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Deep processes

There are two model options for processes deep below ground; regional groundwater flow and aquifers. The regional groundwater flow model simulates groundwater flow between subbasins, but no explicit storage of deep groundwater. The aquifer model simulates aquifer storage and delay before regional groundwater reaches the destination.

Regional groundwater flow

Soil regional groundwater flow creation

Regional groundwater flow from a subbasin is calculated before the subbasin and class loops. This water is added to the downstream subbasin (in the class-loop part of the code) and/or to the outlet lake of the present subbasin (in the routing calculation part of the code). A subbasin dependent variable (grwtolake) governs the allocation between them. The outflow from a subbasin is calculated as a percentage of the water-retention capacity with a recession coefficient (rcgrw) e.g. for the lowest soil layer (k) the groundwater outflow for a class becomes:

matrix{1}{2} {{q_{grw}(t) = rcgrw * (soil(k)-wp(k)-fc(k))} {soil(k)>fc(k)+wp(k)}}

Groundwater flows are summarized for all classes in proportion to their areas and converted to volume flow out of the subbasin.

Soil addition of regional groundwater flow

The groundwater flow to be added to the bottom layer (and mixed) is added in the class loop portion of the code. If the layer then becomes full of water, it will be pressed into the layer above and mixed, etc. Please note that if downstream subbasin is much smaller than the upstream subbasin, groundwater inflows can become large. If part of the groundwater flow is to go to the outlet lake in the subbasin this is added to the volume of the lake in the routing part of the code.

Regional groundwater to outlet lake

With an input variable in GeoData.txt, an outlet lake may be flagged to receive inflow from the regional groundwater. The coefficient (grwtolake) specifies the portion of the produced regional groundwater flow which is led to the subbasins outlet lake.

Section Symbol Parameter/Data File
Soil regional groundwater flow creation grwtolake, grwdownGeoData.txt
rcgrw rcgrwpar.txt
wp(k)wcwp, wcwp1, wcwp2, wcwp3
fc(k)wcfc, wcfc1, wcfc2, wcfc3
Modules (file) Procedures Sections
regional_groundwater_module (regional_groundwater.f90) calculate_regional_groundwater_flow soil regional groundwater flow creation
calculate_soillayer_groundwaterflow_removal soil regional groundwater flow creation
add_regional_groundwater_flow_to_soil soil addition of regional groundwater flow
add_regional_groundwater_flow_to_olake regional groundwater to outlet lake


Aquifers are large water bodies covering several subbasins and located below the soillayers simulated by HYPE. They recieve water by percolation from land and river classes, and they return flow to a river. The aquifer is a large water body that can have a passive volume partaking in mixing but not return flow.

Figure 1 Structure of an aquifer.

Deep percolation

Deep percolation is determined by a recession coefficient (rc_grw). The recession coefficient is set based on the recession parameter (rcgrwst) which is soil type dependent, but if it is not set the general parameter (rcgrw) is used instead. The recession coefficient can also be regionally adjusted by the correction parameter (aqpercorr).

rc_{grw} = rcgrw(or rcgrwst)*(1+aqpercorr)

Deep percolation from lowest soil layer in model is determined by the recession coefficient and the available water for runoff in the soil layer (qperc (mm/timestep)).

matrix{1}{2}{{q_{perc}(t) = rc_{grw}*(sm-fc-wp)}{sm>fc+wp}}

Deep percolation from main river in model is determined by the recession coefficient and the water in the river (qperc (m3/timestep)).

q_{perc}(t) = rc_{grw}*(waterdepth)

The water is removed from river water and the queue in proportion to their relative volumes.

Deep percolation delay and aquifer recharge

Deep percolation from soil and rivers in all recharge subbasins of an aquifer is added together. This total deep percolation (q_{perc}) is delayed before it recharges the aquifer (q_{rech}).

q_{rech}(t) = (1-coeff) * q_{perc}(t) + coeff * q_{rech}(t-1)

coeff = e ^ {-1/delayts}

The delay property (delayts, in timesteps per day) is specified for each aquifer (delay in days). It can be adjusted by the parameter region dependent model parameter aqdelcor.

Return flow from aquifer

Outflow from the aquifer depends on the water in the aquifer (vol) (not including passive volume) and a rate. The rate is determined by a aquifer property (retrate) and a regional adjustment parameter (aqretcor, parameter region dependent).


The outflow may be divided between several receiving subbasins (retfrac). The water is added to the inflow of the main river of these subbasins.

The return flow is added to the main river of the receiving subbasin.

For most models the organic nitrogen, particulate phosphorus, and organic carbon in water percolating from soil and river stay in the soil/river, while temperature, inorganic nitrogen and SRP follow the water to the aquifer. In the case of a model using the simplified soil model based on travel time through soil and half life of pollutants (i.e. using load as input to HYPE) all substances follow the water to the aquifer.

Inorganic nitrogen is subjected to denitrification in the aquifer (see Chapter Nitrogen and phosphorus in land routines - Denitrification). For other substances an exponential decay can be applied in the aquifer. The process is goverened by the model parameter decay (e.g. ppdecaq), which denote the half life of the substance in days. The decay will decrease the concentration (conc) of the substance in the aquifer.

conc = conc *  2 ^ {-ts/decay} where ts is length of time step in days.

Section Symbol Parameter/Data File
Aquifers aquid, subid, parregAquiferData.txt
basedepth, passivedep, inidepth, porosity, area
Deep percolationrc_{grw} calculated fromrcgrw, rcgrwst, aqpercorrpar.txt
wpwcwp, wcwp1, wcwp2, wcwp3
fcwcfc, wcfc1, wcfc2, wcfc3
Deep percolation delay and aquifer recharge rechargeAquiferData.txt
delay delay
aqdelcorr aqdelcorrpar.txt
Return flow from aquiferretrate, retfrac retrate, retfracAquiferData.txt
aqretcorr aqretcorrpar.txt
Nutrient processes related to aquifers topdepth, temp, conc_IN, conc_SPAquiferData.txt
denitaq, hsatINs, wcep, wcfc, wcwppar.txt
decayondecaq, spdecaq, ppdecaq, ocdecaq, ssdecaq, aedecaq, t1decaqpar.txt
Modules (file) Procedures Sections
regional_groundwater_module (regional_groundwater.f90) calculate_aquifer
calculate_regional_groundwater_flow deep percolation
calculate_percolation_delay deep percolation delay and aquifer recharge
calculate_aquifer_outflow return flow from aquifer
calculate_current_flow_from_aquifer return flow from aquifer
nutrient processes related to aquifers
set_percolation_concentration nutrient processes related to aquifers
npc_soil_processes (npc_soil_proc.f90) soil_denitrification nutrient processes related to aquifers
general_functions (general_func.f90) exponential_decay
start/hype_model_description/aquifier.txt ยท Last modified: 2020/12/02 15:48 by cpers