A soil water and salt balance model

Transcription

1 A soil water and salt balance model Reference Manual Version 5.0 June 02 Dirk RAES K.U.Leuven Faculty of Agricultural and Applied Biological Sciences Institute for Land and Water Management Vital Decosterstraat 102, B-00 LEUVEN, Belgium

2 Table of contents Chapter 1. BUDGET 1.1 Input 1.2 Output 1.3 Subroutines 1.4 Installation and structure Chapter 2. Menu Reference 2.1 Main menu 2.2 Climate input (ETo and Rain data) 2.3 Crop data Select/Create crop file Crop Parameters a. Growth stages and Kc coefficients b. Sensitivity stages and Ky coefficients c. Partitioning of ETcrop d. Rooting depth e. Water stress f. Aeration stress g. Water extraction pattern h. Salt tolerance 2.4 Profile description Select/Create soil file Soil Parameters a. Soil layers b. Bunds and surface runoff 2.5 Irrigation options No irrigation (rainfed agriculture) Determination of Irrigation water requirement Irrigation scheduling a. In crop season b. Out of crop season c. Irrigation water quality 2.6 Simulation Period 2.7 Initial conditions Initial soil water and salt conditions Soil cover outside growing period Cumulative counters 2.8 Output files BUDGET Reference Manual Table of Contents ii

3 2.9 Program parameters Display Evaporation Transpiration Salinity Surface runoff Soil compartments Initial conditions Irrigation Effective rainfall 2.10 Run Input section Output section Graphical displays Chapter 3. Calculation procedures 3.1 Simulation of the soil water and salt balance 3.2 Drainage subroutine 3.3 Runoff subroutine 3.4 Infiltration subroutine 3.5 Crop evapotranspiration Evaporation power of the atmosphere Partitioning of ETcrop Evaporation subroutine Transpiration subroutine Actual crop evapotranspiration (ETact) 3.6 Estimation of Relative Yield 3.7 Salt transport Vertical and horizontal salt movement Cells Salt diffusion Soil evaporation Crop transpiration 3.8 Processing of 10-day and monthly climatic data Estimation of surface runoff Estimation of effective rainfall a. 100 percent effective b. USDA-SCS procedure c. Expressed as a percentage of rainfall BUDGET Reference Manual Table of Contents iii

4 References Annex I. Indicative values for Crop Parameters 1. Lengths of crop development stages 2. Crop coefficients 3. Maximum effective rooting depth and maximum depletion factors 4. Maximum crop salt tolerance levels 5. Yield response factors I. 2 I. 7 I.12 I.15 I.17 Annex II. Indicative values and pedo-transfer functions for Hydraulic properties of soil layers 1. Water content at saturation (θ sat ), field capacity (θ FC ) and wilting point (θ WP ) 2. Saturated hydraulic conductivity (K sat ) 3. Drainage characteristic (τ) II.1 II.3 II.5 BUDGET Reference Manual Table of Contents iv

5 The BUDGET program BUDGET is composed of a set of validated subroutines describing the various processes involved in water extraction by plant roots and water movement in the soil profile. During periods of crop water stress the resulting yield depression is estimated by means of yield response factors. By selecting appropriate time and depth criteria irrigation schedules can be generated. The climatic data consists of daily, mean 10-day or monthly ETo (reference crop evapotranspiration) and Rainfall observations. At run time, the 10-day and monthly data are processed to derive daily ETo and Rain data. By specifying and selecting a few appropriate crop parameters in a Menu driven environment, the program creates a complete set of parameters that can be displayed and updated if additional information is available. The soil profile may be composed of several soil layers, each with their specific characteristics. BUDGET contains a complete set of default characteristics that can be selected and adjusted for various types of soil layers. By calculating the water and salt content in a soil profile as affected by input and withdrawal of water and salt during the simulation period, the program is suitable: - to asses crop water stress under rainfed conditions; - to estimate yield response to water; - to design irrigation schedules; - to study the building of salt in the root zone under averse irrigation conditions; - to evaluate irrigation strategies. By visualising the variation of the soil water content and the building up of salt in the soil profile during the simulation process, the program is also valuable as a didactic tool. BUDGET Reference Manual 1. 1

6 Chapter 1. BUDGET 1.1 Input The input consists of: 1. daily, 10-day or monthly Climatic data: - Evaporative demand of the atmosphere (i.e. the Reference evapotranspiration, ETo); - Rainfall 2. Crop parameters: Parameters describing crop development and root water uptake. By selecting the appropriate: - Class of Crop type; - Class of rooting depth (from shallow rooted crops to very deep rooted crops); - Class of sensitivity to water stress (from sensitive to water stress to tolerant to stress); - Class of degree of ground cover at maximum crop canopy (from sparse ground cover to very dense cover); - and specifying the total length of growing period, BUDGET generates a complete set of crop parameters. The generated crop parameters can be adjusted; 3. Soil parameters: The soil profile may be composed of several soil layers, each with their specific characteristics. BUDGET contains a complete set of default characteristics which can be selected and adjusted for various types of soil layers; 4. Irrigation data: Water quality (salinity), Irrigation Intervals and water Application Depths or criteria to generate irrigation schedules; 5. Initial soil water and salt conditions in the soil profile; 1.2 Output With the described input and for the given initial conditions, BUDGET simulates the solute transport and water uptake in the specified Climate/Crop/Soil environment and for the specified irrigation option. 1. During the simulation process the variation of the soil water content and salt build up in the profile is visualised by displaying at the end of each day of the simulation period : - the soil water and salt content at different depths in the soil profile; - the water level in the soil water reservoir; - the root zone depletion. 2. At the end of the simulation process, BUDGET displays: - the final soil moisture profile; - the final salt content of the soil water solution in the soil profile; - the total value for each of the p a r a m e t e r s of the soil water balance; - the expected relative c r o p y i e l d; and - the irrigation water requirement. 3. During the simulation, BUDGET records continuously the daily soil water and salt content in the soil profile, soil water BUDGET Reference Manual 1. 2

7 fluxes, daily values of the various parameters of the soil water and salt balances, the daily root zone depletion and net irrigation requirement. The daily records are stored in various output files, which contents can be displayed at the end of the simulation run with different time aggregations (day, 10-day, month, or year) and saved for further analysis. 1.3 Subroutines BUDGET is composed of a set of validated subroutines describing the various processes involved in water extraction by plant roots and soil water movement in the absence of a water table. The estimation of the amount of rainfall lost by surface runoff is based on the curve number method developed by the US Soil Conservation Service (USDA, 1964; Rallison, 1980; Steenhuis et al., 1995). Since irrigation is assumed to be fully controlled, the runoff sub-model is bypassed when irrigation water infiltrates into the soil. The maximum amount of water that can infiltrate into the soil is however limited by the maximum infiltration rate of the topsoil layer. The infiltration and internal drainage are described by an exponential drainage function (Raes, 1982; Raes et al., 1988) that takes into account the initial wetness and the drainage characteristics of the various soil layers. The drainage function mimics quite realistic the infiltration and internal drainage as observed in the field (Raes, 1982; Feyen, 1987; Hess, 1999; Wiyo, 1999; Barrios Gonzales, 1999.). Irrigation schedules can be generated by time and depth criteria as described by Smith (1985) and used in the irrigation scheduling software packages IRSIS (Raes et al., 1988) and CROPWAT (Smith, 1990). With the help of the dual crop coefficient procedure (Allen et al., 1998) the soil evaporation rate and crop transpiration rate of a well-watered soil is calculated. The actual soil evaporation is derived from soil wetness and crop cover (Ritchie, 1976; Belmans et al. 1983). The actual water uptake by plant roots is described by means of a sink term (Feddes et al., 1978; Hoogland et al., 1981; Belmans et al. 1983) that takes into account root distribution and soil water content in the soil profile. For a given water stress during a specific growth stage, the resulting yield depression is estimated by means of the yield response factor Ky (Doorenbos and Kassam, 1979). With the help of a polynomial function (Kipkorir et al., 02), the Ky values are converted to sensitivity indexes for Jensen s multiplication model. By using the procedure presented by Tsakiris (1982), the effect of water stress on relative yield during a short time period is derived from the relative crop evapotranspiration by means of the empirical model of Jensen (1968). When climatic data consist of 10-day or monthly values, the interpolation procedure presented by Gommes (1983) is used to estimate daily ETo and Rainfall rates. Since it is highly unlikely that rainfall is homogenously distributed over all the days of the 10- day period or month, rainfall data is further processed by means of the USDA-SCS BUDGET Reference Manual 1. 3

8 procedure (1993) to determine the part of rainfall that is stored in the top soil as effective rainfall. In BUDGET it is assumed that solutes move downward in macropores under the effect of gravitational forces. Salts diffuse horizontally from the macropores in the soil matrix and vertical in the matrix as a result of differences in salt concentration of the soil water. To simulate the convection and diffusion of salts, a soil compartment is divided into a number of cells where salts can be stored. The salt module (Raes et al., 01) simulates the building up of salts in a cropped soil profile as a result of irrigation with low quality water. The calculation procedure was validated by comparing simulated with measured soil salinity in irrigated fields in Syria (Van Goidsenhoven, 00), Gaza (Goris and Samain, 01) and Kenia (Hindryckx, 02) and by comparing (Sumith, 02) the simulations with the theoretical solution presented by Ayers and Westcot(1985). 1.4 Installation and structure Download the installation disk and Install BUDGET. If the BUDGET software is correctly installed, the main directory (default C:\Program Files\IUPWARE\BUDGET ) should contain: (i) the following files: - BUDGET1.EXE (the executable file ); - DEFAULT.PAR (a file with initialisation and default parameters); - SOILS.DIR (a file with default values for soil characteristics) (ii) and three subdirectories: - DATA (which contains the ETo (*.ETo), Rain (*. PLU), crop (*.CRO) and soil files (*.Sol); - HELP (which contains help files); - OUTP (which contains the output files (*.OUT). BUDGET Reference Manual 1. 4

9 Chapter 2 Menu Reference 2.1 Main menu The Main Menu (Fig. 2.1) consists of 3 sections: A. Climate/Crop/Soil Data Base: where the user: (1) selects, creates or updates ETo, Rain, Crop and Soil Files from the Climate/Crop/ Soil Data Base; (2) specifies the start of the growing period; (3) specifies an irrigation option (if any) and the quality of the irrigation water; B. Simulation: where the user: (4) specifies the Simulation Period, the Initial Conditions for a run, and the Output Files that need to be created; (5) runs a simulation for the specified environment and conditions. C. Program: where the user: (6) adjust Program Parameters. Figure 2.1 BUDGET Reference Manual 2. 1

10 Main Menu BUDGET Reference Manual 2. 2

11 2.2 Climate input (ETo and Rain data) For each day of the simulation period, the program requires information concerning the weather conditions. Those conditions determine the amount of water that can infiltrate into the soil profile and that can be extracted from the soil profile by soil evaporation and crop transpiration. The climatic conditions are given by: - evaporative demand of the atmosphere for the given weather conditions. The atmospheric demand is given by the evapotranspiration from the reference surface (ETo); - rainfall depth, which is the amount of water collected in rain gauges installed on the field or at a nearby weather station. The amount of water that is actually extracted from the soil profile or infiltrates into the soil profile can only be determined at run time by considering the infiltration rate of the top soil, crop characteristics (Kc) and soil water content in the top soil at that particular day. The data consist of daily, mean 10-day or mean monthly ETo and Rainfall observations. Simulations can run with climatic data of a specific year, mean average data not linked to a specific year, or even with ETo or Rainfall levels than are expected with a given probability (dependable ETo or Rainfall). At run time, the 10- day and monthly data are processed to derive daily ETo and Rain data (See Chapter 3, Processing of 10-day and monthly climatic data ). The climatic data can be - specified by the user at run time (this is the default setting, see 2.10 Run); - specified and saved in a file before running a simulation by selecting the Create Command in the Main Menu, and by subsequently specifying the climatic data for a number of successive days, decades, or months. If the data consists of averages or dependable levels, the data is not linked to a specific year and its use is not restricted to simulations runs for a particular year; - downloaded from an existing climate file by selecting the Select Command in the Main Menu. When selecting this Command, a list is displayed that contains all the files with extension ETo (ETo data) or PLU (rain data) in the specified directory. The user select the appropriate climatic file from this list; - The user can i m p o r t climate files in the data directory with for example the help of Windows Explorer. Climate files are text files with a specific data structure (Table 2.2) and with extension ETo or PLU. The user can create its own climate files (use for example NOTEPAD) as long as the specific data structure is respected. Table 2.2. Structure and Example of Climate file (Tunis.ETo) Structure Example Line 1: Title Mean 10-day ETo data Line 2: (1= daily data, 2 = 10-day and 3 = monthly data) 2 Line 3: day, month and year of first observation (year means that the data is not linked to a specific year) Line 4 to end: each line contains ETo (mm/day) or Rain 1.0 (mm/day, mm/decade or mm/month) data for the 1.1 successive days, decades or months BUDGET Reference Manual 2. 3

12 2.3 Crop data Select/Create crop file To specify the crop parameters, the user either - download the crop parameters from an existing file by selecting the Select Command in the Main Menu. When selecting this Command, a list is displayed that contains all the files with extension CRO in the specified directory (default path is the DATA subdirectory from BUDGET). The user selects the appropriate crop file from this list; - create a new set of parameters that will be saved in a crop file by selecting the Create Command in the Main Menu, and by subsequently specifying the crop parameters. When creating a new file, the user specifies (Fig. 2.3): Figure 2.3 Create CROP file Menu 1. File Name; 2. File Description; 3. Crop Type: Annual (Harvested fresh, Harvested dry, Roots or tubers) or Perennial (Evergreens or Deciduous crop). Corresponding indicative values for the crop coefficients during mid-season and at the end of late season will be displayed; BUDGET Reference Manual 2. 4

13 4. Length of the growing period; 5. Crop coefficient during the mid-season stage. Indicative values for various crops are given in Annex I, Table I.2; 6. Crop coefficient at the end of the late season stage. Indicative values for various crops are given in Annex I, Table I.2; 7. Rooting depth for the fully developed crop: classes ranging from shallow rooted crops to very deep-rooted crops. BUDGET generates the corresponding maximum effective rooting depth (Table 2.3a). The rooting depth can also be specified immediately. Indicative values for various crops are given in Annex I, Table I.3 Table 2.3a. Classes and corresponding default values for effective rooting depth of fully developed crop Class Rooting depth Rooting depth [m] Shallow rooted crops 0.35 Shallow medium rooted 0.60 Medium deep rooted 1.00 Deep rooted crops 1.35 Very deep rooted crops Sensitivity to water stress: classes ranging from sensitive to water stress to tolerant to water stress. BUDGET generates the corresponding p-value, i.e. the soil water depletion fraction for no stress (Table 2.3b). In the absence of sufficient rainfall, crops that are sensitive to water stress will require more frequent irrigations than the more tolerant crops. The p-value can also be specified immediately. Indicative values for various crops are given in Annex I, Table I.3; Table 2.3b. Classes for sensitivity to water stress and corresponding default values for soil water depletion fraction for no stress (p) Class Soil water depletion fraction for no stress Sensitivity to water stress p Sensitive to water stress 0. Moderately sensitive 0. Default 0.50 Moderately tolerant 0.55 Tolerant to water stress Degree of ground cover at maximum crop canopy: classes ranging from sparse to very dense. Under normal field conditions the default value (complete cover) is valid for most agricultural field crops but might be different if the crop is poorly developed as a result of diseases, pests or insufficient fertilisation. In orchards the soil cover is often not complete. The selection has its effect on the partitioning of ETcrop in soil evaporation and crop transpiration (Fig. 2.3c). When the ground cover is sparse, soil evaporation is relative important even at maximum crop canopy, on the other hand when ground cover is very dense, soil evaporation will be small (Table 2.3c). BUDGET Reference Manual 2. 5

14 Table 2.3c. Classes for ground cover and corresponding default values for LAI and soil evaporation at maximum crop canopy Class Ground cover (max. crop canopy) LAI [m 2 /m 2 ] Soil evaporation [% of ETcrop] Sparse cover Sparse complete Complete cover Dense cover Very dense cover Sensitivity to soil salinity: classes ranging from sensitive to soil salinity to tolerant to soil salinity. BUDGET generates the corresponding soil salinity at which growth ceases, ECe(max) (Table 2.3d). Indicative values for various crops are given in Annex I, Table I.4. The selection has its effect when salts enter the soil profile with the irrigation water. Salts in the soil water solution will reduce crop transpiration by making the soil water less available for plant root extraction. When irrigation water does not contain salts, the selection is of no importance. Table 2.3d. Classes for soil salinity and corresponding default values for the maximum soil salinity ECe(max) at which crop growth ceases Class ECe max Sensitivity to soil salinity [ds/m] Sensitive 7 Moderately sensitive 12 Moderately tolerant Tolerant Specifying and selecting the appropriate parameters in fields 4 to 10 creates a complete set of crop parameters. The crop parameters can be adjusted by selecting the Display/Update Command in the Main Menu (see 2.3.2). BUDGET Reference Manual 2. 6

15 Kc coefficient Crop Parameters When creating a crop file, the user has to specify only a few parameters. With the help of this information BUDGET generates the complete set of parameters. The crop parameters can be displayed and adjusted by selecting the Display/Update Command in the Main Menu. The parameters are classified in 9 folders: - Description - Growth stages and Kc coefficients - Sensitivity stages and Ky coefficients - Partitioning of ETcrop - Rooting depth - Water stress - Aeration stress - Water extraction pattern - Salt tolerance a. Growth stages and Kc coefficients Kc,ini(wet) Kc,mid Kc,end Kc,ini(dry) time (days) initial crop development mid-season stage late season total growing period Figure 2.3a Growth stages and Crop coefficient (Kc) curve for a non-stressed annual crop when the soil surface is wet from irrigation or rain (light shaded area) and when the soil surface is dry (dark shaded area) The total growing period is divided in four growth stages; (1) initial, (2) crop development, (3) mid-season and (4) late season stage. General lengths for the four distinct growth stages and the total growing period for various types of climates, locations and various crops are presented in Annex I, Table I.1. BUDGET Reference Manual 2. 7

16 Kc coefficients for various non-stressed, well-managed crops in subhumid climates are presented in Table I.2 of Annex I. As soil evaporation is an integrated part of crop evapotranspiration, the Kc coefficient is affected by wetting frequency and soil cover. BUDGET derives the crop coefficient curve (Fig. 2.3a) for a wet and dry soil surface from information on crop cover (see c Partitioning of ETcrop), the Kc value for a wet bare soil (see 2.9 Program Parameters) and Kc values specified by the user (Table 2.3a and 2.3a(bis)). Table 2.3a Crop coefficients for annual crop types Annual crop types Initial stage Kc,ini (dry) *1 Kc,ini (wet) *2 Crop development stage.. Kc,mid Mid-season stage Kc,mid specify Late season stage Kc,mid Kc,end specify *1 Kc Value determined by ground cover (LAI) at initial stage (see c Partitioning of ETcrop) *2 Kc value for a wet bare soil (see 2.9 Program Parameters) Table 2.3a(bis) Crop coefficients for perennial crop types Perennial crop types Initial stage Kc,ini(wet, shaded) *1 specify Crop development stage.. Kc,mid Mid-season stage Kc,mid specify Late season stage Kc,mid Kc,end specify *1 Kc value for a wet soil shaded by the perennial crop b. Sensitivity stages and Ky coefficients The yield response to water deficit for various sensitivity stages can be specified in this folder. The data will be used to convert water stress in estimates of yield depression (see Chapter 3, Estimation of Relative Yield). The user specifies for each stage its length and the corresponding yield response factor Ky. The higher the Ky value the more sensitive the crop will be to water deficit. In general, crops are more sensitive to water deficit during emergence, flowering and early yield formation than they are during the vegetative growth period and ripening (Doorenbos and Kassam, 1979). Values for yield response factor for various crops and sensitivity stages are presented in Annex I, Table I.5. If data is not available, the default value of 1 will be used for Ky for each of the stages. Consequently the decrease in yield will be directly proportionally with the water deficit throughout the growing season. BUDGET Reference Manual 2. 8

17 c. Partitioning of ETcrop Crop evapotranspiration is the combined effect of crop transpiration and soil evaporation. In a well watered soil, the partitioning of crop evapotranspiration is mainly determined by the leaf area per unit surface of soil below it (LAI). At sowing nearly 100% of ET comes from evaporation, while at full crop a large part of ET comes from transpiration (Fig. 2.3c). 100 % 80 % soil evaporation 60 % partitioning of evapotranspiration % crop transpiration % 0 % sowing time harvest Figure 2.3c The partitioning of evapotranspriation into evaporation (light) and transpiration (dark area) over the growing period for an annual field crop. - Ground cover at maximum crop canopy. By altering the crop cover (from sparse to very dense cover), the user adjusts the partitioning and the corresponding LAI values over the growing season (See Table 2.3c). Altering the ground cover will also alter the average LAI over the initial stage and hence the Kc,ini(dry) (see Table 2.3a). - Plot indicative partitioning ETcrop. By selecting this command typical LAI values over the growing season are selected (determined by the crop type and the LAI value at maximum crop canopy). - Specify specific points of LAI curve. By selecting this command, the generated LAI values at specific points of the growing season are displayed. If data is available the user can adjust the generated LAI values. BUDGET Reference Manual 2. 9

18 rooting depth (m) d. Rooting depth In BUDGET the rooting depth varies between a minimum value (maintained during the initial crop growth stage) and a maximum value (reached at the beginning of the mid-season stage) initial crop development mid season late season time (days) Figure 2.3d Assumed development of the effective rooting depth (shaded area) for an annual crop with a maximum rooting depth of 0.8 m in a uniform deep soil The minimum rooting depth is the effective rooting depth during the initial stage from which the germinating seed and the young seedling (annual crop type) can extract water. A minimum rooting depth of 0.2 (default value) to 0.3 meter is generally considered appropriate for water balance calculations. The minimum rooting depth for perennial crops is normally equal to the maximum rooting depth. When creating a crop file, BUDGET generates the maximum effective rooting depth in function of the selected class (Table 2.3a). Ranges for the maximum rooting depth for various crops are given in Annex I, Table I.3. e. Water stress Crop water stress will occur when the root zone is too dry. - Soil water depletion fraction for no stress (p). The fraction p of the Total Available soil Water (TAW) determines the Readily Available soil Water (RAW). RAW is the maximum amount of water that a crop can extract from its root zone without experiencing water stress (Fig. 2.3e). When creating a crop file, BUDGET generates the soil water fraction for no stress in function of the selected class BUDGET Reference Manual 2. 10

19 (Table 2.3b). Typical values for p for various crops are listed in Annex I, Table I.3. - C o rrection of p for ETcrop. When the crop evapotranspiration is low (ETcrop < 2 mm/day) p is somewhat higher than the specified p value. When the crop evapotranspiration is high (ETcrop > 8 mm) p will be somewhat smaller than the specified value. The user specifies if a correction is required and what type of correction (percentage or FAO adjustment). The FAO adjustment (Allen et al., 1998) is given by: p = pspecified ( ETcrop) The mean ETcrop of the last n days will be considered when adjusting p in the simulation run. The default value for the n days is 5. The value for this program parameter (See 2.9) can be adjusted. soil saturation water stress field capacity threshold wilting point RAW = p TAW Figure 2.3e Zones where root water extraction is not restricted (light area), restricted (grey) as a result of soil water stress and not possible (dark area). f. Aeration stress Crop water stress might also occur when the root zone is waterlogged. The user specifies if the crop is sensitive to water logging and at which soil water content (anaerobiosis point) the aeration of the root zone will be deficient for the crop and starts to affect crop transpiration (Fig. 2.3f). The default value for the anaerobiosis point is the soil water content at saturation minus 5 vol%. BUDGET Reference Manual 2. 11

20 To simulate the resistance of crops to short periods of waterlogging, the full effect will only be reached after a specified number of days. The default value is 3 days. The value for this program parameter (see 2.9) can be adjusted. deficient soil aeration soil saturation anaerobiosis point field capacity threshold wilting point Figure 2.3f Zone (dark area) of restricted soil water extraction as a result of deficient soil aeration g. Water extraction pattern The maximum root extraction rate together with the water extraction pattern over the root zone determines the maximum amount of water that can be extracted by crop roots at a particular depth (Smax). - Maximum root extraction rate. This rate refers to the maximum amount of water that the fully developed crop can extract from a well-watered root zone. Since under very hot and dry weather conditions, the crop is likely to experience some water stress even in a well-watered soil, the value should be kept below a reasonable threshold. The default value is 12 mm/day for root zones deeper than 0.4 m. - Water uptake by plant roots. - Uniform over entire root zone: extraction of water is uniform over entire root zone and in response to the water demand of the atmosphere; - Extraction pattern: since the root density is generally larger near the soil surface and declines with increase depth, the extraction of water is most rapid in the zone of the largest root concentration. The extraction pattern can be adjusted. Almost all plants growing in a well-watered uniform soil have a ///10 percent water extraction pattern (SCS, 1991). The values refer to the percentage of water extracted at the upper, second, third and bottom quarter of the root zone. BUDGET Reference Manual 2. 12

21 upper 1/4 second 1/4 third 1/4 bottom 1/4 % % % 10 % Figure 2.3g Typical water extraction pattern in root zone - Maximum water uptake by roots (Smax). Smax expresses the maximum amount of water (m 3 ) that can be extracted by the roots per unit of bulk volume of soil (m 3 ), per unit of time (day). The values are automatically generated when specifying the maximum root extraction rate and the water extraction pattern. If research data is available the user can adjust the generated Smax values: - If one assumes that the root water uptake is equally distributed over the entire root zone, Smax is defined as the ratio of the potential transpiration rate to the rooting depth. At any moment, the water extraction shall be smaller than or equal to the maximum specified Smax value; - If one assumes that root water uptake takes places in the succeeding compartments from the top downwards, Smax is given as a function of the soil depth. The value at the upper and bottom quarter of the root zone has to be specified. The Smax values are independently of the potential transpiration rate; Typical values for Smax are 0.03 to m 3 /m 3.day when the root density is high (top of a shallow root zone). In regions where the root density is low, Smax values are much lower: m 3 /m 3.day or lower. h. Salt tolerance Too many salts in the soil water solution can reduce evapotranspiration by making water less available for plant root extraction. The reduction in crop transpiration is simulated by taking into account the soil salinity at which growth ceases, ECe(max) (see Table 2.3d). Indicative values for ECe(max) for various crops are given in Annex 1, Table 1.4. ECe is the electrical conductivity of the soil saturation extract and is expressed in ds/m. BUDGET Reference Manual 2. 13

22 2.4 Profile description Select/Create soil file To specify the soil parameters, the user either: - download the soil parameters from an existing file by selecting the Select Command in the Main Menu. When selecting this Command, a list is displayed that contains all the files with extension SOL in the specified directory (default path is the DATA subdirectory from BUDGET). The user selects the appropriate crop file from this list; - create a new set of parameters that will be saved in a soil file by selecting the Create Command in the Main Menu, and by subsequently specifying the soil parameters. When creating a new file, the user specifies (Fig. 2.4): Figure 2.4a. Create SOIL file Menu 1. File Name; 2. File Description; 3. Type of soil surface: either NO bunds when part of intense rainfall might be lost from the field by surface runoff or BUNDS when rain and irrigation water is stored on top of the field between bunds (inundated rice fields) or large clods of earth on the surface of the field; 4. Number of soil layers; 5. Soil type for each of the soil layers: Select soil type from list; 6. Thickness for each of the soil layers; BUDGET Reference Manual 2. 13

23 Specifying and selecting the appropriate parameters in fields 3 to 6 creates a complete set of soil parameters. The soil parameters can be adjusted by selecting the Display/Update Command in the Main Menu (see 2.4.2) Soil Parameters When creating a soil file, the user has to specify only a few parameters. With the help of this information BUDGET generates the complete set of parameters. The soil parameters can be displayed and adjusted by selecting the Display/Update Command in the Main Menu. The parameters are classified in 3 folders: - Description - Soil layers - Bunds and surface runoff a. Soil layers The profile may be composed of up to five Soil Layers. After selecting the number of soil layers, the user selects the s o i l t y p e and specifies the thickness for each of the distinguished soil layer. Soil types are selected from a list. In the list a number of different soil types with typical characteristics are given (Table 2.4a). By selecting the update list soil layer characteristics button, soil layers may be added to the list, removed from the list, or the characteristics of the Soil Layers may be updated. The results are stored in the file SOILS.DIR. Table 2.4a. Default soil parameters listed in Soils.DIR SAT FC WP tau Ksat Soil type vol% vol% vol% - mm/day Sand Loamy sand Sandy loam Loam Silt loam Silt Sandy clay loam Clay loam Silty clay loam Sandy clay Silty clay Clay The characteristics of the soil layer are the following hydraulic properties: SAT : soil water content [vol%] at saturation; FC : soil water content [vol%] at field capacity; WP : soil water content [vol%] at wilting point; tau : drainage characteristic (value between 1 and 0); Ksat : infiltration rate [mm/day] at saturation. Indicative values and pedo-transfer functions for hydraulic properties for various soil types are given in Annex II, Tab. II.1 (SAT, FC, WP), Table II.2 (Ksat) and Table II.3 (tau). BUDGET Reference Manual 2. 14

24 The difference in water content at Field Capacity and Wilting Point determines the total amount of water (TAW) that is available for crop transpiration. Ksat determines the maximum rate with which water can infiltrate in the soil. Its value determines also the default CN value (see Table 2.4b). The amount of water that drains out of a soil layer (when the soil water content is above field capacity) is fully determined by the drainage characteristic, tau. The drainage characteristic is strongly correlated with Ksat (see Annex II) Table 2.4b Infiltration rate of top soil layer and corresponding CN default value Infiltration rate CN default value [mm/day] > < b. Bunds and surface runoff When bunds are present, the user specifies the height of the bunds. In the absence of bunds, part of intense rainfall might be lost by surface runoff. The amount of rainfall lost by surface runoff is based on the Curve Number method of the US Soil Conservation Service (see Chapter 3). The displayed default value for CN is based on the infiltration rate of the top layer (Table 2.4b). The default and specified CN value refer to the value for antecedent moisture class II (AMC II). If requested, BUDGET adjusts the CN value to the relative wetness of the topsoil during the simulation run. As such the estimate of surface runoff takes the variable storage capacity of the soil into account, which will be different when the topsoil is wet (AMC III) or dry (AMC I). The thickness of the topsoil that will be considered for the determination of its soil water content and corresponding adjustment of CN is a Program Parameter that can be altered (see 2.9 Program parameters). By selecting No adjustment the CN value is not corrected in function of AMC. For more information see Chapter 3 (3.3 Runoff subroutine). BUDGET Reference Manual 2. 15

25 2.5 Irrigation options Various irrigation options can be selected and the quality of the irrigation water can be specified after selecting the Irrigation Command in the section Climate/Crop/Soil Data Base of the Main Menu (Fig. 2.1) No irrigation (rainfed agriculture) When selecting this option, no irrigations will be generated when running a simulation Determination of Irrigation water requirement When selecting this option, BUDGET will calculate during the simulation run the amount of water required to avoid crop water stress. When the root zone depletion exceeds a given threshold value (default value is RAW), just enough water will be stored in the soil profile to keep the root zone depletion just above the threshold. The net irrigation water requirement is the sum of the water amount that had to be added during the simulation period. The net requirement does not consider extra water that has to be applied to the field to account for conveyance losses or the uneven distribution of irrigation water on the field. The default value for the maximum allowable root zone depletion (threshold) can be adjusted (see 2.9 Program Parameters) Irrigation scheduling a. In crop season Manuel irrigation scheduling: Specify the date and application depth of each irrigation event. The irrigation depth refers to the net irrigation amount. Extra water applied to the field to account for conveyance losses or the uneven distribution of irrigation water on the field should not be considered. Generation of irrigation schedules: At run time irrigations can be generated by specifying a time and depth criteria. The time criteria specifies When an irrigation has to be applied while the depth criteria determines How much water has to be applied. After the selection of the criteria the values linked with the time and depth criteria has to be specified. The value specified at a specific day of the cropping period will be valid till the date where another value is specified or to the end of the cropping period when no values at later dates are specified. As such one can adjust the values linked with the criteria to crop development (Table 2.5a). The criteria and corresponding values that need to be specified are given in Tables 2.5b and c. Table 2.5a. Example of irrigation schedule where the value of the time criteria (fixed interval) varies over the season while the depth criteria (fixed irrigation application depth) remains constant. Date Daynumber Interval Depth Growth stage 11 March 1 June 1 August [days] [mm] Initial Mid season Late season BUDGET Reference Manual 2. 16

26 Table 2.5b. Time criteria and corresponding parameter to be specified Criteria Specify Fixed interval Interval between irrigations (for example 10 days) Allowable depletion amount Amount of water that can be depleted from the root zone (counting from field capacity) before an irrigation has to be applied (for example mm) Allowable fraction of Readily Percentage of RAW that can be depleted before an Available soil Water (RAW) irrigation has to be applied (for example 100 %) Table 2.5c. Depth criteria and corresponding parameter to be specified Criteria Specify Back to field capacity Additional water depth. The applied irrigation water will set the soil water content in the root zone exactly to field capacity if 0 (mm) is specified. It is possible to plan an over- or under-irrigation by indicating the extra amount that has to be applied on top of the water required to bring the root zone back to field capacity (for example + mm) or the shortage (for example 10 mm) Fixed application depth Net irrigation application depth b. Out of crop season When the simulation period is not fully linked with the cropping period (see 2.6 Simulation Period), irrigation events can also be scheduled before and after the cropping period. This allows the users to simulate a pre-irrigation before the sowing or planting of the crop or to schedule irrigations out of the crop season to leach accumulated salts out of the root zone. c. Irrigation water quality The irrigation water quality (Tab. 2.5d) refers to the salt content of the water. It is expressed on the basis of its electrical conductivity (ECw). Table 2.5d Classification of irrigation water (Ayers and Westcot, 1985) ECw in [ds/m] Degree of restriction in use < 0.7 None slight to moderate > 3.0 severe BUDGET Reference Manual 2. 17

27 2.6 Simulation Period By selecting Simulation Period and subsequently S p e c i f y in the Simulation section of the Main Menu (Fig. 2.1), the user can adjust the simulation period for a simulation run. The menu is composed of 4 sections: - Climatic data: Information on the selected ETo and RAIN file and the period that is covered by the data is displayed in the section. The simulation period can not exceed the range of the available climatic data. If no ETo and RAIN file are selected, the user can select any simulation period but will have to specify the climatic data at run time. There will be also much less restrictions for the specification of the simulation period, if one uses ETo and RAIN file that are not linked to a specific year (containing for example mean climatic data). - Cropping period: The user can adjust the pre-selected day and month for the start of the cropping period in this section. The year has only to be specified if the climatic data is linked to a specific year. The final day, month and year of the cropping period is determined by the length of the cropping period (see 2.3 Crop Data). - Simulation Period: The user adjust the range of the simulation period in this section by specifying the first and last day, month and eventually year. The user can link the simulation period to the cropping period. This will not be possible when the climatic data does not cover the cropping period. - Graphical display (Fig. 2.6): The graph displays on a time axis the length of the period for which climatic data is available (data), the length of the cropping period (crop) and simulation period (simulation). 1 January December 01 ETo : mean 10-day data (no year) Rain : daily observations (1999) climatic data cropping period simulation period 11 March 01 6 September 01 (simulation period) linked to cropping period Figure 2.6 Example of graphical display when specifying the Simulation Period BUDGET Reference Manual 2. 18

28 2.7 Initial conditions By selecting Initial conditions and subsequently Specify in the Simulation section of the Main Menu (Fig. 2.1), the user can adjust the initial conditions for a simulation run Initial soil water and salt conditions Initial soil water content and salt contents can be adjusted: - the initial soil water content of the total profile may be set at Saturation, Field Capacity, Wilting Point or may be specified individually for each of the soil compartments; - the initial salt content of the total profile may be set at a specific value or may be specified individually for each of the soil compartments. The ECe is defined as the electrical conductivity of the soil water solution after the addition of a sufficient quantity of distilled water to bring the soil water content to saturation. The initial conditions in the soil profile are strongly determined by the climatic conditions (ETo and Rain) and irrigation applications in the period before the simulation period. If the simulation period starts at the end of a very rainy season, the soil water content of the soil profile might be close to field capacity. If the simulation starts in the hot dry season, the topsoil might be wet by pre-irrigation but the subsoil will be dry and the water content close to wilting point. If the field is surrounded by bunds the user can additionally adjust: - the water depth on top of the soil surface between the bunds; - the salinity (Electrical Conductivity) of the water stored on the soil surface Soil cover outside growing period If the simulation period is not fully linked with the cropping period (see 2.6 Simulation Period), the user can specify the soil cover (mulch) of the fallow land before and/or after the cropping period Cumulative counters Cumulative counters keep track of the sum of the daily values of parameters of the soil water balance and of the expected relative yield. These counters are, by default, automatically reset at zero (the expected relative yield at 100 %) at the beginning of each simulation run. If the default option is not selected (see 2.9 Program Parameters) the value of the cumulative counters and expected yield are kept for the next run. In the menu Cumulative counters, the user can reset the counters at any time to their initial value. BUDGET Reference Manual 2. 19

29 2.8 Output files By selecting Output Files and subsequently Specify in the section Simulation of the Main Menu (Fig. 2.1), the user can switch the creation of 8 output files individually ON or OFF. The files (see Annex III) contain daily information of: - the water content of each soil compartment (file COMP.OUT); - the water content of each soil layer (file LAYER.OUT); - the value of the different terms of the water balance (file WABAL.OUT); - the fluxes between compartments (file FLUXES.OUT); - the salt content of each soil compartment (file SALT.OUT); - the value of the different terms of the salt balance (file SALTBAL.OUT); - the root zone depletion at the beginning and end of the day (file DEPL.OUT); - the net irrigation requirement (file INET.OUT). BUDGET overwrites previous recorded data, each time a simulation run is started (by pushing on the Run button in the Main Menu). To avoid loss of data, the name as well as the path of output files can be modified. The contents of the Output Files can be displayed at the end of the simulation run by selecting Display in the Simulation section of the Main Menu. The data can be aggregated in 10-day, monthly or yearly intervals. BUDGET Reference Manual 2.

30 2.9 Program parameters When starting BUDGET, default values for program parameters are extracted from the Default.PAR file in the BUDGET main directory. The default values are either selected from literature or are the results of sensitivity analysis. By selecting Parameters at the bottom of the Main Menu (Fig. 2.1), the user can adjust the default values. The adjustments are only valid during the session. (To fix values of program parameters to other default values, the user has to modify the values in the Default.PAR file). The program parameters are classified in 9 folders: - Display - Evaporation - Transpiration - Salinity - Surface runoff - Soil compartments - Initial Conditions - Irrigation - Effective Rainfall Display During the simulation run, the program will plot the value of one particular parameter. The user selects in this menu the specific parameter as well as its maximum value Evaporation The user can adjust parameters, which affects the simulation of soil evaporation: - The two regression coefficients for the Ritchie-type equation; - The crop coefficient for a bare wet soil; - The depth of soil water extraction by evaporation See Chapter 3, Crop evapotranspiration for more information Transpiration The user can adjust parameters, which affects the simulation of crop transpiration: - The number of days over which crop evapotranspiration will be averaged for the adjustment of the maximum depletion factor for no stress (p); - The number of days after which the deficient aeration conditions will have its full effect Salinity The user can adjust parameters, which affects the simulation of salt movement in the soil profile: - The Response factor for global salt diffusion. The value for the global diffusion factor specifies the swiftness with which equilibrium is reached between the salt content of the various cells; - Salt solubility: The salt concentration at which salts will precipitate. The value depends on soil temperature, type of soil solution and type of salts. See Chapter 3, Salt transport for more information. BUDGET Reference Manual 2. 21

31 2.9.5 Surface runoff The user can adjust the depth of the topsoil that will be considered for the determination of its soil wetness (determination of the Antecedent Moisture Class (AMC)) and the corresponding adjustment of the CN value. To estimate surface runoff with 10-day or monthly Rainfall data, one has to know the number of rainy events. The user can adjust the number of rainy days for each of the months, by specifying the number of showers per 10-day. The more rainy days are considered, the smaller the rainfall amount per event and the smaller the runoff will be. More information on the processing of 10-day and monthly rainfall data is given in Chapter Soil compartments To describe accurately the retention, movement and uptake of water and salt in the soil profile, BUDGET divide the soil profile into soil compartments with thickness z (Fig. 2.9). The soil profile may be composed of up to 12 Compartments. After selecting the number of soil compartments, the user specifies the t h i c k n e s s for each of the distinguished compartments. A compartment has the hydraulic characteristics of one soil layer. For more information the user is referred to Chapter 3, Simulation of the soil water and salt balance. Soil Layer 1 Soil Layer Soil compartments Figure 2.9 Soil layers and soil compartments BUDGET Reference Manual 2. 22

32 2.9.7 Initial conditions When starting a new simulation run, the user can either R e s e t the soil water and salinity conditions in a soil profile to the specified initial conditions (see 2.7) or K e e p the values of the previous runs. The selection depends on what one wants to simulate: - RESET: successive simulation runs are not linked in time or apply to different fields; - KEEP: the various runs refer all to one particular field and are successive in time. If in a field one crop after another is grown and the user wants to keep track of the soil water content and/or the building up of salts in the soil profile, the conditions at the end of a simulation run will be the initial conditions for the next run. It is obvious that in such cases the user can not change the soil type between two simulation runs Irrigation When calculating the net irrigation requirement, BUDGET keeps the root zone depletion above a threshold value. The user can vary this threshold from 0 (field capacity) to 100 percent (maximum allowable depletion to avoid water stress) of RAW Effective rainfall Effective rainfall is that part of rainfall that is not lost by surface run-off or deep percolation but is stored in the root zone. If the rainfall data consist of 10-day or monthly values, the rainfall distribution over the period is unknown and the amount of water lost by surface runoff and drainage can not be determined by solving the water balance equation. For such cases, BUDGET determines (after the subtraction of the estimated surface runoff (see e)) the amount of rainfall that will be stored in the root zone by one or another procedure. The ineffective part of the rainfall is assumed to have drained out of the root zone and is stored immediately below the root zone. The following procedures can be selected: percent effective: All rainfall is stored in the root zone; - USDA- SCS procedure: The amount of rainfall stored in the root zone depends on the 10-day or monthly rainfall amount and the crop evapotranspiration rate during that period; - Percentage of 10-day/Monthly rainfall: The percentage of the rainfall that is stored in the root zone is specified by the user. More information on the processing of 10-day and monthly rainfall data is given in Chapter 3. BUDGET Reference Manual 2. 23

33 2.10 Run After specifying the Climate/Crop/Soil environment and adjusting the initial conditions if required, simulations can be run by selecting R u n in the Main Menu (Fig. 2.1). The menu contains an INPUT, OUTPUT and a graphical display section. Figure 2.10 Graphical display of the soil water content in the soil profile (Simulation Run) Input section In the Input section of the Run menu the user can specify for the successive days of the simulation period: - the atmospheric demand (expressed by the reference evapotranspiration ETo). Given the crop characteristics and ground cover, the corresponding amount of water that can be lost by soil evaporation (Epot) and by crop transpiration (Tpot) of a well watered soil and without salt stress is calculated and displayed; - the rainfall depth (Rain); and - the applied irrigation dose (Irri). BUDGET Reference Manual 2. 24

34 If climatic data is retrieved from ETo and RAIN files and if irrigations are generated by the program according to the selected option in the Main Menu, the user is unable to specify any data at run time. By clicking on the Environment button, information on the selected Climate, Crop and Soil file, the irrigation option and on the cropping and simulation periods are displayed. By hitting the START button, the simulation run starts and: - advance with a time step of one day (select day per day as the a d v a n c e option). With this option the user has to click each time on the CONTINUE button to advance one day in the simulation period; - runs through the simulation period (select run as the a d v a n c e option). This option is only possible if climatic data is retrieved from files. By hitting the EXIT button the simulation is terminated and one returns to the Main Menu Output section At the start of each day of a simulation run, the output of the previous day is displayed. Information is given on the: - net irrigation requirement; - actual amount of water lost by soil evaporation (Eact); - actual amount of water lost by crop transpiration (Tact); - amount of water lost by surface runoff (Runoff); - amount of water that has been infiltrated in the soil profile (Infiltrated); - amount of water that drains out of the bottom compartment of the soil profile (Drained); - amount of water that is stored between bunds on top of the soil surface (Surface water); - expected relative yield at the end of the growing period (Expected Yield); and - information on the salt content of the soil water (ECe) at various depths in the soil profile (visual display is soil profile). ECe is calculated by considering only the salts that can be dissolved in the soil saturation extract. Cumulative counters keep track of the sum of Input and Output Parameters Graphical displays The user can select different visual displays: - cross section of the soil profile: The rooting depth, the salt and water content of each soil compartment is displayed; - soil reservoir: The root zone is presented as a reservoir in which the soil water content varies as a result of incoming and outgoing fluxes and of soil water extraction; - root zone depletion: the root zone depletion, as well as RAW and TAW are plotted in function of time; - parameter: During the simulation run, the program will plot the value of the selected parameter (see 2.9a for its selection). BUDGET Reference Manual 2.

35 Cross section of soil profile Soil reservoir Root zone depletion Figure 2.10b Graphical displays BUDGET Reference Manual 2. 26