve the multi-objective optimization problem and derive Pareto-optimal parameter sets. In order to analyze the applicability of the approach, a comparison is made with another calibration routine using the parameter estimator PEST to minimize the model efficiency. The two approaches are evaluated by applying the WetSpa model to the Hornad River (Slovakia) for which observations of daily precipitation, temperature, potential evapotranspiration, and discharge are available for a 10 year period (1991–2000). The first 5 years of the data series are used for model calibration, while the second 5 years for model validation. The results revealed that the quality of the solutions obtained with NSGA-II is comparable or even better to what can be obtained with PEST, considering the same assumptions. Hence, NSGA-II is capable of locating Pareto optimal solutions in the parameter search space and the results obtained prove the excellent performance of the multi-objective model calibration methodology.
Hydrological model performance and parameter estimation in the wavelet-domain
Authors: Schaefli B. , Zehe E.
Published: April 2010

Abstract. This paper proposes a method for rainfall-runoff model calibration and performance analysis in the wavelet-domain by fitting the estimated wavelet-power spectrum (a representation of the time-varying frequency content of a time series) of a simulated discharge series to the one of the corresponding observed time series. As discussed in this paper, calibrating hydrological models so as to reproduce the time-varying frequency content of the observed signal can lead to different results than parameter estimation in the time-domain. Therefore, wavelet-domain parameter estimation has the potential to give new insights into model performance and to reveal model structural deficiencies. We apply the proposed method to synthetic case studies and a real-world discharge modeling case study and discuss how model diagnosis can benefit from an analysis in the wavelet-domain. The results show that for the real-world case study of precipitation – runoff modeling for a high alpine catchment, the calibrated discharge simulation captures the dynamics of the observed time series better than the results obtained through calibration in the time-domain. In addition, the wavelet-domain performance assessment of this case study highlights which frequencies are not well reproduced by the model, which gives specific indications about how to improve the model structure.
On the benefit of high-resolution climate simulations in impact studies of hydrological extremes
Authors: Dankers R. , Feyen L. , Christensen O. B.
Published: April 2010

Abstract. We investigated the effect of changing the horizontal resolution of a regional climate model (RCM) on the simulation of hydrological extremes. We employed the results of three experiments of the RCM HIRHAM using a grid size of approximately 12, 25 and 50 km. These simulations were used to drive the hydrological model LISFLOOD, developed for flood forecasting at European scale. The discharge simulations of LISFLOOD were compared with statistics of observed river runoff at 209 gauging stations across Europe. The largest discrepancies in peak flow occurred in climates with a seasonal snow cover, which may be explained by inaccuracies in the simulated precipitation that accumulate over winter. Although previous studies have found that high resolution climate simulations result in more realistic patterns of extreme precipitation, especially in mountainous regions, we did not find conclusive evidence that the 12-km HIRHAM run generally yields a better simulation of peak discharges. At some gauging stations the model performance is increasing with increasing horizontal resolution of the RCM, while at other stations it is decreasing. However, the differences between the three experiments become less important in larger river basins. Above about 30 000 km2 and 120 000 km2, respectively, the 25- and 50-km runs generally provided a good approximation of the simulations based on the 12-km climatology. Under the A2 scenario of climate change, the changes in extreme discharge levels were similar between the three experiments at continental scale. At the scale of individual river basins, however, there were occasionally important differences. If we assume the 12-km HIRHAM simulation to be more realistic, the use of lower-resolution climate simulations may lead to an underestimation of future flood hazard. This means that results obtained with lower-resolution RCM simulations should be interpreted with care, as the grid scale of the climate model adds to the uncertainty.
The significance and lag-time of deep throughflow: an example from a small, ephemeral catchment with contrasting soil types in the Adelaide Hills, South Australia
Authors: Bestland E. , Milgate S. , Chittleborough D. , VanLeeuwen J. , Pichler M. , Soloninka L.
Published: April 2010

Abstract. The importance of deep throughflow in a small (3.4 km2) ephemeral catchment in the Adelaide Hills of South Australia was investigated by detailed hydrochemical analysis of soil water and stream flow during autumn and early winter rains. In this Mediterranean climate with strong summer moisture deficits, several significant rainfalls are required to generate soil throughflow and stream flow (in ephemeral streams). During Autumn 2007, a large (127 mm) drought-breaking rain occurred in April followed by significant May rains; most of this precipitation occurred prior to the initiation of stream flow in late May. These early events, especially the 127 mm event, had low (depleted) stable water isotope values compared with both later rains and average winter precipitation. Thus, this large depleted early rain event provided an excellent natural tracer. During the June and July rainfall events, daily stream and soil water samples were collected and analysed. Results from major and trace elements, water isotopes (δ18O, δD), and dissolved organic carbon analysis clearly demonstrate that a large component of this early April and May rain was stored and later pushed out of deep soil or regolith zones. This pre-event water was identified in the stream as well as identified in deeper soil horizons due to its different isotopic signature which contrasted sharply with the June–July event water. Based on this data, the regolith and throughflow system for this catchment has been re-thought. The catchment area consists of about half sandy and half clayey soils. Regolith flow is now thought to be dominated by the sandy soil system not the clayey soil system. The clayey duplex soils had rapid response to rain events and saturation excess overland flow. The sandy soils had delayed soil throughflow and infiltration excess overland flow. A pulse of macropore throughflow was observed in the sandy soils three days after the rainfall event largely ended. The macropore water was a mixture of pre-event and event water, demonstrating the lag-time and mixing of the water masses in the sandy soil system. By contrast, the clayey soil horizons were dominated by pre-event water to a much lesser degree, demonstrating the quicker response and shallow flow through of the clayey soil system. Thus, the sandy terrain has a greater vadose zone storage and greater lag time of through-flow than the clayey terrain.
Regional frequency analysis of heavy precipitation in the Czech Republic by improved region-of-influence method
Authors: Gaál L. , Kyselý J.
Published: April 2010

Abstract. The region-of-influence (ROI) approach is implemented for modelling the probabilities of heavy 1-day and 5-day precipitation amounts in the Czech Republic. Compared to the previous study for Slovakia (Gaál et al., 2008), an improved ROI methodology that makes use of a regional homogeneity test for assigning sites to pooling groups, is applied. Two basically different approaches to forming the pooling groups in which groups of sites are gradually built up and cut down, respectively, are combined. A sensitivity analysis, which examines the performance of the ROI models after changing the set of input site attributes that serve for determining the sites' similarity, is carried out. Finally, several frequency models that include the ROI pooling schemes, a conventional model based on fixed regions, and an at-site analysis are compared by means of Monte Carlo simulations. We conclude that, regardless of the duration of precipitation, the ROI pooling scheme based on the actual proximity of sites (latitude and longitude) outperforms the other frequency models in terms of the root mean square error of the growth curves.
Assessment of soil moisture fields from imperfect climate models with uncertain satellite observations
Authors: Schumann G. , Lunt D. J. , Valdes P. J. , de Jeu R. A. M. , Scipal K. , Bates P. D.
Published: April 2010

Abstract. We demonstrate that global satellite products can be used to evaluate climate model soil moisture predictions but conclusions should be drawn with care. The quality of a limited area climate model (LAM) was compared to a general circulation model (GCM) using soil moisture data from two different Earth observing satellites within a model validation scheme that copes with the presence of uncertain data. Results showed that in the face of imperfect models and data, it is difficult to investigate the quality of current land surface schemes in simulating hydrology accurately. Nevertheless, a LAM provides, in general, a better representation of spatial patterns and dynamics of soil moisture. However, in months when data uncertainty is higher, particularly in colder months and in periods when vegetation cover and soil moisture are out of phase (e.g. August in the case of Western Europe), it is not possible to draw firm conclusions about model acceptability. Our work indicates that a higher resolution LAM has more benefits to soil moisture prediction than are due to the resolution alone and can be attributed to an overall intensification of the hydrological cycle relative to the GCM.
Hierarchy theory in hydropedology
Authors: Allen T. F. H. , Allen P. C. , Wixon D. L.
Published: April 2010

Abstract. The challenges that face scientists in the bourgeoning field of hydropedology include many of those that face investigations in complex systems. We suggest hierarchy theory as being particularly helpful in teasing through complexity in hydropedological investigations. We present a brief overview of hierarchy theory highlighting the importance of defining levels of analysis, the role of theory in prediction, and the importance of narrative in science. These concepts are highlighted by references from the hydropological literature. We point out several issues common to scientists faced with complex systems analysis, and suggest several strategies to help hydropedologists deal with them. In order to help bridge the gap between theory and application, we present several specific examples of how hierarchical treatments have helped scientists deal with the modeling and analysis of complex systems related to hydropedology. We conclude that hierarchy theory offers many powerful tools with which to tackle the complexity inherent in soil water interactions, and that its use would benefit a more systematic and robust integration of the hydrologic and soil sciences.
High-resolution satellite-based cloud-coupled estimates of total downwelling surface radiation for hydrologic modelling applications
Authors: Forman B. A. , Margulis S. A.
Published: April 2010

Abstract. A relatively simple satellite-based radiation model yielding high-resolution (in space and time) downwelling longwave and shortwave radiative fluxes at the Earth's surface is presented. The primary aim of the approach is to provide a basis for deriving physically consistent forcing fields for distributed hydrologic models using satellite-based remote sensing data. The physically-based downwelling radiation model utilises satellite inputs from both geostationary and polar-orbiting platforms and requires only satellite-based inputs except that of a climatological lookup table derived from a regional climate model. Comparison against ground-based measurements over a 14-month simulation period in the Southern Great Plains of the United States demonstrates the ability to reproduce radiative fluxes at 4 km/h resolution with good accuracy during all-sky conditions. For hourly fluxes, a mean difference of −2 W m−2 with a root mean square difference of 21 W m−2 was found for the longwave fluxes whereas a mean difference of −7 W m−2 with a root mean square difference of 29 W m−2 was found for the shortwave fluxes. Additionally, comparison against advanced downwelling longwave and solar insolation products during all-sky conditions showed comparable uncertainty in the longwave estimates and reduced uncertainty in the shortwave estimates. The relatively simple form of the model enables future usage in ensemble-based applications including data assimilation frameworks in order to explicitly account for input uncertainties while providing the potential for conditioning estimates from other readily available products derived from more sophisticated retrieval algorithms.
Numerical simulations of the impact of climate variability and change on semiarid watershed response in central New Mexico
Authors: Vivoni E. R. , Aragón C. A. , Malczynski L. , Tidwell V. C.
Published: April 2010

Abstract. Hydrologic processes in the semiarid regions of the southwest United States are considered to be highly susceptible to variations in temperature and precipitation characteristics due to the effects of climate change. Relatively little is known on the potential impacts of climate change on the basin hydrologic response, namely streamflow, evapotranspiration and recharge, in the region. In this study, we present the development and application of a continuous, semi-distributed watershed model for climate change studies in semiarid basins of the southwest US. Our objective is to capture hydrologic processes in large watersheds, while accounting for the spatial and temporal variations of climate forcing and basin properties in a simple fashion. We apply the model to the Río Salado basin in central New Mexico since it exhibits both a winter and summer precipitation regime and has a historical streamflow record for model testing purposes. Subsequently, we utilize a sequence of climate change scenarios that capture observed trends for winter and summer precipitation, as well as their interaction with higher temperatures, to perform long-term ensemble simulations of the basin hydrologic response. Results of the modeling exercise indicate that precipitation uncertainty is amplified in the hydrologic response, in particular for processes that depend on a soil saturation threshold. We obtained substantially different hydrologic sensitivities for winter and summer precipitation ensembles, indicating a greater sensitivity to more intense summer storms as compared to more frequent winter events. In addition, the impact of changes in precipitation characteristics overwhelmed the effects of increased temperature in the study basin. Nevertheless, combined trends in precipitation and temperature yield a more sensitive hydrologic response throughout the year.
Examining the effect of pore size distribution and shape on flow through unsaturated peat using 3-D computed tomography
Authors: Rezanezhad F. , Quinton W. L. , Price J. S. , Elrick D. , Elliot T. R. , Heck R. J.
Published: April 2010

Abstract. The hydraulic conductivity of unsaturated peat soils is controlled by the peat structure which affects the air-filled porosity, pore size distribution and shape. This study investigates how the size and shape of pores affects the flow of water through peat soils. In this study we used X-ray Computed Tomography (CT), at 45 µm resolution under 5 specific soil-water pressure head levels to provide 3-D, high-resolution images that were used to detect the inner pore structure of peat samples under a changing water regime. Pore structure and configuration were found to be irregular, which affected the rate of water transmission through peat soils. The 3-D analysis suggested that pore distribution is dominated by a single large pore-space. At low pressure head, this single large air-filled pore imparted a more effective flowpath compared to smaller pores. Smaller pores were disconnected and the flowpath was more tortuous than in the single large air-filled pore, and their contribution to flow was negligible when the single large pore was active. We quantify the pore structure of peat soil that affects the hydraulic conductivity in the unsaturated condition, and demonstrate the validity of our estimation of peat unsaturated hydraulic conductivity by making a comparison with a standard permeameter-based method. Estimates of unsaturated hydraulic conductivities were made for the purpose of testing the sensitivity of pore shape and geometry parameters on the hydraulic properties of peats and how to evaluate the structure of the peat and its affects on parameterization. We also studied the ability to quantify these factors for different soil moisture contents in order to define how the factors controlling the shape coefficient vary with changes in soil water pressure head. The relation between measured and estimated unsaturated hydraulic conductivity at various heads shows that rapid initial drainage, that changes the air-filled pore properties, creates a sharp decline in hydraulic conductivity. This is because the large pores readily lose water, the peat rapidly becomes less conductive and the flow path among pores, more tortuous.
Local and regional impact of anthropogenic drainage on fen contiguity
Authors: van Loon A. H. , Schot P. P. , Bierkens M. , Griffioen J. , Wassen M. J.
Published: April 2010

Abstract. Knowledge of the hydrological mechanisms behind habitat fragmentation of fen plant communities in intensively managed regions like The Netherlands is essential to improve currently utilized fen restoration and conservation strategies. In this study, we analysed the local and regional impact of anthropogenic drainage on the groundwater supply of fens. For this purpose, we developed fine-scale groundwater flow models and collected empirical data to analyse (1) the differences in groundwater supply between an anthropogenically drained fen and a poorly drained fen in The Netherlands, and (2) the local and regional effects of the elimination of drainage ditches on the groundwater supply of fens. Our results consistently indicated the presence of recently infiltrated precipitation on top of upwelling groundwater across the anthropogenically drained fen, and a mixing gradient of recently infiltrated precipitation and upwelling groundwater across the poorly drained fen. Furthermore, our results showed that the elimination of drainage ditches from the anthropogenically drained fen increased the area and the flux of groundwater supply of both the anthropogenically drained fen and the poorly drained fen. We conclude that anthropogenic drainage not only causes a lowering of groundwater tables, but also (1) enhances the infiltration of local precipitation across fens while simultaneously preventing upwelling groundwater from entering the fen root zone, and (2) reduces the groundwater supply of adjacent fens by intercepting groundwater that is potentially directed to downstream regions. These insights support the need to reconsider the current priorities in hydrological fen restoration strategies.
HESS Opinions "Classification of hydrological models for flood management"
Authors: Plate E. J.
Published: April 2010

Abstract. Hydrological models for flood management are components of flood risk management, which is the set of actions to be taken to prevent flood disasters. It is a cyclical process: initiated by occurrence of an extreme flood it leads through the reconstruction and rehabilitation phase to risk assessment and project planning and implementation, and finally to operation and preparedness for a next extreme flood when the cycle starts again. We subdivide the tasks of flood management into two consecutive parts: planning and operation, which basically require different kinds of hydrological models. For planning, real time runoff is not needed, one works with design scenarios. For this task models should be used appropriate to the tasks at hand, which reflect characteristics of landscape as well as of hydrological scale. For operation, hydrological forecast models are needed which have to meet a different set of conditions. In this paper, requirements for hydrological models as functions of application, geology and topography, and of area size are surveyed. It is suggested that we always should start flood modeling with an analysis of local conditions and select or develop task and locality specific models.
Water availability, water demand, and reliability of in situ water harvesting in smallholder rain-fed agriculture in the Thukela River Basin, South Africa
Authors: Andersson J. C. M. , Zehnder A. J. B. , Jewitt G. P. W. , Yang H.
Published: April 2010

Abstract. Water productivity in smallholder rain-fed agriculture is of key interest for food and livelihood security. A frequently advocated approach to enhance water productivity is to adopt water harvesting and conservation technologies (WH). This study estimates water availability for in situ WH and supplemental water demands (SWD) in smallholder agriculture in the Thukela River Basin, South Africa. It incorporates process dynamics governing runoff generation and crop water demands, an explicit account of the reliability of in situ WH, and uncertainty considerations. The agro-hydrological model SWAT (Soil and Water Assessment Tool) was calibrated and evaluated with the SUFI-2 algorithm against observed crop yield and discharge in the basin. The water availability was based on the generated surface runoff in smallholder areas. The SWD was derived from a scenario where crop water deficits were met from an unlimited external water source. The reliability was calculated as the percentage of years in which the water availability ≥ the SWD. It reflects the risks of failure induced by the temporal variability in these factors. The results show that the smallholder crop water productivity is low in the basin (spatiotemporal median: 0.08–0.22 kg m−3, 95% prediction uncertainty band (95PPU). Water is available for in situ WH (spatiotemporal median: 0–17 mm year−1, 95PPU) which may aid in enhancing the crop water productivity by meeting some of the SWD (spatiotemporal median: 0–113 mm year−1, 95PPU). However, the reliability of in situ WH is highly location specific and overall rather low. Of the 1850 km2 of smallholder lands, 20–28% display a reliability ≥25%, 13–16% a reliability ≥50%, and 4–5% a reliability ≥75% (95PPU). This suggests that the risk of failure of in situ WH is relatively high in many areas of the basin.
Soil moisture active and passive microwave products: intercomparison and evaluation over a Sahelian site
Authors: Gruhier C. , de Rosnay P. , Hasenauer S. , Holmes T. , de Jeu R. , Kerr Y. , Mougin E. , Njoku E. , Timouk F. , Wagner W. , Zribi M.
Published: April 2010

Abstract. This paper presents a comparison and an evaluation of five soil moisture products based on satellite-based passive and active microwave measurements. Products are evaluated for 2005–2006 against ground measurements obtained from the soil moisture network deployed in Mali (Sahel) in the framework of the African Monsoon Multidisciplinary Analysis project. It is shown that the accuracy of the soil moisture products is sensitive to the retrieval approach as well as to the sensor type (active or passive) and to the signal frequency (from 5.6 GHz to 18.8 GHz). The spatial patterns of surface soil moisture are compared between the different products at meso-scale (14.5° N–17.5° N and 2° W–1° W). A general good consistency between the different satellite soil moisture products is shown in terms of meso-scale spatial distribution, in particular after convective rainfall occurrences. Soil moisture values provided by the different products are compared to ground measurements time series. Although soil moisture products obtained from satellite generally over-estimate soil moisture values during the dry season, most of them capture soil moisture temporal variations in good agreement with ground station measurements.
Performance of high-resolution X-band radar for rainfall measurement in The Netherlands
Authors: van de Beek C. Z. , Leijnse H. , Stricker J. N. M. , Uijlenhoet R. , Russchenberg H. W. J.
Published: April 2010

Abstract. This study presents an analysis of 195 rainfall events gathered with the X-band weather radar SOLIDAR and a tipping bucket rain gauge network near Delft, The Netherlands, between May 1993 and April 1994. The high spatial (120 m) and temporal (16 s) resolution of the radar combined with the extent of the database make this study a climatological analysis of the potential for high-resolution rainfall measurement with non-polarimetric X-band radar over completely flat terrain. An appropriate radar reflectivity – rain rate relation is derived from measurements of raindrop size distributions and compared with radar – rain gauge data. The radar calibration is assessed using a long-term comparison of rain gauge measurements with corresponding radar reflectivities as well as by analyzing the evolution of the stability of ground clutter areas over time. Three different methods for ground clutter correction as well as the effectiveness of forward and backward attenuation correction algorithms have been studied. Five individual rainfall events are discussed in detail to illustrate the strengths and weaknesses of high-resolution X-band radar and the effectiveness of the presented correction methods. X-band radar is found to be able to measure the space-time variation of rainfall at high resolution, far greater than can be achieved by rain gauge networks or a typical operational C-band weather radar. On the other hand, SOLIDAR can suffer from receiver saturation, wet radome attenuation as well as signal loss along the path. During very strong convective situations the signal can even be lost completely. In combination with several rain gauges for quality control, high resolution X-band radar is considered to be suitable for rainfall monitoring over relatively small (urban) catchments. These results offer great prospects for the new high resolution polarimetric doppler X-band radar IDRA.
Seasonal evaluation of the land surface scheme HTESSEL against remote sensing derived energy fluxes of the Transdanubian region in Hungary
Authors: Wipfler E. L. , Metselaar K. , van Dam J. C. , Feddes R. A. , van Meijgaard E. , van Ulft L. H. , van den Hurk B. , Zwart S. J. , Bastiaanssen W. G. M.
Published: April 2010

Abstract. The skill of the land surface model HTESSEL is assessed to reproduce evaporation in response to land surface characteristics and atmospheric forcing, both being spatially variable. Evaporation estimates for the 2005 growing season were obtained from satellite observations of the Western part of Hungary and compared to model outcomes. Atmospheric forcing was obtained from a hindcast run with the Regional Climate Model RACMO. Although HTESSEL slightly underestimated the seasonal evaporative fraction, the mean, 10th and 90th percentile of this variable were of the same magnitude as the satellite observations. The initial water as stored in the soil and snow layer did not have a significant effect on the statistical properties of the evaporative fraction. However, the spatial distribution of the initial soil and snow water affected significantly the spatial distribution of the calculated evaporative fraction and the models ability to reproduce evaporation correctly in low precipitation areas in the considered region. HTESSELs performance appears to be less in dryer areas. In Western Hungary these areas are situated in the Danube valley, which is partly covered by irrigated cropland and which also may be affected by shallow groundwater. Incorporating (lateral) groundwater flow and irrigation, processes that are not included now, may improve HTESSELs ability to predict evaporation correctly. Evaluation of the model skills using other test areas and larger evaluation periods is needed to confirm the results. Based on earlier sensitivity analysis, the effect of a number of modifications to HTESSEL was assessed. A more physically based reduction function for dry soils was introduced, the soil depth was made variable and the effect of swallow groundwater included. However, the modification did not lead to significant improved performance of HTESSEL.
CFD modelling approach for dam break flow studies
Authors: Biscarini C. , Di Francesco S. , Manciola P.
Published: April 2010

Abstract. This paper presents numerical simulations of free surface flows induced by a dam break comparing the shallow water approach to fully three-dimensional simulations. The latter are based on the solution of the complete set of Reynolds-Averaged Navier-Stokes (RANS) equations coupled to the Volume of Fluid (VOF) method. The methods assessment and comparison are carried out on a dam break over a flat bed without friction and a dam break over a triangular bottom sill. Experimental and numerical literature data are compared to present results. The results demonstrate that the shallow water approach loses some three-dimensional phenomena, which may have a great impact when evaluating the downstream wave propagation. In particular, water wave celerity and water depth profiles could be underestimated due to the incorrect shallow water idealization that neglects the three-dimensional aspects due to the gravity force, especially during the first time steps of the motion.
Simulation of the soil water balance of wheat using daily weather forecast messages to estimate the reference evapotranspiration
Authors: Cai J. , Liu Y. , Xu D. , Paredes P. , Pereira L. S.
Published: April 2010

Abstract. Aiming at developing real time water balance modelling for irrigation scheduling, this study assesses the accuracy of using the reference evapotranspiration (ETo) estimated from daily weather forecast messages (ETo,WF) as model input. A previous study applied to eight locations in China (Cai et al., 2007) has shown the feasibility for estimating ETo,WF with the FAO Penmam-Monteith equation using daily forecasts of maximum and minimum temperature, cloudiness and wind speed. In this study, the global radiation is estimated from the difference between the forecasted maximum and minimum temperatures, the actual vapour pressure is estimated from the forecasted minimum temperature and the wind speed is obtained from converting the common wind scales into wind speed. The present application refers to a location in the North China Plain, Daxing, for the wheat crop seasons of 2005–2006 and 2006–2007. Results comparing ETo,WF with ETo computed with observed data (ETo, obs) have shown favourable goodness of fitting indicators and a RMSE of 0.77 mm d−1. ETo was underestimated in the first year and overestimated in the second. The water balance model ISAREG was calibrated and validated for both years using ETo, obs by comparing the predicted and observed soil water content relative to various irrigation treatments. The calibrated crop parameters were used in the simulations of the same treatments using ETo,WF as model input. Errors in predicting the soil water balance are small, 0.010 and 0.012 m3 m−3 respectively for the first and second year. Other indicators also confirm the goodness of model predictions. It could be concluded that using ETo computed from daily weather forecast messages provides for accurate model predictions, thus making it viable to use an irrigation scheduling model in real time with daily weather forecast messages.
Catchment conceptualisation for examining applicability of chloride mass balance method in an area of historical forest clearance
Authors: Guan H. , Love A. , Simmons C. T. , Ding Z. , Hutson J.
Published: April 2010

Abstract. Among various approaches for estimating groundwater recharge, chloride mass balance (CMB) method is one of the most frequently used, in particular, for arid and semiarid regions. Widespread native vegetation clearance, common history in many areas globally, has changed land surface boundary condition, posing a question whether the current system has reached new chloride equilibrium for CMB application. To examine CMB applicability for catchments, conceptual catchment types of various chloride equilibrium conditions are defined. The conceptualization, combined with some local climate conditions, is demonstrated to be useful in examining whether a catchment has reached new chloride equilibrium. The six conceptual catchment types are tested with eleven selected catchments in the Mount Lofty Ranges (MLR), a coastal hilly area in South Australia having experienced historical widespread forest clearance. The results show that six of the eleven catchments match type VI chloride balance condition (chloride non-equilibrium with a gaining stream), with the ratio of stream chloride output over atmospheric chloride input (catchment chloride O/I) ranging from 2 to 4. Two catchments match type V chloride balance condition (chloride non-equilibrium with a losing stream), with catchment chloride O/I values about 0.5. For these catchments, the CMB method is not appropriate to apply. The results also suggest that neither a below-one chloride O/I value nor a low seasonal fluctuation of streamflow chloride concentration (a factor below 4) guarantees a chloride equilibrium condition in the study area. But a large chloride O/I value (above one) and a large fluctuation of streamflow chloride concentration (a factor of 10 and above) generally indicates either a chloride disequilibrium, or cross-catchment water transfer, or both, for which CMB is not applicable. Based on the regression between chloride O/I values and annual precipitation for type VI catchments, a catchment with annual precipitation of 900 mm in MLR has most likely reached new chloride equilibrium, for which CMB can be applied given that no cross-catchment water transfer occurs. CMB is applied for one catchment at chloride equilibrium, resulted in a net groundwater recharge estimate of 30 mm, about 4% of annual precipitation.
Combining semi-distributed process-based and data-driven models in flow simulation: a case study of the Meuse river basin
Authors: Corzo G. , Solomatine D. ,   Hidayat, de Wit M. , Werner M. , Uhlenbrook S. , Price R.
Published: April 2010

Abstract. One of the challenges in river flow simulation modelling is increasing the accuracy of forecasts. This paper explores the complementary use of data-driven models, e.g. artificial neural networks (ANN) to improve the flow simulation accuracy of a semi-distributed process based model. The IHMS-HBV model of the Meuse river basin is used in this research. Two schemes are tested. The first one explores the replacement of sub-basin models by data-driven models. The second scheme is based on the replacement of the Muskingum-Cunge routing model, which integrates the multiple sub-basin models, by an ANN. The results showed that: (1) after a step-wise spatial replacement of sub-basin conceptual models by ANNs it is possible to increase the accuracy of the overall basin model; (2) there are time periods when low and high flow conditions are better represented by ANNs; and (3) the improvement in terms of RMSE obtained by using of ANNs is greater than that when using sub-basin replacements. It can be concluded that the presented two schemes based on the analysis of seasonal and spatial weakness of the process based models can improve performance of the process based models in the context of operational flow forecasting.
Prospecting for safe (low fluoride) groundwater in the eastern African Rift: a multidisciplinary approach in the Arumeru District (northern Tanzania)
Authors: Ghiglieri G. , Balia R. , Oggiano G. , Pittalis D.
Published: April 2010

Abstract. This research was aimed at finding fresh and safe groundwater easily deliverable to an area, located in northern Tanzania, within the western branch of the Rift Valley. The study area suffers from water shortage, moreover, due to widespread alkaline volcanism, high fluoride contents (F− up to 70 mg/l) affects the groundwater. The achievement of this goal has been pursued through a multidisciplinary research consisting of geological, hydrogeological, hydro-chemical, geophysical and hydrological investigations. The study area stretches over 440 km2 and lies in the northern part of the Arumeru District, approximately 50 km from Arusha, the capital of the region. The Mount Meru (4565 m a.s.l.) and the Arusha National Park mark the boundary of the area, which includes 9 villages belonging to the Oldonyo Sambu and Ngarenanyuki Wards. The climate is semi-arid, with dry and relatively rainy seasonal alternance. Four principal hydrogeological complexes have been identified within different lithologies. They occur within volcanic formations, singularly or superimposed on each other. Subordinate perched aquifers are present in sedimentary formations with local occurrence. The groundwater flow system has been interpreted on the basis of springs spatial distribution combined with lithological and the geometrical reconstruction of the aquifers. The dominant pattern, consisting of multidirectional flow from the higher elevation area in the south towards the lower area in the north, is complicated by the occurrence of structures such as grabens, faults, lava domes and tholoids. After the identification of the main fluoride source, an interference pattern among groundwater and high F surface water was drawn. Finally, some VES (Vertical Electrical Sounding) were performed that allowed an aquifer to be individuated within a structural high where the fluoride input is prevented. The drilling of a well, able to supply at least 3.8 l/s of low fluoride, drinkable water, successfully concluded the methodological approach for prospecting safe water in a semi-arid, naturally fluoride polluted region.
Numerical study of the evaporation process and parameter estimation analysis of an evaporation experiment
Authors: Schneider-Zapp K. , Ippisch O. , Roth K.
Published: April 2010

Abstract. Evaporation is an important process in soil-atmosphere interaction. The determination of hydraulic properties is one of the crucial parts in the simulation of water transport in porous media. Schneider et al. (2006) developed a new evaporation method to improve the estimation of hydraulic properties in the dry range. In this study we used numerical simulations of the experiment to study the physical dynamics in more detail, to optimise the boundary conditions and to choose the optimal combination of measurements. The physical analysis exposed, in accordance to experimental findings in the literature, two different evaporation regimes, a soil-atmosphere boundary layer dominated regime (regime I) in the saturated region and a hydraulically dominated regime (regime II). During this second regime a drying front forms which penetrates deeper into the soil as time passes. The sensitivity analysis showed that the result is especially sensitive at the transition between the two regimes. By using boundary condition changes it is possible to force the system to switch between the two regimes, e.g. from II back to I. Based on this findings a multistep experiment was developed. The response surfaces for all parameter combinations are flat and have a unique, localised minimum. Best parameter estimates are obtained if the evaporation flux and a potential measurement in 2 cm depth are used as target variables. Parameter estimation from simulated experiments with realistic measurement errors with a two-stage Monte-Carlo Levenberg-Marquardt procedure and manual rejection of obvious misfits lead to acceptable results for three different soil textures.
A comparison of ASCAT and modelled soil moisture over South Africa, using TOPKAPI in land surface mode
Authors: Sinclair S. , Pegram G. G. S.
Published: April 2010

Abstract. In this paper we compare two independent soil moisture estimates over South Africa. The first estimate is provided by automated runs of the TOPKAPI hydrological model. The model has been adapted to run as a collection of independent 1 km cells located on a grid with a spatial resolution of 0.125°, using 3 hourly rainfall estimates and evapotranspiration forcing calculated at 1 h intervals. The rainfall forcing used is the TRMM 3B42RT product, while the evapotranspiration forcing is based on a modification of the FAO56 reference crop evapotranspiration (ET0), which accounts for vegetation health and the availability of surface and soil water, as limiting factors on the potential rate of evapotranspiration. We compare the ET0 estimates, computed using observed meteorological data at a network of weather stations, to those computed using 24 h forecast fields from the South African Weather Service's Unified Model runs. The results show that the ET0 computed using the forecast fields is strongly correlated with and unbiased relative to, the independent values computed (from observed data) at the weather station locations. We therefore conclude that the Unified Model forecasts are suitable for producing an estimate of ET0 instead of observed station data, especially considering the sparse coverage of weather stations in the region. Using the rainfall and evapotranspiration forcing data, the percentage saturation of the TOPKAPI soil store is computed, for each of 6984 uncalibrated TOPKAPI cells at 3 h time-steps, and compared with estimates of surface soil moisture from the ASCAT instrument onboard the METOP polar orbiter. The comparisons indicate a good correspondence in the dynamic behaviour of an exponentially filtered time series of the ASCAT surface soil moisture and the TOPKAPI estimates for several climatic regions in South Africa. The linear agreement in dynamic behaviour for these independent soil moisture estimates suggests that both are correctly capturing the soil moisture dynamics for a significant proportion this region, and could be combined to produce a "best estimate" soil moisture field.
Long-term changes in sediment phosphorus below a rural effluent discharge
Authors: Haggard B. E. , Stoner R. J.
Published: April 2010

Abstract. Effluent discharge often increases the amount of phosphorus (P) in the water column and bed material of receiving water bodies. The goal of this study was to evaluate changes in sediment-P interactions in an effluent-driven stream over a 4-year period where hydrology and watershed P management changed dramatically. Specifically, this study evaluated (i) the equilibrium between benthic sediments and stream water dissolved P; and (ii) the amounts of select P fractions in the bed material within the fluvial channel. Sediment and water samples were collected at Columbia Hollow in northwest Arkansas from October 2003 through September 2007, and the sampling site was approximately 3 km downstream from the Decatur wastewater treatment plant (WWTP). Monthly average effluent total P (TP) concentrations were highly variable (0.30–4.80 mg L−1) from October 2003 until December 2005; however, the Decatur WWTP implemented new P management strategies in 2006 that reduced the variability in effluent TP (0.28–0.95 mg L−1). Soluble reactive P (SRP) concentrations at Columbia Hollow 3 km downstream from the effluent discharge followed the same pattern; these concentrations were positively correlated to the effluent TP (r=0.73; p<0.001). Sediment equilibrium concentrations (EPC0) were significantly less (ln transformed data, p<0.001) after the WWTP effluent reduced TP concentrations, and sediment EPC0 suggested that the stream bed material acted as a P source to the overlying water at Columbia Hollow. The effects of this effluent discharge and the WWTP management changes on sediment P dynamics were profound. Prior to implementation of WWTP P management, the effluent TP concentrations were the driving factor related to SRP concentrations in the water column and sediment EPC0. Conversely, after the P management changes the benthic sediments became the important factor likely regulating dissolved P concentrations in the stream water.