Hydrology

Streamflow Regionalization: Case Study of Turkey

by Ercan Kahya

Co-authored with S. Kalaycı and T.C. Piechota.
Published in Journal of Hydrologic Engineering, Vol. 13, No. 4, 2008.

Homogeneous streamflow regions in Turkey were identified using a multivariate statistical tool; namely, principal... more Homogeneous streamflow regions in Turkey were identified using a multivariate statistical tool; namely, principal components analysis (PCA) with a varimax orthogonal rotation. The rotated PCA was applied to monthly streamflow records of 78 gauging stations during the period 1964–1994. The first seven rotated components were found significant at the 0.05 or lower levels. All accounted for 68% of the total variance of the original data assembly. The different groups of highly intercorrelated streams were defined by the patterns of map drawn for the rotated components loadings. Furthermore, an annual cycle analysis was applied to the same monthly streamflow data. Harmonic vectors extracted from a series of 12 monthly means at each station were marked over a map to readily keep track of streamflow seasonal variations across the study domain. The basic statistical characteristics of streamflow were also investigated within the identified regions; thus, some similarities and dissimilarities between the regions were documented. The identified streamflow regions were shown to be consistent with climate zones in Turkey defined by a recent study based on cluster analysis of temperature and precipitation data.

The analysis of El Ni�o and La Ni�a signals in streamflows of Turkey

by Ercan Kahya

Co-authored with M.Ç. Karabörk
Published in Int. J. Climatol. 21: 1231–1250 (2001)

A group of earlier investigations has identified some specific regions over the world in which hydrometeorological... more A group of earlier investigations has identified some specific regions over the world in which hydrometeorological surface parameters (i.e. precipitation and streamflow) appear to have consistent relationships with the extreme phases of the Southern Oscillation (SO). This paper examines a climatic link between the monthly streamflows in Turkey and the tropical El Nin ̃o and La Nin ̃a events using a large data set and comprehensive empirical methodology. Coherent and significant streamflow responses were detected in two core regions, namely the northwestern Anatolia (NWA) and the eastern Anatolia (EA). The areal extent of both regions with a defined seasonal signal for the case of each tropical event was determined. Some subregions whose indications are considered to be better than those for the whole core region were also isolated within the core region. In the Susurluk subregion identified in the NWA, the April–October seasonal positive streamflow anomalies were found to have a highly significant relation with the El Nin ̃o events. Although there is a possibility that the NWA region is influenced by the El Nin ̃o events, this was not confirmed by the hypergeometric test used to assign a significance level for the relationship. A similar analysis has shown that the La Nin ̃a events have no noticeable influences on streamflows in the NWA. For the EA region, above normal conditions have been observed during the April(0)–November(0) period and the May(0)–February(+) period for the El Nin ̃o and La Nin ̃a events, respectively. Moreover, the annual cycle analysis somewhat implies a modulation of regional streamflow in the core regions during the signal period. In some parts of the core regions, correlation results confirm these signals. The results of this study are discussed and are said to be in agreement with the most relevant previous studies concerning precipitation to understand complex global circulation dynamics. In conclusion, mid-latitude streamflow responses to the extreme phases of SO are detectable over Turkey.

KEY WORDS: annual cycle; correlation analysis; El Nin ̃o; harmonic analysis; La Nin ̃a; Southern Oscillation; streamflow; Turkey

Streamflow and La Nina event relationships in the ENSO-streamflow core areas

by Ercan Kahya

The Relationships Between US Streamflow and La Nina Events

by Ercan Kahya

Critical Drought Analysis: Case Study of Göksu River (Turkey) and North Atlantic Oscillation Influences

by Ercan Kahya

Co-authored with Nermin Şarlak and Osman Anwar Bég.
Published in Journal of Hydrologic Engineering, Vol. 14, No. 8 (2009).

Droughts are complex events which may impair social, economic, agricultural, and other activities of a society. The... more Droughts are complex events which may impair social, economic, agricultural, and other activities of a society. The present hydroclimatological study comprises three stages. First, a Markov chain model based on annual flows at a gauging station located on the Göksu River 􏰀Turkey􏰁 is utilized. Second, a critical drought analysis is conducted. Third, the influences of the North Atlantic Oscillation 􏰀NAO􏰁 on the probability distribution functions 􏰀PDFs􏰁 of critical droughts have been documented. Drought duration is used as a key parameter in the Markov chain model. As the model degree should be defined prior to applying a Markov chain model to a series of observations, a model degree of first order has been selected according to the results of the Akaike information criteria and Bayesian information criteria. The exact PDFs of the critical drought duration in a finite sample that follows the first-order Markov chain have been determined by the enumeration technique. The critical drought duration is the possible maximum duration likely to occur over the economic life of any water resources system. The expectations of the critical drought duration are provided for different transitional probabilities. The influences of NAO on the PDFs of critical droughts have been examined for two opposite cases; namely, the period of negative NAO index values 􏰀1936–1971􏰁 and the period of positive NAO index values 􏰀1972–2000􏰁. The results of this analysis clearly indicate that NAO series have quantifiable influences on the transition probabilities and expectation of the critical drought duration. As a result, the probability distribution functions obtained for various critical drought durations in the observations of the Göksu River may be implemented as a robust tool to determine a design value with the concept of risk.

Trend analysis in Turkish precipitation data

by Ercan Kahya

- Co-authored with T. Partal,
- Published in Hydrological Processes, Vol. 20, No 9, 2011-2026, 2006.

This study aims to determine trends in the long-term annual mean and monthly total precipitation series using non-... more This study aims to determine trends in the long-term annual mean and monthly total precipitation series using non- parametric methods (i.e. the Mann–Kendall and Sen’s T tests). The change per unit time in a time series having a linear trend was estimated by applying a simple non-parametric procedure, namely Sen’s estimator of slope. Serial correlation structure in the data was accounted for determining the significance level of the results of the Mann–Kendall test. The data network used in this study, which is assumed to reflect regional hydroclimatic conditions, consists of 96 precipitation stations across Turkey. Monthly totals and annual means of the monthly totals are formed for each individual station, spanning from 1929 to 1993. In this case, a total of 13 precipitation variables at each station are subjected to trend detection analysis. In addition, regional average precipitation series are established for the same analysis purpose. The application of a trend detection framework resulted in the identification of some significant trends, especially in January, February, and September precipitations and in the annual means. A noticeable decrease in the annual mean precipitation was observed mostly in western and southern Turkey, as well as along the coasts of the Black Sea. Regional average series also displayed trends similar to those for individual stations.

KEY WORDS: precipitation; Mann–Kendall; Sen’s T; trend analysis; Turkey

Trend analysis of streamflow in Turkey

by Ercan Kahya

- Co-authored with S. Kalaycı,
- Published in Journal of Hydrology, Vol. 289, 128-144, 2004.

This paper presents trends computed for the 31-year period of monthly streamflows obtained from 26 basins over Turkey.... more This paper presents trends computed for the 31-year period of monthly streamflows obtained from 26 basins over Turkey. Four non-parametric trend tests (the Sen’s T, the Spearman’s Rho, the Mann-Kendall, and the Seasonal Kendall which are known as appropriate tools in detecting linear trends of a hydrological time series) are adapted in this study. Moreover, the Van Belle and Hughes’ basin wide trend test is included in the analysis for the same purpose. Homogeneity of trends in monthly streamflows is also tested using a procedure developed by Van Belle and Hughes. Thus, this study includes a complete application of both the Van Belle and Hughes’ tests for homogeneity of trends and basin wide trend (originally developed for trend detection in water quality data) on a hydroclimatic variable. As a result, basins located in western Turkey, in general, exhibit downward trend, significant at the 0.05 or lower level, whereas basins located in eastern Turkey show no trend. In most cases, the first four tests provide the same conclusion about trend existence. Use of the Seasonal Kendall, which involves a single overall statistic rather than one statistic for each season, is justified by the homogeneity of trend test. Moreover, some basins located in southern Turkey exhibit a global trend, implying the homogeneity of trends in seasons and stations together, based on the Van Belle and Hughes’ basin wide trend test.

Keywords: Climate change; Mann-Kendall test; Non-parametric tests; Streamflow variability; Trend analysis; Turkey

Assessment of streamflow variability modes in Turkey: 1964-1994

by Ercan Kahya

- Co-authored with "S. Kalaycı and T.C. Piechota"
- Published in Journal of Hydrologic Engineering, Vol. 13, No 4, 205-214, 2008.

Homogeneous streamflow regions in Turkey were identified using a multivariate statistical tool; namely, principal... more Homogeneous streamflow regions in Turkey were identified using a multivariate statistical tool; namely, principal components analysis 􏰀PCA􏰁 with a varimax orthogonal rotation. The rotated PCA was applied to monthly streamflow records of 78 gauging stations during the period 1964–1994. The first seven rotated components were found significant at the 0.05 or lower levels. All accounted for 68% of the total variance of the original data assembly. The different groups of highly intercorrelated streams were defined by the patterns of map drawn for the rotated components loadings. Furthermore, an annual cycle analysis was applied to the same monthly streamflow data. Harmonic vectors extracted from a series of 12 monthly means at each station were marked over a map to readily keep track of streamflow seasonal variations across the study domain. The basic statistical characteristics of streamflow were also investigated within the identified regions; thus, some similarities and dissimilarities between the regions were documented. The identified streamflow regions were shown to be consistent with climate zones in Turkey defined by a recent study based on cluster analysis of temperature and precipitation data.

HYDROLOGIC HOMOGENEOUS REGIONS USING MONTHLY STREAMFLOW IN TURKEY

by Ercan Kahya

Co-authored with. "M.C. Demirel and A.O. Bég"
Earth Sciences Research Journal, Vol. 12, No 2, 181-193, 2008.

Cluster analysis of gauged streamflow records into homogeneous and robust regions is an important tool for the... more Cluster analysis of gauged streamflow records into homogeneous and robust regions is an important tool for the characterization of hydrologic systems. In this paper we applied the hierarchical cluster analysis to the task of objectively classifying streamflow data into regions encompassing similar streamflow patterns over Turkey. The performance of three standardization techniques was also tested, and standardizing by range was found better than standardizing with zero mean and unit variance. Clustering was carried out using Ward’s minimum variance method which became prominent in managing water resources with squared Euclidean dissimilarity measures on 80 streamflow stations. The stations have natural flow regimes where no intensive river regulation had occurred. A general conclusion drawn is that the zones having similar streamflow pattern were not be over- lapped well with the conventional climate zones of Turkey; however, they are coherent with the climate zones of Turkey recently redefined by the cluster analysis to total precipitation data as well as homogenous streamflow zones of Turkey determined by the rotated principal component analysis. The regional streamflow information in this study can significantly improve the accuracy of flow predictions in ungauged watersheds.

Key words: Cluster analysis, Ward’s method, streamflow, homogeneous region, regionalization, Turkey

A Cloud-based Virtual Observatory for Environmental Science

by Yehia El-khatib

Drainage network detection and assessment of network storage capacity in agrarian landscape

by Paolo Tarolli

Published in Hydrological Processes, 2012. Co-authored with Federico Cazorzi, Giancarlo Dalla Fontana, Alberto De Luca, Giulia Sofia

Drainage networks in agrarian landscape within floodplains constitute surface's discontinuities that are expected to... more Drainage networks in agrarian landscape within floodplains constitute surface's discontinuities that are expected to affect hydrological response during floods. Drainage network recognition and quantification of water storage capacity within channels are, therefore, crucial for watershed planning and management. These evaluations require accurate spatial information for the area of interest and in most cases, when studying large catchments, broad datasets of ditches locations and descriptions are not available. In order to characterize drainage networks for large areas, the availability of high resolution topography derived by airborne laser scanner (LiDAR) represents a new and effective tool. Nowadays LiDAR DTMs covering large areas are readily available for public authorities, and there is a greater and more widespread interest in the application of such information for the development of automated methods aimed at solving geomorphological and hydrological problems. While LiDAR DTMs reliability in steep landscape has been proven by several recent studies, only few researches have been conducted to take into account the effectiveness of these data in agrarian low relief landscapes. The goal of this research is to propose a semi-automatic approach based on a LiDAR DTM to (1) detect drainage networks in agrarian/floodplain contexts, and (2) to estimate some of the network summary statistics (network length, width, drainage density and storage capacity). The procedure is applied in two typical alluvial-plain areas in the North East of Italy, and tested comparing automatically derived network with surveyed ones. The results underline the capability of high resolution DTMs for drainage network detection and characterization in the context of agrarian landscapes within floodplains, opening at the same time new challenges to evaluate some hydrological processes in these areas

Analysis of flash flood regimes in the North-Western and South-Eastern Mediterranean regions

by Paolo Tarolli

Published in Natural Hazards and Earth System Sciences, 2012. Co-authored with Marco Borga, Efrat Morin, Guy Delrieu.

This work analyses the prominent characteristics of flash flood regimes in two Mediterranean areas: the North-Western... more This work analyses the prominent characteristics of flash flood regimes in two Mediterranean areas: the North-Western Mediterranean region, which includes Catalonia, France and Northern Italy, and the South-Eastern Mediterranean region, which includes Israel. The two regions are characterized by similarities in the hydro-meteorological monitoring infrastructure, which permits us to ensure homogeneity in the data collection procedures. The analysis is articulated into two parts. The first part is based on use of flood peak data, catchment area and occurrence date for 99 events (69 from the North-Western region and 30 from the South-Eastern region). Analysis is carried out in terms of relationship of flood peaks with catchment area and seasonality. Results show that the envelope curve for the South-Eastern region exhibits a more pronounced decreasing with catchment size with respect to the curve of the North-Western region. The differences between the two relationships reflect changes in the effects of storm coverage and hydrological characteristics between the two regions. Seasonality analysis shows that the events in the North-Western region tend to occur between August and November, whereas those in the South-Eastern area tend to occur in the period between October and May, reflecting the relevant patterns in the synoptic conditions leading to the intense precipitation events. In the second part, the focus is on the rainfall-runoff relationships for 13 selected major flash flood events (8 from the North-Western area and 5 from the South-Eastern area) for which rainfall and runoff properties are available. These flash floods are characterised in terms of climatic features of the impacted catchments, duration and amount of the generating rainfall, and runoff ratio. Results show that the rainfall duration is shorter and the rainfall depth lower in the South-Eastern region. The runoff ratios are rather low in both regions, whereas they are more variable in the South-Eastern area. No clear relationship between runoff ratio and rainfall depth is observed in the sample of floods, showing the major influence of rainfall intensity and the initial wetness condition in the runoff generation for these events.

Can silvicultural treatments improve the water economy?

by Jerome Vanclay

Vanclay, J.K., 2010. Can silvicultural treatments improve the water economy? Forêt Méditerranéenne 31(4):366-368

Four facts about water and forests are well-known and commonly accepted, but conflict with the commonly held view that... more Four facts about water and forests are well-known and commonly accepted, but conflict with the commonly held view that trees use water to the detriment of water catchments: (1) trees can transpire relatively large amounts of water, which is often considered “lost”; (2) cloud condensation nuclei produced by forest canopies mean that forests may be important in clouds formation; (3) the atmosphere holds relatively little moisture, setting a limit to the amount of transpired water than can be retained in the atmosphere; and (4) and most water vapour in the atmosphere does not travel far before it falls back to earth.
The apparent contradictions amongst these four points pose the question: what is the fate all the water “lost” from trees if it is not retained in the atmosphere, doesn’t travel far, and is likely to be condensed over forest? Is evapotranspiration “lost” or does it fall nearby as rain? These are important questions, but are infrequently addressed because relatively few researchers take a broad systems view that includes the atmosphere, and a narrower focus on individual trees can lead to a different (and potentially misleading) conclusion.

FRIEND 2010–footprints of International Cooperation Over a Quarter Century

by Jérôme Gandin

Dynamique d'occupation anthropique et dynamique alluviale du Rhin au cours de l'Holocène: géoarchéologie du site d'Oedenburg (Haut-Rhin, France)

by Vincent Ollive

Human settlement dynamics and alluvial dynamics of the Rhine River during the Holocene: Geoarchaeology of the site of Oedenburg (Haut-Rhin, France).

Author: V. Ollive
PhD Thesis in Earth Sciences, University of Dijon (2007). 305p.
Thèse de Doctorat de Sciences de la Terre, Université de Dijon. 305p.

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A segment of the Upper Rhine River presenting both anastomosed and braided was investigated. This area gives us the... more A segment of the Upper Rhine River presenting both anastomosed and braided was investigated. This area gives us the opportunity to study settlement dynamics during two key periods of the Holocene. The Roman Period characterised by a first major anthropogenic impact on the environment and poorly documented climatic changes. The Little Ice Age, when human settlement and climate may be well documented with high reliability.
A wide methodological approach has been completed to combine results from geomorphology, stratigraphy, geophysics, G.I.S., palynology, dendrochronology and archaeology. This dataset shows:
1/ documentary flood frequency variations of the Rhine River during the last 800 years. These chronological variations show some dissimilarity with other French rivers reconstructed documentary flood series. That attests a primary control of catchment’s characteristics and regime type on long term hydrological dynamics.
2/ changes in the Rhine River and floodplain dynamics during Little Ice Age before major anthropogenic river management of 19th century. Planform changes, riverine forest, land use evolution, and anthropological settlement processes are described and quantified. These results allow the identification of major stratigraphic agent factors role responsible of these changes.
3/ the alluvial landscape character where the Oedenburg site settled, together with the evolution of the riverine forest influenced by the Rhine River hydrodynamics.
4/ settlement dynamics during Antiquity. Four major settlement phases have been evidenced by spatial distribution analysis of roman coins found by pedestrian prospecting during the last twenty years. Settlement dynamics may have been influenced by either historical or environmental impacts.
5/ variations in alluvial dynamics of the Rhine River during Antiquity such as high water table or flooding events.

Watershed-scale analysis of riparian buffer function

by Molly Van Appledorn

Flood Pulses and Restoration of Riparian Vegetation in the American Southwest

by Matt Chew

Ch. 2 in Middleton, Beth A., Ed. 2002. Flood Pulsing in Wetlands: Restoring the Natural Balance. John Wiley & Sons, Inc.

[From CAB Abstracts] Riparian areas in southwestern American deserts support native plant communities that are more... more [From CAB Abstracts] Riparian areas in southwestern American deserts support native plant communities that are more dense, structurally complex, productive and species-rich than those of the adjacent xeric uplands. Riparian areas attract human settlement and, as a result, riparian exotic plants have become an issue. The factors that have facilitated the influx of exotics into riparian corridors and the ways in which riparian ecosystem functions have been affected are discussed (with the impact of saltcedars (Tamarix spp.) as an example). High exotic abundance may be aided by flooding (enhancing dispersal and establishment opportunities), the introduction of grazing animals and other anthropogenic influences, e.g. road development and agriculture. Some woody exotics are now widespread and locally abundant in southwestern riparian areas, including Elaeagnus angustifolia, Ailanthus altissima, Tamarix spp. and Nicotiana glauca. There are probably over 100 naturalized exotic herbaceous species, the most successful of which are probably the grasses, e.g. Cynodon dactylon, Sorghum halepense, Polypogon monspeliensis and Bromus spp. Constructive options for restoring functionality to southwestern desert riparian ecosystems are discussed.

Cycling DDT and homologues at basin scale: the case of Lake Maggiore.

by pietro volta

Galassi, S., P. Volta, L.Guzzella & A. Calderoni. 2006. Journal of Limnology, 65(2): 100-106.

Effects on flood hazard in Marathon plain from the 2009 wildfire in Attica, Greece

by Michalis Diakakis

Effects on flood hazard in Marathon plain from the 2009 wildfire in Attica, Greece

by Michalis Diakakis