ABSTRACT Nitrogen concentrations of drainage, percolation and irrigation were monitored periodically once a week at the experimental small rice-paddy watershed (EPW, 6.96 ha) for three years. Water level of the draining ditch was recorded continuously at the end of EPW. During the cropping period, load L of nitrogen through the drainage were evaluated from the concentrations C and the flow rates Q by integration interval method. During the non-cropping period, L was calculated from L(Q)-equations and Q. The L(Q)-equations were derived from the data measured during storm-runoff events. Concentrations of total nitrogen (TN) of irrigation, drainage and percolation were 0.403, 1.39 and 1.11 mg l-1, respectively, on average for three years. L of the drainage changed in wide range in response to agricultural practices and rainfalls; e.g. puddling, transplanting, fertilizing and plowing. The annual net-load of TN (unit load Ln) was 35.8±4.05 kg ha-1 y-1 on average for three years, of which the non-cropping period occupied 37 %. L for the puddling and transplanting period was discharged 27% of Ln, and strongly affected by the applied volume of irrigation. These results suggest that observations for only cropping periods or for only one year are insufficient for evaluating the precise Ln to assess the effect of the nitrogen discharged from paddy fields on lakes.
Journal of Water and Environment Technology, Vol 6, No.2, 2008 Evaluation of Loading Rate of Nitrogen Rice-Paddies by Small Watershed Method from Yoshitaka SUGIMOTO*, Yukio KOMAI**, Takao KUNIMATSU*** * Environmental Science Graduate School, Graduate School of the University of Shiga Prefecture, 2500 Hassaka, Hikone, Shiga 522-8533, Japan ** Department of Environmental Technology, Faculty of Technology, Osaka Institute of Technology, 5-16-1 Omiya, Asahi-ku, Osaka, 535-8585, Japan *** School of Environmental Sciences, the University of Shiga Prefecture, 2500 Hassaka, Hikone, Shiga 522-8533, Japan ABSTRACT Nitrogen concentrations of drainage, percolation and irrigation were monitored periodically once a week at the experimental small rice-paddy watershed (EPW, 6.96 ha) for three years Water level of the draining ditch was recorded continuously at the end of EPW During the cropping period, load L of nitrogen through the drainage were evaluated from the concentrations C and the flow rates Q by integration interval method During the non-cropping period, L was calculated from L(Q)-equations and Q The L(Q)-equations were derived from the data measured during storm-runoff events Concentrations of total nitrogen (TN) of irrigation, drainage and percolation were 0.403, 1.39 and 1.11 mg l-1, respectively, on average for three years L of the drainage changed in wide range in response to agricultural practices and rainfalls; e.g puddling, transplanting, fertilizing and plowing The annual net-load of TN (unit load Ln) was 35.8±4.05 kg ha-1 y-1 on average for three years, of which the non-cropping period occupied 37 % L for the puddling and transplanting period was discharged 27% of Ln, and strongly affected by the applied volume of irrigation These results suggest that observations for only cropping periods or for only one year are insufficient for evaluating the precise Ln to assess the effect of the nitrogen discharged from paddy fields on lakes Keywords: nitrogen, rice paddy, load INTRODUCTION For more effective and efficient managements of water environments against eutrophication, it is necessary to evaluate the loads of nitrogen and phosphorus from the nonpoint or diffuse sources such as agricultural lands, forests, and urban areas as well as the other point sources Rice-paddy fields occupied 54% of the agricultural area in Japan (Minister of Agriculture, Forestry and Fisheries, 2008) In the case of Lake Biwa, the largest lake in Japan, paddy fields account for 92% of the total agricultural-fields in the catchments There have been many studies about the nutrient loads discharging from rice paddies However, most of them were estimated on the base of the data measured only for the cropping season from April to October (Takamura et al., 1976, 1977, 1979; Kubota et al., 1979; Udo et al., 2000; Kaneki R., 2003; Cho J Y et al., 2008) Takeda et al (1991) reported that paddy fields discharged 52% of total nitrogen (TN) and 14% of total phosphorus (TP) of the annual loads during the non-cropping period This also indicates that the nutrient loads should be estimated on the base of the data monitored through a year It is necessary to estimate storm-runoff loads for evaluation Address correspondence to Takao Kunimatsu, School of Environmental Sciences, the University of Shiga Prefecture, Email: kunimatu@ses.usp.ac.jp Received October 31, 2008, Accepted December 5, 2008 - 113 - Journal of Water and Environment Technology, Vol 6, No.2, 2008 50°N Russia China Japan North Korea Sea JAPAN South Korea 40°N Wakasa Bay 30°N Pacific Ocean 120°E 130°E Lake Biwa 140°E Hikone Kyoto Shiga Pref (EPW) Fig - Experimental small watershed (EPW) and sampling sites □: Site-A was the sampling site of drainage, ●: experimental wells Pa1-Pa4, ∆: site measuring atmospheric deposition of the loads for the non-cropping periods, which are usually induced by rainfalls Only two studies, however, estimated the annual loads on the base of the data obtained by the year-round monitoring (Takeda et al., 1991; Kondoh et al., 1992) In addition, the pollutant loads may be affected by the yearly changes of hydrological conditions and agricultural practices Therefore, the monitoring should be continued all the year round for more than several years In this study, material balance of TN was estimated from the results obtained from continuous observations for three years at a small watershed of rice-paddies MATERIALS AND METHODS Study site The experimental small rice-paddy watershed (EPW) was shown in Fig 1, which is in agricultural areas in Hikone city located around the middle part of Honshu, main island of Japan (Sugimoto et al., 2006) Area of EPW (35° 15' 01" N, 136°12' 43"E) is 6.96 in rice-paddy areas, and consists of 29 small rice-paddy sections (0.1-0.7 ha) Irrigation was pumped up from Lake Biwa (675 km2, 275×108 m3) and distributed to each section via pipeline-pump systems A drainage ditch (540 mm in width and 600 mm in depth) runs through the middle of EPW from the upper end of EPW and not collect any drainage from the outside of EPW The ditch flows at about 200 m downstream into Ezura River The watershed lies between 85.6 and 86.1 m from the sea level (T.P.) and 1.2-1.7 m from the water level of Lake Biwa (84.4 m T.P.) The type of the surface soil of EPW is alluvial gray soil Chemical properties of the paddy soil were summarized in Table The rice plant, Koshihikari, had been cultivated on the all sections for three years since 2004 after crop rotation from wheat In this region, young rice plants are usually transplanted after plowing, fertilizing, flooding and puddling in mid-April to early-May - 114 - Journal of Water and Environment Technology, Vol 6, No.2, 2008 Table - Chemical properties of soils of the experimental paddies depth (cm) 0-20 20-40 pH (H2O) 5.77 5.60 EC (µS cm-1) 85.6 86.8 T-C (%) 2.42 2.09 T-N (%) 0.241 0.203 Average values of four samples collected at Pa1-Pa4 (Fig 1) on Oct and 9, 2003 Nitrogen applied for a crop season was 83 kg ha-1 in chemical fertilizers in 2004 The cultivation under EnFA began from 2005 to 2006 EnFA requires reducing the applying amounts of the chemical fertilizers of nitrogen to less than half of the previous usage amounts, which was allowed to supply with organic fertilizers Then, the amounts of nitrogen applied by chemical fertilizers were reduced to 36 kg ha-1 in 2005, and supplied with additional application of 36 kg ha-1 in organic fertilizers In 2006, the amounts of nitrogen fertilizer were 78 kg ha-1, a half of which was organic fertilizers EnFA also requires preventing rice-paddies from muddy discharge during the puddling and transplanting practices Rice paddies are flooded from mid-April through August with a drainage period for about ten days from late June to early July After harvest in mid-September, paddies are dried up and plowed at the first time in late November and the second time in March Each practice usually continued for duration of about two weeks in EPW, owing to convenience of more than ten tillers Hydrological measurement Long-term water balance in paddy watersheds is usually expressed by the following equation: R + I = D + P + ET + ∆S (1) where R, I, D, and P are depths of rainfall, irrigation, drainage and percolation, respectively, ET is depth of evapotranspiration, and ∆S is the change of flooding water and ground water A gauge having a pressure sensor was installed on the bottom of the ditch at the end of EPW (Site-A in Fig 1), and sequentially recorded the water level of drainage at an interval of 10 minute Flow rates Q were calculated from the water levels by following equation: Q = ah b (2) where h is water level measured at Site-A, and a and b are coefficients obtained from the logarithmic linear regression of the relationships between Q and h, measured during storm-runoff observations, by the least-squares method However, Q were not calculated for some periods (usually 4-5 days) missing the data of the water levels due to backwater from Ezura River caused from heavy rains (5 events for the experimental years) as well as snow covers (5 events) Q was calculated for those periods as following; 1) Irrigation period: calculated with Equation (1) by using I estimated from the volumes of water supply through the pump station 2) Non-irrigation period: estimated on the base of the correlation between rainfalls and surface drainages obtained during periods not affected by the backwater events - 115 - Journal of Water and Environment Technology, Vol 6, No.2, 2008 Table - Conditions carried out storm-runoff observations during non-cropping periods event duration rainfall runoff runoff year date soil condition No (days) (mm) (mm) ratio *1 2003 2003 2004 2004 2004 2004 2006 14 Oct 03 Nov 14 Jan.*1, 07 Mar *2 30 Mar 08 Oct 26 Feb 10 3 before plowing before plowing after plowing after plowing after plowing before plowing after plowing 25.0 17.5 68.5 33.0 33.5 70.5 28.0 19.3 06.6 61.6 17.7 11.7 65.0 13.2 0.77 0.38 0.90 0.54 0.35 0.92 0.47 Composite sample *2 Snowmelt runoff The percolation rates in 2004, 2005, and 2006 were estimated to be 0.23, 0.59, and 0.67 mm d-1 by the water balance of Equation (1) during each non-cropping period The average value of 0.50 mm d-1 was applied to the cropping period Because the irrigation water was supplied through pipe line in the area, as mentioned above, we could not measure directly the irrigation rate Then, the irrigation rates were calculated weekly from the water balance of Equation (1) on the assumption ∆S = Daily precipitations were obtained from Hikone Local Meteorological Observatory, located at km northeast EPW ET was estimated by Thornthwaite method (Kayane I., 1980) Water sampling Water samples of drainage, percolation (around 60 cm below soil surface) and irrigation were collected periodically once a week The observations were started from August 2003 to September 2006 Drainages were taken up at Site-A From mid April to late May including the puddling and transplanting period, the drainage waters were collected at every six hours for a day, and mixed together in proportion to flow rates at each sampling time to make daily composite samples, except 2005 when the drainage waters were collected once a day Storm-runoff observations were carried out during seven events shown in Table 2, and the drainages were collected at every hour The samples obtained during Event were mixed in proportion to flow rates at each sampling time to make one composite sample PVC-pipes (10 cm in diameter and 130 cm in length), having small hales at 10-20 cm from capped bottom and covered by cloth, were buried at Pa1-4 (Fig 1) by using an earth auger The water samples of percolation were collected from the experimental wells, and residual waters were pumped out after sampling The same volumes of the four samples were mixed together and fi1trated through glass fiber filter in the laboratory Irrigation waters were collected from some irrigation valves installed on each paddy section Atmospheric wet and dry depositions were collected monthly by two bulk deposit samplers laid on an open space in the University of Shiga Prefecture (Fig 1) One of them was consisted of a 20-cm diameter PE-funnel and of 20-liter PE-reservoir to analyze EC, pH and NO3-N The other sampler was composed with a 30-cm diameter funnel and the reservoir contained ml of conc.-H2SO4 for measuring other components The water samples were collected after measuring water volumes Water chemistry Total nitrogen (TN) was analyzed non-filtered samples, and the dissolved component - 116 - Journal of Water and Environment Technology, Vol 6, No.2, 2008 Table - Water balance in the experimental small paddy-field watershed (EPW) year 2004 year *1 2005 non-cropping cropping*2 (PuTp)*3 year 2006 non-cropping*1 cropping*2 (PuTp)*3 year non-cropping*1 cropping*2 (PuTp)*3 *1 *2 *3 input duration (days) period precipitation 366) 197) 169) (47) 365) 196) 169) (47) 365) 196) 169) (47) 1538) 612) 926) (319) 1515) 799) 716) (119) 1731) 803) 928) (181) (mm) output irrigation 2586) 0) 2586) (776) 1806) 0) 1806) (454) 2136) 0) 2136) (615) drainage percolation evapotranspiration 3145) 405) 2740) (959) 2294) 523) 1771) (449) 2864) 529) 2335) (675) 131) 46) 85) (24) 201) 116) 85) (24) 216) 131) 85) (24) 849 161 688 (112) 826 160 666 (101) 788 143 645 ((98) Non-cropping period (01 Oct-14 Apr) Cropping period (15 Apr-30 Sep) Puddling and transplanting period (15 April-31 May) (DN) and other dissolved materials were analyzed fi1trated samples (l-µm in pore-size glass fiber filter with a 45 mm diameter) The concentration of particulate nitrogen (PN) was obtained from the difference between the total concentration of the non-filtered sample and that of the filtered one The dissolved organic nitrogen (DON) was calculated by subtracting nitrite nitrogen (NO2-N), ammonium nitrogen (NH4-N) and nitrate nitrogen (NO3-N) from DN Chemical analyses were carried out as follows; TN and DN were assayed by alkaline potassium peroxodisulfate digestion (120°C, 1.5 atm.) - UV absorbance (260 nm) method; nitrite nitrogen by the diazonation method; ammonium nitrogen by the indophenol blue method; nitrate nitrogen by suppressed ion chromatographic method (HPLC) using a CTO-10A attached to Shim-pac IC-A3 (Shimadzu, Kyoto Japan) RESULTS Water balance Water balances of EPW were summarized in Table The depth of the irrigation and precipitation were 2176 (ranged from 1806 to 2586) and 857 (716-928) mm, respectively, on average for the three cropping periods The drainage and percolation were 2282 (1771-2740) and 85 mm The evapotranspiration was 666 (645-688) mm The total input was 3033 mm, 75% of which drained through the ditch to Ezura River For the non-cropping period, the drainage, percolation and evapotranspiration were 486 (405-529), 97 (46-131) and 155 (143-161) mm, respectively, on average for the three years The input precipitation was 738 (612-803) mm, 66% of which discharged as storm-runoff In 2005 and 2006 under EnFA-cultivation, the amount of the irrigation during the puddling and transplanting period (defined from 15th April to 31st May) were reduced from 776 in 2004 to 454 and 615 mm, respectively, even though the precipitation decreased from 319 mm to 119 and 181 mm As the results, the muddy drainage were reduced from 959 mm to 449 and 675 mm The total amounts of the input for the - 117 - Journal of Water and Environment Technology, Vol 6, No.2, 2008 80 Non-cropping Rice Non-cropping Rice Non-cropping Rice (a) 40 20 concentration (mg l-1) 60 cumulative drainage (m) daily drainage (mm d -1) 100 (b) 2003 | 2004 | 2005 | 2006 | Fig - Daily changes of concentrations of TN (a) Daily and cumulative drainage from rice paddy watershed (EPW) : drainage rate, : cumulative load (b) Change of concentration : drainage, : percolation, : irrigation The data of drainage having error bars show the standard deviations of the data during storm-runoff events cropping period decreased from 3512 in 2004 to 2522 and 3064 mm in 2005 and 2006, and the drainage from 2740 mm to 1771 and 2335 mm Water chemistry of paddy fields TN concentrations of drainage, percolation, and irrigation were shown in Fig 2, in which the average concentrations and standard deviations of the storm-runoff observations were included The volume-weighted average concentrations were summarized in Table Long-term fluctuation The concentrations of the irrigation, pumped up from Lake Biwa, were relatively stable, and the annual average concentrations were in narrow range from 0.368-0.414 mg l-1 In contrast, the concentrations of the drainage increased abruptly at the beginning of the puddling in every late-April, and reached the yearly maximums of 7.56, 5.29 and 4.55 mg l-1 in 2004, 2005 and 2006, respectively The period of the high concentration continued for a longer time in 2004 than in 2005 and 2006 Then, the concentration decreased to around mg l-1 in late-May, and reached around 0.5 mg l-1 in August with some small peaks in July and June The concentration increased to around mg l-1, just after the irrigation and following harvest in mid-September In the non-cropping period, the concentrations decreased gradually to around mg l-1 by March, accompanying with many sharp and high fluctuations caused by rainfalls and plowing, and then increased after the second plowing in mid-March to mid-April The concentrations of the percolation changed in the similar manner as the drainage, however the peaks in the puddling and transplanting period were not so high as those of the drainage The average concentrations of the drainage and percolation - 118 - Journal of Water and Environment Technology, Vol 6, No.2, 2008 Table - Average concentrations of TN year 2004 2005 period year non-cropping cropping year 2006 non-cropping cropping year av non-cropping cropping year non-cropping cropping irrigation 0.409 0.409 0.414 0.414 0.368 0.368 0.403 0.403 drainage 1.780 2.140 1.720 1.230 1.250 1.210 1.150 1.720 0.928 1.390 1.710 1.290 (mg l-1) percolation 1.210 1.230 1.200 1.000 0.843 1.20 1.120 1.090 1.16 1.110 1.050 1.190 were 1.39 and 1.11 mg l-1, respectively, in which there were not so large differences Storm-runoff In the non-cropping period from mid-September to mid-April, the ditch intermittently flowed drainage on the effect of rainfalls It is necessary to make clear the characteristics of the storm-runoff load for quantitative estimation of the nutrient load during the non-cropping period Seven storm-runoff events were observed in the non-cropping period (Table 2) The volume-weighted average concentrations of nitrogen were summarized in Table Results obtained from Event before plowing and Event after plowing were shown in Fig The precipitations were 17.5 and 33.5 mm in depth, respectively, and the runoff ratios were similar with each other as to be 38 and 35% In Event 2, the concentrations of the all kinds of nitrogen components were maximum at the beginning of the drainage, and regardless of flow rate changes, decreased rapidly and then gradually and steadily The main components were DON and PN throughout the event The average concentration of TN was 1.87 mg l-1, and DON and PN occupied 38 and 34% of TN Event was one of storm-runoffs after the second plowing in late-March, and showed the typical pattern distinguished clearly from those before the plowings The concentration of TN increased just at the beginning of the drainage, and reached the maximum of 3.33 mg l-1 at around the drainage peak Then, the concentrations decreased, as the drainage decreased, except for slightly increasing NH4-N The increase of TN was caused mainly by the increase of NO3-N and DON The average concentration of TN was 2.18 mg l-1 NO3-N, DON and PN occupied 33, 30 and 25% of TN The changing patterns of the storm-runoff events were clearly different between before and after plowings These results indicated that surface soils of paddy-fields were disturbed by the plowing practices and exposed under oxidative conditions The soil conditions would stimulate the mineralization of organic nitrogen, as Kunimatsu et al (1991) indicated Storm-runoff loads Separate L(Q) method It is well known that loading rates of materials and flow rates - 119 - Journal of Water and Environment Technology, Vol 6, No.2, 2008 0.1 0.0 concentration (mg l-1) 5 10 TN DN NO3-N NH4-N 30 31 10 TN DN November (2003) 0.1 0.2 0.0 concentration (mg l-1) drainage (m3 km2 s-1) 0.2 0.3 rainfall (mm h-1) rainfall (mm h-1) drainage (m3 km2 s-1) 0.3 NO3-N NH4-N 30 31 M arch (2004) April Fig - Changes of drainage and nitrogen concentrations during storm runoff events Left: Event (before- plowing period), right: Event (after-plowing period) of rivers are related with each other by so-called L(Q)-equations As a hysteresis during the increasing and decreasing period of flow rate should be considered, a set of two L(Q)-equations (Separated L(Q)-equation) were derived as follows: LIi = c I QidI (Qi − Qi −1 > ) (3) LDi = c D QidD (Qi − Qi −1 ≤ ) (4) where Li and Qi are loads and flow rates at time i measured during storm-runoffs, c and d are coefficients of the logarithmic linear regression calculated by the least-squares method, and subscripts of I and D represent each L(Q)-equation in the increasing period and the decreasing period of drainages, respectively The detail measurement of flow rates and TN concentrations of the drainage were carried out for the five rain events (Table 2), which included three events before plowing (Event 1, 2, and 6) and two events after plowing (Event and 7) The resulted relationships between these parameters were shown in Fig 4, and the coefficients of L(Q)-equations were summarized in Table After the plowing, the value of d increased Table - Average concentrations weighted with drainage volume during storm-runoff observations in non-cropping periods event No 3*1, 4*2 soil condition before plowing before plowing after plowing after plowing after plowing before plowing after plowing average concentration (mg l-1) NO2-N 0.0040 0.0035 0.0032 0.0042 0.0083 - NO3-N 0.0947 0.2010 0.4900 0.4050 0.7100 - *1 Composite sample *2 Snowmelt runoff - 120 - NH4-N 0.105 0.314 0.154 0.262 0.255 - DON 1.190 0.707 0.520 0.562 0.658 - PN 0.465 0.644 0.353 0.520 0.548 0.599 - TN 1.86 1.87 1.52 1.75 2.18 1.33 3.20 Journal of Water and Environment Technology, Vol 6, No.2, 2008 10-1 10-1 Before-plowing period After-plowing period 10-2 10-3 10-3 10-4 10-4 10-5 load of TN (g l-1 s-1) 10-2 10-5 10-6 10-5 10-4 10-3 10-2 -1 -1 drainage (m s ) 10-1 10-6 -5 10 10-4 10-3 10-2 drainage (m3 ha-1 s-1) 10-1 Fig - Relationships between the loads of TN and the drainage rates obtained from the storm-runoff observations ●: increasing period of drainage rate, ○: decreasing period of drainage rate, : regression line of increasing period, : and decreasing period Table - Constants of L(Q)-equations derived from relationships between rates of load and drainage during the storm-runoff events L = c Qd soil condition before plowing after plowing increasing phase cI dI R2 01.29 0.943 0.980 25.7 1.31 0.992 cD 000.736 031.8 decreasing phase dD R2 0.897 0.986 1.36 0.997 from 0.943 to 1.31 on the increasing phase and from 0.897 to 1.36 on the decreasing phase These results indicated that the loading rate of TN was significantly affected by plowing of paddy soils L(R) method L(R) method was developed in order to evaluate storm-runoff loads by using precipitations in cases of limited data of concentrations and flow-rates (Kunimatsu et al., 1991; Kunimatsu et al., 2006) L(R)-equations were derived from the correlation between the storm-runoff load LSi and corresponding rainfall depth Ri measured during the storm runoff event i f LSi = e (Ri − r ) (5) where e and f are the coefficients obtained from the logarithmic linear regression by the least-square method, and r is the ineffective rainfall, which is estimated from R-intercept of the following equation obtained by linear regression of the relationship between Ri and the storm-runoff discharge Qsi Qsi = k ( Ri − r ) (6) where k and r are the coefficients obtained from the regression It was shown that the load is significantly affected by plowing of paddy soils, so a set of two L(R)-equations corresponding before and after plowing should be derived However, the three and two data corresponding before and after plowing, respectively, were not - 121 - Journal of Water and Environment Technology, Vol 6, No.2, 2008 104 storm runoff load of TN (g ha-1 s-1) 104 Before-plowing period After-plowing period 103 103 102 102 101 100 101 102 101 100 101 102 effective rainfall (R - r) (mm) effective rainfall (R - r ) (mm) Fig - Relationships between rainfalls R and storm-runoff loads L during non-cropping periods ●: observed data, ○: calculated values by Separated L(Q)-equations using hourly flow-rate data during the selected storm events R: rainfall depth, r: ineffective rainfall depth Table 7- Coefficients of L(R)-equations during non-cropping periods Li = e (Ri - r)f soil condition before plowing after plowing e 24.3 13.6 f 0.876 1.320 R2 0.926 0.901 sufficient in number of samples to drive statistically reliable L(R)-equations The observation of storm-runoff events is not so easy to carry out sufficient times of the observation Then, the samples of storm-runoff loads were supplemented with the storm-runoff loads calculated by using L(Q)-equations and hourly flow-rates data selected from automatically recorded water levels The observed storm-runoff loads as well as calculated loads were plotted against the depths of the effective rainfalls on Fig 5, and the coefficients of e and f were summarized in Table The coefficients of determination exceeded 0.9 Evaluation of yearly drainage loads There are some methods for calculating pollutant loads of rivers In this study, TN load of drainage through the ditch was calculated by combining the interval-loads method (ILM) and L(Q)-equation method as following Irrigation period (15th April – 31th August) The drainage load was calculated on the base of ILM-equation as following: n L = ∑ Ci Qi (ti +1 − ti −1 ) / (7) i =1 where Ci and Qi were concentrations and flow-rates measured at Site-A at time ti The data were obtained at a daily interval from the puddling through transplanting, and then weekly till harvest Non-irrigation period The different L(Q)-equations were used between before and - 122 - daily net-load (kg -1 d-1) 2.0 Non-cropping Rice Non-cropping Rice Non-cropping Rice 50 1.6 40 1.2 30 0.8 20 0.4 10 0.0 -0.4 | 2003 2004 | 2005 | 2006 | cumulative net-load (kg -1) Journal of Water and Environment Technology, Vol 6, No.2, 2008 -10 Fig - Daily and cumulative net-loads of TN after plowing (20th November in the case of EPW), as shown in Table The daily load Ld were calculated with Separated L(Q) method, as following: Ld = 24 ∑cQ i =1 d hi (8) where Qhi is hourly flow rate, c and d were coefficients different between the increasing and decreasing periods of flow-rates There were three cases in which the flow-rates were not measured; e.g 1) trouble of the level gouge, 2) backwaters caused by heavy storms, and 3) snow-covers over the ditch In these cases, there were not hourly flow rates but daily precipitations, so the storm-runoff load was calculated with L(R) method DISCUSSION Material balance of paddy-field In the paddy-field watershed irrigating river water, TN in the river flows into the watershed The input of TN is the irrigation load LI The watershed returns drainage and percolation into the river through the ditch and underground The outputs of TN are the drainage load LD and the percolation load LP, respectively The input and output balance of TN, namely the net pollutant load Lnet of the paddy-field watershed to the river is defined as following: Lnet = (LD + LP ) − LI (9) In this study, LI and LP were calculated from the weekly data of Ci and Qi with ILM method In the case of percolation, Ci was not measured from the beginning to November in 2003, so Lp of the period was calculated by using the average concentration of the same period in the second year (2004) Qi was estimated to be 0.50 mm for the irrigation period and 0.23-0.67 mm for the other period, as mentioned above LD was evaluated by the procedure shown in the foregoing section The daily and cumulative net-loads of TN were shown in Fig Because the irrigation pumped up from Lake Biwa contained very low concentration of TN (0.37-0.41 mg l-1 on annual average) (Table 4), the daily net-loads were positive values through the cropping period, except for few small and short-term negative values Fertilizing, puddling and transplanting were continuously carried out from late April to mid May, the net-load drastically increased and reached around 0.6 kg ha-1 d-1 in 2004 and 1.0 kg ha-1 d-1 in 2006 So the cumulative load increased steeply in the period The lower - 123 - Journal of Water and Environment Technology, Vol 6, No.2, 2008 Table - Nitrogen balance of the experimental small paddy-field watershed (EPW) year 2004 2005 (EnFA) 2006 (EnFA) av periods*1 year non-cropping cropping (PuTp) year non-cropping Cropping (PuTp) year non-cropping cropping (PuTp) year non-cropping duration (days) *2 *2 366) 197) 169) (47) 365) 196) 169) (47) 365) 196) 169) (47) fertilizer 83 83 (39) 72 72 (33) 78 78 (39) input R (11.9 (5.22 (6.64 (1.55) (13.0 (6.02 (7.00 (2.31) (12.8 (6.08 (6.75 (1.63) I (9.09 (0.00 (9.09 (4.46) (7.00 (0.00 (7.00 (2.62) (7.56 (0.00 (7.56 (2.50) output D P (45.8 (1.68 (10.0 (0.52 (35.8 (1.15 (18.4) (0.31) (36.2 (2.00 (12.6 (0.98 (23.6 (1.02 (6.81) (0.22) (43.0 (2.49 (14.3 (1.50 (28.7 (0.99 (12.5) (0.20) 365) 196) 78 - (12.6 (5.77 (7.88 (0.00 (41.6 (12.3 (2.06 (1.00 (kg ha-1) net-load*2 (D + P - I) (38.4 (10.6 (27.9 (14.2) (31.2 (13.5 (17.6 (4.40) (37.9 (15.8 (22.1 (10.2) (35.8 (13.3 78 cropping 169) (6.80 (7.88 (29.4 (1.06 (22.5 (37) (PuTp) (47) (1.83) (3.20) (12.6) (0.24) (9.61) *1 Non-cropping period (01 Oct-14 Apr), cropping period (15 Apr-30 Sep) including the puddling and transplanting period (PuTp: 15 Apr-31 May) *2 R: atmospheric deposition, I: irrigation, D: drainage, P: percolation loading peak in 2005 suggested the effect of EnFA The net-loads also showed two or three peaks reaching 0.5-0.6 kg ha-1 d-1 from June to August, which seemed to be caused by the applications of additional fertilizers The drainages flow through the ditch only at rainfalls in the non-cropping season, so that the daily net-loads showed isolated peaks The daily net-loads increased frequently up to around 1.0 kg ha-1 d-1, which was almost the same level as that of the cropping period Unit load of paddy fields The material balance on the yearly base was summarized in Table LR in the table is the atmospheric deposition load, which is not included in Equation (9) but listed for reference LR was calculated from monthly data of Ci of the water sample reserved in the deposit sampler and Qi of the volume by ILM The annual net-load (unit load) of TN was 35.8±4.05 kg ha-1 y-1 on the average of three years The net-load of the cropping period was 22.5 kg ha-1, which occupied 63% of the annual net-load During the puddling and transplanting period, 43% of the net-load of the cropping period was discharged, which corresponded with 27% of the annual net-load The net-load of the non-cropping period was 13.3 kg ha-1, and occupied 37% of the annual net-load The annual net load varied in relatively wide range from year to year These of 2004 and 2006 were similar with each other, however, that of 2005 was apparently smaller than the former two The difference (min.-max.; 31.2-38.4 kg ha-1 y-1) mainly resulted from the difference in the net-load during the cropping periods (17.6-27.9 kg ha-1), - 124 - Journal of Water and Environment Technology, Vol 6, No.2, 2008 especially the puddling and transplanting periods (4.40-14.2 kg ha-1) There were not so large yearly changes in the net-loads of the non-cropping periods (10.6-15.8 kg ha-1) CONCLUSIONS By combine of interval-loads method (ILM) and Separated L(Q)-equations method, the unit load of TN for rice-paddy was evaluated on the base of the data obtained from the experimental small rice-paddy watershed (6.96 ha) located in alluvial low lands around Lake Biwa The unit load (annual net-load) of TN were 35.8 kg ha-1 y-1 (CV: 11%) on the average of three years It became clear that the unit loads of TN fluctuated in relatively wide range from year to year because of yearly changes in hydrological conditions as well as agricultural practices The net-load during the non-cropping period occupied 37% of the total load, which was larger than the load of the puddling and transplanting period (27%) Then, water management during the non-cropping periods as well as the puddling and transplanting periods is important for control over the eutrophication of lakes and inland seas Some results suggested the effect of Environmental-Friendly Agriculture REFERENCES Cho J Y., Son J G., Choi J K., Song C H and Chung B Y (2008) Surface and subsurface losses of N and P from salt-affected rice paddy fields of Saemangeum reclaimed land in South Korea., Paddy Water Environ., 6: 211-219 Kaneki R (2003) Reduction of effluent nitrogen and phosphorus from paddy fields., Paddy Water Environ., 1: 133-138 Kayane I (1980) Evapotranspiration In: Hydrology, Taimeido, Tokyo, pp 94-101 (in Japanese) Kondoh T., Misawa S and Toyota M (1992) Characteristics of Effluent Loads of Nutrient Salts (N, P) from Paddy Fields Located in the Alluvial and Lower Area in Hokuriku Dsictrict., Trans Jpn Soc Irrig Drain Reclam Eng., 159: 17-27 (in Japanese with English abstract) Kubota H., Tabuchi T., Takamura Y and Suzuki S (1979) Water and Material (N P) Balance in the Paddy Fields along Lake Kasumigaura., Trans Jpn Soc Irrig Drain Reclam Eng., 84: 22-28 (in Japanese with English abstract) Kunimatsu T., Rong L., Sudo M and Takeda I (1991) Runoff Loadings of Materials Causing Water Pollution from a Paddy Field during a Non-planting Period., Trans Jpn Soc Irrig Drain Reclam Eng., 170: 45-54 (in Japanese with English abstract) Kunimatsu T., Otomori T., Osaka K Hamabata E and Komai Y (2006) Evaluation of nutrient loads from a mountain forest including storm runoff loads., Water Science and Technology., 53(2): 79-81 Statistics Department, Minister's Secretariat, Ministry of Agriculture, Forestry and Fisheries (2008) Total area and cultivated land area, Statistics of Cultivated Land and Planted Area, Statistics and Information Department, Ministry of Agriculture, Forestry and Fisheries Sugimoto Y., Kunimatsu T Komai T (2006) Change of Nutrient Loads of Paddy-Fields Caused by Crop Rotation from Rice to Wheat., Proceedings of 5th International Conference on Management of Paddy and Water Environment for Sustainable Rice Production (II) in Tochigi, Japan on 10-11 Aug 2006, 02-03 in CD - 125 - Journal of Water and Environment Technology, Vol 6, No.2, 2008 Takamura Y., Tabuchi T., Suzuki S., Harigae Y., Ueno T and Kubota H (1976) The Fates and Balance Sheets of Fertilizer Nitrogen and Phosphorus : Applied to a Rice Paddy Field in the Kasumigaura Basin., Jpn J Soil Sci Plant Nutr., 47 (9) : 398-405 (in Japanese) Takamura Y., Tabuchi T., Harigae Y., Otsuki H., Suzuki S and Kubota H (1977) Studies on Balance Sheets and Losses of Nitrogen and Phosphorus in the Actual Paddy Field in the Shintone River Basin., Jpn J Soil Sci Plant Nutr., 48 (9, 10) : 431-436 (in Japanese) Takamura Y., Tabuchi T., Harigae Y., Nishimura N., Otsuki H., Kubota H., Suzuki S and Osaki K (1979) Behaviour and Balance of Applied Nitrogen and Phosphorus under Rice Field Conditions : (III) Balance and Losses of Nitrogen and Phosphorus in the Well-drained Paddy Field in the Kasumigaura Lake Baisin., Jpn J Soil Sci Plant Nutr., 50 (3) : 211-216 (in Japanese) Takeda I., Kunimatsu T., Kobayashi S and Maruyama T (1990) Storm Runoff Loadings from a Paddy Field Area – Studies on pollution loadings from a paddy field area ( I )., Trans Jpn Soc Irrig Drain Reclam Eng., 147: 79-85 (in Japanese with English abstract) Takeda I., Kunimatsu T., Kobayashi S and Maruyama T (1991) Pollutants Balance of a Paddy Field Area and its Loadings in the Water System – Studies on pollution loadings from a paddy field area ( II )., Trans Jpn Soc Irrig Drain Reclam Eng., 153: 63-72 (in Japanese with English abstract) Udo A., Jiku F., Okubo T and Nakamura M (2000) Mass Balances of Water and Nutrients in a Paddy Field., Jpn Soc Water Environ., 23 (5) : 298-304 (in Japanese with English abstract) - 126 - ... mineralization of organic nitrogen, as Kunimatsu et al (1991) indicated Storm-runoff loads Separate L(Q) method It is well known that loading rates of materials and flow rates - 119 - Journal of Water... balance of TN was estimated from the results obtained from continuous observations for three years at a small watershed of rice-paddies MATERIALS AND METHODS Study site The experimental small rice-paddy... CONCLUSIONS By combine of interval-loads method (ILM) and Separated L(Q)-equations method, the unit load of TN for rice-paddy was evaluated on the base of the data obtained from the experimental small