Quantifying reductions in streamflow due to groundwater pumping (“streamflow depletion”) is essential for conjunctive management of groundwater and surface water resources. Analytical models are widely used to estimate streamflow depletion but include potentially problematic assumptions such as simplified stream‐aquifer geometry and rely on largely untested depletion apportionment equations to distribute depletion from a well among different stream reaches. Here, we use archetypal numerical models to evaluate the sensitivity of five depletion apportionment equations to stream networks with varying drainage densities, topographic relief, and groundwater recharge rates; and statistically evaluate the sources of error for each equation. We introduce a new depletion apportionment equation called web squared which considers stream network geometry, and find that it performs the best under most conditions tested. For all depletion apportionment equations, performance decreases with increases in drainage density, relief, or recharge rates, and all equations struggle to estimate depletion in short stream reaches. Poorly performing apportionment equations tend to underestimate streamflow depletion relative to numerical model results, leading to a negative bias and underpredicted variability, while error in the best performing apportionment equations tends to be due to imperfect correlation. From a management perspective, apportionment equations with error due to bias and variability are preferable as they correctly identify which reaches will be affected and can be statistically corrected. Overall, these results indicate that the web squared method introduced here, which explicitly considers stream geometry, performs the best over a range of real‐world conditions, and will be most accurate in flatter and drier environments.

Plain Language Summary 51Pumping groundwater for human uses such as irrigation can reduce flow in streams by 52 intercepting water which otherwise would have eventually flowed into the river channel or 53 causing water to flow out of the stream into the subsurface. This 'streamflow depletion' reduces 54 the water available to downstream users and ecosystems. Due to a lack of data and resources, 55 relatively simple ('analytical') groundwater models are often used to estimate pumping impacts, 56 but they are based on unrealistic assumptions, such as linear streams. In this study, we introduce 57 a new 'depletion apportionment' equation used to estimate pumping impacts that considers the 58 spatial configuration of real stream networks. By comparing it to more complex ('numerical') 59 groundwater models, we find that our new equation works better than existing equations under a 60 variety of conditions. All of the depletion apportionment equations we test perform best in 61 flatter, drier settings where streams are spaced further apart. Finally, we compare the causes of 62 error among equations, which have different implications for water management decisions. 63Overall, our results show that stream geometry is an important factor to consider when making 64 groundwater pumping decisions, and the new depletion apportionment equation introduced here 65 is a useful tool for water managers. 66