Surface urban energy and water balance scheme – SUEWS

Leena Järvi, Sue Grimmond, Heikki Setälä ja Maija Taka

In urban areas the typically high impervious surface fraction affects the surface energy and water balances by increasing turbulent heat emissions to the atmosphere and surface runoff, and by decreasing evaporation. The energy and water exchanges are not commonly measured so modeling approaches are needed particularly for urban planning and mitigation purposes. The Surface Urban Energy and Water Balance Scheme (SUEWS) is a new model which provides a simple tool to simulate both balances through the use of commonly measured meteorological variables and surface cover information. Rates of evaporation-interception for a single layer with multiple surface types (paved, buildings, coniferous trees/shrubs, deciduous trees/shrubs, irrigated grass, nonirrigated grass and water) are calculated. Below each surface type, except water, there is a single soil layer. At each time step (5 min to 1 h) the moisture state of each surface and soil are calculated. Horizontal water movements at the surface and in the soil are incorporated. In order to minimize the number of required input variables, the model contains several sub-models for e.g. net all-wave radiation (Q*), storage heat (ΔQs) and anthropogenic heat flux (QF) and external irrigation. SUEWS has been tested with three datasets from Vancouver and two from Los Angeles, where it was found to simulate Q*, QH and QE well with rmse between 25-47, 30-59 and 20-55 W m-2, respectively. In addition, the model was able to simulate surface wetness and soil moisture changes. It has also been tested to simulate surface runoff from two catchments in Helsinki with varying surface cover in October – November 2010. For both catchments SUEWS simulated the behavior of measured runoff well but overestimates runoff values slightly when applied in an uncalibrated manner with the default model parameters.