Plants can exert strong controls on water cycling, both locally and across the globe. The impacts are driven by how plants control the exchange of water and energy between the land and the atmosphere, which in turn are influenced by how plants photosynthesize and grow (e.g. biogeochemical cycling). Our team has shown previously that plant responses to increasing CO₂ in the atmosphere can influence the water cycle through changes in rainfall and the amount of water on land with implications for drought. Plant responses can also alter both average and extreme flow of water in rivers, with implications for freshwater availability and flood frequency. Although the impact of plant responses on water cycling has been demonstrated to exist, significant uncertainty remains in our understanding of the magnitude and form of the plant responses themselves, as well as their ultimate impact on water availability for people and ecosystems.

Here we propose to quantify the role of plant processes in regulating water cycling (e.g., precipitation, evapotranspiration, runoff, droughts, etc.).  We will expand on our past findings to address the following objectives: quantify the control of physiological vs. radiative effects on water cycling across many coupled models in idealized and realistic scenarios, as well as to quantify how our assumptions about leaf-level processes (e.g. coupling between stomatal conductance and photosynthesis), and organism-level processes (e.g. leaf area response to high CO₂, hydraulics) contribute to uncertainties and biases in water cycling.

We will focus on the impact of these processes on water cycling by analyzing the water budget (precipitation, evapotranspiration, and runoff) to assess the sensitivities of freshwater availability, drought, and floods to model assumptions about plant processes at the leaf, organism, and community level. As part of this project we will use measurements of stable carbon isotopes, as well as ecosystem water and energy fluxes, in combination with simulations of these quantities to provide additional avenues for validating and choosing between different parameterizations of plant processes. Because tree-ring isotope data provide an observational constraint on stomatal function, we can use tree-ring isotope data as a benchmark of model behavior over the historical period. Similarly, eddy flux data can be used to assess model behavior in recent decades. The tests and benchmarks that we develop will also be added as new metrics to the International Land Model Benchmarking Project (ILAMB) framework, complementing work by the RUBISCO SFA.

There is significant uncertainty about the representation of plant physiology and plant processes in Earth system models (including the Energy Exascale Earth System Model, E3SM) and, in particular, how these processes alter water cycling. To evaluate the influence of plant processes at multiple scales on water cycling, we will leverage simulations from a number of Earth system models from the 6^th Coupled Model Intercomparison Project (CMIP6), and additionally develop sensitivity simulations using E3SM. We will develop idealized simulations in order to isolate individual links in the chain of coupling between the biosphere and overlying atmosphere, and will test assumptions about stomatal conductance, plant growth responses, plant hydraulics, and ecosystem composition. This work is directly relevant to DOE’s primary scientific research questions in the RGMA topic area (b) on Biogeochemical processes, feedbacks, and interactions in the Earth system.