Influences of Land and Water Use on the Water Quality of Canals through Agricultural Areas

Abstract

Shinotsu Canal passes through an agricultural area that contains a high pollutant load from catchments. This pollutant load influences the Ishikari River water quality, which consequently affects downstream aquatic biota. This study was conducted to evaluate the influences of agricultural land and water use activities on Shinotsu Canal water quality. The Shinotsu Canal is 25 km long and contains a 10,864 hectare catchment area, which is divided into 10 sub-catchment areas. The proportions of major land uses such as paddy fields, uplands, and forests were categorized by the supervised classification method using satellite data for each sub-catchment area. Water samples were collected manually from 11 points (P1~P11) upstream to downstream from May 2006 to April 2009. The suspended sediment (SS) concentration was analyzed by suction filtration; nitrate nitrogen (NO₃-N) was measured by ion-chromatography; and total nitrogen (TN), total phosphorus (TP) were measured by UV-spectrophotometry. The SS, TN, and TP concentrations were highest during the puddling period (PP), whereas the highest NO₃-N concentration occurred during the snow melting period (SMP). There was a positive significant relationship between accumulated paddy field area (APA) and SS (r = 0.94) and between the accumulated upland area (AUA) and SS (r = 0.96) at <0.001significance level during the PP. The TP concentration was also significant during the SS. TN was highly correlated with APA (r = 0.94, P <0.001) and AUA (r = 0.98, P <0.001) during the SMP. The SS, TN, and TP concentrations were higher downstream (P11) than upstream (P1) at all periods except for TN during the normal irrigation period. We conclude that land and water use for agriculture, seasonal meteorological characteristics, and fertilizer management affect Shinotsu Canal water quality.