To estimate modern condition of river runoff in Tien Shan alpine basins and to forecast its variability in relation with global and regional climate changes, the mean of the major water cycle characteristics (air temperature, precipitation, evapotranspiration and river runoff) have been simulated using data from 212 hydro-meteorological stations and 304 precipitation gauges. The mean evapotranspiration was calculated using data on air temperature, precipitation, and the topography aspects (including type of vegetation). The findings were simulated over Tien Shan relief using rectangular coordinates of the equivalent cone projection with standard parallels using 1:500 000 scale 100 m grid resolution Digital Elevation Model that covers 800 000 knotted points. Applicable GIS-based distributed River Runoff Models were implemented in regional conditions and tested in the Tien Shan basin, taking into account glacial melt, forest and irrigated areas. The parameterization between measured and simulated runoff occurred by least square method with discrepancy approximation using linear functions with multiple parameters: annual precipitation, snow and glacial runoff and first-derivative of previous year river runoff. The mean square discrepancy between measured and simulated runoff was in accordance with root-mean-square error of measured runoff ranging from 8% to 12%. The level of calculated evapotranspiration revealed the same level as the river runoff. Hypothetical climate-change scenarios in Tien Shan modeled as a stepwise progression predict an increase in air temperature of 1.8¨¬C - 4.4¨¬C (i.e., on average 3¬à¬³) and precipitation at 0.94 to 1.54 times (1.2 times) during the XXI Century, which will lead to a increase of river runoff by 1.047 times. If we assume that precipitation remains constant and air temperature increases by 5¬à¬³, the river runoff can decrease up to 0.66 due to increased evapotranspiration by 1.24 times.