IJERD – International Journal of Environmental and Rural Development (2020) 11-1

Research article
erd

Development of a Simplified Water Storage Operation
Curve at Irrigation Ponds for Flood Mitigation -A Casefrom Tamatsu District in Tottori, JapanRYOMA KUWAE
Graduate School of Sustainability Science, Tottori University, Tottori, JapanKATSUYUKI SHIMIZU*
Faculty of Agriculture, Tottori University, Tottori, Japan
Email: shimizu@tottori-u.ac.jp
Received 18 February 2020

Accepted 10 March 2020(*Corresponding Author)

Abstract In recent years, Japan has been receiving heavy rainfall, and the flooding ofirrigation ponds is a major concern. The irrigation ponds in the Tamatsu district in Tottori,
Japan, are small and overflow easily during heavy rainfall. Pre-releasing water from theseponds was suggested as a simple, safe, and low-cost solution. However, local governments
have prepared no clear guidelines or instructions on the amount of water that should be prereleased or regarding the desired water level after pre-release. Currently, the water volume
that is pre-released from the ponds depends entirely on the pond manager’s experience. To
mitigate this situation, we first observed the water level and discharge of the irrigation ponds
and collected rainfall data from the nearest meteorological station. We then assessed the water
storage operations of pond managers at three irrigation ponds in the Tamatsu district in 2018.
Subsequently, we prepared the height-volume curve (H-V curve) of each pond by level
survey and 3D image analysis, and a cumulative water demand curve based on the volume ofwater released from each pond during no rainfall periods. Considering the cumulative water
demand curve and the height of the intake tap of each pond, we prepared water storage
operation curves for the irrigation ponds. This will assist pond managers to determine theappropriate water level reduction during heavy rainfall events without causing an irrigationwater shortage. We confirmed the extent to which the pond management curve could copewith rainfall. Using the developed water storage operation curve, managers can easily
understand the drop of the water level in the pond. Moreover, the curves are easily constructedfrom the information on the beneficial area, precipitation, water level fluctuations of theirrigation pond during dry days, and the H-V curves of ponds. Finally, the effectiveness of
the water storage operation curve is evaluated for different rainfall events.
Keywords flood mitigation, disaster prevention, irrigation pond, pre-release of water

INTRODUCTION
In recent years, disasters caused by heavy rain have increased throughout Japan. The heavy rains inJuly 2018 caused heavy damage in western Japan, and in Hiroshima Prefecture, the pond broke down.
The pond is small and is in the study area, Tamatsu district, and the possibility of flooding during
heavy rain increases. Disaster prevention and mitigation measures for ponds, therefore, are urgently
needed. The government, after conducting an emergency inspection of ponds nationwide, took
measures such as selecting ponds for disaster prevention, preparing pond maps, developing an
emergency communication system, creating inundation potential area maps, and creating hazardmaps (MAFF, 2018). The government also suggested pre-release in preparation for heavy rainfall by
lowering the water level of the pond in advance and providing sufficient storage capacity. The
spillways of most irrigation ponds in Japan require repairs to safely pass the probability of flood
occurrence, once every 200 years, which is the design criteria set by the Japanese government (MAFF,2015). Because of the large number of ponds and the high repair costs, extensive time, however, will
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IJERD – International Journal of Environmental and Rural Development (2020) 11-1

be required to repair the spillways of every irrigation pond. We, therefore, developed a low-cost prerelease of water. The advantage is that there is no need to build a special facility and no funding isrequired, but there is no specific guideline for the pre-release amount, which depends on theexperience of the manager.
OBJECTIVE
In this study, we evaluated water storage management during heavy rains in July and September
2018 and created a water storage operation curve to guide the pre-discharge at three ponds in Tamatsu
district of Tottori City in 2018. Furthermore, we investigated how much rainfall the ponds could
withstand without flooding when the water level was controlled by the water storage operation curve.
METHODOLOGY
Study Area
Figure 1 shows an overview of the study area. There are six irrigation ponds in the study area,
Tamatsu District, Tottori City, Tottori prefecture, Japan. There are three ponds currently functioning
for water intake management, namely the Yutani, Shinjoji, and Daigo irrigation ponds. These ponds
are stored only by runoff from their basins. A canal connects the Yutani and Shinjoji ponds to theDaigo pond and can be switched between flowing into the Daigo pond and flowing into the river byoperating the weir. Table 1 shows the specification of each pond. The main crop is paddy, and thetotal paddy area is about 6 ha. In this area, there is a rule that water is drawn during the daytime, andeach farmer freely takes water from the ponds. The pond manager performs water pond maintenanceand the pre-release of water except for water intake.

Fig. 1 Study area
Table 1 Information of irrigation ponds
Height of dike（m）
Length of dike（m）
Storage（103 m3）
Catchment area（km2）
Width of spillway（m）
Height of spillway（m）

YutaniShinjoji

Daigo

450
3.8
0.08
0.8
0.5

688
19.0
0.44
5.81.24123
18.0
0.05
0.5
0.5

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IJERD – International Journal of Environmental and Rural Development (2020) 11-1

Hydrological Observation
To clarify the water level fluctuation of the pond, a pressure-type self-recording water level gauge
was installed at the bottom and water surface of the pond, and observations were made hourly. Usinga small UAV, the aerial view of the pond was created and a three-dimensional image was createdbased on the image. The water area for each pond’s water level was calculated to create a water levelvolume curve. From the water level of the pond and the water level-volume curve, the capacity ofthe pond was calculated. To clarify the intake status and the operation status of the weir, acapacitance-type self-recording water level gauge was installed in the canal, and the water level wasobserved at 5-minute intervals. The observation period was from late May to late September 2018,
and the mid-drainage period for paddy rice fields is from late June to mid-July. Precipitation data
were obtained from the Japan Meteorological Agency (Tottori Observatory).
Estimation of Pre-release of Water
As a method of quantitatively indicating the pre-release rate, we estimated the average daily waterrequirement during the irrigation period. This was based on the amount of storage change of eachpond during the continuous drought period (14 days from August 1 to 14) with precipitation of lessthan 5 mm/d. Using the average daily requirement, the cumulative required water volume wascalculated retrospectively to create a pond management curve. The two types of management curves
for each pond are shown based on the land-use status of the target area in 2018 and the assumptionthat all the farmland in the area is paddy rice, that is, with maximum water demand.
Estimation of Water Storage Operation Curve
Flood arrival times in 60-minute rainfall events were calculated for different probability year using
the formula of Talbot, Kadoya–Fukushima, as shown in Eqs. (1) ~ (3), and the water level rise fromfull and from each intake tap position was calculated. From the result, if the water level is maintained
according to the water storage operation curve, it is determined at which probability year the rainfall
will not exceed the top of the embankment until 60 minutes of rainfall.
To simulate the water balance and water level of the pond under heavy rainfall, 60-minuterainfall events at return periods of 10, 20, 30, 40, 50, 60, 70, 100, and 200 years were estimated using
the Iwai method.
aI=(1)
t+b
tp = C∙A0.22 ∙re- 0.35

(2)

re = fp ∙I(3)

where I is the rainfall intensity (mm/h), t is the duration of rainfall (min), a and b are the constantparameters related to the meteorological conditions, tp is the peak flow arrival time (min), C is theparameter related to the land-use conditions, A is the catchment area (km2), re is the average effective
rainfall intensity (mm/h), and fp is the peak runoff coefficient.
The rational equation to determine the peak discharge from the catchment area and average
effective rainfall intensity are expressed in Eq. (4).
Q=1
∙r ∙A
3.6 e(4)The discharge from the spillway was calculated using rectangular weir formulas according to
the water level. The formulas used in the calculation are expressed in Eq. (5).
3(5)

Qs = Cs ∙b∙H2Ⓒ ISERD

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IJERD – International Journal of Environmental and Rural Development (2020) 11-1

where Qs is the discharge from the spillway, Cs is the discharge coefficient for the rectangular weir,b is the width of the spillway (m), H is the water level of the spillway (m). Calculations continued
until the water level reached the top of the spillway.
The water balance and increase rate of the water level of the pond were calculated using thepeak runoff as the inflow element and the discharges from the spillway. Due to the assumption ofshort and heavy rainfall, evaporation and infiltration from the pond were ignored (Shimizu et al.,2016).
The created water storage operation curve of each pond is evaluated for the different rainfall
events. In our analysis, the extra height of each pond was defined as the height of the spillway in caseof seepage collapse of the dike. There are three main reasons for damage to irrigation ponds by heavy
rainfall, namely overflow erosion, sliding, and seepage failure. In the case of seepage failure, internal
erosion occurs from the upstream slope near the full water level. In the case of sliding, a failure starts
at the toe of an embankment. Overflow failure occurs at the crest above the gut of the foundation(Hori et al., 2002). We, therefore, set the upper limit to the height of the spillway.
RESULTS AND DISCUSSION
Evaluation of Water Storage Management during Heavy Rain
Figure 2 shows the water level fluctuation and precipitation of each pond. It is probable that predischarge was not performed at Shinjoji and Daigo ponds because the water level did not drop before
the heavy rain. Pre-discharge was not performed at the Shinjoji pond because the manager judged
there was no necessity to perform pre-discharge because the spillway of the Shinjoji pond was large
enough, as shown in Table 1. Pre-discharge was not performed in the Daigo pond because the water
storage rate of the pond was low, about 60%, and the weir was operated so that water from the Yutaniand Shinjoji Ponds did not enter the pond. Furthermore, the Daigo pond has a larger water storage
capacity for the catchment area than the Yutani and Shinjoji ponds. For a detailed description, Fig. 3shows the water level fluctuation of the Yutani pond during heavy rains in July (308 mm) andSeptember (180 mm). Since the water level dropped before the heavy rainfall, it was confirmed thatthe pre-discharge was performed. The water level of the Yutani pond did not rise more than 30 cm
from the full water level, the peak of the water level was reduced, and the effectiveness of the predischarge was reduced.

Fig. 2 Water level fluctuation at 3 ponds

Fig. 3 Water level fluctuation at Yutani pond

Creating Water Storage Operation Curve
Table 2 shows the daily irrigation requirement based on the paddy rice area in 2018 and the dailyirrigation requirement assuming that paddy rice is grown in all the fields. Figure 4 (a) ~ (c) shows
the water storage operation curves of each pond. The break-line indicates the amount of water storedwith a full water level in each pond. When the water storage operation curve falls below the dottedⒸ ISERD

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IJERD – International Journal of Environmental and Rural Development (2020) 11-1line, pre-release is possible. The dotted line indicates the amount of water storage corresponding tothe position of the intake tap in each pond, and the water level can be lowered to the position of theintake tap by opening the intake tap immediately above the water storage operation curve at the timeof pre-release. Water storage operation curves created from the land-use situation in 2018 show thatthe water storage capacity of the Yutani and Daigo ponds is always lower than the water storage
capacity of the irrigation pond during the irrigation period. It was shown that pre-release was possible.
Regarding the water storage operation curve, assuming the maximum water demand in the area, themanagement curve of the Daigo pond (Fig. 4 (c)) has fallen below the full storage volume of thepond since June 13th. It was shown that there was room for pre-release from the stage. However, inthe management curves of the Yutani (Fig. 4 (a)) and Shinjoji ponds (Fig. 4 (b)), there is no room
for water storage until the end of the irrigation season because of the higher ratio of upland fields
and fallow land in the beneficiary areas at the Yutani and Shinjoji ponds. Because paddy rice fields
are usually irrigated, but upland crop fields are rain-fed, if the ratio of the paddy rice fields increased,
the irrigation water requirement also increased accordingly. The reservoir manager can open theintake tap based on this curve to keep the water level in the vicinity of the opened intake tap positionand prepare for heavy rainfall.
Table 2 Beneficial area
Name of pond
2018
Maximum paddy rice area

Yutani
0.15
1.84

Shinjoji
2.64
3.95

Daigo
2.50
2.90

2018

15.56

391.95206.74

Maximum water demand187.15586.30324.0433(10 m )Paddy rice area（ha）
Average water
demand per day（m3/d）

Fig. 4 Water-storage operation curve (red line)
Evaluation of the Effectiveness of the Water Storage Operation Curve
We investigated how much rainfall each pond could withstand without flooding when the water level
was controlled by the water storage operation curve. Table 3 summarizes the simulation results. In
the Shinjoji pond, the water level did not exceed the upper limit in any rainfall events because thespillway is large enough to pass the flood. In the Yutani pond, the rainfall over the 10-year probability
60-minute rainfall at the full water level exceeds the upper limit. When the water level, however, islowered to the height of the intake tap No.1, the pond withstands flooding with a 50-year probability
60-minute rainfall. The Daigo pond cannot withstand any rainfall at the full water level or when thewater level is at the height of water intake tap No.1. The pond can withstand a 40-year probability
60-minute rainfall if the water level is lowered to the height of water intake tap No.2. Since the waterlevel in the ponds cannot be lowered in a short period of time by opening the intake tap with a smallⒸ ISERD22IJERD – International Journal of Environmental and Rural Development (2020) 11-1

diameter, it is necessary to constantly lower the water level of the ponds or install an emergency
discharge device to increase the ability to lower the water level.
Table 3 Effectiveness of water storage operation curve
Yutani pond
Return period (y)Full water level

Intake tap No.1

Intake tap No.2

Intake tap No.3

10○

○

○

○

20×

○

○

○

30×○

○

○

40×○

○

○

50×○

○

○

60××

○

○

70××

○

○

100××

○

○

200××××

Full water level

Intake tap No.1

Intake tap No.2

Intake tap No.3

10××○

○20××○○30××○

○

40××○

○

50×××

○

60×××

○

70×××

○

100×××○×××

Daigo pond
Return period (y)200×○: water level does not exceed the upper limit
×: water level exceeds the upper limit

CONCLUSION
The pre-discharge of ponds is a simple, safe, and cost-effective method against heavy rainfall in casethe spillway is not constructed, but the pond manager uncertain to what extent the water level of thepond can be lowered. Given this, a water storage operation curve was developed by considering
paddy rice irrigation, and its effectiveness was evaluated based on the probability of rainfall fordifferent years. The developed method is applicable to any ponds and the created curve makes it easy
for pond managers to visually understand how much to lower the water level at what time of irrigation
period. Further refinement of the curve is required by more accurate hydrological observation. Also,
not only short rainfall events but longer rainfall events such as one to two days long should beconsidered.
ACKNOWLEDGEMENTS
This research was supported by the Tottori Prefecture Environmental Science Research Promotion
Project (FY28-H30) and the Tottori University International Dryland Research and Education
Organization.

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REFERENCES
Shimizu, K., Fukuda, Y. and Yoshioka, Y. 2016. Development of an emergency discharging device and an
early warning system for floods at irrigation ponds. International Journal of Environmental and RuralDevelopment, 7 (1), 15-21.
Ministry of Agriculture, Forestry and Fisheries (MAFF), Japan. 2018. Initiatives based on heavy rain in July
2018, Retrived from https://www.maff.go.jp/j/nousin/bousai/bousai_saigai/b_tameike/ (in Japanese).
Ministry of Agriculture, Forestry and Fisheries (MAFF), Japan. 2015. Design criteria of land improvement
project Irrigation pond improvement. JSIDRE, 32, 165-166 (in Japanese).
Japan Meteorological Agency: Search past meteorological record: Retrived from http://www.data.jma.
go.jp/obd/stats/etrn/index.php (in Japanese).
Hori, T., Mohri, Y. and Aoyama, S. 2002. Causes and features of damage to small earth dams induced by heavyrainfall. Irrigation, Drainage and Rural Engineering Journal, 218, 253-263 (in Japanese).

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