Implementation of a multiple flow algorithm into the 1 dynamic ecosystem model LPJ - GUESS

13 Abstract— The dynamic ecosystem model LPJ-GUESS includes 14 explicit representation of vegetation dynamics as well as soil 15 biogeochemistry, and has been widely and successfully 16 implemented in predicting vegetation biomass and carbon cycling 17 at different scales. However, the water cycling for each grid cell in 18 the model is only considering the movement between atmosphere, 19 vegetation and soil, ignoring the lateral water movement between 20 grid cells. A previous study has proposed a distributed scheme in 21 LPJ-GUESS incorporating topographic indices to redistribute 22 lateral water movement, and has demonstrated the impacts on 23 ecological functioning and carbon cycling at the Stordalen 24 catchment, northern Sweden. The topographic indices, extracted 25 based on a Digital Elevation Model (DEM), were based on a single 26 flow (SF) algorithm at 50 m resolution, restricting the flow 27 movement to the downslope cell with maximum gradient. In this 28 study we have incorporated the Triangular Form-based Multiple 29 Flow algorithm (TFM) to redistribute lateral water in LPJ-GUESS 30 and analyzed the influences and differences between the two flow 31 algorithms on runoff prediction as well as carbon cycling 32 estimations. The results indicate that the runoff estimated by the 33 TFM algorithm is more realistic than the SF algorithm. Besides, the 34 comparison with observed runoff data demonstrates the monthly 35 runoff estimated using the SF algorithm tends to overestimate the 36 runoff in May and June as well as in the lower flatter peatland 37 region. For the TFM algorithm, the underestimated runoff during 38 the growing season can be compensated by the decreased soil depth 39 in the elevated area. Moreover, the implementation of the TFM 40 algorithm results in a significant increase of the catchment mean 41 value of vegetation uptake of carbon as well as net ecosystem 42 exchange carbon. We conclude that the advanced multiple flow 43 algorithm (TFM) with more accurate estimation of flow 44 accumulation can improve the hydrological predictions in LPJ45 GUESS. Meanwhile, the results have proved that the flow routing 46 algorithms do influence the vegetation pattern estimations for the 47 study area. 48 INTRODUCTION 49