4.3 A model with solar radiation and air temperature boundary condition
The next model is a more sophisticated one which includes air temperature as a boundary condition and the radiation exchange and convective heat transfer at the soil surface. The outline of the model is shown in Fig. 4.5. In the present model, the soil layer is divided unevenly - that is, thinnest at the surface and thicker toward the bottom. Because the soil temperature does not change so much in deep layers. The top layer is 1 cm thick ls assumed to be a film surface in the balance equation. This assumption is justified cause in practice the soil surface is not smooth and the surface temperature is not well defined and extremely difficult to measure correctly. This thickness can be reduced to 1 mm for example, if it is needed. three new component of heat transfer, all at the surface, are involved. as shown in Fig. 4.5 direct solar radiation (RAD), long- wave radiation exchange between the surface and The sky and convective heat transfer (HO"(TF-T0)). In the present model the atmospheric emissivity ( EPSA )is assumed to be constant. The model is listed in Fig. 4.6, and its result is given in Fig. 4.7
In the Present model. there is a MACRO function, which starts with a MA CRO definition as show" 111 Fla 4.6 and ends with an ENDMACRO (it can be abbreviated ENDM4C) statement. It acts as a subroutine or function subprogram in FORTRAN, and must be placed at the first part of the program as it is in this figure. The difference is that it is fully expanded in the main program, where it is called up when the program is compiled. The first MACRO is used to calculate the absolute temperature. TT is a dummy argument and is replaced when this program is used. The real argument is outside air temperature TO in this program, and T04 is the product of TO four times - that is, TO to the fourth power, Because the Stefan-Boltzmann constant is multiplied by 10 to the eighth power, the temperature involved is divided by 100 before it is multiplied four times. For example, the statement PROCEDURAL (PROCED) is NOSORT in the main block, indicating that the statements following must not be sorted -- that is, they are arranged in the calculation order. The difference between PROCED and NOSORT is that all sections before and after the PROCED section are sorted completely but the NOSORT section separates the programs, and the former section is not sorted throughout the latter. The dynamic section in this program starts with a CLOCK statement. The variable name TIME is a reserved name for time which is an independent variable in CSMP and is automatically from 0 to FINTIM. This section is repeated as it is in real life. model, CLOCK is calculated by using the FORTRAN AMOD function and time clock. CLOCK therefore changes from 0 to 24 hours. Solar radiation as a sine function and starts at 6 am and ends at 6 pm. The maximum value, as RP, but the negative value of the sine function is canceled out by the which performs an AND of TRAJJ and a positive value (1.0). These calculations are conducted in the PROCEDURE section which starts with the PROCED at ENDPRO. the result of the simulation is shown in Fig. 4.7. The temperature boundary conditions are the same in the preceding model except for air temperature at present. Although air temperature, one of the boundary conditions, changes from 5° to 15°C in this case. the soil.