ABSTRACT
In irrigation areas, the terms ditches and tertiary canals are sometimes employed. The actual difference between these two is not clearly explained, although
both refer to small canals. In large irrigation networks, there is normally a single main feeder canal,
from which other, smaller canals are derived. These are secondary, which then produce the tertiary, from which water is directly taken to the plot via plot turnouts. Sometimes there also are quaternary canals, etc. As far as we are concerned, the terms ditches and tertiary canals refer
to those watercourses used for the irrigation of smallholdings by means of irrigation outlets. Normally, when these are small, they are directly excavated in the ground, however, on many occasions they are constructed from concrete. On the other hand, when referring to irrigation watercourses, which are not normally connected to plot turnouts, but are intended to feed water to other smaller watercourses, we simply call them “canals”. This is the definition that we shall employ through this book. This division between the words canal and ditch or tertiary canal there-
fore involves a certain distinction in operation, as will be described in the Chapter 19. Usually, the aim is to obtain operation with a continuous water flow in the main watercourses, which are the canals, while employing a system of turn irrigation in the direct irrigation watercourses consisting of the ditches or tertiary canals. The term “turn” or “rotation” irrigation refers to the fact that each direct
plot turnout from the ditch or tertiary canal is supplied with a flow rate that the irrigator is able to use during a specific number of hours that will be enough to satisfy this person’s irrigation water requirements. Once this time period has expired, the associated plot turnout is closed and another is opened for a time decided by the surface area and the type of crop and so on successively. It should not be necessary to explain that, for operational convenience of the network, rotation and direct irrigation of the plots are tertiary canals, which are therefore the
smallest, so that this concept of the term ditch or tertiary canal basically coincides with the term “small canal”. This concept will be enlarged upon in Chapter 19, which is dedicated to canal regulation. The smaller size of the ditches or tertiary canals does not affect either
the formulas or the hydraulic operation concepts that have been previously described, and they continue to be completely valid. This also applies to the philosophy of the special works, which are identical to the previous ones, with the only difference imposed by the reduced dimensions. The elements that are conceptually new are the direct irrigation plot turnouts, but these can still be basically treated with the same criteria as for the canal turnouts. There is a basic difference between the irrigation canals and the ditches
or tertiary canals, which is a consequence of their direct plot irrigation purpose. We refer to the fact that the ditches or tertiary canals must have the water surface above the ground that is to be irrigated, assuming that only surface irrigation is being carried out, without spray irrigation pumps or localized irrigation. This greater water level over the ground should be around a minimum of
20 cm, but if the tertiary canal is not located connecting the higher points of the plot, then when calculating the canal water surface level, it will be necessary to take into account the highest plot point increased by the required difference in level so that the water can reach it using another provisional ditch. When designing the tertiary canal plan layout, it should always be arranged for it to run along the highest ground points, if it is possible. The water surface levels for the irrigation feed canals are, on the other
hand, fixed by the need to supply water to the derived canals with the levels that these actually require. The other important condition to be fulfilled by the tertiary canals is
that in order to comply with their function of irrigating the plots, they must possess a water velocity that will permit a good level of plot turnout operation. Although special works have been designed to permit the derivation of
water to the plots, even with high speeds, it is recommended that limited velocities, between 0.25 and 1.0m/s, are employed, which should also avoid sedimentation due to very slow speeds. The most usual speeds are around 60 cm/s.
