Using a Land-Surface Hydrology Model to Evaluate the Role of Vegetation Phenology in Altering the Carbon, Energy, and Water Budgets During the 2012 Kansas Flash Drought

In the summer of 2012, a flash drought ravaged the central United States causing billions of dollars of losses to crops. Flash droughts are characterized by the rapid dry down of soil-moisture due to a coupling of low precipitation, high temperatures, and strong winds. Plants play an important role in the movement of water between the atmosphere and soil. We seek to understand how plants alter soil water conditions during extreme droughts. How much water plants extract from the soil is dependent upon where they are in their growing cycle, i.e. phenology. In order to investigate how plant water usage influences the evolution of a flash drought event, we compare results from two land surface hydrology models: one with fixed phenology and one with dynamically changing phenology. We use a 1-D column version of the Duke Coupled Hydrology Model with Vegetation (DCHM-V) and predictive phenology (DCHM-PV) to model vegetation water use and photosynthetic activity during the 2012 flash drought in Lawrence, Kansas. We compare model results with in situ and remotely sensed data at the Ameriflux Kansas Field Station (KFS). The DCHM simulates water, energy, and carbon fluxes between three soil layers, the land-surface, and the atmosphere and incorporates photosynthesis based on the Farquhar model. The model is forced with data from the North American Land Data Assimilation System (NLDAS), Moderate Resolution Imaging Spectroradiometer (MODIS), and Stage IV. Models estimates show that the DCHM-V and -PV matched well against Ameriflux in situ soil moisture measurements and MODIS-derived GPP. The predictive phenology (DCHM-PV) model results show decreases in LAI and FPAR during the 2012 flash drought prior to the phenological changes indicated by MODIS phenology. This results from the use of soil potential in the model formulation to predict LAI and FPAR in the DCHM-PV, and thus reflects how plant water stress affects plant phenology. Differences between forced and predictive phenology results for soil moisture suggest the importance of plant water use strategy. These results have implications for the plant and soil water feedbacks which tie together the carbon and water budgets and which can be used to inform stakeholders on the role of vegetation and water availability during drought and non-drought periods.