1.4 diffuse.aml

This AML enables the user to calculate diffuse solar radiation for a terrain of choice (Kumar et al., 1997). This is a complementary calculation to the direct radiation modeling. The portion of diffuse solar radiation relative to the global radiation varies between summer and winter, and is a function of solar altitude and terrain reflectance, among other. Global radiation (the sum of direct and diffuse radiation) is often used in other biophysical modeling application (e.g. modeling of potential evapotranspiration, 3.1 – 3.3).

This AML is very parsimonious, and thus very fast. The routine does not allow to calculate grids for time periods shorter than a single day. The routine enables the user to generate daily (to many days) grids of diffuse radiation. The user is prompted for:

If the user wants to calculate the radiation for a single day, the same Julian day has to be submitted for start and end of calculations. Southern latitudes have to be entered as negative values. The time interval (submitted in minutes) to increment the solar path indicates how often the sun position is calculated during the day. The program then integrates the energy linearly over time. 30 to 120 minutes are reasonable intervals depending on the size of the DEM (# of pixels).

This is a complementary calculation to the direct radiation modeling. The portion of diffuse solar radiation relative to the global radiation varies between summer and winter, and is a function of solar altitude and terrain reflectance, among other. Global radiation (the sum of direct and diffuse radiation) is often used in other biophysical modeling application (e.g. modeling of potential evapotranspiration, 3.1 – 3.3).
 

Modifications:

This AML had to be adjusted to a bug in ArcInfo 8.0.2, where the raise to fractional powers causes the program to crash. This reported bug is supposed to be fixed for AI 8.2. Calvin Tolkamp proposed an interim "fix", which is included in the version you download here. Attention: This "fix" has been updated Sept. 14th, 2000!
 
 
 

General specifications of the AML:

Command: &r diffuse    (at GRID or ARC prompt)
Required input: DEM
Output units: kJ/m2/time period
Solar constant used: 1.367 kW/m2
Transmittance used: 0.8 (max for vertical beam)
Minimum calculation step: 1 day
Speed of calculations: Very fast due to parsimonious programming
Flexibility of the routine: Medium; can’t go beyond 1 day
User interface: Limited, but simple and easy
Known errors:  -
Corrections 9/14/2000: Adjustments to bug in ArcInfo 8.0.2
Programmer L. Kumar
Available @: fatboy.geog.unsw.edu.au
or download here: diffuse.aml       (use: "save link as")
Contact: lkumar @ une.edu.au

 

References:

Kumar, L., Skidmore, A.K. and Knowles, E., 1997. Modelling topographic variation in solar radiation in a GIS environment. International Journal for Geographical Information Science, 11(5): 475-497.
 
 

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Last Updated: 9/24/03
By Niklaus E. Zimmermann