A DISTRIBUTED WATER QUALITY TANK MODEL FOR NITROGEN LOAD REDUCTION BY ARTIFICIAL WETLANDS

A distributed water quality tank model was developed to evaluate nitrogen load reduction by artificial wetlands in the A10 area, located in the Lake Kasumigaura watershed in Japan. In order to collect data for model calibration and verification, a year-long field investigation was conducted. The study area was divided into a 50 m × 50 m grid, and attributes and the model are added to each cell. The model simulates the runoff and the total nitrogen (TN) concentration with average relative errors of 10% and 2%, respectively, except for no-rain periods. Scenario analyses were conducted considering artificial wetlands at different locations. The three scenarios are as follows: no artificial wetland, cell located at the head of the valley and cell located on a flow path through animal waste cell. Compared with case 1, the annual TN concentration was lower by 4% and 11% in cases 2 and 3, respectively. The results suggest that artificial wetlands on water flow paths with a large nitrogen load effectively reduce these loads and that the model quantitatively evaluates the difference in the nitrogen concentration across locations.

A DISTRIBUTED WATER QUALITY TANK MODEL FOR NITROGEN LOAD REDUCTION BY ARTIFICIAL WETLANDS

T Kato*1, H Kuroda *2 and H Nakasone *3

* Faculty of Agriculture, Ibaraki University, 3-21-1 Chuoh, Ami, Inashiki, Ibaraki 300-0393, Japan

1 tkato@mx.ibaraki.ac.jp, 2 kuroda@mx.ibaraki.ac.jp, and 3 nakasone@mx.ibaraki.ac.jp

ABSTRACT

A distributed water quality tank model was developed to evaluate nitrogen load reduction by artificial

wetlands in the A10 area, located in the Lake Kasumigaura watershed in Japan In order to collect data for

model calibration and verification, a year-long field investigation was conducted The study area was divided

into a 50 m × 50 m grid, and attributes and the model are added to each cell The model simulates the runoff

and the total nitrogen (TN) concentration with average relative errors of 10% and 2%, respectively, except

for no-rain periods Scenario analyses were conducted considering artificial wetlands at different locations

The three scenarios are as follows: no artificial wetland, cell located at the head of the valley and cell located

on a flow path through animal waste cell Compared with case 1, the annual TN concentration was lower by

4% and 11% in cases 2 and 3, respectively The results suggest that artificial wetlands on water flow paths

with a large nitrogen load effectively reduce these loads and that the model quantitatively evaluates the

difference in the nitrogen concentration across locations

KEYWORDS: Diffuse pollutant source; Purification; Scenario analysis; Total nitrogen

INTRODUCTION

Nutrients are often found in high concentrations in agricultural watersheds due to excess fertilizers and animal wastes In general, it is difficult to effectively reduce the amount of diffuse pollutants; however, it is known that paddy fields or wetlands have a purification function, especially for nitrogen pollutants In a small valley, spring water contaminated by a diffuse pollutant load from farmlands located on the surrounding uplands is naturally purified in the paddy fields located on the lowlands; this characteristic is

called the ‘topographic chain’ or ‘topographic sequence’ effect (Tabuchi, 1986; Tabuchi et al., 1991; Nakasone et al., 1996; Kuroda, 1998) and is observed in many farmlands in Japan

In this study, a distributed water quality tank model was developed, and the difference in nitrogen concentration was simulated considering artificial wetlands at different locations An ‘artificial wetland’ is land formerly used as a paddy or bare field that is now filled with water contaminated with diffuse pollutants

A distributed model is generally used to simulate location information (Lu et al., 1989; Goto et al., 1993; Abbott et al., 1996; Vieux, 2001; NIES, 2002); therefore, we modified a ‘water quality tank model classified

by land use’ (Kato et al., 2003) into a distributed model based on detailed data obtained from an

approximately year-long investigation and simulated the nitrogen load under three scenarios considering artificial wetlands at different locations

STUDY AREA AND FIELD INVESTIGATION

Study area

The study area considered was the A10 area, located in the Kasumigaura watershed in Japan (Fig 1) Lake Kasumigaura is the second largest lake in Japan Its water resources are used for municipal, domestic, industrial and irrigational purposes Despite the countermeasures for improving water quality, the annual average total nitrogen (TN) concentration did not improve; rather, it remained at 0.93 mg·L–1 in 2002 (Ibaraki Prefecture environment white paper, 2003)

The area of the A10 is 55.4 ha; in 2001, the human population in the area was 118 and the livestock population was 2,400 Animal wastes, in particular, have accumulated in unlined storage ponds and have

affected the water quality (Shimura et al., 1996) Though the acreage is small, the land use is

diverse—paddy fields, upland vegetable fields, forests and livestock farms—and the TN concentration in the drainage water varies across the points Therefore, we believe that the area is suitable for comparing the effect of artificial wetlands using the distributed model

Field investigation

A field investigation was conducted at four points to measure the nitrogen concentrations and collect runoff data for the period from August 2001 to July 2002 Sampling point A is the mouth of the river; point B, the drainage in the paddy fields surrounded by forests; point C, the drainage of the paddy fields surrounded by the upland fields; and point D, the spring water near the livestock farms (Fig 1) The measurement parameters are the TN, NO3-N, NO2-N and NH4-N concentrations (measurement instruments: YANACO TN-301P; DIONEX DX-100) The water was analyzed by our laboratory staff at Ibaraki University

At point A, a weir was built in the drainage and the runoff was measured using a pressure-type water level gauge (HI-TECHS, HIT-M8) every 10 min for the period from 10 August 2001 to 24 July 2002; in addition, the rating curve (water level discharge curve) was graphed based on the data from an electromagnetic current meter (Tokyo Keisoku SF-5001) every week The water was sampled using an automatic sampling machine (ISCO, ISCO6400) everyday At points B, C and D, the water was manually sampled every week for the period from 1 April to 24 July 2002

Results of the investigation

The average measured data are shown in Table 1 The NO3-N concentrations for points A and D are considerably higher than the maximum legal level of 10 mg·L–1 The TN concentration is lowest at point B because the main land use in this area is forests The TN concentration at point C is higher than that at point

B because upland fields are located behind it The TN concentration at point D is 46.7 mg·L–1 because the water is spring water contaminated by fertilizers and animal wastes

The data measured at point A are illustrated in Fig 2 Data for October 2001 and April 2002 are unavailable due to machine trouble The TN and NO3-N concentrations were diluted by rainfall TN mainly consists of

NO3-N; these results support those of the previous study, which revealed that nitrogen contained in fertilizers and animal wastes accumulates in soil (Kuroda, 1996) The NH4-N concentration is typically high when

N

0 200 m

(ha) Paddy Forest Upland Residence

9.3 23.1 21.5 1.5 Sampling point

A

D

C

B

River

Fig 1 Land use and sampling points in the A10

・

Ibaraki

Tokyo

Lake Kasumigaura Unlined storage pond Drainage

rainfall occurs, indicating that animal wastes directly flow in the drainage along with rainwater We confirmed that a large difference exists in the nitrogen concentrations across locations and that fertilizers and

animal wastes are the main sources of pollutants

DISTRIBUTED WATER QUALITY TANK MODEL

Structure of the model

The study area was divided into a 50 m × 50 m grid with a total of 350 cells Attributes such as elevation, land use and drainage data were added to each cell and were determined by means of a field investigation by using a land-use map and a 1:5,000 map (Fig 3) The Geographical Survey Institute in Japan publishes elevation data and the land-use map A channel network (flow directions) is determined based on the elevation data, and the water and nitrogen load move to the lowest elevation cell among the four adjacent directions A tank model, comprising a set of two tanks, is applied to each cell (Fig 4), and a tank is also applied to a drainage cell Water and the nitrogen load arrive at the drainage cell and then flow into a drainage tank The time step is set to 10 min

The equations used in the model are as follows Symbols used in the equations are illustrated in Fig 5

The equations for the runoff are as follows:

where S is the storage amount in the tank, h is the hole height, r is the coefficient of discharge, q is the

2 2

2 2

2 2 3

3 2 2 3

2 2 3 1

3 2 2 3 1 1

3 2 2 2 3 1 1

3 2 2 3 3 3 1 4

2 2 2 2 3 1 1 3 3 1

1 1 2 2 3 2 2 2 3 3 3 1 1 1

1 2 2 2 3 3 5 2 2 2 3 1 1

3 3 3 3 3 3 3 3 2 3 4 4 1

1 5 3 1 5 3 3 2 2 3 1 1 1 1

1 1 1 3 3 3 2 3 2 3 3 1 1

1 1 1 1 2 2 5 2 2 3 3 1

1 1 1 3 3 3 3 2 3 1 4 1

1 1 1 3 1 1 1 3 3 4 1 1

1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1

0 3

2 4

5 3 5

7 5 4 30

8 6 12 30

11 9 7 12 30 31

13 12 10 12 13 30 31 32

35 34 16 14 14 13 13 14 30 31 32 33 34 35

35 34 16 15 16 16 15 15 16 31 32 33 34

35 34 19 20 31 31 30 26 18 31 32 33 34

35 34 31 32 32 31 30 26 19 31 32 33 34 35

34 33 32 30 26 25 23 20 21 32 33 34 35

33 32 30 28 29 26 26 23 24 33 34 35

35 33 32 31 30 32 32 27 26 33 34 35

37 35 35 35 35 35 30 29 30 31 34 35

37 37 37 37 37 33 33 33 33 34 35

37 37 37 37 37 37

Elevation data (m) Land use data Drainage data

Fig 3 Mesh data

1: upland field 2: paddy field 3: forest 4: residence 5: livestock farm

0 10 20 30 40 50 60

0 50 100 150 200 250

-1 )

NO 3 -N NH4 -N

Fig 2 Results at sampling point A Aug Oct Dec Feb Apr Jun

50

0

100

150

200

250

0

40

60

20

TN 30.45 5.82 10.70 46.73

NO3-N 25.85 4.60 9.38 43.24

NH 4 -N 1.01 0.16 0.11 0.49

NO2-N 0.43 0.01 0.02 0.02

Table 1 Average measured data (mg·L-1)

outflow discharge from each hole, Q is the total outflow discharge to the adjacent tank, i is the tank number (1: upper tank, 2: lower tank), j is the hole number (0: bottom), t is the time step and mesh is the item (0–3:

the number of cells from which water and nitrogen load flow)

The equations for the storage amount are as follows:

where R is rainfall, I is the quantity of irrigation water, E is the amount of evapotranspiration and A is the

total number of cells through which water flows

The equations for the nitrogen load are as follows:

(((1 − β) (R N + I N + αPL + D1) + S N1 )/S 1, t + c 1, j B 1, j ) q 1, i = d 1, i (5)

(((1 − β) (αd1, 0 + D2) + S N2 )/S 2, t + c 2, j B 1, j ) q 2, i = d 2, i (6)

where R N is the nitrogen load in rainfall, I N is the nitrogen load in irrigation water, PL is the pollutant load for each type of land use, D is the total amount of load flowing to the adjacent tank, S N is the load in the tank,

c is the coefficient of cumulative load, B is the amount of cumulative load, d is the outflow load from each

hole, α is the dissolution rate and β is the purification rate (Kato et al., 2003)

The equations for the cumulative load are as follows:

Input data

The input data are rainfall, TN concentration in rainfall (1.1 mg·L–1), quantity of irrigation water, land-use area, human population, livestock population, fertilizer (amount, absorption rate for each crop and schedule) and unit effluent load Hourly rainfall data were drawn from the data collected at the Hokota observatory The evapotranspiration, measured at Ibaraki University, was found to be 3 mm·d–1 The quantity of irrigation water was calculated based on the pump operation records; the average TN concentration of irrigation water was 9.5 mg·L–1 for the period from 20 May to 24 July 2002 Data for fertilizers were obtained from literature (CAES, 1976; Nishio, 2001) The TN load per unit of livestock and septic tank were 28 and 10.35 g·d–1, respectively (IPLEDKCS, 1999)

Model calibration and verification

The trends in the measured data for point A for the period from 10 August to 31 December 2001 were simulated with the parameters manually adjusted to reduce the least squares error (Table 2) Considering that

nitrogen effluent rates differ depending on the land use, the dissolution rate (α) is set to 0.5 for the upland

fields, 0.0 for livestock and 1.0 for others, that is, the nitrogen load from fertilizers is divided into surface runoff load and cumulative load, and the nitrogen load from animal waste constitutes only cumulative load

The model simulates the runoff and the TN concentration with average relative errors of about 10% and 2%, respectively (Fig 6) Although the time step was 10 min, the daily data for only noon are considered representative The verification is shown in Table 3 The monthly relative errors for TN concentration at

β

d1, 2

R N , I N , D1

c1, 2

q1, 2

r1, 2

S1

Fig 5 Symbols for the upper tank Runoff Load

R, I E

B1, 2

S N1

PL

α 1−α 1−β

h1, 2

h1, 2

A

Q1

Tank

Land use

Elevation

Drainage Drainage tank

Fig 4 Schematic diagram of the model

point A are over 30% for the period from January to May, 2002 because this area received little rainfall in winter and the calculated runoff values were lower than the measured values As for nitrogen load, the relative errors become small—the average relative errors are about 10% for point B, 30% for point C and 20% for point D In addition, Fig 7 shows the verification of the TN concentration at point A every 10 min when rainfall occurred (15 and 16 July 2002); the relative error is about 15% Note that the calculated TN values express the difference between the four points according to the measured values and that the model responds to the 10-min time step

0.00E+00

5.00E-05

1.00E-04

1.50E-04

2.00E-04

Calculated Observed

0 10 20 30 40 50 60

Calculated Observed

Fig 6 Simulation of the runoff and the TN concentration

TN concentration (mg·L -1 ) Runoff (10 -4 m·10 min -1 )

0

2.0

1.5

1.0

0.5

Sep Aug

60

40

20

0

Calculated Observed

Calculated Observed

Sep Aug

Upland 1 0.16 0.001 4 0.5 0.3 Residence 1 0.08 0.001 7 1 0.3

Paddy 1 0.16 0.0005 3.5 1 0.6 (ir) Stock 1 0.16 0.001 4 0 0.3

Forest 1 0.16 0.012 4.8 1 0.3 River 1 0.5 0.6 0 0

0 0.01 6.2

2 0.002 0.0001 5.5 0.001 0.0005 5.5

Table 2 Values of the model parameters

Residence

Table 3 Verification

SCENARIO ANALYSIS FOR NITROGEN LOAD REDUCED BY ARTIFICIAL WETLANDS

Scenario analysis

The TN concentration at point A is calculated for artificial wetlands at different locations The three scenarios are as follows: no artificial wetland, cell located at the head of the valley and cell located on a flow path through an animal waste cell

The common calculation conditions are as follows:

・Prediction period: 10 August 2002 to 9 August 2003

・Time step: 10 min

・Precipitation: Data for the period from 10 August 2001 to 9 August 2002

・The parameters for the artificial wetlands are the same as those of the paddy fields, but the purification rate

is 0.6 for the entire year

・Artificial wetlands: The total number of cells in the paddy field cells is 60, of which three cells, i.e., 5%, are used The symbols (i, ii, iii, iv and v) indicate the location of the cell in Fig 8

・Equations: Eq.(5) and (6) are modified to express the purification of the nitrogen load in a tank

((1 − β) (R N + I N + αPL + D1 + S N1 )/S 1, t + c 1, j B 1, j ) q 1, i = d 1, i (9)

((1 − β) (αd1, 0 + D2 + S N2 )/S 2, t + c 2, j B 1, j ) q 2, i = d 2, i (10)

Results Case 1: No artificial wetland The calculated annual nitrogen load is 9.65 kg and the calculated TN concentration is 31.0 mg·L–1 (Table 4) The values are almost the same as the measured values obtained during the field investigation Case 2: The cell is located at the head of the valley The artificial wetlands are located at the head of the valley (i, ii and iii) As the head of the valley is inconvenient to access, paddy fields at this location tend to become fallow Therefore, this location is a possible site for artificial wetlands The calculated annual nitrogen load is 9.30 kg; the calculated TN concentration is 29.9 mg·L–1,which is 4% lower than that of case 1 Case 3: The cell is located on a flow path through an animal waste cell 0 10 20 30 40 50 21:00 3:00 9:00 15:00 0 2 4 6 8 10 Calculated Observed ． Fig 7 Verification at point A every 10 min with rainfall Rainfall (mm·10 min -1 ) TN concentration (mg·L -1 ) 24:00 6:00 12:00 18:00 　 　 　 v 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 　 iii iv 　 　 　 　 　 　 ii 　 　 　 　 　 　 　 i 　 　 　 　 　 　 　 　 　 　 　 Fig 8 Location of artificial wetlands Drainage cell Livestock farm cell Artificial wetland cell 1

1

1

1 1

1 1 1 1

1 1 1

1

1 1 1

1 1

1 1 1

1 1 1 1

1

1

Flow direction

−

The number of cells for which the land use is livestock farms is three for ‘i’ and two each for ‘iv’ and ‘v’ The load from animal wastes is purified by the artificial wetlands The calculated annual nitrogen load is 8.60 kg; the calculated TN concentration is 27.6 mg·L–1, which is 11% lower than that of case 1

Table 4 Results of scenario analyses

Case 1 Case 2 Case 3

N load 10 3 kg·y -1 9.65 9.30 8.60

TN concentration mg·L -1 31.0 29.9 27.6

CONCLUSIONS

A distributed water quality tank model was developed to simulate nitrogen load reduction by artificial wetlands in the A10 area, located in the Kasumigaura watershed in Japan The model simulates the measured runoff and TN concentration with average relative errors of about 10% and 2%, respectively The calculated

TN concentration expresses the difference between the four points according to the measured values Scenario analyses were performed, and the annual TN concentration was found to be lower by 4% and 11%

in cases 2 and 3, respectively The results suggest that, to effectively reduce nitrogen load, the artificial wetlands should be located near flowing water that contains a large nitrogen load The model is capable of quantitatively evaluating the difference in nitrogen concentration across locations

In Japan, paddy fields, especially those at inconvenient locations, tend to be fallow due to the decrease in rice demand However, with paddy fields being converted to artificial wetlands, their purification function is maintained This is one of the methods of conserving the water environment in an agricultural watershed Simulating the appropriate location of artificial wetlands can provide policymakers with the information necessary to set the target water quality and the size of the artificial wetlands

The following steps need to be taken in the future:

- The model must be improved to achieve accurate simulations during no-rain periods One approach is to consider the ground water level, which might affect the runoff

- Verification of the model response to rates such as the cumulative rate and purification rate is required The values used in the developed model are obtained from literature; however, we would like to continuously monitor the study area to obtain values from the measured data

- The number of parameters should be reduced to shorten the calculation time

Acknowledgements

We greatly appreciate Dr Tabuchi Toshio and Dr Wesley W Wallender of University of California, Davis for giving us valuable opinions Thanks go to Mr Kifune Yasunori who helped develop the model This research was supported by the Ministry of Education, Culture, Sports, Science and Technology‚’Grants-in-Aid for Scientific Research’ (subject number: A 14206027)

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