Investigating the impact of irrigation and water storage practices on hydrologic fluxes under climate change in a highly managed river basin

Abstract

Irrigation practices and sources can have significant impacts on water resources and the hydrologic fluxes that control these resources. To better manage water resources and future water supply, the influence of irrigation practices and management on these hydrologic fluxes should be quantified in time and space at varying scales, under potential irrigation management practices. To fulfill this objective, a surface-subsurface modeling approach was applied to simulate watershed-scale hydrologic processes in the Cache la Poudre River Basin, Colorado, USA (4,824 km2), in which both surface water irrigation and groundwater irrigation are prevalent. The model chosen for this study is the watershed model SWAT+, using the spatially distributed, physically based groundwater module gwflow, in which unconfined groundwater storage, flows, and interaction with land surface features are simulated using a collection of grid cells that represent control volumes of the aquifer. Major groundwater inflows and outflows include pumping, recharge, groundwater-channel exchange, groundwater-lake exchange, and tile drainage outflow. To investigate the impact of irrigation practices, detailed surface and groundwater irrigation routines and canal-aquifer interactions were added to the SWAT+ source code, requiring information of irrigation sources and irrigation canal locations throughout the river basin. Model calibration and testing was performed using monthly stream discharge and groundwater head. The calibrated model is used to quantify the impact of surface water and groundwater irrigation scenarios on water availability and hydrologic fluxes within the river basin. A total of 22 scenarios were conducted and grouped into five main groups: irrigation source, irrigation amount, irrigation type, canal bed thickness, and partial or full sealing of earthen irrigation canals. Using groundwater as the only irrigation source decreases groundwater discharge to streams (by 14%) due to lowering groundwater levels; converting flood irrigation to sprinkler irrigation throughout the basin decreases surface runoff by 22%; and sealing earthen canals leads to a lowering of groundwater levels, which decreases groundwater discharge to streams by 9%, leading to an overall decrease in streamflow in the Cache la Poudre River and changes to temporal patterns in streamflow. Overall, irrigation amount and type and canal sealing have a small impact on total groundwater storage, compared to changes in the percent of fields irrigated by groundwater pumping. The potential impacts of climate change on water resources and hydrologic fluxes were analyzed in this study. The calibrated SWAT+gwflow model is run under five different CMIP5 climate models downscaled by MACA, each representing two different climate emission scenarios, RCP4.5 and RCP8.5. Except for the CGCM3 (Warm) model, all climate models and emission scenarios predict an increase in the yearly average temperature. The projected variation in precipitation (that is, snow and rain) depends on the climate model used. However, the average annual precipitation across the entire basin is expected to increase by 6.1% under the RCP8.5 scenario for the NorESM1-M (Mild) model. On the other hand, the IPSL-CM5A-MR (Dry) model shows a maximum decrease rate of 6% from the average climate conditions under the RCP8.5 scenario. The analysis reveals that the IPSL-CM5A-MR-8.5 climate model in the CLP is the most severe, as it combines two climatic stressors: less precipitation and increased temperature. Runoff is observed to be reduced by 47.6%, groundwater recharge to drop by 11%, and a 0.5% reduction in groundwater storage under this climate scenario. Although the climate conditions in the past have been inconsistent, the transboundary water source that flows into the watershed has consistently maintained a stable discharge throughout the investigated historical period. This indicates the existence of regulated water management methods and agreements, irrespective of the impact of climate change. The potential effects of constructing a new reservoir were also assessed in this study, specifically focusing on the influence on streamflow and hydrologic fluxes under changing climatic conditions. The calibrated SWAT+gwflow model was run using two different CMIP5 climate models downscaled by MACA, CNRM-CM5 (Wet) and IPSL-CM5A-MR (Dry) under RCP8.5 emission scenario. The analysis revealed that the CNRM-CM5 (Wet) climate scenario had a higher average monthly diversion rate from the CLP river to the Glade Reservoir during operation months (2.1 m3/s) compared to the IPSL-CM5A-MR (Dry) scenario (1.6 m3/s). Both climate models show a consistent reduction in the average annual streamflow of the CLP river when the reservoir is present. The largest reduction in the average monthly streamflow in CLP river was observed under the IPSL-CM5A-MR (Dry) RCP8.5 with reservoir scenario for the month of June, showing a 78% decrease from the historical average streamflow. The reduction in streamflow, under the reservoir scenario, for both future climate models led to a 13% and 24% reduction in surface water irrigation for the wet and dry climate scenarios, respectively, compared to historical values. Results are helpful for informed decision-making in agriculture water management and can lead to sustainable, efficient, and equitable use of water resources, helping to address the challenges posed by water scarcity and environmental conservation.