The influence of lateral flow on land surface fluxes in southeast Australia varies with model resolution

Abstract. Land surface models (LSMs) used in climate models typically represent surface hydrology as one-dimensional vertical fluxes, neglecting the lateral movement of water within and between grids. It is assumed that lateral flow of water has a negligible impact on land surface states at climate modelling resolutions of a few tens of kilometres. However, with increases in model resolution, it may be necessary to include lateral flow in LSMs as satellite observations indicate the influence of this process on ecohydrological states, particularly in water limited regions. Lateral flow has not been modelled in Australia, but there is some evidence that this process exerts a dominant influence on vegetation variability in arid and semi-arid Australia. Here we use standalone WRF-Hydro simulations to quantify the influence of overland and shallow subsurface lateral flow on surface fluxes in southeast Australia, and the impact of model resolution on the results. We perform LSM simulations at 1-km, 4-km, and 10-km resolutions, with and without lateral flow, to assess the changes in evapotranspiration. Our results show that lateral flow increases evapotranspiration near major river channels in LSM simulations at 4- and 1-km resolutions, consistent with high-resolution observations. The largest changes occur in the warm season after a wet winter, with magnitudes of 50 % or more in some areas. However, the 1-km resolution simulations also exhibit a widespread pattern of drier ridges, different from the coarser resolutions. At 10-km resolution the increases in evapotranspiration are confined to the mountainous regions. Our results suggest that it may be necessary to include lateral flow in LSMs for improved simulations of droughts and future water availability at resolutions higher than 10 km.