Evaluation of water quality in reservoir in the salt affected soil of northeast   Chaiyanam Dissataporn and Pramote Yamclee   ABSTRACT        The statistic model for evaluation the salinity of water in the reservoirs situated in salt-affected soil was developed in order to employ as a guideline for water resources management. Water quality of 178 reservoirs situated from January1999-May1999 in Prayun district, Khonkaen province was statistically related to soil properties, hydrogeologic properties, land use and topographic properties.      Principal component analysis was employed for reducing the number of dependent variables and creating the small number of new variables. The results of the analysis grouped the 16 independent variables into 6 standardized explanatory principal components that can explain 82% of the total variation. Salinity from the apparent electrical conductivity at the depth of 7.5, 15 and 30 meter from soil surface were the most important soil properties and named primary source of salinization. Hydraulic properties of soil, flooding, surface runoff and drainage was the second principal component. Depth of groundwater table, elevation above mean sea level, electrical conductivity of groundwater, subsurface clay layer, landuse, salinity from salt affected soil map, hydrogeological unit and permeability of soil were grouped in other principal component.      The multiple regression of log base 10 of water quality in reservoir and the factor score of the retained principal components can explain about 72% of the total variation of water quality in reservoirs. From the magnitude and sign of the standardized coefficient beta of the principle component, it can seen that the apparent electrical conductivity of soil at the depth of 7.5, 15 and 30 m from soil surface is the major contribution to the salinity of water in reservoir. While few contribution derived from the less of the principal components.      From the statistical model, it can be use as a guideline for planning and management of the water resources in the salt-affected soil. Construction of reservoir in the salt-affected soil must be firstly considered on the apparent electrical conductivity of soil. It can readily and cheaply detect by EM-34. The other parameters such as hydraulic properties of soil, land use, hydrogeological unit and elevation above mean sea level can be interpreted from the available soil map, land use map, hydrogeological map as well as topographic map. The depth of groundwater and the electrical conductivity of groundwater are difficult to obtain, as it needs to collect from piezometer. For this reason the apparent electrical conductivity and hydraulic properties of soil with the elevation above mean sea level and landuse should be enough factors to consider before constructing reservoir in salt affected soil. The measures for preventing contamination of soluble salt into reservoir should be considered in case of construction reservoir in the salt-affected soil.        Keywords : prediction, conductivity, salinity