Zekâi Şen

Run-sums of annual flow series

Abstract A general methodology which is very useful for determining various run-sum properties of a given hydrologic process, has been developed on the basis of the random sum of random variables. Application of the method to some independent and dependent processes has been given.

Autorun analysis of hydrologic time series

Abstract A new methodology which is referred to as the “autorun analysis” has been developed to investigate the sequential properties of a hydrologic time series. The relevance of the methodology as a potential tool in water-engineering applications as well as in statistics, is due to its distribution-free behaviour. The autorun analysis yields the same results as those of the autocorrelation analysis in the case of normally distributed random variables only. An application of the autorun analysis to some mean annual flow series has been performed.

Regional drought and flood frequency analysis: Theoretical consideration

Abstract The problem of regional drought and flood analysis has been approached theoretically on the basis of random fields. Necessary formulations for the regional drought descriptors such as the deficit area, the total areal deficit and the maximum deficit intensity have been derived. Flood descriptors can be obtained accordingly.

Trend Identification Simulation and Application

AbstractTrend analysis occupy a significant role in the climate change studies for almost three decades. It is significant to try and identify monotonic trends in a given time series so as to make future predictions about the possible consequences on the urban environment, water resources, agriculture, and many other socioeconomic aspects of life. Although there are now classically accepted and frequently used trend tests in the open literature, such as Mann-Kendall trend analysis and Spearman’s rho test, they are based on some restrictive assumptions as normality, serial independence, and rather long sample sizes. Also, they search for a single monotonic trend without any specification such as low, medium, and high values, which may have different trend patterns. Many climatological records have serial dependence, and therefore, it is very helpful to provide a methodology that is not affected from such a restriction. It is the main purpose of this paper to provide simulation results and applications of a...

Critical drought analysis of periodic-stochastic processes

Abstract A general method is presented for finding the exact probability distribution function of the longest drought duration in a finite sample of any process on the basis of runs theory and enumeration technique. The methodology developed is applied to some hypothetical and observed monthly river flow sequences at various parts of Turkey.

Climate change impact and runoff harvesting in arid regions

The most significant large-scale environmental challenge that many countries, especially in the arid and semi-arid regions of the world, will face in the middle and long-term are water scarcity problems, which are attributed to climate change impacts such as temperature increase, abundance of high solar radiation, and aridity in addition to population pressure. In many countries, current water resources use already exceeds sustainable and renewable supply. Various methodologies are suggested to increase the sources of water supply, among which one of the alternatives is rainwater and runoff harvesting (ROH). Water scarcity and additional stress are among the most specific problems in arid and semi-arid regions, where vegetation cover is very weak under extensive solar irradiation effects with high evaporation rates. Present global warming, climate change impacts, and their future patterns are expected to cause increase in the evapotranspiration rates and hence reduction in the groundwater recharges. Under such circumstances, any simple but effective water storage augmentation facility as the artificial groundwater recharge gains vital importance for sustainability of water supply and survivals in desert ecosystems. Although intensive and frequent rainfall events are rare they generate significant surface water flow during occasional floods and especially flash floods with huge amounts of surface water. It is, therefore, necessary to enhance artificial groundwater recharge from consequent frequent runoffs through suitable hydraulic structures. This paper aims at assessing the importance of ROH systems for domestic supply in arid regions with specific reference to the Kingdom of Saudi Arabia. For this purpose, it presents ROH from the surface flows in depressions of Quaternary wadi deposits in arid and semi-arid regions.

Geostatistical modelling of the Wasia aquifer in Central Saudi Arabia

Abstract Aquifers in sedimentary basins provide a regional domain for the spatial variabilities in geologic, hydrologic, geomorphologic and hydrochemical phenomena. Their study should account for this spatial variability within the study area prior to any formal modelling. A cumulative semivariogram scheme is adopted in this paper for the spatial variability, which is then incorporated with the kriging technique to provide maps of regional variation concerning variables such as storativity, transmissivity, piezometric levels, total dissolved solids and groundwater flow velocity. It is shown that the classical semivariogram models are not capable of accounting for the spatial variability of the Wasia aquifer. Comparison between the cumulative and classical semivariograms are given on the basis of hydrogeologic variables observed in the field. It is concluded, in general, that the cumulative semivariogram modelling of the spatial variability is more effective and yields realistic regional variables.

Spatio-temporal modeling of 210Pb transportation in lake environments

Radioactive particle movement analysis in any environment gives valuable information about the effects of the concerned environment on the particle and the transportation phenomenon. In this study, the spatio-temporal point cumulative semivariogram (STPCSV) approach is proposed for the analysis of the spatio-temporal changes in the radioactive particle movement within a surface water body. This methodology is applied to the (210)Pb radioactive isotope measurements at 44 stations, which are determined beforehand in order to characterize the Keban Dam water environment on the Euphrates River in the southeastern part of Turkey. It considers the contributions coming from all the stations and provides information about the spatio-temporal behavior of (210)Pb in the water environment. After having identified the radii of influences at each station it is possible to draw maps for further interpretations. In order to see holistically the spatial changes of the radioisotope after 1st, 3rd and 5th hours, the radius of influence maps are prepared and interpreted accordingly.

Adaptive Fourier analysis of periodic-stochastic hydrologic sequences

Abstract General formulation of adaptive parameter and state estimations has been fully developed on the basis of Kalman filters, given a periodic-stochastic model coupled with an observation model. It has been successfully applied to decompose a given monthly flow sequence into periodic and stochastic components. The results show a definite advantage of using adaptive Fourier analysis over conventional analysis.

Potential utilization of the absolute point cumulative semivariogram technique for the evaluation of distribution coefficient.

The classical solid/liquid distribution coefficient, K(d), for radionuclides in water-sediment systems is dependent on many parameters such as flow, geology, pH, acidity, alkalinity, total hardness, radioactivity concentration, etc. in a region. Considerations of all these effects require a regional analysis with an effective methodology, which has been based on the concept of the cumulative semivariogram concept in this paper. Although classical K(d) calculations are punctual and cannot represent regional pattern, in this paper a regional calculation methodology is suggested through the use of Absolute Point Cumulative SemiVariogram (APCSV) technique. The application of the methodology is presented for (137)Cs and (90)Sr measurements at a set of points in Keban Dam reservoir, Turkey.