Uncertainty in mass-balance estimates of regional irrigation-induced return flows and pollutant loads to a river

Time series of the statistics of daily NPS return flows and mass loading rates, within a four-year period, are estimated and discussed. Coefficients of variation in NPS mass loading rates range from about 32% to 134% for the five solutes considered. There is an estimated 95% probability that mass loading rates lie within a range equivalent to 109% to 405% of the temporally-averaged mean value, depending on the solute and the river reach. The sizeable uncertainty quantified in these findings should temper the interpretation, modeling, regulation, and management of irrigation-induced NPS loading in this and similar settings.