A deterministic model will always produce the same answer when given the same input. This approach does not take into account any uncertainty in the input data, the way results are calculated, or other areas that may affect results. This model may be based on physical relationships. Examples of deterministic models include the Antecedent Precipitation Index model and the Sacramento Soil Moisture Accounting model.
With any deterministic model, a given input dataset will always produce the same results. For instance, say a 20 millimeter rainfall happens on a parcel of land. As long as the soil moisture conditions are the same, the resulting hydrograph will always be the same. There is only one possible result.