Methods for developing naturalized monthly flows at gaged and ungaged sites

The state of Texas recently implemented a water availability modeling (WAM) system to support water management activities in its 23 river basins. Hydrology is represented in the modeling system by sequences of naturalized streamflows at all pertinent locations for each month of a several decade long period of analysis. Flows at stream gaging stations are adjusted to remove the effects of historical water resources development and use. The resulting naturalized flows are distributed to numerous ungaged sites of interest in modeling water management. Methods are incorporated into the WAM system for converting gaged flows to naturalized flows and transferring the flows from gaged to ungaged locations. Flow naturalization adjustments consist primarily of removing the effects of historical reservoir storage and evaporation, water supply diversions, and return flows from surface and groundwater supplies, and in some cases other considerations. The WAM system includes several alternative methods for distributing sequences of monthly naturalized flows from gaged to ungaged locations. The option most often used is based on the Natural Resource Conservation Service curve-number-based rainfall-runoff relationship. The methodologies for developing naturalized flows at gaged and ungaged sites incorporated in the Texas WAM system are generally applicable for similar modeling applications in other places.