Reed canarygrass (Phalaris arundinacea) commonly invades meadow wetlands, effectively dominating water use and outcompeting native plants. Objectives of this study were to (i) estimate daily, seasonal and annual water use by reed canarygrass using shallow water table fluctuations; and (ii) calibrate the ALMANAC (Agricultural Land Management Alternative with Numerical Assessment Criteria) model to accurately simulate water uptake by this grass. Using a groundwater well, the water table under an area in Iowa dominated by reed canarygrass was monitored hourly. Differences between water level measurements taken each hour were averaged to determine the hourly water table change in each month. Using these estimates of water use, the ALMANAC model was then calibrated to simulate plant transpiration values close to these water table use rates. Average monthly calculated daily plant water use rates were 3.3 mm d-1 in July and 2.3-2.8 mm d-1 in May, June, August, and September. Simulated bimonthly values for measured water use and plant transpiration simulated by the ALMANAC model differed by 14% or less. From May to October the mean ratio of measured to simulated values was 94%. Thus, the similarity between simulated plant transpiration and water use from the water table showed promise that this process-based model can realistically simulate water use under such grassland systems. ?? 2007 Elsevier B.V. All rights reserved.