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Rivers and lakes

Basic assumptions

The HYPE model simulates most lakes and rivers in a conceptual way. The HYPE model can contain two types of rivers, local river and main river, and two types of lakes, local lakes and outlet lakes (Figure 1). The local river represent all ditches, streams, rivers within a subbasin that recieve only runoff within the subbasin. It is modelled as one conceptual river with a length, regardless of the actual distance runoff from different land has to travel through the subbasin. Similarly a local lake represents all lakes within a subbasin receiving inflow only from within the subbasin. The modelled local lake recieves a fraction of the flow in the local river. The fraction is defined as a fraction of the subbasin area. The main river is the river part(s) of a subbasin that water from upstream subbasins flows through. Depending on subbasin delieation it may be clearly defined or represent several tributaries that comes to the subbasin outlet. Finally the modelled outlet lake represents a lake located at the outlet of a subbasin.

Local and main rivers are present in all subbasins and the length of each watercourse is calculated as the square root of the subbasin area, unless it is given as input. The river can be a SLC class and has then an surface area, but rivers can also be one-dimensional (i.e. no fraction of the subbasin area are occupied by the river and no precipitation added to the river). All runoff enters the local river. Local lakes (ilake) receive a portion of the local flow. The flow leaving the local river and the outflow from local lake, goes to the main river of the same subbasin. If there are upstream subbasins their flow is added to the main river together with the local flow. Outlet lakes (olake) receive the outflow from the main river, i.e. all upstream and local flows.

Figure 1: Schematic representation of streams and lakes i HYPE, and the link between them.

The two lake types are separate classes. The lake classes have characteristics such as land use and soil type, which are defined together with the other classes' characteristics (in GeoClass.txt). Precipitation, atmospheric deposition and evaporation of rivers and lakes are calculated first, while river flow and inflow, transformation processes and the outflow of the lakes is calculated thereafter. Lakes and rivers are calculated in the model’s routing part after all classes are calculated for the subbasin.

An outlet lake can be part of a larger lake. It is then called a lake basin. Lake basins are olakes in nearby subbasins.

A simple outlet lake has a threshold. The outflow ends if the water level drops below the threshold. Lake mean depth below the threshold is specified in GeoData.txt or LakeData.txt as lake_depth in meters. Lake depth can also be set by parameters, i.e general parameter gldepo or olake region parameter olldepth. The threshold is also the the water level of the lake at the start of a simulation. The current water level is denoted as wlm in Fig. 2. For printing, the outlet lake water level (output variable wcom) is calculated in meters and you can set a reference level (w0ref) in LakeData.txt to get the same height system as any observations of the lake's water level. The lake’s w0ref is added to the water level above the threshold. A regulated lake (dam) has two thresholds. One, same as for a simple lake, is used for spill, and one lower threshold were outflow ends completely. The distance between the thresholds are determined by the regulation volume. HYPE assumes the lake/dam has vertical sides in the calculations, thus the observed variation in water level may be larger than the simulated variation. It is therefore possible to adjust the output wcom (and wcav) for the actual amplitude of the regulation volume (wamp). This will make the simulated and recorded water stage comparable below the threshold for a regulated lake.

Figure 2: An outlet lake with some variables.

A local lake also has a threshold depth that is used as start value. The depth is given by general parameter gldepi and is then the same for all the local lakes, or by ilake region parameter illdepth. It is measured in meters. A percentage of flow from the local stream flows into the local lake. The rest of the local flow runs directly to main river watercourse.

The streams may have a delay and an attenuation of the flow. The former shifts the peaks, but leaves them otherwise untouched, while the attenuation both delays and smooths out peaks. The streams have a dead volume in which the residence time of solutes is increased relative to the water and the concentration smoothed out further over time.

Parameter/Data File
rivlen, loc_rivlen, slc_nnGeoData.txt
special class code 1,2,11 and 12GeoClass.txt
lake_depthGeoData.txt or LakeData.txt or DamData.txt
gldepo, olldepth, gldepi, illdepthpar.txt
w0ref, wampLakeData.txt or DamData.txt
Modules (file) Procedures
modelmodule (model_hype.f90)initiate_model_state
surfacewater_processes (sw_proc.f90) calculate_riverlength


Common rivers processes


Daily precipitation is added to the river if it has an area (is a class) and a new concentration is calculated. Precipitation is divided between river water in damping box and queue according to the respective volumes.


If the river has an area (is a class), it evaporates and a new concentration is calculated. Normally the river area is constant over time, but with parameters a reduction of riverarea can be simulated for low volume/flow. The reduced river area is also used for heat exchange calculations.

frac_area = delim{lbrace}{ 
    0 {x<=0}
    {(1+(x-fraxe)/(fraxe-fraxm))*(x/fraxe)^{x/(x-fraxm)}}  {0<x<fraxe}
    1 {x>=fraxe} 

The parameters fraxe and fraxm are general, and fraxe is (minimum) mean river depth (m) where fractional river area = 1 and fraxm is mean river depth (m) where the slope of the fractional river area has its maximum (must be in the range between 0 and fraxe). x is the current mean river depth (based on full area extention).

Pure delay

The delay in the watercourse (transtime) in days is determined by the length of the watercourse (rivlen) and the water’s maximum velocity (rivvel). The maximum velocity is a general parameter with unit m/s. The delay in the river is dependent on subbasin land area if the default river length is used. The delay is a pure translation. The delay is divided into whole days (ttday) and parts of the day (ttpart) (if the time step is daily, other wise the division is made into whole and fractions of that time step). In the following description daily time step is assumed.

transtime = rivlen/{rivvel*8.64*10^4}

ttday = INT(transtime)

ttpart = transtime - REAL(ttday)

The inflow of the river is stored in two arrays (riverq and riverc) until it is time for it to flow out of the river stretch. The outflow is weighted by using the parts of the time step (ttpart) that are to flow out during the time step.

transq = (1-ttpart)*riverq(ttday) + ttpart*riverq(ttday+1)
  transc = ((1-ttpart)*riverq(ttday)*riverc(ttday) + 
             ttpart*riverq(ttday+1)*riverc(ttday+1)) / transq
  transc = 0.

After the calculation of outflow the values in the arrays are shifted forward one time step.

Delay and attenuation

If the delay in the watercourse also includes a damping of the peaks then only part of the delay is considered as translation, while the rest of the delay lies in damping. The translation is calculated first and then the flow goes through a linear box that creates attenuation. The parameter damp indicates how much of the delay that will occur in the attenuation box (or damping box), and translation time is reduced accordingly. Otherwise the translation is calculated in the same manner as above.



The result from the translation of water (transq and transc) flows into the attenuation box, which is assumed to be completely mixed. The delay time in the box (kt) is recalculated to a corresponding recession coefficient (riverrc). The recession coefficient (riverrc) used to calculate the outflow from the box (dampq) is a function of volume in the box (riverbox).

kt = damp * totaltime

riverrc = 1. - kt + kt * e^{-1{/}kt}

dampq = riverrc * riverbox

The delay in the attenuation box is at the centre of gravity of the flow (not the tip) and is not calculated exactly (see Derivation of the equation of recession in the attenuation box below). Therefore there may be a difference in a peak delay between the use of pure delay and delay with damping, although the maximum velocity is set to the same value. In the case of damping the max velocity parameter rivvel may be approximated.

Additional smoothing of dead volume

A dead volume (deadriver) can be used to get extra equalization of concentrations in the watercourse. Then the outflow equation for the attenuation box (riverbox) is:

dampq = riverrc*(riverbox - deadriver)

In this case, the initial value of the attenuation box is equal to the dead volume. Inflows into the box are mixed with the entire volume. At present this represents a dilution of the concentration since the initial value of concentration in the box is zero. At present the estimated dead volume as dependent on the upstream area (inkluding the area of the subbasin itself. Depth is calculated from the parameter dead (deadl or deadm, m^2{/}km^2) and the upstream area (km^2), the width is assumed to be 10 times the depth and the river length (m) is estimated to be the root of the subbasin’s area unless it is set among the input data. This gives the dead volume:

deadriver = dead * upparea * rivlen

Dead volume can be used even without suppression of water (damp = 0).

Derivation of the equation of recession in the attenuation box

The continuous-time equation for the change in volume S in a reservoir is:


where i(t) is influx and q(t) is outflow. If we assume that the outflow is proportional to the volume we get another expression for the change in S. k is in the unit of time.



Combining these two equations gives a first degree equation that can be solved using the method of “integrating factor”:

dq/dt + 1/k~q=i/k,

{d e^{t{/}k}q}/dt=i/k~e^{t{/}k},

q(t)=1/k int{}{}{i(t)e^{delim{(}{tau-t}{)}{/}k}d tau}

In our case, we have a constant influx during a time steps of length 1 day. Assume that the influx during a time steps are I and the outflow Q. These flows are thus the equivalent discrete flows. Because I is constant during the time step (i(t) = I) Q can be calculated. Define y(t)=e^{t/k}q(t) and integrate the second equation above, from 0 to t (t = 1, one time step).

int{y(0)}{y(t)}{dy/dt} d tau = int{0}{t}{{i(tau)}/k} e^{\tau{/}k}d tau,

y(t)-y(0)=I/k int{0}{t}{e^{tau{/}k}} d tau = I~(e^{t{/}k}-1)

e^{t{/}k} q(t) - q(0) = I~(e^{t{/}k}-1)


Calculate Q for time step (the mean of q(t)) and replace the outflow with the volume.

Q = {int{0}{1}{} q(t) dt} / {int{0}{1}{} dt} = int{0}{1}{} q(0)e^{-t{/}k} + I~(1-e^{-t{/}k})dt

{}= q(0) int {0} {1}{} e^{-t{/}k} dt + I~ int{0}{1}{} 1-e^{-t{/}k} dt

{}= q(0)*(-k~(e^{-1{/}k}-1))+I(1-0)-I~(-k~(e^{-1{/}k}-1))

{}= I~(e^{-1{/}k}-1)~(kI - kq(0))


Recession coefficient is defined as the proportion of outflow after an inflow pulse. It is therefore calculated here as r=Q{/}I under the assumption that the starting volume can be neglected, s(0)=0.


For a real flow the volume can of course not be neglected in this way and the relationship is only approximate.

Bank-full flow

Bank-full flow is used for erosion of particulate phosphorus in rivers (see Sedimentation/Resuspension). It is approximated by the second highest daily flow during the last year.

Local river

The local river has a length equal to the square root of the subbasin area, if not specified as input. Runoff from the land area of the subbasin forms the inflow to the local river. The flow in the local river is delayed and attenuated as described above. Of the resulting flow from the stream a constant percentage goes to the local lake (icatch), and the rest directly to the main river.

Main river

A main river is present in all subbasins. The length is equal to the square root of the subbasin area, if it is not specified as input. In areas without incoming water from upstream there is still a main river, but it receives only local river flow after the local lake. In subbasins with upstream incoming water, the flow to the main river will be the sum of outflow from the local lake, the proportion of flow in the local river not flowing into the local lake and the water from upstream.

The return flow from an aquifer is added to the inflow of the main river.

The flow in the main river is delayed and attenuated as described above. The resulting effluent from the river flows to the outlet lake or to the next subbasin if there is no outlet lake.

If the main river length is zero, the inflows to the main river is added to form the main river outflow.

Inflow from upstream subbasins

In input files it is given to which subbasin(s) the outflow from each subbasin flows. The upstream flow enters the main river of the downstream subbasin. Inflow into the main river of a subbasin is calculated by adding outflows from upstream areas. Concentrations are flow-weighted by their relative share. The upstream flow enters the main river, except if the upstream subbasin has a lakebasin that is part of the downstream subbasin's lake. In this case the upstream lakebasin outflow goes directly into the downstream lakebasin.

Hydraulic geometry and river rating curve

River diagnostic variables can be calculated for the main river. A river water depth (d) and river velocity (u) can be calculated from hydraulic geometry for the main river and the calculated main river flow (q). In the case of updating of flow in a subbasin with no outlet lake, the updated (main river) flow is used. The equation coefficients are general parameters (c,f,k,m).



With a given rating curve for main river flow, the water level of the river (wr) can be calculated by the inverse of the rating curve. Basin dependent values for threshold (w0) and the rating curve coefficients are input. Two set of rating parameters may be given for each main river, one for general and ice free condition and one for use with ice on the river. The rating curve for ice conditions are only used if one is given for no ice conditions.


give coefficients a and b for the inverse equation:

a={1/k}^{1/p}  b=1/p

wr = a*q^b + w0.

The transition between ice and no ice rating curves can be smoothed by scaling of the curve in the transition period. The scaling of rating curve parameters are determined by ice porosity and ice thickness thresholds (general model parameters). Between the thresholds the influence of the ice curve increases linearly from zero to one. The minimum influence (of the two) is used as the scaling weight of the winter curve during the transition period. If no winter curve is given, the winter curve is assumed to be composed of summer curve's rate and the exponent one.

Section Symbol Parameter/Data File
Common rivers processesrivlenrivlen, loc_rivlenGeoData.txt
fraxe, fraxm, rivvel, dampfraxe, fraxm, rivvel, damppar.txt
deaddeadm, deadl
Local river icatchGeoData.txt
gicatch, illicatchpar.txt
Main river maindownGeoData.txt
c, f, k, m hygeomc, hygeomf, hygeomk, hygeommpar.txt
ricew0por, ricew1por, ricew0ice, ricew1ice
k, p, w0 mrrivck_noi, mrrivck_ice, mrrivcp_noi, mrrivcp_ice, mrrivcw0GeoData.txt
Modules (file) Procedures Sections
modelmodule (model_hype.f90) initiate_model_state rivers
initiate_model local river
main river
surfacewater_processes (sw_proc.f90) add_precipitation_to_river precipitation
calculate_river_evaporation evaporation
translation_in_river pure delay
calc_qbankbank-full flow
river_water_level main river


Common lake processes


Daily precipitation is added to the lake and a new concentration is calculated. Lakes are assumed to be completely mixed.


The lake is assumed to evaporate at potential evaporation rate (see Processes above ground - Evaporation) and a new concentration calculated. The actual evaporating area is reduced if the lake is (partly) ice covered. Ice is calculated in case water temperature (substance T2) is simulated.

Outflow with rating curve

The rating curve is used for calculation of outflow above a threshold at several occasions.


In the program the equation is solved with linearization (Lindström, 2016). For rating curve exponent equal to one, the solution is exact, while for other exponent an approximation is used.

Local lake (ilake)

The local lake has an initial volume which is determined by its area and threshold depth. The lake depth below the threshold is determined by model parameters (gldepi or illdepth).

The inflow to the lake is a percentage of the flow in the local stream (icatch). The percentage is determined by the percentage of the subbasin area that drains to the lake. This percentage can be given as a fraction in GeoData.txt for each subbasin with an internal lake, or be given by a regional parameter ilicatch, or be given by a general parameter gicatch. If not set at all the default value is 1, i.e. the local river runs through the local lake. The flow from the local river is added to the lake. The lake water is assumed completely mixed.

Water outflow is calculated with the universal rating curve using general or region specific model parameters.

There is an alternative model for local lakes (a model option), the ilake connectivity model. The ilake connectivity model simulates the ilake as a chain of small connected lake sections (lks) with different height of their outflow threshold. Depending on water volume in the ilake one or several of the lake sections can be disconnected from the chain (zero outflow with water level below the threshold). The precipitation and evaporation are divided by the lake sections depending on their fraction of the ilake area, but the runoff from land area is divided by the lake sections depending on their fraction of the catchment area. In case the water level of a lake section is above its threshold, its outflow is calculated. The outflow of each lake section is calculated using the universal rating curve using general or region specific model parameters same as a simple ilake. The outflow of a lake section is of course also added to the receiving lake section before that one’s outflow is calculated. Depending on how far upstream from the last lake section of the chain there is inflow from the above lake section, that fraction of the ilake area is the ilake’s current connectivity. The catchment area of the lake sections above this border is the currently non-contributing catchment area.

Simple outlet lake or dam (olake)


The outlet lake receives both local runoff and inflow from upstream areas via the main river. An outlet lake may receive inflow from regional groundwater flow.

Outflow with universal rating curve

If the water level (wlm) is higher than the threshold (lake_depth) the outflow is calculated with the following equation:

outflow = delim{lbrace}{matrix{3}{3}{{0} {,~for} {wlm<lake_depth} {gratk*uparea^{grata}*ratcorr*(wlm-lake_depth)^{gratp}} {,~for} {grata>0} {gratk*ratcorr*(wlm-lake_depth)^{gratp}} {,~for} {grata=0} }}{}

gratk, gratp and grata are general parameters that apply to all lakes in the model application. ratcorr is parameter region dependent model parameter for adjusting gratk to different regions. The upstream area (uparea, km^2) is included in the equation if parameter grata is >0. If the water level is below the threshold then the outflow is zero.

Outflow with specific rating curve

In LakeData.txt a specific rating curve may be set instead of the general parameters. This is done by setting the rate and exp larger than zero in LakeData.txt.

outflow = rate * (wlm - lake_{depth}) ^ {exp}

Regulated lake

A simple regulation routine can be given in LakeData.txt for selected outlet lakes. There are two main versions of regulations. The first determine a (constant) production flow between the lake threshold and a lower water stage. The second determine different rating curves for two production periods.

Production flow

The regulated outflow is independent of water level (wlm (m)) between the threshold (w0=lake_depth) and a minimum water level (wmin). For this interval the production flow (qprod) is used. Production flow may depend on time of year and the water level. The minimum water stage is not really a water stage, but is calculated from the regulation volume (regvol) and the lake (surface) area. Thus it does not consider the area changing with depth.

Figure 3: Illustration of regulation flow and variables.

For water levels above the threshold, the flow is calculated with a rating curve (typically flow through spillways) or all the water is discharged, but it is at least equal to the production flow (qprod). Compiled together as one equation, the outflow from a regulated lake is:

outflow = delim{lbrace}{matrix{4}{3}{{MAX({rate * (wlm-w0)}^{exp}, qprod)} {,~for} {wlm>w0,rate>0} {MAX((wlm-w0) * 1000 / qunitfactor, qprod)}{,~for}{wlm>w0,rate<=0} {MIN(qprod,(wlm-wmin) * 1000 / qunitfactor)} {,~for} {wmin<wlm<w0}  {0} {,~for} {wlm<wmin}}}{}

Input variables rate, exp, qprod, regvol, w0, qamp and qpha can be found in LakeData.txt. The variable wmin is calculated by the program from regvol and lake area:

wmin = w0 - regvol * 1000000 / area

Production flow can have two different values during the year, which depends on the day of the year. This is determined by the input variables qprod1, qprod2, datum1 and datum2. Regulation period 1 between datum1 and datum2 has production flow qprod1, while the rest of the year has production flow qprod2. Not setting the dates gives the same production flow the whole year (qprod1). Production flow can alternatively be made to vary sinusoidal over the years, with a peak in December, when power output is normally high, and a minimum in June. This is done with the input variable qamp. If you want a different seasonal variation set qpha (default = 102).

qprod = qprod*(1. + qamp * sin(2*pi*(dayno+qpha)/365))

In addition, the production flow is reduced from this level when there is a low water level in the dam. The flow will then be reduced linearly from full production flow for that time of year when water level is over the limit (limqprod (percentage of volume)) to zero when no water is left in the dam over wmin level.

Two rating curves

Regulated outflow can also be calculated from different rating curves for the regulation periods. The rating curve parameters are the specific rate and exponent of is lacking the general rate and exponent.


Rating curve threshold (w0) determined by input variable w0ref is used for regulation period 1, while the rating curve threshold is changed for regulation period 2. The shift is determined by input variable deltaw0:

w0 = w0ref + deltaw0

Dams of specific purpose

In addition to regulated lakes (described above), dams in HYPE can instead be regulated by a specific purpose. Dams are then governed by the main purpose of the dam, e.g. hydropower, flood control. The rules of calculating the outflow of the dam then depends on the specified purpose of each dam. The outlet lakes that are regulated by a specified purpose is given in DamData.txt. A dam without defined purpose may use the methods given in previous sections (and be defined in LakeData.txt).

Dams are characterised by their regulation volume (regvol) between a minimum water stage (wmin) and the spill threshold (w0). The minimum water stage is not a real a water stage, but is calculated from the regulation volume (regvol) and the dam (surface) area. Thus it does not consider the area changing with depth. Below the minimum water level (wmin) no outflow occur.

For dams a preferred production flow is calculated (see below). This is released if the dam in within the regulation volume, but no more water than to the minimum threshold is released. If instead the water level of the dam is above the threshold w0, the spillway equation (rating curve) is used. The outflow is the maximum of the spillway equation and the preferred production flow.

Input variables rate, exp, regvol, qamp and qpha is given in DamData.txt.

General preferred production flow

For all dams the general preferred production flow is calculated from input data on the dam. The flow that is preferred to be released from the dam today is then adjusted depending on purpose. The general production flow (qprod) may be constant or vary between two values per year. Typically it is estimated from the yearly average inflow of the dam, which is given as input data. It can alternatively be determined by input variables qprod1, qprod2, datum1 and datum2, which then has to be estimated. Regulation period 1 between datum1 and datum2 has production flow qprod1, while the rest of the year has production flow qprod2. Not setting the dates gives the same production flow the whole year (qprod1). IF qprod1 is not given qprod is estimated from the yearly average inflow of the dam, which is given as input data.

The production flow may be reduced from this level when there is a low water level in the dam. The flow will be reduced linearly from full production flow for that time of year when water level is over the limit (limqprod (percentage of volume)) to zero when no water is left in the dam over wmin level.

1 - Irrigation dam and 2 - Water supply dam

Dams of these purposes uses the general perferred production flow as it is.

3 - Flood control dam

The flood control dam will try to stay empty and ready to delay large inflows. The preferred production flow is calculated from the inflow of the day (Qinftoday) for low inflow and water stages, but set higher than the inflow for high flows and water levels. The maximum flow (qthresh, the threshold flow) is determined from input data (Qinf is average inflow to dam for each month) and a general parameter (kthr), while the water level threshold (wthres) is determined by a general parameter (klow) for the fraction of the regulation depth.

qthresh = kthr*MAX(Qinf)

wthresh = w0 - klow*(w0-wmin)

If the current inflow is larger than the threshold inflow (qthresh), the maximum allowable discharge is released:


If the current inflow is less than the threshold inflow (qthresh) and the current water level is lower than the threshold level (wthresh), the inflow is released from the dam:

qprod= Qinftoday

If the water level is above the threshold (wthresh), more water is released than inflow in an attempt to empty the dam. The factor multiplied by inflow (krel) is a general parameter:


4 - Hydropower dam

Dams for hydropower uses the general preferred production flow, but it may be adjusted with a seasonal factor. The factor is sinusoidal over the year, with a peak in December, when power output is normally high, and a minimum in June. If you want a different seasonal phase set qpha to another value (default = 102). The seasonal adjustment is set with the input variable qamp (>0,<1) for each dam.

qprod = qprod*(1 + qamp * sin(2*pi*(dayno+qpha)/365))

Alternatively the seasonal variation adjustment can be determined by the climate. If this option is used qamp is set to zero (or left out) in the file, and the value used in the calculations is determined by the fraction of snow fall (snowfrac) for each lake. If the snow fall fraction is above 0.35, a qamp of 0.71 is used.

Outlet lake with two outlets

An outlet lake that is not part of a lake composed of lakebasins, may have two defined outlets in LakeData.txt. The outlets can be divided into different outlet types depending on method for determining the outflow. HYPE will define an outlet's type based on variables present in LakeData.txt. Below the defining (necessary) variables are given for each outlet type. Note: The threshold (w0ref) is given for first outlet. For the second outlet this variable instead is given relative to the threshold of outlet 1 (i.e usually zero).

Outlet types

1. An outlet with outflow as production flow (only). The production flow can be determined by constants for different seasons or with a sinus adjustment; see Production flow above. Defining variables in LakeData.txt: regvol, (rate=0, maxQprod=0, minflow=0)

2. An outlet with outflow as production flow (same as type 1), but with an allowed (higher) maximum production flow. If the outflow from the other outlet is above zero, part of that flow will be sent through this outlet as part of production flow instead. Defining variables in LakeData.txt: regvol, maxQprod.

3. Rating curve determined outflow with constant threshold. Defining variables in LakeData.txt: rate (deltaw0=0, regvol=0, maxQprod=0)

4. Rating curve determined outflow with constant threshold relative to the threshold of the first outlet. This type is only used for the second outlet. Defining variables in LakeData.txt: rate, w0ref (regvol=0, deltaw0=0)

5. Flow determined by production flow below the threshold, and by a rating curve above the threshold. This could be production and overflow from a hydropower plant that goes too the same downstream subbasin. Defining variables in LakeData.txt: regvol, rate (maxQprod=0, minflow=0)

6. An outlet with outflow as production flow determined by different rating curves for different seasons; see Two rating curves above. Defining variables in LakeData.txt: rate, deltaw0 (regvol=0, maxQprod=0)

7. An outlet with outflow as production flow determined by two rating curves (same as type 6), but with an allowed (higher) maximum production flow. If the outflow from the other outlet is above zero, part of that flow will be sent through this outlet as part of production flow instead. Defining variables in LakeData.txt: rate, deltaw0, maxQprod (regvol=0)

8. An outlet with outflow as minimum flow. The minimum flow is determined same as production flow by constants for different seasons or with a sinus adjustment. Defining variables in LakeData.txt: regvol, minflow (rate=0, maxQprod=0)

9. Minimum flow determined by a production flow below the threshold. In addition overflow is determined by a rating curve above the threshold. This could be a minimum environmental flow together with overflow. Defining variables in LakeData.txt: regvol, rate, minflow (maxQprod=0).

10. An outlet with outflow determined from a time series in Xobs (dwtr). This type is only used for the second outlet. Defining variables in LakeData.txt: obsflow=1

Combinations of outlet types

Examples of common combinations of outlets:

- A hydropower plant with an estimated average production flow and a spill overflow at high water levels are simulated by combining production flow (outlet type 1) with rating curve determined overflow (outlet type 3).

- Same as above a hydropower plant with production flow in one branch and overflow in another (outlet type 3), but in this case the maximum allowed production is assumed higher than the average production (outlet type 2). This means that if overflow is calculated due to high water level, part of that flow is lead to the powerplant and through production to the first branch.

- A hydropower plant where the production flow and spill goes to the same downstream subbasin (outlet type 5), but where a second spill outlet goes to another subbasin (outlet type 4).

- A hydropower plant with production flow in one branch (outlet type 1), and a minimum (environmental) flow plus overflow outlet in the old river course that goes to another subbasin (outlet type 9).

- A lake with two outlets with flow determined by a rating curve for each of them (may have different threshold also) are simulated by a combination of outlet type 3 and 4.

Many other combinations can be used, but not all outlet types are natural to combine. The following are not allowed:

Outlet 1 Outlet 2
1 2, 4
2 1, 2, 4, 7, 8, 10
3 5, 9
4 all
5 3, 9
6 4
7 2, 4, 7, 8, 10
8 2, 4, 7-9
9 3-5, 8, 9
10 all

Change of branched flow after updating total flow

After calculating the outflow from the two outlets, which is done separately, the total outflow could be higher than the volume above the threshold. In this case the outflow will be reduced. When updating the total outflow from a lake with two outlets aginst observed total flow, the flow in respective branch has to be adjusted too. Depending on the outlet types this is done by different methods.

Lakes with a clear defined division between production flow and spill branch is handled so that production flow takes priority. For outlets with a maximum production flow, flow are diverted into this branch up to the maximum value and the rest into the other branch. For other lakes with a clear defined division between production flow and spill branch, the same method is applied but with the current production flow as the maximum value. For lakes with a minimum flow in one branch the minimum flow is given highest priority. Second priority is given to the production flow of the other branch (if any). At last the remaining flow is given to the branch for spill (outlet type 9) or the non minimum flow branch (outlet type 8). For the rest of the outlet type combinations the new flows are changed to be proportional to the old flows.

Outlet lake as a multi-basin lake composed of lake basins

An olake can be part of a larger lake. It is then called a lake basin of the larger lake, which is called a multi-basin lake. A multi-basin lake is assumed to have equal water level in all lake basins. The lake basins can have different depths below the outflow threshold, and they all use the same threshold when the water level output is calculated (same reference).


A lake basin can recieve local inflow, inflow from upstream areas (that are not part of the same multi-basin lake) and inflow from regional groundwater. Inflow works in the same way as for a simple olake (see above).

Flow between lake basins

All lake basins of a multi-basin lake are connected by the main downstream path. They can in addition have branches within the multi-basin lake, where a fraction of the flow then goes. Flow through a lake divided into lake basins can occurs in both direction according to the maindown (and possibly branchdown). The implementation prioritize flow from nearby lake basins and flow in the downstream direction when calculating the flow betweeen lake basins. The flow between the lake basins are calculate to reach an equal water level. No internal threshold between lake basins is considered when the flows are calculated.

The flow out of a lakebasin (to another lakebasin) is determined in several steps:

  1. First the net outflow of each lakebasin is calculated. It is calculated as the difference between the current water level of the lakebasin and the water level of the whole lake after outflow from the multi-basin lake. First the average water level of the multi-basin lake is calculated, and from this the outflow(s) of the lake. Considering these outflows leaving the lake, a new average water level is calculated that the lake will have after all flows between lakebasins have been moved. This is the water level compared to the water level of the lakebasin in this step.
  2. The flow out of a lakebasin is the netflow plus all inflows from other lakebasins. Theses inflows are outflows of other lakebasins. Starting upstream in the lake the inflows are added to the outflow to give the flow out of every lakebasin. Note that these flows can be negative.
  3. The last step is to actually move the water. Preferably we want water to move from one lakebasin to the next, without mixing with water incoming from lakebasins further upstream. This we want so that substances do not travel all the way to the lake outlet on one time step. Therefore we try to remove all outflows from the lakebasins before we add the removed water into the downstream lakebasins. This might not be possible so we keep track of how much we can't move and repeat the third step if necessary.

Outflow of lake composed of lake basins

The main outflow of the lake is assumed to be located in the last lake basin. Here outflow is estimated by a rating curve, either a specific equation or with the general equation, or by regulation routine. The threshold may be changing over the year as described in Section Two rating curves above. If the water level is below the threshold then the outflow is zero or if the production flow regulation is used there is production flow. See Section Production flow above. In addition the lake can have outflows out of the multi-basin lake from other lakebasins. These additional outflows are then the flow in the branch of these lakebasins. The outflow in a branch going out of the multi-basin lake is specified in LakeData for that lakebasin. It can be a rating curve or a production flow. If it is a production flow it uses the same thresholds as those for the main outflow of the multi-basin lake, i.e. w0 and wmin.

Initalisation of lake volume

Unless a starting state is given from a file, the lakes start the simulation filled with water to their outflow threshold. That means for most lakes a water level equal to lake_depth. Dams are filled to the dam's maximum elevation (equal to lake_depth), except for flood control dams which are initializied with an empty regulation volume.

For lakes with outflow determined by a rating curve, the water level of the lake will be higher than the outflow threshold level. The equilibrium level will depend on the size of the inflow and the outflow rating curve parameters. Depending on the residence time of water in the lake it may take time for this level to be established, and until then the outflow of the lake will be simulated lower than it should be. Thus a spin-up time is needed for a model simulation.

Section Symbol Parameter/Data File
Local lake (ilake) gldepi, illdepth, gratk, gratp, grata, ilratk, ilratppar.txt
or gicatch, illicatchpar.txt
lks_num, lks_dp_N, lks_fi_N, lks_fa_NGeoData.txt
Simple outlet lake or dam (olake)lakedepthlake_depthGeoData.txt or LakeData.txt or DamData.txt
gldepo, olldepthpar.txt
ratcorr, olratk, olratp, kthr, klow, krel
wmincalculated from regvolLakeData.txt or DamData.txt
rate, exp, qamp, qpha
qprodqprod1, qprod2, datum1, datum2
limqprodLakeData.txt or par.txt
purpose, snowfracDamData.txt
Qinfqinfjan, qinffeb, .., qinfdec
Outlet lake with two outlets ldtype=5 or 6LakeData.txt
w0ref, maxQprod, minflow, obsflow
Modules (file) Procedures Sections
surfacewater_processes (sw_proc.f90) calculate_potential_evaporation evaporation
average_flow_rating_curveouflow with rating curve
calculate_ilake_outflowlocal lake
set_general_rating_ksimple outlet lake
calculate_outflow_from_outlet_lake calculate_branched_flowsimple outlet lake
outlet lake with two outlets
calculate_lake_outlet_outflowoutlet lake with two outlets
calculate_lakebasin_average_waterstageoutlet lake as multi-basin lake
modelmodule (model_hype.f90) initiate_model local lake
calculate_flow_from_undivided_lakesimple outlet lake
outlet lake with two outlets
calculate_flow_for_lakebasin_lakeoutlet lake as multi-basin lake


Lindström, G., 2016. Lake water level for calibration of the S-HYPE model, Hydrology Research 47.4:672-682. doi: 10.2166/nh.2016.019.


Floodplains may be simulated adjacent to a main river or outlet lake. Floodplain is a land area close to the surface water that may be temporary flooded. Water may flow from the surface water to the floodplain and back. The floodplain is a part (fpf) of the classarea of the main river or the lake. The lake/river surface area is the rest of the classarea. The floodplain is flooded when the water in the river or lake reaches above a certain threshold. The water on the floodplain may return to the river lake or may be trapped when the water level of the floodplain sink.

Figure 4: Five possible scenarios of flooding.

Common floodplain processes

The water level of the floodplain and the extent of the flooded area (aplain, m2) are calculated from the volume (volplain, m3), maximum area (amax) and water level at maximum areal extent (fym, m). The actual water level will be compared to the equlibrium water level to determine the water exchange. The equilibrium water level is the water level we would have if water was evenly distributed in the lake and the floodplain. It is solved from a second degree equation.

wl_plain = sqrt{vol_plain * fym * 2 / a_max}

a_plain = wl_plain * a_max /fym

a_max = fpf * classarea

Figure 5: Illustration of floodplain variables of a lake.

The exchange of water between lake (or river) and floodplain is determined by the respective water levels (wlbody and wlplain), the equilibrium water level (wlequil) and the thresholds between the water bodies (flbody/flplain). If the water level is over the threshold and higher than the water level of the floodplain the flow from water body to floodplain is:

q = rc_bodyToPlain * MIN((wl_lake-fl_body)-wl_equil, (wl_lake-fl_body))

If the floodplain water level is over the threshold and higher than the water level of the water body the flow from floodplain to body is similarly:

q = rc_plainToBody * MIN((wl_plain-fl_plain)-wl_equil, (wl_plain-fl_plain))

Floodplain water level (wlplain) and degree of flooded area (a_plain/a_max) are possible to print out from a simulation with an outlet lake or main river with floodplain.

Simple floodplain model

The simple floodplain model simulates precipitation on the floodplain and evaporation from the flooded water, in addition to the exchange of water between the river or lake and the flood plain. All precipitation on the floodplain is contributing to flooded water.

Floodplain model with soil routines

The floodplain model with soil routines is calculating all soil flows for the non-flooded part of the floodplain. The runoff from the floodplain soil goes to the flooded water, and not to the local stream as it does for ordinary land classes. Precipitation and evapotranspiration is calculated for the flooded water and the non-flooded soil. For the soil part, snow, infiltration and surface runoff, percolation, tile drainage and groundwater runoff is calculated as for an ordinary soil. Infiltration from the flooded water to the soil may occur. Regional groundwater percolation is only calculated for the flooded part together with percolation from the river.

Figure 6: Illustration of floodplain soil flows.

Connected floodplains

It is possible for flooded floodplains to limit the water flow from upstream rivers and lakes. This is determined based on the water levels in the floodplains, a higher water level of a downstream floodplain compared to a upstream floodplain may reduced the flow from the upstream compartment (main river or outlet lake) to the downstream compartment (main river or outlet lake). The damming flow is calculated as the flow to reach equilibrium water level between the two floodplains. The relative level of the two floodplains location is determined based om the classes average elevation and thresholds or given as indata separately. The function of connecting floodplains works within a subbasin or between connected subbasins. This floodplain features may be used together with the Floodplain model with soil routines.

Section Symbol Parameter/Data File
fpffpfol, fpfmrFloodData.txt
Common floodplain processesclassareaslc_nn, areaGeoData.txt
fymfymol, fymmrFloodData.txt
rcbodyToPlainrclfp, rcrfp
rcplainToBodyrcfpl, rcfpr
flbodyfloll, flmrr
flplainflolp, flmrp
Connected floodplains hrefl, hrefr
Modules (file) Procedures Sections
surfacewater_processes (sw_proc.f90) add_precipitation_to_floodplain common floodplain processes
calculate_regional_floodplain_flows connected floodplains
floodplain soilmodel (soilmodel4.f90) soilmodel_4 simple floodplain model
floodplain model with soil routines
connected floodplains


Outflow from a subbasin may flow in one or two directions. The main flow follows the main channel, which is the downstream subbasin given by the path in GeoData.txt. If there is a bifurcation, the branch flow goes to another downstream subbasin (with subid branchid). Any of the flows may go outside the model set-ups area, they are then no longer a concern of the model.

There are three ways to determine the flow in the different channels. 1) The division of the total outflow is determined in BranchData.txt. 2) The outflow is calculated for two outlets separately and then divided into the channels. For the second alternative see Outlet lake with two outlets section above. 3) The demanded flow in the branch is prescribed (Qbranch). If the total flow is not enough to fulfil the need, less water goes into the branch. The main channel gets the rest of the total flow.

For the first method the total outflow has been calculated the ordinary way. The flows division into the two channels is the determined by four parameters; mainpart, maxQmain, minQmain and maxQbranch, which are set in BranchData.txt. Zero values of the input data mean they are not used. The main flow (mainflow) is calculated from the totalflow (q) as:

mainflow = delim{lbrace}{matrix{4}{2}{q {q<=minQ_main} {mainpart*(q-minQ_main)+minQ_main} {minQ_main<q<=q_thresh} {maxQ_main} {q>q_thresh \,\ q_thresh={{maxQ_main-minQ_main}/{mainpart}}+minQ_main}  {q-maxQ_branch}  {q>q_thresh\ ,\ q_thresh={{maxQ_branch}/{1-mainpart}}+minQ_main}}}{}


The rest of the total flow goes in the branch. The following figures give some examples of parameter combinations.

  • Serie 1: mainpart = 0.5, minQmain = 2, maxQmain = 10, maxQbranch = 0
  • Serie 2: mainpart = 0.5, minQmain = 2, maxQmain = 0, maxQbranch = 5
  • Serie 3: mainpart = 0.5, minQmain = 0, maxQmain = 10, maxQbranch = 0
  • Serie 4: mainpart = 0, minQmain = 2, maxQmain = 0, maxQbranch = 5
Figure 7: Main flow in relation to total flow for four different parameter combinations.
Figure 8: Branch flow in relation to total flow for four different parameter combinations.
Symbol Parameter/Data File
branchid, Qbranch, mainpartbranchid, Qbranch, mainpartBranchData.txt
maxQmain, minQmain, maxQbranchmaxQmain, minQmain, maxQbranch
Modules (file) Procedures Sections
surfacewater_processes (sw_proc.f90) calculate_branched_flow -
calculate_branched_flow_new -
start/hype_model_description/hype_routing.txt · Last modified: 2021/01/20 09:23 by cpers