Intensive, season-long monitoring of two agricultural drains that discharge into the San Joaquin River revealed significant temporal variability in concentrations of nutrients, salts, and turbidity over intra-daily scales, as well as significant seasonal and diel patterns. Total N values ranged from 0-60 ppm with a standard deviation of 7.81. Total P values ranged from 0-7420 ppb with a standard deviation of 751. Quantifying variability of agricultural tailwaters is of particular pertinence to the Agricultural Discharge Waivers Program which mandates growers and/or coalition groups monitor water quality in agricultural watersheds. Due to the expensive nature of water quality analysis, it is crucial to streamline monitoring protocols to minimize sample number without compromising accuracy. Statistical techniques were applied to our high resolution dataset in order to evaluate the optimum sample size needed to fall within a given confidence interval of the true seasonal mean, as well as to evaluate the efficacy of different sampling strategies (e.g. grab samples vs. composite samples.) The use of daily composite samples presents itself as a viable means to maintain accuracy and resolution, while diminishing required sample number for some constituents by 50%. Significant correlations between electrical conductivity, nitrate nitrogen, and total nitrogen, suggest that electrical conductivity may serve as a proxy for these constituents. The results of this study show that the widely used practice of bi-weekly sampling for water quality parameters is not sufficient to capture the variability of agricultural tail-water systems. This study will provide guidance for growers and water resource regulators to develop economically viable and science-based monitoring protocols for irrigated agriculture.