The Role of Water Subsidy on Vegetation Dynamics in a Semiarid Grassland Catchment: Comparison between Field Measurements and 3-D Ecohydrological Modeling
Abstract
In water-limited regions, water and carbon fluxes are strongly controlled by soil water availability, which may be highly variable even at very small spatial scales (e.g., meters) due to variations in slope, aspect, soils, and vegetation conditions, as well as water redistribution along hillslopes. This paper examines key factors controlling soil water availability for plant growth and surface CO2 fluxes in a small semi-arid experimental watershed (7.92 ha) in southeastern Arizona, using a land surface model (LSM) coupled with a three-dimensional (3-D) process-based hydrological model. The LSM accounts for surface energy, water, and carbon exchanges between the land surface and the atmosphere, while the hydrological model accounts for 3-D subsurface flow in variably saturated porous media and surface flow propagation over hillslopes and in stream channel networks. The coupled model, with minor calibration, simulated the observed surface energy, water, and CO2 fluxes well. The model mirrored the interannual variability of CO2 fluxes without accounting for the transition from native to exotic bunchgrasses that occurred at this site during the observation period, suggesting that water availability exerts a stronger control on the ecosystem dynamics than changes in species. Results from 3-D coupled model experiments with different lateral saturated hydraulic conductivities demonstrated that the model with smaller conductivities improved the simulation of evapotranspiration (ET) and CO2 fluxes, suggesting that lateral subsurface flow was very weak. Experiments with the one-dimensional LSM, which does not explicitly consider either surface or subsurface lateral flows, produced lower-than-observed ET and CO2 fluxes during drought years and in dry-down periods. However, in the 3-D simulations, the wetter soils in lowland areas along stream channels, mainly induced by lateral overland flow and re-infiltration, provides plants with favorable conditions to produce more persistent leaves, CO2, and ET fluxes during the drought years and dry-down periods.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.H43J..03N
- Keywords:
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- 1813 HYDROLOGY / Eco-hydrology;
- 1818 HYDROLOGY / Evapotranspiration;
- 1838 HYDROLOGY / Infiltration;
- 1850 HYDROLOGY / Overland flow