Page 160 - Arabian Studies (II)

152 Arabian Studies II

crescentic arc on the edge of an outwash fan adjacent to the
extension of the lagoon. Some of these gardens have probably been
cultivated for many hundreds of years for Ras al-Khaimah is an old
settlement and port-excavations there have revealed the presence of
potsherds dating back to at least the thirteenth century A.D.
irrigation water for the date gardens was mainly derived from
shallow sub-surface flow, through the gravel fans. This reflected
run-off from the mountains and consequently tended to be un­
reliable. Furthermore, it had to be at shallow depth so that it could
be exploited by primitive lifting devices. However, between Khatt
and Habhab, a falaj (Falaj Usayli) was constructed, while the date
gardens of these two villages were additionally irrigated by spring
water. The spring water is hot, about 39°C., compared with
temperatures of about 30° C. for groundwater beneath the outwash
plains, and issues from limestone of Jurassic-Cretaceous age. While no
significant diurnal or seasonal variations in water temperature have
been observed, the salinity fluctuates between 2.3 and
2.9 mmhos./cm. (Sir W. Halcrow and Partners, 1966).
The shallow groundwater and the springs only provided irrigation
water sufficient for the traditional forms of agriculture. With the
recent rapid development of agriculture, the traditional sources of
water were no longer adequate and new sources of supply were
essential. These occur in the deeper sediments of the Jiri Plain which
are composed of thick deposits of gravel alternating with silty and
sandy horizons. Groundwater is generally found at depths of less
than 50 metres above an aquiclude formed by calcium carbonate
cementation. This groundwater is recharged by runoff from the
mountains — it is disgorged by the mountain wadis on to the outwash
plains, where it quickly sinks below the surface to replenish the
aquifers. In periods of exceptionally heavy rainfall, the runoff may
flow right across the Jiri Plain to be ponded up against the sand
dunes. It slowly evaporates or infiltrates into the soil, leaving behind
silt deposits.
!
As the subsurface flow approaches the coast, not only does the
gradient of flow decrease but also the water becomes more saline
(Fig. 2). This increase in salinity is the result of a number of factors.
The groundwater not only takes up soluble salts present in he
outwash deposits, but also the rapid development of agriculture has
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emphasised the salinisation process. Recirculation of irrigation water,
especially in the Digdaga-Fahlain area has caused an increase in
salinity (though short-term monitoring by Sir W. Halcrow and
i Partners, 1969, gives conflicting evidence) while in certain situations
there has been a local lowering of the water table. This may be