This research had been conducted for calculation of water balance components by Thornthwaite Mather method and simulation model WATBAL model had been used to calculate different water balance components like soil moisture, actual evapotranspiration, surplus, deficit, moisture adequacy index and soil moisture index by using average weekly precipitation and potential evapotranspiration from 1981-2020 for Tarai region of Uttarakhand. In this study, different weather-based indices like, Humidity Index, Aridity Index and Moisture Index also been calculated on the seasonal and annual basis. Based on which climatic classification had been done for Tarai region of Uttarakhand as well as it is of International significance as based on this study water stress condition for the crops and advisories could be done to the farmers for the Tarai region of the world. From, this study it has been concluded that humid climate exists in this region. The results also revealed that there is an increase in the water surplus during 1981-2020 when compared with IMD data for Pantnagar during 1971-2005. Water Surplus was found to be 670.0 mm and water deficit was found to be 440.2 mm. Total potential evapotranspiration was found to be 1339.4 mm, which was calculated by Penman Monteith’s equation during 1981-2020 and was found to be decreased when compared with the PET calculated by IMD for the year 1971-2005 of about 1463.9 mm. When compared with the IMD data over the periods from 1971-2005 with current data from 1981-2020 for Pantnagar region then humidity index had been decreased from 149.0% to 144.7% respectively. Aridity index had been decreased from 73.0% to 50.2%, as water deficit had been decreased from 566.7 mm to 440.2 mm. Moisture index had been increased from 76% to 94.5% as water surplus was increased from 447.7 mm to 670.0 mm respectively on annual basis. Keywords: WATBAL, PET, Thornthwaite Mather, Penman Monteith, Water surplus