ORCHIDEE MICT-LEAK (r5459), a global model for the production, transport and transformation of dissolved organic carbon from Arctic permafrost regions, Part 2: Model evaluation over the Lena River basin

Abstract. In this second part of a two-part study, we perform a simulation of the carbon and water budget of the Lena catchment with the land surface model ORCHIDEE MICT-LEAK, enabled to simulate dissolved organic carbon (DOC) production in soils and its transport and fate in high latitudes inland waters. The model results are evaluated in their ability to reproduce the fluxes of DOC and carbon dioxide (CO 2 ) along the soil-inland water continuum, and the exchange of CO 2 with the atmosphere, including the evasion outgassing of CO 2 from inland waters. We present simulation results over years 1901–2007, and show that the model is able to broadly reproduce observed state variables and their emergent properties across a range of interacting physical and biogeochemical processes, including: 1) Net primary production (NPP), respiration and riverine hydrologic amplitude, seasonality and inter-annual variation; 2) DOC concentrations, bulk annual flow and their volumetric attribution at the sub-catchment level; 3) High headwater versus downstream CO 2 evasion, an emergent phenomenon consistent with observations over a spectrum of high latitude observational studies. (4) These quantities obey emergent relationships with environmental variables like air temperature and topographic slope that have been described in the literature. This gives us confidence in reporting the following additional findings: (5) Of the ~ 34 TgC yr -1 left over as input to terrestrial and aquatic systems after NPP is diminished by heterotrophic respiration, 7 TgC yr -1 is leached and transported into the aquatic system. Of this, over half (3.6 TgC yr -1 ) is evaded from the inland water surface back into the atmosphere and the remainder (3.4 TgC yr -1 ) flushed out into the Arctic Ocean, proportions in keeping with other, empirically derived studies. (6) DOC exported from the floodplains is dominantly sourced from recent, more "labile" terrestrial production, in contrast to DOC leached from the rest of the watershed with runoff and drainage, which is mostly sourced from recalcitrant soil and litter. (7) All else equal, both historical climate change (a spring/summer warming of 1.8 °C over the catchment) and rising atmospheric CO 2 (p85.6 ppm) are diagnosed from factorial simulations to contribute similar, significant increases in DOC transport via primary production, although this similarity may not hold in the future.