MINOR SYSTEMS OF IRRIGATION Water Meadows.—In addition to the two major systems of irrigation, there are a number of minor systems with special objects in view. Nowhere in England can it be said that irrigation is necessary to ordinary agriculture, but it is occasionally em ployed to stimulate the growth of grass and meadow herbage in what are known as water meadows. On the Avon, in Wiltshire, and the Churn, in Worcestershire, these may be traced back to Roman times. This irrigation is not practised in the drought of summer, but in the coldest and wettest months of the year. The water employed being warmer than the natural moisture of the soil, proves a valuable protection against frost. Before the con version of land into water mead ows care must be taken to have good drainage, a sufficient supply of water, and of good quality. It might be thought that drainage would be unnecessary, but porous sub-soils or efficient drains do not act merely by carrying away stagnant water which would otherwise cool the earth, encrust the surface, and retard plant growth. They cause the soil to perform the offices of a filter; thus the earth and the roots of vegetation absorb the useful matter from the water which passes through it.
Warping is the name of a system which pro duces its results by a temporary use of the basin form of irri gation. In this system the suspended solid matters are of im portance as a material addition to the ground. That is to say, the solid matter is used to make a new surface of soil on which, when a satisfactory depth of it is obtained, and where adequate drainage is provided, crops may be grown in the ordinary man ner from rainfall. The warping practised in England is almost exclusively confined to the overflowing of land lying below tide mark within the districts commanded by estuaries or tidal rivers. A good example of the process may be seen when sailing up the Trent from the Humber to Gainsborough. Here the banks of the river were constructed centuries ago to protect the land within them from the encroachment of the tide. The warping begun in the i8th century has become a regular system in recent times. Large masonry sluices, with strong doors to shut off the tide when necessary, may be seen in both banks of the river. From these sluices canals are carried in some cases many miles inland through the flat country to the point previously surrounded by embankments within which the muddy waters are to be allowed to spread. As the waters settle the mud is deposited and the clear water returns with the falling tide to the bed of the river. Spring tides, of course, are preferred. Peat mosses of the most
sterile character have been by this process covered with soil of the greatest fertility and converted into firm and fertile fields. Three years are often spent in this process. One year in warping, one year drying and consolidating, and one year growing the first crop. The latter is generally hoed in by hand, as the mud is at first too soft to admit of horse labour. The drainage of peat soils has the effect of lowering the whole level through shrinkage of the peat. It is said lowering to a depth of io ft. to 12 ft. has occurred in some of the marsh areas drained. Of course warping to some extent makes up for the lowering caused by drainage.
Many towns situated in inland districts, instead of passing their sewage into the nearest stream, deal with it under modern settling tank systems, from which the effluent is run off into small canals, which irrigate suitable cultivable areas. The yield of crop produced is great, as such waters contain a very high percentage of nitrogenous matter. The drainage from the city of Cairo, for instance, is said to be 3o times as valuable as the equivalent of Nile water even in flood time, when the latter bears what is said to be its fertilizing silt.
An absolutely necessary concomitant to irrigation is drainage of the soil. In few areas does nature provide this without further artificial aid. For many years the supply of irri gation water was the only problem considered by engineers; the disastrous results which have followed in many, if not in all, cases, from a neglect of the provision of adequate drainage has, however, awakened everyone to the necessity of supplying it if fields are to continue fertile. Very recent work on this subject in Cambridge shows its importance and, incidentally, that in the case of the Nile it is not alone the addition of Nile silt to the surface of the soil which induces fertility, but the period of rest and drainage which, in particular, the basin system of irrigation provides to a very great degree, that is so valuable. The perennial system can also be so arranged as to give a suitable, though much shorter, period of rest. Only education, experience and scientific teaching can prevent a farmer from overcultivating. In most irrigation systems, however, the collective opinion of the majority can compel the restriction of the supply of water at times when scientific opinion shows that the land should be left fallow.