Soil & Water Assessment Tool

1 Theoretical Documentation Version 2009 Soil & Water Assessment Tool TR-406 COLLEGE OF AGRICULTURE AND LIFE SCIENCES TR-406 2011 Soil and Water Assessment Tool Theoretical Documentation Version 2009 By Neitsch, Arnold, Kiniry, Williams Grassland, Soil and Water Research Laboratory Agricultural Research Service Blackland Research Center Texas AgriLife Research September 2011 Texas Water Resources Institute Technical Report No. 406 Texas A&M University System College Station, Texas 77843-2118 SOIL AND Water Assessment TOOL THEORETICAL DOCUMENTATION VERSION 2009 NEITSCH, ARNOLD, KINIRY, WILLIAMS AUGUST, 2009 GRASSLAND, SOIL AND Water RESEARCH LABORATORY AGRICULTURAL RESEARCH SERVICE 808 EAST BLACKLAND ROAD TEMPLE, TE XAS 76502 BLACKLAND RESEARCH CENTER TEXAS AGRICULTURAL EXPERIMENT STATION 720 EAST BLACKLAND ROAD TEMPLE, TE XAS 76502 TR-406 ACKNOWLEDGEMENTS The SWAT model is a continuation of thirty years of non-point source modeling.

2 In addition to the Agricultural Research Service and Texas A&M University, several federal agencies including the US Environmental Protection Agency, Natural Resources Conservation Service, National Oceanic and Atmospheric Administration and Bureau of Indian Affairs have contributed to the model. We also want to thank all the state environmental agencies (with special thanks to Wisconsin Department of Natural Resources), state NRCS offices (with special thanks to the Texas State Office), numerous universities in the United States and abroad, and consultants who contributed to all aspects of the model. We appreciate your contributions and look forward to continued collaboration. CONTENTS INTRODUCTION 1 DEVELOPMENT OF SWAT 3 OVERVIEW OF SWAT 6 LAND PHASE OF THE HYDROLOGIC CYCLE 9 ROUTING PHASE OF THE HYDROLOGIC CYCLE 21 REFERENCES 24 SECTION 1: CLIMATE CHAPTER 1:1 EQUATIONS: ENERGY 29 1:1.

3 1 SUN-EARTH RELATIONSHIPS 30 DISTANCE BETWEEN EARTH AND SUN 30 SOLAR DECLINATION 30 SOLAR NOON, SUNRISE, SUNSET, AND DAYLENGTH 31 1:1 .2 SOLAR RADIATION 32 EXTRATERRESTRIAL RADIATION 32 SOLAR RADIATION UNDER CLOUDLESS SKIES 34 DAILY SOLAR RADIATION 34 HOURLY SOLAR RADIATION 35 DAILY NET RADIATION 36 1:1 .3 TEMPERATURE 39 DAILY AIR TEMPERATURE 39 HOURLY AIR TEMPERATURE 40 SOIL TEMPERATURE 40 Water TEMPERATURE 45 1:1 .4 WIND SPEED 46 1:1 .5 NOMENCLATURE 47 1:1 .6 REFERENCES 49 CHAPTER 1:2 EQUATIONS: ATMOSPHERIC Water 51 1:2 .1 PRECIPITATION 52 1:2 .2 MAXIMUM HALF-HOUR RAINFALL 53 1:2.

4 3 Water VAPOR 53 1:2 .4 SNOW COVER 56 1:2 .5 SNOW MELT 59 SNOW PACK TEMPERATURE 59 SNOW MELT EQUATION 60 1:2 .6 NOMENCLATURE 61 1:2 .7 REFERENCES 62 CHAPTER 1:3 EQUATIONS: WEATHER GENERATOR 65 1:3 .1 PRECIPITATION 66 OCCURRENCE OF WET OR DRY DAY 66 AMOUNT OF PRECIPITATION 67 1 MAXIMUM HALF-HOUR RAINFALL 68 MONTHLY MAXIMUM HALF-HOUR RAIN 68 DAILY MAXIMUM HALF-HOUR RAIN VALUE 69 1 DISTRIBUTION OF RAINFALL WITHIN DAY 71 NORMALIZED INTENSITY DISTRIBUTION 71 GENERATED TIME TO PEAK INTENSITY 73 TOTAL RAINFALL AND DURATION 74 1:3 .4 SOLAR RADIATION & TEMPERATURE 76 DAILY RESIDUALS 76 GENERATED VALUES 78 ADJUSTMENT FOR CLEAR/OVERCAST CONDITIONS 79 1:3.

5 5 RELATIVE HUMIDITY 81 MEAN MONTHLY RELATIVE HUMIDITY 81 GENERATED DAILY VALUE 82 ADJUSTMENT FOR CLEAR/OVERCAST CONDITIONS 83 1:3 .6 WIND SPEED 85 1:3 .7 NOMENCLATURE 85 1:3 .8 REFERENCES 87 CHAPTER 1:4 EQUATIONS: CLIMATE CUSTOMIZATION 90 1:4 .1 ELEVATION BANDS 91 1:4 .2 CLIMATE CHANGE 93 1 WEATHER FORECAST INCORPORATION 95 1:4 .4 NOMENCLATURE 96 SECTION 2: HYDROLOGY CHAPTER 2:1 EQUATIONS: SURFACE RUNOFF 98 2:1 .1 RUNOFF VOLUME: SCS CURVE NUMBER PROCEDURE 99 SCS CURVE NUMBER 100 2:1 .2 RUNOFF VOLUME: GREEN & AMPT INFILTRATION METHOD 107 2:1 .3 PEAK RUNOFF RATE 110 TIME OF CONCENTRATION 110 RUNOFF COEFFICIENT 113 RAINFALL INTENSITY 114 MODIFIED RATIONAL FORMULA 115 2:1.

6 4 SURFACE RUNOFF LAG 115 2:1 .5 TRANSMISSION LOSSES 117 2:1 .6 NOMENCLATURE 119 2:1 .7 REFERENCES 120 CHAPTER 2:2 EQUATIONS: EVAPOTRANSPIRATION 123 2 CANOPY STORAGE 124 2:2 .2 POTENTIAL EVAPOTRANSPIRATION 125 PENMAN-MONTEITH METHOD 126 PRIESTLEY-TAYLOR METHOD 132 HARGREAVES METHOD 132 2:2 .3 ACTUAL EVAPOTRANSPIRATION 134 EVAPORATION OF INTERCEPTED RAINFALL 134 TRANSPIRATION 135 SUBLIMATION AND EVAPORATION FROM THE SOIL 135 2:2 .4 NOMENCLATURE 140 2:2 .5 REFERENCES 142 CHAPTER 2:3 EQUATIONS: SOIL Water 146 2:3 .1 SOIL STRUCTURE 147 2:3 .2 PERCOLATION 151 2:3 .3 BYPASS FLOW 152 2 PERCHED Water TABLE 158 2:3.

7 5 LATERAL FLOW 160 LATERAL FLOW LAG 162 2:3 .6 NOMENCLATURE 164 2:3 .7 REFERENCES 166 CHAPTER 2:4 EQUATIONS: GROUNDWATER 168 2:4 .1 GROUNDWATER SYSTEMS 169 2:4 .2 SHALLOW AQUIFER 171 RECHARGE 172 PARTITIONING OF RECHARGE BETWEEN SHALLOW AND DEEP AQUIFER 173 GROUNDWATER/BASE FLOW 173 REVAP 176 PUMPING 177 GROUNDWATER HEIGHT 177 2:4 .3 DEEP AQUIFER 178 2:4 .4 NOMENCLATURE 179 2:4 .5 REFERENCES 180 SECTION 3: NUTRIENTS/PESTICIDES CHAPTER 3:1 EQUATIONS: NITROGEN 182 3:1 .1 NITROGEN CYCLE IN THE SOIL 183 INITIALIZATION OF SOIL NITROGEN LEVELS 185 3:1 .2 MINERALIZATION & DECOMPOSITION/ IMMOBILIZATION 187 HUMUS MINERALIZATION 188 RESIDUE DECOMPOSITION & MINERALIZATION 189 3:1.

8 3 NITRIFICATION & AMMONIA VOLATILIZATION 191 3:1 .4 DENITRIFICATION 194 3:1 .5 ATMOSPHERIC DEPOSITION 195 NITROGEN IN RAINFALL 196 NITROGEN DRY DEPOSITION 197 3:1 .6 FIXATION 198 3:1 .7 UPWARD MOVEMENT OF NITRATE IN Water 198 3 LEACHING 198 3:1 .9 NITRATE IN THE SHALLOW AQUIFER 199 3 NOMENCLATURE 201 3 REFERENCES 203 CHAPTER 3:2 EQUATIONS: PHOSPHORUS 206 3:2 .1 PHOSPHORUS CYCLE 207 INITIALIZATION OF SOIL PHOSPHORUS LEVELS 208 3:2 .2 MINERALIZATION & DECOMPOSITION/ IMMOBILIZATION 210 HUMUS MINERALIZATION 211 RESIDUE DECOMPOSITION & MINERALIZATION 212 3:2 .3 SORPTION OF INORGANIC P 214 3:2 .4 LEACHING 216 3 PHOSPHORUS IN THE SHALLOW AQUIFER 217 3:2.

9 6 NOMENCLATURE 217 3:2 .7 REFERENCES 218 CHAPTER 3:3 EQUATIONS: PESTICIDES 221 3:3 .1 WASH-OFF 222 3:3 .2 DEGRADATION 223 3:3 .3 LEACHING 225 3:3 .4 NOMENCLATURE 225 3:3 .5 REFERENCES 225 CHAPTER 3:4 EQUATIONS: BACTERIA 227 3 WASH-OFF 229 3 BACTERIA DIE-OFF/RE-GROWTH 229 3 LEACHING 233 3 NOMENCLATURE 234 3 REFERENCES 236 CHAPTER 3:5 EQUATIONS: CARBON 238 3 SUB-MODEL DESCRIPTION 239 3 CHANGES FROM PREVIOUS VERSION 243 3 ANALYTICAL SOLUTIONS 244 3 NOMENCLATURE 247 3 REFERENCES 248 SECTION 4: EROSION CHAPTER 4:1 EQUATIONS: SEDIMENT 251 4:1.

10 1 MUSLE 252 SOIL ERODIBILITY FACTOR 253 COVER AND MANAGEMENT FACTOR 256 SUPPORT PRACTICE FACTOR 257 TOPOGRAPHIC FACTOR 259 COARSE FRAGMENT FACTOR 259 4:1 .2 USLE 260 RAINFALL ERODIBILITY INDEX 260 4:1 .3 SNOW COVER EFFECTS 263 4:1 .4 SEDIMENT LAG IN SURFACE RUNOFF 263 4:1 .5 SEDIMENT IN LATERAL & GROUNDWATER FLOW 264 4:1 .6 NOMENCLATURE 265 4:1 .7 REFERENCES 266 CHAPTER 4:2 EQUATIONS: NUTRIENT TRANSPORT 268 4:2 .1 NITRATE MOVEMENT 269 4:2 .2 ORGANIC N IN SURFACE RUNOFF 271 ENRICHMENT RATIO 271 4:2 .3 SOLUBLE PHOSPHORUS MOVEMENT 272 4:2 .4 ORGANIC & MINERAL P ATTACHED TO SEDIMENT IN SURFACE RUNOFF 273 ENRICHMENT RATIO 274 4:2.