Page 84 - Arabian Studies (V)

74 Arabian Studies V
three such storms will occur and a 10 per cent probability that four
storms will occur. Maximum total winter rainfall at a point has
exceeded 200 mm. in some years but it is more likely to be of the
order of 75 mm. While heavy rainfall may cause flooding of farms
sited in depressions, its highly erratic nature and low intensity
cannot be regarded as a reliable supplement to irrigation. However,
what limited rainfall does occur constitutes Qatar’s primary water
resource in the form of recharge to groundwater.

4. Soils and Vegetation
The most important arable soils in Qatar are the rawdah (pi.
rawdat/riyad)3 soils of colluvium origin found in the base of most
depressions referred to above. These relatively young soils are
!i composed of fine material forming calcareous loam, clay loam,
sandy loam and sandy clay loam overlying limestone boulders or
outcrop at depths ranging from a mere 30cm. to 150cm. The
rawdah association occupies a total aggregate area of 29,000 ha. and
comprises two series; A—the main arable soil association, covering
an area of some 23,000 ha. and ranging in texture from silty clay to
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clay loam, and A2—a sandy clay loam to sandy loam texture and
covered with aeolian sand to a depth of 10- 14cm. of a total area of
6,000 ha. The remaining soils made up to Sibakh, lithosols or
aeolian sands are of little agricultural importance.
The rawdah soils in their natural condition have certain proper­
ties which give rise to serious management problems. Because of
the low organic matter content, water-holding capacity and struc­
tural stability is poor and causes surface crusting which interferes
with the emergence of seedlings and creates unfavourable plant-air-
water relationships. As much of the irrigation water contains
between 1,000 and 2,000 ppm. of total dissolved solids, surface
crusting inhibits infiltration and leaching causes the soil surface to
become saline within a relatively short time. Furthermore, although
the particle size indicates a high percentage of clay a large propor­
tion is in fact finely divided calcium carbonate which has entirely
different properties in that its contribution to cation and exchange
capacity and resulting moisture and nutrient retention is small
compared to that of clay. Fertility is generally low, particularly
with respect to nitrogen and phosphorous, and there is also a defi­
ciency of iron, manganese and zinc. Recent experimental work has
however shown that with proper land levelling, chisel ploughing,
il pre-leaching of salts, and a well-managed crop rotation, the
productivity of these soils can be greatly increased.
Natural plant growth in Qatar is limited by the seasonal, erratic

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