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Data and Supplementary Information for Spatial variation in transit time distributions for groundwater discharge to a stream overlying the Northern High Plains Aquifer, USA
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| Created: | Dec 20, 2022 at 4:04 a.m. | |
| Last updated: | Jan 04, 2024 at 9:35 p.m. | |
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Abstract
Groundwater transit time distributions (TTDs) describe the spectrum of flow-weighted apparent ages of groundwater from aquifer recharge to discharge. Regional-scale TTDs in stream baseflow are often estimated from numerical models with limited calibration from groundwater sampling and suggest much younger groundwater discharge than has been observed by discrete age-dating techniques. We investigate both local and regional-scale groundwater TTDs in the Upper Middle Loup watershed (5,440 km2) overlying the High Plains Aquifer in the Nebraska Sand Hills, USA. We determined flow-weighted apparent ages of groundwater discharging through the streambed at 88 discrete points along a 99 km groundwater-dominated stream segment using 3H, noble gases, 14C, and groundwater flux measurements at the point-scale (<7.6 cm diameter). Points were organized in transects across the stream width (3-10 points per transect) and transects were clustered in five sampling areas (10-610 m in stream length) located at increasing distances along the stream. Groundwater apparent ages ranged from 0 to 8,200 years and the mean groundwater transit time along the 99 km stream is >3,000 years. TTDs from upstream sampling areas were best fit by distributions with a narrow range of apparent ages, but when older groundwater from downstream sampling areas is included, the regional TTD is scale dependent and the distribution is better described by a gamma model (α ≈ 0.4) which accommodates large fractions of millennial-aged groundwater. Observations indicate: (1) TTDs can exhibit spatial variability within a watershed and (2) watersheds can discharge larger fractions of old groundwater (>1,000 years) than commonly assumed.
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| Explanation of Data Set S1 | Data Set S1. Data Set S1 is a zip file containing the following files: (1) SI_TTD_Plots_MonteCarlo, (2) DataB, (3) Resample, (4) Transects_Gradients_New, and (5) a folder titled FMINSEARCHBND. The description of each of these are as follows: 1. SI_TTD_Plots_MonteCarlo is the MATLAB code used to analyze data, construct TTDs, evaluate the error of various parameters through Monte Carlo simulations, and plot the figures contained in the main text. Extensive metadata is written into the code in the form of comments that will guide users through various sections. All figures produced by this code have been modified post-processing for proper formatting in the manuscript. Note also that MonteCarlo simulations will produce slightly different results each time the simulations are run by nature of randomly perturbing input variables. The code uses many simulations (n=10,000) in the Monte Carlo analysis so differences in results are negligible, but it may result in small changes from the published values. 2. DataB is a data file used by MATLAB code containing groundwater flux (q) and apparent age (τ) for each point used to construct the TTDs. All data points are also contained in table A1 of the main text appendix. 3. Resample is a data file used by MATLAB code containing groundwater flux (q) and apparent age (τ) for each resampled point. These data were not used in the construction of the TTD but are discussed in the main text. These data points are also contained in table A1 of the main text appendix. 4. Transects_Gradients_New is a data file containing the transect location, stream width, point distance from center of transect, and apparent groundwater age for all data points (excluding resample points). This file is used by the MATLAB code to generate Figure 6 in the main text. 5. FMINSEARCHBND is a file folder containing the MATLAB function FMINSEARCHBND (D’Errico, 2022) used to fit the gamma and exponential models to the empirical TTD data by finding the minimum of the sum of chi-squared goodness of fit metric. |
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