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Nutrient runoff from a Korean rice paddy watershed during multiple storm events in the growing season
Journal of Hydrology (2006) 327, 128– 139
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/jhydrol
Nutrient runoff from a Korean rice paddy watershed during multiple storm events in the growing season Jin S. Kim
a,*
, Seung Y. Oh b, Kwang Y. Oh
a
a
Department of Rural Engineering, Chungbuk National University, 12 Gaeshin-dong, Cheongju, 361-763, Republic of Korea Watershed Management Research Center, National Institute of Environmental Research, Gyeongseo-dong, Incheon, 404-708, Republic of Korea
b
Received 5 January 2005; received in revised form 18 October 2005; accepted 4 November 2005
KEYWORDS
Summary The concentrations and loading characteristics of total nitrogen (TN) and total phosphorous (TP) in runoff from a 50.1-ha rice paddy field watershed in South Korea were investigated for eight storm events during the 1998–2001 growing seasons. TN concentrations in total runoff were inversely related to discharge, except in periods with high fertilization rates. In contrast, TP concentrations were strongly proportional to discharge under non-ponded paddy conditions, but not correlated with discharge under most ponded paddy conditions. Stormflow and irrigation return flow were separated from total runoff using the constant-discharge method. The flow-weighted mean TN concentration in stormflow was lower than that in irrigation return flow, mainly because of rainwater dilution, except for periods with a residual fertilizer effect. The flow-weighted mean TP concentration in stormflow, however, was always higher than that in irrigation return flow, likely a result of sediment-associated phosphorus transport. The ratio of mean TP concentration in stormflow to that in irrigation return flow under ponded paddy conditions (1.6) was approximately one-half that under non-ponded conditions (3.1), suggesting that ponding on paddy fields played an important role in reducing soil erosion-related phosphorus export. Relationships between TN loads and stormflow runoff volumes were found except during a storm event in the high fertilization period (p < 0.05). TP loads were also correlated with stormflow runoff volumes (p < 0.05), except for storm events under non-ponded and dry antecedent conditions. These results indicate that nitrogen runoff from paddy field watersheds depends on fertilization rates, while phosphorus runoff is controlled by ponding conditions. c 2005 Elsevier B.V. All rights reserved.
Irrigation return flow; Phosphorus export; Ponding condition; Stormflow; Total nitrogen
* Corresponding author. Tel.: +82 43 261 2573; fax: +82 43 271 2922. E-mail address: [email protected] (J.S. Kim).
0022-1694/$ - see front matter c 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2005.11.062
Nutrient runoff from a Korean rice paddy watershed during multiple storm events in the growing season
Introduction The efflux of nitrogen (N) and phosphorous (P) from agricultural lands is thought to be a major contributor to the accelerated eutrophication of rivers and reservoirs around the world. In South Korea, rice paddy fields are a major feature of the landscape, accounting for 11.6% of the land area in 2002 (Ministry of Agriculture and Forestry, 2003). Paddy fields typically lie between uplands and lower-elevation water bodies and may have a major impact on water quality. The growing season of rice (Oryza sativa) extends from April to October and corresponds with the hydrologically active period. The paddy fields are surrounded by earthen levees 10–30 cm in height and are usually shallowly ponded (3–10 cm) during most of the growing season. Therefore, these fields resemble wetlands, but fertilizers are applied and the fields are both irrigated and drained. Typical paddy soils have a relatively low percolation rate because the less permeable hardpan inhibits deep percolation and facilitates surface ponding. Extremely flat sites (slopes <0.3%) have a large surface flow component compared to sites with slightly steeper slopes (Capece, 1994), and correspondingly, the irrigation return flow from paddy
(a)
IIrrigation rrigation water
Irrig
ditch
atio n
n a ge
di t c h
Drai Tailwater Ponded water
Bypass water
Lateral subsurface irrigation return flow Main drainage canal
Irrigation return flow Watershed outlet
(b)
Rainfall Irrigation water
Drai
Irrig
atio n
h ditc
d it c h
nage
Bypass water Irrigation return flow + Stormflow
Ponded water
Surface stormflow +Tailwater
Lateral subsurface irrigation return flow +Subsurface stormflow Main drainage canal
Watershed outlet
Figure 1 Schematic of runoff from a rice paddy watershed: (a) irrigation return flow during non-storm periods; (b) total runoff during storm events.
129
fields on flat sites consists largely of surface irrigation runoff (i.e., tailwater). In paddy field areas with independent irrigation and drainage ditches, irrigation return flow (normal flow) at the outlet during non-storm periods consists of tailwater, lateral subsurface irrigation return flow, and bypass water (Fig. 1a). In contrast, total runoff at the outlet of paddy field areas during storm events mainly consists of irrigation return flow and stormflow (Fig. 1b). Commonly used hydrologic terms are defined for this study as follows: tailwater refers to the excess water applied to paddy plots that does not infiltrate, but rather runs off into a drainage system; lateral subsurface irrigation return flow is irrigation water that moves laterally through the crop root zone and drains to a drainage system; bypass water denotes irrigation water that does not enter the paddy plots and runs off at irrigation ditch outlets; and stormflow refers to the flow response to a storm event, in excess of irrigation return flow, and consists of surface and subsurface components. Storm events are important in activating hydrological pathways, establishing hydrological connectivity between agricultural land and streams, and hence in the transport of P (Heathwaite, 1997). Hydrologic and chemical responses to storm events have been studied for upland crop watersheds (Pionke et al., 1988, 1996; Cosser, 1989; Owens et al., 1991; DeWalle and Pionke, 1994; Kronvang et al., 1997; Correll et al., 1999), mixed land use watersheds that contain paddy fields (Nakasone and Nakamura, 1984; Suzuki and Tabuchi, 1984; Gao et al., 2004), and predominantly paddy field watersheds (Takeda et al., 1990; Feng et al., 2004). Based on previous studies, we hypothesized that in watersheds dominated by paddy fields, total nitrogen (TN) and nitrate nitrogen (NO3–N) concentrations in storm runoff would not always increase, but total phosphorus (TP) concentration would increase with increased discharge. However, only limited field data were available to verify such characteristics. The objectives of this study were to characterize nutrient (TN and TP) concentration patterns in total runoff from a Korean rice paddy watershed during multiple storm events in the growing season, to investigate mean nutrient concentration relationships between stormflow and irrigation return flow, and to evaluate relationships between runoff volume and nutrient loads in stormflow.
Materials and methods Study site The study was conducted in a 50.1-ha watershed in Soro, Chungbuk, in central South Korea (3641 0 N, 12725 0 E; 30– 32 m above sea level; Fig. 2). Rice paddy fields account for more than 80% of land use in the area, and the rest of the watershed consists of farm roads, irrigation and drainage ditches, and stream banks. The irrigation season in the study area ranges from midApril to late September, overlapping with the growing season. The 5-year mean growing season precipitation level was about 980 mm for 1998–2002. Typical (30-year mean) temperatures during the growing season range from 12.1 C in April to 27.2 C in August.
130
J.S. Kim et al. season and a single P application of 22 kg ha1 yr1 occurs in early to mid-May as a basal dressing.
l
na
n
ca
Methods
io
t ga
i
n
ai
r ir
Irrigation ditch Drainage ditch
M
Water samples were collected immediately prior to or during storm events by manually filling a polyethylene bottle at the inlets of irrigation ditches (i.e., irrigation water), two paddy plots (i.e., ponded water), a study watershed outlet (i.e., irrigation return flow and total runoff), and a rain gauge site (i.e., rainwater; Fig. 2). Water samples and discharge measurements for total runoff were taken at 2- to 6-h intervals per event. Rainfall data were also collected from a rain gauge near the study watershed. Water samples were analyzed for TN and TP using the persulfate and ascorbic acid methods (APHA, 1998), respectively. The amount and timing of fertilizer applications were determined by questioning farmers in the study area. A power function (Betson and McMaster, 1975; Kronvang, 1992; Webb and Walling, 1998) was used to determine the regression relationship between specific discharge (discharge per unit area) and dependent nutrient concentration in total runoff: C ¼ aqb ;
Main drainage canal
Figure 2 Location of the study site in South Korea. d Sampling point of irrigation water. j Sampling point of total runoff and irrigation return flow. m Rain gauge site. s Sampling point of ponded water.
The study watershed, located in an alluvial plain, consists of loamy soils with a shallow (1–2 m) water table. The paddy soils of the study area have a percolation rate of about 1–2 mm day1. The surface paddy soils are usually puddled (or mixed with standing water) to reduce percolation and to suppress weed growth just prior to transplanting. Rice seedlings are transplanted from a nursery to the paddy fields in mid to late May. The paddy fields are drained in early to mid-September to facilitate mechanized combine harvesting. The study area is largely irrigated with surface water released from an agricultural reservoir by a diversion weir in the Miho River, and the return flows are drained back to the same river. The Miho River is a tributary of the Geum River, which forms the third largest watershed area (9886 km2) in South Korea. The study paddy field area was replotted in 1996 to increase land and labor productivity. The main irrigation canals, irrigation ditches, and main drainage canals are concrete-lined, but drainage ditches are vegetated. The standard size of a unit lot is 100 · 100 m; one side is adjacent to an irrigation ditch and another side is next to a drainage ditch. In each paddy plot, ponded water slowly moves from inlets in an irrigation ditch to outlets in a drainage ditch. On average, three split N applications (basal dressing and top dressings at the tillering and panicle formation stages) of 172 kg ha1 yr1 occur during the growing
ð1Þ
where C is the nutrient concentration of total runoff, q is specific discharge, and a and b are a constant and an exponent of the function, respectively. When the exponent b > 0 or <0, the dependent nutrient concentration increases or decreases exponentially, respectively. When b = 0, the resulting nutrient concentration remains constant. The power function was also used to determine the regression relationship between runoff volume and dependent nutrient load in stormflow. This was done to characterize loading patterns of nutrients from the study rice paddy watershed. Regression analyses were performed using SAS (SAS Institute, 1999) and the regression curves were considered significant if p < 0.05.
Results and discussion Outline of storm events, water management, and fertilization Eight storm events during the 1998–2001 growing seasons, each resulting in 27.8–194.5 mm of rainfall (Table 1), were used in this study to characterize the loading patterns of nutrients from the study watershed. Event 1 occurred shortly after irrigation ended in late September. Event 2 occurred when some rice paddies were not ponded and no rice paddies were planted. Event 8 occurred after a two-month drought. During heavy storms, a ditch rider stops the irrigation supply to irrigation canals to reduce the risk of crop damage by inundation. Events 2, 3, and 8 were such storms during which irrigation was halted. Irrigation was stopped prior to Events 5 and 7 as a precaution because of the high antecedent rainfall. Event 2 coincided with fertilizer application in the study watershed at rates of 55 kg N ha1 and 12 kg P ha1, and Event 6 coincided with the application of 25 kg N ha1.
Summary of storm events during the 1998–2001 growing seasons Status of ponding and rice growth stage
Status of irrigation water
Remark
0
Non-ponded, ripening
Stopped
8.7
0
Non-ponded in some paddies, not transplanted
Stopped on the way
16
10.7
0
Ponded, tillering
50.6 41.0 27.8
36 43 27
6.7 10.3 7.6
9.7 26.7 0
Ponded, booting Non-ponded, ripening Ponded, rooting
Stopped on the way Supplied Stopped Supplied
Non-irrigation period Basal dressing (55 kg N ha1, 12 kg P ha1) Top dressing (N/Aa)
80.0 93.8
48 25
12.6 20.4
55.4 0
Ponded, tillering Ponded, tillering
Storm event
Date
Amount of rainfall (mm)
Duration hours of rainfall (h)
Maximum rainfall intensity (mm/h)
1
September 29–October 2, 1998
194.5
54
15.8
2
May 3–5, 1999
55.1
24
3
June 16–18, 1999
67.3
4 5 6
July 27–30, 1999 September 19–21, 1999 May 26–28, 2000
7 8
June 26–28, 2000 June 18–20, 2001 a
Not available for application rate of fertilizer.
Total 3-day antecedent rainfall (mm)
Stopped Stopped on the way
Top dressing (25 kg N ha1)
Nutrient runoff from a Korean rice paddy watershed during multiple storm events in the growing season
Table 1
131
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J.S. Kim et al.
Concentrations in irrigation water, rainwater, and irrigation return flow Irrigation water was supplied during five storm events (Events 2–4, 6, and 8) with mean nutrient concentrations ranging from 1.3 to 3.3 mg TN L1 and from 0.08 to 0.30 mg TP L1 (see Table 2). Irrigation return flow had specific discharges ranging from 0.25 to 4.97 L s1 ha1, with particularly low values during the non-irrigation season (Event 1) and drought (Event 8; Table 2). Irrigation return flow had nutrient concentrations similar to irrigation water: 1.3–4.8 mg L1 for TN and 0.05–0.19 mg L1 for TP. The mean concentrations of TN and TP in irrigation return flow were 2.5 and 0.17 mg L1, respectively, for a 7.4-ha rice paddy watershed in Japan (Feng et al., 2004). The mean concentrations of NO3–N and TP in streamwater during irrigation periods were reported to be 1.5 and 0.20 mg L1, respectively, for a 205-ha mixed land use watershed composed of 25% paddies in Japan (Nakasone, 2003). TN concentrations in streamwater in a mixed land use watershed that contained paddies in China seldom exceeded 2.0 mg L1 from early July to late October (rice full growing season; Gao et al., 2004). The mean TN concentration was the highest (4.8 mg L1) at the lowest specific discharge (0.25 L s1 ha1), in Event 1. Typically, tailwater dramatically decreases after the irrigation season ends, and a small lateral subsurface irrigation return flow gradually decreases over 15–20 days. Thus, as the lateral subsurface irrigation return flow is reduced by evapotranspiration, TN concentra-
tions increase against a nearly constant TN load. Also, mean TN and TP concentrations for Event 2 were high (4.5 and 0.19 mg L1, respectively) because of the effect of fertilization. The mean nutrient concentrations in rainwater, 0.1– 1.4 mg L1 for TN and 0.01–0.04 mg L1 for TP, were generally lower than in irrigation water or irrigation return flow. Therefore, rainwater may function to dilute irrigation water and irrigation return flow.
Temporal variation in TN concentration in total runoff TN concentrations in total runoff increased with increased specific discharge during Events 2 and 3 when fertilization was applied to the study watershed. However, overall TN concentrations decreased with increased discharge during non-fertilization periods, such as during Events 1, 4, 5, 7, and 8, and top-dressing periods, such as Event 6, although concentrations partially increased with rapidly increased discharge during Events 1 and 4 (see Fig. 3). For example, during the basal dressing period of Event 2, TN concentrations in the total runoff water greatly increased with discharge from 4.5 to 11.3 mg L1, and then decreased to 6.3 mg L1. Most irrigation water becomes irrigation return flow in the form of bypass water during storms, because farmers usually do not withdraw water during storm events (Fig. 1b). When irrigation water at a relatively low TN concentration (2.8 mg L1) was cut off, the total runoff discharge rapidly decreased and then TN
Table 2 Specific discharge of irrigation return flow and mean nutrient concentrations in irrigation water, irrigation return flow, and rainwater Storm event
Specific discharge of irrigation return flow (L s1 ha1)
Parameter
1
0.25
2
Mean concentration (mg L1) Irrigation watera
Ponded water in paddy plotsb
Irrigation return flowa
Rainwaterb
TN TP
N/Ac N/A
N/A N/A
4.8 0.10
0.2 0.01
3.54
TN TP
2.8 0.18
N/A N/A
4.5 0.19
N/A N/A
3
4.43
TN TP
3.3 0.15
5.4 0.22
2.3 0.05
0.9 0.01
4
2.02
TN TP
1.3 0.08
1.3 0.11
2.0 0.06
0.6 0.01
5
1.44
TN TP
N/A N/A
N/A N/A
1.3 0.07
0.1 0.01
6
4.97
TN TP
2.6 0.14
2.9 0.09
2.9 0.09
1.4 0.04
7
1.13
TN TP
N/A N/A
N/A N/A
2.1 0.09
0.9 0.01
8
1.04
TN TP
2.9 0.30
1.7 0.12
3.6 0.14
0.9 0.02
a b c
Flow-weighted mean. Arithmetic mean. N/A, not available.
(mg L-1)
TP
10
1.2 6
0.3
0
c 10
6 20
20 25
15 10
0.4 2
TP No irrigation
q
0.0 0
16
4
TN 12 0.1
8
2
TP
0.0
4
q
Irrigation stops
0
0 17
16
Oct. 1998
June 1999
10
0.3 3 20 25
20
20 15 10
Irrigation stops
5
q 0
0
5
TP
q 0.0 0
Ju1y 1999 (mg L-1)
TP
TN
10
0.3 3
0
g
TN
0
e
Rainfall (mm)
(mg L-1)
May 1999
TP
0
30
29
28
27
4
3
10
0.1 1
10
0.3 3
20
20
25 20 15
TN TP
0.1 1
10 5
q No irrigation
0.0 0
19
20
20
Specific discharge (L s-1 ha-1)
0.2 2
0
TN 0.2 2 15
10 0.1 1
TP 5 Stormflow Irrigation return flow
0.0 0
21
27
26
Sept. 1999
No irrigation 28
0
TN
(mg L-1)
TP
TN
10
0
h
TN
0
Rainfall (mm)
(mg L-1)
TP
q
June 2000
f
0.3 3
Rainfall (mm)
0.0
15
TN
Specific discharge (L s-1 ha-1)
5
20
0.2 2
10
0.4 4 20
20
Rainfall (mm)
0.4
Specific discharge (L s-1 ha-1)
25
TP
0.8 10
Rainfall (mm)
0
d
Specific discharge (L s-1 ha-1)
TN
TP
TN
10
1.2 15
Rainfall (mm)
0
b
TN (mg L-1)
(mg L-1)
Sept. 1998
TP
0
2
1
30
29
5
0.2
Specific discharge (L s-1 ha-1)
20
20
Specific discharge (L s-1 ha-1)
TN 0.8 4
Rainfall (mm)
0
133
TN
TP
TN
a
Rainfall (mm)
(mg L-1)
Nutrient runoff from a Korean rice paddy watershed during multiple storm events in the growing season
TP 0.1 1
q
0.0 0
5
0 26
27
May 2000
28
16 0.2 2
TN
12
TP
8
0.1 1
Irrigation stops
4
q
Specific discharge (L s-1 ha-1)
10
0.2 2
Specific discharge (L s-1 ha-1)
20 0.3 3
0
0.0 0 18
19
20
June 2001
Figure 3 Temporal variation in nutrient concentrations and discharge in total runoff: (a) Event 1; (b) Event 2; (c) Event 3; (d) Event 4; (e) Event 5; (f) Event 6; (g) Event 7; (h) Event 8.
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J.S. Kim et al.
concentration sharply increased again to a peak value of 14.7 mg L1 because of decreased dilution by irrigation water. Also, the TN concentrations increased with discharge for Event 3 during the top dressing period. The TN concentrations in stormflow may be greatly affected by the N concentrations in ponded water in rice fields. During fertilizer application periods, like Events 2 and 3, TN concentrations in ponded water in paddy plots may be much higher than in the irrigation return flow because of the applied nitrogen fertilizer retained in the ponded water. Thus, the concentrations in total runoff may increase with discharge overall, despite peak concentrations on the rising limb or during an early peak, because of the ponded water runoff with high TN concentrations caused by rainwater influx. For Event 3, the TN concentration in ponded water (5.4 mg L1) was much higher than that in irrigation return flow (2.3 mg L1; Table 2). In China, the storm event immediately after fertilization produced high ammonium nitrogen (NH4–N) concentrations in stream water from a 122-ha mixed land use watershed that contained paddy fields (Gao et al., 2004).
For other storms, the TN concentrations in ponded water and irrigation return flow differed slightly. Therefore, the concentrations in total runoff may decrease overall with increased discharge, perhaps because of dilution by rainwater influx, but rise partially with rapidly increased discharge in cases like Events 1, 4, and 7. The TN concentrations in ponded water were equal to or somewhat lower than those in irrigation return flow for Events 4, 6, and 8 (Table 2). These results were consistent with previous observations (Feng et al., 2004) in that TN concentrations in total runoff decreased with discharge overall, despite a small early peak on the rapidly rising limb of the hydrograph during a storm in a non-fertilization period in a Japanese paddy rice watershed. The small early rise in TN concentration on the rapidly rising limb may have been a result of the organic nitrogen transport from soils to watercourses. The concentrations of particulate organic nitrogen (PON) or organic nitrogen greatly increased during storm events in mixed land use watersheds, including paddies (Gao et al., 2004), or uplands (Correll et al., 1999).
100.0
1.00 b
a
Peak
-1
-1
TN (mg L )
Peak Peak
1.0
Peak Peak
Event Rising Falling No. 1
TP (mg L )
Peak
10.0
Peak
0.10
Event Rising Falling No. 1
No. 2
No. 2
No. 3 No. 4
No. 3 No. 4
0.1 0.1
1.0 10.0 Specific discharge (L s-1 ha-1)
100.0
0.01 0.1
100.0
1.00
Event Rising Falling No. 5
a
Peak
1.0 10.0 Specific discharge (L s-1 ha-1)
100.0
b
No. 6
No. 7 No. 8
Peak
TN (mg L )
10.0
Peak
TP (mg L-1)
-1
Peak Peak
1.0
Peak
0.10 Peak Event Rising Falling No. 5
Peak
No. 6
Peak
0.1 0.1
1.0
10.0
Specific discharge (L s-1 ha-1)
No. 7 No. 8
100.0
0.01 0.1
1.0
10.0
100.0
Specific discharge (L s-1 ha-1)
Figure 4 Relationship between nutrient concentrations and specific discharge in total runoff. Open and solid symbols indicate rising and falling limbs of hydrographs, respectively: (a) TN; (b) TP.
Nutrient runoff from a Korean rice paddy watershed during multiple storm events in the growing season
discharge were observed in another paddy field watershed (Feng et al., 2004), four mixed land use watersheds (1.7– 28 km2), including 10–23% paddies (Suzuki and Tabuchi, 1984), and watersheds with other land uses (Pionke et al., 1988; Quinton et al., 2001). When antecedent rainfall levels were low (e.g., Events 1–3, 6, and 8), TP concentrations usually peaked before total runoff peaked, whereas they peaked during or after peak runoff rate when antecedent rainfall levels were medium to high (e.g., Events 4, 5, and 7; Table 1). The TP concentration markedly peaked before peak runoff for events with high rainfall intensity (e.g., Events 1 and 8). The peaking of TP concentrations prior to runoff peaks may occur because the sediment and particulate P that accumulated in paddy fields and drainage ditches during the drier weather quickly washes away at the beginning of a storm. This phenomenon was observed during several storms in paddy field watersheds (Takeda et al., 1990; Feng et al., 2004) and mixed land use watersheds containing paddies (Suzuki and Tabuchi, 1984). The results suggest that P transport in storm runoff from paddy fields is highly affected by antecedent rainfall.
However, the results of this study disagreed with results from another 11.6-ha rice paddy watershed in Japan (Takeda et al., 1990). In Takeda et al. (1990), the TN concentration in the total runoff during storms increased with discharge during non-fertilizer application periods, whereas it decreased with discharge during the top-dressing period (late July); the TN and NO3–N concentrations did not always increase with discharge for mixed land use watersheds that contained paddy fields in Japan (Nakasone and Nakamura, 1984; Suzuki and Tabuchi, 1984), but they did increase with discharge in a mixed land use watershed containing paddy fields in China (Gao et al., 2004).
Temporal variation of TP concentrations in total runoff Overall, the TP concentrations generally increased with discharge for many storm events (Fig. 3). For Event 1 under non-ponded conditions, the TP concentrations in the total runoff water increased sharply (from 0.10 to 0.80 mg L1) prior to peak discharge, and then greatly decreased to 0.24 mg L1 around the discharge peak. For Event 2, the TP concentrations in the total runoff greatly increased (from 0.19 to 0.52 mg L1) with discharge, and then slowly decreased with decreasing discharge. However, after irrigation stopped, the runoff discharge rapidly decreased and the TP concentration sharply increased to 0.81 mg L1. For Event 4 under ponded conditions, the TP concentrations increased (from 0.06 to 0.12 mg L1) with discharge, and then decreased with decreasing discharge. The increase in TP concentration with increased discharge may be a result of the transport of particulate phosphorus. Phosphate phosphorus (PO4–P) concentrations increased much more gradually with increased discharge in a paddy field watershed, relative to TP concentrations (Takeda et al., 1990). Particulate organic phosphorus (POP) constituted the largest fraction of TP in storms in watersheds of differing land uses in Maryland, USA (Correll et al., 1999). Increases in TP concentrations with increased
Table 3
No. of samples
29 16b 10c 14b 10c 25 23 12 16 16b 7c
4 5 6 7 8 a b c d
Fig. 4 and Table 3 indicate the relationships between nutrient concentrations (C) and specific discharges (q) for total runoff in the eight investigated storm events. The nutrient concentration in the total runoff water ranged from 0.6 to 14.7 mg L1 for TN and from 0.03 to 0.81 mg L1 for TP (Fig. 4). At the same discharge level, TN concentrations for Event 2 and TP concentrations for Events 1 and 2 were much higher than those from other storm events. The relationship between TN and TP concentration and discharge during Event 1 showed anticlockwise and clockwise hystereses, respectively (Fig. 4). Here, clockwise hysteresis indicated higher concentrations during the rising limb of the hydrograph compared with those measured
TN
TP
C q equationa
3
Relationship between specific discharge and concentration in total runoff
Regression equations for nutrient concentrations and specific discharge in total runoff
Storm event
1 2
135
0.13
C = 3.17q – C = 2.01q0.66 – C = 1.07q0.43 – C = 1.33q0.26 C = 3.59q0.20 C = 1.97q0.08 – C = 3.78q0.19
r2 0.38 0.23 0.61 0.04 0.70 0.08 0.46 0.36 0.38 0.05 0.78
p <0.001 NSd <0.01 NS <0.01 NS <0.001 <0.05 <0.05 NS <0.01
C, nutrient concentration is in mg L1; q, specific discharge is in L s1 ha1. Regression equation including the period of irrigation stoppage. Regression equation excluding the period of irrigation stoppage. Not significant.
C q equationa 0.38
C = 0.224q – C = 0.052q1.00 – – C = 0.033q0.22 C = 0.055q0.47 – – – –
r2
p
0.61 0.16 0.72 0.27 0.32 0.16 0.91 0.03 0.07 0.12 0.37
<0.001 NS <0.01 NS NS <0.05 <0.001 NS NS NS NS
136
Mean nutrient concentration in stormflow The boundary between stormflow and irrigation return flow in total runoff from paddy field during a storm event is difficult to define because total runoff may sometimes contain ponded water artificially released by farmers to prevent levee collapse. In this study, the total runoff hydrograph was assumed to consist of stormflow and irrigation return flow, and the two components were separated using a constantdischarge method (McCuen, 1998; Fig. 3g). Flow-weighted mean nutrient concentration in stormflow Cs was obtained by: Ls Lt Lr Cs ¼ ¼ ; ð2Þ Vs Vt Vr where Ls is stormflow load, Vs is stormflow volume (depth), Lt is total runoff load, Lr is load of irrigation return flow, Vt is total runoff volume (depth), and Vr is volume (depth) of irrigation return flow. Fig. 5 shows the mean TN and TP concentrations in stormflow during storm events obtained from Eq. (2). Flow-weighted mean TN concentrations in stormflow for Events 2 and 3 were high, 9.6 and 3.3 mg L1, respectively, likely because of the effects of N fertilization (Fig. 5a). However, mean TN concentrations in stormflow for the other storms ranged from 0.7 to 2.5 mg L1. These values were similar to the mean TN concentration (1 mg L1) in
10.0
2
a
-1
Stormflow (mg L )
3 6 1.0
1 8
7 4 5 2:1 line
Ponded Non-ponded (dry) Non-ponded (wet)
1:1 line 1:2 line 0.1 0.1
1.0
10.0 -1
Irrigation return flow (mg L )
1.00
b 2
1 -1
Stormflow (mg L )
during the falling limb. The TN and TP concentrations– discharge relationships during Events 4 and 8 showed clockwise hystereses, but it is still unclear under what conditions the clockwise hysteresis occurs. The clockwise hystereses of the relationship between TP concentration and discharge have been reported elsewhere, for a mixed land use watershed containing paddies (Suzuki and Tabuchi, 1984). Statistically significant relationships between discharge and TN concentration (Eq. (1)) were obtained for seven of eight events (Table 3). Irrigation management practice greatly affected the C q rating equation. Thus, the relationship between discharge and TN concentration was not significant when data were included from the irrigation cutoff periods, i.e., Events 2, 3, and 8. In general, TN concentration decreased with specific discharge (b < 0 in Eq. (1)). However, TN increased with specific discharge (b > 0) for Events 2 and 3 during the fertilizer application period. In comparison, a correlation between N concentration and streamflow (r2 = 0.70) was observed during storms from July to September (the main rice-growing season) for a mixed land use watershed containing rice paddies in China (Gao et al., 2004). For mixed land use watersheds, including the uplands in the United States, the concentrations of NO3–N (Schnabel et al., 1983) and PON (Correll et al., 1999) were positively correlated with discharge. The relationship between TP concentration and discharge was significant for storm events under non-ponded conditions, i.e., Events 1, 2, and 5. However, TP concentration was not related to discharge for storms under ponded conditions, with the exception of Event 4. Similarly, concentrations of particulate P and POP, the dominant forms of TP in storm runoff, increased significantly with storm discharge on other land use types in Australia (Cosser, 1989) and the USA (Correll et al., 1999), respectively.
J.S. Kim et al.
5 0.10
3 2:1 line
6
8
7 4 Ponded
1:1 line
Non-ponded (dry) 1:2 line 0.01 0.01
Non-ponded (wet) 0.10
1.00
Irrigation return flow (mg L-1)
Figure 5 Relationship between stormflow and irrigation return flow for: (a) TN and (b) TP mean concentrations. Numbers next to symbols refer to the respective storm events.
stormflow from another paddy field watershed (Feng et al., 2004), but much lower than the mean NO3–N concentrations (7.0 and 7.5 mg L1, respectively) in stormflow from an upland watershed and a mixed land use watershed in Pennsylvania, USA (Pionke et al., 1996). Flow-weighted mean TP concentrations in stormflow under non-ponded conditions, such as in Events 1 and 2, were much higher (0.48 and 0.49 mg L1, respectively) than under ponded conditions (Fig. 5b). Prior to transplanting (e.g., Event 2), bare paddy soils were tilled and fertilized; thus, a combined effect of fertilization and vulnerability to soil erosion may have caused high mean TN and TP concentrations in stormflow. However, the mean TP concentration in stormflow for other storms was relatively low (<0.16 mg L1). The mean TP concentration for Event 8 under ponded conditions was 0.15 mg L1 despite the high rainfall intensity (20.4 mm h1). The lowest mean TP concentration was 0.06 mg L1 for Event 4 under ponded and abundant crop cover conditions. Overall, the mean TP concentrations in stormflow from the study area, except for non-ponded conditions, were almost identical to the mean concentration (0.15 mg L1) from another paddy field watershed (Feng et al., 2004), but much lower than those (0.24–0.55 mg L1) from mixed land use watersheds in Pennsylvania (Sharpley et al., 1999) or the mean POP con-
Nutrient runoff from a Korean rice paddy watershed during multiple storm events in the growing season 10.0
a
This study 0.88
TN load (kg ha-1)
y = 0.029x r 2 = 0.60, p <0.05 2
1
8 7 3
1.0 6
4
Takeda et al.(1990) y = 0.040x0.63
5
Ponded Non-ponded (dry) Non-ponded (wet) 0.1 10
100 Stormflow runoff (mm)
1000
1.00
b
1
-1
TP load (kg ha )
Takeda et al.(1990) y = 0.0076x0.80
0.10
8
2 5
7
This study 0.99
6
0.01 10
y = 0.001x 2 r = 0.72, p <0.05 3
4
Ponded Non-ponded (dry) Non-ponded (wet)
100 Stormflow runoff (mm)
1000
Figure 6 Relationship between runoff and TN and TP load of stormflow. Numbers next to symbols refer to the respective storm events: (a) TN; (b) TP.
centration (>1.50 mg L1) in storm events from mostly cropland watershed in Maryland (Correll et al., 1999), USA.
137
der ponded conditions. Despite a small crop canopy, the mean TP concentration ratio for Event 6 was as low (1.6) as that for other ponded conditions. Moreover, mean TN and TP concentration ratios between stormflow and irrigation return flow from the ponded study area were much smaller than those between stormflow and base flow from other land use watersheds. The mean TN and TP concentrations in stormflow from mixed land use watersheds, including paddy fields in Japan, were 2.4 and 5.3 times higher than those in base flow, respectively (Suzuki and Tabuchi, 1984). The median TN concentration in streamwater in Chinese mixed land use watersheds, including paddy fields during storm events, was three times higher than in regular weekly samples (Gao et al., 2004). The mean TP concentrations in stormflow were at least 10 times higher (Pionke et al., 1988) and 4–13 times higher (McDowell et al., 2001) than those in base flow for an upland watershed and a mixed land use watershed in Pennsylvania, USA, respectively. The mean total organic phosphorus (TOP) concentrations in stormflow were 9.1 times higher than those in base flow for a mixed land use watershed and 3.7 times higher for a mostly cropland watershed in Maryland, USA (Correll et al., 1999). The small increase in stormflow TP concentration relative to that in irrigation return flow under ponded conditions may occur because the shallow water layer protects the soil from the direct impact of the rain and decreases flow velocity with large roughness coefficients, thereby greatly reducing the transport of P associated with soil erosion. When dry soils are suddenly wetted, water molecules rapidly displace the absorbed O2 and N2 molecules. These gases join entrapped air, causing pressure forces sufficient to burst soil aggregates (Carter, 1990). The rice paddies are ponded and are rapidly covered by the crop canopy during most of the growing season after transplanting, thereby controlling TP.
TN and TP stormflow loads Ratio of mean nutrient concentration in stormflow to that in irrigation return flow The ratio of mean TN concentration in stormflow to that in irrigation return flow ranged from 0.5 to 2.1, and the mean TP ratio ranged from 1.0 to 4.4 (Fig. 5). The mean TN ratio of stormflow to irrigation return flow was 0.6, except for Events 2 and 3, indicating that the TN mean concentrations in stormflow were usually lower than in irrigation return flow. Conversely, the mean TP concentrations in stormflow were usually higher than in irrigation return flow. Similar trends were observed in previous research on a paddy field watershed (Feng et al., 2004). However, the mean concentrations of N and P were usually higher in stormflow than in base flow from mixed land use watersheds, including paddy fields (Suzuki and Tabuchi, 1984; Gao et al., 2004), or uplands (Owens et al., 1991). The ratio of mean TN concentration in stormflow to that in irrigation return flow under non-ponded conditions (1.0) was nearly identical to that under ponded conditions (0.9), indicating that ponding did not have much affect on the mean TN concentrations in stormflow. In contrast, the stormflow to irrigation return flow ratio for mean TP under non-ponded conditions (3.1) was about twice that (1.6) un-
The TN and TP stormflow loads were highest (3.9 kg N ha1 and 0.77 kg P ha1, respectively) in Event 1 under drained paddy and highest runoff conditions (Fig. 6). The TN load for Event 2 was high (2.1 kg N ha1) under fertilized and tilled paddy conditions, despite relatively low storm runoff (22.3 mm). For other storms, the TN and TP loads were less than 1.7 kg N ha1 and 0.15 kg P ha1, respectively. The relationships between TN loads and stormflow runoff volumes were significant (p < 0.05), except for Event 2 when basal dressing of N fertilizer occurred at a high application rate (55 kg N ha1). Similar relationships between TN loads and stormflow runoff volumes except during periods of fertilization were reported in ponded paddy fields on sandy loam soils in Japan (Takeda et al., 1990). Staver et al. (1988) reported that the greatest potential for N transport in surface runoff from an Atlantic Coastal Plain watershed in the United States occurred during extreme storm events soon after N application. The TP stormflow loads were significantly (p < 0.05) correlated with stormflow volume, excluding Events 1 and 2 under non-ponded and dry antecedent conditions. The exponent for TP was somewhat higher (b = 0.99) than for TN (b = 0.90), indicating that TP stormflow load increased with discharge more than TN load
138 did. Overall, the TN loads in this study for the same stormflow volume, were higher, but the TP loads were lower than in other ponded paddy field areas in Japan (Takeda et al., 1990). This may be related to differences in annual mean fertilizer application rates. The annual mean nitrogen application rate in this study area was higher (172 kg N ha1), but the phosphorus application rate was lower (22 kg P ha1), than the respective values (106 kg N ha1 and 51 kg P ha1) in Takeda et al. (1990). Hallberg et al. (1986) and Randal et al. (1990) showed that the concentrations and amounts of NO3–N lost in tile drainage water were influenced by the N fertilizer rate. Sharpley (1999) reported that larger additions of phosphorus in fertilizer and manure increased P loss in surface runoff. In this study, TP stormflow loads from ponded paddies or non-ponded paddies with wet antecedent conditions were much lower than from non-ponded paddies with dry antecedent conditions. Therefore, paddy field ponding may be an important factor in regulating TP export during storm events.
Conclusions The nutrient runoff potential of a rice paddy watershed during storm events is influenced by numerous factors, including amount and intensity of rainfall, antecedent rainfall conditions, timing and rate of fertilization, ponding, and irrigation management practices. This study revealed contrasting patterns between nitrogen and phosphorus behavior in storm runoff. Overall, TN concentrations in total runoff were significantly correlated with decreasing discharge because of dilution effects, except for storms occurring when fertilization effects still remained. In contrast, TP concentrations in total runoff water under non-ponded conditions were significantly related to discharge, but there was little relationship with discharge under ponded conditions. Mean TN concentrations in stormflow were lower than the concentrations in irrigation return flow except during periods when fertilization effects were high. The mean TN concentrations in stormflow from a paddy field watershed were much lower than reported in stormflow from other land use types. However, the mean TP concentrations in stormflow were always higher than concentrations in irrigation return flow, but much lower than reported for other land use types. The mean TP concentration ratio of stormflow to irrigation return flow under ponded conditions (1.6) was about one-half that under non-ponded conditions (3.1). The relationships between TN loads and stormflow volumes were significant (p < 0.05), except for a storm event coinciding with basal dressing with a high application rate of N. TP loads were significantly (p < 0.05) correlated with volumes of stormflow except for storm events under non-ponded and dry antecedent conditions. Paddy fields are generally shallowly ponded during the growing season, overlapping with the rainy season (June– August), and this could help reduce the P export associated with soil erosion during storm events, thereby helping to control eutrophication of the downstream waters. More research is needed to better clarify the mechanism of nutrient
J.S. Kim et al. transport from rice paddy watersheds during storm events by investigating organic and inorganic components of N and P.
References APHA (American Public Health Association), 1998, 20th ed.Standard Methods for Water and Wastewater Examination APHA (American Public Health Association), Washington, DC, USA. Betson, R.P., McMaster, M., 1975. Non-point source mineral water quality model. J. Water Pollut. Con. Feder., 47. Capece, J.C., 1994. Hydrology and Contaminant Transport on Flatwood Watersheds. Ph.D. dissertation, University of Florida, Gainesville, FL, USA, 225pp. Carter, D.L., 1990. Soil erosion on irrigated lands. In: Steward, B.A., Nielson, D.R. (Eds.), Irrigation of Agricultural Crops, ASA, CSSA, and SSSA Monograph, Madison, WI, USA, pp. 1143–1171. Correll, D.L., Jordan, D.L., Weller, D.E., 1999. Transport of nitrogen and phosphorus from Rhode River watersheds during storm events. Water Resour. Res. 35, 2513–2521. Cosser, P.R., 1989. Nutrient concentration–flow relationships and loads in the South Pine River, eastern Queensland. I. Phosphorus loads. Aust. J. Mar. Fresh. Res. 40, 613–630. DeWalle, D.R., Pionke, H.B., 1994. Streamflow generation on a small agricultural catchment during autumn recharge: II stormflow periods. J. Hydrol. 163, 23–42. Feng, Y.W., Yoshinaga, I., Shiratani, E., Hitomi, T., Hasebe, H., 2004. Characteristics and behavior of nutrient in a paddy field area equipped with a recycling irrigation system. Agric. Water Manage. 68, 47–60. Gao, C., Zhu, J.G., Zhu, J.Y., Gao, X., Dou, Y.J., Hosen, Y., 2004. Nitrogen export from an agriculture watershed in the Taihu Lake area, China. Environ. Geochem. Hlth. 26, 199–207. Hallberg, G.R., Baker, J.L., Randall, G.W., 1986. Utility of tile-line effluent studies to evaluate the impact of agricultural practices on ground water. In: Proceedings of the Conference on Agricultural Impacts Ground Water, Omaha, NE, 11–13 August. Water Well J. Publ. Co., Dublin, OH, USA. Heathwaite, A.L., 1997. Sources and pathways of phosphorus loss from agriculture. In: Tunney, H., Caton, O.T., Brookes, P.C., Johnston, A.E. (Eds.), Phosphorus Loss to Water from Agriculture. CAB International, UK, pp. 205–224. Kronvang, B., 1992. The export of particulate matter, particulate phosphate and dissolved phosphorus from two agricultural river watersheds: implication on estimating the non-phosphorus load. Water Res. 26, 1347–1358. Kronvang, B., Laubel, A., Grant, R., 1997. Suspended sediment and particulate phosphorus transport and delivery pathways in an arable catchment, Gelbaek stream, Denmark. Hydrol. Process. 11, 627–642. McCuen, R.H., 1998. Hydrologic Analysis and Design. Prentice-Hall, Upper Saddle River, NJ, USA. McDowell, R.W., Sharpley, A.N., Kleinman, P.J.A., 2001. Phosphorus export from an agricultural watershed: linking source and transport mechanisms. J. Environ. Qual. 30, 1587–1595. Ministry of Agriculture and Forestry, 2003. Yearbook of Agricultural Land and Water Development Statistics, 21, Seoul, Korea (in Korean). Nakasone, H., 2003. Runoff water quality characteristics in a small agriculture watershed. Paddy Water Environ. 1, 183–188. Nakasone, H., Nakamura, R., 1984. Fluctuation of water quality and runoff of pollutant load in the Karase River during rainfall. Trans. Jpn. Soc. Irri. Drain. Reclam. Eng. 111, 35–42 (in Japanese). Owens, L.B., Edwards, W.M., Keuren, R.W., 1991. Baseflow and stormflow transport of nutrients from mixed agricultural watersheds. J. Environ. Qual. 20, 407–414.
Nutrient runoff from a Korean rice paddy watershed during multiple storm events in the growing season Pionke, H.B., Hoover, J.R., Schnabel, R.R., Gbruek, W.J., Urban, J.B., Rogowski, A.S., 1988. Chemical–hydrologic interactions in the near-stream zone. Water Resour. Res. 24, 1101–1110. Pionke, H.B., Gburek, W.J., Sharpley, A.N., Schnabel, R.R., 1996. Flow and nutrient export pattern for an agricultural hill-land watershed. Water Resour. Res. 32, 1795–1804. Quinton, J.N., Catt, J.A., Hess, T.M., 2001. The selective removal of phosphorus from soil: is event size important?. J. Environ. Qual. 30, 538–545. Randal, G.W., Malzer, G.L., Anderson, B.W., 1990. Nitrate Losses to Tile Drainage as Affected by Nitrogen Fertilization of Corn in a Corn–Soybean Rotation. MN Agric. Exp. Stn. Misc. Publ. 71-1991. St. Paul, MN, USA. SAS Institute, 1999. The SAS System for Windows. Version 8. Release 8.1. SAS Institute, Cary, NC, USA. Schnabel, R.R., Urban, J.B., Gburek, W.J., 1983. Hydrologic controls in nitrate, sulfate and chloride concentrations. J. Environ. Qual. 22, 589–596. Sharpley, A.N., 1999. Global issues of phosphorus in terrestrial ecosystems. In: Reddy, K.R., O’Connor, G.A., Schelske, C.L.
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(Eds.), Phosphorus Biochemistry in Subtropical Ecosystems. Lewis Publishers, Boca Raton, FL, USA, pp. 15–46. Sharpley, A.N., Gburek, W.J., Folmar, G., Pionke, H.B., 1999. Sources of phosphorus exported from an agricultural watershed in Pennsylvania. Agric. Water Manage. 41, 77– 89. Staver, K., Brinsfield, R., Magette, W., 1988. Nitrogen Export from Atlantic Coastal Plain Soils. Paper No. 88-2040. ASAE, St. Joseph, MI, USA. Suzuki, S., Tabuchi, T., 1984. Study of the direct runoff of nutrients to streams in an agricultural area. Trans. Jpn. Soc. Irri. Drain. Reclam. Eng. 114, 21–31 (in Japanese). Takeda, I., Kunimatsu, T., Kobayashi, S., Maruyama, T., 1990. Storm runoff loadings from paddy field areas. Tran. Jpn. Soc. Irri. Drain. Reclam. Eng. 147, 79–85 (in Japanese). Webb, B.W., Walling, D.E., 1998. Water quality II. Chemical characteristics. In: Calow, P., Petts, G.E. (Eds.), The River Handbook, vol. 1. Blackwell Scientific Publications, Boston, MA, USA, pp. 81–82.
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Nutrient runoff from a Korean rice paddy watershed during multiple storm events in the growing season Jin S. Kim
a,*
, Seung Y. Oh b, Kwang Y. Oh
a
a
Department of Rural Engineering, Chungbuk National University, 12 Gaeshin-dong, Cheongju, 361-763, Republic of Korea Watershed Management Research Center, National Institute of Environmental Research, Gyeongseo-dong, Incheon, 404-708, Republic of Korea
b
Received 5 January 2005; received in revised form 18 October 2005; accepted 4 November 2005
KEYWORDS
Summary The concentrations and loading characteristics of total nitrogen (TN) and total phosphorous (TP) in runoff from a 50.1-ha rice paddy field watershed in South Korea were investigated for eight storm events during the 1998–2001 growing seasons. TN concentrations in total runoff were inversely related to discharge, except in periods with high fertilization rates. In contrast, TP concentrations were strongly proportional to discharge under non-ponded paddy conditions, but not correlated with discharge under most ponded paddy conditions. Stormflow and irrigation return flow were separated from total runoff using the constant-discharge method. The flow-weighted mean TN concentration in stormflow was lower than that in irrigation return flow, mainly because of rainwater dilution, except for periods with a residual fertilizer effect. The flow-weighted mean TP concentration in stormflow, however, was always higher than that in irrigation return flow, likely a result of sediment-associated phosphorus transport. The ratio of mean TP concentration in stormflow to that in irrigation return flow under ponded paddy conditions (1.6) was approximately one-half that under non-ponded conditions (3.1), suggesting that ponding on paddy fields played an important role in reducing soil erosion-related phosphorus export. Relationships between TN loads and stormflow runoff volumes were found except during a storm event in the high fertilization period (p < 0.05). TP loads were also correlated with stormflow runoff volumes (p < 0.05), except for storm events under non-ponded and dry antecedent conditions. These results indicate that nitrogen runoff from paddy field watersheds depends on fertilization rates, while phosphorus runoff is controlled by ponding conditions. c 2005 Elsevier B.V. All rights reserved.
Irrigation return flow; Phosphorus export; Ponding condition; Stormflow; Total nitrogen
* Corresponding author. Tel.: +82 43 261 2573; fax: +82 43 271 2922. E-mail address: [email protected] (J.S. Kim).
0022-1694/$ - see front matter c 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2005.11.062
Nutrient runoff from a Korean rice paddy watershed during multiple storm events in the growing season
Introduction The efflux of nitrogen (N) and phosphorous (P) from agricultural lands is thought to be a major contributor to the accelerated eutrophication of rivers and reservoirs around the world. In South Korea, rice paddy fields are a major feature of the landscape, accounting for 11.6% of the land area in 2002 (Ministry of Agriculture and Forestry, 2003). Paddy fields typically lie between uplands and lower-elevation water bodies and may have a major impact on water quality. The growing season of rice (Oryza sativa) extends from April to October and corresponds with the hydrologically active period. The paddy fields are surrounded by earthen levees 10–30 cm in height and are usually shallowly ponded (3–10 cm) during most of the growing season. Therefore, these fields resemble wetlands, but fertilizers are applied and the fields are both irrigated and drained. Typical paddy soils have a relatively low percolation rate because the less permeable hardpan inhibits deep percolation and facilitates surface ponding. Extremely flat sites (slopes <0.3%) have a large surface flow component compared to sites with slightly steeper slopes (Capece, 1994), and correspondingly, the irrigation return flow from paddy
(a)
IIrrigation rrigation water
Irrig
ditch
atio n
n a ge
di t c h
Drai Tailwater Ponded water
Bypass water
Lateral subsurface irrigation return flow Main drainage canal
Irrigation return flow Watershed outlet
(b)
Rainfall Irrigation water
Drai
Irrig
atio n
h ditc
d it c h
nage
Bypass water Irrigation return flow + Stormflow
Ponded water
Surface stormflow +Tailwater
Lateral subsurface irrigation return flow +Subsurface stormflow Main drainage canal
Watershed outlet
Figure 1 Schematic of runoff from a rice paddy watershed: (a) irrigation return flow during non-storm periods; (b) total runoff during storm events.
129
fields on flat sites consists largely of surface irrigation runoff (i.e., tailwater). In paddy field areas with independent irrigation and drainage ditches, irrigation return flow (normal flow) at the outlet during non-storm periods consists of tailwater, lateral subsurface irrigation return flow, and bypass water (Fig. 1a). In contrast, total runoff at the outlet of paddy field areas during storm events mainly consists of irrigation return flow and stormflow (Fig. 1b). Commonly used hydrologic terms are defined for this study as follows: tailwater refers to the excess water applied to paddy plots that does not infiltrate, but rather runs off into a drainage system; lateral subsurface irrigation return flow is irrigation water that moves laterally through the crop root zone and drains to a drainage system; bypass water denotes irrigation water that does not enter the paddy plots and runs off at irrigation ditch outlets; and stormflow refers to the flow response to a storm event, in excess of irrigation return flow, and consists of surface and subsurface components. Storm events are important in activating hydrological pathways, establishing hydrological connectivity between agricultural land and streams, and hence in the transport of P (Heathwaite, 1997). Hydrologic and chemical responses to storm events have been studied for upland crop watersheds (Pionke et al., 1988, 1996; Cosser, 1989; Owens et al., 1991; DeWalle and Pionke, 1994; Kronvang et al., 1997; Correll et al., 1999), mixed land use watersheds that contain paddy fields (Nakasone and Nakamura, 1984; Suzuki and Tabuchi, 1984; Gao et al., 2004), and predominantly paddy field watersheds (Takeda et al., 1990; Feng et al., 2004). Based on previous studies, we hypothesized that in watersheds dominated by paddy fields, total nitrogen (TN) and nitrate nitrogen (NO3–N) concentrations in storm runoff would not always increase, but total phosphorus (TP) concentration would increase with increased discharge. However, only limited field data were available to verify such characteristics. The objectives of this study were to characterize nutrient (TN and TP) concentration patterns in total runoff from a Korean rice paddy watershed during multiple storm events in the growing season, to investigate mean nutrient concentration relationships between stormflow and irrigation return flow, and to evaluate relationships between runoff volume and nutrient loads in stormflow.
Materials and methods Study site The study was conducted in a 50.1-ha watershed in Soro, Chungbuk, in central South Korea (3641 0 N, 12725 0 E; 30– 32 m above sea level; Fig. 2). Rice paddy fields account for more than 80% of land use in the area, and the rest of the watershed consists of farm roads, irrigation and drainage ditches, and stream banks. The irrigation season in the study area ranges from midApril to late September, overlapping with the growing season. The 5-year mean growing season precipitation level was about 980 mm for 1998–2002. Typical (30-year mean) temperatures during the growing season range from 12.1 C in April to 27.2 C in August.
130
J.S. Kim et al. season and a single P application of 22 kg ha1 yr1 occurs in early to mid-May as a basal dressing.
l
na
n
ca
Methods
io
t ga
i
n
ai
r ir
Irrigation ditch Drainage ditch
M
Water samples were collected immediately prior to or during storm events by manually filling a polyethylene bottle at the inlets of irrigation ditches (i.e., irrigation water), two paddy plots (i.e., ponded water), a study watershed outlet (i.e., irrigation return flow and total runoff), and a rain gauge site (i.e., rainwater; Fig. 2). Water samples and discharge measurements for total runoff were taken at 2- to 6-h intervals per event. Rainfall data were also collected from a rain gauge near the study watershed. Water samples were analyzed for TN and TP using the persulfate and ascorbic acid methods (APHA, 1998), respectively. The amount and timing of fertilizer applications were determined by questioning farmers in the study area. A power function (Betson and McMaster, 1975; Kronvang, 1992; Webb and Walling, 1998) was used to determine the regression relationship between specific discharge (discharge per unit area) and dependent nutrient concentration in total runoff: C ¼ aqb ;
Main drainage canal
Figure 2 Location of the study site in South Korea. d Sampling point of irrigation water. j Sampling point of total runoff and irrigation return flow. m Rain gauge site. s Sampling point of ponded water.
The study watershed, located in an alluvial plain, consists of loamy soils with a shallow (1–2 m) water table. The paddy soils of the study area have a percolation rate of about 1–2 mm day1. The surface paddy soils are usually puddled (or mixed with standing water) to reduce percolation and to suppress weed growth just prior to transplanting. Rice seedlings are transplanted from a nursery to the paddy fields in mid to late May. The paddy fields are drained in early to mid-September to facilitate mechanized combine harvesting. The study area is largely irrigated with surface water released from an agricultural reservoir by a diversion weir in the Miho River, and the return flows are drained back to the same river. The Miho River is a tributary of the Geum River, which forms the third largest watershed area (9886 km2) in South Korea. The study paddy field area was replotted in 1996 to increase land and labor productivity. The main irrigation canals, irrigation ditches, and main drainage canals are concrete-lined, but drainage ditches are vegetated. The standard size of a unit lot is 100 · 100 m; one side is adjacent to an irrigation ditch and another side is next to a drainage ditch. In each paddy plot, ponded water slowly moves from inlets in an irrigation ditch to outlets in a drainage ditch. On average, three split N applications (basal dressing and top dressings at the tillering and panicle formation stages) of 172 kg ha1 yr1 occur during the growing
ð1Þ
where C is the nutrient concentration of total runoff, q is specific discharge, and a and b are a constant and an exponent of the function, respectively. When the exponent b > 0 or <0, the dependent nutrient concentration increases or decreases exponentially, respectively. When b = 0, the resulting nutrient concentration remains constant. The power function was also used to determine the regression relationship between runoff volume and dependent nutrient load in stormflow. This was done to characterize loading patterns of nutrients from the study rice paddy watershed. Regression analyses were performed using SAS (SAS Institute, 1999) and the regression curves were considered significant if p < 0.05.
Results and discussion Outline of storm events, water management, and fertilization Eight storm events during the 1998–2001 growing seasons, each resulting in 27.8–194.5 mm of rainfall (Table 1), were used in this study to characterize the loading patterns of nutrients from the study watershed. Event 1 occurred shortly after irrigation ended in late September. Event 2 occurred when some rice paddies were not ponded and no rice paddies were planted. Event 8 occurred after a two-month drought. During heavy storms, a ditch rider stops the irrigation supply to irrigation canals to reduce the risk of crop damage by inundation. Events 2, 3, and 8 were such storms during which irrigation was halted. Irrigation was stopped prior to Events 5 and 7 as a precaution because of the high antecedent rainfall. Event 2 coincided with fertilizer application in the study watershed at rates of 55 kg N ha1 and 12 kg P ha1, and Event 6 coincided with the application of 25 kg N ha1.
Summary of storm events during the 1998–2001 growing seasons Status of ponding and rice growth stage
Status of irrigation water
Remark
0
Non-ponded, ripening
Stopped
8.7
0
Non-ponded in some paddies, not transplanted
Stopped on the way
16
10.7
0
Ponded, tillering
50.6 41.0 27.8
36 43 27
6.7 10.3 7.6
9.7 26.7 0
Ponded, booting Non-ponded, ripening Ponded, rooting
Stopped on the way Supplied Stopped Supplied
Non-irrigation period Basal dressing (55 kg N ha1, 12 kg P ha1) Top dressing (N/Aa)
80.0 93.8
48 25
12.6 20.4
55.4 0
Ponded, tillering Ponded, tillering
Storm event
Date
Amount of rainfall (mm)
Duration hours of rainfall (h)
Maximum rainfall intensity (mm/h)
1
September 29–October 2, 1998
194.5
54
15.8
2
May 3–5, 1999
55.1
24
3
June 16–18, 1999
67.3
4 5 6
July 27–30, 1999 September 19–21, 1999 May 26–28, 2000
7 8
June 26–28, 2000 June 18–20, 2001 a
Not available for application rate of fertilizer.
Total 3-day antecedent rainfall (mm)
Stopped Stopped on the way
Top dressing (25 kg N ha1)
Nutrient runoff from a Korean rice paddy watershed during multiple storm events in the growing season
Table 1
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J.S. Kim et al.
Concentrations in irrigation water, rainwater, and irrigation return flow Irrigation water was supplied during five storm events (Events 2–4, 6, and 8) with mean nutrient concentrations ranging from 1.3 to 3.3 mg TN L1 and from 0.08 to 0.30 mg TP L1 (see Table 2). Irrigation return flow had specific discharges ranging from 0.25 to 4.97 L s1 ha1, with particularly low values during the non-irrigation season (Event 1) and drought (Event 8; Table 2). Irrigation return flow had nutrient concentrations similar to irrigation water: 1.3–4.8 mg L1 for TN and 0.05–0.19 mg L1 for TP. The mean concentrations of TN and TP in irrigation return flow were 2.5 and 0.17 mg L1, respectively, for a 7.4-ha rice paddy watershed in Japan (Feng et al., 2004). The mean concentrations of NO3–N and TP in streamwater during irrigation periods were reported to be 1.5 and 0.20 mg L1, respectively, for a 205-ha mixed land use watershed composed of 25% paddies in Japan (Nakasone, 2003). TN concentrations in streamwater in a mixed land use watershed that contained paddies in China seldom exceeded 2.0 mg L1 from early July to late October (rice full growing season; Gao et al., 2004). The mean TN concentration was the highest (4.8 mg L1) at the lowest specific discharge (0.25 L s1 ha1), in Event 1. Typically, tailwater dramatically decreases after the irrigation season ends, and a small lateral subsurface irrigation return flow gradually decreases over 15–20 days. Thus, as the lateral subsurface irrigation return flow is reduced by evapotranspiration, TN concentra-
tions increase against a nearly constant TN load. Also, mean TN and TP concentrations for Event 2 were high (4.5 and 0.19 mg L1, respectively) because of the effect of fertilization. The mean nutrient concentrations in rainwater, 0.1– 1.4 mg L1 for TN and 0.01–0.04 mg L1 for TP, were generally lower than in irrigation water or irrigation return flow. Therefore, rainwater may function to dilute irrigation water and irrigation return flow.
Temporal variation in TN concentration in total runoff TN concentrations in total runoff increased with increased specific discharge during Events 2 and 3 when fertilization was applied to the study watershed. However, overall TN concentrations decreased with increased discharge during non-fertilization periods, such as during Events 1, 4, 5, 7, and 8, and top-dressing periods, such as Event 6, although concentrations partially increased with rapidly increased discharge during Events 1 and 4 (see Fig. 3). For example, during the basal dressing period of Event 2, TN concentrations in the total runoff water greatly increased with discharge from 4.5 to 11.3 mg L1, and then decreased to 6.3 mg L1. Most irrigation water becomes irrigation return flow in the form of bypass water during storms, because farmers usually do not withdraw water during storm events (Fig. 1b). When irrigation water at a relatively low TN concentration (2.8 mg L1) was cut off, the total runoff discharge rapidly decreased and then TN
Table 2 Specific discharge of irrigation return flow and mean nutrient concentrations in irrigation water, irrigation return flow, and rainwater Storm event
Specific discharge of irrigation return flow (L s1 ha1)
Parameter
1
0.25
2
Mean concentration (mg L1) Irrigation watera
Ponded water in paddy plotsb
Irrigation return flowa
Rainwaterb
TN TP
N/Ac N/A
N/A N/A
4.8 0.10
0.2 0.01
3.54
TN TP
2.8 0.18
N/A N/A
4.5 0.19
N/A N/A
3
4.43
TN TP
3.3 0.15
5.4 0.22
2.3 0.05
0.9 0.01
4
2.02
TN TP
1.3 0.08
1.3 0.11
2.0 0.06
0.6 0.01
5
1.44
TN TP
N/A N/A
N/A N/A
1.3 0.07
0.1 0.01
6
4.97
TN TP
2.6 0.14
2.9 0.09
2.9 0.09
1.4 0.04
7
1.13
TN TP
N/A N/A
N/A N/A
2.1 0.09
0.9 0.01
8
1.04
TN TP
2.9 0.30
1.7 0.12
3.6 0.14
0.9 0.02
a b c
Flow-weighted mean. Arithmetic mean. N/A, not available.
(mg L-1)
TP
10
1.2 6
0.3
0
c 10
6 20
20 25
15 10
0.4 2
TP No irrigation
q
0.0 0
16
4
TN 12 0.1
8
2
TP
0.0
4
q
Irrigation stops
0
0 17
16
Oct. 1998
June 1999
10
0.3 3 20 25
20
20 15 10
Irrigation stops
5
q 0
0
5
TP
q 0.0 0
Ju1y 1999 (mg L-1)
TP
TN
10
0.3 3
0
g
TN
0
e
Rainfall (mm)
(mg L-1)
May 1999
TP
0
30
29
28
27
4
3
10
0.1 1
10
0.3 3
20
20
25 20 15
TN TP
0.1 1
10 5
q No irrigation
0.0 0
19
20
20
Specific discharge (L s-1 ha-1)
0.2 2
0
TN 0.2 2 15
10 0.1 1
TP 5 Stormflow Irrigation return flow
0.0 0
21
27
26
Sept. 1999
No irrigation 28
0
TN
(mg L-1)
TP
TN
10
0
h
TN
0
Rainfall (mm)
(mg L-1)
TP
q
June 2000
f
0.3 3
Rainfall (mm)
0.0
15
TN
Specific discharge (L s-1 ha-1)
5
20
0.2 2
10
0.4 4 20
20
Rainfall (mm)
0.4
Specific discharge (L s-1 ha-1)
25
TP
0.8 10
Rainfall (mm)
0
d
Specific discharge (L s-1 ha-1)
TN
TP
TN
10
1.2 15
Rainfall (mm)
0
b
TN (mg L-1)
(mg L-1)
Sept. 1998
TP
0
2
1
30
29
5
0.2
Specific discharge (L s-1 ha-1)
20
20
Specific discharge (L s-1 ha-1)
TN 0.8 4
Rainfall (mm)
0
133
TN
TP
TN
a
Rainfall (mm)
(mg L-1)
Nutrient runoff from a Korean rice paddy watershed during multiple storm events in the growing season
TP 0.1 1
q
0.0 0
5
0 26
27
May 2000
28
16 0.2 2
TN
12
TP
8
0.1 1
Irrigation stops
4
q
Specific discharge (L s-1 ha-1)
10
0.2 2
Specific discharge (L s-1 ha-1)
20 0.3 3
0
0.0 0 18
19
20
June 2001
Figure 3 Temporal variation in nutrient concentrations and discharge in total runoff: (a) Event 1; (b) Event 2; (c) Event 3; (d) Event 4; (e) Event 5; (f) Event 6; (g) Event 7; (h) Event 8.
134
J.S. Kim et al.
concentration sharply increased again to a peak value of 14.7 mg L1 because of decreased dilution by irrigation water. Also, the TN concentrations increased with discharge for Event 3 during the top dressing period. The TN concentrations in stormflow may be greatly affected by the N concentrations in ponded water in rice fields. During fertilizer application periods, like Events 2 and 3, TN concentrations in ponded water in paddy plots may be much higher than in the irrigation return flow because of the applied nitrogen fertilizer retained in the ponded water. Thus, the concentrations in total runoff may increase with discharge overall, despite peak concentrations on the rising limb or during an early peak, because of the ponded water runoff with high TN concentrations caused by rainwater influx. For Event 3, the TN concentration in ponded water (5.4 mg L1) was much higher than that in irrigation return flow (2.3 mg L1; Table 2). In China, the storm event immediately after fertilization produced high ammonium nitrogen (NH4–N) concentrations in stream water from a 122-ha mixed land use watershed that contained paddy fields (Gao et al., 2004).
For other storms, the TN concentrations in ponded water and irrigation return flow differed slightly. Therefore, the concentrations in total runoff may decrease overall with increased discharge, perhaps because of dilution by rainwater influx, but rise partially with rapidly increased discharge in cases like Events 1, 4, and 7. The TN concentrations in ponded water were equal to or somewhat lower than those in irrigation return flow for Events 4, 6, and 8 (Table 2). These results were consistent with previous observations (Feng et al., 2004) in that TN concentrations in total runoff decreased with discharge overall, despite a small early peak on the rapidly rising limb of the hydrograph during a storm in a non-fertilization period in a Japanese paddy rice watershed. The small early rise in TN concentration on the rapidly rising limb may have been a result of the organic nitrogen transport from soils to watercourses. The concentrations of particulate organic nitrogen (PON) or organic nitrogen greatly increased during storm events in mixed land use watersheds, including paddies (Gao et al., 2004), or uplands (Correll et al., 1999).
100.0
1.00 b
a
Peak
-1
-1
TN (mg L )
Peak Peak
1.0
Peak Peak
Event Rising Falling No. 1
TP (mg L )
Peak
10.0
Peak
0.10
Event Rising Falling No. 1
No. 2
No. 2
No. 3 No. 4
No. 3 No. 4
0.1 0.1
1.0 10.0 Specific discharge (L s-1 ha-1)
100.0
0.01 0.1
100.0
1.00
Event Rising Falling No. 5
a
Peak
1.0 10.0 Specific discharge (L s-1 ha-1)
100.0
b
No. 6
No. 7 No. 8
Peak
TN (mg L )
10.0
Peak
TP (mg L-1)
-1
Peak Peak
1.0
Peak
0.10 Peak Event Rising Falling No. 5
Peak
No. 6
Peak
0.1 0.1
1.0
10.0
Specific discharge (L s-1 ha-1)
No. 7 No. 8
100.0
0.01 0.1
1.0
10.0
100.0
Specific discharge (L s-1 ha-1)
Figure 4 Relationship between nutrient concentrations and specific discharge in total runoff. Open and solid symbols indicate rising and falling limbs of hydrographs, respectively: (a) TN; (b) TP.
Nutrient runoff from a Korean rice paddy watershed during multiple storm events in the growing season
discharge were observed in another paddy field watershed (Feng et al., 2004), four mixed land use watersheds (1.7– 28 km2), including 10–23% paddies (Suzuki and Tabuchi, 1984), and watersheds with other land uses (Pionke et al., 1988; Quinton et al., 2001). When antecedent rainfall levels were low (e.g., Events 1–3, 6, and 8), TP concentrations usually peaked before total runoff peaked, whereas they peaked during or after peak runoff rate when antecedent rainfall levels were medium to high (e.g., Events 4, 5, and 7; Table 1). The TP concentration markedly peaked before peak runoff for events with high rainfall intensity (e.g., Events 1 and 8). The peaking of TP concentrations prior to runoff peaks may occur because the sediment and particulate P that accumulated in paddy fields and drainage ditches during the drier weather quickly washes away at the beginning of a storm. This phenomenon was observed during several storms in paddy field watersheds (Takeda et al., 1990; Feng et al., 2004) and mixed land use watersheds containing paddies (Suzuki and Tabuchi, 1984). The results suggest that P transport in storm runoff from paddy fields is highly affected by antecedent rainfall.
However, the results of this study disagreed with results from another 11.6-ha rice paddy watershed in Japan (Takeda et al., 1990). In Takeda et al. (1990), the TN concentration in the total runoff during storms increased with discharge during non-fertilizer application periods, whereas it decreased with discharge during the top-dressing period (late July); the TN and NO3–N concentrations did not always increase with discharge for mixed land use watersheds that contained paddy fields in Japan (Nakasone and Nakamura, 1984; Suzuki and Tabuchi, 1984), but they did increase with discharge in a mixed land use watershed containing paddy fields in China (Gao et al., 2004).
Temporal variation of TP concentrations in total runoff Overall, the TP concentrations generally increased with discharge for many storm events (Fig. 3). For Event 1 under non-ponded conditions, the TP concentrations in the total runoff water increased sharply (from 0.10 to 0.80 mg L1) prior to peak discharge, and then greatly decreased to 0.24 mg L1 around the discharge peak. For Event 2, the TP concentrations in the total runoff greatly increased (from 0.19 to 0.52 mg L1) with discharge, and then slowly decreased with decreasing discharge. However, after irrigation stopped, the runoff discharge rapidly decreased and the TP concentration sharply increased to 0.81 mg L1. For Event 4 under ponded conditions, the TP concentrations increased (from 0.06 to 0.12 mg L1) with discharge, and then decreased with decreasing discharge. The increase in TP concentration with increased discharge may be a result of the transport of particulate phosphorus. Phosphate phosphorus (PO4–P) concentrations increased much more gradually with increased discharge in a paddy field watershed, relative to TP concentrations (Takeda et al., 1990). Particulate organic phosphorus (POP) constituted the largest fraction of TP in storms in watersheds of differing land uses in Maryland, USA (Correll et al., 1999). Increases in TP concentrations with increased
Table 3
No. of samples
29 16b 10c 14b 10c 25 23 12 16 16b 7c
4 5 6 7 8 a b c d
Fig. 4 and Table 3 indicate the relationships between nutrient concentrations (C) and specific discharges (q) for total runoff in the eight investigated storm events. The nutrient concentration in the total runoff water ranged from 0.6 to 14.7 mg L1 for TN and from 0.03 to 0.81 mg L1 for TP (Fig. 4). At the same discharge level, TN concentrations for Event 2 and TP concentrations for Events 1 and 2 were much higher than those from other storm events. The relationship between TN and TP concentration and discharge during Event 1 showed anticlockwise and clockwise hystereses, respectively (Fig. 4). Here, clockwise hysteresis indicated higher concentrations during the rising limb of the hydrograph compared with those measured
TN
TP
C q equationa
3
Relationship between specific discharge and concentration in total runoff
Regression equations for nutrient concentrations and specific discharge in total runoff
Storm event
1 2
135
0.13
C = 3.17q – C = 2.01q0.66 – C = 1.07q0.43 – C = 1.33q0.26 C = 3.59q0.20 C = 1.97q0.08 – C = 3.78q0.19
r2 0.38 0.23 0.61 0.04 0.70 0.08 0.46 0.36 0.38 0.05 0.78
p <0.001 NSd <0.01 NS <0.01 NS <0.001 <0.05 <0.05 NS <0.01
C, nutrient concentration is in mg L1; q, specific discharge is in L s1 ha1. Regression equation including the period of irrigation stoppage. Regression equation excluding the period of irrigation stoppage. Not significant.
C q equationa 0.38
C = 0.224q – C = 0.052q1.00 – – C = 0.033q0.22 C = 0.055q0.47 – – – –
r2
p
0.61 0.16 0.72 0.27 0.32 0.16 0.91 0.03 0.07 0.12 0.37
<0.001 NS <0.01 NS NS <0.05 <0.001 NS NS NS NS
136
Mean nutrient concentration in stormflow The boundary between stormflow and irrigation return flow in total runoff from paddy field during a storm event is difficult to define because total runoff may sometimes contain ponded water artificially released by farmers to prevent levee collapse. In this study, the total runoff hydrograph was assumed to consist of stormflow and irrigation return flow, and the two components were separated using a constantdischarge method (McCuen, 1998; Fig. 3g). Flow-weighted mean nutrient concentration in stormflow Cs was obtained by: Ls Lt Lr Cs ¼ ¼ ; ð2Þ Vs Vt Vr where Ls is stormflow load, Vs is stormflow volume (depth), Lt is total runoff load, Lr is load of irrigation return flow, Vt is total runoff volume (depth), and Vr is volume (depth) of irrigation return flow. Fig. 5 shows the mean TN and TP concentrations in stormflow during storm events obtained from Eq. (2). Flow-weighted mean TN concentrations in stormflow for Events 2 and 3 were high, 9.6 and 3.3 mg L1, respectively, likely because of the effects of N fertilization (Fig. 5a). However, mean TN concentrations in stormflow for the other storms ranged from 0.7 to 2.5 mg L1. These values were similar to the mean TN concentration (1 mg L1) in
10.0
2
a
-1
Stormflow (mg L )
3 6 1.0
1 8
7 4 5 2:1 line
Ponded Non-ponded (dry) Non-ponded (wet)
1:1 line 1:2 line 0.1 0.1
1.0
10.0 -1
Irrigation return flow (mg L )
1.00
b 2
1 -1
Stormflow (mg L )
during the falling limb. The TN and TP concentrations– discharge relationships during Events 4 and 8 showed clockwise hystereses, but it is still unclear under what conditions the clockwise hysteresis occurs. The clockwise hystereses of the relationship between TP concentration and discharge have been reported elsewhere, for a mixed land use watershed containing paddies (Suzuki and Tabuchi, 1984). Statistically significant relationships between discharge and TN concentration (Eq. (1)) were obtained for seven of eight events (Table 3). Irrigation management practice greatly affected the C q rating equation. Thus, the relationship between discharge and TN concentration was not significant when data were included from the irrigation cutoff periods, i.e., Events 2, 3, and 8. In general, TN concentration decreased with specific discharge (b < 0 in Eq. (1)). However, TN increased with specific discharge (b > 0) for Events 2 and 3 during the fertilizer application period. In comparison, a correlation between N concentration and streamflow (r2 = 0.70) was observed during storms from July to September (the main rice-growing season) for a mixed land use watershed containing rice paddies in China (Gao et al., 2004). For mixed land use watersheds, including the uplands in the United States, the concentrations of NO3–N (Schnabel et al., 1983) and PON (Correll et al., 1999) were positively correlated with discharge. The relationship between TP concentration and discharge was significant for storm events under non-ponded conditions, i.e., Events 1, 2, and 5. However, TP concentration was not related to discharge for storms under ponded conditions, with the exception of Event 4. Similarly, concentrations of particulate P and POP, the dominant forms of TP in storm runoff, increased significantly with storm discharge on other land use types in Australia (Cosser, 1989) and the USA (Correll et al., 1999), respectively.
J.S. Kim et al.
5 0.10
3 2:1 line
6
8
7 4 Ponded
1:1 line
Non-ponded (dry) 1:2 line 0.01 0.01
Non-ponded (wet) 0.10
1.00
Irrigation return flow (mg L-1)
Figure 5 Relationship between stormflow and irrigation return flow for: (a) TN and (b) TP mean concentrations. Numbers next to symbols refer to the respective storm events.
stormflow from another paddy field watershed (Feng et al., 2004), but much lower than the mean NO3–N concentrations (7.0 and 7.5 mg L1, respectively) in stormflow from an upland watershed and a mixed land use watershed in Pennsylvania, USA (Pionke et al., 1996). Flow-weighted mean TP concentrations in stormflow under non-ponded conditions, such as in Events 1 and 2, were much higher (0.48 and 0.49 mg L1, respectively) than under ponded conditions (Fig. 5b). Prior to transplanting (e.g., Event 2), bare paddy soils were tilled and fertilized; thus, a combined effect of fertilization and vulnerability to soil erosion may have caused high mean TN and TP concentrations in stormflow. However, the mean TP concentration in stormflow for other storms was relatively low (<0.16 mg L1). The mean TP concentration for Event 8 under ponded conditions was 0.15 mg L1 despite the high rainfall intensity (20.4 mm h1). The lowest mean TP concentration was 0.06 mg L1 for Event 4 under ponded and abundant crop cover conditions. Overall, the mean TP concentrations in stormflow from the study area, except for non-ponded conditions, were almost identical to the mean concentration (0.15 mg L1) from another paddy field watershed (Feng et al., 2004), but much lower than those (0.24–0.55 mg L1) from mixed land use watersheds in Pennsylvania (Sharpley et al., 1999) or the mean POP con-
Nutrient runoff from a Korean rice paddy watershed during multiple storm events in the growing season 10.0
a
This study 0.88
TN load (kg ha-1)
y = 0.029x r 2 = 0.60, p <0.05 2
1
8 7 3
1.0 6
4
Takeda et al.(1990) y = 0.040x0.63
5
Ponded Non-ponded (dry) Non-ponded (wet) 0.1 10
100 Stormflow runoff (mm)
1000
1.00
b
1
-1
TP load (kg ha )
Takeda et al.(1990) y = 0.0076x0.80
0.10
8
2 5
7
This study 0.99
6
0.01 10
y = 0.001x 2 r = 0.72, p <0.05 3
4
Ponded Non-ponded (dry) Non-ponded (wet)
100 Stormflow runoff (mm)
1000
Figure 6 Relationship between runoff and TN and TP load of stormflow. Numbers next to symbols refer to the respective storm events: (a) TN; (b) TP.
centration (>1.50 mg L1) in storm events from mostly cropland watershed in Maryland (Correll et al., 1999), USA.
137
der ponded conditions. Despite a small crop canopy, the mean TP concentration ratio for Event 6 was as low (1.6) as that for other ponded conditions. Moreover, mean TN and TP concentration ratios between stormflow and irrigation return flow from the ponded study area were much smaller than those between stormflow and base flow from other land use watersheds. The mean TN and TP concentrations in stormflow from mixed land use watersheds, including paddy fields in Japan, were 2.4 and 5.3 times higher than those in base flow, respectively (Suzuki and Tabuchi, 1984). The median TN concentration in streamwater in Chinese mixed land use watersheds, including paddy fields during storm events, was three times higher than in regular weekly samples (Gao et al., 2004). The mean TP concentrations in stormflow were at least 10 times higher (Pionke et al., 1988) and 4–13 times higher (McDowell et al., 2001) than those in base flow for an upland watershed and a mixed land use watershed in Pennsylvania, USA, respectively. The mean total organic phosphorus (TOP) concentrations in stormflow were 9.1 times higher than those in base flow for a mixed land use watershed and 3.7 times higher for a mostly cropland watershed in Maryland, USA (Correll et al., 1999). The small increase in stormflow TP concentration relative to that in irrigation return flow under ponded conditions may occur because the shallow water layer protects the soil from the direct impact of the rain and decreases flow velocity with large roughness coefficients, thereby greatly reducing the transport of P associated with soil erosion. When dry soils are suddenly wetted, water molecules rapidly displace the absorbed O2 and N2 molecules. These gases join entrapped air, causing pressure forces sufficient to burst soil aggregates (Carter, 1990). The rice paddies are ponded and are rapidly covered by the crop canopy during most of the growing season after transplanting, thereby controlling TP.
TN and TP stormflow loads Ratio of mean nutrient concentration in stormflow to that in irrigation return flow The ratio of mean TN concentration in stormflow to that in irrigation return flow ranged from 0.5 to 2.1, and the mean TP ratio ranged from 1.0 to 4.4 (Fig. 5). The mean TN ratio of stormflow to irrigation return flow was 0.6, except for Events 2 and 3, indicating that the TN mean concentrations in stormflow were usually lower than in irrigation return flow. Conversely, the mean TP concentrations in stormflow were usually higher than in irrigation return flow. Similar trends were observed in previous research on a paddy field watershed (Feng et al., 2004). However, the mean concentrations of N and P were usually higher in stormflow than in base flow from mixed land use watersheds, including paddy fields (Suzuki and Tabuchi, 1984; Gao et al., 2004), or uplands (Owens et al., 1991). The ratio of mean TN concentration in stormflow to that in irrigation return flow under non-ponded conditions (1.0) was nearly identical to that under ponded conditions (0.9), indicating that ponding did not have much affect on the mean TN concentrations in stormflow. In contrast, the stormflow to irrigation return flow ratio for mean TP under non-ponded conditions (3.1) was about twice that (1.6) un-
The TN and TP stormflow loads were highest (3.9 kg N ha1 and 0.77 kg P ha1, respectively) in Event 1 under drained paddy and highest runoff conditions (Fig. 6). The TN load for Event 2 was high (2.1 kg N ha1) under fertilized and tilled paddy conditions, despite relatively low storm runoff (22.3 mm). For other storms, the TN and TP loads were less than 1.7 kg N ha1 and 0.15 kg P ha1, respectively. The relationships between TN loads and stormflow runoff volumes were significant (p < 0.05), except for Event 2 when basal dressing of N fertilizer occurred at a high application rate (55 kg N ha1). Similar relationships between TN loads and stormflow runoff volumes except during periods of fertilization were reported in ponded paddy fields on sandy loam soils in Japan (Takeda et al., 1990). Staver et al. (1988) reported that the greatest potential for N transport in surface runoff from an Atlantic Coastal Plain watershed in the United States occurred during extreme storm events soon after N application. The TP stormflow loads were significantly (p < 0.05) correlated with stormflow volume, excluding Events 1 and 2 under non-ponded and dry antecedent conditions. The exponent for TP was somewhat higher (b = 0.99) than for TN (b = 0.90), indicating that TP stormflow load increased with discharge more than TN load
138 did. Overall, the TN loads in this study for the same stormflow volume, were higher, but the TP loads were lower than in other ponded paddy field areas in Japan (Takeda et al., 1990). This may be related to differences in annual mean fertilizer application rates. The annual mean nitrogen application rate in this study area was higher (172 kg N ha1), but the phosphorus application rate was lower (22 kg P ha1), than the respective values (106 kg N ha1 and 51 kg P ha1) in Takeda et al. (1990). Hallberg et al. (1986) and Randal et al. (1990) showed that the concentrations and amounts of NO3–N lost in tile drainage water were influenced by the N fertilizer rate. Sharpley (1999) reported that larger additions of phosphorus in fertilizer and manure increased P loss in surface runoff. In this study, TP stormflow loads from ponded paddies or non-ponded paddies with wet antecedent conditions were much lower than from non-ponded paddies with dry antecedent conditions. Therefore, paddy field ponding may be an important factor in regulating TP export during storm events.
Conclusions The nutrient runoff potential of a rice paddy watershed during storm events is influenced by numerous factors, including amount and intensity of rainfall, antecedent rainfall conditions, timing and rate of fertilization, ponding, and irrigation management practices. This study revealed contrasting patterns between nitrogen and phosphorus behavior in storm runoff. Overall, TN concentrations in total runoff were significantly correlated with decreasing discharge because of dilution effects, except for storms occurring when fertilization effects still remained. In contrast, TP concentrations in total runoff water under non-ponded conditions were significantly related to discharge, but there was little relationship with discharge under ponded conditions. Mean TN concentrations in stormflow were lower than the concentrations in irrigation return flow except during periods when fertilization effects were high. The mean TN concentrations in stormflow from a paddy field watershed were much lower than reported in stormflow from other land use types. However, the mean TP concentrations in stormflow were always higher than concentrations in irrigation return flow, but much lower than reported for other land use types. The mean TP concentration ratio of stormflow to irrigation return flow under ponded conditions (1.6) was about one-half that under non-ponded conditions (3.1). The relationships between TN loads and stormflow volumes were significant (p < 0.05), except for a storm event coinciding with basal dressing with a high application rate of N. TP loads were significantly (p < 0.05) correlated with volumes of stormflow except for storm events under non-ponded and dry antecedent conditions. Paddy fields are generally shallowly ponded during the growing season, overlapping with the rainy season (June– August), and this could help reduce the P export associated with soil erosion during storm events, thereby helping to control eutrophication of the downstream waters. More research is needed to better clarify the mechanism of nutrient
J.S. Kim et al. transport from rice paddy watersheds during storm events by investigating organic and inorganic components of N and P.
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