The differences in the geographical distribution of precipitation vary from year to year, as may be illustrated by referring to that for the year 1908, which may be compared with the normal usual distribution as given above. In the northern portion of the country more rain than usual was reported; for example, there was over 1,000 mm. on the littoral east of Algiers, and over 500 mm. on the High Plateau, but on the desert the amount was somewhat less. At Laghouat it was 161 mm., at Ghardaia it was 89.2 mm., and at Ouargla it was 28 mm.
Besides the differences in amount of yearly rainfall, well-marked seasonal amounts of precipitation are also to be noted. In the northern portion of the colony rains are likely to occur in winter and spring, the summer and early autumn being dry; and as one goes south of the Saharan Atlas nearly the same conditions obtain; that is, the rains usually fall during the rainy season of the coast. The seasonal distribution of rain for the Tell, including the stations of the littoral, the High Plateau, the Saharan Atlas, and the desert, for a series of years including 1908, is given in table 1.
Table 1.—Seasonal distribution of rain.
| Season. | Tell (10). | High Plateau (8). | Saharan Atlas (10). | Desert (6). | Remarks. | |
|---|---|---|---|---|---|---|
| mm. | mm. | mm. | mm. | |||
| Winter | 469.9 | 119.3 | (50.8) | 113.5 | 37.4 | The Tell is represented by FortNational; the High Plateau by Geryville (with Ain Sefra, for 6years, in parentheses); the Saharan Atlas by Djelfa; the desert byOuargla. The number of yearly records on which averages arecomputed are given at the head of each column in parentheses. |
| Spring | 300.0 | 80.5 | (95.0) | 88.2 | 39.0 | |
| Summer | 38.1 | 66.5 | (35.8) | 50.4 | 3.7 | |
| Autumn | 287.2 | 139.5 | (58.5) | 123.1 | 19.6 | |
The seasonal percentages of precipitation give a more graphic conception of the rainfall conditions for the four physiographic provinces. In the Tell this percentage in winter is 42, in spring 27, in summer 4, and in autumn 27. On the High Plateau the percentages are 30, 20, 16, and 34 for the four seasons respectively. In the Saharan Atlas 30 per cent of the rain occurs in winter, 24 in spring, 13 in summer, and 33 in autumn. In the desert the percentages of rainfall are 37, 39, 4, and 20[3] for the four seasons.
It is of interest also to note the number of days on which the rain fell on an average each year over a period running from 7 to 12 years. Thus at two typical stations on the Tell rain was reported on 102 and 118 days; at two stations on the High Plateau it rained 65 and 83.8 days; at a station in the Saharan Atlas rain was reported on 70 days; at desert stations, at Ouargla rain fell on an average 14.2, and at Laghouat 49 days each year. As a comparison, it may be mentioned that for ten years at Wady Halfi, Egyptian Sudan, there were only 22 days on which rain-drops were seen to fall. (Engler, loc. cit.)
The amount of precipitation varies greatly for the different desert stations, usually becoming less as one goes south from the High Plateau. As has already been mentioned, the average rainfall at Laghouat, which lies at the southern base of the Saharan Atlas, is 198 mm., the average at Ghardaia is 114 mm., while that at El Golea is 47 mm. The latter station is about 225 miles south of Laghouat, in the midst of the Sahara. The amount of rainfall, however, is greatly influenced by altitude, although lack of adequate precipitation data for the desert makes impossible a detailed presentation of this phase of the subject. As the amount of the yearly precipitation is less in the extreme southern part of Algeria than it is nearer the Saharan Atlas, where the altitude also is greater, it might be expected that the number of rainy days would vary in a like manner. Such records as are at hand, however, do not show this to be the case. For instance, at Ouargla rain falls on an average 14.2 days, average of 7 years, while the rainfall is 90.2 mm.; yet at El Golea, with a rainfall of 47 mm., there are 23.4 rainy days each year.[4]
On the desert the rains are often of a torrential nature, as facts presented above would indicate, and sometimes as much rain falls within a few hours, or even a few minutes, as usually occurs in an entire year. How much of the annual precipitation is of this character and how much is of the non-torrential kind the usual summaries leave entirely out of the account. It is well known that the former type of storm is more destructive and less useful to plants than the latter type. To illustrate the irregularity of the rainfall in the northern Sahara the monthly precipitation at Ouargla for several years is presented in table 2.
Table 2.—Rainfall at Ouargla, in millimeters.
| Year | Amt. | No. of days rain | Jan. | Feb. | Mar. | Apr. | May | June | July | Aug. | Sept. | Oct. | Nov. | Dec. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1900 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 1901 | 18 | |||||||||||||
| 1902 | 70.5 | 8 | 0 | 0 | 11.3 | 0 | 0 | 0 | 0 | 1.8 | 0 | 46.4 | 11.0 | 0 |
| 1903 | 135.6 | 14 | 55.0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 30.6 | 50.0 |
| 1904 | 35.0 | 26 | 20.0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 9.0 | 4.0 | 2.0 |
| 1905 | ||||||||||||||
| 1906 | 15.0 | 0 | 2.0 | 4.0 | 0 | 0 | 4.0 | 80.0 | 41.0 | 80.0 | ||||
| 1907 | 210.0 | 18 | 0 | 68.2 | 67 | 325.0 | 16.2 | 20.0 | 0 | 0 | 5.0 | 3.3 | 3.8 | 1.3 |