Water loss in canals

According to an interesting study carried out by the International Commission for Irrigation and Drainage (56), around a third of the water that uses irrigation canals all over the world is lost during transportation. Another third of the water is lost on the plot of land or the farm when it is used for irrigation. Therefore, only one-third may be usefully applied. These horrifying figures refer to worldwide average values, since there

are cases, such as those quoted by Kennedy in 1981, where canals lose 45% of the water they are transporting. There are the individual cases in the USA where losses during water

transporting in canals reach 60% and on average they reach 23%. In many countries throughout the world water is scarce, therefore it must

be rationally used. The cases of North Eastern Brazil, South Eastern Spain, areas in the Middle East, etc. where the lack of water restricts production are only examples known by the public in general, but they form part of a much more widespread problem. Canals used for town water supply or for hydroelectrical use also lose a

significant amount, although we do not have any statistics for them. When water is lost in canals we must ask ourselves what benefits would

have been obtained with the construction from the beginning of canals that were less permeable or the improvement of the watertightness of existing ones and therefore the profitability of this investment. In the case of irrigation, for example, it is clear that saving of these

water losses would mean irrigation in new areas, with significant increase in production. But similar considerations may be made for energy production canals, for town water supply canals, etc. In the cases of using purified waste water, desalinization of sea water,

expensive water transfer work from other basins, the interest in decreasing the losses to the maximum economically possible is obvious, since a significant amount of water could be saved and could be transferred to other uses. Moreover, even in those countries where there is no lack of water, its

its elevation are expensive operations,

therefore preventing useless losses during the transport is an economically interesting operation. You only have to imagine a canal that has to transport a certain amount of

water to a certain place. If, during this operation, it suffers from significant losses, the water flow that we have to introduce through its main intake must be greater, and equal to the sum of the required flow and the predicted leakage amount. This without doubt makes not only the energy to raise water from a well (if this is the supply system) but also the construction of a possible regulating reservoir (if surface water is taken) and even the work on the canal itself more expensive, which must have a greater transport capacity, since as well as the final water requirements, it must transport the leakage itself that will be lost en route. Any of these considerations have justified the decision to line canals to

decrease their losses. In Spanish legislation there have been many economic aid rules for irrigators, wanting to line their tertiary irrigation canals. The Decree of the 15 December 1939 is a simple example. The water saving that it represented and the profitability its use on other applications meant it easily compensated the State for the economic expenditure carried out. Another important example is that of the Water Supply Consortium of Tarragona, which used a flow of almost 5m3/s saved thanks to the lining of the canals of the Bajo Ebro. When reaching this point an important question arises: given that the

civil work that we can build, repair and handle cannot be perfect and will always have some losses, what maximum value can we aspire to and what technical possibilities have we to obtain this? Frequently it is considered that a canal that has losses due to leakage

between 25 and 50 l/m2 in 24 hours adequately fulfils its undertaking. This value is not clearly defined, but it represents an order of magnitude that is generally internationally accepted (56). It is obvious that many canals, because they are located in highly imper-

meable land or because they are covered with linings that are very well conceived and very well made, can have lower leakage rates (at times almost none), but it is no less true that in many cases, if greater precautions are not taken, the losses are much greater, due to the high permeability of the land, due to performance faults or simply due to cracking of the linings. This demands a very delicate care by the project engineer to study the solution to be adopted and to carefully specify the conditions that must be complied with during the work. Only in this way may the leaks be kept within the abovementioned minimums. If, to bring our ideas together, we consider a real canal that transports

60m3/s, with a speed of 1.5m/s, a wet perimeter of 20m2 per lineal metre and total length of 100 km, the leakage surface area (wet perimeter) in the case of the dimensions of the cross section being kept throughout the layout metres. If the canal were telescopic,

the wet surface area would be half of this, i.e. one million square metres. With the abovementioned leakage values, the losses would be from 25000 to 50000m3 per day. The volume of water transported by this canal per day will be

60m3/s×86400s/day, i.e. 5140000m3; therefore the relative loss is around 0.5-1%. If the water speed were higher (as normally happens with canals in hydro-

electric plants), the loss percentage would be even lower. This percentage might seem small, but other causes that increase it con-

siderably must be taken into account. A canal such as this one that supplies an irrigation area supplies a network

of irrigation canals and derived canals the length of which is around ten times greater (in the specific case that we are talking about, the length of the canal represents 2m/ha and the secondary channels around 20m/ha). Though the latter are much smaller (a tenth in size of the canal), their losses due to leakage are approximately equivalent to the losses of the main canal and the total losses are doubled. There is another important cause for water loss in canals, unfortunately

about which we can do nothing. Here we are referring to losses due to evaporation. In the real canal we are considering, the surface width is approximately

15m; therefore the evaporation surface area over the 100 km of the canal will be 1500000m2 or 150 ha, which would be reduced by half if the canal were to be telescopic. With a fictitious evaporation of 1 l/s per ha (which can be real on many occasions, but which in general errs on the side of safety), the loss would be 150 l/s, equivalent to 12960m3/day, or half of this if the canal were telescopic, i.e. a value around a quarter or eighth part of the values to which the losses would be reduced if we were to limit the leakages to 25-50 l/m2 in 24 hours. We have no weapons to fight against evaporation. But we have to be

aware that the maximum desired leakage of 25-50 l/m2 in 24 hours is equivalent to a loss percentage of around 1-2%, to which another 0.5% must be added due to evaporation. There is a third cause that produces water losses in canals that is worth

mentioning. We are referring to involuntary spills due to problems of bad regulation in the handling of the canal. It is a subject we will talk about in this book and that requires suitable facilities to be foreseen in the canal and careful planning in its handling. With the correct precautions the losses due to this reason, together with the leakage, make a total of 5% of the water flow, which is an admissible value, particularly if we compare it to the 33% that we have given as the current average in the world. In this book we will try to fight against losses due to leakage and due to

However, since we are aware that we will not be able to design and construct canals in general that have no losses, we must oversize their capacity to carry not only desired flow, but also the losses. Traditionally, with a safety margin and whenever great care in the design and the building of the canal were taken, this concept means the calculated flow is increased by 10%.