Comparison of measured and calculated loads using quadratic LQ equation for Hii River flows, eastern Shimane Prefecture, Japan

We sampled river water from 1 July 2010 to 30 June 2011 in the Hii River. Suspended solids (SS), chemical oxygen demand (COD), total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP) showed increasing trends during floods. In contrast, dissolved silica (DSi) had a decreasing trend during floods. By comparing estimated loading amounts from a loading-flow (LQ) equation and actual loading amounts from multiplying a given days chemical species and discharge, TN, nitrate and D-Si showed quantitative agreement between the two estimations. However, loading amounts of SS, COD, TOC, TP and phosphate from a linear LQ equation had lower values than those of actual loadings. The reason for such underestimation is an increased concentration of certain chemical species during flooding or high discharge. Therefore, we applied quadratic or cubic LQ equations. After applying the quadratic LQ equation to certain chemical species that showed low coefficients of determination, values improved. For more accurate loading amounts from an LQ equation, we require observation data from periods of high discharge.