Salvatore Grimaldi

Panta Rhei-Everything Flows: Change in hydrology and society-The IAHS Scientific Decade 2013-2022

Abstract The new Scientific Decade 2013–2022 of IAHS, entitled “Panta Rhei—Everything Flows”, is dedicated to research activities on change in hydrology and society. The purpose of Panta Rhei is to reach an improved interpretation of the processes governing the water cycle by focusing on their changing dynamics in connection with rapidly changing human systems. The practical aim is to improve our capability to make predictions of water resources dynamics to support sustainable societal development in a changing environment. The concept implies a focus on hydrological systems as a changing interface between environment and society, whose dynamics are essential to determine water security, human safety and development, and to set priorities for environmental management. The Scientific Decade 2013–2022 will devise innovative theoretical blueprints for the representation of processes including change and will focus on advanced monitoring and data analysis techniques. Interdisciplinarity will be sought by increased efforts to connect with the socio-economic sciences and geosciences in general. This paper presents a summary of the Science Plan of Panta Rhei, its targets, research questions and expected outcomes. Editor Z.W. Kundzewicz Citation Montanari, A., Young, G., Savenije, H.H.G., Hughes, D., Wagener, T., Ren, L.L., Koutsoyiannis, D., Cudennec, C., Toth, E., Grimaldi, S., Blöschl, G., Sivapalan, M., Beven, K., Gupta, H., Hipsey, M., Schaefli, B., Arheimer, B., Boegh, E., Schymanski, S.J., Di Baldassarre, G., Yu, B., Hubert, P., Huang, Y., Schumann, A., Post, D., Srinivasan, V., Harman, C., Thompson, S., Rogger, M., Viglione, A., McMillan, H., Characklis, G., Pang, Z., and Belyaev, V., 2013. “Panta Rhei—Everything Flows”: Change in hydrology and society—The IAHS Scientific Decade 2013–2022. Hydrological Sciences Journal. 58 (6) 1256–1275.

Design hyetograph analysis with 3-copula function

Abstract A design hyetograph is a synthetic rainfall temporal pattern associated with a return period, usually determined by means of statistical analysis of observed mean rainfall intensity through intensity—duration—frequency (IDF) curves. Since the univariate approach is simple to apply and data availability is scarce, only the mean intensity of a rainfall storm is usually analysed statistically. The other characteristics of a rainfall storm, such as peak (maximum intensity), total depth and duration, are found indirectly throughout the several phases of hydrological analysis by suitable work assumptions. The aim of this paper is to apply a multivariate approach in order to analyse jointly observed data of critical depth, peak and total depth. In particular, bivariate analysis of peak—total depth conditioned on critical depth is developed using a 3-copula function to define the trivariate joint distribution. Following the proposed procedure, once design return period and related critical depth are selected, it is possible to determine—in a probabilistic way—peak and total depth, without advancing a priori hypotheses on the design hyetograph pattern.

Hydrogeomorphic properties of simulated drainage patterns using digital elevation models: the flat area issue

Abstract Flat areas are a critical issue for the characterization of drainage patterns using digital elevation models (DEM). In this work, flat area removal and flow direction algorithms are implemented, and also a physically-based DEM correction model is introduced, for investigating their influence on the topological properties of the channel network, the Hortonian parameters and the hillslope width function. Differences of results, as compared to the standard procedures implemented in widely-used GIS-based hydrological packages, show the importance for hydrogeomorphic modellers to consider the use of more detailed approaches.

Time of concentration: a paradox in modern hydrology

Abstract The time of concentration is a primary parameter for a variety of modern hydrological models adopted in professional and scientific communities. Nevertheless, a universally accepted working definition of this parameter is currently lacking and several definitions can be found in the technical literature along with related estimation procedures. This study brings to light the inherent variability of these definitions through the empirical analysis of four small basins. These case studies demonstrate that available approaches for the estimation of the time of concentration may yield numerical predictions that differ from each other by up to 500%. Editor D. Koutsoyiannis Citation Grimaldi, S., Petroselli, A., Tauro, F. and Porfiri, M., 2012. Time of concentration: a paradox in modern hydrology. Hydrological Sciences Journal, 57 (2), 217–228.

Synthetic Design Hydrographs Based on Distribution Functions with Finite Support

The primary characteristics that influence the potential of defining a synthetic design hydrograph (SDH), are the hydrograph shape, peak discharge (Qp), volume (V), and duration (D). This paper studies the advantages and shortcomings of using simple distribution functions with finite support (namely, beta and generalized standard two-sided power distributions) to represent and synthesize direct runoff hydrographs. The relationships among Qp, V, D, and distribution parameters are explored on a few flood events selected by a recursive digital filter algorithm and an overthreshold approach. The results obtained indicate that the adopted procedure provides a good compromise between simplicity and accuracy for building SDHs with two assigned flood characteristics (e.g., Qp and V) and a defined shape.

A continuous simulation model for design-hydrograph estimation in small and ungauged watersheds

Abstract The estimation of design hydrographs for small and ungauged watersheds is a key topic in hydrology. In this context, event-based procedures, which transform design storms deduced from intensity–duration–frequency curves by lumped rainfall–runoff models, are commonly applied. This study introduces a continuous simulation model that involves a two-stage rainfall generator, a geomorphological rainfall–runoff model and a flood frequency analysis applied to simulated runoff time series. The resulting design hydrograph with an assigned return period preserves peak and volume information. The case study results indicate that the continuous model is able to return a range of flood scenarios corresponding to a wide range of possible watershed physical conditions. Moreover, the rainfall–runoff model applied using empirical calibration without observations appears to be as accurate as other models based on regionalized information. Editor D. Koutsoyiannis Citation Grimaldi, S., Petroselli, A., and Serinaldi, F., 2012. A continuous simulation model for design-hydrograph estimation in small and ungauged watersheds. Hydrological Sciences Journal, 57 (6), 1035–1051.

Large-Scale Particle Image Velocimetry From an Unmanned Aerial Vehicle

Large-scale particle image velocimetry (LSPIV) enables nonintrusive and continuous characterization of surface flow velocities in natural watersheds. However, current LSPIV implementations are based on hxed cameras that only allow for surface flow monitoring at a limited number of locations on the water stream. This paper seeks to leverage the growing held of unmanned aerial vehicles to transform LSPIV practice, by enabling rapid characterization of large water flow systems in areas that may be difhcult to access by human operators. Toward this aim, a lightweight and low cost quadrotor is developed to host a digital acquisition system for LSPIV. A gimbal is realized in house to maintain the camera lens orthogonal with respect to the water surface, thus preventing image orthorectihcation. Field experiments demonstrate that the vehicle is able to stably hover above an area of 1 × 1 m2 for 4 min with a payload of 532 g. The feasibility of quadrotor-based LSPIV is demonstrated through tests in an outdoor laboratory setting and over a natural stream.

Orienting the camera and firing lasers to enhance large scale particle image velocimetry for streamflow monitoring

Large scale particle image velocimetry (LSPIV) is a nonintrusive methodology for continuous surface flow monitoring in natural environments. Recent experimental studies demonstrate that LSPIV is a promising technique to estimate flow discharge in riverine systems. Traditionally, LSPIV implementations are based on the use of angled cameras to capture extended fields of view; images are then orthorectified and calibrated through the acquisition of ground reference points. As widely documented in the literature, the identification of ground reference points and image orthorectification are major hurdles in LSPIV. Here we develop an experimental apparatus to address both of these issues. The proposed platform includes a laser system for remote frame calibration and a low-cost camera that is maintained orthogonal with respect to the water surface to minimize image distortions. We study the feasibility of the apparatus on two complex natural riverine environments where the acquisition of ground reference points is prevented and illumination and seeding density conditions are challenging. While our results confirm that velocity estimations can be severely affected by inhomogeneously seeded surface tracers and adverse illumination settings, they demonstrate that LSPIV implementations can benefit from the proposed apparatus. Specifically, the presented system opens novel avenues in the development of stand-alone platforms for remote surface flow monitoring.

Characterization of buoyant fluorescent particles for field observations of water flows.

In this paper, the feasibility of off-the-shelf buoyant fluorescent microspheres as particle tracers in turbid water flows is investigated. Microspheres’ fluorescence intensity is experimentally measured and detected in placid aqueous suspensions of increasing concentrations of clay to simulate typical conditions occurring in natural drainage networks. Experiments are conducted in a broad range of clay concentrations and particle immersion depths by using photoconductive cells and image-based sensing technologies. Results obtained with both methodologies exhibit comparable trends and show that the considered particles are fairly detectable in critically turbid water flows. Further information on performance and integration of the studied microspheres in low-cost measurement instrumentation for field observations is obtained through experiments conducted in a custom built miniature water channel. This experimental characterization provides a first assessment of the feasibility of commercially available buoyant fluorescent beads in the analysis of high turbidity surface water flows. The proposed technology may serve as a minimally invasive sensing system for hazardous events, such as pollutant diffusion in natural streams and flash flooding due to extreme rainfall.

Comparing expert judgement and numerical criteria for hydrograph evaluation

Abstract This paper investigates the relationship between expert judgement and numerical criteria when evaluating hydrological model performance by comparing simulated and observed hydrographs. Using a web-based survey, we collected the visual evaluations of 150 experts on a set of high- and low-flow hydrographs. We then compared these answers with results from 60 numerical criteria. Agreement between experts was found to be more frequent in absolute terms (when rating models) than in relative terms (when comparing models), and better for high flows than for low flows. When comparing the set of 150 expert judgements with numerical criteria, we found that most expert judgements were loosely correlated with a numerical criterion, and that the criterion that best reflects expert judgement varies from expert to expert. Overall, we identified two groups of 10 criteria yielding an equivalent match with the expertise of the 150 participants in low and high flows, respectively. A single criterion common to both groups (the Hydrograph Matching Algorithm with mean absolute error) may represent a good indicator for the overall evaluation of models based on hydrographs. We conclude that none of the numerical criteria examined here can fully replace expert judgement when rating hydrographs, and that both relative and absolute evaluations should be based on the judgement of multiple experts. Editor D. Koutsoyiannis