Difference in Water Quality Depending on the Catchment-area the Vegetation Cover of the Irrigation Reservoirs in the Hiki Hilly Areas: Comparison of the Reservoirs Varying Degree of Dominance of Deciduous Forest or Coniferous Forest Dominance

Abstract

The objective of this work was to elucidate the impact of different kinds of vegetation in the catchment of a pond on the water quality. The study area covered the irrigation reservoirs distributed over the Hiki hilly areas in Saitama Prefecture. Five reservoirs were chosen for the investigation based on a local survey after pre-selection using the geographic information system （GIS） software. The catchments were composed of different types of vegetation such as deciduous forest （e.g., sawtooth oak “Kunugi”） and coniferous forest （e.g., Japanese cedar “Sugi” or Japanese cypress “Hinoki”）. The concentration ranges （mg・l^－1） of DOC, COD_Mn, total inorganic nitrogen （TIN）, and Chl－a in the surveyed reservoirs were found to be 1.6–3.0, 3.9–6.4, 0.52–2.4, and 0.0024–0.011, respectively. In the reservoirs with deciduous forest-dominated catchments, NH₄－N/TIN and NO₃－N/TIN were ＞30％ and ～60％. However, in the reservoirs with coniferous forest-dominated catchments, NH₄－N/TIN and NO₃－N/TIN were ＜20％ and ～70％. Thus, the water quality of the reservoirs is dependent on the catchment vegetation, with typical abundance ratios of inorganic nitrogen more likely in those with conifer forest or deciduous forest-dominated catchments.