Jihane Ahattab et al. Int. Journal of Engineering Research and Applications www.ijera.com ISSN : 2248-9622, Vol. 5, Issue 4, ( Part -1) April 2015, pp.01-07 www.ijera.com 1 | Page Mapping Gradex values on the Tensift basin (Morocco) Jihane Ahattab*, Najat Serhir**, El Khadir Lakhal* *(Department of Physics, Semlalia sciences Faculty, Cadi Ayyad University, Marrakech, Morocco) ** (Department HEC, Ecole Hassania des Travaux Publics, Casablanca, Morocco) ABSTRACT The aim of this study is to elaborate the cartography of Gradex parameter used in the Gradex method for estimating flood peaks in order to size hydraulic structures. Map of spatial variation is elaborated using the geostatistical method of kriging. Several reference functions (exponential model, spherical, linear, Gaussian and cubic) were used for modeling the kriging variogram. Cross- validation enabled a comparison between the results of these models and choice of spherical model with anisotropy and trend fit by a second-order polynomial as the most suitable. The use of available series of annual maximum daily rainfall recorded at 23 rainfall stations, distributed over the Tensift basin, led also to develop the cartography of standard prediction errors‟ values associated to the predicted parameter for each point of Tensift basin. These errors vary from acceptable values (16.8%) to very high ones depending on the density of the rainfall stations at the desired site. Keywords - Cartography, flood peaks, Gradex, IDF, kriging. I. INTRODUCTION Gradex method is one of the most used methods for estimating flood peaks in order to size hydraulic structures in Morocco. It is based on a regional parameter calculated from the information provided by the rain. But precipitation data usually comes from observations registered at rainfall stations and which do not allow calculating this parameter at any given point (the site of a project). Thus, the engineer resorts to the usual interpolation methods for estimating this parameter for the desired site: Thiessen polygons, linear interpolation IDWA (weighting with the inverse square distance). These interpolation methods ignore the spatial structure of the variable and may omit very specific local situations (areas of high or very low values). Also, no statistical criterion to judge the accuracy of the results obtained is formulated [1]. So the aim of this study is developing an accurate mapping of the Gradex parameter using the method of kriging. The overall purpose is to make the usual methods used to estimate flood peaks more reliable in order to have a more secure design of hydraulic structures (dams, bridges, Road crossing…) [2]. The choice of this geostatistical tool is based on the fact that the Kriging is a spatial interpolation method that takes into account both the geometrical configuration of the observed points and the spatial dependence of the data structure, unlike interpolations regression. This method has also the advantage of quantifying the errors related to the predicted values [3]. II. STUDY AREA AND DATA The Tensift basin is located between latitudes 32°10' and 30°50' North and longitudes 9°25' and 7°12' West, around the city of Marrakech in the western center of Morocco. It is drained by the Tensift River which flows from east to west for over 260 km. The basin extends over 19400 km 2 . Its vegetation is generally poor and depends on the topography and the nature of land. The climate is semi-arid influenced by the presence of high altitudes (the High Atlas). The altitude ranges from 0 to 4167m NGM with an average altitude of 2014m. "Fig.1" Rainfall is generally low and characterized by high spatial and temporal variability. The annual average rainfall is about 200 mm in the plains and more than 800 mm on the peaks of the Atlas [4]. Available data used are the series of annual maximum rainfall available for durations of 1day, 2 days, 3 days, 4 days and 5 days. These series were recorded using rain gauges at the 23 rainfall stations that are located all aver the Tensift basin at altitudes ranging from 53m to 2230m NGM. The series‟ lengths vary from 14 years to 44 years (since 1967 until 2011). The location of these stations is shown in "Fig.1". RESEARCH ARTICLE OPEN ACCESS