Note 24 — GHARDAIA.

It has already been shown that the number of days on which rain may be expected to fall each year is greater at Laghouat than at Ouargla, and probably at Ghardaia also. According to reports, the rainy days at Laghouat vary from 20 to 84 (seven years’ observation), with an average of 49 each year. The average number of days on which rain falls at Ouargla is 14.2. The amount of rain at Laghouat is 200 mm., at Ouargla 90.2 mm. It would appear, therefore, that the average rain at Laghouat is less in amount than the average rain at Ouargla; or, in other words, it points to the torrential as being the type of the desert storm. Since, other things being equal, the greater storms would penetrate the ground the most deeply, we may here have an explanation of the emphasis at Ghardaia on the tap-root as against the generalized root as the type of the root-system.