H146-03
A model-based study of the effects of spatial recharge patterns on groundwater transit time distributions in the Nebraska Sand Hills
Monday, 14 December 2020: 05:38
Virtual
Caner Zeyrek1, Troy E Gilmore2,3, Aaron R. Mittelstet2, Vitaly A Zlotnik4, Douglas Kip Solomon5, David P Genereux6, Eric Humphrey5, Markus R. MacNamara6 and Craig R Jensen6, (1)University of Nebraska Lincoln, School of Natural Resources, Lincoln, NE, United States, (2)University of Nebraska - Lincoln, Biological Systems Engineering Department, Lincoln, NE, United States, (3)University of Nebraska - Lincoln, Conservation and Survey Division - School of Natural Resources, Lincoln, NE, United States, (4)University of Nebraska - Lincoln, Earth and Atmospheric Sciences, Lincoln, NE, United States, (5)University of Utah, Department of Geology and Geophysics, Salt Lake City, UT, United States, (6)North Carolina State University, Marine, Earth, and Atmospheric Sciences, Raleigh, NC, United States
Abstract:
Determining groundwater transit time distributions (TTDs) is critical in assessing the quantity and quality of water resources. Groundwater TTDs can be constructed by analyzing age-dating tracers in groundwater collected from beneath streambeds or from wells. Groundwater flow models, coupled with particle tracking algorithms, can also simulate the travel times of groundwater discharge to any streambed or well grids. Previously, numerical groundwater flow models yielded TTDs with more young groundwater compared to streambed-based groundwater sampling. The objective of this modeling study is to investigate one potential driver of this discrepancy between flow models and tracer-based studies: the effects of spatial patterns in groundwater recharge on the shape of TTDs in the Upper Middle Loup River (UMLR, in the Sand Hills region, Nebraska, USA). This is a first step in a detailed comparison of numerical modeling and age-dating tracer studies conducted in five stream and river reaches along a 99 km portion of UMLR (baseflow index = 0.90).
MODFLOW is used to develop a steady-state, three-dimensional numerical model utilizes actual catchment geometry with 5436 km2 area and hypothetical recharge scenarios. TTDs of the particles located at the different stream reach cells are calculated using MODPATH. Effects of variations in groundwater recharge on TTDs are evaluated for five different stream reaches. Hypothetical model scenarios are based on (1) a regional west-to-east gradient in recharge and (2) a lateral gradient, where recharge is lower near the stream due to higher evapotranspiration in sub-irrigated hay meadows and floodplains. TTDs obtained from different recharge scenarios are compared using descriptive statistics and the non-parametric Kolmogorov-Smirnov test. The results of this study will help in understanding factors controlling discrepancies between numerical modeling- and tracer-based TTDs. In particular, and if the shape of the distributions can be explained by the spatial patterns in groundwater recharge. This hypothetical model will also serve for improving the understanding of the recharge-discharge distributions and constraining the input parameters of numerical groundwater flow models.
