Effects of sampling strategy in rivers on load estimation for Nitrate-Nitrogen and total Phosphorus in a lowland agricultural area

•Longer sampling intervals lead to a bigger error of the load estimation.•The sampling frequency should be decided, respectively, for different elements based on the autocorrelation.•Both periodic sampling of discharge and nutrients leads to unreliable results.•The best sampling strategy for load estimation is a combination of continuous discharge monitoring and periodic nutrient concentration measurements.