184 GNATS OK MOSQUITOES — CHAPTER VIII untoward influence of canal irrigation on the public health through the concomitant increase of malarial disease. The matter has been the subject of several special investigations, and as a matter of fact has been fully admitted alike by the profession and the Indian Government long before we had any exact knowledge of the way in which irrigation is responsible for such evil effects.
In some cases these effects have been so serious as to raise the question of the advisability of abandoning the system ; but these have occurred mainly in situations where old native canals have been utilised, and in which the ahgnment contravenes the principles that will be explained below ; but it may be at once admitted that no suggestion involving the prohibition, or even restriction of irrigation will be considered worthy of serious consideration by practical men.
Starvation is a worse disease to bear than malarial fever, and over much of the irrigated area, the very existence of the population is dependent upon irrigated crops. In much of the Punjab and Eajputana, and the whole of Sind, the natural rainfall is so small that, but for irrigation from canals or wells, the whole country would be an uninhabitable desert ; and the entire prosperity of the country and the prevention of famine depend entirely on the energetic extension of canals.
Though admittedly, as we -have seen, no unmixed blessing, irrigation is in such places a necessary postulate to the existence of any population at all, whether fever stricken or healthy, and the question resolves itself merely into how the malarious influences of canals can best be prevented, or at least minimised.
Irrigation is so little needed in Europe, that in order to convey to those who have not seen it in practice, why and how far it favours the spread of malarial disease, some few words of explanation as to how this form of cultivation is carried out are necessary. In the greater part of Europe, the natural surface soil is rather too wet than too dry for the farmer's purposes, and his efforts at improvement naturally take the form of surface and subsoil drainage, and
CONDITIONS INFLUENCING PEEVALENCE 185 SO are distinctly anti-malarial. In subtropical regions on the contrary, especially far inland, the problem the cultivator has to solve is how to keep the surface soil damp enough to keep his crops alive.
Subsoil drainage would double the labours of his well bullocks, and to prevent the escape of the share of the scanty showers that fall on his patch of ground he subdivides his holding into small, carefully levelled patches, each but a few yards square, surrounding each patch with a low ridge of earth a few inches high.
Naturally m such regions cultivation favours malaria. Irrigation may be divided into two kinds : (a) Where the water is raised to the surface by water lifts of various kinds ; and (b) where it is brought on to the land by gravitation, or in other words, by canals.
The first is always a domestic operation ; the second can only be successfully carried out by large and expensive engineering works. Whichever plan be adopted, the method of applying the water to the land is the same. The cultivator having levelled and subdivided his land in the manner described above, the water is made to flow in turn into each little square until the ground within its boundaries is covered to a depth suitable to the particular crop. In practice (except for rice) the amount given is usually absorbed in a few hours, and the careful levelling of each patch is distinctly unfavourable to the formation of puddles. In domestic irrigation the water is raised from wells by bullock or human labour, or it may be scooped up in adroitly swung baskets from rivers or swamps, but whatever its source, the water is too laboriously gained to be wasted, either in leakage or puddles. The quantity raised is usually too small to perceptibly affect the level of the ground water; but so far as it goes the tendency must be to lower the spring leveJs, and on the whole I believe that the influences of domestic irrigation are rather unfavourable to malaria than otherwise.
In the case of canal irrigation the results are quite different, as the enormous quantity of water used usually seriously raises the level of the ground water and ma}' even
186 GXATS OR MOSQUITOES — CHAPTER VIII cause water-logging, while the freedom of the supply, and especially its independence of the individual efforts of the user, leads to carelessness in its use, and hence to leakages and puddles ; but it must be distinctly understood that, for all crops except rice, the puddles are the result of the abuse and not of the use of irrigation, and that the reason why these puddles are sufficiently permanent to be harmful is that the water is not absorbed on account of the saturation of the soil with canal water, nearly three-quarters of which, as will be seen, represents waste of some sort. In recording my beliefs as to the harmlessness of domestic irrigation, it must be understood that I refer only to field irrigatioi]. Gardening as carried on in the parts of India with which I am familiar is conducted in a way that makes each garden a paradise for Mosquitoes.
In considering the influence of canals in favouring the production of puddles, and therefore of malaria, it must be remembered that but a comparatively small proportion of the water entering the canal at its head works ever reaches the fields. In a departmental note " On the Irrigating Duty of Bari Doab Canal," Mr. K. G. Kennedy, of the Indian Public Works Department, states that out of every 100 cubic feet of water entering the canal during the winter months of 1881-82, twenty were lost in the canal proper, six in the larger distributories, twenty-one in water-courses, twenty-five by waste in various ways, so that only twentyeight was left to do all the useful work of irrigation. That is to say, that considering the canal as a machine, its efficiency was but 28 per cent.
Now in this canal alone, which is by no means the largest of our Indian irrigation works, the average flow of water is over thirteen millions of gallons per minute, and this enormous amount of water is distributed, in work and waste, over 1,200 square miles, or, in other words, is equivalent to a rainfall of about 40 inches per annum. But this gives no adequate idea of what may be the local effects of the system. In the case of the canal in question, for example, the distribution of the water is very different at different seasons of the year.
CONDITIONS INFLUENCING PREVALENCE 187 During the dry weather some 26 inches are spread over a very large area, the security of the spring crops in the Punjab being almost entirely dependent on irrigation (either from canals or wells). For the autumn crops, however, the cultivator relies mainly on the natural rainfall, and it is mainly in the case of certain valuable crops, such as rice and sugar, that he requires to supplement it by irrigation. Hence, for the cultivation of the autumn crops, which commences in the dry weather and is continued through the* rains, some 56 inches of water are given, although the area so treated is much smaller than that irrigated for the spring crops ; and this, it must be remembered, is in addition to a natural rainfall of some 20 inches, which is mainly concentrated in the same season of the year. Except in the case of rice, however, the water never lies long enough on the ground to admit of the development of a generation of Mosquitoes, as for most crops only a few waterings and those at long intervals are given, as may be seen from the table given below, taken from Mr. Kennedy's above-quoted paper.
Mr. H. Frost, the present executive engineer of the same canal, tells me that except in the case of rice it is rare for the water given to the land to remain visible on the surface for more than a day, and my own casual observation of irrigated land has given me the same impression. It will be observed in the above table that the number of waterings given to rice is not stated. In other words, during the earlier and more critical stages of the crop, it is kept continuously under water. The water is fairly clean, though
188 GNATS OR MOSQUITOES — CHAPTER VIII rich ill vegetable growths, and the stems protect the surface from the wind, so that we have here absolutely ideal conditions for the development of the larvae of Anopheletes. Personally I have seen but little of rice cultivation, as irrigated rice is not greatly cultivated in the parts of India in which I have served, but Captains James and Cornwall, of the Madras branch of our service tell me that there, the rice fields absolutely swarm with Aiiopheles larvae, and this in spite of the presence of numbers of small fish, though the latter do not appear to be often seen in the irrigated rice of the Punjab. Eice is therefore the only irrigated crop in which the used water is instrumental in the propagationof Mosquitoes.
Putting aside the quantity lost by evaporation, whicli must form a considerable proportion of the used water, the whole of the canal intake, whether used or wasted, must ultimately find its way to the subsoil and so raise the level of the subterranean waters.
This raising of the spring level, with its attendajit increase of malaria, does not take place at once, but is gradual and progressive for many years after the introduction of canals, until the subsoil waters are raised to a level at which canal supply and drainage are at an equilibrium, but this may mean the absolute water-logging of the soil and the progress of these changes has been made the subject of careful observation by the irrigation branch of the Indian Public Works Department. As an example I may quote a note by Mr. T. Higham, Chief Engineer for irrigation in the Punjab, on the " Spring Levels in the Dari Doab Circle," dated March 19th, 189G.
Summary of Kesults, Upper Sutley Canals Division. " {a) On the upper two-thirds of the Katora canal, the spring level is rising at the rate of 0.81 feet per annum, the average depth from the surface being 16.2 feet in 1890-91. After a lapse of fourteen years, the spring level may be expected to average but five feet.
" (6) In the lower third of the same canal the spring level is rising at the rate of 6.32 feet per annum, but as it is still
CONDITIONS INFLUENCING PEEVALENCE 189 on the average, about 20 feet from the surface, water-logging is not likely to occur until a lapse of forty-seven years and probably not even then.
"(c) There has been on the whole, during the last ten years a general rise of spring level on the other canals of the division but water-logging need not be apprehended even in the remote future."
The departmental literature on this subject, would alone more than fill the present book, and indisputably establish that everywhere the introduction of canal irrigation has brought about a serious raising of the spring level, but the above quotation shows sufficiently for our purpose the amount and considerable variation in extent of the changes within a given small area.
The European engineer endeavours as far as possible, to take his canals along the lines of the water-sheds, but the reverse plan was followed as a rule by our native predecessors, and when the old canals made by them have been utilised the results on the health of the population have often been so serious as to necessitate heavy expenditure in altering their alignment ; as the untoward effects, of canal irrigation in intensifying malarial diseases, has been thoroughly recognised by the Indian Government for many years, and the prevention of water-logging has been the subject of anxious consideration by their canal engineers. As regards the direct production of puddles, I suspect that most of the 25 per cent, of waste " in various ways" is spent in their production, as a very large proportion of this item results from leakages due to carelessness on the part of the cultivators. The water is allowed to flow on to the land from the ultimate distribution channels by simply breaching the bank, and is stopped by repairing the opening by means of a few handfuls of mud. Although when carefully done the plan is much more efficient than one would expect, leakages, which necessarily result in the production of puddles are certainly very common, and owing to the water-logged condition of the soil resulting from the high spring level, such puddles are likely to last for a considerable time.
190 GNATS OR MOSQUITOES— CHAPTER VIII Of course the canal authorities do their best to lesson such wastage of the precious fluid, and a cultivator may be charged double rates if detected in permitting any considerable waste, but the areas administered are so large, that numbers of such cases must pass undetected, and a leakage promptly repaired, would probably not be punished, though it may easily have lasted long enough to produce a considerable puddle.
The means proposed by the Canal Department to prevent water- logging of the land they irrigate are : — (a) To carry their canals along the line of water-shed. (h) To so arrange the minor channels as to avoid their being carried across the natural lines of drainage. (c) To limit the supply both as to quantity and time to the amount absolutely required for the success of the crops. (d) The making of drainage cuts along the natural lines of outfall.
Of these the last is probably by far the most efficient, but the attempts that have been made in this direction have hitherto been of a tentative character and on no very large scale.
With the water brought so near the surface as it often is, it is obvious that it would be quite possible to use it over again by raising it by pumping ; but under the present conditions of Indian agriculture the cost of such a system would obviously be prohibitory.
It will be observed that none of the above propositions can have any influence in preventing loss of water from the main canal and its permanent branches.
This loss, as we have seen, amounts to over a quarter of the entire intake, and as the amount of water that can be taken from the supplying river is limited, cannot be met by increasing the speed of the flow, so that the water thus wasted has a large and definite money value ; and the stoppage of this loss becomes an object in which considerable capital might profitably be expended. It is needless to say this aspect of the question can hardly have escaped the consideration of the administration of so successful a " productive " department as that of our Indian canals, and I
CONDITIONS INFLUENCING PREVALENCE 191 understand that the question is not so much as to the desirabihty of taking measures to stop the waste, as of how to do it. At first sight the remedy seems obvious, namely, to "revet" the channels with a layer of masonry or concrete, but unless the work is of a very solid description it has been found that such a lining soon cracks and becomes pervious. Doubtless, however, the ingenuity of our engineers will ere long find a method of meeting the difficulty, and whoever answers the question will not only have solved an important economic problem, but will have done much to diminish the untoward effect of canals in water-logging the soil, with its inevitable concomitant of intensification of malaria.
An occasional way in which canals may directly produce breeding places for Mosquitoes is their being left empty for a period long enough to admit of the rearing of a generation of imagines from the egg. Necessarily a temporarily disused canal is really a chain of pools ; and I have met with an instance where a small canal supplying a tract of thin loam lying on boulder alluvium, furnished the only situation in which I could discover any Anopheles larvae, in the form of pools in the beds of canal channels not in use. Moreover the village in question was notoriously malarious. It is obvious that the flushing of each empty channel every week or ten days would prevent their serving as Mosquito nurseries.
The above remarks apply exclusively to the large perennial canals, but in many places another system is in use, viz., irrigation by " inundation canals." There are channels carried from the river bank to any portions of the country which may happen to lie lower than the flood level of the river, and are so graded that they are filled only during times of flood. Their object is in fact, exactly the opposite of the system of protecting low-lying lands by dykes that have been so efficient in diminishing malaria in Holland and in the English fen country, and as neither the time nor quantity of the supply can be controlled, their effect on health can hardly be otherwise than disastrous. Another reason why inundation canals specially favour