Soil Moisture: Simulation and reporting of soil moisture as $pF$ values

Introduction

$pF$ values quantify the force with which water is held by the soil matrix. LISFLOOD offers the possibility to calculate $pF$ values from the moisture content of both soil layers. The calculation of $pF$ values is optional, and it can be activated by adding the following line to the lfoptions element in the LISFLOOD settings file:

    <setoption name="simulatePF" choice="1" /> 

Using this option does not influence the actual model results in any way, and it is included only to allow the model user to report pF time series or maps. The actual reporting of the computed pF values (as time series or maps) can be activated using separate options (which are discussed further down).

Calculation of pF

A soil’s $pF$ is calculated as the logarithm of the capillary suction head, $h$:

\[pF = \log_{10}(h)\]

with $h$ in cm (positive upwards). Values of $pF$ are typically within the range 1.0 (very wet) to 5.0 (very dry). The relationship between soil moisture status and capillary suction head is described by the Van Genuchten equation (here again re-written in terms of mm water slice, instead of volume fractions):

\[h = \frac{1}{\alpha} [(\frac{w_s - w_r}{w - w_r})^{\frac{1}{m}} - 1]^{\frac{1}{n}}\]

where $h$ is the suction head $[cm]$, and $w$, $w_r$ and $w_s$ are the actual, residual and saturated amounts of moisture in the soil, respectively, all in $mm$. Parameter $\alpha$ is related to soil texture. Parameters $m$ and $n$ are calculated from the pore-size index, $\lambda$, which is related to soil texture as well:

\[m = \frac{\lambda }{\lambda + 1} \\ n = \lambda + 1\]

If the soil contains no moisture at all ($w=0$), $h$ is set to a fixed (arbitrary) value of $1 \cdot 10^7$ cm.

Reporting $pF$

$pF$ can be reported at the locations defined on the “sites” map or as average values upstream each gauge locatio. The report may be as time series or as maps. To generate time series at the “sites”, add the following line to the lfoptions element of your settings file:

	<setoption name="repPFTs" choice="1" />

To generate maps, use the following lines instead (for the upper and lower soil layer, respectively):

	<setoption name="repPF1Maps" choice="1" />
	<setoption name="repPF2Maps" choice="1" />

In either case, the reporting options should be used in addition to the simulatePF option. If you do not include the simulatePF option, there will be nothing to report and LISFLOOD will exit with an error message.

Preparation of settings file

The naming of the reported time series and maps is defined in the settings file. The two Tables at the end of this page list the settings variables default output names. If you are using a default LISFLOOD settings template, all file definitions are already defined in the lfbinding element.

• Time series:

    <comment>                                                        
    PF TIMESERIES, VALUES AT SITES                                     
    </comment>

    <textvar name="PF1TS" value="$(PathOut)/pFTop.tss">         
        <comment>                                                        
        Reported pF upper soil layer [-]                                 
        </comment>                                                       
    </textvar>

    <textvar name="PF2TS" value="$(PathOut)/pFSub.tss">         
        <comment>                                                        
        Reported pF lower soil layer [-]                                 
        </comment>                                                       
    </textvar>

    <comment>                                                        
    PF TIMESERIES, AVERAGE VALUES UPSTREAM OF GAUGES                   
    </comment>

    <textvar name="PF1AvUpsTS" value="$(PathOut)/pFTopUps.tss"> 
        <comment>                                                        
        Reported pF upper soil layer [-]                                 
        </comment>                                                       
    </textvar>

    <textvar name="PF2AvUpsTS" value="$(PathOut)/pFSubUps.tss"> 
        <comment>                                                        
        Reported pF lower soil layer [-]                                 
        </comment>                                                       
    </textvar>                                                       

• Map stacks:

    <comment>                                              
    PF MAPS                                                  
    </comment>

    <textvar name="PF1Maps" value="$(PathOut)/pftop"> 
        <comment>                                              
        Reported pF upper soil layer [-]                       
        </comment>                                             
    </textvar> 

    <textvar name="PF2Maps" value="$(PathOut)/pfsub"> 
        <comment>                                              
        Reported pF lower soil layer [-]                       
        </comment>                                             
    </textvar>                                             

Table: $pF$ map outputs.

Description	Option name	Settings variable	Default prefix
pF upper layer	repPF1Maps	PF1Maps	pftop
pF lower layer	repPF2Maps	PF2Maps	pfsub

Table: $pF$ timeseries outputs.

Description	Settings variable	Default name
pF at sites (option repPFSites)
pF upper layer	PF1TS	pFTop.tss
pF lower layer	PF2TS	pFSub.tss
pF, average upstream of gauges (option repPFUpsGauges)
pF upper layer	PF1AvUpsTS	pFTopUps.tss
pF lower layer	PF2AvUpsTS	pFSubUps.tss

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