Linking floodplain processes to hydrologic modeling with SWAT+ gwflow in the Lower Arkansas River basin

Abstract

Floodplain landscapes play a significant role in hydrologic fluxes, including connectivity to the alluvial aquifer and the biogeochemical processing of solutes from irrigation return flows. Variable spatial extents and limited temporal occurrence of active floodplains make quantifying their hydrologic and biogeochemical impacts problematic. To investigate, a surface-subsurface modeling practice was implemented to simulate hydrologic processes at the watershed scale in the heavily managed Lower Arkansas River Valley (LARV) (Colorado, USA). The SWAT+ model accounts for spatial variability of landscape features while simulating the fundamental physical principles that govern hydrologic processes within a watershed, such as runoff, infiltration, soil water routing, crop uptake, soil lateral flow, groundwater storage and flow, and streamflow. Using the gwflow module of SWAT+ simulates groundwater head, storage, and fluxes in response to hydrology and irrigation at the surface, replacing the original groundwater module. The primary objective of this thesis is to improve the implementation of floodplain landscapes in a modified version of SWAT+ with gwflow; and to assess the role of floodplains in aquifer recharge in the LARV. The model is run for the 1992-2020 period, with fifteen parameters calibrated for streamflow. Most years, flooding is insignificant in the managed LARV, and few floodplain-linked cells become active along the Arkansas River corridor. Flood scenarios for 100-year and 500-year events were run to observe the effects of including floodplain-exchange in SWAT+ gwflow models. Water balances reveal that the hydrologic process with the largest daily groundwater flux may occur through active floodplains, with implications on the annual change in storage for an aquifer system. Groundwater contributes 13% to streamflows through the standard simulation period. During the month of a 500-year flood scenario, groundwater produced 8% of surface flows, up from 3% without integrating floodplains. Activating floodplains in the 500-year flood scenario provided an additional flux, increasing groundwater storage by 3.5 % that year. Results are largely dependent on how floodplain landscapes are delineated; findings show that incorporating floodplains and floodplain-channel interaction into models likely brings the simulation into a stronger accordance with the real stream-aquifer system, as seasonal groundwater head and fluxes (groundwater saturation excess flow, groundwater evapotranspiration, groundwater return flows) are influenced by river water seeping to the aquifer during periods of flooding. Doing so allows the LARV model, and other models that use the floodplain option, to be used for quantifying the effects of flooding events on hydrological processes, nutrient processes, and management of wetlands and cropping systems within the floodplain of a river valley.