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HYPE simulate nitrogen and phosphorus in soil both as immobile soil pools and as fractions dissolved in soil water. The pools are changed by outside sources and internal soil processes. The following fractions are simulated:
The figure below give an overview of the nitrogen and phosphorus in soil in HYPE. The internal processes are shown with broad arrows and described in the section Nutrient soil processes below. Sources and sinks are shown with arrows and text, and are further described in sections Nutrient sources and Vegetation and soil surface processes
Fertilization can be specified to be applied up to two times per year for each of the two fertilization forms, (commercial) fertilizer (fert) and manure (man). For each fertilization event the application date is specified, the amount of N and P (fert(inut,ife), man(inut,ife), where inut=N,P and ife=1,2 the fertilization event) to be added, and a management indicator (fdown(ife), mdown(ife)). The last information indicates how much of the fertiliser is to be added to the second soil layer (the rest is added to the first soil layer). These parameters are given in CropData.txt and are crop and region dependent. For manure 50% of the nutrients are assumed to be in organic form (inorgpart = 0.5).
Fertilizer and the inorganic part of the manure are added to the IN and SP pools of the soil water. The total amount is spread out over a specified period (given by the general parameter fertdays) after the date of fertilization. An equal amount is applied each day for the specified period. The organic part of the manure is added to the fastN and fastP pools.
Each class can have two crops (icrop=1,2), the main crop and the secondary crop. The secondary crop do not have to cover (a) the whole class, e.g. if only 50% of the spring wheat (main crop) area have catch crop (second crop) (a(1)=1, a(2)=0.5). The total amount of added nutrients () is calculated as the sum of fertiliser additions from the main and secondary crops.
Inorganic nutrients to be added to IN and SP pools:
Organic nutrients to be added to fastN and fastP pools:
Note that the terms for application of fertilizer and manure only are added during the active time, which is from the application date and for fertdays number of days thereafter.
Plant residues are added at a specified date each year. They are added to the immobile soil pools (fastN, fastP, humusN, and humusP) to get a delay of the leakage and an accumulation of humus in the soil. The amount of crop residues (resamount(inut), inut=N,P) and the date they are added on depends on crop and region and is given in the CropData.txt file. Part of the plant residues are added to the fast pool (resfast), and part to the humus pool. The plant residues are added to the upper two soil layers with a distribution (resdown) also specified in CropData. For each class the plant residues are weighted () for main and secondary crop together every day with the help of share for each crop (a(icrop), where icrop=1,2).
All crop residues are transferred into the soil at a given date as determined by the user in CropData.txt. Note that the terms for application of residues only are added that day.
Information on rural household diffuse source (private sewers) is located in the file GeoData.txt. The source has a flow and concentrations of total nitrogen and phosphorus and fraction of IN and SP. The diffuse source is divided into two parts, where the distribution is determined by a general parameter (locsoil). One part is added directly to the local river. The other part is added to soil water in the bottom soil layers in the catchment area of all land classes with a distribution proportional to the classes’ area.
|Fertilizer||fert||fn1, fp1, fn2, fp2||CropData.txt|
|man||mn1, mp1, mn2, mp2|
|reg, cropid, fday1, fday2, mday1, mday2|
|Plant residues||resamount||resn, resp||CropData.txt|
|resday, resfast, resdown|
|Rural household diffuse source||loc_tp, loc_tn, loc_vol, loc_sp, loc_in||GeoData.txt|
The CropData.txt file contains information on the various crops that are simulated. Each soil type-land use class (SLC-class) can have a main crop and a secondary crop. To reduce the number of SLC-classes in a simulation the individual crops with similar properties are brought together in a grouped crop (e.g. row crops). In CropData.txt there is information such as the amount of fertilizer, manure and crop residues introduced into the soil for different crops in different regions. The file also provides coefficients for nutrient uptake capabilities.
The potential uptake is used to calculate the absorption of IN and SP from the soil water (see section Vegetation nutrient uptake). The actual uptake will be limited by the nutrients available. Potential uptake of nitrogen by plants is based on a three-parameter equation (logistic growth) from SOILN (Eckersten et al., 1994). Nitrogen uptake (uptake) is calculated for the main and secondary crop and added to the common potential absorption function (common_uptake, mg/m2/d) using the percentage of the area of the main crop (part = 1.0) and secondary crop (part<1). The potential uptake is divided between the upper soillayer and second soillayer by a crop dependent input variable for the fraction of the upper soil layer. If the crop is growing in a class of only one soil layer, only the upper soil layer fraction of the potential uptake will be used in the calculations. The growing period with simulated nutrient uptake is determined by the sowing date (bd2) and continues until harvest (bd3). Potential uptake depends on three parameters (up1, up2 and up3) and the number of days after sowing date. The uptake for a specific crop is calculated with the parameter values for that crop as
for the growing period. Outside the growing period the uptake is assumed to be zero.
|Figure 2 Uptake function over a year with growing period bd2=100 to bd3=230. Effect of changing parameters up1-up3.|
Autumn-sown crops may take up IN and SP for a while after sowing in autumn. The same potential uptake of nitrogen as the main growing season are used, but uptake is limited by a temperature function. This uptake will run from the autumn sowing date (bd5) to the mid winter (31 December or 30 June depending on the autumn sawing date).
where the temperature function (tmpfcn) is calculated as
The growing period will then continue next season from bd2 as described above.
The sowing date (bd2) may be given as a constant, or calculated dynamically depending on temperature. If it is dynamically determined it is set to the first day of the year which has a degreeday sum (GDD) above a threshold (gddsow). The degreeday sum (GDD) is calculated as
where d is day of year, T is air temperature (degree Celcius), basetemp is a temperature threshold. The GDD is accumulated for each day after firstday with day length larger than daylength. The GDD is zeroed at firstday.
The erosion of soil particles is calculated by HYPE for transport of particulate phosphorus (PP) or for the simulation of sediment material (SS). For transport of PP an erosion model depending on mobilization of particles by rain and by surface runoff is often used. This formulation is the default model for sediment transport, but another erosion model based on catchment erosion index can also be used.
In the first model PP can leave the land by two means, by surface runoff transport and by macropore flow through drainage pipes. The calculation of PP transport is done in three steps: first the erosion (mobilization) of soil particles from the land surface is calculated, secondly how much of the mobilized particles that are leaving the field is calculated, finally the amount of soil particles is converted to phosphorus.
Mobilization (MobilisedRain and MobilisedSR) occurs due to energy from falling raindrops () and/or from surface runoff. The kinetic energy in rainfall is calculated as a function of rainfall and day of the year (dayno). If the precipitation falls as snow, or if it falls on snow-covered ground or if it is smaller than 5 mm/day no mobilization occurs in the model. Some of the rain drop's energy can be absorbed by vegetation. Cropcover is defined as the portion of land that is sheltered from raindrops; for a description of how this is calculated, see next chapter. The factor is the total fraction of cover that the main and secondary crop give together. It varies over the year due to crop growth and management. The mobilization is also influenced by soil erodibility (soil dependent parameter soilerod (g/J)).
When surface runoff occurs, soil particles are eroded and carried away as the soil surface is exposed to shear forces. The mobilization is calculated from the surface runoff (sflow), land slope, a parameter for soil cohesion (soilcoh (kPa) soil type dependent), and a general parameter (sreroexp). This type of erosion can be mitigated by protective vegetation or vegetation residues that are in contact with the ground. The calculation of this factor (groundcover) is described in next chapter. The variable is the total fraction of cover that the main and secondary crop give.
All mobilized particles is not removed from the field, because sometimes the transport capacity of the particle-bearing water (eflow) will not suffice for the task. A transportfactor will reduce the particle amount:
Eroded sediment (kg/km2) is calculated as:
The alternative erosion model calculates eroded sediment (erodedSed (kg/km2)) based on precipitation (prec) and a number of model parameters and subbasin input data.
The parameters erodslope, erodexp and erodindex are general. The parameters erodluse and erodsoil are land-use and soil type dependent. Subbasin input is needed on slope, the subbasins average slope, and an erosion index, EI.
A selective process is affecting the soil erosion of phosphorus. Smaller and lighter particles are eroded easier than larger ones. The tiny particles contain more P per unit weight than the average particle of the soil. Therefore an enrichment factor (enrichment) is used. The enrichment factor is calculated from three parameters (ppenrmax, ppenrstab, ppenrflow), one of which is soil type dependent (ppenrmax), and the particle bearing flow. Typical values of the parameters, here called max, stab and flowstab, are given in the example in Figure 3.
Eventually the eroded PP (kg/km2) is calculated as:
The calculated eroded sediment (erodedSed) and eroded phosphorus (erodedP) are now available to add to the suspended sediment (SS) or particulate phosphorus (PP) of surface runoff.
A portion of SS and PP in surface runoff (sflow) is filtered out (for example by buffer zones). Filtering (srfilt) is determined by land use dependent parameters (bufferfilt, innerfilt, otherfilt), percentage of agricultural land close to a watercourse (alfa), and proportion of agricultural land near the rivers which have a protective buffer zone (bufferpart).
Similarly, part of the SS and PP which is transported by macropore flow (mflow) is filtered away between the soil surface and drainage pipes. The parameter that determines this effect (macrofilt) is soil dependent. The SS or PP transported (transp) by surface runoff and macropore flow is collected in a temporary storage pool (relpool (kg/km2)) together with SS and PP in tile runoff (tilep).
From the temporary pool sediment and phosphorus is released (release (kg/km2)) and then transported to the local river depending on the size of the total runoff (runoff (mm)). The parameters pprelmax and pprelexp are general.
Erosion can be mitigated by protective vegetation or vegetation residues that are in contact with the ground. Crop cover and ground cover erosion reduction are used for particulate phosphorus (PP) and the default sediment transport model (SS). The reduction is given by the fraction of cover, with full cover (1) halting the erosion completely. Harvested crops have seasonally varying ground and crop cover, while permanent vegetation (e.g. forest) has constant values for these parameters. The input data needed for calculations (ccmax1, ccmax2, gcmax1, gcmax2) are given in CropData.txt. Parameters ccmax1 and gcmax1 describe the maximum crop and ground cover during spring-summer growing season, parameters ccmax2 and gcmax2 are corresponding maxima for winter crop's growth. These maximum ratios are reached at maxday1 and maxday2, which are defined as halfway between planting and harvest, and halfway between autumn planting and midwinter (1 January or 30 June depending on autumn planting date), respectively. From sowing the coverage fractions increase linearly up to their maximum values. After these dates maximum coverage is maintained to the next ploughing, harvest, or until the growing season starts again in the spring (for winter crops) (Figure 4). During the period between harvesting and ploughing, crop cover is equal to ground cover (gcmax1). At ploughing, ground and crop cover are reduced to zero. Parameters bd1 and bd4 describe the dates of spring and autumn ploughing. In the case of spring sowing, when no winter crop is crowing, either one of the ploughing parameters can be set for ploughing date.
|Figure 4: Crop cover and ground cover development for four different crop configurations.|
The parameter ponatm indicates that some of the nitrogen deposition will be organic instead of only inorganic. For wet deposition and deposition on snow, this is adjusted when there is infiltration of rain and melted snow into the soil. A portion (ponatm) of IN in the infiltration are added to fastN in the top layer instead of following the water to the soil water.
|Potential vegetation uptake of nitrogen||part||scr_nn||GeoData.txt|
|upupper, pnupr, bd2, bd3, bd5, up1, up2, up3 gddsow, basetemp, firstday, daylength||CropData.txt|
|Soil erosion||soilerod, soilcoh, seroexp, erodslope, erodluse, erodsoil, erodindex, erodexp, bufferfilt, innerfilt, otherfilt, macrofilt, pprelmax, pprelexp||par.txt|
|Crop cover and ground cover||ccmax1, ccmax2, gcmax1, gcmax2, bd1, bd4||CropData.txt|
|maxday1, maxday2||calculated from bd2, bd3, bd5|
|Transformation of nitrogen from atmospheric deposition||ponatm||par.txt|
|npc_soil_processes (npc_soil_proc.f90)||calculate_plant||potential vegetation uptake of nutrients|
|crop cover and ground cover|
|atmdep_in_loss||transformation of nitrogen from atmospheric deposition|
The initial pools of immobile soil nutrients are dependent on land use and is determined by the user and very important because they are changing slowly. The initial pools of the soil layers are calculated from a parameters. The parameters (humusn0, humusp0, partp0, fastn0, fastp0) are representative for the uppermost soil layer; the unit for these parameters is mg/m3. The initial humusN, humusP and partP-pools decreases with depth, with half depth determined by land use dependent parameters (hnhalf, hphalf and pphalf).
The equation for a depth dependent soil nutrient is:
where d is depth below zero-level, nut0 is nutrient at zero-level (i.e. model parameter for uppermost soil layer) and par is calculated from respective nutrient half depth model parameter (NNhalf) as:
As the model parameters that represent zero-level nutrient (nut0) is defined for the uppermost soil layer, the zero-level is the middle of uppermost soillayer. The following depths (d) represent the soillayers (1-3):
The nutrient is transformed into model soil nutrient pools (unit kg/km2) by taking into account the thickness of the soil layers.
Pools with rapid turnover (fastN, fastP) have initial values independent of depth.
The nutrients dissolved in soil water is initialized independent of depth. The initial IN and SP concentrations are zero. The initial ON and PP concentration are determined by land use dependent parameters (onconc0 and ppconc0, mg/L) – valid for all three soil layers.
Many soil processes depend on temperature and soil moisture. The following equations are used in these cases. The temperature function (figure 5) depends on the estimated soil layer temperature (soiltemp). The soil temperature requires some parameters to be simulated, see Section Soil temperature and snow depth.
tmpfcn = 2**((soiltemp - 20.0) / 10.0) IF(temp < 5.0) tmpfcn = tmpfcn * (soiltemp / 5.0) IF(temp < 0.0) tmpfcn = 0.0
The humidity function (figure 6) depends on soil moisture (soil) in the soil layer and the parameters of wilting limit (wp), field capacity (fc) and effective porosity (ep) transformed to unit mm. All these humidities are specified as percentages. The function includes coefficients thetaupp = 0.12, thetalow = 0.08, thetapow = 1.0 and satact = 0.6. Note that another function is used in the calculation of denitrification. For soil layers k = 1..3 the equation is:
IF(soil >= wp + fc + ep) THEN smfcn = satact ELSE smfcn = min(1.,(1-satact)*((wp+fc+ep-soil) /(thetaupp*soilthick)) **thetapow + satact,((soil-wp)/(thetalow*soilthick) **thetapow) ENDIF IF(soil < wp) THEN smfcn=0.0 ENDIF
The humidity function (figure 6) is always less than or equal to one while the temperature function may be greater than one when the temperature exceeds 20 degrees.
Vegetation nutrient uptake is a sink of the IN and SP concentrations in the two upper soil layers. The uptake (uptake / uptakeP, mg/m2/d) depends on the potential uptake of nitrogen (), calculated for the combination of main crop and any secondary crop of the specified class. Vegetation nutrient uptake also depends on uptsoil1 which gives the distribution of uptake between the two layers for a crop, the pool of IN (INpool) and SP (SPpool) and for phosphorus by P / N ratio (pnratio). The pool of dissolved nutrients are calculated as the concentration multiplied by the water in the soil layer (in mm). The main and secondary crop of a class is specified in GeoClass.txt.
The plants can take up a maximum factor a of the pool. This factor is calculated as a percentage of soil water over the wilting point. The equations for the potential uptake of nitrogen () are described under that section above. The constants uptsoil1 (percentage of plant nutrients absorbed top layer) and pnratio (ratio of N and P uptake) are given for each crop.
Denitrification is a sink for IN in all three soil layers. Denitrification (denitr, mg/m2/d) depends on the denitrification rate and the pool of IN (INpool) in each soil layer (k = 1.3). The denitrification rate depends on a rate coefficient (drate), soil temperature, IN concentration and soil moisture.
The coefficient drate is determined by land use dependent model parameters denitrlu and denitrlu3. The temperature dependence (tmpfcn) is described above. The soil moisture function (figure 7) is exponential and thus different from the general soil moisture function.
The function depends on soil moisture (soil) and pore volume (pw). Is also depends on two constants; the limit where moisture is high enough to allow denitrification to occur (dlim= 0.7) and the exponent (exp= 2.5). These cannot currently be changed. The dependence of the denitrification rate on the IN concentration is described by a function with a half-saturation concentration (general parameter hsatINs was in earlier HYPE versions a constant equal to 1 mg/L) (Figure 8).
Turnover of fastN is a sink for fastN and a source of IN in all three soil layers (k = 1..3). Turnover (transfN, mg/m2/d) depends on a general parameter (minerfn), temperature function (tmpfcn), humidity function (smfcn) and the pool of fastN (fastN).
Turnover of fastP is a sink for fastP and a source of SP in all three soil layers (k = 1..3). Turnover (transfP, mg/m2/d) depends on a general parameter (minerfp), temperature function (tmpfcn), humidity function (smfcn) and the pool of fastP (fastP).
Turnover of humusN is a sink for humusN and a source of fastN in all three soil layers (k = 1..3). It is supposed to be a very slow process. Turnover (degradhN, mg/m2/d) depends on a general parameter (degradhn), temperature function (tmpfcn), humidity function (smfcn) and the pool of humusN (humusN).
Turnover of humusP is a sink for humusP and a source of fastP all three soil layers (k = 1..3). It is supposed to be a very slow process. Turnover (degradhP, mg/m2/d) depends on a general parameter (degradhp), temperature function (tmpfcn), humidity function (smfcn) and the pool of humusP (humusP).
All organic soil pools of nutrient (humusN, fastN, humusP and fastP) contribute to dissolved organic nutrients in soil water (i.e. ON and PP). This process is a sink of the soil pools and a source of ON and PP in all three soil layers (k = 1..3). Dissolution (mg/m2/d) depends on a model parameter (one for each soil pool, dispar), temperature function (tmpfcn), humidity function (smfcn) and the size of the soil pool (soilpool).
The dissolution model parameters (dispar) is land use dependent and named dissolfp, dissolfn, dissolhp, and dissolhn for dissolution of nutrient pools fastN, fastP, humusN and humusP respectively.
ON and PP concentration are reduced as the water percolates down through the soil layers. The decrease in concentration depends on land use dependent model parameters (par).
SP is in a dynamic equilibrium with P adsorbed to soil particles (partP). The equilibrium is described by the Freundlich equation. From the total amount of P (SP + partP) in the soil an equilibrium concentration of SP is calculated in the soil solution using the iterative Newton-Raphson method.
coeff = Kfr * bulkdensity * soilthick DO WHILE fxn = xn * vol + coeff * (xn ** Nfr) - totalP fprimxn = vol + Nfr * coeff * ( xn **(Nfr-1)) xn_1 = xn - fxn / fprimxn xn = xn_1 ENDDO
The coefficients Kfr, and Nfr are soil dependent, bulkdensity is the dry density of soil (= 1300 kg/m3) and soilthick is the soil layer thickness. fxn is function value for the concentration xn, fprimx is the function value of the derivative of fxn, and is the concentration of the next iteration. The resulting SP equilibrium concentration () is used to calculate the PP eqiuilibrium concentration (). A new SP concentration () is calculated from the previous concentration () and the equilibrium concentration, taking into account the balance is not reached during the time step.
Adsdes is the amount of adsorbed/desorbed P, Kadsdes is a soil dependent rate constant, equi_concsolid is the solid phase equilibrium concentration, and concsolid is the initial concentration the solid phase.
The soil pools of nutrients are shared by crops on rotation on a field. This is modelled by averaging the soil pools once per year for the classes which are in the same crop rotation group.
|Soil pools - initial values||nut0||humusn0, humusp0, partp0, fastn0, fastp0||par.txt|
|soilthick||calculated from soillayerdepth||GeoClass.txt|
|Common functions||wp, fc, ep||calculated from wcwp, wcwp1-3, wcfc, wcfc1-3, wcep, wcep1-3||par.txt|
|Vegetation nutrient uptake||cropid, cropid2||GeoClass.txt|
|Denitrification||denitrlu, denitrlu3, hsatINs||par.txt|
|Immobile soil nutrient pool transformations||minerfn, minerfp, degradhn, degradhp||par.txt|
|Dissolution of soil organic pools||dispar||dissolfp, dissolfn, dissolhp, dissolhn||par.txt|
|Balance SP - PartP||Kfr||freuc||par.txt|
|Crop rotation effect on soil pools||crop rotation||GeoClass.txt|
|initiate_soil_npc_state||soil pools - intial values|
|plant_uptake||vegetation nutrient uptake|
|soil_pool_transformations||immobile soil pool transformation|
|general_functions (general_func.f90)||tempfactor||common functions|
IN, ON, SP and PP comes with water when it flows through the soil, but remain when water evaporates. Different flow paths contribute to the runoff of water and concentration of IN, ON, SP and PP from the corresponding soil layer. The flow paths possible today are: surface runoff (soil layer 1), runoff (soil layer 1, 2 and 3) and runoff through drainage pipe (soil layer 1, 2 or 3).
Some additional output variables are calculated from the soil variables. Gross and net soil load is calculated for the “root zone”, e.g. soil layer one and two together, and for soil layer three separate. A third option for soil load is calculated not strictly based on the soil layers but as an approximation for “groundwater” based on third soil layer but manipulated with tile drainage.
Gross load to soil layer one and two (root zone). Components included: atmospheric deposition (wet and dry), fertilizers, crop residues, rural load if number of soil layers is less than three. HYPE variable IDs: sl01=IN, sl03=ON, sl05=sl01+sl03=TN, sl07=SP, sl09=PP, sl11=sl07+sl09=TP
Gross load to soil layer three (lower soil). Components included: percolation from soil layer two to three, rural load if number of soil layers is three. HYPE variable IDs: sl13=IN, sl15=ON, sl17=sl13+sl15=TN, sl19=SP, sl21=PP, sl23=sl19+sl21=TP
Gross load to “groundwater” (modified soil layer three). Components included: percolation from soil layer two to three, rural load if number of soil layers is three, tile drain runoff if tile is in soil layer one or two. HYPE variable IDs: sl25=IN, sl27=ON, sl29=sl25+sl27=TN, sl31=SP, sl33=PP, sl35=sl31+sl33=TP
Net load from soil layer one and two (root zone). Components included: percolation from soil layer two to three, runoff from soil layer one and two, tile drain runoff if tile is in soil layer one or two, surface runoff. HYPE variable IDs: sl02=IN, sl04=ON, sl06=sl02+sl04=TN, sl08=SP, sl10=PP, sl12=sl08+sl10=TP
Net load from soil layer three (lower soil). Components included: runoff from soil layer three, tile drain runoff if tile is in soil layer three. HYPE variable IDs: sl14=IN, sl16=ON, sl18=sl14+sl16=TN, sl20=SP, sl22=PP, sl24=sl20+sl22=TP
Net load from “groundwater” (modified soil layer three). Components included: runoff from soil layer three, tile drain runoff if tile is in any soil layer. HYPE variable IDs: sl26=IN, sl28=ON, sl30=sl26+sl28=TN, sl32=SP, sl34=PP, sl36=sl32+sl34=TP
There is a model option to skip calculating nutrients with HYPE and instead use nutrient soil leakage from an outer source. In this case the calculations above is skipped, while soil water and runoff processes is calculated as usual. HYPE output variables related to soil nutrients are set to missing values.
The first implemented soil leakage model was
1. montly seasonal variation of concentrations for each subbasin
The soil leakage is introduced into the model as inorganic and organic nitrogen, and soluble reactive phosphorus and particulate phosphorus concentrations of the total runoff. Different possibilities exist on how to introduce these regarding time variations (montly seasonal variation, daily), spatial variations (subbasin, regions) and land use dependence (class groups). So far the only implemented alternative is the monthly seasonal variation.
The nutrient concentration of total runoff (HYPE variable ID
crun) is set, and then used for all further calculations in the streams and lakes.
The default is to use HYPE calculated runoff concentrations with the processes described above (
modeloption soilleakage 0).
Eckersten, H., P.-E. Jansson, and H. Johnsson 1994. SOILN model – user’s manual 2nd edition, Division of Agricultural Hydrotechnics Communications 94:4, Department of Soil Sciences, Swedish University of AgriculturalSciences, 58pp, Uppsala.