Soil water supply plays a key role in driving switchgrass (Panicum virgatum L.) yield, and therefore is an important parameter for crop-model accuracy. ALMANAC (Agricultural Land Management Alternative with Numerical Assessment Criteria) model has been applied to simulating switchgrass growth with mixed results. The objective was to develop and test a calibration for ALMANAC simulating soil water dynamics in a switchgrass (‘Alamo’) stand under Arkansas conditions. Soil volumetric water content (SW) profiles were measured daily in switchgrass from May 2009 to February 2013. Soil, crop, and weather input data were developed based on in situ measurements. After identifying the most sensitive parameters in SW simulation, a calibration method was proposed, and the parameters initial soil water (FFC), permanent wilting point (U), field capacity (FC), sand content (SAN), silt content (SIL), pH, and maximum stomatal conductance (GSI) were modified. Daily SW simulation outputs from default and calibrated runs were compared to SW observations. Default Willmott agreement d-index values were lower than the calibrated d-index values in all years. Therefore, calibration improved simulation accuracy. Calibration accuracy was greater in 2009 and 2010 than in 2011 and 2012. Lower root mean square error of calibrated versus observed SW data confirmed the elevated d-index values. Lower accuracy in the latter years was related to drought periods when ALMANAC was unable to mimic switchgrass drought adaption by lowering GSI. Calibration of SW in ALMANAC was aided by using site-specific soil data. Improvement of SW calibration for drought-resistant plants may be achieved by quantifying GSI dynamics.