A method for evaluating continuous daily discharge, loads of N, P, and inorganic ions in a watershed on the Shimanto River basin, Shikoku, Japan

In the Shimanto River basin, Shikoku, Japan, agricultural activities are suspected to lead pollution in water quality of the Shimanto River. However, causal relationship between the agricultural activities and the water quality is not evident. So, we developed a method for evaluating continuous daily discharge and effluent loads of N, P, and inorganic ions to analyze their effluent characteristics in a watershed (815 ha) located on the Shimanto River basin. The results are as follows. 1) As for discharge and concentration, complement of continuous daily data from discontinuously measured data such as once at two weeks or from automatically measured ones including failed-observation periods was revealed to be practicable by a method we designed in the present paper. 'Systematic biases' arising in the complemented values was corrected by modification factors defined in each of discharge classes. 2) Relative errors in the complemented values were +- 40% for the discharge, from -13% to +30% for the concentration, and +-40% for the effluent load. Cumulative errors when the complemented values linked with automatically measured ones were practically +-17% for the discharge, and +-18% for the effluent load. 3) The discharge and concentration of the whole forested area (622 ha) in the watershed were able to be estimated from the partial forested catchment A (43 ha), using correlative relationships between mean specific discharge, or mean concentration of five forested catchment samples and the complemented discharge of the forested catchment A (forested area extending method). 4) Annual discharge calculated from the data complementing and forested area extending methods was concluded to be an appropriate value, because evapo-transpiration obtained from a subtraction of the complemented discharge from precipitation coincided with that estimated from the automatically measured discharge.