Understanding Ecohydrology in a Managed Watershed through Integrated Modeling and Observations

Both natural (e.g., ground- and surface water exchange) and anthropogenic (e.g., irrigation) processes modulate ecohydrological dynamics in management watersheds. To understand such dynamics, coordinated team efforts are required to monitor key components of the watershed and employ results in a novel modeling framework.

Our watershed modeling framework features the coupling between PFLOTRAN, a massively parallel subsurface reactive transport model, the land model in the Energy Exascale Earth System Model (ELM), and the river routing model from the Soil and Water Assessment Tool (SWATR). We applied the modeling framework in the Upper Columbia-Priest Rapids (UCPR) watershed, a typical semi-arid and productive crop growing region located in the north-western United States dominated by cropland, where long-term observations from three eddy covariance stations as part of the AmeriFlux network, groundwater monitoring well, and discharge from USGS streamflow gages are available.

Our results show that the upland and riparian ecosystems exhibit drastically different water and carbon dynamics. The upland ecosystem relies heavily on rainfall for water supply, and evapotranspiration (ET) and net ecosystem exchange (NEE) peak in late spring when both precipitation and energy inputs are high. In contrast, the riparian ecosystems are dominated by exchanges between river water and groundwater, and ET and NEE peak in the summer when plants are physiologically active and less water-stressed. Irrigation substantially alters water, carbon, and nitrogen budgets in the watershed. Such an external water source leads to elevated ET and runoff over cropland, accompanied by an increase in total water storage and shallower groundwater table. Irrigation also increases gross primary productivity which results in elevated carbon storage. Soil organic carbon (SOC) shows drastically large fluctuations in its seasonality in response to changes in soil temperature and soil moisture. Additionally, irrigation intensifies the rate of denitrification and mineralization during growing season, thereby enhancing the interactions between soil mineral nitrogen and SOC.

Future work in improving the coupling among model components and collecting additional observations to inform and validate the model will be discussed.