134 research outputs found
The Geomorphological Origin of Recession Curves
No full textThis study is motivated by the fact that natural basins share some striking similarities in their basic organization as well as in the way they respond to rainfall input. The central objective here is thus to link the patterns observed in morphology of basins with the patterns that we observe in their hydrologic responses. This study particularly attempts to uncover hidden links between basin morphology and recession curves through a simple conceptual model describing dynamics of saturated channel network or active drainage network (ADN).
Analysis of recession curves is performed by following the framework proposed in the paper by Brutsaert and Neiber, i. e. to interpret –dQ/dt as having a power law relationship with Q (-dQ/dt = kQα), where Q is the discharge at the outlet of a basin at time t. It is shown here that N(l), number of links in a channel network at distance l from their channel heads exhibits a power law relationship with G (l), length of the ADN or length of the channel network at a distance greater than or equal to l from channel heads. And by assuming that the rate of discharge produced by the AND per unit length, q, and the rate at which channel network desaturates towards downstream, c, are constant in space and time, it is argued here that the power law relationship between –d/dt and Q essentially originates from the power law relationship bewteen N(l) and G(l).
It is observed that there is no unique relationship between –dQ/dt and Q for a basin, which implies, the popular assumption that there exists a unique relationship between storage and discharge of a basin is wrong. Also a typical –dQ/dt vs Q curve possess different regimes, because of its association with flood response that we observe shortly after rainfall and observational errors that particularly dominate during very low flow periods. One can observe a fairly good power law relationship between –dQ/dt vs Q, shortly after a rainfall event, continuing for several days. This regime for each individual hydrograph is identified by means of visual interpretation. The power law exponent α for each individual hydrograph of a basin is then computed and the median value is considered as the representing α for the basin . A good agreement is observed between the observed power law exponents, α0, with the geomorphic power law exponents, αg, for 67 United States basins of different sizes and situated in different climatic zones. The correlation is strong, particularly, for the steep basins without having significant human influences, like the presence of dams, cities, extensive agriculture practice etc. Also it is found that the power law exponent α is closely related to the well known Hack’s exponent .
The coefficient k is observed to have power law relationship with any characteristics discharge chosen deonated as Qn, which objectively means the discharge observed after n days of a hydrograph peak. Therefore, one can make different recession curves of a basin collapse on one another by using a suitable multiplicative function. Also it is observed that the power law exponent, γ, as well as the coefficient of determination increases as n increases. Thus the collapse gets more prominent with increase in n.
N(l)/A vs G(l)/A graphs of different basins are found to be collapsing on another, implying that natural basins follow some universal geomorphological themes. Recession curves on the other hand are dependent on many factors like geology, land use, topographic characteristics. It is observed that the recession curves of the basins that have similar geological and topographical characteristics collapse on one another. This observation essentially implies that one can classify natural basins by just observing their recession curves.
This finding further strengthens the assumptions of constant q and constant c made earlier. And finally, it is stressed that the argument, recession curves bearing signatures of basin morphology, can be safely stretched to conclude that the incision of the channel network may be due, to a significant degree, to subsurface flow, in such a way as produce an approximately uniform drainage of the local groundwater system and thus a uniform distribution of q.Questo studio è motivato dal fatto che i bacini naturali sono accomunati da importanti similarità relativamente alla loro organizzazione ed al modo in cui essi rispondono agli impulsi di pioggia. L’obiettivo centrale di questa tesi è quindi quello di individuare una relazione tra alcune caratteristiche osservabili nella morfologia dei bacini e la loro risposta idrologica. In particolare con questo studio si cerca di scoprire analogie nascoste tra la morfologia dei bacini e le curve di recessione attraverso un semplice modello concettuale in grado di descrivere le dinamiche delle reti in saturazione o “Active Drainage Networks” (ADN).
L’analisi delle curve di recessione è effettuata seguendo un approccio proposto in Brutsaert and Neiber (1977), in cui -dQ/dt è rappresentato in funzione di Q attraverso una legge di potenza (-dQ/dt = k Qα), dove Q è il deflusso alla sezione di chiusura di un bacino al tempo t. Nella tesi si mostra che il numero di connessioni (links) in una rete, N(l), ad una distanza l dalle origini (channel heads) è correlato attraverso una legge di potenza con la lunghezza della ADN, cioè la lunghezza delle connessioni ad una distanza maggiore o uguale ad l dalle origini. Assumendo che il deflusso, q , prodotto dalla ADN per unità di lunghezza e la velocità di desaturazione della rete, c, siano costanti nel tempo e nello spazio si discute l’ipotesi che la legge di potenza che lega -dQ/dt e Q abbia origine dalla legge di potenza che lega N(l) e G(l).
Si osserva che non c’è un'unica relazione tra -dQ/dt e Q per un bacino, il che suggerisce che la nota assunzione di una singola relazione tra deflusso e volume di immagazzinamento sia inappropriata. Inoltre, una tipica curva -dQ/dt vs. Q possiede regimi differenti dovuti al suo legame con la risposta superficiale che si osserva subito dopo un evento di pioggia e agli errori di misura effettuati in particolare in periodi di deflusso moderato. Si può osservare una relazione di potenza discretamente buona tra -dQ/dt e Q subito dopo un evento di pioggia e persistente per diversi giorni. L’esponente α della legge di potenza per ciascun idrogramma di un bacino è quindi calcolato e il valore mediano è considerato rappresentativo di tutti i valori di α per il bacino. I valori osservati dell’esponente αo sono in buon accordo con gli esponenti della relazione geomorfologica, αg, per 67 bacini statunitensi di dimensioni diverse e situati in diverse zone climatiche. In particolare si osserva una forte correlazione per bacini con elevata pendenza che non presentano aspetti antropici significativi, come ad esempio dighe, città, diffuse aree coltivate, ecc. Si è inoltre osservato che l’esponente α è significativamente correlato con il noto esponente di Hack.
Si è visto che il coefficiente k è legato attraverso una legge di potenza con qualsiasi deflusso caratteristico selezionato: k=k’Qn-γ, dove Qn è il deflusso osservato dopo n giorni dal picco dell’idrogramma. Di conseguenza, diverse curve di recessione possono essere fatte collassare in un’unica curva. Si osserva inoltre che l’esponente γ della legge di potenza e il coefficiente di determinazione aumentano all’aumentare di n.
Si osserva come le curve N(l)/A vs. G(l)/A di diversi bacini collassino in un'unica curva, il che implica che i bacini naturali seguono una qualche legge geomorfologica universale. Le curve di recessione, invece, dipendono da molti fattori come la geologia, l’uso del suolo, le caratteristiche topografiche. Si osserva che le curve di recessione di bacini con simili caratteristiche topografiche e geologiche collassano in un’unica curva. Questa osservazione implica che è possibile classificare bacini naturali solo sulla base delle curve di recessione. Questa scoperta rafforza ulteriormente l’assunzione di q e c costanti fatta in precedenza. Infine viene messo in evidenza che il fatto che le curve di recessione rappresentano una sorta di “firma” della morfologia dei bacini suggerisce che il deflusso subsuperficiale influenza la rete, in modo da produrre un drenaggio approssimativamente uniforme, e perciò una distribuzione uniforme di q
'Universal' recession curves and their geomorphological interpretation
No full textThe study of recession flows offers fundamental insights into basin hydrological processes and, in particular, into the collective behavior of the governing dominant subsurface flows and properties. We use here an existing geomorphological interpretation of recession dynamics, which links the exponent in the classic recession curve -dQ / dt = kQα to the geometric properties of the time-varying drainage network to study the general properties of recession curves across a wide variety of river basins. In particular, we show how the parameter k depends on the initial soil moisture state of the basin and can be made to explicitly depend on an index discharge, representative of initial sub-subsurface storage. Through this framework we obtain a non-dimensional, event-independent, recession curve. We subsequently quantify the variability of k across different basins on the basis of their geometry, and, by rescaling, collapse curves from different events and basins to obtain a generalized, or 'universal', recession curve. Finally, we analyze the resulting normalized recession curves and explain their universal characteristics, lending further support to the notion that the statistical properties of observed recession curves bear the signature of the geomorphological structure of the networks producing the
Predicting streamflow distributions and flow duration curves from landscape and climate
No full textCharacterizing the probability distribution of streamflows in catchments lacking in discharge measurements represents an attractive prospect with consequences for practical and scientific applications, in particular water resources management. In this paper, a physically-based analytic model of streamflow dynamics is combined with a set of water balance models and a geomorphological recession flow model in order to estimate streamflow probability distributions based on catchment-scale climatic and morphologic features. The models used are described and the novel parameterization approach is elaborated on. Starting from rainfall data, potential evapotranspiration and digital terrain maps, the method proved capable of capturing the statistics of observed streamflows reasonably well in 11 test catchments distributed throughout the United States, east of the rocky mountains. The method developed offers a unique approach for estimating probability distribution of streamflows where only climatic and geomorphologic features are known
Dynamic hydrologic modeling using the zero-parameter Budyko model with instantaneous dryness index
No full textLong-term partitioning of hydrologic quantities is achieved by using the zero-parameter Budyko model which defines a dryness index. However, this approach is not suitable for dynamic partitioning particularly at diminishing timescales, and therefore, a universally applicable zero-parameter model remains elusive. Here an instantaneous dryness index is proposed which enables dynamic hydrologic modeling using the Budyko model. By introducing a "decay function" that characterizes the effects of antecedent rainfall and solar energy on the dryness state of a basin at a time, I propose the concept of instantaneous dryness index and use the Budyko function to perform continuous hydrologic partitioning. Using the same decay function, I then obtain discharge time series from the effective rainfall time series. The model is evaluated by considering data form 63 U.S. Geological Survey basins. Results indicate the possibility of using the proposed framework as an alternative platform for prediction in ungagued basins
Incorporating channel network information in hydrologic response modelling: Development of a model and inter-model comparison
No full textIncorporation of channel network information in streamflow modelling is a well-accepted scientific practice now. In particular, channel network morphology based instantaneous unit hydrographs (IUHs) are widely used for modelling of flood response. However, very few attempts have been made so far to use channel networks for modelling total flow, not just flood flow. In this study, total flow is partitioned into pure surface flow (PSF) and mixed surface-subsurface flow (MSSF), which are then modelled separately by constructing channel network morphology based IUHs. For modelling total flow, the combined IUH is then obtained by introducing a splitting parameter that represents the relative proportions of PSF and MSSF. We compare the performance of the proposed geomorphology based routing structure and a variant with a commonly used routing structure – two linear reservoirs in parallel. The three routing structures are then integrated with a well-known water balance model to perform continuous streamflow modeling. We perform inter-model comparison quantitatively by considering eight performance metrics within a multi-objective framework as well as qualitatively by observing the simulated storage-discharge relationships. By performing the inter-model comparison for 71 catchments across the US, we find that the geomorphology based models perform better than the linear model for low flow related metrics. They are also better at capturing non-linear and dynamic relationship between catchment water storage and discharge. The geomorphology based models perform particularly well in northeastern and midwestern US, while no such region of dominance emerges for the linear routing based model. Results also indicate the possibility of using the proposed models to capture the dominant flow generation processes in a basin
Assessing the Impact of Climate Change on Water Resources: The Challenge Posed by a Multitude of Options
No full textGlobal warming due to anthropogenic emissions of greenhouse gases is already altering the Earth’s climate. A changing climate implies shifts in long-term temperature and precipitation patterns, which in turn will affect the spatiotemporal distribution of water resources. It is imperative to study the impact of climate change on water resources so that adequate adaptation actions can be planned well in advance. We, therefore, need reliable methods to estimate how the timing and magnitude of available fresh water in a region may change in response to a changing climate. This chapter summarizes the main approaches that are used to achieve this goal. We focus on the numerous choices that a modeler faces when attempting to quantify the impact of climate change on water resources of a region. We discuss the relative strengths and weaknesses of each approach. These choices vary from the choice of model structures representing global climate and local hydrology, possible trajectories of greenhouse gas emissions in the future, to methods for model evaluation. Wherever feasible, we provide recommendations that can help a modeler in choosing an appropriate course of action. We conclude the chapter with a discussion on recent techniques developed to deal with large uncertainties in projections of climate change and possible research directions that will benefit the impact assessment community
Can a Calibration-Free Dynamic Rainfall‒Runoff Model Predict FDCs in Data-Scarce Regions? Comparing the IDW Model with the Dynamic Budyko Model in South India
No full textConstruction of flow duration curves (FDCs) is a challenge for hydrologists as most
streams and rivers worldwide are ungauged. Regionalization methods are commonly followed to
solve the problem of discharge data scarcity by transforming hydrological information from
gauged basins to ungauged basins. As a consequence, regionalization-based FDC predictions are
not very reliable where discharge data are scarce quantitatively and/or qualitatively. In such a
scenario, it is perhaps more meaningful to use a calibration-free rainfall‒runoff model that can
exploit easily available meteorological information to predict FDCs in ungauged basins. This
hypothesis is tested in this study by comparing a well-known regionalization-based model, the
inverse distance weighting (IDW) model, with the recently proposed calibration-free dynamic
Budyko model (DB) in a region where discharge observations are not only insufficient
quantitatively but also show apparent signs of observational errors. The DB model markedly
outperformed the IDW model in the study region. Furthermore, the IDW model’s performance
sharply declined when we randomly removed discharge gauging stations to test the model in a
variety of data availability scenarios. The analysis here also throws some light on how errors in
observational datasets and drainage area influence model performance and thus provides a better
picture of the relative strengths of the two models. Overall, the results of this study support the
notion that a calibration-free rainfall‒runoff model can be chosen to predict FDCs in discharge
data-scarce regions. On a philosophical note, our study highlights the importance of process
understanding for the development of meaningful hydrological models
Flow Duration Curve Prediction Using a Physical Deterministic Model for Indian Catchments
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