NUTRIENT RESIDUAL IN AN IRRIGATION POND -A CASE STUDY OF HIGASHIIKE -

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1 NUTRIENT RESIDUAL IN AN IRRIGATION POND -A CASE STUDY OF HIGASHIIKE - Ken HIRAMATSU 1, Akihiro SATO 2, Toshihiko KAWACHI 2 and Hiroshi ITAGAKI 1 1 Department of Land and Water Engineering, Gifu University, Gifu, Japan 2 Division of Environmental Science and Technology, Kyoto University, Kyoto, Japan ABSTRACT: Irrigation ponds called Tameike that are a kind of Japanese traditional RHW are built to store precipitations and supply them to farmland. About 11.4% of the total irrigation water is supplied from irrigation ponds in Japan. It has, however, been known that they work not only for water storage but also purification of water. The catchment area of most ponds consists of forests and mountains, but some ponds have paddy field or dry field in their catchment area. In such a case, the runoff contains considerable nutrients of phosphorus and nitrogen due to fertilizers and therefore, quite large amount of nutrients drift into the pond. Meanwhile, some of them settle down to the bottom and are consumed by phytoplankton and aquatic plants during the retention time. Consequently, nutrients move away from the water and stay in the pond. Then, the water that is reused for irrigation and miscellaneous purposes is purified. It is, however, still unclear that how much they stay behind. In this research, their residual ratios are discussed by investigating the balance of nutrients in a study pond, Higashi-Ike that is a small shallow pond with a capacity of 2,4 cubic meters and has paddy field with an area of about 5,2 square meters in its catchment area. As a result, 37.61kg of nitrogen and 11.9kg of phosphorus flowed into the pond and 7.5% and 5 % of them remained, respectively, in the research period. In addition, it becomes clear that the residual ratio is closely related with the fertilizing terms. INTRODUCTION Irrigation ponds that are often called Tameike are a kind of Japanese traditional rainwater harvesting facilities. The number of existing irrigation ponds in Japan is about 1,. Each impoundment of them is certainly small, i.e. mostly less than 5, m 3, but the sum of becomes about 3,42 million m 3 and corresponds to about 11.4 % of the total irrigation water impoundments in Japan. A fair percentage of them are seen in the region with low- precipitations like Seto inland region where annual precipitation is less than 1,5 mm. The main purpose of irrigation pond is to store and supply water to farmland, but some other roles are also found as; 1) cut of the peak in flood runoff, 2) bio-habitat or biotope, 3) fire-fighting use, 4) recreation and 5) purification of water. These roles are reevaluated recently, while the number of irrigation ponds is decreasing on the other hand as the domestic agricultural industry regresses. The catchment area of irrigation pond usually consists of forest and mountain, but some of Irrigation ponds have paddy or dry field in their catchment area. In such a case, the runoff contains considerable nutrients of phosphorus and nitrogen due to fertilizers and therefore, quite large amount of nutrients drifts into the pond. Meanwhile, some of them settle down to the bottom and also some of them are consumed by phytoplankton and aquatic plants during the retention time. Consequently, nutrients move away from the water and stay in the pond. Then, it can be considered that the water that will be reused for irrigation and miscellaneous purposes is purified by the impoundment. It is, however, still unclear that how much they stay behind. In this research, their residual ratios are discussed by Fig.1 Overview of Higashiike

2 investigating the balance of nutrients in a study pond. STUDY SITE Higashiike is a small and shallow irrigation pond located in the northeast part of Minakuchi-town, Shiga prefecture. Its maximum surface area is about 3, m2, its average water depth is about.82 m and its capacity is about 2,4 m3. Fig. 1 shows the overview of Higashiike. Fig.2 shows schematic view of Higashiike, its catchment area, the position of the spillway, and the position of inlets. Higashiike has only two major inlets as seen, however infiltration through banks implicitly exists. The inflows from the western and southern inlets that are named inflow1 and inflow2, respectively, are measured continuously using pressure sensors as well as the fluctuation of water level of the pond. Figs. 3, 4 and 5 show inflow1, inflow2 and spillway, respectively. Spillway N Higashi-ike P1 P2 Inflow1 P6 S4 S6 The catchment of Higashiike consists of paddy field and forest. The total area of the catchment for inflow1 is 13,691 m2 of which the area of paddy field is 2,75 m2. That for inflow2 is 58,237 m2 and the area of paddy field included is 2,978 m2. From here onwards, it is clear that the catchment area of inflow2 is larger than that of inflow1, but each area of paddy field is about the same. Meanwhile, the command area, most of which is paddy field, is about 5, m2. In terms of biota, there are only killifish, small shrimps, and Rhinogobius sp. Species diversity of biota in Higashiike is not so high. This may be greatly influenced by environmental factors such as low dissolved oxygen and the condi- P : Paddy field S : Catchment slope P3 P4 Irrigation tank P5 S1 S5 Inflow2 S2 S3 Fig 5 Spillway on the north bank Fig 2 Schematic view of Higashiike Fig 3 Inlet 1 (Inflow 1) with triangular weir tion of aquatic vegetations. There are a few kinds of aquatic plants, i.e., Chinese water chestnut and reed. However, in summer almost water surface is covered with Chinese water chestnut and in early autumn Chinese water chestnut rots away and sinks to the bottom. Transparency of the pond is not so high, but even the bottom layer is included in the euphotic zone, because of the shallowness. In addition, strong thermocline does not appear throughout the year. The paddy and dry field in the command area is usually irrigated by the water of Yasu River and the impounded water is withdrawn through the downspout subsidiarily, only when the river water comes short. Then, the retention period of the pond becomes quite long as about 28 days, considering that the volume of the pond is small. METHOD The volume of water is estimated by using observed water level, precipitation and discharge of inflows. By the previous work (Kirihata et al. 21), the impoundment V and discharge of outflow qout are formulated as; V = 113.1h h and Fig 4 Inlet 2 (Inflow 2) with rectangular weir (1)

3 dv dt 3 = q ( t) q ( t) (2) in where h is water level, q in is discharge of inflow. Water is sampled at the inlets and at the center of the pond for chemical and physical analyses from December 2 to December 21. At the center sampling is executed both for shallow layer (surface) and deep layer (near bottom). Sampling frequency is once a month during non-irrigation period and twice a month during irrigation period. At the center of the pond, dissolved oxygen, ph, EC, water temperature, transparency and so on are also measured by probes (Horiba, W-2XD) and some instruments on site. Nutrients such as total phosphorus (T-P), total nitrogen (T-N), nitrate nitrogen (NO 3 - -N), ammonia nitrogen (NH 4 + -N) and ortho-phosphate (PO P) and chlorophyll a (chl-a) are measured with a spectrometer (Shimadzu UVmini 124) and an ion chromatography (Shimadzu PIA 1) in the experimental laboratory after bringing back the samples. The nutrient that remains within the pond is estimated from the difference between a mass in inflows and that in outflow. In this surrey, the concentration of nutrients in the upper of the center is substituted for that in the outflow, because their values are almost similar in this pond. RESULTS OF SURVEY Fig.6 shows the precipitation and storage volume in the study pond. The survey period is roughly divided into three terms as non-irrigation period I (from December 2 to March 21), irrigation period (from April to September 21) and non-irrigation period II (from October to December 21). As seen Fig.6, the impounded water is withdrawn twice in the irrigation period. precipitation(mm/day) Total nitrogen and Total phosphorus (T-N, T-P) Figs. 7 and 8 show temporal change of T-N and T-P concentration. T-N and T-P vary from.2 to 2.1 mg/l and from.2 to 2. mg/l irregularly. This complex fluctuation may be due to the size of the pond. Even a small disturbance can change the water quality of the pond considerably. The great fluctuations are due to the fertilizer into the farmland in out Dec- Jan-1 Feb-1 Mar-1 Apr-1 May-1 Jun-1 Jul-1 Aug-1 Sep-1 Oct-1 Nov-1 Dec-1 Fig. 6 Precipitation and storage volume Storage volume(m 3 ) Storage volume(m ) storage volume(m3) the catchment area. According to our survey, the organic fertilizer that contains 15% of nitrogen, 1% of phosphorus and 15% potassium, is applied to paddy field three times in T-N(mg/L) T-P(mg/L) Fig 7 Total nitrogen at the center of the pond (upper and lower levels) and at the inlets Fig 8 Total phosphorus at the center of the pond (upper and lower layers) and at the inlets the irrigation period, i.e. 2 g/m 2 on 2 April and 2 June and 1-15g/m 2 on 1 July. In the non-irrigation period I, the average concentrations of T-N are.57 mg/l in upper layer and.6 mg /L in lower layer, respectively and those of T-P are.46 mg/l in upper layer and.93 mg/l in lower layer, respectively. The concentrations do not fluctuate largely. T-P in the lower layers is twice as high as that in the upper layer, while there is not much difference between T-N in upper and lower layers in this period. From here onward, it is estimated that phosphorus may be eluting from the bottom mud and sediment and detritus that contains much phosphorus are settling abundantly. In the irrigation period, the average concentrations of T-N are.67 mg/l in upper layer and.74 mg/l in lower layer, respectively and those of T-P are.211 mg/l in upper layer and.185 mg/l in lower layer respectively. Both T-N and T-P of the inflows 1 and 2 increase drastically, when fertilizer was applied. And in proportion as they increase, the T-N and T-P in the center of pond also widely fluctuate, except the T-P in July.

4 In the non-irrigation period II, the average concentrations of T-N are.98 mg/l in upper layer and 1.16 mg/l in lower layer, respectively and those of T-P are.44 mg/l in upper layer and.5 mg/l in lower layer, respectively. The values of T-N are still larger than those of the non-irrigation period I. That may be because autumn is a rainy (typhoon) season in Japan and nutrients come in with soils as well as runoff. In addition, it is considered that crops consume phosphorus thoroughly in the irrigation period. From 1991 to 1993, water quality of 896 irrigation tanks in Shiga prefecture had been surveyed. In this survey, the average concentration of T-N is 1.5 mg/l. In comparison with these data, the concentration of T-N in the study pond is relatively low. Nitrate and ammonia nitrogen (NO 3 - -N, NH 4 + -N) Fig.9 and 1 indicate temporal change of nitrate nitrogen NO 3 - -N and ammonia nitrogen NH 4 + -N, respectively. 4. In the non-irrigation periods I and II, the average concentrations of NO 3 - -N are.62 mg/l in upper layer and.46 mg/l in lower layer, respectively. In the irrigation period, the average concentrations of NO 3 - -N rise higher to.99 mg/l in upper layer and.94 mg/l in lower layer, respectively, which is also due to the influence of the fertilizer application. In addition those of inflows show remarkable rises that are more than ten times of the usual. NH 4 + -N shows a tendency of fluctuation almost similar to NO 3 - -N. The concentrations in the non-irrigation periods are not so high but increasing at the irrigation period. The average concentration in the non-irrigation periods is.26 mg/l in upper layer and.35 mg/l in lower layer, respectively. That in the irrigation period is.61 mg/l in upper layer and.81 mg/l in lower layer, respectively. Ortho-phosphate (PO P) Fig.11 shows fluctuation of ortho-phosphate PO P. In the non-irrigation periods I and II, the average concentrations of PO P are.9 mg/l in upper layer,.13 mg/l in lower layer, respectively and in the irrigation period, they are.15 mg/l in upper layer,.16 mg/l in lower layer, respectively Nitrogen in nitric acid ion(mg/l) Nitric acid ion(mg/l) Inorganic phosphorus(mg/l) Inorganic phousphorus(mg/l) Nitrogen in ammonium ion(mg/l) Anmonium ion(mg/l).5. 1/9 2/28 4/19 6/8 7/28 9/16 11/5 12/ Fig.9 Nitrate nitrogen at the center of the pond (upper and lower levels) and at the inlets Fig.1 Ammonia nitrogen at the center of the pond (upper and lower layers) and at the inlets Compared with T-P in the non-irrigation period I, the difference between the upper and lower layers is not distinct. It indicates that the influence of the sedimentation of some particulate inorganic matters or some organic matters is superior to that of the elution. Enhancements of the concentration in the influxes are also due to the application of fertilizer. Among them, the enhancement in April is remarkable, while other indices of water quality do not respond at that time. Chlorophyll a (chl-a) The fluctuation of chlorophyll a that is an index of phytoplankton is shown in Fig. 12. In the non-irrigation period I, the average concentration of Chlorophyll a is 4.15 µg/l in upper layer and 4.73 µg/l in lower layer, respectively. Chl-a( g/l) Chl-a (ìg /L) Fig.11 Ortho-phosphate at the center of the pond (upper and lower levels) and at the inlets Upper layer Lower layer Fig. 12 Chlorophyll a at the center of pond (upper and lower layers)

5 In the irrigation period, those in upper and lower layers are 8. µg/l and 7.98 µg/l, respectively. In non-irrigation period II, those in upper and lower layers are 7.31 µg/l and 5.55 µg/l, respectively. It seems that there is no depth-wise variance, because both upper and lower layers are within the euphotic zone, as mentioned above. MASS BALANCES Nitrogen and phosphorus that accumulate in the pond are estimated from the difference between the product of water quality indices by influx and that by efflux. Table 1 shows the mass balances of nitrogen and phosphorus for every month. Total weights of T-N in the inflow and outflow are kg and kg, respectively, during the research period. Those of T-P are 11.9 kg and 5.9 kg, respectively. Thus, 2.93 kg of T-N and 6.kg of T-P are concluded to remain in the pond. Those correspond to about 7.5 % of the total incoming nitrogen and 5 % of the total incoming phosphorus, respectively. When details are examined, it becomes clear that the residual of T-N is negative in the non-irrigation period and the residual of T-N increases as the concentration of T-N in the inflows rises. On the other hand, the residual of T-P is mostly positive throughout the research period and its rate becomes biggest in June and August that are one month after the application of the fertilizer. This may indicate that the influence of fertilizer on phosphorus appears later than that on nitrogen. The amount of residuals of both nitrogen and phosphorus are closely related with the application of fertilizer. The nutrients remain positively in the pond when the fertilizer is applied and drifts into the pond. Schematic flow of phosphorus Phytoplankton plays an essential role in the nutrients recycling of the irrigation pond. Some of inorganic nutrients are fixated by the uptake of the phytoplankton to organic matters. They are preyed by the superordinate or settle down as detritus. In general, this uptake is formulated by a simple equation as; dm dt vm = µ M kd M M (3) z where M is the concentration of phytoplankton that is given by chlorophyll a, t is time, µ is growth rate of phytoplankton, k d is mortality of phytoplankton, v m is settling rate of phytoplankton, and z is depth of the pond. If the amount of phytoplankton moves on a year cycle and that per annum does not change, the left side member of yearly averaged Eq. (3) should be zero. Under this presumption, the term of growth becomes same as the sum of the rest terms of the right side member of Eq. (3) that are expected to be a residual in the pond. Hereafter, phosphorus is considered to find the production of phytoplankton, because the average ratio of nitrogen and phosphorus is more than 7:1 in the research period and then phosphorus often plays as a limiting nutrient for production of phytoplankton. The following Monod s equation gives the growth of phytoplankton in phosphorus Table 1: Inflow, outflow and residual of T-N and T-P (bold face means remarkable value.) Month-Year Inflow (m 3 ) Inflow of TN Inflow of TP Outflow of TN Outflow of TP Residual of TN Residual of TP Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total

6 δp M P = µ max MλPM (4) P + K where δp M is the variation of phosphorus in phytoplankton, µ max is maximum growth rate of phytoplankton, P is the concentration of T-P, K P is a half-saturation coefficient and λ PM is the ratio of phosphorus in phytoplankton. The dominant phytoplankton is a small diatom and commonly used values are cast into the parameters of Eq. (4) referring to e.g. Hiramatsu et al. (1999). Fig.13 shows the schematic flow of phosphorus in the pond. As a result, roughly 13 % of remaining phosphorus and 31 % of nitrogen are fixated through physiological action of phytoplankton. P in the pond. However, the influence does not appear in the non-irrigation period. (d) Phytoplankton plays an essential role in the nutrient recycling. Roughly 7 % of the phosphorus that remains in the pond is fixated by the phytoplankton. The results of the survey show the mass balance of the nutrients in the pond roughly. However, the survey frequency is still low and no survey during rainy days is included in the results. Intensive surveys are required to make the mass-flow clear. The irrigation pond catches the nutrients in the runoff and the drained water from the paddy and dry field quite efficiently. It works like a wetland that is artificially built to control the water quality. However, it can be said that the pond is eutrophicating in return of the residuals on the other hand. It indicates that dredging or some other adequate management will be required to maintain the sound water environment of the pond. Fig. 13 Schematic view of T-P flow CONCLUSIONS The conclusions are summarized below; (a) About 7.5% of nitrogen and 5 % of phosphorus in the influx are accumulated in the pond. The ratio for phosphorus is quite large and that for nitrogen is not so high, when comparing the previous works. E.g. Okubo et al. (2a, b) reports that 19 % of phosphorus and 14 % of nitrogen remains in the Ekainuma and little phosphorus and nitrogen remain in the Daimonike in Shiga prefecture, and Nagasaka et al. (21) shows that 4-43 % of phosphorus and 46-5 % of nitrogen remains in a small irrigation pond in Kyoto Prefecture. The retention periods of the reference ponds are shorter than that of Higashiike. It may cause the differences. (b) Application of fertilizer has strong influence on the concentration of nutrients in the pond as well as in the inflows. Moreover, the residual of nutrients increases as the concentration of nutrients rises. Then, the application of fertilizer and the residual have a strong relation. (c) Elution of nutrients from the bottom mud certainly exists, but its speed is slower than that of sedimentation. This fact also indicates that nutrients are likely to remain ACKNOWLEDGEMENTS This research was partially supported by Grant- in-aid for Scientific Research (B), and that for Young Scientists (B), 2-22 of the Ministry of Education, Culture, Sports, Science and Technology, Japan. REFERENCES Braskerud, B. C. (1995) Factors affecting phosphorus retention in small constructed wetlands treating agricultural non-point source pollution, Ecological Engineering, 19, Hiramatsu, K., Kawachi, T. and Nada, Y. (1999) Side-View Modelling in Thermally Stratified Reservoirs, Trans. JSIDRE, 199, Kirihata, H., Kawachi, T., Unami, K. (21) Stochastic identification of runoff process in the catchment of an irrigation pond, Proc. Annual Congress of JRCSA, Nagasaka, S (21) Studies on water quality and environment in the rural area with irrigation ponds, PhD thesis of Kyoto University. Okubo,T.,Tsujimura, S. and Sudo, M. (2a) Purification of water in endorheic -A case study of Ekainuma, Adogawa town-, Report of Lake Biwa Research Institute, 18, (in Japanese). Okubo,T.,Tsujimura, S. Tanaka, H. and Higashi, E. (2b) Mass budget in an irrigation pond occurring blue-green algae -A case study of Daimonike, Taga town-, Report of Lake Biwa Research Institute, 18, (in Japanese). Rural Development Bureau, Ministry of Agriculture, Forestry and Fisheries (1981) Database of irrigation ponds in Japan (in Japanese).