HYPE has many possibilities to adjust the precipitation and temperature given in the Pobs.txt and Tobs.txt files. Adjustment is done because the supplied data can be of different reference height or (known to be) wrong in some areas, or because data vary over the subbasin but the input is given as one value, and or to compensate for non modelled processes.

Subbasin air temperatures () in the input file are normally assumed to be for the subbasin’s average elevation (elev). The same adjustments are made for maximum and minimum temperatures if available.

(1) If data with other reference height is used it is possible to adjust for that. The subbasin temperature can be adjusted depending on subbasin elevation with general parameter tcelevadd (° / 100m). This adjustment assumes observations are located at sea level.

(2) Alternatively it can be adjusted based on the difference between subbasin elevation and temperature observation elevation by general parameter tcobselev (° / 100m). The temperature observation elevation () then need to be supplied to the model.

(3) It can be adjusted based on month with the monthly dependent parameter monthlapse (° / 100m)). These three subbasin elevation adjustments should not be used together. Subbasin temperature can be adjusted equally over all subbasins within a region with the parameter region dependent parameter tempcorr.

The temperature can also be adjusted for each class depending on their deviation from the subbasin average elevation (). The class-dependent temperature (T) is calculated using the parameter tcalt. The temperature lapse rate often has a value of 0.6 (° / 100m).

Subbasin input precipitation () can be adjusted equally over all subbasins with the general parameter pcaddg or for some subbasins with the parameter region dependent parameter preccorr. Additionally it is possible to adjust precipitation for undercatch with different parameters (pcurain,pcusnow) depending on if the precipitation falls as snow or rain.

Subbasin precipitation () is for subbasin average elevation (elev), but can be adjusted for elevation variations within the subbasin. The precipitation of a class (P) is adjusted for classes where the class average elevation is greater than a threshold (general model parameter pcelevth). The adjustment is determined by a general parameter (pcelevadd) that is the correction per 100m. The class elevation adjustment can alternatively be determined from the basin standard deviation of elevation () and a parameter pcelevstd. The class height adjustment is limited by a general parameter pcelevmax. The precipitation of a class can additionally be adjusted with land-use dependent parameter pcluse, e.g. for interception evaporation.

Where is a class's elevation deviation from the subbasin average elevation and snfrac is the average fraction of precipitation that falls as snow calculated from subbasin temperature () and temperature thresholds (see equation below), or from input.

The rain/snow fraction of precipitation is calculated based on temperature or given as an input time series. Different temperatures can be used in the equation, i.e. basin average or class temperature. When the air temperature (T) is around the threshold temperature for mixed precipitation (land-use dependent parameter ttmp plus general parameter ttpd) both rain and snow. The interval for mixed precipitation is given by the parameter ttpi. For temperature below threshold minus ttpi, the precipitation is assumed to be in solid form only and is added to the snowpack. If the air temperature is greater than the threshold temperature plus ttpi, the precipitation is assumed to be solely in liquid form. For intermediate temperatures, the precipitation is assumed to be a mixture of liquid and solid forms i.e. as both rain and snow. The proportion () of precipitation that falls as rain depends linearly on the temperature.

Alternatively snowfall fraction (sffrac) may be read from input file and calculated as:

The amount of rainfall and snowfall is calculated from the corrected precipitation (P).

The optional snowfall distribution models redistribute the snowfall of a catchment among the classes within a subbasin or between subbasins depending on wind. The model uses Winstral coefficients. For each subbasin and class and wind direction a Winstral coefficient (WSF) is given as input data to the model. Wind direction is calculated from forcingdata of westerly (U) and southerly (V) wind. The wind direction is translated to the closest quadrant or octant, and the Winstral coefficient of that direction is used.

The Winstral coefficient is, with some scaling parameters, used to calculate the relative snowfall for each class this time step. There are two models; one that use a linear function (Gustafsson et al, 2015) and one that is log-linear (Clemenzi et al., 2023):

WSFscale and WSFbias are general parameter, WSFluse is a scaling parameter depending on land-use. The relative snowfall is limited to the interval [1 - sfdmax, 1 + sfdmax] for the linear model and [0,sfdmax] for the log-linear. sfdmax is a general parameter. The relative snowfall is normalized within each subbasin so that the subbasin mean snowfall is preserved. The snowfall distribution is applied only if snow is falling for a fraction of the subbasin area that is larger than a limit (limit).

For the option of distributing snowfall also between subbasins, the log-linear Winstral coefficients are used. They are used to calculate the relative snowfall for each class in both the upwind and the downwind subbasin separately. The distribution of the snowfall of the upwind subbasin between the upwind and down wind subbasin is then calculated based on windspeed (wind) and distance (d). The two subbasins share the sharefrac of snowfall, and the rest goes to the upwind subbasin only.

The sndrscale parameter is a general length scale parameter (m per m/s). The distance is calculated for the subbasins X/Y coordinates.

Gustafsson et al., 2015

Ilaria Clemenzi, David Gustafsson, Wolf-Dietrich Marchand, Björn Norell, Jie Zhang, Rickard Pettersson, Veijo Allan Pohjola, 2023. Impact of snow distribution modelling for runoff predictions. Hydrology Research 1 May 2023; 54 (5): 633–647. https://doi.org/10.2166/nh.2023.043

Potential evaporation is calculated for each class based on land use and atmopheric variables. The potential evaporation is then used to calculate the current actual evapotranspiration from each land class (see Soil water - Evaporation. A river with an area (i.e. is a class), flooded floodplains and lakes are assumed to evaporate at the potential rate, when the air temperature is above the threshold temperature (ttmp). Evaporation is limited by the water body's volume.

Potential evaporation (epot in mm) is calculated based on the temperature if it is not read in from file (repo in Xobs.txt). Alternative PET models exist, and is described below. When the air temperature (T) is greater than the threshold temperature ttmp evaporation is assumed to occur. Snow melting, snow density and evaporation use the same threshold temperature. The basic potential evapotranspiration () depends on the land use dependent rate parameter cevp.

A seasonal factor cseason adjusts the potential evaporation rate (cevp) e.g. making it higher in the spring when the air is often dry, and lower in autumn when the air is often more humid than in spring. The factor is sinusoidal with two parameters cevpam and cevpph. It is not used if cevpam is zero. A cevpph around 45 days give a maximum correction in mid May (dayno=45+91=136). The minimum correction will then be a half year later in September (dayno=136+182). For an earlier maximum, reduce cevpph.

The basic potential evapotranspiration may be adjusted with a regional correction factor (cevpcorr) equally over the year depending on parameter region.

HYPE give the option to exchange the default potential evaporation model for another model. Only the basic potential evaporation () differs between models. Models 0-2 only use air temperature forcing. Models 3-5 want shortwave radiation and minimum and maximum daily air temperature, although if lacking approximations are made (see section Input to alternative potential evaporation models below). In addition, model 5 wants relative humidity and wind speed if available. Note, regardless of potential evaporation model, the actual evaporation is limited to temperatures above parameter ttmp.

As described above; evapotranspiration depends on the rate parameter cevp and air temperature (T) above a threshold ttmp. If the variable repo is given in Xobs.txt those values are used.

Model 1 is the same as model 0, but it will not be using repo from input data, even if it is present.

The modified Jensen-Haise/McGuiness model follow Oudin et al. (2005). The potential evaporation depends on extraterrestrial radiation (radext), latent heat of vaporization () and temperature (T). Two general parameters (jhtadd and jhtscale) are used and one land use dependent (crop coefficient kc2 or kc).

The Hargreaves-Samani evaporation is modified to limit the “turbidity-factor”. The potential evaporation depends on extraterrestrial radiation (radext), latent heat of vaporization (), temperature (T) and turbidity (turbidity). One general parameter (krs) is used and one land use dependent (crop coefficient kc3 or kc).

The Priestly-Taylor potential evaporation depends on net downward radiation (netrad), slope of saturated vapour pressure curve (dsatvap), latent heat of vaporization () and a psychrometric constant (). One general parameter (alfapt) are used and one land use dependent (crop coefficient kc4 or kc).

The FAO Penman-Monteith potential evaporation depends on net downward radiation (netrad), slope of saturated vapour pressure curve (dsatvap), saturated and actual vapour pressure (satvap and actvap), temperature (T), wind speed (wind) and a psychrometric constant (). One land use dependent parameter (crop coefficient kc5 or kc) is used.

Summary of alternative input to PET models, and link to file reference.

Actual vapour pressure (actvap) is calculated following FAO recommended procedure and function/data priority. Depending on availability of minimum, mean and maximum relative humidity (rh) and minimum, mean and maximum air temperature () equations with different combinations of saturated vapour pressure (calculated from temperature) times relative humidity is used. For example:

In case not enough data is available, minimum temperature is calculated from turbidity and a general parameter (krs) and the last equation of those above is used.

Eventually actual vapour pressure is limited by the calculated saturated vapour pressure.

Air pressure (pa) is calculated as a function of elevation for the class (elev).

Shortwave radiation (swrad) is supplied as input forcing time series or otherwise calculated from extraterrestrial radiation.

Extraterrestrial solar radiation (radext) is estimated from day of year and latitude. The equations used comes from FAO. The day of the year are used to estimate distance to the sun and declination. The sunset hour angle are calculated from latitude, but with special care for high latitudes (polar night and midnight sun). These variables together with latitude and the solar constant eventually give the current extraterrestrial radiation for each subbasin.

Latent heat of vaporization () is a function of temperature (T).

The net downward radiation (netrad) is used explicitly for PET model 4 and 5 (Priestly-Taylor and FAO Penman-Monteith). The net radiation is calculated following FAO recommended procedure. It is calculated as net shortwave radiation minus net longwave radiation. Net shortwave radiation () is calculated from the shortwave radiation (swrad) and the land use dependent albedo parameter (alb). Net longwave radiation () is calculated using temperature (), actual vapour pressure (actvap) and relative shortwave radiation (relsh) if those are available, otherwise it is set to zero. The relative shortwave radiation is shortwave radiation in relation to clear sky shortwave radiation.

The psychrometric constant () is a function of air pressure (pa) and latent heat of vaporization () following FAO.

Saturated vapour pressure (satvap, kPa) is calculated from temperature (temp) following FAO. If daily minimum and maximum air temperature is available satvap is set to the average of the saturated vapour pressure calculated for each of those two temperatures, otherwise it is calculated from daily average air temperature.

The slope of saturated vapour pressure temperature function (dsatvap) is used explicitly for PET model 4 and 5 (Priestly-Taylor and FAO Penman-Monteith). The slope is calculated from daily mean air temperature (T).

The turbidity factor is used explicitly for the PET model 3 - modified Hargreaves-Samani. If shortwave radiation (swrad) has been given as forcing data time serie, the turbidity factor is calculated as

but limited by a minimum turbidity value (0.25) and an estimated clearsky turbidity. The clearsky turbidity (clearturb) is estimated by the Ångström formula (FAO):

where elev is subbasin elevation in meter above sea level. If no shortwave radiaton time series are given, but montly cloudiness climatology is provided (cloudiness), the turbidity is calculated as

If no cloudiness are given either, but time series of daily minimum and maximum temperature are, the turbidity is calculated as in the “ordinary” Hargreaves-Samani turbidity function:

but still limited by the minimum turbidity value and the calculated clearsky turbidity.

Wind speed is used in PET model 5 - FAO Penman-Monteith. Wind speed (wind) may be given as a constant general parameter (mwind) or as a forcing data time series. The time serie wind is given for each subbasin (). It is possible to adjust the time serie wind speed to different height than observations. If the general parameters zwind, zwish, roughness, and zpdh is set, wind speed is adjusted with the transformation factor windtrans.

Richard G. Allen, Luis S. Pereira, Dirk Raes & Martin Smith, 1998. Crop Evapotranspiration – Guidelines for Computing Crop Water Requirements. FAO Irrigation and drainage paper 56. Rome, Italy: Food and Agriculture Organization of the United Nations. ISBN 92-5-104219-5

Oudin, L., F. Hervieu, C. Michel, C. Perrin, V. Andreassian, F. Anctil and C. Loumagne 2005. Which potential evapotranspiration input for a lumped rainfall–runoff model?: Part 2—Towards a simple and efficient potential evapotranspiration model for rainfall–runoff modelling, Journal of Hydrology 303(1-4):290-306.

Atmospheric deposition can be added to the model as load or as concentration of precipitation.

Atmospheric deposition in the form of wet deposition of IN and SP is added as a concentration of rainfall. You can specify a time series of the concentration in precipitation in Xobs.txt (implemented for IN, SP, and T1). Otherwise wet deposition (as a concentration) is specified in AtmdepData.txt for each subbasin.

If parameter (aloadconst) is set, the wet deposition load in considered constant and not dependent on precipitation corrections. Thus the concentration of precipitation is changed to keep the load constant, when precipitation correction is applied.

Dry deposition is specified in AtmdepData.txt for each subbasin. It can be specified for different vegetation types or land uses.

Dry deposition of nitrogen and phosphorus is added to the snow or, if there is no snow, directly to the ground. Phosphorus deposition is added to PartP-pool in the upper soil layer and nitrogen deposits to the IN dissolved in soil water if the parameter ponatm is not set. A new concentration of IN in the soil water is then calculated. The parameter ponatm indicates that some of the nitrogen deposition will be added to organic nitrogen pool (fastN) instead of to the IN pool.

For both nitrogen and phosphorus wet deposition is added through the concentration of precipitation (as described above). Precipitation may fall as snow and be added to the snow pack or if it is rain added to the potential infiltration. Depending on the fate of the infiltration, the wet atmospheric deposition will mix with the recieving water (e.g. surface runoff, soil layer water, macropore flow).

Deposition is specified in AtmdepData.txt for each subbasin, as load and/or as concentration for wet deposition. It can be specified for different vegetation types or land uses, and thus water surfaces may have a different deposition than land surfaces. In addition monthly deposition can be specified.

For both nitrogen and phosphorus wet deposition is added through the concentration of precipitation (as described above), while for dry deposition an amount of the nutrient is added to the river (if it has a class-area) or lake water.