GPFARM is a USDA-ARS decision support system (DSS) for strategic (long-term) planning.  This study evaluated GPFARM performance for comparing alternative dryland no-till cropping systems and established limits of accuracy for eastern Colorado locations.  Data used in the GPFARM evaluation were collected in 1987 through 1999 from an on-going long-term experiment at three locations along: 1) a gradient of potential evapotranspiration (PET) [Sterling, low PET; Stratton, medium PET; and Walsh, high PET]; and 2) across summit, sideslope, and toeslope landscape positions at all three sites.  Crops included winter wheat (W) [Triticum aestivum (L.)], corn (C) [Zea mays (L.)], sorghum (S) [Sorghum bicolor (L.) Moench], proso millet (M) [Panicum miliaceum (L.)], and varying fallow (F) periods; crop rotations included WF, WCF, WCMF, WSF, and WSMF.  Ranges of relative error (RE) of simulated mean and root mean square error (RMSE) were calculated for total soil profile water content, dry mass grain yield, dry mass crop residue, and total soil profile residual nitrate-N. GPFARM simulations agreed with observed trends and showed that productivity and water use efficiency increased with cropping intensification and downslope landscape position.