The Cover Crop Nitrogen Availability Calculator (CCNAC) is a simulation model used to estimate the nitrogen (N) contribution of cover crops to subsequent cash crops, and can be a useful tool to determine optimum N fertilizer recommendations. Research on CCNAC with surface cereal rye (Secale cereale L.) and crimson clover (Trifolium incarnatum L.) residues found that the water release curves, which model the relationship between residue water content and water potential, changed as decomposition progressed, and that osmotic potential contributed negligibly to total residue potential when residues reached 40 days of decomposition. Rain also affected residue water content, which increased when exposed to increasing amounts of rain. This research included a validation of CCNAC, which indicated that the model could be used to obtain reasonable estimates of dry biomass and N for crimson clover and cereal rye residues in Georgia.