The introduction of the noria in any district has always had revolutionary consequences upon agricultural productivity. If introduced into an area where river irrigation has been practiced (for example, Ptolemaic Egypt), it lessens the dependence of gravity-flow canals on fluctuations in the level of the river (the Nile, in the case of Egypt) by using nongeared, water-driven wheels to raise the water.(52) The geared wheel, moved by harnessing animal traction to it, may have an even more dramatic economic effect. Since it is relatively inexpensive to build and simple to maintain, the noria enabled the development of entire huertas, intensively irrigated on the basis of individually owned smallholds. The diffusion of both kinds of wheels had the effect of increasing agricultural production, allowing for a greater density of population, and of providing agricultural surpluses with which to underwrite urbanization.
In al-Andalus, water-driven wheels were found in conjunction with canal systems in Murcia, where the mammoth wheel at La Nora was driven by the current of the Aljufia canal, and most typically in Toledo,  described by al-Idrisi (twelfth century) as having around it gardens interlaced with canals on which were established wheels for irrigation, and Córdoba, where al-Shaqundi (thirteenth century) described 5,000 norias (probably including both lifting and milling devices) on the Guadalquivir.(53) The extensive aeronomical literature of eleventh- and twelfthcentury Andalusi writers strongly accentuates the role of irrigation agriculture to the detriment of dry-farming techniques. But the irrigation described is not fluvial but from wells from which the water is lifted by a noria with a chain of pots and deposited directly into a channel or into a holding basin. Indeed, these writers came largely from Seville and Toledo, two cities to which the cultural center of gravltv had shifted after the fall of the Caliphate and where Valencian-style irrigation was not practiced.
In the ancient Mediterranean world, generally only winter crops were grown, with each field yielding one harvest every other year. The Arabs introduced a variety of new crops of Indian origin (of which the Andalusi agronomists were fully aware -- al-Tignari of Granada makes reference to "Indian agriculture").(54) Since these crops required heat, they were grown in the summer. Thus a rotation of crops became the norm, and irrigated fields yielded as many as four harvests yearly. The far greater number of annual plowings required by the new crop succession and the resultant water loss tended to make Muslim irrigators meticulous in their regard for the water-bearing capacities of each kind of soil. More kinds of soil were used than had been the custom in antiquity, and the aeronomical handbooks indicate that each soil type should be fully exploited.(55) Ibn Bassal, whose treatise was based completely on practical experience, distinguished between ten classes of soil, assigning to each a different lifesustaining capability, according to the season of the year. He was insistent that fallow be plowed four times between January and May and, in certain cases (for example, cotton, when planted in the thick soils of the Mediterranean littoral), he recommended as many as ten plowings.(56)
The agronomists believed that irrigation was necessary, along with fertilizer, as an improver of soil and, according to prevailing natural philosophical notions, as a regulator of soil temperature. Reflecting the highly refined practical techniques of eastern garden agriculture, they stressed the necessity of cultivating in such a way as to preserve the maximum amount of moisture in the soil. In irrigated fields, this involved a carefully planned field system of raised ridges and furrows, oriented in such a way as to make the best use of the slope of the land. In some cases, this  involved "correcting the land" so that the water would run equally through all the furrows, so that submersion could be achieved with maximum benefit to all plants without letting the water stagnate. These notions were standard throughout the Islamic world wherever irrigation was practiced and gave rise to distinctive patterns of rectilinear fields approaching a square shape, containing low irrigation levees to regulate the flow of water, and with ridges and furrows typically arranged in the form of a comb. The purpose of the comb, one of a number of furrowing patterns used, was to slow the velocity of flow on fields whose slope exceeded one degree or more, thus enabling a farmer to irrigate without terracing his land. This was the common form of furrowing for vegetable crops, with serpentine furrows used for row crops, and its diffusion has been traced from Central Asia to southern Spain.(57)
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