The most important wastewater operation for farmers using lagoons to treat swine waste is maintaining the liquid level within the operational levels to avoid overflows and to satisfy the water demand for irrigation. This study describes a model developed to determine liquid levels of single-stage anaerobic lagoons. Using this model, swine producers can determine the best liquid handling for their operations. Inputs required are lagoon dimensions; hog numbers, sizes, sex, and place an breeding cycle; records of management decisions; and accurate measurements of temperature, precipitation, humidity, wind speed, and solar radiation. The model was calibrated and validated for three single-stage lagoons in different climatic regions within the state of Oklahoma. Simulation results indicate that the model predicted liquid level within 3% of the observed values. Determining evaporation proved to be the most critical component of the water balance for lagoons. Evaporation was found to follow the modified Penman equation. A linear equation to relate net radiation to solar radiation was found to be sufficiently accurate to estimate evaporation from the lagoons used in calibration and validation. A single set of evaporation coefficients accurately predicted lagoon evaporation for the examined set climatic regions, lagoon depths, and lagoon colors. Sensitivity of the model to errors in nine input parameters was determined for two sites by simulating model performance over a one year period. If accurate estimates of farm water use are available, then the model is most sensitive to errors in estimating precipitation and evaporation. Solar radiation proved to be the most sensitive single parameter for estimating surface evaporation.