Factors affecting the efficiency of some estimators of fluvial total phosphorus load

Abstract

The accuracy of estimating total phosphorus (TP) loads to receiving waters usually is constrained by availability of concentration data, as discharge (flow) data normally are comparatively abundant. Using 4 years of daily observations from three tributaries to the Great Lakes (Grand, Saginaw, and Sandusky Rivers), annual TP loads were tested for accuracy by five methods, including regression, ratio, and robust estimators. Monte Carlo methods were employed to simulate replicated flow‐stratified sampling of the datasets with various allocations of samples to flow strata. Each of the load calculation methods was applied to each group of simulated samples, and response was quantitated as load estimation error (computed minus “true” load). The results show the most consistently accurate estimator was Beale's stratified ratio estimator. It was, however, the only stratified estimator tested and should have been more accurate. Most accurate of the unstratified estimators was a straightforward least squares regression (log‐log) method. The response of estimation bias to flow cut point and sample allocation manipulations indicated (1) beneficial results generally (but not always) obtained from high flow weighting of sampling and (2) postsampling stratification generally may yield improved accuracy for load estimation and deserves additional research.

Errors of flux integration methods for solutes in grab samples of runoff water, as compared to flow-proportional sampling

“…Several authors found the stratified Beale's ratio estimator (BRE) to have small biases (Dolan et al, 1981;Preston et al, 1992;Young et al, 1988). These studies were based either on sediment or on phosphorus concentrations, both showing a positive correlation with discharge.…”

Choice of Sampling Strategy and Estimation Method for Calculating Nitrogen and Phosphorus Transport in Small Lowland Streams

“…Walling and Webb, 1981;, nitrate-N (N03-N) transport (Smith and Stewart, 1977;Stevens and Smith, 1978) and total P (TP) transport (Dolan et al, 1981;Young et al, 1988;Preston et al, 1989;Rekolainen et al, 1991). Walling and Webb (1981), working on suspended sediment data from the river Creedy, UK, reported that the precision of transport estimates decreased with increasing sampling interval, and that regression methods were more precise than interpolation methods.…”

Estimation of Catchment Nutrient Loads in New Zealand Using Monthly Water Quality Monitoring Data

“…The Ratio method is also affected by poor characterization of the tail of the concentration distribution. Because the method is based on the mean of the unit loads, the weight that any individual observation has on the load estimate is reduced compared to the Rating method (Young et al ., ). However, in our study, loads estimated using the Ratio method at some sites had lower precision (larger SE P values) than those estimated using the L5 and L7 methods, particularly for TP (Figure ).…”