The high variability of wet troposphere delay significantly degrades the precision of Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI). In order to remove effects of atmospheric gradient several anisotropic mapping functions (MacMillan, 1995; Chenand Herring, 1997) have recently been proposed. In these mapping functions atmospheric gradients are assumed to have a simple linear form. On the other hand, meso-scale and local-scale phenomena such as the passage of cold fronts, severe storms and land-sea breezes occur frequently in Japan. Thus, we have to evaluate the space-time variability of water vapor under these meteorological conditions quantitatively. For this purpose, we have been carrying out observations for detecting and characterizing variations in water vapor by using water vapor radiometers (WVRs) in the Kanto district of central Japan. Results of our observations are compared with precipitable water fields obtained by the Key Stone Project observation sites of Communications Research Laboratory (CRL) and the nation-wide GPS array of Geographical Survey Institute (GSI). Moreover, results from WVRs will be used to evaluate possible anisotropic mapping functions. Analysis of our results will be available for developing an optimum method to handle the anisotropy of the atmosphere.