Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/379

 363 of the plants. Stagnation of water is inimical to the action of the roots, and does away with the advantageous processes of flowing and percolating currents. Some of the best water-meadows in England have but a thin soil resting on gravel and flints, this constituting a most effectual system of natural drainage. The fall of the water supply must suffice for a fairly rapid current, say 10 inches or 1 foot in from 100 to 200 yards. If possible the ater should be taken so far above the meadows as to have sufficieat fall without damming up the river. If a dam be abso lutely necessary, care must be taken so to build it as to secure the fields on both sides from possible inundation ; and it should be constructed substantially, for the cost of repairing accidents to a weak dam is very serious. Quantity of Water. Even were the objects of irriga tion always identical, the conditions under which it is carried on are so variable as to preclude calculations of quantity. Mere making up of necessary water in droughty seasons is one thing, protection against frost is another, while the addition of soil material is a third. Amongst causes of variation in the quantity of water needed will be its quality and temperature and rate of flow, the climate, the season, the soil, the subsoil, the artificial drainage, the slope, the aspect, and the crop. In actual practice the amount of water varies from 300 gallons per acre in the hour to no less than 28,000 gallons. Where water is used, as in dry and hot countries, simply as water, less is generally needed than in cold, damp, and northerly climates, where the higher temperature and the action of the water as manure are of more consequence. But it is necessary to be thoroughly assured of a good supply of water before laying out a water-meadow. Except in a few places where unusual dryness of soil and climate indicate the employment of water, even in small quantity, merely to avoid the consequences of drought, irrigation works are not to be commenced upon a large area, if only a part can ever be efficiently watered. The engineer must not decide upon the pj.an till he has gauged at different seasons the stream which has to supply the water, and has ascertained the rain- collecting area available, and the rainfall of the district, as well as the proportion of storable to percolating and evaporating water. Reservoirs for storage, or for equaliz ing the flow, are rarely resorted to in England ; but they are of absolute necessity in those countries in which it is just when there is least water that it is most wanted. It is by no means an injudicious plan before laying out a system of water-meadows, which is intended to be at all extensive, to prepare a small trial plot, to aid in determin ing a number of questions relating to the nature and quantity of the water, the porosity of the soil, &c. Quality of Water. The quality of the water employed for any of the purposes of irrigation is of much importance. Its dissolved and its suspended matters must both be taken into account. Clear water is usually preferable for grass land, thick for arable land. If it is to be used for warping, or in any way for adding to the solid material of the irrigated land, then the nature and amount of the suspended material are necessarily of more importance than the character of the dissolved substances, provided the latter are not positively injurious. For use on ordinary water- meadows or on rice-fields, however, not only is very clear water often found to be perfectly efficient, but water having no more than a few grains of dissolved matter per gallon answers the purposes in view satisfactorily. Water from moors and peat-bogs or from gravel or ferru ginous sandstone is generally of small utility so far as plant food is concerned. River water, especially that which has received town sewage, or the drainage of highly manured land, would naturally be considered most suitable for irrigation, but excellent results are obtained also with waters which are uncontaminated with manurial matters, and which contain but 8 or- 10 grains per gallon of the usual dissolved constituents of spring water. Experienced English irrigators generally commend as suitable for water- meadows those streams in which fish and waterweeds abound. But the particular plants present in or near the water-supply afford further indications of quality. Water cress, sweetflag, flowering rush, several potamogetons, water milfoil, water ranunculus, and the reedy sweet watergrass (Glyceria aquatica) rank amongst the criteria of excellence. Less favourable signs are furnished by such plants as Arundo Donax (in Germany), Ciciita virosa, and Typha latifolia, which are found in stagnant and torpid waters. Water when it has been used for irrigation generally be comes of less value for the same purpose. This occurs with clear water as well as with turbid, and obviously arises mainly from the loss of plant food which occurs when water filters through or trickles over poor soil. By passing over or through rich soil the water may, however, actually be enriched, just as clear water passed through a charcoal filter which has been long used becomes impure. It has been contended that irrigation water suffers no change in composition by use, since by evaporation of a part of the pure water the dissolved matters in the remainder would be so increased as to make up for any matters removed. But it is forgotten that both the plant and the soil enjoy special powers of selective absorption, which remove and fix the better constituents of the water, and leave the less valuable. The Influence of Mining and other Refuse. In some of the districts of Devonshire and of Wales, in which the sloping sides of narrow valleys have been converted into small catchwork irrigated meadows, the injurious effects of water from mines have been most marked. A stranger visiting the district in early spring would notice, along the sides of a valley, a number of small irrigated fields. Some of these, watered directly from little streams behind and above them, would show grass of great luxuriance, especially close to the main and secondary carriers. But where the river-water, contaminated by mining refuse, had been used, the grass bordering the water-courses would show a sickly yellow tint, and be generally less developed than the herbage of the rest of the field. This difference between the fields irrigated by small local brooks and those watered by the river cannot be explained by any inferiority in the river water as river water ; for above the entrance of the refuse from the first mine it was every thing that could be wished. But just below the place of entrance of the mine water the grass on the banks looked as if it had been burnt up with vitriol, while in the stream itself not a vestige of a living waterweed could be detected. The injurious effects thus caused by the mine water have led to its partial disuse for the purposes of irrigation. Some of the most profitable water-meadows are no longer irrigated : the herbage in these is now of inferior character, and mosses and weeds, suppressed by total immersior, have reappeared, to the detriment of the more valuable grasses. Besides, there is now no early feed. Manure, not before wanted, has now to be applied, and the yield of grass is reduced in annual value by 30s. to 60s. per acre. To get a fair growth of grass the plant-food which the water formerly brought at little expense has to be furnished by costly farmyard manure ; and even with this the crop is late and light. It is clear in the particular instance to which reference has been made that water pumped from copper mines ^ or used in dressing the ores, is the origin of the mischief. Several changes in the composition of the water have been