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671 | werner | 2 | /******************************************************************************************** |
3 | ** iLand - an individual based forest landscape and disturbance model |
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4 | ** http://iland.boku.ac.at |
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5 | ** Copyright (C) 2009- Werner Rammer, Rupert Seidl |
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6 | ** |
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7 | ** This program is free software: you can redistribute it and/or modify |
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8 | ** it under the terms of the GNU General Public License as published by |
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9 | ** the Free Software Foundation, either version 3 of the License, or |
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10 | ** (at your option) any later version. |
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11 | ** |
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12 | ** This program is distributed in the hope that it will be useful, |
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13 | ** but WITHOUT ANY WARRANTY; without even the implied warranty of |
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14 | ** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
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15 | ** GNU General Public License for more details. |
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16 | ** |
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17 | ** You should have received a copy of the GNU General Public License |
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18 | ** along with this program. If not, see <http://www.gnu.org/licenses/>. |
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19 | ********************************************************************************************/ |
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20 | |||
237 | werner | 21 | #include "global.h" |
22 | #include "watercycle.h" |
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23 | #include "climate.h" |
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24 | #include "resourceunit.h" |
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25 | #include "species.h" |
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239 | werner | 26 | #include "model.h" |
626 | werner | 27 | #include "helper.h" |
646 | werner | 28 | #include "modules.h" |
237 | werner | 29 | |
697 | werner | 30 | /** @class WaterCycle |
31 | @ingroup core |
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32 | simulates the water cycle on a ResourceUnit. |
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33 | The WaterCycle is simulated with a daily time step on the spatial level of a ResourceUnit. Related are |
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34 | the snow module (SnowPack), and Canopy module that simulates the interception (and evaporation) of precipitation and the |
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35 | transpiration from the canopy. |
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36 | The WaterCycle covers the "soil water bucket". Main entry function is run(). |
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37 | |||
38 | See http://iland.boku.ac.at/water+cycle |
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39 | */ |
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40 | |||
237 | werner | 41 | WaterCycle::WaterCycle() |
42 | { |
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266 | werner | 43 | mSoilDepth = 0; |
496 | werner | 44 | mLastYear = -1; |
237 | werner | 45 | } |
46 | |||
239 | werner | 47 | void WaterCycle::setup(const ResourceUnit *ru) |
237 | werner | 48 | { |
49 | mRU = ru; |
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50 | // get values... |
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266 | werner | 51 | mFieldCapacity = 0.; // on top |
281 | werner | 52 | const XmlHelper &xml=GlobalSettings::instance()->settings(); |
53 | mSoilDepth = xml.valueDouble("model.site.soilDepth", 0.) * 10; // convert from cm to mm |
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338 | werner | 54 | double pct_sand = xml.valueDouble("model.site.pctSand"); |
55 | double pct_silt = xml.valueDouble("model.site.pctSilt"); |
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56 | double pct_clay = xml.valueDouble("model.site.pctClay"); |
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572 | werner | 57 | if (fabs(100. - (pct_sand + pct_silt + pct_clay)) > 0.01) |
338 | werner | 58 | 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)); |
59 | |||
60 | // calculate soil characteristics based on empirical functions (Schwalm & Ek, 2004) |
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61 | // note: the variables are percentages [0..100] |
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62 | mPsi_ref = -exp((1.54 - 0.0095*pct_sand + 0.0063*pct_silt) * log(10)) * 0.000098; // Eq. 83 |
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63 | mPsi_koeff_b = -( 3.1 + 0.157*pct_clay - 0.003*pct_sand ); // Eq. 84 |
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64 | mRho_ref = 0.01 * (50.5 - 0.142*pct_sand - 0.037*pct_clay); // Eq. 78 |
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240 | werner | 65 | mCanopy.setup(); |
339 | werner | 66 | |
67 | mPermanentWiltingPoint = heightFromPsi(-4000); // maximum psi is set to a constant of -4MPa |
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379 | werner | 68 | if (xml.valueBool("model.settings.waterUseSoilSaturation",false)==false) { |
69 | mFieldCapacity = heightFromPsi(-15); |
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70 | } else { |
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71 | // =-EXP((1.54-0.0095* pctSand +0.0063* pctSilt)*LN(10))*0.000098 |
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72 | double psi_sat = -exp((1.54-0.0095 * pct_sand + 0.0063*pct_silt)*log(10.))*0.000098; |
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73 | mFieldCapacity = heightFromPsi(psi_sat); |
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431 | werner | 74 | if (logLevelDebug()) qDebug() << "psi: saturation " << psi_sat << "field capacity:" << mFieldCapacity; |
379 | werner | 75 | } |
76 | |||
266 | werner | 77 | mContent = mFieldCapacity; // start with full water content (in the middle of winter) |
431 | werner | 78 | if (logLevelDebug()) qDebug() << "setup of water: Psi_ref (kPa)" << mPsi_ref << "Rho_ref" << mRho_ref << "coeff. b" << mPsi_koeff_b; |
566 | werner | 79 | mCanopyConductance = 0.; |
496 | werner | 80 | mLastYear = -1; |
621 | werner | 81 | |
82 | // canopy settings |
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83 | mCanopy.mNeedleFactor = xml.valueDouble("model.settings.interceptionStorageNeedle", 4.); |
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84 | mCanopy.mDecidousFactor = xml.valueDouble("model.settings.interceptionStorageBroadleaf", 2.); |
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85 | mSnowPack.mSnowTemperature = xml.valueDouble("model.settings.snowMeltTemperature", 0.); |
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237 | werner | 86 | } |
87 | |||
331 | werner | 88 | /** function to calculate the water pressure [saugspannung] for a given amount of water. |
338 | werner | 89 | returns water potential in kPa. |
90 | see http://iland.boku.ac.at/water+cycle#transpiration_and_canopy_conductance */ |
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266 | werner | 91 | inline double WaterCycle::psiFromHeight(const double mm) const |
92 | { |
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93 | // psi_x = psi_ref * ( rho_x / rho_ref) ^ b |
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94 | if (mm<0.001) |
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95 | return -100000000; |
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96 | double psi_x = mPsi_ref * pow((mm / mSoilDepth / mRho_ref),mPsi_koeff_b); |
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338 | werner | 97 | return psi_x; // pis |
266 | werner | 98 | } |
99 | |||
331 | werner | 100 | /// calculate the height of the water column for a given pressure |
338 | werner | 101 | /// return water amount in mm |
102 | /// see http://iland.boku.ac.at/water+cycle#transpiration_and_canopy_conductance |
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266 | werner | 103 | inline double WaterCycle::heightFromPsi(const double psi_kpa) const |
104 | { |
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105 | // rho_x = rho_ref * (psi_x / psi_ref)^(1/b) |
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106 | double h = mSoilDepth * mRho_ref * pow(psi_kpa / mPsi_ref, 1./mPsi_koeff_b); |
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107 | return h; |
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108 | } |
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109 | |||
331 | werner | 110 | /// get canopy characteristics of the resource unit. |
111 | /// It is important, that species-statistics are valid when this function is called (LAI)! |
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237 | werner | 112 | void WaterCycle::getStandValues() |
113 | { |
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246 | werner | 114 | mLAINeedle=mLAIBroadleaved=0.; |
237 | werner | 115 | mCanopyConductance=0.; |
553 | werner | 116 | const double ground_vegetationCC = 0.02; |
237 | werner | 117 | double lai; |
720 | werner | 118 | foreach(const ResourceUnitSpecies *rus, mRU->ruSpecies()) { |
455 | werner | 119 | lai = rus->constStatistics().leafAreaIndex(); |
120 | if (rus->species()->isConiferous()) |
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237 | werner | 121 | mLAINeedle+=lai; |
122 | else |
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123 | mLAIBroadleaved+=lai; |
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455 | werner | 124 | mCanopyConductance += rus->species()->canopyConductance() * lai; // weigh with LAI |
237 | werner | 125 | } |
126 | double total_lai = mLAIBroadleaved+mLAINeedle; |
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502 | werner | 127 | |
128 | // handle cases with LAI < 1 (use generic "ground cover characteristics" instead) |
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129 | if (total_lai<1.) { |
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553 | werner | 130 | mCanopyConductance+=(ground_vegetationCC)*(1. - total_lai); |
502 | werner | 131 | total_lai = 1.; |
237 | werner | 132 | } |
502 | werner | 133 | mCanopyConductance /= total_lai; |
134 | |||
368 | werner | 135 | if (total_lai < Model::settings().laiThresholdForClosedStands) { |
136 | // following Landsberg and Waring: when LAI is < 3 (default for laiThresholdForClosedStands), a linear "ramp" from 0 to 3 is assumed |
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299 | werner | 137 | // http://iland.boku.ac.at/water+cycle#transpiration_and_canopy_conductance |
368 | werner | 138 | mCanopyConductance *= total_lai / Model::settings().laiThresholdForClosedStands; |
237 | werner | 139 | } |
431 | werner | 140 | if (logLevelInfo()) qDebug() << "WaterCycle:getStandValues: LAI needle" << mLAINeedle << "LAI Broadl:"<< mLAIBroadleaved << "weighted avg. Conductance (m/2):" << mCanopyConductance; |
237 | werner | 141 | } |
142 | |||
502 | werner | 143 | /// calculate responses for ground vegetation, i.e. for "unstocked" areas. |
144 | /// this duplicates calculations done in Species. |
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145 | /// @return Minimum of vpd and soilwater response for default |
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146 | inline double WaterCycle::calculateBaseSoilAtmosphereResponse(const double psi_kpa, const double vpd_kpa) |
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147 | { |
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148 | // constant parameters used for ground vegetation: |
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503 | werner | 149 | const double mPsiMin = 1.5; // MPa |
502 | werner | 150 | const double mRespVpdExponent = -0.6; |
151 | // see SpeciesResponse::soilAtmosphereResponses() |
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152 | double water_resp; |
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153 | // see Species::soilwaterResponse: |
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154 | const double psi_mpa = psi_kpa / 1000.; // convert to MPa |
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155 | water_resp = limit( 1. - psi_mpa / mPsiMin, 0., 1.); |
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156 | // see species::vpdResponse |
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157 | |||
158 | double vpd_resp; |
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159 | vpd_resp = exp(mRespVpdExponent * vpd_kpa); |
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160 | return qMin(water_resp, vpd_resp); |
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161 | } |
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162 | |||
367 | werner | 163 | /// calculate combined VPD and soilwaterresponse for all species |
164 | /// on the RU. This is used for the calc. of the transpiration. |
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165 | inline double WaterCycle::calculateSoilAtmosphereResponse(const double psi_kpa, const double vpd_kpa) |
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166 | { |
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167 | double min_response; |
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502 | werner | 168 | double total_response = 0; // LAI weighted minimum response for all speices on the RU |
169 | double total_lai_factor = 0.; |
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720 | werner | 170 | foreach(const ResourceUnitSpecies *rus, mRU->ruSpecies()) { |
171 | if (rus->LAIfactor()>0.) { |
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502 | werner | 172 | // retrieve the minimum of VPD / soil water response for that species |
455 | werner | 173 | rus->speciesResponse()->soilAtmosphereResponses(psi_kpa, vpd_kpa, min_response); |
174 | total_response += min_response * rus->LAIfactor(); |
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502 | werner | 175 | total_lai_factor += rus->LAIfactor(); |
367 | werner | 176 | } |
177 | } |
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455 | werner | 178 | |
502 | werner | 179 | if (total_lai_factor<1.) { |
180 | // the LAI is below 1: the rest is considered as "ground vegetation" |
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181 | total_response += calculateBaseSoilAtmosphereResponse(psi_kpa, vpd_kpa) * (1. - total_lai_factor); |
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182 | } |
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183 | |||
377 | werner | 184 | // add an aging factor to the total response (averageAging: leaf area weighted mean aging value): |
185 | // conceptually: response = min(vpd_response, water_response)*aging |
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503 | werner | 186 | if (total_lai_factor==1.) |
187 | total_response *= mRU->averageAging(); // no ground cover: use aging value for all LA |
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188 | else if (total_lai_factor>0. && mRU->averageAging()>0.) |
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189 | total_response *= (1.-total_lai_factor)*1. + (total_lai_factor * mRU->averageAging()); // between 0..1: a part of the LAI is "ground cover" (aging=1) |
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502 | werner | 190 | |
720 | werner | 191 | DBGMODE( |
192 | if (mRU->averageAging()>1. || mRU->averageAging()<0. || total_response<0 || total_response>1.) |
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193 | qDebug() << "water cycle: average aging invalid. aging:" << mRU->averageAging() << "total response" << total_response << "total lai factor:" << total_lai_factor; |
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484 | werner | 194 | ); |
482 | werner | 195 | |
196 | //DBG_IF(mRU->averageAging()>1. || mRU->averageAging()<0.,"water cycle", "average aging invalid!" ); |
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367 | werner | 197 | return total_response; |
198 | } |
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199 | |||
200 | |||
237 | werner | 201 | /// Main Water Cycle function. This function triggers all water related tasks for |
202 | /// one simulation year. |
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299 | werner | 203 | /// @sa http://iland.boku.ac.at/water+cycle |
237 | werner | 204 | void WaterCycle::run() |
205 | { |
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496 | werner | 206 | // necessary? |
207 | if (GlobalSettings::instance()->currentYear() == mLastYear) |
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208 | return; |
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626 | werner | 209 | DebugTimer tw("water:run"); |
646 | werner | 210 | WaterCycleData add_data; |
626 | werner | 211 | |
237 | werner | 212 | // preparations (once a year) |
367 | werner | 213 | getStandValues(); // fetch canopy characteristics from iLand (including weighted average for mCanopyConductance) |
237 | werner | 214 | mCanopy.setStandParameters(mLAINeedle, |
215 | mLAIBroadleaved, |
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216 | mCanopyConductance); |
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217 | |||
218 | // main loop over all days of the year |
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239 | werner | 219 | double prec_mm, prec_after_interception, prec_to_soil, et, excess; |
338 | werner | 220 | const Climate *climate = mRU->climate(); |
221 | const ClimateDay *day = climate->begin(); |
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222 | const ClimateDay *end = climate->end(); |
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237 | werner | 223 | int doy=0; |
224 | double total_excess = 0.; |
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225 | for (; day<end; ++day, ++doy) { |
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226 | // (1) precipitation of the day |
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227 | prec_mm = day->preciptitation; |
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228 | // (2) interception by the crown |
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229 | prec_after_interception = mCanopy.flow(prec_mm, day->temperature); |
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230 | // (3) storage in the snow pack |
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231 | prec_to_soil = mSnowPack.flow(prec_after_interception, day->temperature); |
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646 | werner | 232 | // save extra data (used by e.g. fire module) |
233 | add_data.water_to_ground[doy] = prec_to_soil; |
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234 | add_data.snow_cover[doy] = mSnowPack.snowPack(); |
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237 | werner | 235 | // (4) add rest to soil |
236 | mContent += prec_to_soil; |
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266 | werner | 237 | |
238 | excess = 0.; |
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239 | if (mContent>mFieldCapacity) { |
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240 | // excess water runoff |
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241 | excess = mContent - mFieldCapacity; |
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242 | total_excess += excess; |
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243 | mContent = mFieldCapacity; |
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244 | } |
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245 | |||
367 | werner | 246 | double current_psi = psiFromHeight(mContent); |
247 | mPsi[doy] = current_psi; |
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553 | werner | 248 | |
335 | werner | 249 | // (5) transpiration of the vegetation (and of water intercepted in canopy) |
367 | werner | 250 | // calculate the LAI-weighted response values for soil water and vpd: |
680 | werner | 251 | double interception_before_transpiration = mCanopy.interception(); |
367 | werner | 252 | double combined_response = calculateSoilAtmosphereResponse( current_psi, day->vpd); |
253 | et = mCanopy.evapotranspiration3PG(day, climate->daylength_h(doy), combined_response); |
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680 | werner | 254 | // if there is some flow from intercepted water to the ground -> add to "water_to_the_ground" |
255 | if (mCanopy.interception() < interception_before_transpiration) |
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256 | add_data.water_to_ground[doy]+= interception_before_transpiration - mCanopy.interception(); |
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238 | werner | 257 | |
241 | werner | 258 | mContent -= et; // reduce content (transpiration) |
338 | werner | 259 | // add intercepted water (that is *not* evaporated) again to the soil (or add to snow if temp too low -> call to snowpack) |
260 | mContent += mSnowPack.add(mCanopy.interception(),day->temperature); |
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241 | werner | 261 | |
338 | werner | 262 | // do not remove water below the PWP (fixed value) |
266 | werner | 263 | if (mContent<mPermanentWiltingPoint) { |
271 | werner | 264 | et -= mPermanentWiltingPoint - mContent; // reduce et (for bookkeeping) |
266 | werner | 265 | mContent = mPermanentWiltingPoint; |
237 | werner | 266 | } |
266 | werner | 267 | |
546 | werner | 268 | |
271 | werner | 269 | //DBGMODE( |
239 | werner | 270 | if (GlobalSettings::instance()->isDebugEnabled(GlobalSettings::dWaterCycle)) { |
240 | werner | 271 | DebugList &out = GlobalSettings::instance()->debugList(day->id(), GlobalSettings::dWaterCycle); |
239 | werner | 272 | // climatic variables |
605 | werner | 273 | out << day->id() << mRU->index() << mRU->id() << day->temperature << day->vpd << day->preciptitation << day->radiation; |
368 | werner | 274 | out << combined_response; // combined response of all species on RU (min(water, vpd)) |
239 | werner | 275 | // fluxes |
271 | werner | 276 | out << prec_after_interception << prec_to_soil << et << mCanopy.evaporationCanopy() |
540 | werner | 277 | << mContent << mPsi[doy] << excess; |
239 | werner | 278 | // other states |
279 | out << mSnowPack.snowPack(); |
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364 | werner | 280 | //special sanity check: |
281 | if (prec_to_soil>0. && mCanopy.interception()>0.) |
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282 | if (mSnowPack.snowPack()==0. && day->preciptitation==0) |
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283 | qDebug() << "watercontent increase without precipititaion"; |
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239 | werner | 284 | |
285 | } |
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271 | werner | 286 | //); // DBGMODE() |
239 | werner | 287 | |
237 | werner | 288 | } |
646 | werner | 289 | // call external modules |
290 | GlobalSettings::instance()->model()->modules()->calculateWater(mRU, &add_data); |
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496 | werner | 291 | mLastYear = GlobalSettings::instance()->currentYear(); |
292 | |||
237 | werner | 293 | } |
294 | |||
295 | |||
296 | namespace Water { |
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297 | |||
298 | /** calculates the input/output of water that is stored in the snow pack. |
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299 | The approach is similar to Picus 1.3 and ForestBGC (Running, 1988). |
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300 | Returns the amount of water that exits the snowpack (precipitation, snow melt) */ |
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301 | double SnowPack::flow(const double &preciptitation_mm, const double &temperature) |
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302 | { |
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621 | werner | 303 | if (temperature>mSnowTemperature) { |
237 | werner | 304 | if (mSnowPack==0.) |
305 | return preciptitation_mm; // no change |
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306 | else { |
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307 | // snow melts |
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308 | const double melting_coefficient = 0.7; // mm/°C |
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621 | werner | 309 | double melt = qMin( (temperature-mSnowTemperature) * melting_coefficient, mSnowPack); |
240 | werner | 310 | mSnowPack -=melt; |
237 | werner | 311 | return preciptitation_mm + melt; |
312 | } |
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313 | } else { |
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314 | // snow: |
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315 | mSnowPack += preciptitation_mm; |
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316 | return 0.; // no output. |
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317 | } |
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318 | |||
319 | } |
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320 | |||
321 | |||
334 | werner | 322 | inline double SnowPack::add(const double &preciptitation_mm, const double &temperature) |
323 | { |
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324 | // do nothing for temps > 0° |
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621 | werner | 325 | if (temperature>mSnowTemperature) |
334 | werner | 326 | return preciptitation_mm; |
327 | |||
328 | // temps < 0°: add to snow |
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329 | mSnowPack += preciptitation_mm; |
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330 | return 0.; |
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331 | } |
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332 | |||
237 | werner | 333 | /** Interception in crown canopy. |
334 | Calculates the amount of preciptitation that does not reach the ground and |
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335 | is stored in the canopy. The approach is adopted from Picus 1.3. |
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299 | werner | 336 | Returns the amount of precipitation (mm) that surpasses the canopy layer. |
337 | @sa http://iland.boku.ac.at/water+cycle#precipitation_and_interception */ |
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237 | werner | 338 | double Canopy::flow(const double &preciptitation_mm, const double &temperature) |
339 | { |
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340 | // sanity checks |
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341 | mInterception = 0.; |
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271 | werner | 342 | mEvaporation = 0.; |
237 | werner | 343 | if (!mLAI) |
344 | return preciptitation_mm; |
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345 | if (!preciptitation_mm) |
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346 | return 0.; |
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347 | double max_interception_mm=0.; // maximum interception based on the current foliage |
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348 | double max_storage_mm=0.; // maximum storage in canopy |
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349 | |||
350 | if (mLAINeedle>0.) { |
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351 | // (1) calculate maximum fraction of thru-flow the crown (based on precipitation) |
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352 | double max_flow_needle = 0.9 * sqrt(1.03 - exp(-0.055*preciptitation_mm)); |
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353 | max_interception_mm += preciptitation_mm * (1. - max_flow_needle * mLAINeedle/mLAI); |
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354 | // (2) calculate maximum storage potential based on the current LAI |
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621 | werner | 355 | double max_storage_needle = mNeedleFactor * (1. - exp(-0.55*mLAINeedle) ); |
237 | werner | 356 | max_storage_mm += max_storage_needle; |
357 | } |
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358 | |||
359 | if (mLAIBroadleaved>0.) { |
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360 | // (1) calculate maximum fraction of thru-flow the crown (based on precipitation) |
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299 | werner | 361 | double max_flow_broad = 0.9 * pow(1.22 - exp(-0.055*preciptitation_mm), 0.35); |
237 | werner | 362 | max_interception_mm += preciptitation_mm * (1. - max_flow_broad * mLAIBroadleaved/mLAI); |
363 | // (2) calculate maximum storage potential based on the current LAI |
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621 | werner | 364 | double max_storage_broad = mDecidousFactor * (1. - exp(-0.5*mLAIBroadleaved) ); |
237 | werner | 365 | max_storage_mm += max_storage_broad; |
366 | } |
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367 | |||
368 | // (3) calculate actual interception and store for evaporation calculation |
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369 | mInterception = qMin( max_storage_mm, max_interception_mm ); |
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370 | |||
335 | werner | 371 | // (4) limit interception with amount of precipitation |
372 | mInterception = qMin( mInterception, preciptitation_mm); |
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373 | |||
374 | // (5) reduce preciptitaion by the amount that is intercepted by the canopy |
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237 | werner | 375 | return preciptitation_mm - mInterception; |
376 | |||
377 | } |
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378 | |||
239 | werner | 379 | /// sets up the canopy. fetch some global parameter values... |
380 | void Canopy::setup() |
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237 | werner | 381 | { |
240 | werner | 382 | mAirDensity = Model::settings().airDensity; // kg / m3 |
237 | werner | 383 | } |
384 | |||
385 | void Canopy::setStandParameters(const double LAIneedle, const double LAIbroadleave, const double maxCanopyConductance) |
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386 | { |
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387 | mLAINeedle = LAIneedle; |
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388 | mLAIBroadleaved=LAIbroadleave; |
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389 | mLAI=LAIneedle+LAIbroadleave; |
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239 | werner | 390 | mAvgMaxCanopyConductance = maxCanopyConductance; |
553 | werner | 391 | |
392 | // clear aggregation containers |
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562 | werner | 393 | for (int i=0;i<12;++i) mET0[i]=0.; |
553 | werner | 394 | |
237 | werner | 395 | } |
396 | |||
397 | |||
398 | |||
256 | werner | 399 | /** calculate the daily evaporation/transpiration using the Penman-Monteith-Equation. |
400 | This version is based on 3PG. See the Visual Basic Code in 3PGjs.xls. |
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401 | Returns the total sum of evaporation+transpiration in mm of the day. */ |
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367 | werner | 402 | double Canopy::evapotranspiration3PG(const ClimateDay *climate, const double daylength_h, const double combined_response) |
256 | werner | 403 | { |
404 | double vpd_mbar = climate->vpd * 10.; // convert from kPa to mbar |
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405 | double temperature = climate->temperature; // average temperature of the day (°C) |
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406 | double daylength = daylength_h * 3600.; // daylength in seconds (convert from length in hours) |
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407 | double rad = climate->radiation / daylength * 1000000; //convert from MJ/m2 (day sum) to average radiation flow W/m2 [MJ=MWs -> /s * 1,000,000 |
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408 | |||
561 | werner | 409 | // the radiation: based on linear empirical function |
410 | const double qa = -90.; |
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411 | const double qb = 0.8; |
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412 | double net_rad = qa + qb*rad; |
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413 | |||
256 | werner | 414 | //: Landsberg original: const double e20 = 2.2; //rate of change of saturated VP with T at 20C |
415 | const double VPDconv = 0.000622; //convert VPD to saturation deficit = 18/29/1000 |
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416 | const double latent_heat = 2460000.; // Latent heat of vaporization. Energy required per unit mass of water vaporized [J kg-1] |
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417 | |||
368 | werner | 418 | double gBL = Model::settings().boundaryLayerConductance; // boundary layer conductance |
419 | |||
420 | // canopy conductance. |
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421 | // The species traits are weighted by LAI on the RU. |
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422 | // maximum canopy conductance: see getStandValues() |
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423 | // current response: see calculateSoilAtmosphereResponse(). This is basically a weighted average of min(water_response, vpd_response) for each species |
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367 | werner | 424 | double gC = mAvgMaxCanopyConductance * combined_response; |
256 | werner | 425 | |
367 | werner | 426 | |
256 | werner | 427 | double defTerm = mAirDensity * latent_heat * (vpd_mbar * VPDconv) * gBL; |
428 | |||
561 | werner | 429 | // with temperature-dependent slope of vapor pressure saturation curve |
430 | // (following Allen et al. (1998), http://www.fao.org/docrep/x0490e/x0490e07.htm#atmospheric%20parameters) |
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431 | // svp_slope in mbar. |
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621 | werner | 432 | //double svp_slope = 4098. * (6.1078 * exp(17.269 * temperature / (temperature + 237.3))) / ((237.3+temperature)*(237.3+temperature)); |
561 | werner | 433 | |
621 | werner | 434 | // alternatively: very simple variant (following here the original 3PG code). This |
435 | // keeps yields +- same results for summer, but slightly lower values in winter (2011/03/16) |
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436 | double svp_slope = 2.2; |
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437 | |||
256 | werner | 438 | double div = (1. + svp_slope + gBL / gC); |
561 | werner | 439 | double Etransp = (svp_slope * net_rad + defTerm) / div; |
335 | werner | 440 | double canopy_transpiration = Etransp / latent_heat * daylength; |
256 | werner | 441 | |
562 | werner | 442 | // calculate reference evapotranspiration |
443 | // see Adair et al 2008 |
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444 | const double psychrometric_const = 0.0672718682328237; // kPa/degC |
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445 | const double windspeed = 2.; // m/s |
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446 | double net_rad_mj_day = net_rad*daylength/1000000.; // convert W/m2 again to MJ/m2*day |
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447 | double et0_day = 0.408*svp_slope*net_rad_mj_day + psychrometric_const*900./(temperature+273.)*windspeed*climate->vpd; |
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448 | double et0_div = svp_slope+psychrometric_const*(1.+0.34*windspeed); |
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449 | et0_day = et0_day / et0_div; |
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450 | mET0[climate->month-1] += et0_day; |
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553 | werner | 451 | |
256 | werner | 452 | if (mInterception>0.) { |
453 | // we assume that for evaporation from leaf surface gBL/gC -> 0 |
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562 | werner | 454 | double div_evap = 1. + svp_slope; |
620 | werner | 455 | double evap_canopy_potential = (svp_slope*net_rad + defTerm) / div_evap / latent_heat * daylength; |
456 | // reduce the amount of transpiration on a wet day based on the approach of |
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457 | // Wigmosta et al (1994). see http://iland.boku.ac.at/water+cycle#transpiration_and_canopy_conductance |
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458 | |||
459 | double ratio_T_E = canopy_transpiration / evap_canopy_potential; |
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460 | double evap_canopy = qMin(evap_canopy_potential, mInterception); |
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461 | |||
462 | // for interception -> 0, the canopy transpiration is unchanged |
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463 | canopy_transpiration = (evap_canopy_potential - evap_canopy) * ratio_T_E; |
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464 | |||
562 | werner | 465 | mInterception -= evap_canopy; // reduce interception |
466 | mEvaporation = evap_canopy; // evaporation from intercepted water |
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620 | werner | 467 | |
256 | werner | 468 | } |
335 | werner | 469 | return canopy_transpiration; |
256 | werner | 470 | } |
471 | |||
237 | werner | 472 | } // end namespace |