IJERD – International Journal of Environmental and Rural Development (2019) 10-2

Research article
erd

Functional Evaluation of Groundwater Level Decrease inNon-sloped Subsurface Drainage Systems in Upland Field
on Peatland
NAHO NOHARA
Graduate School of Agriculture, Hokkaido University, Sapporo, JapanMINORU YOKOCHI
Graduate School of Global Food Resources, Hokkaido University, Sapporo, JapanTADAO YAMAMOTO*
Research Faculty of Agriculture, Hokkaido University, Sapporo, Japan
Email: tady@env.agr.hokudai.ac.jp

TAKASHI INOUE
Research Faculty of Agriculture, Hokkaido University, Sapporo, Japan
Received 1 February 2019

Accepted 20 October 2019(*Corresponding Author)

Abstract In lowland farmland, the maintenance cost of conventional inclined subsurface
drainage systems is high because the drainage pipes must be laid at depth, or the tile
deepens as the farm field enlarges. To reduce the maintenance cost, the Hokkaido
Provincial Government of Japan has introduced a subsurface drainage system with asmaller inclination angle than the conventional method. However, the drainage effects of
non-sloped subsurface drainage systems have been rarely reported, and insufficientevidence has prevented their widespread acceptance. The present study investigates how theslope of the drainage pipe influences the groundwater-level decrease in poorly drainedfields. The survey was conducted in the upland field on peatland located in the Ishikari
River Basin, Hokkaido. In 2015, two types of subsurface drainage systems were constructedin the same lot: a conventional subsurface drainage (sloped area) system, and a subsurface
drainage system with low inclination (non-sloped area). After measuring the groundwaterlevel at 12 points and the precipitation from 2016 to 2017, we found that: i) thegroundwater level was higher in the non-sloped area than in the sloped area, ii) between
2016 and 2017, the groundwater level decreased in the sloped area and rose in the nonsloped area. The above results suggest that the efficiency of decreasing the groundwaterlevel during 3 years after the construction was lower in non-sloped subsurface drainage thanin sloped subsurface drainage, which is different from that observed in the previous study.
Keywords subsurface drainage, groundwater level, large-scale field, peatland

INTRODUCTION
In some developed countries, including Japan, the economic growth of the agricultural industry has
declined while that of the secondary and tertiary industries has increased. This problem has
severely affected rural communities in Japan. Maintaining agricultural production and enhancing
the competitiveness of agriculture are urgent demands in Japan’s agricultural policy. As thefarming population decreases, agricultural production must be maintained by astute agriculturalbusiness, improvement of work efficiency, and labor-saving measures. As a precondition ofarresting this decline, the enlargement of farm fields has been promoted. Subsurface drainage has
been widely implemented for improving the yield and quality of farm products. In the future, thecompetitiveness of agriculture must be bolstered by lowering the installation cost of facilities andconstructing irrigation and drainage systems, including subsurface drainage systems.
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In Hokkaido, northern Japan, lowland farmlands developed from peatland are widespread, andsubsurface drainage is employed to improve the soil condition. Recently, the average field size has
progressively enlarged to enhance the productivity. Deeper pipes and ditches are needed to installconventionally sloped subsurface drainage pipes in large fields, which increases theimplementation cost. In an attempt to alleviate this problem, subsurface drainage pipes with noinclination (<1/600) have been trialed in a suitable geographical feature (Koshiba et al., 2005).
However, as very few studies have evaluated the function of non-sloped subsurface drainage, thepracticality of this approach is currently unknown. Non-sloped subsurface drainage reduces overtime as the drainpipes become uneven and accumulate soil sediments. Koshiba et al. (2005)
reported no significant malfunction caused by pipe sedimentation and unevenness after five years’
implementation of non-sloped subsurface drainage in a farm field with sandy loam soil. They also
reported the same drainage through non-sloped and sloped pipes. They concluded that non-sloped
pipe drainage effectively reduces the installation cost (Koshiba et al., 2005).
However, the functioning of non-sloped subsurface drainage has not yet been investigated in
Hokkaido, where peat layer is distributed. Peat is an organic-rich soil consisting mainly of
undecomposed plant material, which accumulates in saturated environments. Such environments
require intense drainage to reach suitable conditions for agriculture, but owing to their uniquecomposition, inevitably subside under agricultural drainage and artificial loading. Armstrong andCastle (1999) stated that peat subsidence frequently misaligns the drainpipes and inverts thegradient along the pipes. They also mentioned that because peat is rich in iron, pipes laid inpeatland are often clogged with deposits of iron ochre, an orange-colored mud made of ironcompounds (Armstrong and Castle, 1999). The question remains whether non-sloped drainage
performs well in peatlands, where the drainage conditions are difficult. Therefore, to evaluate thefunction of non-sloped subsurface drainage on peatland fields, this study compares thegroundwater-level changes in conventional (sloped) and non-sloped pipe subsurface drainage
systems installed in the same field.
METHODOLOGY
Overview of the Research Site
The study was carried out in an upland field in Iwamizawa (43°12′ N, 141°43′E), located in the eastof the Ishikari plains in Hokkaido (Fig. 1). The mean annual temperature and precipitation are
7.6°C and 1,163 mm, respectively. In the research field, the peat layer underlies a 30 cm-thickupper artificial soil. In August of 2015, subsurface drainage pipes with low inclination wereinstalled in one-half of the field (forming the non-sloped area), and conventional subsurface
drainage was implemented in the other half (sloped area) (Fig. 2). This setup permits a performancecomparison between the drainage experiment and a control experiment with a different drainpipe
slope in the same lot. The field size is 1.1 ha, and onions were grown from May to August in 2016
and 2017. The drainpipes are buried 70 cm below the surface at 9-m intervals. In the sloped area,
the drainpipes are inclined at 1/500, and their diameter is 60 mm (versus 80 mm in the non-slopedarea). This difference in diameter is based on the design criteria. The drainpipes are filled withgravels. Prior to cultivation, the subsoil was broken to improve the drainage.
Survey Equipment
The precipitation in the field was measured hourly by a tipping-bucket rain gauge with a diameterof 20 cm (HOBO event, Climatec Incorporated, Tokyo, Japan) placed near the study field.
Groundwater-level gauges for absolute pressure measurements (S & DL water level, OyoCorporation, Tokyo, Japan) were set at six points (five between the drainpipes, and one just abovethe drainpipe) in both the sloped and the non-sloped areas, giving 12 observation points in total(Fig. 2). The groundwater level was measured every 30 minutes. The measured absolute pressures
were converted to gauge pressures by a barometer installed near the field.
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IJERD – International Journal of Environmental and Rural Development (2019) 10-2Fig. 1 Location of the research site

Fig. 2 Overview of the research site

Evaluation Method
The groundwater level generally rises immediately after rainfall, and gradually declines after therain. The drop-down phase depends on the characteristics of the draining process. From theobtained rainfall data and the groundwater levels in the field investigations, we evaluated thefollowing contents: i) groundwater-level dynamics, ii) groundwater-level drop response after a
rainwater-induced rise in the groundwater level, and iii) achievement rate of the design target set by
Hokkaido Prefecture Agricultural Administration Department. The subsurface drainage efficiency
was evaluated by comparing the rainfall-related contents and groundwater table fluctuations in thesloped and non-sloped areas. We also evaluated the time-series changes in the function of eachdrainage system by comparing the results of 2016 and 2017. The observation data from June tomid-August in 2016 and 2017 were used for the drainage evaluation. For determining the drainage
efficiency, we defined the following terms.
Groundwater level: Vertical distance from the ground surface to the groundwater table at eachobservation point.
Rain event: Rainfall within 24 hours of the rainfall start is regarded as a rainfall series. Rainfall of
1 mm/h or more within 24 hours after the rain stopped was included in the same event. Rain eventswith a total rainfall of 10 mm or more were used in the evaluation.
Amount of groundwater level increase: The groundwater-level difference between immediately
before the rain event and the highest water level during the rain event.
Amount of groundwater level drop: The groundwater-level difference between the highest waterlevel during the rain event and the lowest water level after the rain event.
Groundwater level drop rate: The ratio of the amount of groundwater level drop to the amount ofgroundwater-level rise after the rain event.
RESULTS AND DISCUSSION
Fluctuation of Groundwater Level
The drainage was evaluated by comparing the fluctuations in the groundwater levels at eachobservation point. The results are presented as box plots. Fig. 3 shows the distribution ofgroundwater levels at six observation points located at the same distance from the drainage ditch(N2, N5, N6, S2, S5, S6) in 2017. Comparing the fluctuations in groundwater levels in 2017, thefluctuation range of groundwater levels at N5 tended to be narrower than other points. Thegroundwater fluctuations except N5 remained at a higher level in the non-sloped area than in theⒸ ISERD112

IJERD – International Journal of Environmental and Rural Development (2019) 10-2

sloped area. Fig. 4 compares the distributions of groundwater level at the above-mentioned six
observation points in 2016 and 2017. The 2016 survey result confirmed the high performance ofthe non-sloped subsurface drainage relative to the sloped system, similar to previous studies (e.g.
Koshiba et.al., 2005). However, the groundwater distribution in the non-sloped area was higher in2017 than in 2016. In contrast, the groundwater in the sloped area was lower in 2017 than in 2016,
possibly because the subsoil was broken by subsoiler before tilling in 2017, and the precipitation
was lower in that year than in 2016. However, despite the same climate conditions and the same
farming method in both areas, subsoil breaking exerted no effect in the non-sloped area. Thissuggests that, for some reason, the subsurface drainage functioning (maintenance of low-levelgroundwater) was lower in the non-sloped area than in the sloped area.

The top and bottom extremes of the whiskers indicate the maximum and minimum groundwater levels, respectively, andthe segment inside the box is the median groundwater level. The lower and upper base lines of the boxes indicate the firstquartile (25th percentile) and third quartile (75th percentile), respectively.

Fig. 3 Groundwater level changes at
6 observation points in 2017

Fig. 4 Comparison of groundwater-level fluctuations
between 2016 and 2017 at 6 observation points

Rain Response of Groundwater Level
In a properly functioning drainage system, the raised groundwater table quickly falls after therainfall cessation. The drainage function was evaluated by relating the rises and falls in thegroundwater level after several rain events. The evaluation was performed at 10 observation points(N1, N2, N3, N5, N6, S1, S2, S3, S5, S6) located between the drainage pipes. Points located above
the drainage pipes (N4, S4) were excluded because their locations relative to the drainpipe differedfrom those of the other points. Figs. 5 and 6 plot the groundwater-level drop versus thegroundwater-level rise at 24 and 72 hours, respectively, at all observation points after each rain
event. The average groundwater-level drop rate over all rain events and observation points can bedetermined from the regression coefficient; when the regression line is steep, the soil is welldrained after the rainfall-induced rise in groundwater level. The drainage efficiencies werecompared by comparing the steepness of the regression lines between the sloped and the nonsloped areas, or between 2016 and 2017. The statistical significances of the groundwater-level drop
rates were assessed by a 2-sample t-test at the 95% confidence level.
We first compared the drainages in the sloped and non-sloped areas in 2017. As shown in Fig.
5, the gradient of the regression line was smaller in the non-sloped area than in the sloped area. In
the sloped area, 67% of the groundwater level raised by rainfall (on average) was withdrawn within
24 hours, versus 52% in the non-sloped area. The groundwater-level drop rate was significantly
higher in the sloped than in the non-sloped area (2-sample t-test; p <0.01). At 72 hours following
the rain events, the groundwater drop rates were statistically the same (2-sample t-test; p >0.1). The
results imply that immediately after the rain events, the drainage system with sloped pipes better
withdrew the groundwater level than the drainage system with level pipes.
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Next, the time-series changes of drainage function in the non-sloped area were compared
between 2016 and 2017. At 24 hours following the rain events, the groundwater-level drop rate wassmaller in 2017 than in 2016, and the reduction was significant (2-sample t-test; p <0.01). The same
result was obtained at 72 hours following the rain events (Fig. 6). Specifically, the groundwater levelat 72 hours following the rain events dropped by 100% in 2016 (meaning that the groundwater level
had recovered to its pre-rain level within 72 hours), but reduced by only 82% in 2017. In the slopedarea, a reduction in the groundwater-level drop rate between 2016 and 2017 (from 96% to 85%) was
found only at 72 hours following the rain events. If it is assumed that there are no differences in soil
physical properties based on farming operations, the decline of the subsurface drainage function after
rainfall was more conspicuous in the non-sloped area than in the sloped area, implying that inclining
the drainpipes plays an important role in maintaining drainage function.

Fig. 5 Relationship between rise and fall ingroundwater level at 24 hours afterrain events in 2017

Fig. 6 Relationship between rise and fall ingroundwater level at 72 hours afterrain events in the non-sloped area

Achievement Rate of the Groundwater Level Standard
Table 1 Achievement rates of groundwater-level standard

Non-sloped
area

Sloped area

N1
N2
N3
N4
N5
N6Avg.
S1
S2
S3
S4
S5
S6Avg.

2016
100%
86%
100%100%100%
100%
97%
100%
43%
71%
100%
100%
86%
83%

201763%
38%
63%
－100%
0%
53%
100%
63%
100%
100%
100%
100%
94%

The desired groundwater level has been set as a design target of subsurface drainage systems.
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designed groundwater level should be −60 to −50 cm under dry conditions, and −50 to −40 cm at
two or three days after cessation of a rain event. However, during the observation period of thepresent study, the no-rainfall periods rarely continued for more than seven days; therefore, wedecided to set the standard for a well-functioning groundwater level below −40 cm at 72 hours after
cessation of a rain event. The achievement rate of this standard was calculated at each observation
point, and used in the drainage-efficiency evaluation. Table 1 lists the achievement rates at allobservation points in 2016 and 2017. The achievement rates and averages in the sloped area
improved from 2016 to 2017. On the other hand, many points in the non-sloped area showed low
achievement rates in 2017, and the average achievement rate was considerably decreased from thatof 2016 in spite of subsoil breakage. Furthermore, the achievement rate was higher in non-slope
area than in the sloped area in 2016; therefore, it seems that pipe inclination is not a direct cause of
the decrease in the subsurface drainage function. From these results, we considered that somefactors, except for soil physical changes, influencing drainage function decline may have occurredby the elapsed time since the construction. Thus, the cause of drainage function decline may berelated to changes in conditions inside the pipe, similar to water pass prevention due to sedimentaccumulation.
CONCLUSION
The present study investigated how the slope of the drainage pipe influences the groundwater-level
decrease in poorly drained fields on peatland located in the Ishikari River Basin, Hokkaido. Aftermeasuring the groundwater level at 12 points and the precipitation from 2016 to 2017, we found
that: (i) the groundwater level was higher in the non-sloped area than in the sloped area; (ii)between 2016 and 2017, the groundwater level decreased in the sloped area and increased in the
non-sloped area; and (iii) the drainage efficiency in the non-sloped area tended to decrease after 3years of installation.
The 2017 findings revealed a large difference between the groundwater behavior in the sloped
and non-sloped areas, although the same farming method was implemented in both areas, and thedrainage function obviously declined in the non-sloped area. These results contradict a previous
field study conducted on loam soil (Koshiba et al., 2005). Unlike loam soil, peatlands cause special
problems in subsurface drainage, such as subsidence-associated misalignment of the pipes anddeposition of iron ochre (Thiobacillus ferrooxidans colony) in the pipe interiors. Subsurface
drainage through sloped pipes may tolerate these problems because the pipe gradient increases theflow velocity of the drainage water. At sufficiently high flow velocities, the deposit is easily
washed away. The gradient might also prevent misalignment and inverse gradients of the pipes. On
the other hand, the non-sloped drainage may be adversely affected by these peatland characteristics.
Our results implied that inclining or periodic cleaning the drainpipes is important for maintaining
the performance of the drainage system.
ACKNOWLEDGEMENTS
This research was conducted under the cooperation agreement between Hokkaido Prefecture
Agricultural Administration Department, and Fishers and Forest Department, andFaculty/Graduation School/Research Faculty of Agriculture, Hokkaido University. The authors
would like to thank the staffs of Hokkaido Prefecture Agricultural Administration Department andSorachi Subprefecture, and Central Agricultural Experiment Station of Hokkaido Research
Organization for their supports.
REFERENCES
Armstrong, A.C. and Castle, D.A. 1999. Drainage of organic soils. In Skaggs, R.W. and Schilfgaarde, J. van(Eds.) Agricultural Drainage, 1083-1105, Madison, Wisconsin, USA.
Hokkaido Prefecture Agricultural Administration Department. 2010. Design guidelines for subsurface drainage.
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IJERD – International Journal of Environmental and Rural Development (2019) 10-233-34, Retrived from http://www.pref.hokkaido.lg.jp/ns/jcs/grp/03/ankyosisin2206.pdf
Koshiba, N., Okuyama, T., Iwabuchi, Y. and Arihara, K. 2005. Drainage performance of a shallow pipe drainwith no slope. Jour. JSIDRE, 73 (10), 917-918.

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