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The State of Texas has identified a number of water bodies having water quality problems and classified them for Total Maximum Daily Load (TMDL) development. A TMDL program has been initiated for the North Bosque River Watershed in Texas, USA, where point and nonpoint sources of pollution are of a concern. Impairment was determined under narrative water quality criteria related to excessive aquatic plant growth.

A watershed modeling approach was used to quantify the effects of phosphorus control measures on stream water quality. The watershed simulation model, Soil and Water Assessment Tool (SWAT), was validated for flow, sediment, and nutrients. The validated model was applied to quantify the effects of management practices related to dairy manure management and municipal wastewater treatment plant effluent. This paper describes the impact of these practices on phosphorus. The benefits of phosphorus control measures for wastewater treatment plants resulted in greater improvement in stream concentrations, while control measures on dairies made greater difference on phosphorus loadings. Results were used to inform the TMDL stakeholder group of possible controls that could reduce phosphorus loadings and concentrations in the Bosque River Watershed. This SWAT application and these assessment methods are adaptable to similar TMDL/water quality projects in other parts of the United States.