Continuous data of unconfined groundwater level recorded at Isawa Fan (Iwate Prefecture, northern Japan) from January 2008 to July 2021

We present the continuous data of unconfined groundwater level recorded at Mizusawa Campus, National Astronomical Observatory of Japan (hereafter referred to as NAOJ Mizusawa), located in the central part of Isawa Fan, Iwate Prefecture, northern Japan. In 2008, we installed a water pressure gauge (Mini-Diver DI501, Van Essen Instruments B.V.) at each of four shallow wells located around the gravity observation building of NAOJ Mizusawa, to monitor temporal variations in the water level at NAOJ Mizusawa. We also installed a barometer (Baro-Diver DI500, Van Essen Instruments B.V.) above the water level at the northern well, to subtract the effect of air pressure changes from the water pressure changes observed in the groundwater. The unconfined groundwater levels have been continuously measured with these instruments at 5-minute intervals, and the water level data have been used in some studies (Kazama et al., 2012; Tamura et al., 2023) to model gravity changes due to spatiotemporal variations in underground water such as soil water and groundwater.

PNG files in map.zip shows the location of NAOJ Mizusawa with the aerial photographs obtained from Google Earth Pro. The red pin denoted by SG indicates the gravity observation building, in which a superconducting gravimeter have been recording gravity continuously (Kazama et al., 2012; Tamura et al., 2023). Yellow pins around the gravity observation building indicate the shallow wells to monitor unconfined groundwater levels.

JPG files in photo.zip are the photographs taken on October 3, 2008, when the water pressure gauges were installed at the four wells in NAOJ Mizusawa. A water pressure gauge was connected to the top of a well by a metal wire of about 5 m in length, and installed into the groundwater in the well.

data.zip contains the time series of unconfined groundwater level recorded at NAOJ Mizusawa from January 2008 to July 2021. The time series from 2008 to 2010 indicates the average water level of the four water wells (see Kazama et al., 2012 in detail). In and after 2011, however, we were unable to obtain the average water level, because some of the four water gauges failed. Alternatively, we here used the water level data observed at the eastern well from 2011 to 2013, assuming that the variation of the water level at each well is consistent with that of the average water level. We also used the water level data observed at the northern well from 2014 to 2021. Note that we corrected for artificial steps in the water level caused by the switching of the water gauges to be used.

Each file of 20xx.txt in data.zip stores the times series of water level in the year of 20xx (xx = 08, 09, ..., 21). The water level was recorded every 5 minutes and time-stamped in Japan Standard Time (JST). Each column of 20xx.txt means as follows: ($1) year, ($2) month, ($3) day, ($4) day of year, ($5) hour, ($6) minute, ($7) time [day] from 00:00 in January 1, ($8) water level [cm] of unconfined groundwater relative to ground surface. For example, the value of -51.88 for the eighth column means that the water table is located at a depth of 51.88 cm below the ground surface.

Note that the file of 2011.txt stores incorrect water level values from March 28 to April 25, 2011, because we were unable to collect the data during this period due to the 2011 Tohoku earthquake. Therefore, we additionally provide a file of 2011_modified.txt, in which synthetic water level values are stored for the period from March 28 to April 25. The synthetic water level was calculated by assuming a linear response to precipitation of 16.5 cm/cm and a constant decrease in water level of -3 cm/day. These parameters were determined from the water level and precipitation data observed at NAOJ Mizusawa in March 2011.

Each file of 20xx.png in figure.zip shows the time variation of unconfined groundwater level at NAOJ Mizusawa in the year of 20xx. The water level rises rapidly due to rainfall and decreases exponentially after rainfall. The water level typically changes in the range of 1-3 m below the ground surface, but it becomes shallower than 1 m during heavy rainfall events. It also becomes deeper than 3 m below the ground surface during extreme dry periods such as in August 2011 and September 2012.