Surface discharge of heavy metals from low farmland.

Runoff heavy metals from farmland were examined using the field data for the summer of 2005. The observation farmland is located on lowland where the irrigation water was contaminated with the drained water from the upstream farmlands. The area of the farmland is 11.2 ha, of which 6.0 ha and 4.5 ha have been used for rice paddy fields and soybean cultivation, respectively. During the observation, heavy metal concentrations at the downstream end were usually found to be higher than those in the irrigation water. That is, the heavy metal concentrations increased due to the passage of the water through the farmland. This increase in the heavy metal concentrations is not equal to the discharge of the heavy metal because the evaporation on the surface of the paddy field and the absorption by plants makes the surface water volume small. The discharged load from the farmland generally indicates the gross surface load from the farmland. When the effects of circulation irrigation on the heavy metal concentrations are estimated, the discharged load from the farmland should be calculated as the net surface load. When the runoff heavy metals from the circulation irrigation farmland are estimated, it is important to consider the inflowing heavy metals with irrigation water. All the heavy metal types observed in this study were discharged from the farmland. The net surface loads of Cr, Fe, Cd, and Pb were 371 microg m(-2) day(-1), 14.9 mg m(-2) day(-1), 0.26 microg m(-2) day(-1), and 3.3 microm( -2) day(-1), respectively.

Nitrogen removal function of recycling irrigation system.

The purpose of this study was to clarify the nitrogen (N) purification capacity of a paddy field in a recycling irrigation system. Irrigation water was sampled at 12-h intervals during the irrigation period from April to September 2003. In addition, ponded water in a paddy field was collected at three points (inlet, centre and outlet). Total amounts of N were 30.7 kg ha(-1) in inflow and 27.8 kg ha(-1) in outflow. Thus, the net outflow load was -2.9 kg ha(-1). The N removal rate constant when N removal is expressed as a 1st-order kinetic was 0.017-0.024 m d(-1). This value is close to values of wetlands and paddy fields in the literature. We found a good correlation between recycling ratio and N removal effect. These results indicate that the recycling irrigation system accumulates N in the irrigation/drainage system, and thus the paddy field does a good job of water purification by removing N.

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.

Model analysis for nitrogen effluent from upland field constructed with under-drain.

A mathematical model to estimate nitrogen (N) effluent from an upland field cultivated with barley, where under-drain pipes were installed 60 cm below the field surface, was developed and N effluents for several rainfall patterns during the cultivation period were analysed. The model is composed of the water drainage model and N cycle model. The water drainage model is made up of Sugawara's tank model in which the field is divided into two soil types, permeable soil and impermeable soil, and macro pore. The N cycle model can calculate the N reactions including nitrification, denitrification, mineralization, immobilization, urea hydrolysis and N transportation in the field. By using this model, N effluents caused by rainfall were analysed and characteristics of N effluent were clarified. The under drainages caused by heavy rains which occur around 60 days after fertilization contribute greatly to the amount of N effluent load during a cultivation period. Also, split application of fertilizer is not effective for the reduction of N effluents under any cultivation conditions, especially in winter crops. A large quantity of N effuses out of the field because the major portion of rainfall in a cultivation period occurs in the latter half of the period.

Nutrient balance in a paddy field with a recycling irrigation system.

We studied nutrient balance in a paddy field that had a recycling irrigation system and evaluated the effect of the irrigation system on nutrient balance during the irrigation period, from April to August 2002. Chemical fertilizer was the main input of phosphorus; the soil absorbed about 56% of it. The amount of nitrogen supplied by the irrigation system was higher than in a representative paddy field, and the amount of nitrogen fertilizer used was decreased because the irrigation water was partly reused. About 20% of applied nitrogen was lost by denitrification. The net outflows of phosphorus and nitrogen were -0.37 and -3.98 kg ha(-1), respectively. These results indicate that our study paddy field performed well in removing phosphorus and nitrogen compounds from runoff water. A recycling irrigation system can be considered an effective way of reducing the amounts of water and fertilizer used and reducing the outflow nutrients.

Scenario analysis for reduction of effluent load from an agricultural area by recycling the run-off water.

We reviewed rates of nitrogen (N) removal in paddy fields and wetlands. Then we developed a numerical model to simulate N flow in an agricultural paddy field area and analyzed scenarios for recycling the agricultural run-off, including field drainage, from an agricultural area with an irrigation/drainage system. In it, we considered N removal in paddy fields, a regulating reservoir, and canals. The review showed that the rate constant for N removal in paddy fields and wetlands was 0.025 m/d under natural conditions and 0.01 m/d under dark conditions. The scenario analysis of recycling paddy field run-off within the model area indicated that a large amount of the effluent load occurred during the transplanting period and that recycling could reduce the effluent N load. The effluent N load would be equal to the inflowing N load at a 48% recycling rate. In this scenario, paddy fields accounted for most of the reduction in the effluent N load, and the mean removal rates per unit area of paddy field and regulating reservoir were 53.6 g ha(-1)d(-1) and 222.6 g ha(-1)d(-1) , respectively.