1,721,029 research outputs found
ARTIFICI PER MIGLIORARE LA CONSISTENZA DELLE MISURE CON IL PERMEAMETRO DI GUELPH
No full textIl permeametro di Guelph (PG) è un noto strumento per la misura in campo dell'infiltrazione da cui ottenere una stima della conducibilità idraulica alla saturazione di campo. Diversi autori hanno proposto modifiche alla procedura da seguire per la preparazione del pozzetto di infiltrazione con lo scopo di migliorare l'attendibilità delle misure. In questa memoria si descrivono alcuni artifici ideati per ottenere, in modo pratico, una forte standardizzazione delle caratteristiche del pozzetto. Le misure effettuate con queste varianti, in posizioni vicine ed entro alcuni giorni di tempo, hanno mostrato una buona riproducibilità dei valori (scarto dalla media nell'ordine del 6%)
A composite statistical method for the detection of multiple undocumented abrupt changes in the mean value within a time series
No full textThe time series of measurements of hydro-meteorological variables often suffer from imperfections such as missing data, outliers and discontinuities in the mean values. The discontinuity in the mean can be the effect of: instrumental offsets and of their corrections, of changes in the monitoring station or in the surrounding environment. If the discontinuities can be identified with a reasonable precision, a correction of the erroneous data can be made. Several authors have put their great effort into developing techniques to identify non-climatic inhomogeneities; the resulting statistical methods are especially effective when the series contains a single change point, while their performances decline when the series contains multiple change points or inhomogeneous segments (a portion of the series bounded by two complementary shifts). These limitations also affect the standard normal homogeneity test (SNHT), one of the most effective and widely applied tests. We present a composite method of homogeneity testing, standard normal homogenization composite method (SNHCM), including the SNHT as one component, which improves the SNHT performances with multiple change points
and inhomogeneous segments. A number of comparisons among the new method, the SNHT and a powerful optimal segmentation method (OSM-CM), are illustrated in the paper. SNHCM demonstrates their performances in change-point detection similar to, or better than, the SNHT and very close to the OSM-CM. The SNHCM is effective in recognizing complex patterns of discontinuities, especially inhomogeneous segments, which represent a severe problem for SNHT; on the contrary, SNHT performs slightly better only when the series contains a single change point, but the difference
between the two methods is negligible. Compared to the OSM-CM, SNHCM provides very similar performances, with some favourable features deriving from the fact that it is computationally lighter, simpler to implement, can easily handle very long series and is based on statistical hypothesis tests with a well-defined and adjustable significance level
Effects of under-vacuum and vessel soil sample saturation on soil hydraulic conductivity estimated by the core method in the case of paddy soils
No full textIn paddy fields most of the percolation flux is regulated by the saturated soil hydraulic conductivity (Ks) of the hardpan. The core method performed on large undisturbed soil samples generally allows a good estimation of Ks. However, only little attention is paid in the literature to the saturation process adopted to prepare soil samples for analysis. Two methods are available: vessel saturation (AtmSat), and saturation under vacuum (VacSat).
In autumn 2015, ten large undisturbed soil samples were collected from the hardpan layers of three paddy fields located in Lombardy (Italy). In particular, five soil profiles were opened and two samples were collected from the hardpan of each profile. Ks was determined through the core method by adopting the two saturation methods. The first outcome of the research is that the saturation method strongly influences the obtained Ks value. In order to evaluate the accuracy of the estimated Ks values and to determine which saturation method is advisable in the case of paddy fields, the percolation flux estimated by the Darcy law (applied considering measured Ks) for the three paddy fields was compared to the flux obtained as the residual in the water balance equation for the same fields (where all the other terms were measured during the agricultural season 2015). Results show that VacSat provides better results with respect to AtmSat; however, vacuum must be applied slowly and the final Ks value is reached after a long convergence time
A procedure for the detection of undocumented multiple abrupt changes in the mean value of daily temperature time series of a regional network
No full textThis paper presents the new procedure MAC-D for the automated detection of undocumented Multiple Abrupt Changes in the mean value of Daily temperature series, recorded in a network of meteorological stations. MAC-D can be applied to series containing seasonality, multiple change points, outliers, and with a noise component that can be autocorrelated and non-normally distributed. The main novelties of the procedure are (1) the pretreatment of the observed series, to derive a series of daily values that complies with the theoretical requirements of the change point detection tests and in (2) the combined use of the reference series and pairwise comparison approaches. MAC-D consists of three phases in sequence. In phase 1, the seasonal and climatic fluctuations are estimated and removed, using the reference series approach. Phase 2 combines a linear filtering with a change point detection test in an iterative algorithm, which runs until full compliance between the characteristics of the filtered series and the test requirements is achieved. Phase 3 is aimed at removing the false change points, due to error propagation in the reference series analysis, by double checking the detected change points with the pairwise comparison approach. Monte Carlo estimations of the actual significance and overall performance of the procedure for different series features and test resolutions are provided. Results demonstrate that MAC-D performs very well with daily series having a wide range of different characteristics
Application of the changepoint detection to daily temperature series with the MAC-D procedure
No full textInvestigation of spatial and temporal variability of saturated soil hydraulic conductivity at the field-scale
No full textThough soil hydrologists agree that field saturated
conductivity (Ks) is a key parameter in modelling the dynamics of water flow and solute transport in soils, they also recognize that its variability in space and time is far from being completely understood. In order to highlight the variability of Ks at the plot scale we performed ten measurement campaigns in three parcels within a 10 ha maize field during two subsequent crop seasons and in the fallow periods following them, in uniform conditions of crop, agricultural practices and, to a large extent, of pedological characteristics. This paper reports the
outcomes of the measurements, conducted with the Guelph permeameter (GP) and with the tension infiltrometer (TI), along with detailed information on the data and a thorough description of the experimental field and of the measurement techniques. Based on a careful statistical analysis of the dataset and an extensive discussion of the results, the following conclusions were reached.
GP Ks show changes in time and space, both between and within the parcels, with a different temporal behaviour for the different parcels, and no evident seasonal cycle. Mean and standard deviation of the transformed GP data samples are shown to be linearly related. This allowed the definition of a model of Ks statistical distribution that elucidates the distinct contributions of soil matrix and macropores, and provides a validation of the Morales et al. (2010) concept of
biologically-driven macropore dynamics.
TI estimations of Ks vary in space in agreement with the soil texture and
show a stable seasonal pattern. However, in presence of macropores, they
are not representative of the actual values of the saturated conductivity. On the other hand, TI Ks could provide an estimate of the conductivity of the soil matrix. The comparison with the soil Matrix conductivity values deriving from the proposed model of Ks statistical distribution seem to support this possibility.
These results, that shall be corroborated by further experiments, support the importance of thoroughly investigating the interactions between soil biota, vegetation and the soil hydraulic properties
Changepoint detection in case of step change and inhomogeneous segments (platform-like inhomogeneities)
No full textUN NUOVO TEST STATISTICO PER LA RICERCA DI ANOMALIE NELLE SERIE TEMPORALI
No full textLe serie temporali di misure delle grandezze idro-meteorologiche risentono spesso di imperfezioni, quali dati mancanti, outliers e discontinuità nei valori medi. Se per le elaborazioni statistiche si opta spesso per concentrare le analisi sui soli dati validi, per gli scopi modellistici è in genere indispensabile affrontare e risolvere i problemi menzionati, al fine di ottenere serie complete di tutte le grandezze necessarie per la calibrazione e validazione dei modelli. Le discontinuità nella media, su cui questo lavoro si concentra, possono essere effetto di offset strumentali e delle loro correzioni, di modifiche nella stazione di rilevamento (posizione, tipologia degli strumenti) o anche di alterazioni dell’ambiente che circonda la stazione. Queste discontinuità si presentano come gradini o trend all’interno delle serie; come per gli outliers, la registrazione indica un valore differente dal valore reale della grandezza misurata ma, a differenza di questi, l’errore è sistematico e modifica i dati di una quantità che persiste per un certo periodo, mantenendo approssimativamente stabili le altre caratteristiche della serie, quali ad esempio la differenza di passo uno e parte degli eventuali andamenti stagionali. Accade sovente che gli offset non siano rilevati o che, quando lo sono, le azioni correttive non vengano documentate. E’ quindi consigliabile, prima di utilizzare le serie di misure, sottoporle ad un test statistico per la verifica dell’eventuale presenza di punti di discontinuità. In letteratura sono stati proposti diversi test di questo tipo; generalmente essi sono efficaci soprattutto quando la serie contiene un singolo punto di discontinuità, mentre riducono notevolmente le loro prestazioni quando l’intervallo di valori anomali si colloca all’interno della serie o quando i punti di discontinuità sono molteplici. Nella memoria viene presentato un nuovo test (Rienzner & Gandolfi, 2009), che supera in larga misura queste limitazioni, e viene illustrato un esempio di applicazione a serie di grandezze meteorologiche di alcune stazioni della rete agrometeorologica della Regione Lombardia
Capitoli: 5 (valutazione dei fabbisogni irrigui), 6 (fabbisogni idrici colturali stimati nelle simulazioni), 7 (bilancio idrologico), 8 (fabbisogni aggregati per canale), 9 (confronto fabbisogni idrici / disponibilità idrica superficiale). Appendice 1.
No full textANALISI STATISTICA DELLA CONDUCIBILITÀ IDRAULICA SATURA STIMATA IN CAMPO PER UN SUOLO SUPERFICIALE
No full textNella memoria si mostrano analisi statistiche di valori di conducibilità idraulica satura (Kfs) stimati a partire da infiltrazione misurata in campo con il permeametro di Guelph (PG) e l'infiltrometro a depressione (ID). La base dati proviene da 6 campagne di misura lungo l'anno 2010 (da giugno a febbraio) che hanno interessato 3 piccole porzioni di un appezzamento a mais in Lombardia. Le analisi mostrano che: i) pur avendo i tre siti caratteristiche pedologiche molto simili, i campioni di dati non sono omogenei tra loro; ii) le differenze nei valori medi dei campioni trasformati (log) non assecondano le variazioni di granulometria; iii) nessuna distribuzione tra le più comuni (inclusa la log-normale) si adatta a tutti i campioni di valori; iv) le stime ottenute con l'ID e con il PG non sono affatto intercambiabili tra loro, possano invece essere considerate complementari. Questi risultati, sono utili in quanto indicano, o ribadiscono, punti aperti verso i quali dirigere la ricerca
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