Evaluation of Nutrient Loads from a Citrus Orchard in Japan

We estimated four methods of calculating loads of nutrients from a citrus orchard in Japan. River water discharge and nutrient loads were examined every week for 2 years and more intensively at the time of one rainfall event. River water level was recorded automatically at 5-minute intervals. Nutrient concentrations and water discharge data obtained for the load varied with the evaluation methods, because the estimations made during rain events differed. Nutrient loads calculated by the four methods ranged from 2004 kg/year to 4194 kg/year for total nitrogen and from 30.3 kg/year to 44.6 kg/year for total phosphorus from the watershed area of 78.3 ha with citrus orchard and forest. Particulate form nutrients from farmland were loaded by runoff during rainfall events, and nutrient load increased quickly more so with phosphorus than with nitrogen during rainfall events. There is thus a need to quantify the degree of nutrient load from farmland in rainfall events. Continuous observation data will be needed to obtain appropriate values for load from diffuse pollution sources

Address correspondence to Kouji Tsushima, Department of Bioenvironmental and Agricultural

Evaluation of Nutrient Loads from a Citrus Orchard in Japan

Kouji TSUSHIMA* ,† , Testsuya KOIKE*, Yoko HORINOUCHI*, Takanobu INOUE*, Toshiro YAMADA**

*Department of Architecture and Civil Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho, Toyohashi, Aichi 441-8580 Japan

**Department of Water Supply Engineering, National Institute of Public Health, 2-3-6 Minami, Wako, Saitama 351-0197 Japan

†Present address: Department of Bioenvironmental and Agricultural Engineering, College of Bioresource Sience, Nihon University, 1866 Kameino, Fujisawa, Kanagawa 252-8510 Japan

ABSTRACT

We estimated four methods of calculating loads of nutrients from a citrus orchard in Japan River water discharge and nutrient loads were examined every week for 2 years and more intensively

at the time of one rainfall event River water level was recorded automatically at 5-minute intervals Nutrient concentrations and water discharge data obtained for the load varied with the evaluation methods, because the estimations made during rain events differed Nutrient loads calculated by the four methods ranged from 2004 kg/year to 4194 kg/year for total nitrogen and from 30.3 kg/year to 44.6 kg/year for total phosphorus from the watershed area of 78.3 ha with citrus orchard and forest Particulate form nutrients from farmland were loaded by runoff during rainfall events, and nutrient load increased quickly more so with phosphorus than with nitrogen during rainfall events There is thus a need to quantify the degree of nutrient load from farmland

in rainfall events Continuous observation data will be needed to obtain appropriate values for load from diffuse pollution sources

Keywords: nitrogen load, phosphorus load, evaluation method, sequential observation

INTRODUCTION

Sources of nutrients entering water bodies are divided into two categories: point and non-point (diffuse) sources The control of point-source nutrient pollution is moving toward a solution thanks to remarkable developments in the techniques used for water treatment Actions being taken to control point-source pollution are proving to be effective against eutrophication However, 1974-2004 measurements of water quality in public water bodies in Japan have revealed that the environmental quality standard for chemical oxygen demand—a water-quality indicator for organic contamination—in lakes and reservoirs has only been met in a small proportion of lakes and reservoirs (i.e.,

by only about 40 to 50% of sampling sites) (Ministry of the Environment, 2006) Diffuse pollution is the leading remaining cause of water quality problems Since inadequate action has been taken to control non-point-source pollution, improvement of these water environments has been incomplete

Diffuse pollution sources are divided into three categories: urban area, forest, and farmland Levels of nutrient loads from farmland are greater than those from the other sources (Ebise and Inoue, 1991) Here, we focused on fertilizers applied in farmland Nutrients in these fertilizers are discharged from farmland into water bodies Some highly soluble nutrients are discharged constantly, whereas nutrients held in particulate matter are discharged irregularly, mainly during rainfall events (Inoue and Ebise, 1991;

Inoue et al., 2003; Yamada and Inoue, 2005)

In the present study, we aimed to identify the characteristics of nitrogen and phosphorus loads from a Japanese citrus orchard (a type of farming enterprise that usually needs intensive application of nutrients), and to compare the loads derived from the four methods

MATERIALS AND METHODS

Study Area

Study area including in a citrus orchard is located in Shizuoka Prefecture, in central Japan (Fig 1) This area is famous in Japan for the production of high quality mandarin oranges Study area is a region where forest had been changed to orchard, and the land use is simply composed of forest and orchard The study site downstream of the orchard receives the orchard discharge; the area of the watershed at the downstream site is 78.3

ha The citrus orchard occupies 30.4 ha (39%) of the watershed, the remainder being forest The study site upstream of the orchard is a control site to the downstream site The upstream site has a watershed area of 6.7 ha, which is covered with forest It is assumed that the forest is almost uniformity in the area

Fig 1 - Study Area The brown and green lines delineate the watershed of the downstream (○) and upstream (□) sites, respectively The upstream site is located on a branch river The watershed of the upstream site is a watershed covered with forest

Sample Collection

Water samples were collected at the upstream and downstream sites every week from November 2005 to October 2007 Intensive sampling was performed at the downstream site on 8 and 9 October 2007 during a rainfall event The total rainfall during the sampling period was 16.5 mm (on the morning of 8 October), and water samples were collected 15 times from 9:00 on 8 October to 11:00 on 9 October

Observation methods

The water levels at the downstream site were recorded automatically at 5-min intervals during the research period Water discharges were measured with an electromagnetic current meter at the downstream site when the water samples were collected Continuous at 5-min intervals water discharge data at the downstream site were

Japan

Downstream site Upstream site

calculated using the relationship between the measured discharges with the current meter and the automatically measured water levels The relationships between the measured discharges (Q) and the water levels (H) on the observation show in Fig 2 This H-Q relationship was approximated by two quadratic equations as follows;

10 09 4 10

92

10 92 4 10

42

where Q is water discharge (m3/s) at the sampling site, H is water level (mm) at the site

0

0.1

0.2

0.3

0.4

Water level (mm)

3 /s)

Fig 2 - The measured discharge versus the automatically measured water level at the

observation The gray curve lines show the H-Q quadratic equations

Water discharge at the upstream site was compared with that at the downstream site, and each was determined in proportion to the watershed area Laboratory analyses of the collected water samples included concentrations of total nitrogen (TN) and total phosphorus (TP), which were determined by standard methods with colorimeter Suspended solids (SS) were collected on a pre-combusted Whatman GF/F glass-fibre filter (mean pore size: 0.7 µm) The concentrations of SS were determined from the differences in the dry weights of the filters before and after filtration

Estimations in 4 Cases of Nutrient Loads

We assumed that the nutrient concentrations and water discharges were constant over a period of half the observation interval (half of a week before and after the observation), because our evaluation was made only by intermittent observation Nutrient load was obtained by multiplying nutrient concentration by water discharge during the same

period (Case 1)

In the next case (Case 2)—evaluation from weekly researched concentrations and automatically recorded water levels—we obtained continuous water discharge data from the automatic water level records As in Case 1, we had to assume that the concentrations were constant during periods for which we lacked continuous data However the continuous water discharge data calculated from the automatically measured water levels were used for Case 2 The water discharge calculations of Case 2, unlike those of Case 1, included the contribution of rainfall events

Daily nutrient loads from farmland can change not only seasonally and with

anthropogenic activity, but also during rainfall events Diffuse pollution loads from farmland can accelerate dramatically during even short rainfall events (e.g., Ebise and

Inoue, 1991; Inoue and Ebise, 1991; Inoue et al., 2003), though observations and

evaluation of water discharge and nutrient concentrations at these times are difficult Loads from diffuse pollution sources cannot be calculated accurately by weekly research, which is normally researched at the time when the water discharge was almost same as the base flow We therefore need a way of accurately quantifying nutrient load from farmland during rainfall events We can obtain sequential nutrient load data by empirical estimations made from sequential water discharge data (i.e., nutrient load is predictable from water discharge) The relationship between water discharge and nutrient load can be analyzed by the regression model,

n

Q a

where L is nutrient load at the sampling site, Q is water discharge at the site, and a and n are coefficients This equation is called the L-Q equation model (e.g., Komai et al., 2003; Kunimatsu et al., 2006) and shows the relationships between nutrient load (L) and water discharge (Q) We decided the optimum coefficients (a and n) of two sets (Cases 3

and 4) by applying the least-squares method to antilogarithmic and logarithmic regression curves The Case 3 evaluation was made on an antilogarithm scale, and Case

4 used a logarithmic scale Kawara et al (1984) showed utility to which the optimum coefficients (a and n) of a L-Q equation model were decided by applying the

least-squares method to logarithmic regression curve This decision method decreases the influence of small number of high load values upon the coefficients

RESULTS

Nutrient Concentrations

TN and TP concentrations at the upstream site were low, and those at the downstream site were high (Figs 3 and 4), because nitrogen and phosphorus were discharged from the citrus orchard There was no seasonal trend in total nitrogen concentration, which was about 3-4 mg/L downstream and 0.5 mg/L upstream

0

2

4

6

8

10

Research period (2006-2007)

0 100 200 300 400 500

Upstream site Downstream site Rain

Fig 3 - Total nitrogen (TN) concentrations at research sites every week Rain data quoted daily precipitation in the Automated Meteorological Data Acquisition System of

Mikkabi observation station by Japan Meteorological Agency (2008)

0.1

0.2

0.3

0.4

Research period (2006-2007)

0 100 200 300 400 500

Upstream site Downstream site Rain

Fig 4 - Total phosphorus (TP) concentrations at research sites every week Rain data quoted daily precipitation in the Automated Meteorological Data Acquisition System of

Mikkabi observation station by Japan Meteorological Agency (2008)

Load Estimations

Load evaluation of Case 1 was calculated by water discharge and nutrient load from our only weekly research data (Table 1) In the Case 2 evaluation, only the water discharge calculations considered the contribution of all rainfall events The results of our evaluation by weekly field research and the automatic water level records showed that the Case 2 evaluation gave a higher discharge than in Case 1 Kunimatsu et al (2006) calculated cv (coefficient of variation) values of the annual loads in Mano River (watershed area of 16.4 km2), under various conditions of observation frequency (from every day to every 30 days) by the same method as Case 2 of our study Kunimatsu et al (2006) also reported the cv values of the annual loads calculated from the data of weekly (7-days interval) observation were 0.54 for total nitrogen and 0.74 for total phosphorus, and that of discharge was 0.23 They designated that the annual loads calculated with Case 2 vary widely depending on only the observation frequency The

cv values will change depending on at least the size and land use of watershed, therefore

we avoided simply referring to those values

The sequential nutrient load data for the research period for Cases 3 and 4 were summed and are shown in Table 1 Fitted curves of the L-Q equations were drawn for nitrogen and phosphorus in accordance with our observations (Figs 5 and 6) The L-Q equations estimated in Cases 3 and 4 are shown in Table 2

Table 1 - Evaluation of nutrient loads and water discharge for each case at the

downstream site (watershed area of 78.3 ha)

(kg/ha/yr) 25.6 40.5 53.6 47.2

(kg/ha/yr) 0.39 0.55 0.57 0.52

Case

TN load

TP load

0.2

0.4

0.6

0.8

Case 3 Case 4

Fig 5 - Evaluation of nitrogen load in accordance with two L-Q equations Observed data of weekly research (from November 2006 to October 2007) are shown as dots

Black line, for Case 3; gray line, for Case 4

0

0.002

0.004

0.006

0.008

Case 3 Case 4

Fig 6 - Evaluation of phosphorus load in accordance with two L-Q equations Observed data of weekly research (from November 2006 to October 2007) are shown as dots

Black line, for Case 3; gray line, for Case 4

Table 2 - L-Q equation models (L = a Q n) for Cases 3 and 4 (L: g/s, Q: m3/s)

DISCUSSION

Nitrogen Load

The relationships between loads and water discharges can be classified into three

categories in accordance with the value of the coefficient n of the L-Q equation model, namely as a washout type (n > 1), dilution type (n < 1), and stable type (n = 1) (Yamada

et al., 2000) In the washout type, nutrient concentration increases in direct proportion

to water discharge as a result of eluviation; in the dilution type, the concentration increases in inverse proportion to water discharge; and in the stable type, the concentration remains constant

The coefficient n for total nitrogen loads in Cases 3 and 4 was >1 (Table 2) The total

nitrogen loads were therefore classified as the washout type Most of total nitrogen load occurs in the form of nitrate Yamada et al (2007) reported an annual average nitrate concentration of 3.39 mg/L at the same sampling point of our downstream site In this study of the Case 4, an average total nitrogen concentration of 4.6 mg/L was calculated from TN load (3693 kg/yr) and water discharge (80.4×104 m3/yr) Nitrate would be the predominant form in total nitrogen of our samples, in comparison between the average nitrate of 3.39 mg/L and the average TN of 4.6 mg/L Nitrate is a kind of stable nitrogen species under naturally oxidizing conditions (Stumm and Morgan, 1981) and forms compounds that are highly soluble in water (Hook, 1983) Nitrate can move with water discharge and be easily eluviated from sources, thus nitrate load will be evaluated precisely from water discharge It suggested that total nitrogen load was evaluated appropriately by the methods used in Cases 3 and 4 When the water discharge was high, the load evaluation in Cases 3 and 4 differed (Fig 5) High concentration data influence the coefficients of the equation model The evaluated TN load by the Case 3 was more near to the observed load in the range of high water discharge than that by the Case 4 Annual load will be influenced by large load in the period of high water discharge To estimate annual nitrogen load, preferable method in our study was Case 3

Phosphorus Load

Some of the observation data for phosphorus did not fit the equations (Fig 6) In the intensive sampling around the time of the rainfall event, the TP concentrations rose more dramatically than the nitrogen concentrations at the time when the water discharge accelerated (Figs 7 and 8) TP concentrations in the period of increasing of water discharge differed greatly from those in the period of their decreasing

During a rainfall event, the SS concentrations increase by more than one or two orders

of magnitude compared with that on a normal day (e.g., Ebise and Inoue, 1991; Inoue

and Ebise, 1991; Inoue et al., 2003; Yamada and Inoue, 2005) Observed concentrations

of SS also increased at the peak of water discharge in the rainfall event (Fig 9) Phosphate is strongly adsorbed by soil particles, so most of the total phosphorus load was in particulate form, and little dischargeable phosphorus would have been left in the latter part in the sampling period of rainfall event

TP concentrations in the weekly research showed an increasing trend in summer and autumn at the downstream site (Fig 4) In these seasons, high phosphorus concentration did not totally correspond to high water discharge The higher loads (0.002-0.007 g/s) were observed at the low discharges (0.02-0.03 m3/s), as shown in Fig 6 These increases may be caused by fertilizer application, and they suggest that phosphorus in

applied fertilizer remaining on the soil surface was discharged (Yamada et al., 2007)

The L-Q equation used to estimate phosphorus loads seemed to differ with the season (Fig 4), and so evaluation by the L-Q equation may not be as easily applicable to

phosphorus as to nitrogen Discussion concerning the coefficient n of the L-Q equations

of TP was avoided in this study Evaluation of phosphorus loads will require the establishment of a seasonally separated equation model that takes into account rainfall events and seasonal changes

0

1

2

3

4

0.0

1.0

2.0

3.0

4.0

5.0

Intensive sampling Period (8-9 October 2007)

0.00 0.05 0.10 0.15

3 /s)

TN conc.

Water discharge

Fig 7 - TN concentration and water discharge at the downstream site during the intensive sampling period Rain data quoted 10-minute interval precipitation in the Automated Meteorological Data Acquisition System of Mikkabi observation station by

Japan Meteorological Agency (2008)

0.0

0.1

0.2

0.3

0.4

Intensive sampling Period (8-9 October 2007)

0.00 0.05 0.10 0.15

3 /s)

TP conc.

Water discharge

Fig 8 - TP concentration and water discharge at the downstream site during the

intensive sampling period

50

100

150

Intensive sampling Period (8-9 October 2007)

0.00 0.05 0.10 0.15

3 /s)

SS conc.

Water discharge

Fig 9 - SS concentration and water discharge at the downstream site during the

intensive sampling period

CONCLUSIONS

The calculated loads differed with the evaluation methods even when the same data set was used, because the estimations made during the rainfall period were different We thus need to establish a preferable evaluation method that uses continuous research results, including those for rainfall events Observation data during times of high water discharges are important because they influence the results obtained by the L-Q equation model A separate L-Q equation for total phosphorus should be established to take into account rainfall events and seasonal changes

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