Geoscience Reference
In-Depth Information
2
P 0.5489
P
=
max
P
+
U* 0.2188
P
+
1.5852 (1
e
) ;0
12 94
(The maximum function [ ;] means that it is possible to work out rainfall levels.)
This method was also used to create the 360 monthly precipitation maps for the
period between September 1965 and August 1995. Figure 7.11 is an example of one
of these maps that was created.
7.2.3.4. A summary of rainfall mapping
It is not possible to highlight the exact effect that relief or distance from the sea
has on rainfall levels for a particular month. However, these two factors influence
the average monthly rainfall values. By applying the process of interpolation to the
reduced variables it is possible to generate maps that are coherent with the maps that
show information on monthly median rainfall levels.
The application of the method of interpolation to the reduced variables has
improved estimations of average rainfall levels in the drainage basins. This
improvement is important for drainage basins whose rain gauges are located at the
bottom of a valley. If the relief of the local area is not taken into consideration, then
rainfall levels tend to be underestimated and this underestimation means that the
variables influencing rainfall levels need to be modified. Before carrying out this
study, which focuses on monthly rainfall levels, the model known as LOIEAU was
tested on 55 drainage basins from which average rainfall was recorded on rain
gauges that were found either in the basin or near it. The LOIEAU was derived from
the GR2M model. After slightly adjusting the values of the coefficients, the average
of the 55 X1 coefficients was 1.13 and its standard deviation was 0.69. By taking
measurements of rainfall levels that were recorded by rain gauges located only in
the drainage basins, the average of the coefficients was very similar at 1.14,
however, the standard deviation fell to a value of 0.47. A large part of the variance
of the parameters used to adjust the values of the coefficients was used to
compensate for errors that were created whenever rainfall levels were estimated
[LAB 03]. Similar results were recorded in the French regions of Corsica and
Provence by the CEMAGREF [LAV 97; ASS 00]. The CEMAGREF is a public and
agricultural research institute that exists in France.
7.3. The problems associated with recording rainfall and average spatial
rainfall
In the previous section we saw that it is possible to estimate rainfall for any part
of the country by using approximate measurements, such as radar and remote
sensing. However, using these methods is a relatively modern process and they are
seldom used as they often lead to inaccurate results. The most common method used
is interpolation, which is applied to isolated rainfall, as was described in the
previous section. Hydrologists are more interested in the average spatial rainfall that
falls in a particular drainage basin rather than levels of rainfall. In order to show
 
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