IJERD – International Journal of Environmental and Rural Development (2013) 4-1

Research article
erd

Impacts of an Agricultural Water Use Restructuring
Project on the Water Quality of Oxbow LakeYAMAMOTO TADAO*Research Faculty of Agriculture, Hokkaido University, Hokkaido, Japan
Email: tady@env.agr.hokudai.ac.jp

OKAZAKI HIROKI
Graduate School of Agriculture, Hokkaido University, Hokkaido, Japan
Received 14 December 2012

Accepted 27 March 2013(*Corresponding Author)

Abstract Oxbow lakes in the Ishikari River Basin, Japan, are used as irrigation watersources, flood control ponds, and waterside parks, etc. In this study, we monitored water
quality over the course of the restructuring project at Lake Chashinainuma. In thiscatchment area, irrigation water was pumped from the lake until 2010, but is now drawn
from a distant river. As a result, the total nitrogen load balance changed from an outflow
type to a storage type due to decreased removal of water from the lake. The lake water
retention time also increased significantly. However, total nitrogen concentration
decreased after the pumping station was abolished. We suggest that this reduction in totalnitrogen concentration after 2011 reflected decreased particulate nitrogen due to biologicalpurification effects that are expected with increased chlorophyll concentration. Hence, thechange in load balance to an accumulation type may have been influenced by changes inagricultural water management and transformation of paddy fields into upland.
Keywords water quality paddy field, load balance, agricultural land improvement project

INTRODUCTION
Oxbow lakes are remnants of meandering floodplain rivers that have been cut off and physicallyisolated from their respective main river channels (Cullum et al., 2006). There are many oxbow
lakes in the Ishikari River basin, and both naturally and artificially formed lakes are used as
irrigation water sources, flood control ponds, and waterside parks. Hence, they are recognized asregional resources with multiple functions (Yamamoto et al., 2001). Many oxbow lakes are closed
type waterways in which eutrophication occurs easily. Because oxbow lakes originate from rivers,
much drainage water flows into them, and their water quality is strongly influenced by humanactivities, including agriculture in the catchment area. Yamamoto et al. (2002, 2004) studied theeffects of nutrient inflow from an agricultural area in Hokkaido on the water quality of an oxbow
lake.
Currently, land improvement projects, including land partition, enlargement, andrearrangement, creation of multipurpose paddy fields, and restructuring of irrigation and drainage
facilities are ongoing in the Lake Chashinainuma watershed in Bibai, Hokkaido. These projects
incur changes of land use and water management, and hence, influence water quality and thehydrological environment. However, few studies have examined the relationship between humanactivities, including land improvement projects, and water quality of oxbow lakes.
OBJECTIVEThis study object is to determine the impacts of the water use restructuring project on the physicaland   chemical   water   properties   of   the   oxbow   lake   “Chashinainuma”   in   terms   of;;   (1)   Changes   of  nitrogen concentration in the lake and drainage water, (2) Impact of pumping station on waterbalance and water quality in the lake and (3) Evaluation of water balance and total nitrogen loadⒸ ISERD220

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balance of the lake.
And this investigation contributes to the evaluation of changes in water quality following land
improvement projects.
METHODOLOGYThis survey was performed at Lake Chashinainuma between 2007 and 2012. Lake Chashinainuma
is located in the middle stream of the Ishikari river Basin. It has a water surface area of 13.1 ha andan average depth of 1.02 m. The size of the catchment area changed from 106 to 127 ha with theimprovement project (Table 1, Fig.1), and the land is mainly used as farmland. In this watershed,irrigation water was pumped up from the lake until 2010, when the pumping station was abolished,
and is now drawn from a distant river via open channels and pipelines. In addition, a new drain(D4) was constructed and the land use and crops were changed each year of the project. There are
four direct drainage and inflow points from the farmland. In the restricting area, the drainage waterfrom the fields cannot flow directly into the lake. The pressure-controlled tank for pipeline
irrigation (T) and the surplus water from this tank drain into the lake via the D3 water management
facility.

Fig. 1 Outline figure of catchment area and sampling points
To measure water quality, water was drawn from outflow drainage and inflow drainage
sampling points in the lake. Water sampling was conducted about two times per month during
irrigation periods, and once per month during non-irrigation periods from April 2007 to August
2012. Water sampling was performed two times a day during the paddling period (May) and once
per day during normal irrigation periods (June–August) using an automatic water sampler (ISCOInc.) from 2009 to 2012. Total nitrogen (TN), dissolved total nitrogen (DTN), nitrate nitrogen(NO3-N), nitrite nitrogen (NO2-N), and ammonium nitrogen (NH4-N) contents were determined.
These analyses conformed to Japanese Industrial Standards (JIS). Dissolved inorganic nitrogen(DIN) was calculated as the sum of NO3-N, NO2-N, and NH4-N. Total organic nitrogen (TON) wasⒸ ISERD221

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estimated by subtraction of DIN from TN. Particulate organic nitrogen (PON) was estimated bysubtraction of DTN from TN. Dissolved organic nitrogen (DON) was calculated as the difference
between DTN and DIN.
Table 1 Changes of land use and projects in the investigated area
Year
2007
2008
2009
2010
2011
Catchment area (ha)
112
112
112
112
106
Crop type (ha) Paddy
64
N.D.*
4245
24
Upland crop
42N.D.*
59
60
71
Water body2009; Water supply changes in a part of D1 catchment area
Plantation
2011; Pumping station stopped and land arrangement in D4 catchment area
Upland paddy 2012; Land arrangements in parts of D1 and D3 catchment area

2012
1276067* Crop type investigation was not carried out in 2008 because it was considered same in 2007.
Precipitation data were used in the Automated Meteorological Data Acquisition System(AMeDAS) at Bibai and Iwamizawa cities. Evapotranspiration was calculated according toPenman’s  method.
RESULTS AND DISCUSSION
Changes of water quality in the lake

Fig. 2 Changes in TN concentration in the lake and its drains
Figure 2 shows monthly changes in TN concentration in the drains (D1–D4) and in the lake. When
more than three samples were taken in a month, data are presented as the mean ± standard
deviation. The concentration of TN in the lake changed periodically, with high values during Apriland May, low values during the normal irrigation period, and a concentration rise again in thewinter. This trend was reflected in samples from the drains. Compared with D1 and D3, theconcentration of TN in D2 was markedly increased. In the D2 catchment area, paddy fields areprevalent in the downstream and influence the water quality easily. Accordingly, the concentrationsof TN in samples from D4 were high during the paddling period.
Figure 3 shows the changes in concentration of each nitrogen form in lake water. TN iscomprised mainly of TON and NO3-N. Concentrations of TN exceeded those allowed by theJapanese agricultural water use standard and environmental standard for environmentalⒸ ISERD

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conservation  (≤1.00  mgL−1) for most of the study period. Comparison of TN concentrations before
and after the pumping station was abolished show that TN concentrations decreased. Figure 3shows that this is reflected by decreased TON concentrations. Changes in NO3-N concentration
could not be confirmed.
During the years 2011 and 2012, approximately 70% of TON (40% of TN) was composed ofPON, indicating a significant influence of PON on TN concentrations in this lake.

After pumping
station abolishment

Fig. 3 Changes in nitrogen component of the lake water
Changes of water balance and total nitrogen balance during irrigation periods
Water balance during irrigation periods was calculated using the relation expression (Equation 1).
The inflow discharge in 2011 decreased compared with levels seen in 2009 and 2010 (Fig. 4) due
to advance of the pipeline and consequent decreases in paddy field area. Prior to 2010,
approximately 40% of the lake water was left via the pumping station, thus abolishment of thepumping station caused significant changes in water balance during 2011. In addition, thetransformation from paddy fields to cultivated area was accompanied by decreased inflow andoutflow, and the retention time of the lake water increased from 3.37 day in 2009 to 5.98 day in2011.

'h Qin  Qout  R  E r G  S  P(1)∆h, change in storage discharge; Qin, discharge of inflow into the lake; Qout, discharge from thelake outlet; R, precipitation; E, evapotranspiration; S, surplus water from pressure control tank; G,groundwater balance; P, discharge of intake water by pumpstation.

Fig. 4 Water balance of the lake

Fig. 5 Nitrogen Load balance in the lake

Total nitrogen balance during the irrigation periods of 2009–2011 was calculated from waterbalance and concentration. The concentration of TN in the lake water was used to calculate that of
the intake water for the pumping station, and changes in nitrification and outflow through theground were considered constant. The difference between input and output loads wasapproximately 110 kg day−1 and did not differ with that in 2009 and 2010. TN load balance duringⒸ ISERD223

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irrigation periods changed from an outflow type to a storage type due to decreased removal bypumping (Fig. 5), suggesting that TN may accumulate in the future.
Changes in water quality factors under the agricultural water use restriction project
We predicted that decreased water removal after the pumping station was abolished would cause
increased TN concentrations in the lake water. Though removal of water by pumping tends to takepollution from the lake, total nitrogen concentrations fell in 2010. Thus, changed inflow volume
appears to be the main factor that influenced water quality in the lake. Although inflow water
volume and load decreased after 2011, TN retention in the lake increased. Therefore, anotherprocess in the lake may also affect changes in water quality.
Consistent with the phenomena observed in Lake Chashinainuma, a study of Lake
Kasumigaura confirmed that increasing retention time caused load removal from the lake
(Nakamura and Amano, 2007). We suggest that extension of retention time may cause (1)sedimentation of particulate matter, (2) nitrogen fixation by phytoplankton or aquatic plants, and(3) organic matter decomposition due to microbes. Moreover, drawdown of the water level in thelake during the irrigation period was confirmed after 2011, and expansion of the habitation area for
emerging plants was observed. On the other hand, increasing retention time may advance theorganic decomposing effect, thereby increasing DIN by decomposing DON or organic matter inbottom sediments. However, exuberant reed growth was seen in 2011 and 2012 on the northernpart of the lake and may have lowered concentrations of dissolved water items by absorbing DTN.
CONCLUSION
The results indicate that the water inflow volume from the catchment area decreased after theproject, and the TN load balance of the lake changed to storage type despite decreased input load atLake Chashinainuma. This decreasing of water inflow volume could be caused by changes of landuse. The pumping station abolishment and the changing of water balance could affect the change ofTN load balance. Interestingly, the concentration of TN in the lake water decreased after the project,largely due to fixation of TON. This decrease in TN concentration is attributed to physicalbiological phenomenon that stem from changes in the hydrological environment and crop types.
ACKNOWLEDGMENTS
We thank the Foundation of Hokkaido Agricultural Modernization Technology Research Center
for making arrangements with the Hokkaido prefecture and the Naie branch office of the Hokkai
land improvement district. Analysis was supported by Akiya ODA was our graduate student.
This work was supported by a Kurita Water and Environment Foundation Research Grant, anda JSPS KAKENHI Grant Number 23580324.
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