ESTIMATING NITRATE–N LOSSES FROM SUBSURFACE DRAINS USING VARIABLE WATER SAMPLING FREQUENCIES

Abstract

Growing concern about nitrate–N in effluent from subsurface drains requires technically and statistically valid water quality data. However, intensive monitoring with frequent (daily or more) sampling and analysis of nitrate–N concentrations is expensive and difficult. The objective of this study was to determine the accuracy and precision of nitrate–N loss estimates from subsurface drainage plots based on varying sampling frequencies. A case study was performed for the 5, 10, and 20 m drain spacings at the Southeast Purdue Agricultural Center for three years. Concentration values representing 7–, 30–, and 90–day sampling frequencies were combined with continuous flow data to obtain mass loss estimates, using all possible combinations or random (Monte Carlo) selections of samples, depending on the number of all combinations. Estimates were compared to the “true” mass loss (based on flow–proportional sampling using all available concentration measurements) to obtain probability distributions and 95% confidence intervals of mass loss estimates. On average over these three spacings for three years, the probability of estimating the annual mass loss within .15% of the “true” mass loss was 92% for the weekly sample frequency, 68% for the monthly (30–day) sampling frequency, and 51% for the 90–day frequency. Results of this study can help researchers select cost–effective sampling frequencies based on their specific accuracy requirement.