The SL Scheme: A Simplified Land Surface Scheme

by Stefan Hagemann and Lydia Dümenil Gates

 

The Simplified Land surface (SL) scheme (Hagemann and Dümenil Gates, 2003) is mainly used to compute the appropriate input for the hydrological discharge model (Hagemann and Dümenil, 1998) in cases where no daily values of surface runoff and drainage are archived separately by the corresponding atmospheric model. Its structure is shown in Fig. 1. The SL scheme uses daily time series of precipitation and temperature as input, and its time step is one day. It uses the same horizontal resolution as the corresponding input data of precipitation and temperature which, e.g., may be taken from the ECMWF re-analysis or from global or regional climate model output. The SL scheme incorporates the main components of the hydrological cycle at the land surface and primarily uses relations which are functions of temperature and precipitation. The soil is represented by a single soil layer and the following processes are included:

 

# Surface runoff and infiltration according to the improved Arno scheme (Hagemann and Dümenil Gates, 2003)

# Drainage according to the Arno scheme (Dümenil and Todini, 1992; Roeckner et al., 1992)

# Separation of precipitation into rain and snow according to Wigmosta (1994)

# Snowmelt using a daily degree formula according to the HBV model (Bergström, 1992)

# Potential evapotranspiration using the Thornthwaite formula (Chebotarev, 1977)

# Actual evapotranspiration based upon formulations used in the ECHAM model (Roeckner et al., 1992) and on Warrilow et al. (1986) and Bauer et al. (1983)

# A day length correction factor for potential evapotranspiration according to Kramer (personal communication, 1995)

 

The land surface is represented by the following datasets:

 

# Land sea mask, glacier mask, total (field capacity) and plant-available soil water capacity according to Hagemann et al. (1999)

# Seasonally varying vegetation ratio according to Hagemann (2002).

# Lake fraction according to Cogley (1987, 1991, 1994), corrected in several places

# Permafrost areas according to Cogley (1987, 1991, 1994)

# Wetlands fraction according to Matthews and Fung (1987)

 

For special applications where the total water balance should be constrained by the atmospheric model input data, a special version of the SL scheme exists where also the daily evapotranspiration is taken from the atmospheric input.

 

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