A method and system for providing hydraulic modelers with the ability to accurately produce a surface representation of flow data that incorporates transitional elements in the underlying gravity hydraulic network is provided. The method includes triangulating surface techniques and smooth surface techniques that employ functions that enforce rules that preserve monotonicity as well as to preserve Q=0 when TW=HW. The value of Q can be calculated as well as the partial derivatives in the TW and HW directions for a point of interest. A smooth surface interpolation of the data, which maintains the above-noted conditions, and allows the quick evaluation of flow for any point in the data set, is provided.