Soil and water are the two major resources in the Earth’s hydro biological and geological systems. The hydrology of arid areas
has become a topic of interest recently for hydrologists as water shortage at these areas can affect the agriculture, irrigation, and
industry as a whole. This has also prompted water resource planners to more thoroughly investigate water resource crisis at arid
areas. In this respect, the Soil and Water Assessment Tool (SWAT), a semi-distributed hydrological model, can be a subsidiary
tool to be used in the prediction of surface runoff (blue water). This paper presents the application of SWAT on the Roodan
watershed, which is located in the southern part of Iran and has 215 mm of annual precipitation. SWAT was engaged to know
more about the daily flow and to evaluate the runoff volume. Three continuous scenarios were defined over the 21 years (1988-
1992, 1993-2001, 2002-2008) for the land use map as it was found that continuous update of this layer were basically done
during these periods. Results of sensitivity analysis showed that parameters related to transmission losses are most sensitive for
this watershed. Furthermore, the SWAT had also visualized from the input data that the sub basins which have been designated
for agricultural activities from 1988 to 2008 were at the southwest, center and northeast parts of Roodan watershed. Strength of
modeling was evaluated by percentage of observations covered by the 95 Percentage prediction uncertainty (P-factor) and
relative width of 95 % probability band (R-factor). The P and R factors in this study were recorded at, for calibration and
validation periods, 50 % and 0.18 (calibration), and 50 % and 0.17 (validation) respectively. Nash-Sutcliffe and PBIAS obtained
for calibration period were 0.75 and 1.5 %, and those for validation period were 0.64 and 21 %. However, results showed an
underestimation trend for most peak flows during the modeling of daily stream flow. Nevertheless, the annual runoff volume for
calibration and validation periods depicted a promising performance and thus validated the usage of SWAT as a subsidiary
hydrological tool for water management projects attributed with stream flow and runoff volume.