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Evaluation of Capture Analysis Bias Associated with Use of Nonlinear Groundwater Flow Models
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Water law in the western U.S. typically recognizes surface and groundwater as different sources of water. Because these two systems are often truly connected, groundwater pumping can deplete surface water and is contributing to more frequent litigation by surface water right holders. Surface and groundwater interactions can be quantified through capture analysis, which assesses the decrease in surface water flow rates caused by groundwater pumping. Capture can be described in volumetric terms, or it can be expressed as a fraction of the change in surface water flow rate to the pumping causing the change in flow rate and is often determined through use of groundwater flow models. Capture fractions are a useful means for characterizing the impact of groundwater pumping on streamflow depletion when models behave linearly. However, nonlinear models are often needed to adequately simulate flow processes such as groundwater evapotranspiration and unconfined aquifers. This study quantifies and analyzes the capture bias associated with use of nonlinear groundwater flow models. Generally, bias associated with nonlinear groundwater flow models simulating a connected stream is not affected by selection between the MODFLOW Time-Variant Specified-Head (CHD) and Streamflow-Routing (SFR2) Packages, nor is it significantly affected by variations of streambed hydraulic conductivity of up to two orders of magnitude. Capture fraction bias is significantly altered by disconnected streams, variations in pumping rates, well proximity to capture sources of interest, and by altering the depth at which a well is screened. Understanding the behavior and magnitude of capture fraction bias associated with nonlinear groundwater flow models can help with water management of surface and groundwater interactions.