Estimating sediment and particulate organic nitrogen and particulate organic phosphorous yields from a volcanic watershed characterized by forest and agriculture using SWAT model

Abstract

The study was conducted in the Shibetsu River watershed (SRW), Hokkaido, Japan, in order to examine the possibility of using the soil and water assessment tool (SWAT) to provide an understanding of sediment and particulate organic nitrogen (PON) and particulate organic phosphorous (POP) yields between 2003 and 2008. The SRW is a non-conservative catchment (the surface catchment lying on a continuous impervious horizon) and it is recognized that it receives external groundwater (EXT) from other watersheds. The EXT yield from each hydrologic response unit (HRU) was added to streamflow in the SWAT model. Simulated daily sediment and PON and POP yields from the SWAT model showed a strong agreement with the observed values. The simulated annual sediment yield ranged from 5 to 45 tonnes.km−2.yr−1 (annual mean of 24 tonnes.km−2.yr−1). Annual PON yield ranged from 0.1 to 0.3 tonnes.km−2.yr−1 (annual mean of 0.18 tonnes.km−2.yr−1). Annual POP yield ranged from 0.01 to 0.03 tonnes.km−2.yr−1 (annual mean of 0.02 tonnes.km−2.yr−1). Snowfall, snowmelt and rainfall seasons contributed about 10, 20 and 70% respectively to total sediment and associated PON and POP yields. The SWAT model identified that sub-basins located in the upper part of the watershed were critical source area of land surface erosion. This research demonstrates the ability of the SWAT model to estimate sediment and associated PON and POP yields, and to improve the understanding of soil erosion mechanisms at catchment scale receiving external water.