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).

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_top = TRUE

JULES_HYDROLOGY::zw_max

Type:

real

Default:

None

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

This is the depth from the soil surface to the bottom of an additional layer that is used to track water tables below the standard soil model (which has layer thicknesses given by dzsoil_io). A value of ~10m can often be used (though the previous default value was 6m) - the suitability of any value depends on values of the ancillary variable fexp (see List of TOPMODEL parameters) and the sum of the soil layer thicknesses (denoted sum_dzsoil here). The saturated hydraulic conductivity declines exponentially with depth in the additional deep TOPMODEL layer, with decay parameter fexp, and should be sufficiently small at depth zw_max that the flow at this depth can be neglected, that is EXP(-fexp(zw_max-sum_dzsoil)) should be sufficiently small at all locations. (As a minimum guide, the code tests that the value of this expression is <= 0.05 and a warning is printed where this condition is not met; users should check model output logs for these messages.)

JULES_HYDROLOGY::ti_max

Type:

real

Default:

None

The maximum possible value of the topographic index. A value of 10.0 can be used.

JULES_HYDROLOGY::ti_wetl

Type:

real

Default:

None

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. A value of 1.5 can be used.

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:

None

The number of values tried when fitting wetland and saturation fractions to water table depth in the initialisation. A value of 20 can be used.

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.

Only used if l_pdm = TRUE

JULES_HYDROLOGY::dz_pdm

Type:

real

Default:

None

The depth of soil considered by PDM (m).

A value of ~1m can be used.

JULES_HYDROLOGY::b_pdm

Type:

real

Default:

None

PDM shape parameter (exponent) of the Pareto distribution controlling spatial variability of storage capacity. A value ~1 can be used. b=0 implies a constant storage capacity at all points.

JULES_HYDROLOGY::l_spdmvar

Type:

logical

Default:

F

TRUE

Use a linear function of topographic slope to calculate S0/Smax (the minimum soil water storage below which there is no saturation excess runoff from PDM, expressed as a fraction of the maximum storage Smax): S0/Smax=MAX(0.0,1-(slope/slope_pdm_max)). The slope is read as an ancillary field (see JULES_PDM).

This function will result in high S0/Smax values for flatter regions and low values for steeper regions, and has been tested for catchments in Great Britain.

FALSE

Use a fixed value for S0/Smax, specified in s_pdm.

Only used if l_spdmvar = TRUE

JULES_HYDROLOGY::slope_pdm_max

Type:

real

Default:

None

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.

Only used if l_spdmvar = FALSE

JULES_HYDROLOGY::s_pdm

type:

real

permitted:

0-1

default:

None

Minimum soil water storage below which there is no saturation excess runoff from PDM, expressed as a fraction of the maximum storage Smax)

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