In the southwestern U.S. the North American Monsoon (NAM) delivers summer precipitation from July – September and the rest is provided as snowmelt in early spring. Both periods serve as an important water source for plants, but due to the heterogeneous nature of the NAM, not all locations within the region receive precipitation, leading to differences in soil moisture. Understanding how spatially heterogeneous soil water storage controls plant water use is needed, especially with projected changes in western U.S. hydroclimate. We examined the importance of NAM precipitation as a water source in different populations of Pinus ponderosa (PIPO) growing in the northern boundary of the NAM. We asked: 1) Which populations of PIPO switch from winter precipitation to NAM precipitation as a water source throughout the year? 2) Does the amount of winter precipitation affect how PIPO use NAM precipitation? 3) How does tree size and location impact water use? We measured xylem water isotopes in trees from eight sites across southern Utah and northern Arizona for two years. We found the southern sites had a distinct separation in water use before and after the NAM, while the northern sites had a less distinct separation. We also found that differences within populations seem to be a function of tree size, elevation, and aspect. By understanding how the isotopic signal of xylem water changes throughout the growing season, we can better recognize how different hydroclimatic years can impact tree growth.