Temporal and spatial distribution of water components in watersheds, estimation of water quality, and uncertainties
associated with these estimations are important issues in freshwater studies. In this study, Soil and Water Assessment
Tool (SWAT) model was used to estimate components of freshwater availability: blue water (surface runoff plus deep
aquifer recharge), green water flow (actual evapotranspiration) and green water storage (soil water), in Hamadan-Bahar
watershed. Also, the Sequential Uncertainty Fitting program (SUFI2) was used to calibrate and validate the SWAT
model and do the uncertainty analysis. Degree of uncertainty is calculated by R-factor and P-factor parameters. In this
paper, results of calibration and validation are given for the river monthly discharge. In most stations, especially in
outlet of the watershed (Koshkabad station), simulation of river discharge was satisfactory. Values of R-factor in
calibration of monthly runoff were 0.4-0.8. These small values show good calibration of runoff in this watershed.
Values of P-factor were 20-60%. These small values show high uncertainty in estimations. For most stations of the
watershed, lack of data on river-water withdrawal caused poor simulation of base-flow and therefore the P-factor values
were low. Nash-Sutcliff (NS) coefficient was 0.3-0.8 after calibration, which shows good model calibration of outlet.
This study provided good information on the components of freshwater availability at spatial (sub-basin) and temporal
(monthly) scales with 95% prediction uncertainty ranges. The results of uncertainty analysis of components of
freshwater availability show that uncertainty ranges of average monthly blue water are larger than the other
components, because of its sensitivity to more parameters.
