CHAPTER I.

INTRODUCTION

1. =Preliminary Remarks.=--The largest irrigation canals are fed from perennial rivers. When the canal flows throughout the year it is called a “Perennial Canal.” Chief among these are the canals of India and particularly those of Northern India, some of which have bed widths ranging up to 300 feet, depths of water up to 11 feet and discharges up to 10,000 c. ft. per second. Other large canals as for instance many of those in Scinde, Egypt and the Punjab, though fed from perennial rivers, flow only when the rivers are high. These are called “Inundation Canals.” Many canals, generally of moderate or small size, in other countries and notably in the Western States of America, in Italy, Spain, France and South Africa, are fed from rivers and great numbers of small canals from reservoirs in which streams or rain-water have been impounded. Sometimes water for irrigation is pumped from wells and conveyed in small canals. In Australia a good deal of irrigation is effected from artesian wells. Irrigation works on a considerable scale are being undertaken in Mexico and the Argentine. In this book, irrigation works of various countries are referred to and to some extent described, but the perennial canal of Northern India, with its distributaries, is the type taken as a basis for the description of the principles and methods which should be adopted in the design, working and improvement of irrigation channels and it is to be understood that such a canal is being referred to where the context does not indicate the contrary. Any reader who is concerned with irrigation in some other part of the world will be able to judge for himself how far these principles and methods require modification. The branches and distributaries--all of which are dealt with--of a large perennial canal cover all possible sizes.