Porres, Maru 
  
3.2.3. Potential Solar Radiation Maps. To calculate an solar radiation map it is necessary to use two types of 
information (Ruiz ef al. 1999): 
Q data that vary in time, that is depending on the position of the sun in each instant, and 
the incident solar radiation at a specific place and a particular time depends on: 
- the solar constant (o) : the radiation received by a 1 cm? surface at the top of the atmosphere, 
- the position of the sun : the height of the sun above the horizon (}) and its azimuth (4), 
Q data that vary in space, referring to the position of the point receiving the solar radiation. 
- latitude (9), slope (B) and orientation (0) 
The solar radiation at the top of the atmosphere (R;) can be deduced from the solar constant and the height of the sun: 
R,= 0 -sin h 
The incident radiation on an area on the surface of the Earth (R,), taking into account the alterations due to the effect of 
the atmosphere, can be calculated using the expression: 
R, = Ra [ (0,29 cos q) + (0,54 n/Na) ] 
in which the top of the atmosphere radiation is seen to be modified by the latitude and the cloudiness (n/Na). Although 
this formula includes a correction to the solar radiation due to cloudiness, since the study area is not extensive, and as 
there were no meteorological stations that could provide reasonable cloudiness measurement, it was considered that the 
clouds would have affected the whole area to the same degree and that, therefore, cloud cover did not cause spatial 
variations. The map calculated shows the maximum direct solar radiation over the area, which is considered 
proportional to the real radiation taking into account the effect of clouds. 
The data referring to each point where the solar radiation is incident correct the values in function of the slope and 
orientation of each point. 
Using the formula proposed by Lambert the following expression is obtained: 
R; = R; [(cos ^ sin B cos (4 —9)) + (sin h cos B)] 
by which is calculated the energy received (R;) at any time for each cell. However, this formula does not take into 
account the effect of shadows caused by the relief, but rather it calculates for each cell whether it receives radiation and 
how much. For this reason the projected shadows were deduced for each instant calculated. 
In order to know the importance of the effect of the solar radiation on the distribution of the vegetation it is necessary to 
create an annual solar radiation map. For this it is necessary to calculate the direct radiation for every instant in the 
year. However this calculation would generate a huge and unmanageable volume of data, making it reasonable to 
determine representative moments that would explain the radiation behaviour with a smaller volume of data. From the 
study of the optimisation of the calculation of the annual potential solar radiation presented by Urbano (1999) it has 
been deduced that with 105 sun positions it could be generate a significant map of this variable. 
The map of potential solar radiation calculated by a program developed by us, is created by the integration of each of 
the moments selected, and its geometric properties (25 metre grid step) coincide with the digital elevation model used to 
produce it. 
4. RESULTS 
Once the ten parameters of topographic origin are calculated, the values are correlated in each of the xx homogeneous 
units of study with the NDVI corresponding to the two analyzed dates, to include the differences between both. First, 
the results which demonstrate hydrological parameters are presented, then those associated with thermal parameters. 
4.1. Study of the curvatures 
The correlation coefficients show a very bad relationship between the curvature parameters and NDVI values. These 
bad correlation index are observed for each of the curvature parameters, types of material, and fires studied. All of the 
cases yield very low values, which shows that neither the profile nor the surface of the curvature present a significant 
relationship with the recovery process of the vegetation. As well, the tendencies, which should presumably always be 
negative, are not constant, which reaffirms the weak significance that this parameter displays. 
It is interesting to observe that none have improved as a consequence of the different location considered. In fact, due 
to the slight significance neither an improvement nor a deterioration are noticed. 
4.2. Upslope Catchment area 
The parameter of the upslope catchment area (CA) does not present a significant degree of correlation —its indexes 
  
1174 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000.