P. La Barbera

Rainfall intermittency and the sampling error of tipping-bucket rain gauges

Abstract This work amplifies upon the influence of rainfall intermittency on pluviometric time series. Intermittency is here defined as the percentage of no-rain periods within a rainfall event, and can be interpreted as a sort of ‘stochastic intermittency’. When measuring an intermittent or, in general, ‘erratic’ signal, a relevant source of error is associated with the sampling procedure. By using numerical simulation of intermittent rainfall events, we focus on the deriving pattern of sampling errors within rainfall measurements obtained from any classic tipping-bucket rain gauge. The analysis, performed in both the time and frequency domain, reveals a strong inverse linear dependence between rainfall intermittency and the sampling errors, while a weak dependence on the autocorrelation of synthetic rainfall events is shown. The average absolute error is around 30% for the events analysed while the error at each sampling period peaks values higher than 100%. A direct influence of intermittency on the statistical characteristics of measured rainfall events is observed.

Morphological characterization of channel initiation

Abstract The system of hillslopes and channels that form a drainage basin and shape its morphology and dynamics is the result of a complex interaction between climate and soil through erosion and sediment transport. The identification of sediment sources and pathways must therefore be carried out taking into account system genesis and dynamics. In this framework, drainage density assumes a major significance: it relates morphology to climate and soil properties and cannot be assumed constant throughout the basin. Empirical evidence and theoretical results are available to describe hydrodynamical and morphological conditions that are expected to hold at the boundary between hillslopes and streams. However, the variance of the physical processes and the low accuracy achievable in the estimation of the local slope value from available elevation data sets are limiting factors in the practical application of such concepts. These limiting factors can be removed by means of a global approach in which drainage density is linked to the relative elevation of the subcatchment draining to the site under consideration. Moreover, this can be viewed as a function of morphology through contributing area, local slope, and the fractal dimension of single rivers. The mathematical form that describes this function is equal, in structure, to that of the equations that describe the hydrodynamical and morphological conditions. This approach leads to a well connected and coherent network which is able to describe the boundary between hillslopes and channels and to reproduce the spatial variation of the drainage density.

Integrating Meteosat satellite and rain-gauge information to estimate rainfall patterns

This paper presents the development and the application of techniques for the integration of information coming from Meteosat satellite images and rain-gauge measurements, with the purpose of estimating the rainfall pattern on a certain river basin. The proposed integration techniques are based on the definition of a data coherence problem and on the application of mathematical programming methods. The quality of the estimation procedure is evaluated by using a rainfall/runoff model for the basin which allows the generation of a hydrograph at a given section of the river, on the basis of the above estimated rainfall pattern in the upstream watershed. The comparison of the hydrograph with the observed one allows the parametric tuning of the integration procedure.

Hydrologic influence on stormwater pollution at two urban monitoring sites

The pollutant transport process operated by stormwater runoff on urban paved surfaces and the relationship between the parameters of the wash-off function and the controlling hydrologic variables are investigated in the present work. Data collected during two monitoring campaigns carried out at the plot scale within a residential area and an auto dismantler site are used to this aim. The observed runoff events are classified into different mass delivery processes and the occurrence of the first flush phenomenon is also investigated. The maximum flow discharge obtained as the average value over the time of concentration of the drainage network is proposed as the controlling factor for the total mass of pollutant that is made available for wash-off during each runoff event.

On the cumulative area distribution of natural drainage basins along a coastal boundary

The natural drainage network in a region is analyzed in order to investigate scaling properties and the form of the probability distribution for basin areas along a coastal boundary. The Horton/Strahler approach used in the classical description of the drainage network within a single basin is extended to a series of independent though conterminous basins with outlet to the sea, assuming that a regional area ratio RA, analogous to the Hortonian area ratio, holds for independent basins. A multiplicative factor Rc is also defined as the average ratio between the number of independent basins of a given order ω and those of order ω+1. The cumulative area distribution (CAD) for drainage basins that drain along the coastal boundary is obtained as a power law in the form P[A > a] ∝ a−γ, with γ = log Rc/log RA. The derived form and parameters of the CAD are shown to hold for the Liguria region of Italy (≃5700 km2 in the northern Mediterranean) as well as for the whole continental Italy (≃250,000 km2). The fractal dimension of the regional system encompassing all drainage areas of basins with outlet to the sea is derived in the form 𝒟 = 1/γ. The analytical results obtained are shown to agree with the theoretical expressions already available in the literature for optimal channel networks.

MIDA, a distributed hydrological model: Its behavior under some different parameter and data discretization conditions

MIDA is a distributed model for hydrological processes at the basin scale, the structure of which is suited to small basins of mountainous Mediterranean coastal areas. The underlying philosophy of the model is a distributed conceptual approach in which geomorphological scaling relationships are used, fully supported by a Geographical Information System. The sensitivity of the model response to some variable conceptual and global parameters and its performance across some different land-use and climatological conditions are analyzed. The analysis reveals a high sensitivity to some parameters related to infiltration and runoff generation (soil cover thickness on hillslope and saturated hydraulic conductivity) and to land-use features, while change in discretization levels seems to have less influence on predicted discharge.

Multisensor Analysis of the Flood Event of November 23–25th, 1987 on the Arno Basin

The event of November 23–25th, 1987 on the Arno basin was observed by ground raingauges and a hydrometric reporting network, the Meteosat geosyncronous satellite, and the Special Sensor Microwave/Imager (SSM/I) aboard a polar orbiting satellite. In the present paper the three data sets derived from the different information sources are used as an input to a coherence assessment analysis; information provided by raingauges, geosyncronous and polar satellites are indeed intrinsecally different as far as space and time scales and reliability are concerned. The proposed approach has two main purposes: i) to integrate effectively different data sources to obtain a rain rate estimate which minimises the overall deviation from the set of available measurements; ii) to assess the relative reliability of a source with respect to the others. Hydrographs in Florence at Uffizi, simulated by a distributed model using the Meteosat information and raingauge data, have been compared to the observed one in order to assess the use of multisensor observations to predict rainfall ground effects during the considered extreme meteorological event.

Analysis of Spatial Variability of River Network Morphology, Flow and Potential Energy

Spatial variability of morphological characteristics and flow in river networks, and its relation to power distribution are analytically and empirically investigated. It is assumed, and positively tested, that Horton-type laws describe the downstream change in link morphological and topological characteristics. Accordingly, surrogates to the traditional stream length and area ratios are provided by the link number ratio, and the total link number ratio, respectively. The opposite statistical behaviour of stream and link length, as being dependent and independent variables, respectively, is found to be reversed in the case of link and stream heights. This property leads to an identical trend in the spatial variability of slope in both cases. On the other hand, assessment the of the self-similar model of link altitudinal geometry [Gupta & Waymire, 1989] reveals that previous testing, upon which the model has been refuted by Tarboton et al. [1989] was inadequately performed. However, our results show an increasing structured departure from simple-scaling for the n-th order moments. Finally, using Horton-type laws for height and flow yields the distribution of power to be characterized by a state of maximum spatial uniformity for a given flow quantile, for which the scaling exponent of mean link slope equals the one that describes mean flow pattern. This result is found to be implicitly explained by using the informational entropy principles as introduced by Kapoor [1990] for river networks.