the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Observation-based sowing dates and cultivars significantly affect yield and irrigation for some crops in the Community Land Model (CLM5)
William J. Sacks
Danica L. Lombardozzi
Alan Robock
Abstract. Farmers around the world time the planting of their crops to optimize growing season conditions and choose varieties that grow slowly enough to take advantage of the entire growing season while minimizing the risk of late-season kill. As climate changes, these strategies will be an important component of agricultural adaptation. Thus, it is critical that the global models used to project crop productivity under future conditions are able to realistically simulate growing season timing. This is especially important for climate- and hydrosphere-coupled crop models, where the intra-annual timing of crop growth and management affects regional weather and water availability. We have improved the crop module of the Community Land Model (CLM) to allow the use of externally-specified crop planting dates and maturity requirements. In this way, CLM can use alternative algorithms for future crop calendars that are potentially more accurate and/or flexible than the built-in methods.
Using observation-derived planting and maturity inputs reduces bias in the mean simulated global yield of sugarcane and cotton but increases bias for corn, wheat, and especially rice. These inputs also reduce simulated global irrigation demand by 15 %, much of which is associated with particular regions of corn and rice cultivation. Finally, we discuss how our results suggest areas for improvement in CLM and, potentially, similar crop models.
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Sam S. Rabin et al.
Status: open (until 30 Jun 2023)
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RC1: 'Comment on gmd-2023-66', Anonymous Referee #1, 02 Jun 2023
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The aims of the presented study are to 1) incorporate externally-prescribed growing season data into the Community Land Model (CLM) and 2) use the modified model to explore the effects of the Global Gridded Crop Model Intercomparison (GGCMI) growing season data on historical yield and irrigation water demand simulations.
The research conducted is interesting and beneficial to the agricultural and modeling field. It sheds light on the new prescribed calendar functionality of the CLM model, but also shows limitations of the simulated global phenology results. The methodology is well described, and the conclusions are well derived. However, there are some minor issues that hinder the clarity of the study. Minor revision is recommended before acceptance.
General comments:
- The study often refers to wheat in general (e.g., in text and figures 9 and 10 labels), but for CLM5, wheat is only represented by spring wheat. The authors mention this several times but there are still areas which may suggest that winter wheat is considered in the analysis. I also find it difficult to draw conclusions for global wheat phenology when vernalization is not considered since it plays a key role in determining planting date for wheat farmers and the length of the growing season. Further clarification should be added throughout the manuscript to ensure readers understand that only spring wheat is considered.
- What is the reason for the huge difference in sugarcane shown in S15? Most areas are > 200 days different which is an entirely different season. Why even include sugarcane and cotton if they are not used in most global studies due to limited confidence (as stated in lines 304-308)? It is challenging to have confidence in the simulated results with this large of a difference. A detailed section on model uncertainties should be included in the Discussion.
- The median of the prescribed calendar GDD for cotton are much higher than the CLM default in Fig. S17. This results in the large yield increases seen, but how are these GDD values justified? Additionally, the prescribed calendar GDD variation for all crops is unrealistic (e.g., < 500 or > 3000 GDD, > 5000 GDD for sugarcane), and should be checked. How are these simulated results justified? Improved justification and uncertainty explanation is needed in the Discussion to have confidence in the model results.
Specific comments:
- Line 25-26: “…can reduce yield because they have less time to photosynthesize.” Yield reduction from temperature driven early maturity is mainly because of less time for grain filling/reproductive growth and resource partitioning. Please clarify.
- Line 100: what is the resolution of the simulations?
- Line 239 to 240: Remove parenthesis around sentence.
Technical Corrections:
Line 37: Please also reference the Agricultural Model Intercomparison and Improvement Project (AgMIP, https://agmip.org/) when first mentioning GGCMI since GGCMI is a part of this larger project.
Citation: https://doi.org/10.5194/gmd-2023-66-RC1
Sam S. Rabin et al.
Model code and software
CLM code used for 1850–1957 period and GDD-Generating run Sam S. Rabin, CTSM team, and contributors https://doi.org/10.5281/zenodo.7724212
CLM code used for spinup CTSM team and contributors https://doi.org/10.5281/zenodo.7724294
CLM code used for experimental runs Sam S. Rabin, CTSM team, and contributors https://doi.org/10.5281/zenodo.7724225
Sam S. Rabin et al.
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