Relation between Land Use and Nitrate Concentration of Outflow Water from Watersheds of Agricultural and Forest Areas

Abstract

The relation between NO₃-N concentration of outflow water and land use in agricultural area was investigated in Hokkaido. Five areas were selected as follows: 1. Furano; forest area, 2. Kushiro; grass land area, 3. Nemuro; grass land area with high cow density, 4. Obihiro; agricultural area including upland field and grassland, 5. Abashiri; large pig farm area.
Nitrate concentrations and flow rates of stream water were measured. In the forest area, the values of NO₃-N concentration were less than 0.3mg/l. In the agricultural area the values were higher than the values found in the forest area. They depend on the percentage of the agricultural land, consisting of upland field and grass land. The concentration of NO₃-N (y) increases with increase in percentage of the agricultural land (x). The estimated regression line was given by the equation y=0.029x. All values of y (mg/l) are included in the range (0.01-0.05) x. The values measured in Kanto districts were higher than that of this investigated area. It was caused by the larger supply of chemical fertilizer and high density of pig farms without an adequate treatment system for fecal matters.