## 4 Conclusions  
This study quantified growing season WUE of maize and alfalfa crops, which resulted in higher harvest WUE (HWUE) estimates in maize but higher ecosystem WUE (EWUE) in alfalfa. In addition, it shows the importance of input variable choice, as well as the timescale over which WUE is determined, when interpreting WUE values due to the inconsistencies observed between these methods. Patterns observed between methods do not necessarily correlate, because above ground biomass and GPP estimates, used as WUE calculation inputs, are influenced differently by plant physiology and farming practices. For example, above ground biomass carbon content did not correlate with GPP due to differences which photorespiration associated with C3 and C4 photosynthetic pathways and allocation of carbon to below ground carbon storage. Similarly, ET and T are influenced disproportionately by these two drivers due to differences in soil exposure, canopy development, aging effects, and growth rates. This was the result of differences in physiology between species, maize being planted in rows, and alfalfa undergoing four cuts throughout the season. Furthermore, two input-data processing methods were used in this study to produce the same flux-based WUE calculation, which provided substantially different results and results using the same method showed that the timescale over which data is collected is important when making comparisons. Since the different methods for calculating WUE did not agree with one another, this suggests that inter-method comparisons of WUE values are ill-advised. Due to the complexity of physical and physiological mechanisms involved in these WUE estimates, it is recommended that future studies focus on the individual water use and carbon uptake variables rather than the seemingly arbitrary WUE measure. The results of this study have implications for shifts in environmental conditions leading to potential applications in crop selection under future climate scenarios. Should growing season length increase, the results of this study indicate that selecting crops that establish earlier in the season, such as alfalfa, could improve ecosystem water use efficiency (i.e., EWUE) over the growing season. However, should temperatures increase, there could be a shift from C3 crops to C4 crops due to the improved EWUE at higher temperatures of the C4 pathway becoming more prominent. Furthermore, the results of this study affirm that hastened canopy development could be important to reducing water losses from evaporation by reducing soil exposure, which could be important under strained water resources, and work to increase canopy EWUE. This is particularly important for row-crops, such as maize, which have larger evaporation due to the exposure of bare soil in row gaps. Selecting crops that undergo multiple harvests, such as alfalfa, could reduce senescence effects and increase ecosystem and canopy level EWUE, especially under lengthened growing seasons. However, this does not equate to improved HWUE, since GPP and AGB were not well correlated. This complicates future agricultural responses to changing climate, since the results of this study suggest that selecting crops which accumulate greater biomass, such as maize, would be beneficial to optimize HWUE. This highlights the need for a more robust approach to quantify water resource use, which accounts for both plant (stomata) responses (GPP, T), abiotic field responses (ET), and agricultural production (AGB). Future research into the agricultural water use resources must consider all three of these components to promote a holistic approach to climate change agricultural adaptability, and ensure a valid interpretation of WUE results. Conceptualization, R.M.P. and K.D.H. ; methodology, R.M.P., A.G., M.K. ([PERSON]) and K.D.H. ; formal analysis, K.D.H. and M.K. ([PERSON]). ; investigation, K.D.H., R.M.P., W.H. and M.L.M. ; resources, R.P. and M.L.M. ; data curation, A.G. and M.K. ([PERSON]), writing--original draft preparation, K.D.H. and R.M.P.