Discharge and nutrient uncertainty: implications for nutrient flux estimation in small streams

Abstract

13The measurement of discharge is fundamental in nutrient load estimation. Due to our ability to 14 monitor discharge routinely, it is generally assumed that the associated uncertainty is low. This paper 15 challenges this preconception, arguing that discharge uncertainty should be explicitly taken into 16 account to produce robust statistical analyses. In many studies, paired discharge and chemical 17 datasets are used to calculate 'true' loads and used as the benchmark to compare with other load 18 estimates. 19This paper uses two years of high frequency (daily and sub-hourly) discharge and nutrient 20 concentration data (nitrate-N and total phosphorus (TP)) collected at four field sites as part of the 21Hampshire Avon Demonstration Test Catchment (DTC) programme. A framework for estimating 22 observational nutrient load uncertainty was used which combined a flexible non-parametric approach 23 to characterising discharge uncertainty, with error modelling that allowed the incorporation of errors 24 which were heteroscedastic and temporally correlated. 25The results showed that the stage-discharge relationships were non-stationary, and observational 26 uncertainties from ±2-25% were recorded when the velocity-area method was used. The variability in 27 nutrient load estimates ranged from 1.1-9.9% for nitrate-N and from 3.3-10% for TP when daily 28 laboratory data were used, rising to a maximum of 9% for nitrate-N and 83% for TP when the sensor 29 data were used. However, the sensor data provided a better representation of the 'true' load as storm 30 events are better represented temporally, posing the question: is it more beneficial to have high 31 frequency, lower precision data or lower frequency but higher precision data streams to estimate 32 nutrient flux responses in headwater catchments? 33 34

Prediction of storm transfers and annual loads with data-based mechanistic models using high-frequency data

“…River water level was measured at 15 min resolution in the three catchments, with rating curves developed for discharge estimation (Outram et al, 2014;Perks et al, 2015;Lloyd et al, 2016b). TP concentration was determined in situ at 30 min intervals with a Hach Lange combined Sigmatax sampling module and Phosphax analyser using acid digestion and colorimetry (Jordan et al, 2007(Jordan et al, , 2013Perks et al, 2015).…”

“…A rigorous treatment of the uncertainties in highfrequency nutrient data and its subsequent impact on loads is given by Lloyd et al (2016b). For Newby Beck, where stage-discharge gaugings were available, the discharge uncertainty was estimated using the method of McMillan and Westerberg (2015), fitting multiple plausible rating curves and weighting with a likelihood function.…”

“…This method accounts for a mix of systematic and random measurement errors. The uncertainty of the phosphorus concentration measurements was estimated by comparing the time series from the bank-side analyser with the laboratory spot samples taken for ground-truthing (Lloyd et al, 2016b), fitting multiple regression curves and weightings according to McMillan and Westerberg (2015). The time series of discharge and TP concentration, with their uncertainty distributions, were then combined by resampling to give the measurement data uncertainties on the TP loads.…”

“…The time series of discharge and TP concentration, with their uncertainty distributions, were then combined by resampling to give the measurement data uncertainties on the TP loads. For the Wylye, discharge measurement uncertainties were estimated using a standard deviation of 10 %, the maximum value calculated by Lloyd et al (2016b) for the gauging site at Brixton Deverill using the method of Coxon et al (2015). Wylye discharges were combined with a standard deviation of 0.11 mg L −1 for the uncertainty of the TP concentration from the bank-side analysers (Lloyd et al, 2016b) to give uncertainty bounds on the TP load.…”

“…For the Wylye, discharge measurement uncertainties were estimated using a standard deviation of 10 %, the maximum value calculated by Lloyd et al (2016b) for the gauging site at Brixton Deverill using the method of Coxon et al (2015). Wylye discharges were combined with a standard deviation of 0.11 mg L −1 for the uncertainty of the TP concentration from the bank-side analysers (Lloyd et al, 2016b) to give uncertainty bounds on the TP load. For the Blackwater, discharge uncertainties were estimated by the DTC team and supplied with the DTC data, with uncertainty bounds of approximately ±20 % for low flows rising to ±30 % for high flows.…”