Economic valuation of reduction in nitrogen outflow from a paddy field area equipped with a recycling irrigation facility.

We estimated the reduction in nitrogen outflow load from a paddy field that had a recycling irrigation facility and, by using a replacement cost method, evaluated the economic effect of nitrogen removal by the paddy field during the irrigation period in the Yoshinuma region of Tsukuba City, Japan. The recycling ratio of outflow water (proportion of outflow reused) was 13.5%. The nitrogen (N) outflow load was reduced by about 45 kg ha(-1) by the N removal function of the paddy field and by about 39 kg ha(-1) by the recycling irrigation facility. The paddy field equipped with a recycling irrigation facility as an N removal facility was valued at 32.6 million Japanese yen (JPY) ha(-1) and 0.72 million JPY ha(-1) per year, which compare it with the construction and maintenance costs, respectively, of a water quality improvement facility. The recycling irrigation facility was costed at 17.3 million JPY ha(-1) for construction and 0.21 million JPY ha(-1) for maintenance per year. The cost for constructing and maintaining a recycling irrigation facility was 53% of the value of the paddy field area equipped with a recycling irrigation facility as an N removal facility.