[GRASS-user] Weird results with r.sun

[Odoardo Zecca]

Hi all,
running r.sun on a mountain area with time=8 and day=1, I got negative
values for the diffuse radiation fraction.
I even get negative global radiation!
Where am I wrong?

 +----------------------------------------------------------------------------+
 | Layer:    1_8_d                          Date: Fri Oct 12 07:53:50
2007    |
 | Mapset:   zodoardo                       Login of Creator:
zodoardo        |
 | Location:
Valledaosta                                                      |
 | DataBase:
//media/sda7/zodoardo/cartografia/grassdb                        |
 | Title:     ( r.sun solar model output
)                                    |
 | timestamp:
none                                                            |
 |----------------------------------------------------------------------------|
 |
|
 |   Type of Map:  raster               Number of Categories:
255             |
 |   Data Type:
FCELL                                                      |
 |   Rows:
864                                                        |
 |   Columns:
1290                                                       |
 |   Total Cells:
1114560                                                    |
 |        Projection: UTM (zone
32)                                           |
 |            N:  5096987.5    S:  5032187.5   Res:
75                     |
 |            E:   422587.5    W:   325837.5   Res:
75                     |
 |   Range of data:    min = -18.926418  max = 53.797997
|
 |
|
 |   Data
Source:                                                             |
 |
|
 |
|
 |
|
 |   Data
Description:                                                        |
 |    generated by r.sun
|
 |
|
 |
Comments:                                                                |
 |
----------------------------------------------------------------       |
 |     Day [1-365]:
1                            |
 |     Local (solar) time (decimal hr.):         8.0000
|
 |     Solar constant (W/m^2):
1367                         |
 |     Extraterrestrial irradiance (W/m^2):      1412.689562
|
 |     Declination (rad):                        -0.401581
|
 |     Latitude min-max(deg):                    45.4214 - 46.0199
|
 |     Sunrise time (hr.):                       7.74
|
 |     Sunset time (hr.):                        16.26
|
 |     Daylight time (hr.):                      8.52
|
 |     Solar altitude (deg):                     2.1959
|
 |     Solar azimuth (deg):                      127.0877
|
 |     Linke turbidity factor:                   3.1
|
 |     Ground albedo:                            0.200
|
 |
-----------------------------------------------------------------      |
 |
|
 |    r.sun -s elevin="DEM" aspin="DEM_asp" slopein="DEM_sl" lin=3.1alb=0\   |
 |    .2 beam_rad="1_8_b" diff_rad="1_8_d" refl_rad="1_8_r" day=1 step=0.5\
|
 |
time=8                                                                 |
 |
|
 +----------------------------------------------------------------------------+

 +----------------------------------------------------------------------------+
 | Layer:    1_8_g                          Date: Fri Oct 12 08:20:06
2007    |
 | Mapset:   zodoardo                       Login of Creator:
zodoardo        |
 | Location:
Valledaosta                                                      |
 | DataBase:
//media/sda7/zodoardo/cartografia/grassdb                        |
 | Title:     ( 1_8_g
)                                                       |
 | timestamp:
none                                                            |
 |----------------------------------------------------------------------------|
 |
|
 |   Type of Map:  raster               Number of Categories:
255             |
 |   Data Type:
FCELL                                                      |
 |   Rows:
864                                                        |
 |   Columns:
1290                                                       |
 |   Total Cells:
1114560                                                    |
 |        Projection: UTM (zone
32)                                           |
 |            N:  5096987.5    S:  5032187.5   Res:
75                     |
 |            E:   422587.5    W:   325837.5   Res:
75                     |
 |   Range of data:    min = -10.351157  max = 327.495667
|
 |
|
 |   Data
Source:                                                             |
 |
|
 |
|
 |
|
 |   Data
Description:                                                        |
 |    generated by r.mapcalc
|
 |
|
 |
Comments:                                                                |
 |    1_8_b + 1_8_d +
1_8_r                                                   |
 |
|
 +----------------------------------------------------------------------------+

Thanks in advance for your help.
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