10:45 AM - 12:15 PM
[AHW20-P04] Long Term Variation of Mean Transit Time in Spring and Groundwater in a Forested Headwater Catchment
Keywords:Mean Transit Time, Groundwater, Tracers, Stable Isotopes, SF6
The estimation of the age of spring and groundwater in mountainous headwaters is important for evaluating the origins, flow path and storage volume of groundwater. Particularly in mountainous headwaters where relatively young spring and groundwater with ages of several years to decades predominate, the inert gas sulfur hexafluoride (SF6) is useful as a tracer for age-dating.
The mean transit time (MTT) is also an important aspect of research related to water age. It is the time that elapsed between water entering a system and then leaving it, such as from the infiltration of precipitation into a catchment until its spring water discharge. In order to find the MTT, stable isotopes of water (18O and 2H) are most frequently used, and can estimate the MTT of water from the unsaturated zone to the outflow of rivers and springs.
In this study, we applied a new methodology that utilized a combination of stable isotopic concentrations of water and SF6 atmospheric concentrations to investigate temporal variations in the spring water and groundwater MTTs of a forested headwater catchment in Japan.
The main objective of this study was to clarify how the characteristics of a forest alter its short and long-term hydrological processes in a headwater catchment. To do that, understanding the change of MTT of spring water and groundwater is essential. We applied a new multi-tracer methodology that included analyses of stable isotopes of water and SF6 atmospheric concentrations. Previous studies have mainly used trial-and-error approaches to identify the water age. We propose a new method that provides more reliable MTT estimation, as the model parameters are calibrated beforehand using apparent age information based on the SF6 data. Using past and current hydrological and tracer data, we sought to effectively understand how short and long-term temporal changes in MTT after forest thinning in Japan.
The study area is at Mt. Karasawa, Tochigi Prefecture, Japan, which had a 50% linear thinning in October 2011. Stable isotopes of water and SF6 analysis and hydrological observations were applied to investigate the temporal variation of MTT on three different periods: “Before Thinning”, from June 2010 to September 2011, “Soon After Thinning”, from November 2011 to December 2013 and “Long After Thinning”, from August 2017 to November 2021.
The SF6 concentrations were used to estimate the apparent age of the spring water and groundwater, which were used to initiate a parameter calibration and model fitting process. This apparent age is an initial parameter that can be found from previous data analysis and, therefore, it is an educated estimate. The MTT was evaluated by estimating the d-excess variations of the stable isotopes of water using the exponential piston flow model.
The MTT of spring water and groundwater showed a tendency to become older soon after the forest was thinned, and then younger approximately 4 to 8 years after thinning occurred. Considering that it is possible to assess the groundwater storage volume with the MTT and spring water discharge knowledge, we found that the average volume of the catchment increased “Soon After Thinning”, and decreased “Long After Thinning”. Therefore, the higher MTT values indicate an increase in groundwater volume storage, and shorter MTT values indicate a decrease in groundwater volume storage.
The results indicate that the characteristics of the hydrological processes in the catchment underwent long-term changes after the forest was thinned. This study demonstrates that using combined tracer methods to investigate the hydrological response to forest treatment practices improved the results and can be used for better forest and subsurface water resources management.
The mean transit time (MTT) is also an important aspect of research related to water age. It is the time that elapsed between water entering a system and then leaving it, such as from the infiltration of precipitation into a catchment until its spring water discharge. In order to find the MTT, stable isotopes of water (18O and 2H) are most frequently used, and can estimate the MTT of water from the unsaturated zone to the outflow of rivers and springs.
In this study, we applied a new methodology that utilized a combination of stable isotopic concentrations of water and SF6 atmospheric concentrations to investigate temporal variations in the spring water and groundwater MTTs of a forested headwater catchment in Japan.
The main objective of this study was to clarify how the characteristics of a forest alter its short and long-term hydrological processes in a headwater catchment. To do that, understanding the change of MTT of spring water and groundwater is essential. We applied a new multi-tracer methodology that included analyses of stable isotopes of water and SF6 atmospheric concentrations. Previous studies have mainly used trial-and-error approaches to identify the water age. We propose a new method that provides more reliable MTT estimation, as the model parameters are calibrated beforehand using apparent age information based on the SF6 data. Using past and current hydrological and tracer data, we sought to effectively understand how short and long-term temporal changes in MTT after forest thinning in Japan.
The study area is at Mt. Karasawa, Tochigi Prefecture, Japan, which had a 50% linear thinning in October 2011. Stable isotopes of water and SF6 analysis and hydrological observations were applied to investigate the temporal variation of MTT on three different periods: “Before Thinning”, from June 2010 to September 2011, “Soon After Thinning”, from November 2011 to December 2013 and “Long After Thinning”, from August 2017 to November 2021.
The SF6 concentrations were used to estimate the apparent age of the spring water and groundwater, which were used to initiate a parameter calibration and model fitting process. This apparent age is an initial parameter that can be found from previous data analysis and, therefore, it is an educated estimate. The MTT was evaluated by estimating the d-excess variations of the stable isotopes of water using the exponential piston flow model.
The MTT of spring water and groundwater showed a tendency to become older soon after the forest was thinned, and then younger approximately 4 to 8 years after thinning occurred. Considering that it is possible to assess the groundwater storage volume with the MTT and spring water discharge knowledge, we found that the average volume of the catchment increased “Soon After Thinning”, and decreased “Long After Thinning”. Therefore, the higher MTT values indicate an increase in groundwater volume storage, and shorter MTT values indicate a decrease in groundwater volume storage.
The results indicate that the characteristics of the hydrological processes in the catchment underwent long-term changes after the forest was thinned. This study demonstrates that using combined tracer methods to investigate the hydrological response to forest treatment practices improved the results and can be used for better forest and subsurface water resources management.