ESSDEarth System Science DataESSDEarth Syst. Sci. Data1866-3516Copernicus PublicationsGöttingen, Germany10.5194/essd-10-2295-2018A database of water and heat observations over grassland in the north-east of JapanA database of water and heat observations over grassland in the north-east of JapanMaWenchaowma@ied.tsukuba.ac.jpAsanumaJunhttps://orcid.org/0000-0003-1258-5491XuJianqingOndaYuichiCenter for Research in Isotopes and Environmental Dynamics, University of Tsukuba, Tsukuba, 305-8577, JapanFaculty of Life and Environmental Sciences, University of Tsukuba, Tsukuba, 305-8577, JapanCollege of Business Administration, Kanto Gakuin University, Yokohama, 236-0037, JapanWenchao Ma (wma@ied.tsukuba.ac.jp)18December20181042295230923April20186August201822October201829October2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://essd.copernicus.org/articles/10/2295/2018/essd-10-2295-2018.htmlThe full text article is available as a PDF file from https://essd.copernicus.org/articles/10/2295/2018/essd-10-2295-2018.pdf
A highly valuable database of long-term hydrometeorological measurements is
presented, containing in situ observations for a period of 37 years from a
well-maintained grassland in the north-east of Japan. The observations
include shortwave radiation, net radiation, air and dew point temperatures at
three elevations, soil temperature at four depths, sensible heat flux, soil
heat flux, wind speed, relative humidity, air pressure and precipitation. The
heights of measurements are 1.6, 12.5 and 29.5 m above ground, with the
soil-layer observations at depths of 0.02, 0.1, 0.5 and 1 m. This
high-quality database includes four temporal resolutions of 10 s, 0.5 h,
1 h and 24 h, with the hourly data
presented here. Monthly and annual statistics are presented at the database
web page of the Center for Research in Isotopes and Environmental Dynamics
and Prediction of the University of Tsukuba,
http://doi.org/10.24575/0001.198108. We validated the data by comparing
them with published data from the local meteorological agency in Tateno operated
by the Japan Metrological Agency, including the average, maximum and minimum
values of air temperature, shortwave radiation, wind speed, relative
humidity and precipitation. We have generated a daily downward longwave
radiation time series with a method developed by Kondo and Xu (1997) based on
the observations from the database. This constructed time series agrees well
with observations collected between 2002 and 2006, as evaluated based on the
values of the Nash–Sutcliffe efficiency (=0.947) and percent bias (=1.486). For the whole database, annually averaged values show a positive
trend in precipitation, air temperature, shortwave radiation, net radiation
and sensible heat flux over the past 37 years, with a negative trend detected
for wind speed, soil heat flux and soil temperature.
Introduction
In situ observational databases play a dominant role in all
research disciplines. In hydrometeorological studies, water and temperature
observations from field work are essential for validating differently sourced
data (Jackson et al., 2009; Liang et al., 2011; Guillevic et al., 2012),
supporting theoretical modeling (Grayson and Blöschl, 2001), estimating
the energy budget (Hirschi and Seneviratne, 2017; Makarieva et al., 2018) and
recording historical climatic variation (Godsey et al., 2018; Kormos et
al., 2018) for different subjects (Qu et al., 2016; Godsey et al., 2018). A
full set of in situ hydrometeorological observations is introduced here from
a well-maintained grassland located in the north-east of Japan, which has
been continuously operated since 1981 by the Environmental Dynamics &
Prediction (EDP) department of the Center for Research in Isotopes and
Environmental Dynamics of the University of Tsukuba, Japan. This
observational site has a long history of providing important contributions to
multidisciplinary investigations in many aspects of geological, hydrological,
biological and meteorological studies, including micrometeorology both within
and outside of the plant community as well as investigations into the
transportation of turbulence, evapotranspiration, the soil energy budget and
groundwater movement (Kawamura, 1991).
For decades, the EDP department has provided a high-quality database for many
valuable studies assessing the energy balance and the degree of
evapotranspiration. For example, Nakagawa (1983) tested the possibility for
estimating actual evapotranspiration using an equilibrium evaporation model.
Sugita et al. (1985) developed an apparatus for measuring the heat pulse
velocity for estimation of the transpiration flux by employing the
energy-budget method, while investigating the effect of stemflow and vegetation storage on
the evapotranspiration. Sugita and Kotoda (1985) tested the
effects of the soil-water deficits on forest evapotranspiration based on the
observations from the EDP database, from which the effective rate of
soil-moisture consumption was estimated and shown to be more than half the
amount controlling the suppression of the evapotranspiration. In that same
year, another advanced technology was employed (Sugita and Kotoda, 1985),
which combined using the remotely sensed, land-surface temperature as well
as Priestley–Taylor-type equations, for estimating the regional
evapotranspiration. By evaluating the observations in 2001, Yubasaki et
al. (2005) tested whether a reduction factor for pasture can be used for the turf site
by comparing the evapotranspiration estimated by Penman, energy-budget eddy
covariance and energy-balance Bowen ratio methods. Saito and Yamanaka (2005)
carried out a quality control of the data and analyzed the
evapotranspiration data observed with a weighing lysimeter between 1981 and
2002, with the results of the data quality summarized. In 1983, a model was
developed by Tase and Majima (1983) for estimating precipitation under the
influence of interception. This model showed good adaptability with the
canopy, stem and evapotranspiration components. In addition, latent heat flux
was assessed by Hiyama et al. (1993), with flux behavior compared before and
after the precipitation event as well as an investigation into temporal
variation and an assessment of measurement accuracy.
Investigations into the water and energy transfer, not only above ground,
but also within the soil layers, have also been extensively carried out. For
example, Sakura (1979) tested the infiltration effect for soil-water
movement and soil temperature. Taniguchi (1990) specified the distribution
of heat transport within soil layers by using observations of soil
temperature. Sakura and Taniguchi (1983) conducted experiments at the EDP
site to test the characteristics of soil-water movement during infiltration.
Measurement errors were investigated by Iwata and Sugita (2006), who
demonstrated the reliability of the observed heat fluxes.
A variety of studies on ecology and vegetation were conducted because of the
unique land surface covered by grassland. For example, Kotada and Hayashi
(1980) investigated the micrometeorology of the vegetation, with
specifying empirical parameters describing the vegetation growth state based
on physical modeling. While Nasuno et al. (1989) estimated the turbulent
fluxes with the eddy-covariance method to investigate the effect of forest
and vegetation on the exchange of energy and water vapor. Another study was
carried out by Hayashi et al. (1989) to investigate the water vapor and
temperature profile on this grassland. Being a well-maintained observation
site, the grassland provides good conditions for fundamental ecological
research. For example, Yasui and Oikawa (1993) investigated the CO2
flux resulting from soil respiration, showing a high correlation between
the soil temperature, moisture and respiration. Based on the unique
land-surface characters, Yamanaka et al. (2005) conducted experiments to
investigate the biomass and root characteristics at the site.
As a newly developed and reliable approach, the isotope method has also been
applied extensively in the investigations of the water and temperature
observations within the EDP database. Generally, branches of isotopic
research in hydrological applications include the fields of
hydrometeorology, eco-hydrology, groundwater hydrology, watershed
hydrology and the use of isoscapes (Yamanaka, 2012), with isotopic
techniques being well accepted as a powerful and stable method for
partitioning the transpiration from the evapotranspiration. In particular,
Shimizu and Yamanaka (2005) tested the spatial structure of the isotopic
composition of the atmospheric water vapor at the micrometeorological scale
over this observational site and analyzed the mixing processes of water
vapor with different sources. Wang and Yamanaka (2014) developed a new method
based on a two-source model for the growing season and demonstrated the
behavior of vegetation affected by physiological responses. Since then,
numerical models of isotopic tracers have been developed (Wang et al., 2015),
from which the Iso-SPAC model has evolved, with a steady-state assumption for
the transpiration flux successfully reproducing seasonal variations of all
the components of the surface-energy balance.
EDP observational site: (a) location,
(b) projected view of the EDP grassland, (c) the
observation tower and (d) results of the soil content analysis.
In recent decades, more attention has been paid to climate change, with the
need for more comprehensive and diverse data sources. The advantage of a
single-site database is the ability to provide refined, high-quality data,
since a large-scale database can easily overlook specific procedures
occurring within certain events involving both water and energy transport.
The purpose of this paper is to present the available data and the processes
involved in their collection for providing high-quality observations over
multiple frequency scales for assisting climate studies and energy-balance
estimations in multiple ways, such as (1) providing a complete data set for
water and temperature observations for modeling studies or theoretical
investigations; (2) helping to compare forecasted and estimated values of the
climate trend; (3) showing a unique data set representing the meteorological
and hydrological variations located in the north-east of Japan; (4) providing
a valuable and unique database for biological and ecological studies in a
long-term, continually well-maintained grassland.
Data descriptionObservational site, data and instrumentation
The data were collected from an observational site located at the department
of Environmental Dynamics and Prediction at the Center for Research in
Isotopes (36∘06′35′′ N, 140∘06′00′′ E) in the
grounds of the University of Tsukuba (Fig. 1a). This region has a semihumid
marine climate with a long-term-average annual precipitation of
1200–1600 mm (Hamada et al., 1998). The observational site consists of a
grass-covered circular field that is 160 m in diameter at an altitude of 27 m above
sea level (a.s.l.), a meteorological observation tower at a height of 30 m
equipped with sensors at heights of 1.6, 10 and 29.5 m, and underground
sensors at depths of 0.02, 0.1, 0.5 and 1 m, which monitor the soil
temperature and heat flux (Fig. 1b and Table 1). All observed time series
have been aggregated at frequencies of 30 min, 1 h and 24 h since 1981
and at a frequency of 10 s since 2003. Being a continuous, long-term
observational site, the maintenance and modernization of instrumentation has
taken place to ensure high-quality observations, with the date on which each
instrument was introduced found in Table 1.
The observational site was artificially filled with loam and volcanic ash
soil in the top of 1∼2 m and a clay layer with thickness of 4∼5 m, underneath (Sakura, 1977). Each soil layer consists of brown to dark
sand, clay and loam in different ratios. A soil-particle-size analysis was
carried out using a Shimadsu SALD-3100 particle analyzer, with the results
shown in Fig. 1d. During the growing season, the lowest soil layer encounters
groundwater at a depth of around 2 m. The observational site is covered by
multiple species of vegetation, with a similar percentage of species each
year since 1977 (Imasu et al., 2002) providing consistent meteorological and
hydrological research conditions throughout this period.
The vegetation is naturally grown C3 and C4 vegetation, such as
Imperata cylindrica, Andropogon virginicus,
Miscanthus sinensis as C4 and Solidago altissima,
Artemisia princeps, Lespedeza cuneata, Lespedeza pilosa, Equisetum arvense, Festuca arundinacea,
Potentilla freyniana, Lysimachia clethroides as C3. The
similarities in grass species, depth and leaf-area index (LAI) were confirmed
annually by two different surveys. Tanaka and Oikawa (1998) conducted a
survey of the seasonal dynamics of the LAI for both C3 and C4 vegetation
types, with results showing that the percentage of the species and the height
of the vegetation had not changed dramatically since 1977. Another survey was
carried out between 2000 and 2002 to directly measure the LAI and height,
with similar results found as before, such as the value of the LAI, which
ranged from less than 0.1 in winter to nearly 5 during September, while the
height of the grass is about 1 cm during winter and around 1.8 m in
September and October (Imasu et al., 2002). Since 2006, the grass has been
mown twice each year (summer and winter); dead plants and grass clippings
were redistributed.
Instrumentation information of the EDP observational system.
ItemElevation (m)PeriodInstrumentMaker (model)Shortwave radiation1.5Dec 2015 ∼Four component radiation balance meterHukseflux (CHF-NR01)1.5Aug 1981 ∼aThermocouple type total solar pyrometerEKO (MS-402F)EKO (MS-43F)EKO (A75510(MS43F))EKO (A84517)Net radiation1.5Mar 2014 ∼Four component radiation balance meterHukseflux (CHF-NR01)Draft-type thermocouple type radiationEKO (MF-11, CN-11)Aug 1981 ∼budget meterAir temperatureb1.6, 12.3, 29.5Aug 1981 ∼Ventilated platinum resistance thermometerVaisala (CVS-HMP155D)Humidity temperature probeVaisala (CVS-HMP45D)Nakaasa (E-731)Dew point temperature1.6, 12.3, 29.5Aug 1981 ∼ Jan 2006Calculated from relative humidityVaisala (CVS-HMP)(relative humidity)2006 ∼Lithium chloride dew point thermometerNakaasa (E-771)Sensible heat flux1.6Aug 1981 ∼Ultrasonic wind speed thermometerKAIJO SONIC (DA650, TA-61A)12.3Aug 1981 ∼ 1987KAIJO (DA600)29.5NWAug 1981 ∼Kaijyo Denki29.5SEAug 1981 ∼ Mar 2015Jul 2015 ∼Soil temperature-0.02, -0.1,Aug 1981 ∼Waterproof type platinum resistanceC-PTG-10-0.5, -1.0thermometerNakaasa (E-751)Soil heat flux-0.02Aug 1981–∼Thermocouple type ground heat flow plateEKO (CN-81)-0.1May 2013∼Heat flow plateHukseflux (HFP01-10)Wind speed1.6Aug 1981 ∼Ultrasonic wind speed thermometerKAIJO SONIC (DA650, TA-61A)12.3Aug 1981 ∼ 1987KAIJO SONIC (DA600)29.5NWcAug 1981 ∼Kaijyo Denki29.5SEcAug 1981 ∼ Mar 2015Jul 2015 ∼30.35Feb 2014 ∼Propeller type wind direction anemometerYOUNG (CYG-5103LM)Precipitation0.3Aug 1981 ∼Tipping falls separated type self-recordedYokokawa (WB0013-05,rain gaugeB-011-00)Atmospheric pressure1.5Aug 1981 ∼BarometerVaisala (PTB210)Nakaasa (F-401)
a Frequency for all observation periods: 0.5, 1 and
24 h. A frequency of 10 s was used starting in 2003. b Air
temperature observations included average, maximum and minimum values.
c NW is northwest; SE is
southeast.
Most observations we present were observed directly, except for the dew point
temperature Td, which, according to the historical record, has
been collected by different instrumentation. While the initial
instrumentation for the dew point temperature was a lithium chloride dew
point thermometer, since 15 December 2006 this has been changed
to a hygrometer (CVS-HMMP45D, Climatec), with the value of the dew point
temperature calculated from the observed specific humidity by Watarai and
Yamanaka (2007).
Td=b×log10e6.11a-log10e6.11,e=esat×RH100,
where a=7.5, b=237.3, RH is the relative humidity expressed as a
percentage, and the saturated vapor pressure esat is obtained
from
esat=6.11×10aTb+T,
where the temperature T is given in ∘C. Data collected from
supersonic anemometer–thermometers were used to obtain heat and momentum
fluxes using the eddy-correlation method. The data are freely available to
download from the Center for Research in Isotopes and Environment Dynamics
(CRiED) website (http://www.ied.tsukuba.ac.jp/en/edps/database-doi/,
last access: 22 October 2018) (formerly known as TERC) as hourly, monthly and
annual summaries. Since 2003 the temporal resolution has been at 10 s,
30 min, 60 min and 24 h intervals.
When calculating averaged data (Asanuma et al., 2004) at least 24 records at
30 min were required. Readings with less than 20 records were discarded
(marked with “***” in the supplement data file) and data with between
20 and 24 records were annotated as incomplete (Ohba and Yamanaka, 2008;
marked with “*” in the supplement data file). In addition to the
missing data, the dates of equipment maintenance, construction and mowing
information are recorded in the maintenance log
(http://www.ied.tsukuba.ac.jp/yosoku/kansoku/hojyo_log/, last access:
22 October 2018).
Estimation of downward longwave radiation
Using a method developed by Kondo and Xu (1997), the downward longwave
radiation flux is estimated from the routine meteorological observations,
including the solar radiation, air temperature, relative humidity, air
pressure, wind speed and precipitation (see Table 1 for instrumentation
information), with the temporal resolution of the calculation chosen based on
both the observed forcing data and the estimated downward longwave radiation.
This method was successfully applied to the Tibetan Plateau (Xu and Haginoya,
2001; Xu et al., 2005). In this study, the results of the estimation have
been validated against the downward longwave radiation as directly observed
by the EDP department between 2002 and 2007. To assess the model
performance, the coefficient of determination (R2), Nash–Sutcliffe
efficiency (NSE) and the percent bias (PBIAS) are considered.
Estimation of downward longwave radiation
The downward longwave radiation at the ground is estimated following Kondo et
al. (1994) as
L↓=σT41-1-Lf↓σT4C,Lf↓=0.74+0.19x+0.07x2σT4,x≡log10w*,
where L↓ is the downward longwave radiation for a cloudy day,
Lf↓ is the downward longwave radiation for a fine
day, T (K) is the 1 h air temperature, σ=5.670×10-8 W m-2 K-4 is the Stefan–Boltzmann constant, and
w* is the effective precipitable water expressed as
w*=1g∫0Psqpp0dp.
Here, p0=1013 hPa is the standard atmospheric pressure, and
ps is the surface pressure (hPa). The precipitable water (cm) is
estimated as
log10w≈log10w*+0.10,
and C is the coefficient expressing the effect of clouds by
C=0.03B3-0.03B2+1.25B-0.04,B≥0.0323=0,B<0.0323,
where B≡Sobs↓Stop↓, Here, Sobs↓
is the observed solar radiation flux, Stop↓ is the
mean downward solar radiation at the top of the atmosphere,
Stop↓=S00πdζsinϕsinδ+cosϕcosδsinζ,
where
ζ=cos-1(sinα-sinϕsinδcosϕcosδ),d=1.00011+0.034221cosη+0.00128sinη+0.000719cos2η+0.000077sin2η,δ=sin-1[0.398×sin4.871+η+0.033sinη],η=2π365Day,S00 is the solar constant, ζ is the half-day angle, ϕ is the
latitude, δ is the solar declination, and Day is the total number of
days from 1 January to the day of observation. According to Kondo et
al. (1994), the estimated downward longwave radiation is accurate to
within ±10 W m-2, with differences resulting from the effects of
snow in winter.
Parameter information within the EDP database. Locations A, B and C
are located within the observation site of the EDP department as shown in
Fig. 1.
a Data items are compared with data from the Japan Meteorological
Agency (JMA_Tateno). b The data coverage for relative humidity
was estimated starting in 2003. c The observation of longwave
downward radiation was between 2002 and 2007. Data coverage, average, maximum
and minimum were determined using estimated longwave radiation based on other
observation items. The estimated longwave radiation data comprise dates since
1983.
Model evaluation
Evaluating the agreement between observed and simulated data is a basic
requirement for the data quality control (Moriasi et al., 2007; Singh et
al., 2005) for which the following indices are chosen: the coefficient of
determination (R2) for describing the ratio of variance in the
observations, the Nash–Sutcliffe efficiency (NSE) for indicating the error
in the units of the constituent between observed and simulated data, and the
percentage bias (PBIAS) for measuring the average tendency of the simulated
data to be larger or smaller than the corresponding observed data (Gupta et
al., 1999). Here,
NSE=1-[∑i=1nYiobs-Yisim2∑i=1nYiobs-Ymean2]PBIAS=[∑i=1nYiobs-Yisim⋅100∑i=1nYiobs],
where n is the total number of observations, Yiobs is the
observed value for the ith step, Yisim is the simulated
value, Ymean is the mean value for all the observed value (Moriasi et
al., 2007). Here, a NSE value of unity represents the optimal value, while
the optimal value is zero for the PBIAS, with low-magnitude values indicating
an accurate simulation. These modeling indices are evaluated for checking the
accuracy of the time series of the estimated downward longwave radiation.
Data quality control
The data quality was checked for the entire observation period, and
errors were deleted that resulted from maintenance activities or abnormal system
behavior and marked as missing values in the EDP database, with Table 2
showing the data availability within the database after error deletion. To
preserve only original observational data from the EDP site, we do not
conduct any gap filling. For most of the parameters, the daily values are
average values from hourly data, except the daily precipitation value is the
accumulation from hourly data.
Comparisons of the EDP data with the Tateno data from the Japan
Meteorological Agency from 1981 to 2017. Further parameter information is
found in Table 2.
This database includes three versions: ver. 1.0, 1.1 and 2.0. For ver. 1.0,
the data were collected in integer data format following a former system
standard, which applied to the observed data until April 2003. Then, the new
system was started from May 2003 and the data set was updated as ver. 1.1.
The data quality is guaranteed by the consistent quality control of all raw
observation data. The quality control includes removing error data due to
instrumental problems and missing data caused by observed values out of the
specified range (http://www.ied.tsukuba.ac.jp/yosoku/terc/, last
access: 22 October 2018). The data format in ver. 1.1 were established in
accordance with Asanuma et al. (2004). Ver. 2.0 is the newest version, which
is a comprehensive version containing both ver. 1.0 and ver. 1.1 for the
purpose of improving data reliability by performing quality evaluation and
quality control. Ver. 2.0 has two main sections. The first section is
composed of the hourly, monthly and annual average values with highly
consistent quality control from August 1981 to December 2005. The other one
is composed of the raw data, which include data in time frequencies of
30 min, 60 min, 24 h and 10 s from 2003 to the present
(http://www.ied.tsukuba.ac.jp/yosoku/kansoku/rawdata/, last access:
22 October 2018).
Comparison with the Japan Meteorological Agency
After the deletion of error values, the reliability of the observations from
the EDP database is assessed with respect to the reference database from the
nearest local meteorological agency in Tateno, which belongs to the Japan
Meteorological Agency. The Tateno observations are at a height of 2 m above
ground level, with only a few items available for data-quality checking,
including shortwave radiation, air temperature (average, maximum and
minimum), wind speed, relative humidity and precipitation, at a daily
resolution between 1981 and 2017 (see Table 2, Fig. 2). For the EDP database,
observations from the 1.6 m height are compared with the Tateno data, with
the data at other heights compared with the adjacent measurement heights.
Both of these two data sets are compared in Fig. 2 along with the
corresponding values of R2.
As the value of R2>0.99 for the average, maximum and minimum values of
the air temperature in correlations between the Tateno and EDP databases, a
high similarity exists between the two closely located stations separated by
<10 km and an elevation difference of 1.8 m (the Tateno and EPD sites
are 25.2 m and 27 m a.s.l., respectively). However, slight differences
between these two databases were found, which is caused by the different land
surface, such as the presence of different vegetation and artificial
structures. Because of the diverse land-surface cover and varied moisture
distribution, the value of the relative humidity RH differs slightly, but is
still well correlated. Differences in shortwave radiation between the EDP and
Tateno are mainly governed by solar radiation, whereas absorption and
reflection may be caused by atmospheric conditions (clouds). Overall, most of
the variables are highly correlated, except wind speed and precipitation.
According to the regression analysis, a lower correlation is found for the
precipitation and wind speed between these two stations (Fig. 2), because of
the highly local characteristics of these two variables. In the case of the
wind speed, the sensor of the EDP site located at a height of 1.6 m is
surrounded by grass during the growing season and prior to mowing when the
height of the grass reaches 1.8 m, which generates enhanced local
turbulence, resulting in a reduced wind speed. The precipitation amounts are
significantly affected by the local conditions depending on the humidity,
temperature, cloud distribution and wind speed (Shuttleworth, 2011). In
general, after error deletion, the EDP database compares reasonably well to the
Tateno database.
Downward longwave radiation
The downward longwave radiation was calculated by using the method introduced
in Sect. 2.2.1 as estimated from routine meteorological observations from
1983 to 2017 and compared with a 5-year data set collected by the EDP
department from 2002 to 2006 (Fig. 3), giving R2=0.974, NSE =0.947
and PBIAS =1.486 and indicating good agreement of the estimated values
with the observed values according to Moriasi et al. (2007). Although the
estimated values are slightly higher than the observed values, this slight
bias of estimation is consistent with the statement by Kondo et al. (1994)
regarding the effect of winter resulting from the presence of snow. However,
most of the estimated values correspond well with the observed values.
Comparison of estimated and observed downward longwave radiation at
the EDP site from 1 January 2002 to 31st December 2006.
Statistical analysis
Plotted in Fig. 4 are daily observed values of the air and soil temperatures
for all layers, the precipitation, air pressure, humidity, longwave
radiation, solar radiation, net radiation (Rn), the soil heat
flux (G) and sensible heat flux (H) after data quality control. For each
soil layer, the temperature varies between the deep and shallow layers, with
the long-term-average values varying from 14.7 to 15 ∘C, the maximum
values from 25.2 to 32.3 ∘C and the minimum values from -1.5 to
5 ∘C. A regular annual variation is found for the four soil layers,
with the response of the deeper layers being slower than the shallow layers. The
larger amplitude variation is evident in the shallowest soil layer, with the
amplitude gradually decreasing for deeper soil layers. Some extremely high
values occurred in 1990, 1991 and 2005, and extremely low values were found from
1983 to 1985 and 2011 to 2013. In contrast to the temperature of the
different soil layers, the three air-temperature measurements show similar
patterns of variation, with the 37-year average values for the heights of
1.6, 12.5 and 29.5 m corresponding to 13.9, 14.2 and 14.5 ∘C,
respectively. The maximum value is 31.1 ∘C for all three layers, but
the minimum values show obvious differences of -5.3∘C at 1.6 m,
-3.8∘C at 12.9 m and -3.0∘C at 29.5 m. For the dew
point temperature, the three layers show very similar ranges of observation
values. From 1.6 to 29.5 m, the long-term average varies from 8.5 to
8.9 ∘C, the maximum value from 26.3 to 27.9 ∘C and the
minimum value from -14.7 to -19.5∘C. The minimum values for the
air and dew point temperature at 1.6 m are more different than the other
layers, since the 1.6 m measurement height is more easily affected by any
dramatic temperature or moisture exchanges with the ground.
The 37-year average value of precipitation is 1183.8 mm year-1, and
the maximum precipitation event occurred in 1986 with an amount of
181.5 mm day-1. The relative humidity takes an average value of
75.2 % for the 37-year period. The long-term-average value of the air
pressure is 1010.4 hPa.
The long-term-average value of the net radiation is 66.9 W m-2, while
the maximum value is 237.2 W m-2, and the minimum is
-15 W m-2. The interannual variation indicates an increasing
tendency for both the summer and winter from 1981 to 2004, but which has
reduced since 2005. The average value of the shortwave radiation is about
146.6 W m-2, while the maximum value is 356.5 W m-2, and the
minimum is -3.5 W m-2. The average value of the longwave radiation
is 346.5 W m-2, while the maximum value is 450.8 W m-2, and the
minimum is 227.4 W m-2. The soil heat flux and sensible heat flux show
less regular variations compared with the values of the temperatures
presented above, because of the difficulties in the accurate monitoring of
the soil heat flux resulting from the high spatial variation in the soil
properties. Furthermore, the soil heat flux is easily influenced by the soil
temperature, air temperature and heat capacity as well as the thermal
peculiarities of heterogeneous soil layers. However, since the observation of
these thermal components is rare, they are highly valuable for
hydrometeorological investigations. Further analysis of the database is
highly encouraged for specific research topics.
Daily observed values of the air temperatures for all layers,
maximum, minimum and mean air temperature at a height of 1.6 m, soil and dew
temperatures for all layers, the precipitation, air pressure, humidity, wind
speed, longwave radiation, solar radiation, net radiation, sensible heat flux
and the soil heat flux at the EDP site from 1981 to 2017.
Results
The complete long-term hydrometeorological data set at our grassland site
represents a valuable resource for climate studies. For example, Duan et
al. (2015) demonstrated the uniqueness of this area, which indicates that changes
in the precipitation amounts in Japan are in the opposite direction to the
extremes seen in the surrounding countries. Moreover, the database is capable
of providing data for assessing both the energy and water budgets. Here, the
main tendencies in the last 37 years are summarized in Fig. 5, where the
annual values represent the years with data availability >90 %
(328 days for 1 year), because gap filling has not been performed.
Annual average values of the observational database showing the
(a) shortwave radiation (W m-2), (b) net radiation
(W m-2), (c) longwave radiation (W m-2),
(d) sensible heat flux (∘C m s-1), (e) soil
heat flux (W m-2), (f) wind speed (m s-1),
(g) air pressure (hPa), (h) precipitation (mm day-1),
(i) relative humidity (%), (j) average, maximum and
minimum air temperature (∘C) at a height of 1.6 m, with the average
values at the heights of 12.3 and 29.5 m, (k) dew point temperature
(∘C) at heights of 1.6, 12.3 and 29.5 m, and (l) soil
temperature (∘C) at the depths of 0.02, 0.1, 0.5 and 1 m.
Based on the annually averaged values, most variables show a positive trend,
except the soil temperature, soil heat flux, wind speed and precipitation,
which show a slightly negative trend. First, the shortwave, longwave and net
radiation increased between 1981 and 2017 (Fig. 5a–c). The shortwave
radiation was compared with data from the Tateno database above, from which a
similar positive tendency was found. That the estimated longwave radiation
has a positive trend is consistent with the air-temperature trend with
reference to the Stefan–Boltzmann law (Shuttleworth, 2011). Since the
shortwave radiation has increased, it is reasonable to expect the net
radiation to have increased as well, because the balance of net radiation is
governed by the balance of the shortwave and longwave radiation. An observed
positive air-temperature trend is evident since 1981 (Fig. 5j), which is
consistent with the global trend related to climate change and an increasing
temperature. The average, maximum and minimum values of the air temperature
all show a positive trend with an increase of 0.04 ∘C year-1.
Here, the minimum value increased more slowly than the other two statistics,
because the minimum air temperature is highly affected by urbanization (Kondo
et al., 1994), where the main construction period of Tsukuba occurred within
the 1980s and slowed down thereafter, so that the minimum air temperature
shows a relatively smaller positive trend compared with the maximum and
average values. The dew point temperature mainly followed a similar positive
trend as the air temperature, with similarly increasing values at all three
heights, including the first height, which is often surrounded by grass (see
Figs. 5k, 1). The variation in the surface soil layer (-0.02 m) shows a
higher amplitude than the lower soil layers, which are more exposed to the
energy exchange occurring in the lower atmosphere, and since the EDP site is
covered by bushy grass, the energy transport received has a strong influence.
Therefore, the temperature of the soil layer does not show any simple
deviation from the annual average values (Fig. 5l). The explanation of the
negative tendency of the soil heat flux (Fig. 5e) relates to the energy
balance as pointed out by Brutsaert (1982), where Rn=LeE+H+G,Le is the latent heat of vaporization, and Lp is the thermal
conversion factor for the fixation of carbon dioxide, so that the variation
in the soil heat flux (G=Rn-LeE-H) is directly related to the
variation in the net radiation, and the latent and sensible heat fluxes. The
tendency of evapotranspiration is possibly positive since the temperature,
which is the dominant factor, shows a significant positive trend.
Furthermore, although there is a slightly positive trend of net radiation as
well as a negative trend of the sensible heat flux, the soil heat flux shows
a negative trend. The corresponding increase in potential evapotranspiration
is very possibly caused by the air temperature rising, which is in accordance
with global climate change. As the temperature is the most important
parameter for evapotranspiration, the increase in temperature probably leads
to an increase in evapotranspiration, meaning the decrease in the soil heat
flux is reasonable, although no data exist for the observations of the latent
heat flux. The sensible heat flux and the relative humidity show positive
trends (Fig. 5d, i). The main variation in the trend of precipitation is
relatively moderate because of the more extreme low and high values recorded
in recent decades, which is consistent with the more extreme precipitation
observed globally (Donat et al., 2016, Fig. 5h). Compared with precipitation,
a similar trend may be found in the relative humidity, although the analysis
period is relatively short, beginning in 2004 (Fig. 5i). For the wind speed
(Fig. 5f), there is negative tendency shown for the long-term observations,
which is easily affected by the variation in the surrounding land cover.
During these decades, two minor decreases in the signal are found between
1993 and 2002, and a decrease is also found in the value of the air pressure
(Fig. 5g).
The database described here has a digital object identifier
(10.24575/0001.198108, Asanuma and Ma, 2017) and is freely available at
the home page of the EDP data center (10.24575/0001.198108). The data
must be fully referenced for every use as introduced at
http://www.ied.tsukuba.ac.jp/en/edps/database-doi/ (last access:
22 October 2018). Supplemental materials may be found by checking the
observational data website at
http://www.ied.tsukuba.ac.jp/yosoku/kansoku/ (last access: 22 October
2018). Maintenance information is found from the database log at
http://www.ied.tsukuba.ac.jp/yosoku/kansoku/hojyo_log/ (last access:
22 October 2018).
Conclusions
A high-quality database covering 37 years is presented from the
north-east of Japan, encompassing four observational frequencies, three
measurement levels above ground and four layers below the ground. The daily
and annual average values were presented, including the daily values of
shortwave radiation, average, maximum and minimum air temperatures, wind
speed, relative humidity and precipitation, which were compared with data
from the local meteorological agency at Tateno for validation of the data
quality. Highly valuable data are also presented, such as for the net radiation,
soil temperature at four depths, the air and dew point temperatures at three
heights, the net radiation, soil heat flux and sensible heat flux, especially
because the soil heat flux and sensible heat flux are relatively rarely
observed or available. Furthermore, a time series of longwave radiation has
been generated for this site based on the reliable observations.
The annual average values were analyzed for the years with data
availability >90 %. The annual average values show a positive
tendency for the shortwave radiation, net radiation, longwave radiation,
sensible heat flux, air pressure, relative humidity, the air and dew point
temperatures at the three heights and a negative trend of the soil heat
flux, wind speed, precipitation and soil temperature. These trends provide
an important database and evidence for understanding the variations
occurring within the study area. At the same time, the specific
characteristics of the database may be found for the grassland with respect
to the values of the wind speed and precipitation based on the comparisons
between the daily values from the EDP database and Tateno as well as the
regression of annual average values. As the in situ observational site and
database have contributed to many previous studies, we shall continue to
maintain it, and we wish to make the data available for further research and
analysis.
The supplement related to this article is available online at: https://doi.org/10.5194/essd-10-2295-2018-supplement.
JA created the observation system and data collection system,
and controls the measurement. WM prepared the database and wrote this
manuscript. JX provided the theoretical support of downward longwave
radiation. WM and JX estimated the downward longwave radiation. JA and YO
designed this work. All authors provided critical feedback and helped to
shape the research, analysis and manuscript.
The authors declare that they have no conflict of
interest.
Acknowledgements
This study was supported by “Interdisciplinary Project on Environmental
Transfer of Radionuclides”. The authors would like to acknowledge Prof.
Tsutomu Yamanaka for his valuable suggestions. We thank Tomohiro Sekiguchi
and Kentaro Aida for their assistance in the system maintenance. We thank
Hideo Iijima, Shiraishi Izumi and Naomi Nakajima for providing digital and
paper-based bulletins for this review. We thank two anonymous reviewers for
their thoughtful and constructive comments, which helped to improve the quality
of this work.
Edited by: David Carlson
Reviewed by: two anonymous referees
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