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The role of paddy rice in recharging urban groundwaterin the Shira River Basin
Kenji Tanaka • Yoshitaka Funakoshi •
Takaomi Hokamura • Fumihiko Yamada
Received: 3 August 2009 / Revised: 15 September 2009 / Accepted: 18 February 2010 / Published online: 7 March 2010
� Springer-Verlag 2010
Abstract Agricultural fields in the middle Shira River
basin play an important role as a source of groundwater
recharge; however, the water balance between the agri-
cultural water and river water is unclear. This study was
conducted to investigate the water balance in the fields by
measuring the stream flow of agricultural water channels,
which draw water from the Shira River. The flow rate of
water channels was found to increase in the beginning of
May, which corresponded to the cultivation of paddy rice
fields. During summer, the total agricultural intake was
comparable to the river flow observed in the middle Shira
River Basin. Determination of the water budget for the
targeted area revealed that most of the recharged water was
dependent on agricultural irrigation from the river. The
annual recharge of the overall target area was estimated to
be as high as 15,300 mm. In addition, the infiltration rate
was as high as 170 mm/day in the paddy fields during
summer, and as high as 30 mm/day in the upland fields
during winter. In order to recover the groundwater recharge
in this region, it is necessary to extend the submerged
period to include periods in which the stream water in the
Shira River is not subject to heavy rainfall as well.
Keywords Kumamoto � Shira River � Agricultural water �Groundwater recharge � Water balance
Introduction
The Kumamoto Urban Region, which comprises the city of
Kumamoto and its surrounding districts, is known as the
largest urban groundwater region in Japan. Indeed, 100%
of the water supply for nearly one million people in this
region is dependent on groundwater. The quality of
groundwater in Kumamoto is excellent, and it is renowned
as the best-tasting water in Japan (Hashimoto 1989).
However, a decline in the amount of water in the region has
occurred over the last few decades in response to a
decrease in the recharge area (Tsuru et al. 2006). Accord-
ingly, local governments in the Kumamoto Urban Region
have developed a plan designed to preserve the ground-
water resources in Kumamoto (Kumamoto Prefecture
2009). These governments have estimated that the
groundwater recharge in the region in 2007 was
6.00 9 108 m3/year, but that this will have decreased to
5.63 9 108 m3/year by 2024 if no countermeasures are
implemented. Additionally, it has been determined that a
recharge volume of 6.36 9 108 m3/year is necessary to
preserve the groundwater, assuming that the groundwater
pumping rate does not change from the rate that was
observed in 2006 (1.862 9 108 m3/year). Hence, this pro-
ject is designed to increase the amount of groundwater
recharge in the region by 7.3 9 107 m3 by the year of
2024.
The middle Shira River Basin is a key area for
groundwater recharge. In this area, the geological structure
of the surface layer has a high permeability. Specifically,
the top several meters of surface soil consist of alluvium on
a pyroclastic flow deposit known as Aso-III (approximately
132,000 years old) and Aso-IV (approximately 89,000
years old). Additionally, a large groundwater path extends
from this area to the Kumamoto Plain, in which there is a
K. Tanaka (&) � Y. Funakoshi � T. Hokamura � F. Yamada
Department of Civil and Environmental Engineering,
Graduate School of Science and Technology, Kumamoto
University, Kumamoto, Japan
e-mail: [email protected]
123
Paddy Water Environ (2010) 8:217–226
DOI 10.1007/s10333-010-0201-y
mean velocity of 40 m/day (Kumamoto Prefecture and
Kumamoto City 2005). Field surveys have shown that the
groundwater recharge area has a high capacity for recharge,
with the soil infiltration rate ranging from 30 to 500 mm/
day (Kiriyama and Ichikawa 2004; Takemori and Ichikawa
2007). Based on these findings, it is believed that urbani-
zation and increase in crops other than paddy rice have
caused a decline in groundwater recharge.
Although the importance of the Shira River middle area
has been demonstrated, the surface water balance in the
region is still unclear, especially the balance between river
flow and agricultural water use. The effect of agricultural
water use on river flow has been reported by several
authors (Shimotsu 1987; Arai 2004), but there is a lack of
quantitative evaluation based on the measurement of
agricultural water channels. Because of this uncertainty,
the water requirements for river flow in the Shira River
have not yet been confirmed (MLIT 2002). Determination
of this balance will enhance understanding of the capacity
for groundwater recharge throughout the agricultural area.
Many studies have been conducted to evaluate the water
balance in the agricultural area, and these studies have
revealed that the transfer of water from agricultural to
urban use is an important issue (Revine et al. 2007; Mat-
suno et al. 2007; Huang et al. 2007). However, studies
conducted to date have determined the quantitative balance
using numerical models such as the Soil and Water
Assessment Tool (SWAT) (Van Liew et al. 2003; Schilling
et al. 2008), while information based on field measure-
ments is very limited.
Therefore, this study was conducted to investigate the
water balance over the agricultural area and river flow in
the middle Shira River Basin by measuring water flow in
the irrigation channels. Specifically, the annual cycle and
interannual variation in stream water over the middle Shira
River Basin was analyzed using an MLIT observation data
set. This information was then used to estimate the area-
averaged groundwater recharge rate for one of the local
areas, Sako. When evaluating the recharge rate, the effects
of evaporation flux on the surface energy balance were
estimated with consideration of the land use cover.
Materials and methods
Study area
Figure 1 shows the location of the station and agricultural
water channel system in the middle Shira River Basin,
which contains the towns of Ozu, and Kikuyo and the
northeastern portion of the City of Kumamoto. The bold
line indicates the watershed boundary. The watershed of
the middle Shira River Basin is narrow; therefore, the river
receives very little inflow from tributaries. Seven intake
weirs are in place for agricultural use in the study area:
Hata-ide, Uwa-ide, Shimo-ide, Sako-Tamaoka, Tsukure,
Babagusu, and Toroku. The hatched area shows the irri-
gation area of each agricultural waterway. Some agricul-
tural water from the Shira River is returned to the river
after use, while some is directed to other river systems
including the Hori River from Uwa-ide and the Kase River
from Toroku. In the Shira River, the Jinnai flow station
operated by the MLIT (36.05 km from the mouth of the
river) is located immediately downstream of the Sako-
Tamaoka Weir.
In this study, Sako was treated as the target area for the
water budget, because both irrigation water (Sako 1, 2) and
runoff water (Sako 3, 4) are measured in this area. Figure 2
shows the land use conditions in Sako determined during a
field survey conducted 31 July 2008. The area of each land
Fig. 1 Location of intake weirs on the Shira River and of the
agricultural water observation stations used in this study. Stations 1–
11 in this figure correspond to the first column in Table 1. Major
waterways from each intake weir are shown as bold lines. The boldbreak line indicates the boundary of Shira River Basin. Jinnai
Waterstream Station maintained by the MLIT, Mashiki Station
maintained by the JMA-AMeDAS, and Kumamoto JMA Observatory
are also shown
218 Paddy Water Environ (2010) 8:217–226
123
use type was calculated using GIS (Kumamoto GP Map)
and is summarized in Table 1. In the upland fields, soy-
beans are primarily cultivated during summer. The land use
category of the irrigation without planting represents the
area that is seasonally submerged only for groundwater
recharge. Paddy rice fields cover about 33%, average
conditions during mid-summer (August 8) and low water
conditions following the harvest of paddy rice (October 21,
November 19–29).These data were then used in the fol-
lowing regression function (Tamura et al. 2006):ffiffiffiffi
Qp
¼ aH þ b ð1Þ
where the values of a and b are those shown in Table 2. For
the Sako 4 station, the flow rate was obtained from
Manning’s equation (Yen and Tsai 2001):
Q ¼ 1
nI1=2R2=3Ac ð2Þ
where n is Manning’s roughness coefficient (0.02), I the
slope coefficient (1/345), R the hydraulic radius, and Ac a
cross section of the open channel.
In order to obtain the water level from the pressure
gauge, it was necessary to correct the atmospheric pressure.
The atmospheric pressure for each site, Pa, was calculated
as follows (Kondo 1994):
Pa ¼ Pref
Ts
Tref
� �g=CRd
ð3Þ
where Pref is the reference atmospheric pressure at Ba-
bagusu, Tref the reference atmospheric temperature in
Kelvin, Ts the atmospheric temperature at the target station,
C the environmental temperature lapse rate (=6.5 K km-1),
Rd the gas constant (287 J kg-1 K-1), and g the accelera-
tion of the gravity. Atmospheric temperature can be cal-
culated using the lapse rate, which is given by
T = T0 ? C(z - z0), where T0 and z0 (=37.0 m) represent
the atmospheric temperature and altitude at Kumamoto
station. The corrected water level is nearly 10 cm higher at
Hata-ide 1 station with the correction than without the
correction.
River flow measurement data set
River flow data for Japanese national rivers (class 1) from
recent years is available from the Water Information Sys-
tem (WIS) (URL http://www1.river.go.jp) maintained by
the MLIT. The authors obtained hourly data regarding
water levels and river flow observed at Jinnai Station
(Fig. 1) from the WIS for Jan. 2004 to Apr. 2009. Because
of the strict data quality check by MLIT, it takes several
years for the flow rate to be released. In that period, the
flow rate data were only released for 2004 and 2006.
Hence, we calculated the flow rate for other periods (2005,
2007–2009) based on the hourly water level using the
relationship described in Eq. 1, in which coefficients
a = 5.4015 and b = 5.2609 were derived from flow rates
and water levels observed during 2006. After determining
the hourly flow rate, the daily average value was calcu-
lated. It should be noted that water level data for later than
July 2008 are preliminary values.
In addition to the aforementioned hourly data set, daily
flow rate data from 1979 to 2003 were obtained from the
Fig. 2 Land use in the Sako
area in July 2008. The dottedline shows the area boundary
Table 1 Land use condition in the Sako area in July 2008
Land use Covered area (ha)
Paddy rice field 33.17
Upland field 32.07
Irrigation without planting 7.31
Residences and buildings 10.17
Others (Roads, etc.) 19.34
Total 102.06
Paddy Water Environ (2010) 8:217–226 219
123
rain and river flow database provided by the Japan River
Association. These data were then used to investigate the
interannual characteristics of river flow over the last
30 years. We first separated the 30-year data set by date-of-
year (DOY), after which we computed the 30-year average
for each DOY. Statistical values such as the median, 25%
lower flow and 10% lower flow were also extracted for
each DOY by numerical sorting.
Water budget for the targeted area
The basic balance of the surface water budget integrated
over the targeted area, A, can be given as:
Qirr � Qroffð Þ þZ
A
Pr�X
EL � Rc
� �
dAþZ
A
�dgdt
� �
dA
¼ 0;
ð4Þ
where Qirr irrigation from the river, Qroff surface runoff, Pr
precipitation, EL net evapotranspiration from each land
cover, Rc recharge into the ground, and g temporal water
storage by the surface (e.g., inside the paddy fields). The
first term on the left-hand side represents the water budget
between the river basin and inside the agricultural field.
The second term represents the vertical budget term and
includes precipitation, evapotranspiration, and groundwater
recharge. The third term, temporal storage, might be sig-
nificant in short time scales (e.g., days), but is negligible on
an annual scale.
For the Sako Area, the first term on the left-hand side of
Eq. 4 was determined based on field observations. Ground
precipitation data are available from the Mashiki station
(32�50.2N, 130�51.3E, 193 m a.s.l.) and AMeDAS, which
is 2.5 km southwest of the Sako Area. However, the
ground observations from 1600 JST 16 August to 1200 JST
17 August were missing; therefore, the precipitation
intensity obtained from the 1-km Grid Point Value of the
JMA Radar was inserted. The amount was estimated to be
70 mm during the missing period of ground observation.
Evapotranspiration from the area was estimated using the
monthly meteorological data obtained from Kumamoto
Observatory, JMA (32�48.8N, 130�51.3E, a.s.l.; see
Fig. 1b) because no observations of humidity or radiation
were available at Mashiki Station and no micrometeoro-
logical observations were available for the target area.
Estimation of evapotranspiration for each land use
Evapotranspiration from the surface was computed based
on the surface energy budget using the following equation
(Arai 2004; Kondo 1994):
E ¼ Rn � G
L 1þ Bð Þ ð5Þ
where E the evapotranspiration flux, Rn the net radiation
flux, G the soil heat flux, L the latent heat of water, and
B = H/LE is the Bowen ratio.
Net radiation flux can be computed as follows (Arai
2004):
Rn ¼ 1� að ÞS� 1� cLWn2� �
1� aLW � bLW
ffiffiffiffiffi
ev
pð ÞrT4;
ð6Þ
where a the surface albedo, S the incoming shortwave
radiation, r = 5.67 9 10-8 Wm-2 K-4 the Stefan–Boltz-
mann constant, T the atmospheric temperature in Kelvin, n
the cloud cover ratio, ev the water vapor pressure (hPa),
aLW = 0.51, bLW = 0.0061, and cLW = 0.64 (Arai 2004).
The observed daily variables, S, n, ev and T, were obtained
from the Kumamoto Observatory (Fig. 3). The difference
in the monthly mean temperature between the observatory
and the station (0.3–0.4 K) was about 0.01–0.02 MJ/day,
Table 2 List of stations at which flow rate was measured
No. Station Name Latitude Longitude Altitude (m) a b
1 Hata-ide 1 32�52024.500N 130�57010.800E 181 1.3191 0.4001
2 Hata-ide 2 32�52004.000N 130�56004.100E 164 2.1784 -0.4416
3 Uwa-ide 32�52017.400N 130�55059.100E 145 4.8548 0.5186
4 Shimo-ide 32�52012.500N 130�55040.900E 143 2.4339 -0.9868
5 Sako-1 32�51037.300N 130�53026.700E 95.5 1.0113 0.1097
6 Sako-2 32�51037.700N 130�53026,800E 95.5 1.4711 0.0707
7 Sako-3 32�51016.700N 130�52008.700E 83.5 4.2112 0.1869
8 Sako-4 32�51007.800N 130�52003.100E 88.2 N/A N/A
9 Tsukure 32�51018.000N 130�51032.800E 77.2 2.9338 -0.1045
10 Babagusu 32�51007.700N 130�50056.600E 65.7 2.7112 -0.177
11 Toroku 32�48044.600N 130�44006.800E 38.5 1.8274 0.1407
The number in the first column shows the station number plotted in Fig. 1. The right two columns indicate the regression coefficient in Eq. 1
220 Paddy Water Environ (2010) 8:217–226
123
which is negligible when compared with the shortwave
radiation.
Soil heat flux was estimated using the thermal diffusion
equation with the daily mean atmospheric temperature as a
surface boundary condition. The soil heat flux was found to
range from 7 to 10 Wm-2 during the warm season and
from -5 to -12 Wm-2 during the cold season.
In order to determine the Bowen ratio and the surface
albedo, we used the typical value of each land use condi-
tion listed in Table 3 (Inoue 2008; Kondo 1994). These
values were used because there are no recorded measure-
ments for the atmospheric surface layer over the paddy
fields in the middle Shira River Basin.
Results and discussion
Annual cycle of agricultural intake
Figure 4 shows the variation in the daily average flow rates
observed at each agricultural site. The annual cycle of
agricultural water flow can be seen clearly, except for the
Babagusu water channels. The irrigation rate into the
agricultural area increases in the beginning of May,
approximately 1 month before the paddies are sown
(Fig. 5). The inflow rate for each agricultural channel
fluctuates during the rainy season due to control of the
inner flood water and is higher from July to late September
than during the rest of the year. During this period, the
irrigation rate ranges from 6 to 9 m3/s at Uwa-ide and
Shimo-ide, from 4 to 5 m3/s at Babagusu and Toroku, and
from 1.5 to 2.5 m3/s at Sako and Tsukure, respectively. The
flow rate decreases rapidly in the beginning of October,
which corresponds to the harvest period of rice. The irri-
gation rate is maintained at a low level from late October
until the following April, during which time wheat is
grown in the paddy fields.
Based on the results shown in Fig. 4, the water balance
can be separated into three phases: increased irrigation in
May and June (Phase-I), high irrigation from July to early
October (Phase-II), and no rice cropping from mid October
to the following April (Phase-III). Figure 6 shows a flow
diagram of the middle Shira River averaged for each phase.
The total intake water above Jinnai Station (Hata-ide 1,
Uwa-ide, Shimo-ide and Sako-1, 2) was 10.38 m3/s during
Phase-I, 15.27 m3/s during Phase-II, and 4.29 m3/s during
Phase-III. Additionally, the total intake below Jinnai Sta-
tion (Tsukure, Babagusu and Toroku) was 5.93 m3/s during
Phase-I, 9.50 m3/s during Phase-II, and 3.03 m3/s during
Phase-III. The total intake from Shira River was 16.31 m3/s
(8.611 9 107m3 for amounts), 24.77 m3/s (2.183 9
108m3), and 7.32 m3/s (1.277 9 108 m3) for each phase,
which was 35.2, 90.6, and 32.8% of the river flow at Jinnai
05
101520253035
Jan−08 Apr−08 Jul−08 Oct−08 Jan−09 Apr−09
(a) Incoming solar radiation (MJ/m2day)
05
101520253035
Jan−08 Apr−08 Jul−08 Oct−08 Jan−09 Apr−09
(b) Atmospheric temperature (degC)
05
101520253035
Jan−08 Apr−08 Jul−08 Oct−08 Jan−09 Apr−09
(c) Vapor pressure (hPa)
0.00.20.40.60.81.0
Jan−08 Apr−08 Jul−08 Oct−08 Jan−09 Apr−09
Date
(d) Fractional cloud cover
Fig. 3 Meteorological variables observed at Kumamoto Observa-
tory: a incoming solar radiation (MJ/day), b daily mean temperature
(�C), c vapor pressure (hPa) and d normalized cloud cover ratio (0–1)
Table 3 Surface albedo and Bowen ratio for land use as determined
by Inoue (2008) and Kondo (1994)
Land use Albedo Bowen Ratio
Paddy field (Summer) 0.08–0.20 0.13
Upland field (Summer)
(Winter)
0.14 0.29
0.27
Irrigation without planting (Summer) 0.06–0.09 0.24
Residences and buildings (Summer)
(Winter)
0.20 0.79
1.70
Others (Summer)
(Winter)
0.25 0.79
1.70
Paddy Water Environ (2010) 8:217–226 221
123
Station, respectively. The annual amount of intake water
was 2.643 9 108 m3 on the upstream side of Jinnai and
1.678 9 108 m3 on the downstream side, while the annual
river flow at Jinnai was 8.777 9 108 m3.
It should be noted that the river flow rates were sensitive
to rainfall (Fig. 3). The hydrograph of the middle Shira
River is shown in Fig. 7. The bars show the daily
precipitation observed at Mashiki Station. High water days
in June and early October, when the flow rate was greater
than 100 m3/s, were found to correspond to rainfall levels
of greater than 60 mm/day. The stream water flow rate
under the high water conditions (9 days) in 2008 was
1.425 9 108 m3, or about 16.3% of the annual amount of
stream water (32.2% of the amount during the rice-crop-
ping period). Except for these high water days, the river
streamflow ranged from 15 to 30 m3/s. The streamflow
decreased between late April and early May in 2008, with
the minimum streamflow being observed in the beginning
of June.
The statistical properties of river flow in the middle
Shira River are plotted in Fig. 8. The interannual variation
in river flow can be clearly seen from May to October. The
maximum flow rate for each DOY depends on the indi-
vidual mesoscale systems, such as the front system of the
extratropical cyclone and typhoon. The median flow rate
ranged from 15 to 20 m3/s from August to the following
April. The median flow decreased to less than 10 m2/s in
the beginning of June and then increased to 40 m3/s during
0123456789
10
Flo
w r
ate
(m3 /
s)
Jan−08 Apr−08 Jul−08 Oct−08 Jan−09 Apr−09
(a) Upper Middle Shira River
Hata−ide 1 Hata−ide 2 Uwa−ide Shimo−ide
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Flo
w r
ate
(m3 /
s)
Jan−08 Apr−08 Jul−08 Oct−08 Jan−09 Apr−09
(b) Sako Area
Inflow (Sako−1,2) Runoff (Sako−3,4)
0123456789
10
Flo
w r
ate
(m3 /
s)
Jan−08 Apr−08 Jul−08 Oct−08 Jan−09 Apr−09
(c) Lower Middle Shira River Basin
Tsukure Babagusu Toroku
Fig. 4 Variation in the daily average flow rate observed at each
station. a Hata-ide 1,2, Uwa-ide, Shimo-ide, b Sako 1–4 and cTsukure, Babagusu, Toroku
Fig. 5 Standard water level
control over the paddy field
according to the cropping cycle
in the middle Shira River Basin.
The original agricultural
calendar is provided by the
Kikuchi branch office of Japan
Agriculture (JA)
Fig. 6 Flow diagram of the middle Shira River basin separated by
cropping phase: beginning of the rice-cropping season (from May to
June 2008), middle of the rice-cropping season (from July to 10
October 2008) and off-season cropping (from 11 October 2008 to
April 2009)
222 Paddy Water Environ (2010) 8:217–226
123
the rainy season of June and July. The 25% lower flow and
10% lower flow falls below 10 m3/s during the rice-crop-
ping season, while these lower flow values are greater than
10 m3/s during the off-season. Such depression of the
lower flow values implies that the water intake is too high
during dry periods. Therefore, further studies should be
conducted to evaluate the interannual variation in the water
balance between agricultural channels and the river.
Water budget of the local area
Table 4 summarizes the water budget of the targeted area
from May 2008 to April 2008. Each component of the
water budget in Eq. 4 was translated into millimeters
(Table 4). The annual precipitation at Mashiki Station was
2463 mm in 2008, which was comparable to the
precipitation observed during the study period (2493 mm).
The water level in the paddy fields only varied by several
centimeters (Fig. 5), which was much smaller than the
variation in net irrigation that was observed over the entire
study area. The average annual groundwater recharge for
the targeted area was estimated to be 15,290 mm.
The annual evapotranspiration was estimated to be
657.9 mm, which was approximately 15% lower than in
previous studies. For example, Suekane and Kayane (1980)
estimated the annual evapotranspiration to be 777 mm
using Penmann’s method. The estimated evapotranspira-
tion of July and August was nearly the same as that esti-
mated by Suekane and Kayane (1980); however, in winter
and spring, the results of the present study were 10–20 mm/
month lower than in previous studies. Since Penmann’s
method is for homogeneous open-wet surfaces, it tends to
overestimate the heterogeneous land cover; therefore, it
must be corrected using a reduction factor obtained by field
observations (Yabusaki et al. 2005). The daily evapo-
transpiration from each land use condition is shown in
Fig. 9. Under fine weather conditions, the daily evapo-
transpiration was higher than 6 mm/day from paddy fields
that were irrigated without planting during summer,
whereas it was as low as 3 mm/day for residential areas.
The average daily evapotranspiration for the entire study
area was about 5 mm/day under fine weather conditions
during summer. For all land use conditions, the daily var-
iation in evapotranspiration fluctuated with the incoming
solar radiation as shown in Fig. 3a. Under cloudy and rainy
conditions, the evapotranspiration was smaller than 1 mm/
day, even in June. Considering the complex land cover in
the targeted area, the present estimation is reasonable.
The recharge rate Rc was estimated assuming that the
storage term was negligible. The dependency in Table 4
represents the ratio of net irrigation to the residual recharge
Rc. The recharge rate during the rice-cropping period
050
100150200250300350400450500550600
Flo
w r
ate
(m3 /
s)Jan−08 Apr−08 Jul−08 Oct−08 Jan−09 Apr−09
Date
0
40
80
120
160
200
240
280
320
360
400
Pre
cipi
tatio
n (m
m/d
ay)
Flow rate at Jinnai Daily precipitation at MashikiFig. 7 Hydrograph of the
middle Shira River Basin. The
flow rate observed by Jinnai
Station and daily rainfall at
Mashiki are shown
0.10.2
0.512
51020
50100200
5001000
Flo
w r
ate
(m3 /
s)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Date
Statistical properties (Jinnai) (1979−2008)Maximum MinimumAverage Median25% Low 10% Low
Fig. 8 Statistical properties of river flow observed at Jinnai in the
middle Shira River plotted by date
Paddy Water Environ (2010) 8:217–226 223
123
(2200–2600 mm) was approximately three or four times
higher than that of the no rice-cropping period (600–
700 mm). Most of the recharge water depends on the irri-
gation water, and the recharge rate ranges from 80 to 97%
throughout the year. Here, we attempt to estimate the daily
recharge rate over the paddy fields and temporal changes in
the water surface during irrigation. In order to accomplish
this, the following equations were used:
Reff ¼RcrAall �Wl
Ap þ Aw
ð7Þ
Wl ¼ RcnAall
Au
Agr
ð8Þ
where Rcr the Rc during the rice-cropping period, which
was determined to be 76.1 mm/day based on the infor-
mation in Table 4, Rcn the Rc during the off-season crop-
ping period, which was determined to be 22.2 mm/day
based on the average from Nov. 2008 to Mar. 2008, Aall the
total area of the targeted area, Ap the area of the paddy
fields, Aw the area of irrigation without planting during
summer, Au the area of the upland fields during summer,
and Agr = Ap ? Aw ? Au the total agricultural field area.
Wl in Eq. 8 represents the infiltration water mass over the
upland fields shown in Fig. 2 assuming that the recharge
rate is constant throughout the year. Reff was estimated to
be 167.5 mm/day. Takemori and Ichikawa (2007) demon-
strated that the infiltration rate of paddy fields in this area
was 130–300 mm/day. Hence, the estimation obtained in
this study is reasonable. In addition, the daily recharge rate
during winter was estimated to be 31.3 mm/day. Taken
together, these results indicate that the annual total
recharge in the paddy fields is 27,740 mm/year (120 days
for paddy rice irrigation), while it is 11,380 mm/year in the
upland fields.
Figure 10 shows the variation in area coverage of the
paddy rice field and the total submerged area (paddy field
and irrigation without planting) for the towns of Ozu and
Kikuyo in the middle Shira River Basin. The acreage of the
paddy rice fields in this area was as high as 1600 ha during
the 1970s, which resulted in a contribution to groundwater
recharge of about 3.0 9 108 m3; however, this value has
decreased gradually since the 1980s. The acreage of paddy
rice has been lower than 1000 ha since 2000, and in recent
years the acreage was half that observed in the 1970s (i.e.,
about 1.5 9 108 m3). The reduction of paddy fields in the
middle Shira River over the last few decades has affected
the reduction of groundwater by about 1.5 9 108 m3,
which is about 25% of the annual recharge in 2008 esti-
mated by Kumamoto Prefecture.
Since 2004, the seasonal flooding campaign has been
extended to fallow fields during summer to increase the
groundwater recharge. Specifically, 291 ha were flooded in
2004, while 576 ha were flooded in 2008. It is estimated
that an additional 1.637 9 107 m3 of water was recharged
by this campaign in 2008, assuming 30 days of submersion
with a recharge rate of 100 mm/day (Kumamoto Prefecture
2009). The areal extension of the submerged surface for
groundwater recharge in the agricultural area will com-
pensate the required water to some extent, but the period
should be made as long as possible in accordance with the
agricultural cycle.
In order to increase the groundwater recharge over the
agricultural area, the intake from the Shira River must be
increased by at least 5 m3/s during the rice-cropping per-
iod. However, increasing the intake prior to the rainy
season could lead to critically low water levels, as shown in
Figs. 6 and 8. Furthermore, the irrigation gates are con-
trolled to shut off under high water conditions such occur
Table 4 Monthly water budget for the Sako area from May 2008 to April 2009
Month Qin–Qout (mm) P (mm) E (mm) P–E (mm) Rc (mm) Dependency (%)
May 2008 1412.2 227.5 77.7 149.8 1562.0 90.4
Jun. 2008 1910.1 764.5 71.1 693.4 2603.5 73.4
Jul. 2008 2150.1 229.0 133.4 95.6 2245.8 95.7
Aug. 2008 2325.6 177.5 104.2 73.3 2398.9 96.9
Sep. 2008 1977.7 331.0 69.0 262.0 2239.7 88.3
Oct. 2008 655.8 88.5 50.7 37.8 693.6 94.5
Nov. 2008 593.5 119.0 19.1 99.9 693.3 85.6
Dec. 2008 482.0 114.0 4.9 109.1 591.2 81.5
Jan. 2009 680.7 57.5 11.4 46.1 726.8 93.7
Feb. 2009 529.5 152.0 16.5 135.5 665.0 79.6
Mar. 2009 560.7 149.0 39.4 109.6 670.3 83.6
Apr. 2009 177.8 83.5 60.6 22.9 200.8 88.6
Total 13455.8 2493.0 657.9 1835.1 15290.9 88.0
224 Paddy Water Environ (2010) 8:217–226
123
during the Baiu season. For example, in 2008, 1/3 of the
stream water was not available during summer. An increase
in heavy rainfall events as a result of recent climate change
may lead to further reductions in the available stream water
during summer. Hence, it is more efficient to recharge the
groundwater during winter using idle fields than by
increasing the irrigation area during summer.
Concluding remarks
This study investigated the water balance between agri-
cultural water and river flow by measuring water flow in
water channels in the Shira River Basin.
The annual cycle of water flow into the agricultural area
can be clearly seen for each water channel except
Babagusu, in which the control of the gate is more com-
plex. The agricultural intake of water has significant
impacts on river water in the middle Shira River Basin. The
period of no-precipitation days during the paddy rice-
cropping season led to a severe reduction in the main
streamflow. The total agricultural intake during summer
was as high as the flow observed in the middle of the Shira
River. Even during winter, the amount of agricultural water
collected was about 30% of the water flow in the main
river. The annual amount of intake water was 2.643 9
108 m3 on the upstream side of Jinnai and 1.678 9 108 m3
on the downstream side, while the annual river flow at
Jinnai was 8.777 9 108 m3.
Development of a water budget of the targeted area
revealed that most of the recharged water was dependent
on agricultural irrigation from the Shira River. The daily
recharge rate was *166.9 mm/day in the paddy rice fields
during summer and about 30 mm/day in the upland fields
during winter. The annual recharge over the targeted area
was 15,000 mm, while it was 27,800 mm/year in the paddy
fields. The current recharge in the paddy field in the middle
Shira River Basin was 1.5 9 108 m3, which was about half
of the intake water from the Shira River reported for 2008.
The reduction of the cropping area in the middle Shira
River led to a reduction in groundwater recharge of
1.5 9 108 m3 when compared with the level of recharge in
the 1970s. Such a reduction is as high as 25% of the annual
recharge estimated by Kumamoto Prefecture for 2008.
In order to increase the groundwater recharge to pre-
serve water resources in the Kumamoto Urban Region, it is
difficult to depend only on the middle Shira River. Most of
the water that recharges the agricultural area originates
02468
10
Pad
dy fi
eld
Daily Evapotranspiration (mm)
02468
10
Upl
and
field
02468
10
Irr.
w.o
. Pla
nt.
02468
10
Res
iden
ce
02468
10
Oth
ers
02468
10
Are
a A
vg.
Jan−08 Apr−08 Jul−08 Oct−08 Jan−09 Apr−09
Date
Fig. 9 Daily evapotranspiration from each land use condition as
listed in Table 2
0
500
1000
1500
2000
Acr
eage
(ha
)
1960 1970 1980 1990 2000 2010Year
Total Seasonally flooded areaPaddy rice in OzuPaddy rice in Kikuyo
Fig. 10 Interannual variation of rice-paddy acreage of the towns of
Ozu and Kikuyo in the middle Shira River Basin, provided by the
Kyushu Regional Agricultural Administration Office. The area of
seasonal flooding is based on data corresponding to Kumamoto
Prefecture (2009)
Paddy Water Environ (2010) 8:217–226 225
123
from surface runoff in the upper basin (e.g., from inside the
caldera of Mt. Aso). If the recharge over this area must be
increased, it would be more efficient to irrigate idle fields
during winter because the river flow in winter is much
more stable than in summer.
It should be noted that Tomiie et al. (2009) found that
the concentration of Nitrate–Nitrogen (NO3–N) in
groundwater has increased gradually throughout the
Kumamoto Urban Region, especially in northeastern
Kumamoto. However, the primary cause of such contam-
ination is still not clear. Therefore, the effects of irrigation
on water quality should be addressed in future studies.
Acknowledgments This study was supported by a grant from the
Center for Politics Study, Kumamoto University. The authors also
thank Mr. Hirano of the Sea-Bass Planning Co. Ltd. for providing the
flow measurement data for the water channels.
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