In order to clarify how groundwater leakage and river runoff occur in a catchment under tectonic movement, the water balance was estimated in the forested (88.3% in area) Oikamanai River catchment (area, 62.6 km 2), Hokkaido, Japan. The catchment’s geology is early Miocene to Pliocene sedimentary bedrock of partly high permeability, accompanied by currently active faults. Daily evapotranspiration, E, in water balance was calculated by applying the one-layer model to meteorological data in the rainfall season of 2011 and 2012, with the topographic influence on heat balance of the catchment considered. The coupling with the short-term water balance method for river runoff events allows us to estimate groundwater leaking to the other catchments through the faults and bedrock. As a result, the leakage corresponded to 50% - 80% of effective rainfall (= P - E: P, rainfall) in 2011, whereas it was lower or negative in 2012. The estimate of leakage then included variability of ca. 80%. In 2012, shallow groundwater storage seems to retain high baseflow during non-rainfall.
Focusing on the forest and land management and the water resource assessment on the time scale of one to 100 years order, it is important to know how the water balance is built up in a forested catchment and consequently the river runoffs occur. For estimating the water balance in the catchment, it is necessary to reasonably evaluate actual evapotranspiration, groundwater storage change, and groundwater leakage to the other catchments or coastal regions. With respect to the groundwater storage and leakage, there are many hydrogeological studies of groundwater flow system in bedrocks with fractures or faults [
The groundwater flow in a catchment or region of 102 to 105 km2 order has been explored on the monthly to annual scale by welling observations and providing some models such as MODFLOW [
In this study, water balance in the geologically active and forested (88.3% area) Oikomanai River catchment, Hokkaido, Japan, is estimated on a daily basis, and how effective rainfall produces groundwater leakage and storage in the catchment is explored.
The Oikamanai River is a main influent river of the Oikamanai Lagoon in the Tokachi coastal region of southeastern Hokkaido, Japan (
143˚28'36''E; altitude, 6 m to 330 m asl) upstream of site R1 has the area of 62.6 km2, the mean slope angle of 34.7˚ (gradient, 0.692) and the mean riverbed gradient of 0.033 (
The catchment upstream of site R1 is covered by 88.3% forest in the mountainous region, 10.6% farmland (mostly, grassland) on the lowest alluvial plain, etc. (
River stage at site R1 was recorded at 30 min intervals by an air pressure logger and an air pressure-plus-water pressure logger with temperature sensors (HOBO U20, Onset Computer, Inc., USA; the range of 69 - 207 kPa and the accuracy of ±0.62 kPa for pressure, and the range of −20˚C to 50˚C and the accuracy of ±0.2˚C for temperature). The water pressure logger was set at near the deepest point in the cross-section of site R1 in Novem-
ber 2010. The water level, h (m), at site R1 was changed into river discharge, Q (m3/s), by the h-Q rating curve (Q = 11.72h1.373, R2 = 0.966, P < 0.01). The rating curve was obtained by frequent discharge measurements in April-November of 2011 and 2012 during the stage recording. River discharge was calculated by measuring the depth averaged velocity at about 20 sections partitioned in the cross-section and then summing the sectional discharge defined by the product of the sectional area. Then, the unchanged positions of the two loggers were ascertained by frequent surveys with three benchmarks. The Oikamanai River is frozen in December-March, when the h-Q rating curve is not applicable. Hence, the discharge obtained in the non-frozen season of April-November is applied for discussing the characteristics of discharge. For estimating water balance of the catchment, discharge and meteorological data in the rainfall season in the catchment of no snow cover are utilized, because the snow-covered period in April is very short (about two weeks).
Meteorological data were obtained at site M (altitude, 6 m asl; rainfall and air temperature) near the Oikamanai Lagoon [
Dates for the start and end of snow cover within the catchment were suggested by using the RGB images of MODIS/Terra or MODIS/Aqua with the resolution of 500 m (URL: http://www.eorc.jaxa.jp/cgi-bin/adeos/modis_frame.cgi?year=2014&month=10&prov=eoc&type=500mchla) and snow depth data at Taiki town. Considering a topographic effect on the solar radiation, the DEM in
The baseflow after the snowmelt season of 2012 was consistently high compared with that in 2011 (
The ratio of runoff rate to total rainfall for the runoff events in
No. | Time period of river runoff event | Mean Q (m3/s) | Max Q (m3/s) | (a) Total rainfall (mm) | (b) Total runoff rate (mm) | (c) Mean runoff rate (mm/h) | (b)/(a)×100 (%) |
---|---|---|---|---|---|---|---|
2011 | |||||||
1 | 0000 h, 24 Apr. - 0200 h, 25 Apr. | 7.03 | 11.2 | 71.5 | 10.60 | 0.407 | 14.8 |
2 | 0800 h, 28 Apr. - 2300 h, 28 Apr. | 5.72 | 6.81 | 47.0 | 4.89 | 0.330 | 10.4 |
3 | 1400 h, 13 May - 0100 h, 14 May | 3.50 | 3.96 | 37.0 | 2.19 | 0.201 | 5.9 |
*4 | 0200 h, 29 May - 1500 h, 30 May | 2.26 | 2.80 | 40.5 | 4.81 | 0.130 | 11.9 |
5 | 2100 h, 16 Jul. - 0000 h, 18 Jul. | 2.44 | 3.18 | 32.5 | 3.81 | 0.141 | 11.7 |
6 | 2300 h, 4 Sep. - 1000 h, 5 Sep. | 2.26 | 3.01 | 18.0 | 1.42 | 0.132 | 7.9 |
7 | 1300 h, 5 Sep. - 0900 h, 8 Sep. | 8.62 | 25.1 | 21.0 | 33.70 | 0.496 | 160.5 |
8 | 2000 h, 17 Sep. - 1300 h, 20 Sep. | 3.00 | 4.44 | 42.5 | 11.20 | 0.172 | 26.4 |
9 | 0000 h, 22 Sep. - 0500 h, 23 Sep. | 9.84 | 22.4 | 88.0 | 18.20 | 0.570 | 20.7 |
10 | 0100 h, 24 Sep. - 0900 h, 24 Sep. | 3.55 | 3.74 | 12.0 | 1.63 | 0.232 | 13.6 |
11 | 2100 h, 22 Oct. - 1500 h, 23 Oct. | 6.19 | 9.36 | 59.5 | 6.46 | 0.359 | 10.9 |
12 | 0500 h, 24 Nov. - 1800 h, 26 Nov. | 1.80 | 4.07 | 24.0 | 6.31 | 0.103 | 26.3 |
Sum 493.5 mm | Mean 26.7% | ||||||
2012 | |||||||
13 | 0300 h, 4 May - 0200 h, 6 May | 18.7 | 36.4 | 221 | 50.80 | 1.080 | 23.0 |
14 | 1500 h, 6 May - 2200 h, 7 May | 6.43 | 8.58 | 30.5 | 11.40 | 0.369 | 37.4 |
15 | 1800 h, 11 May - 0300 h, 13 May | 4.63 | 5.36 | 32.0 | 8.74 | 0.265 | 27.3 |
16 | 1900 h, 15 May - 0700 h, 17 May | 3.35 | 3.86 | 19.5 | 6.90 | 0.192 | 35.4 |
17 | 1500 h, 20 Jun. - 0100 h, 22 Jun. | 3.32 | 3.85 | 48.5 | 6.46 | 0.190 | 13.3 |
18 | 0700 h, 12 Jul. - 0900 h, 13 Jul. | 2.41 | 2.95 | 35.5 | 3.60 | 0.138 | 10.1 |
19 | 0100 h, 10 Aug. - 1800 h, 12 Aug. | 3.74 | 5.39 | 44.5 | 13.93 | 0.214 | 31.3 |
20 | 1500 h, 13 Aug. - 1100 h, 14 Aug. | 3.24 | 3.86 | 17.0 | 3.72 | 0.186 | 21.9 |
21 | 1600 h, 9 Sep. - 0600 h, 10 Sep. | 2.11 | 2.42 | 18.0 | 1.70 | 0.212 | 9.4 |
22 | 0400 h, 1 Oct. - 0200 h, 2 Oct. | 9.32 | 19.5 | 89.5 | 11.99 | 0.545 | 13.4 |
23 | 1900 h, 11 Oct. - 1600 h, 12 Oct. | 6.30 | 12.4 | 40.0 | 7.70 | 0.367 | 19.3 |
24 | 0600 h, 29 Oct. - 2000 h, 29 Oct. | 8.16 | 12.6 | 53.0 | 6.68 | 0.477 | 12.6 |
25 | 2300 h, 1 Nov. - 1700 h, 3 Nov. | 5.23 | 7.27 | 42.0 | 12.60 | 0.300 | 30.0 |
26 | 0900 h, 7 Nov. - 1800 h, 8 Nov. | 14.7 | 25.6 | 77.0 | 28.15 | 0.853 | 36.6 |
27 | 1400 h, 12 Nov. - 1200 h, 15 Nov. | 6.12 | 10.3 | 16.5 | 24.53 | 0.350 | 148.7 |
28 | 2200 h, 28 Nov. - 2000 h, 29 Nov. | 3.17 | 3.47 | 9.5 | 3.99 | 0.182 | 42.0 |
29 | 1400 h, 4 Dec. - 0500 h, 6 Dec. | 10.9 | 20.0 | 75.5 | 24.60 | 0.631 | 32.6 |
Sum 869.5 mm | Mean 32.0% |
*composite of two sequential runoff events, runoff events utilized for the short-term water balance method.
160.5%. The very high ratios of 160.5% and 148.7% were recorded for the No. 7 and No. 27 runoff events of 5 - 8 September 2011 and 12 - 15 November 2012, respectively. Meanwhile, the other events have the ratios of 5.9% - 26.2 % in 2011 and 9.4% - 41.8% in 2012. This suggests that high groundwater storage in the catchment sporadically produces relatively large discharge.
The time series of discharge and meteorology in
The DEM and land-use maps of the catchment in
Water balance of a catchment for a certain period is given as follows:
where ΔS is groundwater storage change (mm) for the water-balance period Δt (day), P is precipitation (mm/d), E is evapotranspiration (mm/d), R is runoff rate (mm/d) at the “exit” of the catchment (here, site R1 in
and L is groundwater leakage (mm/d). The runoff rate, R, is calculated from daily discharge (m3/d) divided by the catchment area (6.26 × 107 m2). Here, the evapotranspiration, E, is calculated by the one-layer model for the forest or grassland area (
where Kθ↓ is downward shortwave radiation (W/m2) onto the catchment of slope angle θ, α is albedo, L↓ is downward long wave radiation (W/m2), G is soil heat flux (W/m2), σ is the Stefan-Bolzmann constant (=5.670 × 10−8 W/m2/K4), QH is sensible heat flux (W/m2), QE is latent heat flux (W/m2), λ is vaporization heat (J/kg), cp is specific heat (J/kg/K) of water at constant pressure, ρa is air density (~1.2 kg/m3), ε is the ratio of water vapor density to dry air density (=0.622), CH is a dimensionless bulk transfer coefficient for sensible heat flux, Tz is air temperature (K) at z, uz is wind speed (m/s) at the height, z, above the earth surface, p is air pressure (Pa) at z, ez is the vapor pressure (Pa) at z, e0(Te) is saturated vapor pressure (hPa) at leaf-surface temperature Te, and β is evaporation efficiency (=CE/CH; CE, dimensionless bulk transfer coefficient for latent heat flux).
Here, the albedo α is supposed to be constant at 0.10 or 0.15 for the forested (88.3% area) Oikamanai River catchment, since α = 0.05 - 0.15 for broadleaf forest, α = 0.1 - 0.2 for needleleaf forest and α = 0.15 - 0.25 for grassland (
Downward shortwave radiation Kθ↓onto the basin slope is calculated by the following equations [
where Kd is direct shortwave radiation onto a normal surface, Ks is scattered shortwave radiation onto a plane, K↓ = Kd sinh + Ks, i is an incident angle of shortwave radiation onto the slope, h is solar elevation, As is solar azimuth, and A is slope aspect (clockwise from A = 0˚ for the south-facing slope). Kd and Ks in Equation (5) were obtained by the following equations.
where J0 is the solar constant (=1353 W/m2 as annual average), ζ is atmospheric transmissivity, and here is assumed to be constant at 0.7. The downward shortwave radiation Kθ↓ onto basin slope was calculated by using hourly K↓ data (θ = 0˚) at the Taiki Aerospace Research Field. Here, Kθ↓ in Equation (5) was calculated at θ = 34.66˚ (averaged slope angle) and A = 356.6˚ (averaged slope aspect) (
Daily mean values (Ld↓) of downward long wave radiation L↓ in Equation (2) were calculated by the following equations [
where B = Kd ↓/Kdf ↓ (Kd ↓and Kdf ↓, daily mean shortwave radiation observed on the plane and in fair weather, respectively), Ldf ↓ is daily mean longwave radiation in fair weather, and T is daily mean air temperature (K) at z. C is a factor of cloud effect with C = 1 in fair weather. Ldf ↓ is calculated as follows:
where w is total amount (cm) of effective water vapor, and Tdew is daily mean dew point temperature (˚C) at z, respectively. Daily mean shortwave radiation Kdf↓ in fair weather is calculated by using a relation with daily mean downward shortwave radiation K0d↓on a horizontal surface in the upper end of atmosphere [
When Equations (2)-(4) are applied at the daily base, the soil heat flux G could then be negligibly small compared with the other heat fluxes. Here, the daily mean Kθ↓ calculated and the wind speed, U10 (Uz at z = 10 m) at the Taiki Aerospace Research Field, and daily mean air temperature and relative humidity at site F are adopted to obtain daily mean T10 and e10 values and the Tdew values at z = 10 m. Then, for daily mean air temperature, its spatial distribution or the elevation effect in the catchment should be taken into account. However, the daily mean air temperature at sites M, R1, F and P (
where ρw is water density (kg/m3) at Te, and E is evapotranspiration (mm/day).
The groundwater leakage, L, in Equation (1) is estimated, since the catchment has many faults in addition to the relatively porous sedimentary bedrock (
Actual evapotranspiration E evaluated by the one-layer model (α = 0.1, CH = 0.008), the daily rainfall P at site P and the runoff rate R at site R1 are shown in
Relations between the leakage, L, and the effective rainfall (or infiltration), (P ? E), are shown in
The application of the short-term water balance method in 2012 was limited to the small effective rainfalls, and the leakage was then negative for the 4 time periods. The negative values are probably due to large variability of the actual evapotranspiration estimated and the relatively high runoff rate during non-rainfall in 2012 (
The downward shortwave radiation onto slope angle θ = 34.66˚ (mean μ) averaged over the catchment was by 24% larger than that at the Taiki Aerospace Research Field. Consequently the actual evapotranspiration E increased by 29% and 23% in 2011 and 2012, respectively (
The catchment slope aspect A takes μ = 356.6˚ (almost southward) and σ = 79.0˚ (
356.6˚ offers a nearly peaked E. In contrast, the Kθ↓ at θ = 58.86˚ and A = 277.6˚ may be smallest at ca. 47% of the Kθ↓ values at θ = 34.66˚ and A = 356.6˚, and total E values at θ = 34.66˚ and A = 356.6˚ decreased by ca. 61%. Then, total E values at the albedo α = 0.1 for 12 April-31 November 2011 and 20 April-30 November 2012 were by 0.5% smaller and by 7.0% larger than those at α = 0.15, respectively. Thus, the difference of the albedo does not greatly influence actual evapotranspiration from the catchment.
It was found that actual evapotranspiration estimated by the one-layer model varies mainly by the angle and aspect of the catchment slope. Then, there is the variability of decreasing up to ca. 61% from nearly peaked E. The standard deviation of rainfall P in the catchment is within 15% at less than 100 mm/d (
The characteristics of discharge were specified in the geologically active and forested Oikamanai River catchment using data sets obtained in the rainfall season of 2011 and 2012, and the estimate of water balance for the catchment suggested that the groundwater leakage and storage changed inter-annually. In 2011, the groundwater leakage corresponded to ca. 50% - 75% of the effective rainfall, whereas in 2012, the leakage was negative for the 4 time periods when the short-term water balance method was applicable. The negative values were probably due to large variability of the actual evapotranspiration estimated and the relatively high runoff rate during non- rainfall in 2012. The actual leakage and storage of groundwater seem to be linked to the bedrock and faults in the catchment. The leakage estimated by water balance has the variability of ca. 80%. Thus, the inner structure and hydrological role of bedrocks and their faults should be explored by geophysical prospecting, drilling and groundwater modelling. On larger scale, studies on the submarine groundwater discharge (SGD) into the sea could be useful for ascertaining the leakage, because some catchments accompanied by sedimentary rocks with faults facing the Pacific Ocean. With respect to the forested-land management, the investigation of sediment loading processes is important, because landslides and bank collapse frequently occur due to the active tectonic movement. As a next step, a model for simulating river runoff and sediment load will be proposed.
We are indebted to the Taiki City Government and associated staffs for the kind permission of our field observations in the Oikamanai River catchment. We express our gratitude to the farmers, Messrs. Mori, Takahashi and Yamamori, at the Seika and Bansei Villages for the frequent water sampling and instrument management. We are also grateful to Mr. T. Usutani, Japan Weather Association, for his welcome supply of analytical rainfall data. This study was supported partly by the Grant for Joint Research Program of the Institute of Low Temperature Science, Hokkaido University.