6.8. jules_hydrology.nml

This file sets the hydrology options. It contains one namelist called JULES_HYDROLOGY.

6.8.1. JULES_HYDROLOGY namelist members

JULES_HYDROLOGY::l_top

Type:logical Default:F

Switch for a TOPMODEL-type model of runoff production.

TRUE

Use a TOPMODEL-type scheme. This is based on Gedney and Cox (2003); see also Clark and Gedney (2008).

FALSE

No TOPMODEL scheme.

See also

References:

• Gedney, N. and P.M.Cox, 2003 , The sensitivity of global climate model simulations to the representation of soil moisture heterogeneity, J. Hydrometeorology, 4, 1265-1275.
• Clark and Gedney, 2008, Representing the effects of subgrid variability of soil moisture on runoff generation in a land surface model, Journal of Geophysical Research - Atmospheres, 113, D10111, doi:10.1029/2007JD008940.

JULES_HYDROLOGY::l_pdm

Type:logical Default:F

Switch for a PDM-type model of runoff production.

PDM is the Probability Distributed Model (Moore, 1985), implemented in JULES following Clark and Gedney (2008).

TRUE

Use a PDM scheme.

FALSE

No PDM scheme.

See also

References:

• Moore, R. J. (1985), The probability-distributed principle and runoff production at point and basin scales, Hydrol. Sci. J., 30, 273-297.
• Clark and Gedney, 2008, Representing the effects of subgrid variability of soil moisture on runoff generation in a land surface model, Journal of Geophysical Research - Atmospheres, 113, D10111, doi:10.1029/2007JD008940.

Note

Setting l_top = FALSE and l_pdm = FALSE selects a more basic runoff production scheme. In this scheme, surface runoff comes only from infiltration excess runoff (no saturation excess runoff), and subsurface runoff comes only from free drainage from the deepest soil layer (no lateral flow from mid-layers), as described in Essery et al. (2001, HCTN 30).

Only used if l_top = TRUE

JULES_HYDROLOGY::zw_max

Type:real Default:6.0

The maximum allowed depth to the water table (m).

This is the depth to the bottom of an additional layer below the sm_levels soil layers and hence should be set to a value greater than SUM(dzsoil).

JULES_HYDROLOGY::ti_max

Type:real Default:10.0

The maximum possible value of the topographic index.

JULES_HYDROLOGY::ti_wetl

Type:real Default:1.5

A calibration parameter used in the calculation of the wetland fraction.

It is used to increment the “critical” value of the topographic index that is used to calculate the saturated fraction of the gridbox. It excludes locations with large values of the topographic index from the wetland fraction.

Note

When TOPMODEL is on (i.e. l_top = TRUE), JULES follows Gedney & Cox (2003, J Hydromet, eqn 14) in assuming that wetlands occur where gridcell elevation is low enough (assumed to be where topographic index is large enough) that the water table is above the land surface (topidx > ti_wetl) but not above the land surface by enough that streamflow may be assumed to occur (topidx < ti_max). Both ti_wetl and ti_max are levels calibrated from observed wetland fractions. So, if the water table is above the surface then JULES can calculate an areal fraction of total inundation (fsat) and also the areal fraction that is inundated but shallow enough to be stagnant/non-flowing (fwetl, with fwetl<=fsat), which is the ‘wetland fraction’.

JULES_HYDROLOGY::nfita

Type:integer Default:20

The number of values tried when fitting wetland and saturation fractions to water table depth in the initialisation.

This controls the range of cfit values tried in calc_fit_fsat.F90 where cfitmax = 0.15 * nfita

JULES_HYDROLOGY::l_wetland_unfrozen

Type:logical Default:F

TRUE

Treat the calculations of wetland and surface saturation fractions more like those of an unfrozen soil.

FALSE

Use standard wetland and surface saturation fraction calculations.

JULES_HYDROLOGY::l_limit_gsoil

Type:logical Default:F

TRUE

Limit the soil conductance to the value when the top layer soil moisture is at the critical soil moisture. Below this threshold, the soil conductance follows Best et al. (2011) equation 7.

FALSE

Allow the soil conductance to increase as the top layer soil moisture goes above the critical soil moisture, as in Best et al. (2011) equation 7.

Only used if l_pdm = TRUE

JULES_HYDROLOGY::dz_pdm

Type:real Default:1.0

The depth of soil considered by PDM (m).

A value of ~1m can be used.

JULES_HYDROLOGY::b_pdm

Type:real Default:1.0

PDM shape parameter / exponent of the Pareto distribution controlling spatial variability of storage capacity. Value b=0 implies a constant storage capacity at all points.

JULES_HYDROLOGY::s_pdm

Type:real Permitted:0-1 Default:0.0

Minimum soil water content below which there is no surface runoff saturation excess production by PDM (fraction of maximum storage, as S0/Smax)

Default value is 0 - i.e. surface saturation can occur for any value of water storage. A value of, e.g., 0.5 would indicate that no surface runoff is produced until the soil is 50% saturated.

JULES_HYDROLOGY::l_spdmvar

Type:logical Default:F

Use a linear function of topographic slope to calculate S0/Smax: s_pdm=MAX(0.0,1-(slope/slope_pdm_max)).

The function has been tested for Great Britain catchments.

This function will result in high S0/Smax values for flatter regions and low values for steeper regions.

TRUE

Use a linear function of slope to calculate S0/Smax.

FALSE

Use a fixed S0/Smax parameter, specified in s_pdm.

Only used if l_spdmvar = TRUE

JULES_HYDROLOGY::slope_pdm_max

Type:real Default:6.0

The maximum topographic slope (deg) in the linear function of slope to calculate S0/Smax. Slopes above this value will result in a S0/Smax value of zero.

A value of 6.0 has been tested for slope fields calculated from a high resolution DEM dataset (50m IHDTM for Great Britain).

For slopes calculated from coarser DEM datasets, a lower value might be more appropriate as fine-resolution features of the terrain are not included.