Differences in Net Ecosystem Exchange for an intensely managed watershed using a lumped, regional model and a mechanistic, hillslope-scale model

Abstract:

In this study, Net Ecosystem Exchange (NEE), and its components Gross Ecosystem Exchange (GEE) and Ecosystem Respiration (RESP), were compared from a lumped, regional model and a mechanistic, hillslope-scale model to determine if the effects of land management on the carbon cycle are captured by larger-scale biosphere models that determine CO₂ sources and sinks. WRF-VPRM (Weather Research & Forecasting – Vegetation Photosynthesis & Respiration Model) is a regional-scale model that uses simulated downward shortwave radiation and surface temperatures, along with satellite-derived land cover indices and eddy flux tower-derived parameters to estimate biosphere CO₂ fluxes with empirical equations. The DAYCENT biogeochemical model coupled with the Watershed Erosion Prediction Project model (WEPP), which simulates changes in soil carbon stocks due to different land management and the resulting enhanced erosion, can also quantify biosphere CO₂ fluxes. Both models (i.e., WRF-VPRM and WEPP-DAYCENT) were used to quantify GEE, RESP, and NEE for the summer of 2008 in the IML-CZO Clear Creek watershed of the U.S. Midwest to examine the role of land management heterogeneity in CO₂ exchanges between the biosphere and atmosphere. Comparing average daily GEE rates from WRF-VPRM (-11.0 ± 5.2 g C/m²/d) with WEPP-DAYCENT average values weighted per land use area in the watershed (-10.2 ± 1.5 g C/m²/d) showed no significant differences (t-test; p=0.08). In contrast, daily RESP values were different between the two models. Daily respiration rates were relatively constant for WRF-VPRM (6.0 ± 0.8 g C/m²/d), while WEPP-DAYCENT values for each management practice were significantly greater (7.2 ± 1.8 g C/m²/d; t-test, p<0.001) with the land uses experiencing tillage having the widest ranges. WEPP-DAYCENT accounts for the effects of land management and net erosion/deposition on total SOC stocks and tillage impacts on respiration by increasing decomposition from the breaking of soil aggregates and enhanced mineralization. In WRF-VPRM, respiration is calculated with a regression equation based on air temperature. As a result, comparison of average daily NEE rates from WRF-VPRM (-5.0 ± 5.3 g C/m²/d) with WEPP-DAYCENT average weighted values (-3.0 ± 1.8 g C/m²/d) also showed significant differences (t-test; p<0.001).