Rock moisture can be an important contributor to forest transpiration and growth. Limited work has been done studying the effects of rock moisture (subsurface water stored in fractured, weathered rock) on transpiration rates — especially in water-limited environments. Semi-arid forests like the Gordon Gulch catchment (west of Boulder, CO) exhibit complex water budget systems where water sources are not completely understood. Here, we compare transpiration rates from plots on opposing aspects with regard to soil moisture and potential rock moisture storage as inferred from shallow seismic refraction surveys. We calculated the transpiration rates of ponderosa pine and lodgepole pine trees with sap flow data collected from June to September 2014. Potential storage for rock moisture is estimated based on qualitative analysis of shallow seismic refraction line data. While one would expect areas with higher soil moisture on average to have higher transpiration rates, our results showed the contrary: the plot with less soil moisture on average exhibited 25% higher transpiration rates. By qualitatively analyzing the seismic line images, we found that this phenomenon could possibly be explained by rock moisture. The plot with higher transpiration also had more fractured, weathered bedrock below that could potentially store more water in rock moisture. Rock moisture is an important component of the complex water budget system in Gordon Gulch. Further imaging of the subsurface is key to advance our understanding on how water is being used and moved in similar environments. Our research provides insight into rock moisture’s potential effects on water usage via transpiration in water-limited environments.