Effects of subsurface flow and paddy fields on water and nutrient dynamics in the Nasu alluvial fan I. Water dynamics

Abstract

To measure water discharge, we instrumented 23 river basins ranging from 1.2 to 341 km² in a 1280 km² study area that included the Nasu alluvial fan (497 km²), lying between the Naka and Hoki Rivers, and surrounding areas in Tochigi, northern Honshu, Japan, for a period of 3 years and 4 months (May 2007 to September 2010). In tandem, we developed a model of the water dynamics of this area based on the tank model and used it to calculate water movement in the basins from actual meteorological data. The model estimated that subsurface flow production from the whole study area was 678 mm y⁻¹, corresponding to 59％ of the 1150 mm y⁻¹ difference between precipitation and evapotranspiration. The model calculated that subsurface flow was high in the mountainous northern portion of the study area and river basins located on the upper slopes of the alluvial fan, involving the Sabi, Kuma, and Hoki Rivers. The model also indicated the existence of a subsurface water storage zone in the upper portion of the alluvial fan (at an altitude of 250–350 m a.m.s.l) and that a large proportion of the flow into the basins in the middle–lower portion of the alluvial fan was from subsurface flow, involving the Yusaka, Momura, Kabuchu, Fuka, and Maki Rivers. The model results indicated that subsurface flow is the principal pathway governing water dynamics in the Nasu alluvial fan. The amount of subsurface outflow from the Nasu alluvial fan was 848 mm y⁻¹, considerably less than the inflow of 1630 mm y⁻¹, but measurement of the subsurface water movement from the Takaku colline, outside of the Naka River, is required before these figures can be confirmed. The amount of subsurface water used for paddy field irrigation across all basins was 725 mm y⁻¹, representing 72％ of the difference between precipitation and evapotranspiration and 44％ of subsurface inflow to stream channels, indicating that paddy fields exert a strong influence on the water dynamics of the Nasu alluvial fan.