Analysis of the level of dissolved Oxygen in the water of several places of one Dam

Water oxygen concentration is often used for the contamination control and to study the age of a reservoir. By following this variable we can determine the temporal evolution and the spatial differences in the environmental conditions of the reservoirs. To carry out the analysis of the oxygen concentration, several sites are selected in the reservoir, and measures of the interest variable are taken at certain periods. The analysis of these data provides the conclusions about the reservoir conditions. Now, the characteristics of these data don't allow using techniques like classic regression analysis, because the dependence among the observations of a same site. This paper suggests the use of the Hierarchical Linear Models (Mason, Wong and Entwistle, 1983; Bryck and Raudenbush, 1992) as a useful statistical tool for the analysis of this kind of problems. Its application on sample data for the surface of 9 sites of the Pun china reservoir, showed that the mean level of the oxygen seems to change with the site position. We found that the proportion of the variance in oxygen concentration explained by the site location was about 25%