WFM 6204: Hydrologic Statistics © Dr. Akm Saiful IslamDr. Akm Saiful Islam WFM-6204: Hydrologic Statistics Akm Saiful Islam Lecture-1: Characteristics of Hydrologic Data December, 2006 Institute of Water and Flood Management (IWFM) Bangladesh University of Engineering and Technology (BUET) WFM 6204: Hydrologic Statistics © Dr. Akm Saiful IslamDr. Akm Saiful Islam

WFM 6204: Hydrologic Statistics © Dr. Akm Saiful IslamDr. Akm Saiful Islam Review of Probability and statistics Population and Sample Central tendency: Mean, Median and Mode Range, Standard Deviations Skew ness and kurtosis Quartile and Percentile Histogram, Bar diagram, Box plots

WFM 6204: Hydrologic Statistics © Dr. Akm Saiful IslamDr. Akm Saiful Islam Characteristics of hydrologic data Observed hydrological data may be either deterministic or random.  Deterministic data can be described by an explicit mathematical relationship.  The bulk of hydrological data relating to the time history of the process is random or stochastic.  Random data has the property that each particular record is unique.  A set of time history records (x t ) is called an ensemble and it is said to be a random process if it can be described by statistical properties.

WFM 6204: Hydrologic Statistics © Dr. Akm Saiful IslamDr. Akm Saiful Islam Hydrological data are generally neither purely deterministic nor purely random. Most records contain both deterministic (periodic) components and random components. Again the random component is not often normally distributed (i.e., pure chance dependent), instead there may be a strong persistence or internal dependence effect. For example, a low flow corresponding to a very severe drought condition will never follow after a maximum flood observation.

WFM 6204: Hydrologic Statistics © Dr. Akm Saiful IslamDr. Akm Saiful Islam Hydrologic process Hydrologic processes are thought of as stochastic processes. Stochastic in this sense means involving a variate at each instant of time where the variate is a variable that may take on any of the values of a specified set with a certain probability. An example of stochastic hydrologic process is the annual maximum daily rainfall or peak flow of a river over a period of several years. The stochastic nature of the process however means that one can never estimate with certainty the exact value of the process (peak discharge) based solely on the past observation.