Subversion Repositories public iLand

#include "global.h"

#include "watercycle.h"

#include "climate.h"

#include "resourceunit.h"

#include "species.h"

#include "model.h"

WaterCycle::WaterCycle()

{

    mSoilDepth = 0;

}

void WaterCycle::setup(const ResourceUnit *ru)

{

    mRU = ru;

    // get values...

    mFieldCapacity = 0.; // on top

    const XmlHelper &xml=GlobalSettings::instance()->settings();

    mSoilDepth = xml.valueDouble("model.site.soilDepth", 0.) * 10; // convert from cm to mm

    double pct_sand = xml.valueDouble("model.site.pctSand");

    double pct_silt = xml.valueDouble("model.site.pctSilt");

    double pct_clay = xml.valueDouble("model.site.pctClay");

    if (pct_sand + pct_silt + pct_clay != 100.)

        throw IException(QString("Setup Watercycle: soil composition percentages do not sum up to 100. Sand: %1, Silt: %2 Clay: %3").arg(pct_sand).arg(pct_silt).arg(pct_clay));

    // calculate soil characteristics based on empirical functions (Schwalm & Ek, 2004)

    // note: the variables are percentages [0..100]

    mPsi_ref = -exp((1.54 - 0.0095*pct_sand + 0.0063*pct_silt) * log(10)) * 0.000098; // Eq. 83

    mPsi_koeff_b = -( 3.1 + 0.157*pct_clay - 0.003*pct_sand );  // Eq. 84

    mRho_ref = 0.01 * (50.5 - 0.142*pct_sand - 0.037*pct_clay); // Eq. 78

    mCanopy.setup();

    mPermanentWiltingPoint = heightFromPsi(-4000); // maximum psi is set to a constant of -4MPa

    mFieldCapacity = heightFromPsi(-15);

    mContent = mFieldCapacity; // start with full water content (in the middle of winter)

    qDebug() << "setup of water: Psi_ref (kPa)" << mPsi_ref << "Rho_ref" << mRho_ref << "coeff. b" << mPsi_koeff_b;

}

/** function to calculate the water pressure [saugspannung] for a given amount of water.

    returns water potential in kPa.

  see http://iland.boku.ac.at/water+cycle#transpiration_and_canopy_conductance */

inline double WaterCycle::psiFromHeight(const double mm) const

{

    // psi_x = psi_ref * ( rho_x / rho_ref) ^ b

    if (mm<0.001)

        return -100000000;

    double psi_x = mPsi_ref * pow((mm / mSoilDepth / mRho_ref),mPsi_koeff_b);

    return psi_x; // pis

}

/// calculate the height of the water column for a given pressure

/// return water amount in mm

/// see http://iland.boku.ac.at/water+cycle#transpiration_and_canopy_conductance

inline double WaterCycle::heightFromPsi(const double psi_kpa) const

{

    // rho_x = rho_ref * (psi_x / psi_ref)^(1/b)

    double h = mSoilDepth * mRho_ref * pow(psi_kpa / mPsi_ref, 1./mPsi_koeff_b);

    return h;

}

/// get canopy characteristics of the resource unit.

/// It is important, that species-statistics are valid when this function is called (LAI)!

void WaterCycle::getStandValues()

{

    mLAINeedle=mLAIBroadleaved=0.;

    mCanopyConductance=0.;

    double lai;

    foreach(const ResourceUnitSpecies &rus, mRU->ruSpecies()) {

        lai = rus.constStatistics().leafAreaIndex();

        if (rus.species()->isConiferous())

            mLAINeedle+=lai;

        else

            mLAIBroadleaved+=lai;

        mCanopyConductance += rus.species()->canopyConductance() * lai; // weigh with LAI

    }

    double total_lai = mLAIBroadleaved+mLAINeedle;

    if (total_lai>0.) {

        mCanopyConductance /= total_lai;

    } else {

        mCanopyConductance = 0.02;

    }

    if (total_lai < 3.) {

        // following Landsberg and Waring: when LAI is < 3, a linear "ramp" from 0 to 3 is assumed

        // http://iland.boku.ac.at/water+cycle#transpiration_and_canopy_conductance

        mCanopyConductance *= total_lai / 3.;

    }

    qDebug() << "WaterCycle:getStandValues: LAI needle" << mLAINeedle << "LAI Broadl:"<< mLAIBroadleaved << "weighted avg. Conductance (m/2):" << mCanopyConductance;

}

/// Main Water Cycle function. This function triggers all water related tasks for

/// one simulation year.

/// @sa http://iland.boku.ac.at/water+cycle

void WaterCycle::run()

{

    // preparations (once a year)

    getStandValues(); // fetch canopy characteristics from iLand

    mCanopy.setStandParameters(mLAINeedle,

                               mLAIBroadleaved,

                               mCanopyConductance);

    // main loop over all days of the year

    double prec_mm, prec_after_interception, prec_to_soil, et, excess;

    const Climate *climate = mRU->climate();

    const ClimateDay *day = climate->begin();

    const ClimateDay *end = climate->end();

    int doy=0;

    double total_excess = 0.;

    for (; day<end; ++day, ++doy) {

        // (1) precipitation of the day

        prec_mm = day->preciptitation;

        // (2) interception by the crown

        prec_after_interception = mCanopy.flow(prec_mm, day->temperature);

        // (3) storage in the snow pack

        prec_to_soil = mSnowPack.flow(prec_after_interception, day->temperature);

        // (4) add rest to soil

        mContent += prec_to_soil;

        excess = 0.;

        if (mContent>mFieldCapacity) {

            // excess water runoff

            excess = mContent - mFieldCapacity;

            total_excess += excess;

            mContent = mFieldCapacity;

        }

        // calculate the relative water content

        mRelativeContent[doy] = currentRelContent();

        mPsi[doy] = psiFromHeight(mContent);

        // (5) transpiration of the vegetation (and of water intercepted in canopy)

        et = mCanopy.evapotranspiration3PG(day, climate->daylength_h(doy));

        mContent -= et; // reduce content (transpiration)

        // add intercepted water (that is *not* evaporated) again to the soil (or add to snow if temp too low -> call to snowpack)

        mContent += mSnowPack.add(mCanopy.interception(),day->temperature);

        // do not remove water below the PWP (fixed value)

        if (mContent<mPermanentWiltingPoint) {

            et -= mPermanentWiltingPoint - mContent; // reduce et (for bookkeeping)

            mContent = mPermanentWiltingPoint;

        }

        //DBGMODE(

            if (GlobalSettings::instance()->isDebugEnabled(GlobalSettings::dWaterCycle)) {

                DebugList &out = GlobalSettings::instance()->debugList(day->id(), GlobalSettings::dWaterCycle);

                // climatic variables

                out << day->id() << day->temperature << day->vpd << day->preciptitation << day->radiation;

                // fluxes

                out << prec_after_interception << prec_to_soil << et << mCanopy.evaporationCanopy()

                        << mRelativeContent[doy]*mSoilDepth << mPsi[doy] << excess;

                // other states

                out << mSnowPack.snowPack();

            }

        //); // DBGMODE()

    }

}

namespace Water {

/** calculates the input/output of water that is stored in the snow pack.

  The approach is similar to Picus 1.3 and ForestBGC (Running, 1988).

  Returns the amount of water that exits the snowpack (precipitation, snow melt) */

double SnowPack::flow(const double &preciptitation_mm, const double &temperature)

{

    if (temperature>0.) {

        if (mSnowPack==0.)

            return preciptitation_mm; // no change

        else {

            // snow melts

            const double melting_coefficient = 0.7; // mm/°C

            double melt = qMin(temperature * melting_coefficient, mSnowPack);

            mSnowPack -=melt;

            return preciptitation_mm + melt;

        }

    } else {

        // snow:

        mSnowPack += preciptitation_mm;

        return 0.; // no output.

    }

}

inline double SnowPack::add(const double &preciptitation_mm, const double &temperature)

{

    // do nothing for temps > 0°

    if (temperature>0.)

        return preciptitation_mm;

    // temps < 0°: add to snow

    mSnowPack += preciptitation_mm;

    return 0.;

}

/** Interception in crown canopy.

    Calculates the amount of preciptitation that does not reach the ground and

    is stored in the canopy. The approach is adopted from Picus 1.3.

    Returns the amount of precipitation (mm) that surpasses the canopy layer.

    @sa http://iland.boku.ac.at/water+cycle#precipitation_and_interception */

double Canopy::flow(const double &preciptitation_mm, const double &temperature)

{

    // sanity checks

    mInterception = 0.;

    mEvaporation = 0.;

    if (!mLAI)

        return preciptitation_mm;

    if (!preciptitation_mm)

        return 0.;

    double max_interception_mm=0.; // maximum interception based on the current foliage

    double max_storage_mm=0.; // maximum storage in canopy

    if (mLAINeedle>0.) {

        // (1) calculate maximum fraction of thru-flow the crown (based on precipitation)

        double max_flow_needle = 0.9 * sqrt(1.03 - exp(-0.055*preciptitation_mm));

        max_interception_mm += preciptitation_mm *  (1. - max_flow_needle * mLAINeedle/mLAI);

        // (2) calculate maximum storage potential based on the current LAI

        double max_storage_needle = 4. * (1. - exp(-0.55*mLAINeedle) );

        max_storage_mm += max_storage_needle;

    }

    if (mLAIBroadleaved>0.) {

        // (1) calculate maximum fraction of thru-flow the crown (based on precipitation)

        double max_flow_broad = 0.9 * pow(1.22 - exp(-0.055*preciptitation_mm), 0.35);

        max_interception_mm += preciptitation_mm *  (1. - max_flow_broad * mLAIBroadleaved/mLAI);

        // (2) calculate maximum storage potential based on the current LAI

        double max_storage_broad = 2. * (1. - exp(-0.5*mLAIBroadleaved) );

        max_storage_mm += max_storage_broad;

    }

    // (3) calculate actual interception and store for evaporation calculation

    mInterception = qMin( max_storage_mm, max_interception_mm );

    // (4) limit interception with amount of precipitation

    mInterception = qMin( mInterception, preciptitation_mm);

    // (5) reduce preciptitaion by the amount that is intercepted by the canopy

    return preciptitation_mm - mInterception;

}

/// sets up the canopy. fetch some global parameter values...

void Canopy::setup()

{

    mHeatCapacityAir = Model::settings().heatCapacityAir; // J/kg/°C

    mAirDensity = Model::settings().airDensity; // kg / m3

    double airPressure = Model::settings().airPressure; // mbar

    double heat_capacity_kJ = mHeatCapacityAir / 1000; // convert to: kJ/kg/°C

    const double latent_heat_water = 2450;// kJ/kg

    // calc psychrometric constant (in mbar/°C): see also http://en.wikipedia.org/wiki/Psychrometric_constant

    mPsychrometricConstant = heat_capacity_kJ*airPressure/latent_heat_water/0.622;

}

void Canopy::setStandParameters(const double LAIneedle, const double LAIbroadleave, const double maxCanopyConductance)

{

    mLAINeedle = LAIneedle;

    mLAIBroadleaved=LAIbroadleave;

    mLAI=LAIneedle+LAIbroadleave;

    mAvgMaxCanopyConductance = maxCanopyConductance;

}

/** calculate the daily evaporation/transpiration using the Penman-Monteith-Equation.

   The application of the equation follows broadly Running (1988).

   Returns the total sum of evaporation+transpiration in mm of the day. */

double Canopy::evapotranspirationBGC(const ClimateDay *climate, const double daylength_h)

{

    double vpd_mbar = climate->vpd * 10.; // convert from kPa to mbar

    double temperature = climate->temperature; // average temperature of the day (°C)

    double daylength = daylength_h * 3600.; // daylength in seconds (convert from length in hours)

    double rad = climate->radiation / daylength * 1000000; //convert from MJ/m2 (day sum) to average radiation flow W/m2 [MJ=MWs -> /s * 1,000,000

    const double aerodynamic_resistance = 5.; // m/s: aerodynamic resistance of the canopy is considered being constant

    const double latent_heat = 2257000.; // Latent heat of vaporization. Energy required per unit mass of water vaporized [J kg-1]

    // (1) calculate some intermediaries

    // current canopy conductance: is calculated following Landsberg & Waring

    // note: here we use vpd again in kPa.

    double current_canopy_conductance;

    current_canopy_conductance = mAvgMaxCanopyConductance * exp(-2.5 * climate->vpd);

    // saturation vapor pressure (Running 1988, Eq. 1) in mbar

    double svp = 6.1078 * exp((17.269 * temperature) / (237.3 + temperature) );

    // the slope of svp is, thanks to http://www.wolframalpha.com/input/?i=derive+y%3D6.1078+exp+((17.269x)/(237.3%2Bx))

    double svp_slope = svp * ( 17.269/(237.3+temperature) - 17.269*temperature/((237.3+temperature)*(237.3+temperature)) );

    double et; // transpiration in mm (follows Eq.(8) of Running, 1988).

    // note: RC (resistance of canopy) = 1/CC (conductance of canopy)

    double dim = svp_slope + mPsychrometricConstant*(1. + 1. / (current_canopy_conductance*aerodynamic_resistance));

    double dayl = daylength*mLAI / latent_heat;

    double upper = svp_slope*rad + mHeatCapacityAir*mAirDensity * vpd_mbar/aerodynamic_resistance;

    et = upper / dim * dayl;

    // now calculate the evaporation from intercepted preciptitaion in the canopy: 1+rc/ra -> 1

    if (mInterception>0.) {

        double dim_evap = svp_slope + mPsychrometricConstant;

        double pot_evap = upper / dim_evap * dayl;

        double evap = qMin(pot_evap, mInterception);

        mInterception -= evap; // reduce interception

        mEvaporation = evap;

    }

    return et;

}

/** calculate the daily evaporation/transpiration using the Penman-Monteith-Equation.

   This version is based on 3PG. See the Visual Basic Code in 3PGjs.xls.

   Returns the total sum of evaporation+transpiration in mm of the day. */

double Canopy::evapotranspiration3PG(const ClimateDay *climate, const double daylength_h)

{

    double vpd_mbar = climate->vpd * 10.; // convert from kPa to mbar

    double temperature = climate->temperature; // average temperature of the day (°C)

    double daylength = daylength_h * 3600.; // daylength in seconds (convert from length in hours)

    double rad = climate->radiation / daylength * 1000000; //convert from MJ/m2 (day sum) to average radiation flow W/m2 [MJ=MWs -> /s * 1,000,000

    //: Landsberg original: const double e20 = 2.2;  //rate of change of saturated VP with T at 20C

    const double VPDconv = 0.000622; //convert VPD to saturation deficit = 18/29/1000

    const double latent_heat = 2460000.; // Latent heat of vaporization. Energy required per unit mass of water vaporized [J kg-1]

    double gBL  = 0.2; // boundary layer conductance

    // canopy conductance scales linearly from 0 to LAI=3 and is constant for LAI > 3.

    double gC = mAvgMaxCanopyConductance * exp(-2.5 * climate->vpd);

    if (mLAI<3.)

        gC *= mLAI / 3.;

    double defTerm = mAirDensity * latent_heat * (vpd_mbar * VPDconv) * gBL;

        // saturation vapor pressure (Running 1988, Eq. 1) in mbar

    double svp = 6.1078 * exp((17.269 * temperature) / (237.3 + temperature) );

    // the slope of svp is, thanks to http://www.wolframalpha.com/input/?i=derive+y%3D6.1078+exp+((17.269x)/(237.3%2Bx))

    double svp_slope = svp * ( 17.269/(237.3+temperature) - 17.269*temperature/((237.3+temperature)*(237.3+temperature)) );

    double div = (1. + svp_slope + gBL / gC);

    double Etransp = (svp_slope * rad + defTerm) / div;

    double canopy_transpiration = Etransp / latent_heat * daylength;

    if (mInterception>0.) {

        // we assume that for evaporation from leaf surface gBL/gC -> 0

        double div_evap = 1 + svp_slope;

        double evap = (svp_slope*rad + defTerm) / div_evap / latent_heat * daylength;

        evap = qMin(evap, mInterception);

        mInterception -= evap; // reduce interception

        mEvaporation = evap; // evaporation from intercepted water

    }

    return canopy_transpiration;

}

} // end namespace