This variation in the reports from ʿUmar indicates that capacity must be taken into account. Likewise, the assessment of kharaj should be determined with due regard for the capacity of every parcel of land to bear it. Lands differ in three respects, each of which affects whether the kharaj is increased or reduced.
The first is the quality specific to the land: its fertility, which causes the crops to flourish, or its poor quality, which reduces its yield. The second is the variation in the types of crops. Some grains and fruits command a high price, while others command a lower price; accordingly, the kharaj should correspond to them. The third concerns irrigation and watering, since land whose irrigation requires the expense of water wheels and animal-drawn water-lifting devices cannot bear the same kharaj as land irrigated by flowing water and rain.
The watering of crops and trees is divided into four categories. The first is water supplied by people without the use of equipment, such as water from springs and rivers channeled to the land, flowing over it when needed and being diverted from it when no longer needed. This is the most beneficial form of water and the least costly.
The second category is water supplied by people using animal-drawn water-lifting devices, water wheels, or other wheels. This requires the greatest expense and the most arduous labor. The third is water supplied by the sky in the form of rain, snow, or dew; this is called ʿadhī (rain-fed irrigation). The fourth is water supplied by the earth through its moisture and through whatever water its soil retains. Its crops and trees drink through their roots, and this is called baʿl (naturally irrigated land).
As for ghayl, it is water by which land is irrigated through a canal. If it flows naturally, it belongs to the first category; if it does not flow naturally, it belongs to the second. As for the kẓāʾim, they are wells from which land is irrigated. If the water is drawn from them with buckets, it belongs to the second category; but if it is extracted through qanāts, it is ghayl and is assigned to the first category.
Once this is established, the assessor of kharaj must take into account the three aspects we have described: the differences among lands, the differences among crops, and the differences in their sources of water, so that he may determine the amount of kharaj the land can bear. He should seek justice between the people liable for kharaj and those entitled to the fayʾ, without imposing an increase that would wrong the kharaj payers or making a reduction that would harm the beneficiaries of the fayʾ.