Spatial Precipitation Assessment With Elevation By Using Point Cumulative Semivariogram Technique

Water Resources Management 14: 311–325, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.

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Spatial Precipitation Assessment with Elevation by Using Point Cumulative Semivariogram Technique
ZEKAI SEN1∗ and ZEYAD HABIB2 ¸
1 Technical University of Istanbul, Civil Engineering Faculty, Hydraulics Division, Maslak 80626,

Istanbul, Turkey
2 University of Al-Fateh,Faculty of Science, Department of Meteorology, P.O. Box 13488, Tripoli,

Libya (∗ author for correspondence, e-mail: zsen@itu.edu.tr) (Received: 7 August 1997; in final form: 10 November 1998) Abstract. Depiction of precipitation change by elevation in an area indicates the possibilities of orographic rainfall occurrences in the mountainous regions. In general, precipitation increases with elevationbut sometimes inverse cases occur locally due to orographic and meteorological features of the area. In practice, prior to any quantitative modeling, qualitative assessments and interpretations help to have sound foundations in search for a suitable model. In this article, standardized point cumulative semivariogram (SPCSV) methodology is employed for identification of the precipitationelevationrelationship. According to relative positions of the precipitation and elevation SPCSVs four different precipitation categories are suggested. The application of the methodology proposed is presented for the precipitation records from Yucca Mountain, Nevada, U.S.A. Key words: areal precipitation, elevation, orography, point semivariogram, spatial variation

1. Introduction Water resourcesdevelopment and high-level nuclear-waste repository location require assessment of the precipitation potential of concerned area. In rather flat areas, areal average precipitation calculations play a significant role in such assessments but in mountainous areas, vertical variations of precipitation must be taken into consideration. In addition, as the hills are permanent features, it is the orography whichdetermines the distribution of the average annual precipitation (AAP) according to different but suitable patterns to the areal synoptical, topographical and hydrological conditions. In this article, the word ‘orography’ is used rather than the ‘altitude’ because we consider the effect of the shape of the hills in producing rising air. Clearly, an isolated hill will not give as high aprecipitation as a ridge of the same height extending for a considerable distance and set at right angles to the prevailing wind direction. For instance, with an isolated hill much of the wind can slip round on either side rather than be forced to rise. However, a ridge forms a barrier over which the air must rise and the rainfall increases to the summit and then decreases when the air is no longer forced torise. In the case of

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a series of ridges parallel to each other, the rainfall at the same height decreases starting from the first ridge because some rain is abstracted by the first ridge and the moisture content of the air decreases after each forced rise. The dependence of the AAP distribution on the orography is an important feature. The orographicaldistribution is so well marked that the AAP distribution in each annual map is similar to each other. It further requires a close inspection to see the areas with more or less than the regional average precipitation. Indeed, if two nearby station records are examined, it is observed that there is high correlation between them in addition to some local variations due to local thunderstorm precipitation.Matheron (1963) laid the foundations of quantifying regional variabilities within any event provided that there are sample measurements at distinct sites. It has been applied in mining for estimating the ore grades, recoverable reserves and in planning optimum operations. Collectively, these techniques are referred to as the ‘geostatistical’ methods and they are applicable throughout the earth...