Integrated basin-scale long-term water resources modeling serves as a valuable tool for supporting future water management strategies. This study explores the impacts of climate change on water resources and hydrologic fluxes in the Cache la Poudre River, a semiarid highly managed basin located in northeastern Colorado, under two scenarios: (1) climate change with the current reservoir infrastructure, and (2) climate change with a new reservoir. The soil and water assessment tool (SWAT)+gwflow model, calibrated and tested using historical monthly streamflow data from 2009 to 2015 and from 2002 to 2009, respectively, was used to forecast future streamflow, groundwater storage, and hydrologic fluxes. To examine the effects of irrigation techniques, the SWAT+ code was adjusted to incorporate detailed surface water and groundwater irrigation practices, as well as canal-groundwater interaction. The model was run with five Coupled Model Intercomparison Project Phase 5 (CMIP5) climate models downscaled by multivariate adaptive constructed analogs (MACA), encompassing two emission scenarios, namely, representative concentration pathways (RCPs): RCP4.5 and RCP8.5. Specifically, the IPSL climate model, version 5A, medium resolution (IPSL-CM5A-MR) (dry) model, under RCP8.5, exhibited the most severe conditions, with a 47.6% runoff reduction, 11% groundwater recharge decrease, and a minor 0.5% decline in groundwater storage. The addition of a new reservoir is projected to impact streamflow, particularly during peak diversion months, potentially leading to reduced water availability downstream. The analysis suggests that annual diversion to the reservoir, intended to meet a demand of 40,000 acre-ft (approximately 49 million m3), could result in a 78% June streamflow decrease under the dry scenario compared with historical levels. Furthermore, the reservoir scenario reduces watershed average runoff by 39% and 49% in wet and dry scenarios, respectively, lowering irrigation water availability. In the wet scenario, groundwater recharge rises by 8%, and surface irrigation and canal seepage decline by 13% and 15%, respectively. In the dry scenario, groundwater recharge decreases by 12%, with surface irrigation and canal seepage decreasing by 24% and 22%, respectively. These findings emphasize the importance of assessing the risks associated with reservoir development and climate change for water management and agriculture.