Modular Crop Growth Model and Simulation Suggested Language C++ Maximum Team Size Skill Set C++, GUI 4 time step within this loop. The weather object is also called from within the time step loop, but only once per day (i.e. in the rate calculation part.) 1. Introduction Simulation Main Program Crop growth model is an excellent example of object oriented paradigm. Such a model is composed of a number of parts like plant, soil, weather, etc. Being able to handle all of these separately as well as together is the focus of this project. Start Plant Module 2. Problem Description Initialization Rate Calculations Integration Output Close Initialization The model is intended to simulate the growth of a particular type of plant while interacting with soil and weather under various conditions. Rate Calculations 3. Solution Design (Tentative, Incomplete) Weather Module Initialization Rate Calculations Close Integration Output Soil Water Module Initialization Rate Calculations Integration Output Close Close End The suggested model has four main classes: the simulation class, plant class, soil water balance class and weather class. Figure 1 illustrates the classes; where each class has two or more of the following five functionalities: 1. The initialization (constructor) section is used to input data and initialize variables at start of simulation 2. The 'rate calculation functionality computes process rates and rates of change of state variables based on conditions at the end of the previous day of simulation. This method is called once per time step (day) of simulation. 3. The 'integration' method updates state variables using the rates previously calculated 4. The 'output' method is called once per day to generate daily output reports. 5. The close' functionality is called once at the end of simulation to close output files and generate summary reports. The simulation object makes calls to methods inside each other object to accomplish the various components of processing. Each object (weather, soil water, plant) is called once at the beginning of simulation to initialize, resulting in execution of the initialization portion of the object. During the daily time loop, each object is called three times: for rate calculation, for integration calculations, and for output of daily computed data. A final call to each object is made to write summary output files and to close input and output files after the simulation is complete. Rate calculation, integration and output methods are within the time step loop and the soil water and plant objects are each called three times per Figure 1. Class/Message Structure 3.1 Plant Class The plant class computes plant growth and development based on daily values of maximum and minimum temperatures, radiation and the daily value of two soil water stress factors, SWFAC1 and SWFAC2. This class also simulates leaf area index (LAI), which is used in the soil water class to compute evapotranspiration. Details of its functionality are as under: Initialization PT = 1 -0.0025 ((0.25 TMIN -0.75 TMAX )-26) Input variables, as listed in Table 1, are read from file PLANT.INP. File PLANT.OUT is opened and a header is written to this output file. Rate calculations The plant object calls three methods: 1. Method PTS to calculate the effect of temperature on daily plant growth rate and rate of leaf number increase. The growth rate reduction factor (PT) is calculated every day using the following equation: where TMIN and TMAX are the minimum and maximum daily temperatures, respectively. 2. Method PGSO to calculate the daily plant weight increase; The method calculates PG, the potential daily total dry matter increase (g/plant) as: maturity is accumulated as an int. Method LAIS is called for both phases to compute the change in leaf area index (LAI). During vegetative period, LAI increases as a function of the rate of leaf number increase. The potential rate is limited by soil water stress (both deficit and saturation) through SWFAC, and temperature, through PT. Its value is given by: dLAI = SWFAC PT. PD EMP 1. dN THA where PD is the plant density (plants/m), EMP1 is the maximum leaf area expansion per leaf, (0.104 m2/leaf) and a is given by: EMP 2.N - ) a = e 2.1. SRAD. (1.0-e-Y1441) 3. where EMP2 and nb are coefficients in the expolinear equation and N is the development age of the plant (leaf number). After plant has reached the maximum number of leaves,i.e. during reproductive phase, LAI starts to decrease as a function of the daily thermal integral, di. The rate of decrease is given by: LAI = - PD .dipl. SLA where SRAD is the daily solar radiation (MJ/m) and PD the plant density (plant/m?). Y1 is obtained by: Y1 = 1.5 -0.768 (ROWSPC : 0.01). PD ) Units where ROWSPC is the row spacing in (cm). Table 1 - Input data read for plant module Variable Definition Name EMPI Empirical coefficient for LAI computation, maximum leaf area expansion per leaf EMP2 Empirical coefficient for LAI computation fc Fraction of total crop growth partitioned to canopy intot Duration of reproductive stage lai Lfmax Maximum imum number of leaves where pl is the dry matter of leaves removed per plant per unit development after maximum number of leaves is reached and SLA is the specific leaf area. In the vegetative phase the assimilates are partitioned between canopy and roots (dwc and dwr) whereas in the reproductive phase all growth occurs in the grain (dwf). All whole plant weight increases (dw) are converted to area based values by multiplying by the plant density value (PD). m leaf Leaf area index degree-days mm Integration Leaf number -- PD rm sla tb Changes to leaf area index (dLAI), plant weights (dw, dwc, dwr and dwf) and leaf number (N) are integrated into the appropriate state variables (LAI, W, wc, wr, wf and N) at the beginning of the 'integration' section. When the accumulated value of the rate of development towards maturity (int) reaches a genetically determined value (intot), the plant is matured and the simulation is complete. w WC wr Output nb Empirical coefficient for LAI computation pl Dry matter of leaves removed per plant per unit development after maximum number of leaves is reached Plant density plants/m Maximum rate of leaf appearance leaf/day Specific leaf area Base temperature above which reproductive growth PC occurs Total plant dry matter weight Canopy dry matter weight Root dry matter weight 4. Method LAISO to calculate increase in leaf area index dLAI. The plant cycle is divided into vegetative and reproductive phases. The vegetative phase continues until the plant reaches a genetically determined maximum leaf number (Lfmax). During the vegetative phase, leaf number increase (DN) is calculated based on a maximum rate (rm) and a temperature based limiting factor (PT). During reproductive phase, di, the difference between daily mean temperature and a base temperature (tb), is used to calculate the rate of plant development. Total rate of development towards Daily output is written to the PLANT.OUT file. Close The PLANT.OUT output file is closed. 3.2 Soil Water Class A single, homogeneous soil layer underlain by a relatively impervious layer is assumed for the model. A simple water Cumulative values of rainfall (TRAIN), irrigation (TIRR), soil evaporation (TESA), plant transpiration (TEPA), runoff (TROF), vertical drainage (TDRN), and infiltration (TINF) are set to zero for the beginning of the simulation. Rate calculations balance is used to update the soil water content on a daily basis using computed values of infiltration, evaporation, transpiration and drainage. The soil characteristics defined are soil water content at wilting point (WPp), field capacity (FCp) and saturation (STP) all in units of cm/cm?; soil profile depth (DP in cm), daily drainage fraction (DRNP), curve number (CN) and initial soil water content (SWC INIT in mm). The soil water class calculates two parameters (SWFAC1 and SWFAC2) which express the effects of drought and excess soil water respectively on crop growth rate. These factors vary from 1.0 (minimum stress) to 0.0 (maximum stress). In addition to soil characteristics and weather data, this class requires the value of leaf area index (LAI), which is computed in the plant class, to calculate potential evapotranspiration. Details of functionality of the class are as under: Irrigation rates are read from file IRRIG.INP. Potential infiltration (POTINF) is calculated as the sum of rainfall (RAIN) and irrigation (IRR). Cumulative irrigation and rainfall depths are summed to TIRR and TRAIN, respectively. Method DRAINEO is called to compute vertical drainage of soil water (DRN in mm) based on a daily fraction of the soil water content above field capacity: DRN = (SWC - FC) * DRNp If potential infiltration (POTINF) is greater than zero, method RUNOFF() is called to compute daily surface water runoff rates (ROF) using the SCS curve number method: Initialization IF (POTINF > 0.2 * S) THEN ROF = ((POTINF-0.2 * S)**2/(POTINF+ 0.8 *S) ELSE ROF = 0 ENDIF In the 'initialization' portion of the code, input files SOIL.INP and IRRIG.INP and output file SW.OUT are opened. The required soil input data is read from file SOIL.INP and the file is closed. These input variables are listed in Table 2. Headers are written to output file SW.OUT. Units for the soil parameters are converted from volumetric fractions to mm of water as: WP = DP * WPp * 10.0 FC = DP * FCp * 10.0 ST = DP * STp * 10.0 where WP, FC and ST are the wilting point, field capacity and saturation content in mm of water, respectively. Other variables are as defined in Table 2. Table 2 - Input data read for soil water balance module Variable Name Definition Units CN Runoff curve number DP Depth of soil profile DRN Daily drainage percentage (fraction of void space) 1/day FCP Soil water content at field capacity (fraction of void space) cm cm STO Soil water content saturation (fraction of void space) cm cm SWC Soil water content in the profile (value read from file represents initial soil water content) WPp Soil water content at wilting point (fraction of void space) cm cm cm Infiltration (INF) is calculated as the difference between potential infiltration and runoff, i.e. INF=POTINF-ROF Method ETPSO calculates the daily potential evapotranspiration rate (ETP) based on the Priestly-Taylor method. The surface albedo (ALB) is estimated as a weighted average (based on LAI) of the albedo of the soil (0.1) and crop (0.2). ALB = 0.1 *EXP(-0.7 * LAI) + 0.2* (1 - EXP(-0.7 * LAI)) The average temperature during the day (Tmed) and the equilibrium evaporation (EEQ) are calculated as: Tmed = 0.6 * TMAX + 0.4 * TMIN EEQ = SRAD * (4.88E-03 - 4.37E-03 * ALB) * (Tmed + 29) The equilibrium evaporation rate is adjusted by a coefficient (f) resulting in the final value of ETp. Next, the potential soil evaporation (ESP) and plant transpiration (EPp) rates are calculated using the same weighting coefficient used for albedo: ESp = ETp * EXP(- 0.7 * LAI) EPp = ETp * (1 - EXP(-0.7 * LAI)) Method ESS() calculates the actual daily soil evaporation rate (ESa) based on current soil water availability. No evaporation occurs if the soil water content is less than the wilting point, and the potential evaporation is met if soil water content is greater than field capacity. Between the wilting point and field capacity, the actual evapotranspiration varies linearly between 0.0 and the potential evapotranspiration rate: For initialization, method RUNOFFO is called to compute soil storage capacity (S) based on the Soil Conservation Service runoff curve number method: S = 254 * (100/CN - 1) Then, method STRESSO is called to calculate the threshold soil water content below which drought stress will occur (THE). This is approximated as: THE = WP +0.75 * (FC - WP) Initial stress factors (SWFAC1 and SWFAC2) are then calculated based on initial soil water content. This is discussed in more detail later. IF (SWC FC) THEN a = 1 ELSE a =(SWC - WP)/(FC - WP) ENDIF ESa = ESp * a The potential plant transpiration rate (EPp) is reduced by the minimum soil water stress factor (SWFAC1 or SWFAC2) to obtain the actual plant transpiration rate (EP) capacity is met. The depth to the water table (DWT in mm) is then computed. WTABLE =(SWC - FC)/(ST - FC) * DP * 10. DWT = DP * 10. - WTABLE Minimum excess soil water stress (SWFAC2 = 1.0) occurs when the water table thickness is zero. Maximum stress (SWFAC2 = 0.0) occurs when the depth to the water table (DWT) is greater than 250 mm (STRESS DEPTH = 250 mm). The excess water stress factor is linearly interpolated between these water table conditions. IF (DWT > STRESS DEPTH) THEN SWFAC2 = 1.0 ELSE SWFAC2 = DWT/STRESS_DEPTH ENDIF Integration Output Daily values are written to output file SW.OUT. Close At the end of simulation, method WBALOis called to check that the seasonal water balance is zero, i.e., that changes in soil water content are equal to cumulative inflows and outflows. A water balance report is written (WBAL.OUT). Files SW.OUT and IRRIG.INP are closed. 3.3 Weather class The integration portion of the soil water class updates the value of the soil water content based on the computed values of infiltration (INF), soil evaporation (ESa), plant transpiration (EP), and vertical drainage (DRN): SWC = SWC + (INF - ESa - EP - DRN) The computed value is limited to a maximum of the saturation content (ST) and a minimum of zero. If the computed soil water content exceeds saturation, runoff rates and soil water content are adjusted. IF (SWC>ST) ROF=ROF+(SWC- ST) SWC=ST END IF An additional adjustment factor (SWC_ADJ) is introduced if the computed soil water content is less than zero. IF (SWC THE THEN SWFAC1 = 1.0 ELSE SWFAC1 = (SWC - WP)/(THE - WP) SWFAC1 = MAX(MIN(SWFAC1, 1.0), 0.0) ENDIF The thickness of the water table measured from the bottom of the soil profile (WTABLE in mm) is calculated using the excess water available after field Upon initialization, the weather file (WEATHER.INP) is opened. In the rate calculations section, weather values are read into the model on a daily basis from WEATHER.INP. Table 3 lists the weather input data read. The close section is invoked to close the weather input file. Table 3 - Input data read for weather module Variable Name Definition Units DATE Julian date in YYDDD format PAR Daily photosynthetically active radiation mol[photon]/m2-day RAIN Daily rainfall mm SRAD Daily solar radiation M/m2 TMAX Daily maximum temperature C TMIN Daily minimum temperature -C Modular Crop Growth Model and Simulation Suggested Language C++ Maximum Team Size Skill Set C++, GUI 4 time step within this loop. The weather object is also called from within the time step loop, but only once per day (i.e. in the rate calculation part.) 1. Introduction Simulation Main Program Crop growth model is an excellent example of object oriented paradigm. Such a model is composed of a number of parts like plant, soil, weather, etc. Being able to handle all of these separately as well as together is the focus of this project. Start Plant Module 2. Problem Description Initialization Rate Calculations Integration Output Close Initialization The model is intended to simulate the growth of a particular type of plant while interacting with soil and weather under various conditions. Rate Calculations 3. Solution Design (Tentative, Incomplete) Weather Module Initialization Rate Calculations Close Integration Output Soil Water Module Initialization Rate Calculations Integration Output Close Close End The suggested model has four main classes: the simulation class, plant class, soil water balance class and weather class. Figure 1 illustrates the classes; where each class has two or more of the following five functionalities: 1. The initialization (constructor) section is used to input data and initialize variables at start of simulation 2. The 'rate calculation functionality computes process rates and rates of change of state variables based on conditions at the end of the previous day of simulation. This method is called once per time step (day) of simulation. 3. The 'integration' method updates state variables using the rates previously calculated 4. The 'output' method is called once per day to generate daily output reports. 5. The close' functionality is called once at the end of simulation to close output files and generate summary reports. The simulation object makes calls to methods inside each other object to accomplish the various components of processing. Each object (weather, soil water, plant) is called once at the beginning of simulation to initialize, resulting in execution of the initialization portion of the object. During the daily time loop, each object is called three times: for rate calculation, for integration calculations, and for output of daily computed data. A final call to each object is made to write summary output files and to close input and output files after the simulation is complete. Rate calculation, integration and output methods are within the time step loop and the soil water and plant objects are each called three times per Figure 1. Class/Message Structure 3.1 Plant Class The plant class computes plant growth and development based on daily values of maximum and minimum temperatures, radiation and the daily value of two soil water stress factors, SWFAC1 and SWFAC2. This class also simulates leaf area index (LAI), which is used in the soil water class to compute evapotranspiration. Details of its functionality are as under: Initialization PT = 1 -0.0025 ((0.25 TMIN -0.75 TMAX )-26) Input variables, as listed in Table 1, are read from file PLANT.INP. File PLANT.OUT is opened and a header is written to this output file. Rate calculations The plant object calls three methods: 1. Method PTS to calculate the effect of temperature on daily plant growth rate and rate of leaf number increase. The growth rate reduction factor (PT) is calculated every day using the following equation: where TMIN and TMAX are the minimum and maximum daily temperatures, respectively. 2. Method PGSO to calculate the daily plant weight increase; The method calculates PG, the potential daily total dry matter increase (g/plant) as: maturity is accumulated as an int. Method LAIS is called for both phases to compute the change in leaf area index (LAI). During vegetative period, LAI increases as a function of the rate of leaf number increase. The potential rate is limited by soil water stress (both deficit and saturation) through SWFAC, and temperature, through PT. Its value is given by: dLAI = SWFAC PT. PD EMP 1. dN THA where PD is the plant density (plants/m), EMP1 is the maximum leaf area expansion per leaf, (0.104 m2/leaf) and a is given by: EMP 2.N - ) a = e 2.1. SRAD. (1.0-e-Y1441) 3. where EMP2 and nb are coefficients in the expolinear equation and N is the development age of the plant (leaf number). After plant has reached the maximum number of leaves,i.e. during reproductive phase, LAI starts to decrease as a function of the daily thermal integral, di. The rate of decrease is given by: LAI = - PD .dipl. SLA where SRAD is the daily solar radiation (MJ/m) and PD the plant density (plant/m?). Y1 is obtained by: Y1 = 1.5 -0.768 (ROWSPC : 0.01). PD ) Units where ROWSPC is the row spacing in (cm). Table 1 - Input data read for plant module Variable Definition Name EMPI Empirical coefficient for LAI computation, maximum leaf area expansion per leaf EMP2 Empirical coefficient for LAI computation fc Fraction of total crop growth partitioned to canopy intot Duration of reproductive stage lai Lfmax Maximum imum number of leaves where pl is the dry matter of leaves removed per plant per unit development after maximum number of leaves is reached and SLA is the specific leaf area. In the vegetative phase the assimilates are partitioned between canopy and roots (dwc and dwr) whereas in the reproductive phase all growth occurs in the grain (dwf). All whole plant weight increases (dw) are converted to area based values by multiplying by the plant density value (PD). m leaf Leaf area index degree-days mm Integration Leaf number -- PD rm sla tb Changes to leaf area index (dLAI), plant weights (dw, dwc, dwr and dwf) and leaf number (N) are integrated into the appropriate state variables (LAI, W, wc, wr, wf and N) at the beginning of the 'integration' section. When the accumulated value of the rate of development towards maturity (int) reaches a genetically determined value (intot), the plant is matured and the simulation is complete. w WC wr Output nb Empirical coefficient for LAI computation pl Dry matter of leaves removed per plant per unit development after maximum number of leaves is reached Plant density plants/m Maximum rate of leaf appearance leaf/day Specific leaf area Base temperature above which reproductive growth PC occurs Total plant dry matter weight Canopy dry matter weight Root dry matter weight 4. Method LAISO to calculate increase in leaf area index dLAI. The plant cycle is divided into vegetative and reproductive phases. The vegetative phase continues until the plant reaches a genetically determined maximum leaf number (Lfmax). During the vegetative phase, leaf number increase (DN) is calculated based on a maximum rate (rm) and a temperature based limiting factor (PT). During reproductive phase, di, the difference between daily mean temperature and a base temperature (tb), is used to calculate the rate of plant development. Total rate of development towards Daily output is written to the PLANT.OUT file. Close The PLANT.OUT output file is closed. 3.2 Soil Water Class A single, homogeneous soil layer underlain by a relatively impervious layer is assumed for the model. A simple water Cumulative values of rainfall (TRAIN), irrigation (TIRR), soil evaporation (TESA), plant transpiration (TEPA), runoff (TROF), vertical drainage (TDRN), and infiltration (TINF) are set to zero for the beginning of the simulation. Rate calculations balance is used to update the soil water content on a daily basis using computed values of infiltration, evaporation, transpiration and drainage. The soil characteristics defined are soil water content at wilting point (WPp), field capacity (FCp) and saturation (STP) all in units of cm/cm?; soil profile depth (DP in cm), daily drainage fraction (DRNP), curve number (CN) and initial soil water content (SWC INIT in mm). The soil water class calculates two parameters (SWFAC1 and SWFAC2) which express the effects of drought and excess soil water respectively on crop growth rate. These factors vary from 1.0 (minimum stress) to 0.0 (maximum stress). In addition to soil characteristics and weather data, this class requires the value of leaf area index (LAI), which is computed in the plant class, to calculate potential evapotranspiration. Details of functionality of the class are as under: Irrigation rates are read from file IRRIG.INP. Potential infiltration (POTINF) is calculated as the sum of rainfall (RAIN) and irrigation (IRR). Cumulative irrigation and rainfall depths are summed to TIRR and TRAIN, respectively. Method DRAINEO is called to compute vertical drainage of soil water (DRN in mm) based on a daily fraction of the soil water content above field capacity: DRN = (SWC - FC) * DRNp If potential infiltration (POTINF) is greater than zero, method RUNOFF() is called to compute daily surface water runoff rates (ROF) using the SCS curve number method: Initialization IF (POTINF > 0.2 * S) THEN ROF = ((POTINF-0.2 * S)**2/(POTINF+ 0.8 *S) ELSE ROF = 0 ENDIF In the 'initialization' portion of the code, input files SOIL.INP and IRRIG.INP and output file SW.OUT are opened. The required soil input data is read from file SOIL.INP and the file is closed. These input variables are listed in Table 2. Headers are written to output file SW.OUT. Units for the soil parameters are converted from volumetric fractions to mm of water as: WP = DP * WPp * 10.0 FC = DP * FCp * 10.0 ST = DP * STp * 10.0 where WP, FC and ST are the wilting point, field capacity and saturation content in mm of water, respectively. Other variables are as defined in Table 2. Table 2 - Input data read for soil water balance module Variable Name Definition Units CN Runoff curve number DP Depth of soil profile DRN Daily drainage percentage (fraction of void space) 1/day FCP Soil water content at field capacity (fraction of void space) cm cm STO Soil water content saturation (fraction of void space) cm cm SWC Soil water content in the profile (value read from file represents initial soil water content) WPp Soil water content at wilting point (fraction of void space) cm cm cm Infiltration (INF) is calculated as the difference between potential infiltration and runoff, i.e. INF=POTINF-ROF Method ETPSO calculates the daily potential evapotranspiration rate (ETP) based on the Priestly-Taylor method. The surface albedo (ALB) is estimated as a weighted average (based on LAI) of the albedo of the soil (0.1) and crop (0.2). ALB = 0.1 *EXP(-0.7 * LAI) + 0.2* (1 - EXP(-0.7 * LAI)) The average temperature during the day (Tmed) and the equilibrium evaporation (EEQ) are calculated as: Tmed = 0.6 * TMAX + 0.4 * TMIN EEQ = SRAD * (4.88E-03 - 4.37E-03 * ALB) * (Tmed + 29) The equilibrium evaporation rate is adjusted by a coefficient (f) resulting in the final value of ETp. Next, the potential soil evaporation (ESP) and plant transpiration (EPp) rates are calculated using the same weighting coefficient used for albedo: ESp = ETp * EXP(- 0.7 * LAI) EPp = ETp * (1 - EXP(-0.7 * LAI)) Method ESS() calculates the actual daily soil evaporation rate (ESa) based on current soil water availability. No evaporation occurs if the soil water content is less than the wilting point, and the potential evaporation is met if soil water content is greater than field capacity. Between the wilting point and field capacity, the actual evapotranspiration varies linearly between 0.0 and the potential evapotranspiration rate: For initialization, method RUNOFFO is called to compute soil storage capacity (S) based on the Soil Conservation Service runoff curve number method: S = 254 * (100/CN - 1) Then, method STRESSO is called to calculate the threshold soil water content below which drought stress will occur (THE). This is approximated as: THE = WP +0.75 * (FC - WP) Initial stress factors (SWFAC1 and SWFAC2) are then calculated based on initial soil water content. This is discussed in more detail later. IF (SWC FC) THEN a = 1 ELSE a =(SWC - WP)/(FC - WP) ENDIF ESa = ESp * a The potential plant transpiration rate (EPp) is reduced by the minimum soil water stress factor (SWFAC1 or SWFAC2) to obtain the actual plant transpiration rate (EP) capacity is met. The depth to the water table (DWT in mm) is then computed. WTABLE =(SWC - FC)/(ST - FC) * DP * 10. DWT = DP * 10. - WTABLE Minimum excess soil water stress (SWFAC2 = 1.0) occurs when the water table thickness is zero. Maximum stress (SWFAC2 = 0.0) occurs when the depth to the water table (DWT) is greater than 250 mm (STRESS DEPTH = 250 mm). The excess water stress factor is linearly interpolated between these water table conditions. IF (DWT > STRESS DEPTH) THEN SWFAC2 = 1.0 ELSE SWFAC2 = DWT/STRESS_DEPTH ENDIF Integration Output Daily values are written to output file SW.OUT. Close At the end of simulation, method WBALOis called to check that the seasonal water balance is zero, i.e., that changes in soil water content are equal to cumulative inflows and outflows. A water balance report is written (WBAL.OUT). Files SW.OUT and IRRIG.INP are closed. 3.3 Weather class The integration portion of the soil water class updates the value of the soil water content based on the computed values of infiltration (INF), soil evaporation (ESa), plant transpiration (EP), and vertical drainage (DRN): SWC = SWC + (INF - ESa - EP - DRN) The computed value is limited to a maximum of the saturation content (ST) and a minimum of zero. If the computed soil water content exceeds saturation, runoff rates and soil water content are adjusted. IF (SWC>ST) ROF=ROF+(SWC- ST) SWC=ST END IF An additional adjustment factor (SWC_ADJ) is introduced if the computed soil water content is less than zero. IF (SWC THE THEN SWFAC1 = 1.0 ELSE SWFAC1 = (SWC - WP)/(THE - WP) SWFAC1 = MAX(MIN(SWFAC1, 1.0), 0.0) ENDIF The thickness of the water table measured from the bottom of the soil profile (WTABLE in mm) is calculated using the excess water available after field Upon initialization, the weather file (WEATHER.INP) is opened. In the rate calculations section, weather values are read into the model on a daily basis from WEATHER.INP. Table 3 lists the weather input data read. The close section is invoked to close the weather input file. Table 3 - Input data read for weather module Variable Name Definition Units DATE Julian date in YYDDD format PAR Daily photosynthetically active radiation mol[photon]/m2-day RAIN Daily rainfall mm SRAD Daily solar radiation M/m2 TMAX Daily maximum temperature C TMIN Daily minimum temperature -C