Improve the Understanding of Drylands Water and Ecosystem Responses to Droughts through Data Analysis and Modeling Studies

Abstract

Drylands include arid, semi-arid, and dry sub-humid regions covering ~41% of global land surface, where long-term precipitation is substantially lower than atmospheric water demands. It supports nearly 2.5 billion people with limited water resources and dominates the trend and interannual variability of the global carbon cycle. Drylands are becoming more vulnerable to frequent and severe water scarcity due to climate change exacerbated by increasing anthropogenic and ecological water demands. The intensified droughts under a warming climate substantially reduce vegetation carbon uptakes and even exacerbate the warming through land-atmosphere interactions. Therefore, it is imperative to investigate, realistically model, and project ecohydrological changes over drylands. Compared to observations, vegetation in most Earth System Models (ESMs) is more susceptible to water anomalies and shows a lower drought resilience. This dissertation aims to understand the discrepancy in vegetation drought responses between ESMs and observations through data analysis and modeling. The first part of this dissertation studies lateral flow movement in ESMs. Due to the computation constraint, ESMs typically only describe vertical soil water movement without considering lateral subsurface flow. I incorporated the hillslope-storage Boussinesq (hsB) scheme into the land model of DOE’s Energy Exascale Earth System Model (ELM) to explicitly represent lateral groundwater movement. I applied this newly developed model over California and found better model performance against ELM through the explicit yet simplified representation of lateral flow along hillslopes. Most importantly, our new model outperforms the default ELM in reproducing the seasonal variations, interannual variabilities, and a declining trend of terrestrial water storage anomaly in California. The better terrestrial water loss associated with lateral flow is primarily through enhanced vegetation drought resilience (e.g., enhanced plant transpiration). However, modeled enhancements of plant transpiration decline with increasing grid size and almost disappear with a grid size of 1°, potentially resulting from unrepresentative climates and unresolved land surface properties on the subgrid scale. The second part of the dissertation investigates the controls of concerted plant and soil hydraulics on the interannual variability (IAV) of vegetation carbon uptakes (GPP) over the central US through multiple model experiments. A land surface model explicitly representing plant hydraulics and groundwater capillary rise with an adequate soil hydraulics well captures the observed GPP IAV. The sensitivity experiments indicate that, without representations of plant hydraulics and groundwater capillary rise or using an alternative soil hydraulics, the land model substantially overestimates the GPP IAV and the GPP sensitivity to water in the central US. This highlights the importance of plant and soil hydraulics to Earth system modeling for projections of future climates over regions that may experience more intense and frequent droughts. The last part of this dissertation explores whether vegetation responses (greening/stomatal closure) to the escalating CO2 concentration and warming exacerbate or ameliorate future runoff yields in the dry western US. Water shortage in the western US is becoming increasingly serious due to increasing socioeconomic demands and climate change. Although previous studies have projected various degrees of runoff changes, they neglect the impact of rising CO2 on runoff projections. To explore the possible role that CO2 may play in the hydrologic cycle, I conducted three experiments with the newly improved Noah-MP land model including vegetation dynamics and plant hydraulics. Consistent with previous studies, the western US tends to be drier toward the end of the 21st Century. CO2-induced leaf area index increases (surface “greening”) contribute considerably to the projected widespread transpiration increases and runoff reductions; however, these changes are nearly compensated by the stomatal closure effect of CO2 on transpiration, leaving the warming effect to remain the major cause to these transpiration and runoff changes. This study suggests that both surface “greening” and stomatal closure effects are important factors and should be considered together in water resource projections. In summary, this dissertation enhances the understanding of water and ecosystem responses to droughts over the drylands in the US.