Water, energy, and mass balance in infertile pastureland in the Setouchi area, western Japan

Abstract

To assess the effect of land-use changes on water quality in the Inland Sea of Japan (Seto-naikai) and its surrounding area (Setouchi area), an integrated evaluation of water, energy, and mass balance in pastureland was carried out based on field monitoring over a 1-year period from August 1999. The pastureland is characterized by less productive soil originated from weathered granite, which is a common type of soil in the Setouchi area. The annual amount of groundwater recharge (523 mm) occupies approximately 40% of annual precipitation (1, 262 mm), and half of the amount was attributed to infiltration caused by heavy rains in September. The annual total of latent heat flux due to evapotranspiration occupying 60% of precipitation is approximately equivalent to two-thirds of the annual net radiation (2,712 MJ/m2). The evapotranspiration rate showed dependence on seasonal variation in solar radiation and vegetation conditions. Solute transport into deeper soil layers (i.e., leaching flux) estimated for each chemical element is approximately 2 to 9 times greater than atmospheric input (including wet and dry depositions). The leaching fluxes were dependent on fertilization and succeeding ion exchanges with the soil matrix, while the nitrogen leaching flux showed a small value of less than 7% of the fertilization flux. This result suggests that moderate fertilization on less productive pastureland will directly cause neither nitrate contamination in groundwater nor eutrophication in surface water bodies. However, it should be noted that the ratio of leaching flux to fertilization flux is variable depending on the management of vegetation conditions and the time schedule of fertilization.