Watershed soils and land use characteristics affecting the river runoff processes in terms of P and N concentrations

Abstract

Phosphorus (P) and nitrogen (N) concentrations were measured at 75 points in the upper reaches of rivers draining into the agricultural watersheds south of the Ibaraki prefecture during the summer and winter of year 2009. A geographic information system (GIS) was used to combine the environmental information regarding the watersheds such as land use, topography, and soil with water quality data for a statistical analysis. As andosols have a stronger tendency to adsorb P than other soil types, watersheds were classified into areas with andosol and non-andosol soils. No correlation was observed between land use or topography and the P levels in the rivers for andosol areas, whereas a positive correlation was observed between the proportion of the built areas or paddy fields in the watersheds and the P levels in the rivers for non-andosol soil areas. These results indicate that a difference in the adsorption property among soils is an extremely important environmental factor affecting the P discharge process. Although the differences related to the soil type were not observed for N, there was an extremely strong positive correlation between the N levels in the rivers and the proportion of fields in the watersheds: summer N concentration declined greatly for watersheds with a high proportion of dry fields. These results clarified that dry fields are the major sources of N, but they could also act as N sink during summer. In watersheds with soils other than andosols, the effects of land use and seasonal changes on the load sources were relatively large in terms of the P and N discharge processes. It was difficult to discern the effects of topography, and a correlation between the nutrient concentrations and TWI (Topographic Wetness Index) was observed only for a few cases.