IJERD – International Journal of Environmental and Rural Development (2023) 14-2

Research article
erd

Development of Indexes to Evaluate the Effectiveness of
Low Water Level Control in Irrigation Ponds – A Case
Study of Irrigation Ponds in Tottori, JapanRYO HARASAWA*
Graduate School of Sustainability Science, Tottori University, Tottori, Japan
Email: ryoharasawa2018@outlook.jp

KATSUYUKI SHIMIZU
Faculty of Agriculture, Tottori University, Tottori, JapanYURI YAMAZAKI
Faculty of Agriculture, Tottori University, Tottori, Japan
Received 27 September 2023

Accepted 3 December 2023(*Corresponding Author)

Abstract The spillways of most irrigation ponds in Japan need additional repairs to safely
pass the 200-year flood event, which is the design criteria set by the government. At the sametime, low water level control (LWLC) in irrigation ponds is currently recognized as one of
the countermeasures for mitigating floods and avoiding dam collapse. However, the pond
managers do not know to what extent the water level of the pond should be lowered. In thisstudy, the flood mitigation function of LWLC in each pond was quantitatively evaluated, andindexes were proposed to determine whether it is worth practicing LWLC in each pond orwhether the spillway needs to be repaired. Seventy-two irrigation ponds in Tottori prefecture
were selected for analysis and the water balance of each pond was calculated with 10-, 50-,
and 200-year rainfall events. The results showed that half of the ponds cannot safely copewith a 10-year rainfall event even if they are empty prior to the event. The ratio of thecatchment area to water surface area at full capacity in such ponds often exceeds 50 to 1. On
the other hand, ponds with a catchment to water surface area ratio of less than 50 show a highflood mitigation function due to LWLC. In addition to this, priority for repair work can beevaluated in terms of whether the spillway can pass peak runoff or not, because the size andthe type of spillway are different among ponds. The effectiveness of LWLC can be evaluatedin terms of the ability of the spillway to pass peak runoff from the catchment when the pond
is full. The three indicators, namely, the ratio of catchment area to water surface area at full
capacity, difference of peak flood reduction rate, and peak discharge ratio, are simple anduseful indexes for LWLC to prevent dam collapse.
Keywords flood mitigation, spillway, water management, water balance

INTRODUCTION
In recent years, unexpected, short, and heavy rain has been increasing in Japan. Especially in 2021
one of the strongest rainfall events occurred at many meteorological observing stations in Japan. Someirrigation ponds have been damaged and broken down due to heavy rainfall. For instance, Hutago
Pond in Tottori was broken down because of a heavy rainfall event in July 2021. An irrigation pondis generally constructed to irrigate agricultural water which is mainly used for the paddy rice fields
if there is no river near the irrigated district. The number of irrigation ponds in Japan is about 150
thousand and most of them are constructed over 100 years ago (MAFF). The spillways of someirrigation ponds in Japan are too small and require repairs to safely pass the 200-year flood event
which means the probability of flood occurrence, is once per 200 years, which is the design criteria
set by the Japanese government (MAFF, 2015). However, due to the large number of ponds and thehigh repair costs, extensive time will be required to repair the spillways of every irrigation pond.
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Therefore, the low water level control (LWLC) was suggested as one of the low-cost and nonstructural solutions to prevent dam collapse. However, the water control entirely depends on the
manager’s experience and the municipality doesn’t have clear guidelines or indicators on the amountof water that should be released in advance. Moreover, no assessment of the flooding mitigation
function of irrigation ponds when LWLC is practiced has been done.
OBJECTIVESIn this study, the flood mitigation function of LWLC in irrigation ponds was quantitatively evaluated,
and indexes were proposed to determine whether it is worth practicing LWLC in each pond orwhether the spillway needs to be repaired. This study was conducted on the irrigation ponds certified
as priority irrigation ponds for disaster prevention in Tottori Prefecture, Japan.
METHODOLOGY
Study Area
In this study, the water levels of the irrigation
ponds in Tottori were estimated using a waterbalance model under the different heavy
rainfall events at each rainfall station. The
number of irrigation ponds in Tottori is around
950 (Tottori Prefecture, 2022). Of these 72
irrigation ponds were selected for analysis.
Figure 1 shows selected 72 irrigation pondsthat are indicated as black spots. Most of themwere constructed in mountainous areas andstore water only from the runoff from thecatchment areas. Therefore, when LWLC isFig. 1 Selected irrigation ponds in Tottori
practiced, the farmers will take on the risk
Location source: Tottori web map
increase that the storage water would be dried
up. They are located near residences which could be damaged if the ponds are flooded. Besides, allof them cannot safely pass the peak runoff with a 200-year rainfall event, which is a design criterion
set by the government. Table 1 shows the features of irrigation ponds in the study area. The irrigation
ponds are classified by storage capacity.
The distribution of the storage capacities is biased to a small, less than 10,000 m 3. Of the 72
selected irrigation ponds, the number of the small-sized ones is 55. Those with a storage capacity ofless than 3,000 m3 account for 36. The number of beneficiary farmers in these irrigation ponds isquite small, and some of them are often not well maintained. The capacity of medium-sized irrigation
ponds ranges from 10,000 m3 to 100,000 m3. The number of medium irrigation ponds is 15. The
capacity of large-sized irrigation ponds is more than 100,000 m3 and there are two such irrigation
ponds in the study area, with capacities of 101,000 m3 and 120,000 m3. The number of beneficiary
farmers is large, and downstream damage due to a potential dam collapse on these ponds would beenormous.
Table 1 Features of irrigation ponds in the study area
Irrigation Ponds

Capacity (m3)

Numbers of ponds

Small<10,000 m3

55

Medium

10,000~100,000 m3

15

Large

> 100,000 m3

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Water Balance Analysis
Outline of Irrigation Pond Water Balance Model
The irrigation pond water balance model was developed by referring to the Guidance of flood
control function intensify of irrigation ponds (MAFF, 2018). The difference points are estimation
formulas of coefficient K for the storage function method, effective rainfall intensity, outflow water
estimation from the spillway, and height-volume curve of the irrigation pond (H-V curve).
To determine and compare the effectiveness of LWLC from the aspect of the return period, 10year, 50-year, and 200-year return period hourly and daily rainfall was estimated by Iwai formulaand Talbot formula. The hourly and daily return period was estimated at 9 stations such as Iwai,
Tottori, Shikano, Kurayoshi, Shiotsu, Daisen, Ebi, Yonago, and Chaya. The rainfall duration was
assumed to be 3 hours, and the rainfall distribution was assumed to have a rainfall intensity peak in
the latter half of the rainfall. The inflow of water to the ponds is estimated by the storage function
method. It needs to estimate coefficient 𝐾, 𝑝, and effective rainfall intensity. 𝐾 is referred from Nagai
et al. (1987). p is set to 0.6 as Sugiyama et al. (1988) suggested. Effective rainfall is estimated tomultiply rainfall intensity by the theoretical peak outflow rate referred from the internal survey
documents of Tottori Prefecture. Only the outflow from the pond is considered for the outflow fromthe spillway of the pond. In this study, the outflow from intake taps is not considered because it isnegligibly small compared to that from the discharge from the spillway. In addition to this, due to theassumption of short and heavy rainfall, evaporation and infiltration from the irrigation pond are also
ignored (Shimizu et al, 2016). To estimate the amount of released water, the relationship between thewater level and the storage capacity of a pond is required, but these data are not available. Therefore,
in this study, a simplified method is employed to make an H-V curve with the assumption that theshape of the irrigation pond is approximated as frustum (Tanakamaru et al., 2015).
Validation
To validate the results of the water balance model, the actual water level fluctuation was observed
at 3 irrigation ponds in Tottori using the heavy rainfall data at 2 rainfall stations from 7th July to 9th
July 2021.
Indexes Proposal and Classification
In the analysis, the extra height of each irrigation pond was defined as the height of the spillway incase of seepage collapse of the irrigation pond. There are three main reasons for dam collapse
following heavy rainfall, namely overflow erosion, sliding, and seepage collapse. In the case of
seepage collapse, internal erosion occurs from the upstream slope near the full water level (Hori etal., 2002). Moreover, the interval of intake taps is supposed to be 0.5 m and release water level 0.5
m each, and estimate released water previously that peak water level is less than the height of thespillway at each irrigation pond. To calculate how many meters of water level should be lowered,storage water was released corresponding to 0.5m each from full water level until the peak waterlevel in case the irrigation pond conducts LWLC would not over the height of the spillway. The waterlevel reduction rate (R) can be expressed as a ratio of released water level and water storage depth(the height difference between the bottom and full water level) shown in Eq. 1.
𝐷

(1)

𝑅=𝐻

Where R is the water level reduction rate, D is the depth after the water is released from the fullwater level, and H is the water storage depth.
As follows, we proposed 3 indicators that are useful to classify the irrigation ponds based on thecalculation from the data of irrigation ponds and rainfall data. Catchment to storage ratio (CSR) is
one of the indexes that can be calculated to divide the catchment area with the storage area (fullsurface water area) of each irrigation pond shown in Eq. 2.
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𝐴(2)

𝐴𝑟 = 𝑆

1

Where Ar is the catchment to storage ratio, A is the catchment area, and S1 is the full water
surface area.
Design discharge to peak discharge ratio (DPR) is the index that can be calculated to divide thedesign discharge of the spillway (Qout) by peak discharge (Qp) when the water level is full of eachirrigation pond shown as Eq. 3.
𝐷=

𝑄𝑜𝑢𝑡
𝑄𝑝

(3)

Where 𝐷 is the design discharge to peak discharge ratio, Qout is the design discharge of thespillway, and Qp is peak discharge when the water level is full.
In addition to this, to understand how much water discharge could be reduced by conducting
LWLC, the flood peak mitigation rate was calculated when the initial water level was 0.5m lower
than the full water level. The difference in flood peak reduction ratio (PRRd) is expressed as the ratioof the difference in peak discharges with and without LWLC implementation to the peak runoff fromthe catchment area shown in Eq. 4.
𝑄𝑑 =𝑄𝑝 −𝑄𝑝 ′(4)

𝑄𝑛

Where Qd (%) is a difference in flood peak reduction ratio. Qp is the peak water flow rate when
the irrigation pond is full, Qp’ is the peak water flow rate when the irrigation pond conducts LWLC,
and Qn is the peak inflow water flow rate from the catchment area.
RESULTS AND DISCUSSIONS
Validation
The difference between the estimated peak water levels and observed ones of the irrigation ponds arearound 10 cm, 1 cm, and 13 cm. Figure 2 shows observed and simulated water level data at the
Kanaguri irrigation pond. The irrigation pond is located in the middle of Tottori prefecture. The peakwater level difference is 13cm at Kanaguri irrigation pond.
The observed water level until the peak water level seems to be lower than the calculated peakwater level. It is considered that because of the degree of soil saturation. If the soil is dried, theoutflow rate is affected to be lower inevitably.

Fig. 2 Comparison with observed water level and calculated water level
Relationship between DPR, CSR, and WLRR

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Figure 3 shows the relationship between design discharge to peak discharge ratio (DPR), catchment
to storage ratio (CSR), and water level reduction rate (WLRR) in the case of 10-year rainfall. From
Fig. 3, half of the irrigation ponds need to release water level until empty because to spillway size iscomparatively small to consider their catchment area. Especially under the conditions that the DPR
is smaller than 1 and CSR is relatively higher (>50), WLRR gets almost 100 % which means if thewater level is released to empty, the theoretical peak water level would exceed the height of thespillway of each irrigation pond. It can be considered that the effectiveness of LWLC has a strong
relationship with the inflow of water from the catchment area while it has almost no relationship withthe size of the spillway, but the priority of LWLC for preventing dam collapse can be judged by DPR.

Fig. 3 Relationship between DPR, CSR, and WLRR in case of a 10-year return period
Relationship between CSR and PRRd
Figure 4 shows the relationship between CSR and the difference in flood peak reduction ratio (PRRd).
The figure shows the difference when the irrigation pond is full and, the irrigation pond has released
storage water corresponding to 0.5m from the full water level. If the CSR is under 50, PRRd tends tobe higher. However, in the case of heavy rainfall, PRRd becomes lower. Therefore, it needs toconsider the PRRd and rainfall intensity carefully when the CSR is under 50. Tanakamaru. et al (2019)selected 1,902 irrigation ponds with a large effect on flood mitigation by water release in advance at
Awaji district in Hyogo prefecture, Japan. The effectiveness of LWLC was calculated by theirrigation pond water balance model. After that, the outflow during the flood period was estimated ateach irrigation pond. As a result, the study showed that if normalized storage becomes higher, theeffectiveness of LWLC will be higher. However, it needs to be careful when calculating the flood
control function using normalized storage since the reliability of capacity is low.

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Classification of Irrigation Ponds
Based on the above results and discussion, studied irrigation ponds are classified. We proposed 3classifications as groups A, B, and C. Group A is characterized by lower CSR (<50) and lower DPR(<1). Irrigation ponds categorized in group A are recommended to implement LWLC or rehabilitate
the spillway immediately due to the high risk of dam collapse. Group B is characterized by lower
CSR (<50) and sufficient DPR (>1). It is recommended to implement LWLC to prevent dam collapse
and the overflow of the downstream canal even if the pond is safe. Group C is characterized by higher
CSR (>50). It is recommended to rehabilitate the spillway immediately due to the high risk of dam
collapse. Thus, it is of no use in implementing LWLC due to its quite small effect (PRR d≒0). The
number of irrigation ponds classified as groups A, B, and C is shown in Table 2.
Table 2 Classification of irrigation pondsGroup

10-year return period rainfall

50-year return period rainfall

200-year return period rainfall

A

20

28

31

B

17

9

6

C

35

35

35

CONCLUSION
There are a lot of irrigation ponds in Japan, and some of these do not have large enough spillways tosafely pass the 200-year return period. In this study, we proposed a method to classify irrigation
ponds concisely using relatively easy-access data. As a result of calculation, even if low water level
control is implemented, some of these have very little effectiveness against flood mitigation. In thecase of the ponds with high irrigation use frequency, i.e., high dependence, immediate rehabilitationof spillways should be considered, while in the case of the ponds with very low irrigation use
frequency, the abolition of irrigation ponds should be considered to prevent flood damage in case ofheavy rainfall events.
ACKNOWLEDGEMENTS
The authors would like to express their deepest gratitude to the Tottori Prefecture of Japan for thedata provision on irrigation ponds in Tottori Prefecture and the financial support for this study.
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