82 



MONTHLY JOURNAL OF AGRICULTURE. 



rashes and coarse aquatic plants, as ma^ be 

 seen in all marshy spots. The circulation of the 

 water, therefore, appears to be as necessary- as 

 its presence ; and, pro\-ided there be a sufBcieut 

 supply of water of a proper quality, the more 

 porou^ the soil, and especially the subsoil, is, 

 the more ^^gorous is the vegetation. It is on 

 this principle alone that we can rationally ac- 

 count % the great advantage of irrigation in 

 those climates where rain is abundant, and 

 where the soil, which is most benefited by hav- 

 ing a supply of water running through it. is of a 

 nature to require artificial draining as an indis- 

 pensable preliminary to being made fertile by 

 irrigation. By keeping tliese principles in ^-iew, 

 great light will be throv\-n on the practical part 

 of irrigation, which, ha\-iug been long estab- 

 lished by experience, before these principles 

 were thought of depends not on their correct- 

 ness, but only confirms tiieir truth. 



The whole art of irrigation may be deduced 

 from two simple rules, which are. first to give 

 a sufficient supply of water during all the time 

 the plants are growing, and, secondly, never to 

 allow it to accumulate so long as to stagnate. — 

 ■We shall see hereafter one apparent exception 

 to this la-st rule, but it ^^-ill be readily explained. 



The supply of water must come from natural 

 lakes and rivers, or from artificial wells and 

 ponds, in \\hich it is collected in sufficient quan- 

 tity to disperse it over a certain surface. As the 

 ■water must flow over the land, or in channels 

 through it. the supply must be above the level 

 of the land to be irrigated. This is generally 

 the principal object to be considered. If no 

 water can be conducted to a reservoir above die 

 level of the land, it cannot be irrigated. But 

 there must al.so be a readj' exit for the water, 

 and, therefore, the land must not be so low a.s 

 the natural level of the common receptacle of 

 the waters, -whether it be a lake or the .sea, to 

 which they run. The taking of the level is, 

 therefore, the first step towards an attempt to 

 irrigate any lands. 



Along the banks of running streams Xature 

 points out the dechvity. A channel, which re- 

 ceives the water at a point higher than that to 

 which the river flows, may be dug \vith a much 

 smaller declivity than that of the bed of the riv- 

 er, and made to carrj- the water much higher 

 than the natural banks. It may thence be dis- 

 tributed so as to descend slowly, and ^vater a 

 considerable extent of ground in its ^vaJ' to re- 

 join the stream. This is. by far, the most com- 

 mon mode of irrigation, and the .shape, size, and 

 direction of the channels are regulated by the 

 nature of the surface and other circumstances, 

 which vary in almost every situation. A few 

 examples will give to those who are not ac- 

 quainted with the best mod&s of irrigating land 

 a pretty accurate notion of the sj-stem. 



We shall suppose a river to run widi a rapid 

 current between high banks. At some point of 

 its course a portion of the \vater is diverted into 

 a canal dug along tlie bank, with a very small 

 declivity. The water in this canal will flow 

 with less rapidity than the river, but will keep 

 the same level as that part of the river where it 

 has its origin. Thus the water may be carried 

 over lands which are situated considerably above 

 the bed of the river farther down. All the lands 

 between Uiis canal and tlie river may be irri- 

 gated, if there is a .sufficient supply of water. 

 The canal may be carried to conidderable dis- 

 tance from the river. The size of tiie canal and 

 its declivity depend on the quantity of water 

 (190) 



which may be made to flo^v into it. A dam is 

 often constructed across a river, in order that as 

 much of its water as is possible may be diverted, 

 and the original channel is often laid quite dry, 

 to take advantage of all the water at the time 

 when it is advantageous to irrigate the land. To 

 have an entire command of the water, there are 

 flood-gates on the main channel and on the lesser 

 branches. By opening or shutting these, the 

 ■water may be stopped or made to flow, as may 

 be required. It must be remembered, that to 

 carry water to a considerable distance, and in 

 great quantity, a larger channel and more rapid 

 declivity are required ; and it is a matter of cal- 

 culation whether it is most advantageous to bring 

 a smaller quantitj' to a higher point, or a greater 

 abundance somewhat lo^wer. Having a certain 

 command of water, it may be caiTied from the 

 main channel by smaller branches to dilTerent 

 points, so as to irrigate the whole equally. — 

 These branches should be nearly horizontal, that 

 the water may overflo-w the sides of them, and 

 be equally distributed over the land immediate- 

 1}' below. Every branch which brings water 

 over the land should have a corresponding chan- 

 nel below to carry it off; for the water must 

 never be allowed to stop and stagnate. When 

 it has run 15 or 20 feet, according to the declivi- 

 ty-, over the laud situated below the feeder, or 

 the channel which brings the ■water, it should 

 be collected into a drain to be carried off, unless 

 it can be used to irrigate lands which lie still 

 lower. Finally it runs back into tbe river from 

 which it was taken, at a lower point of its 

 course. 



When there is a considerable fall and a suffi- 

 cient supply of water, a series of channels may 

 be made, so situated below each otlier. that the 

 second collects the water which the first has 

 supplied, and in its turn becomes a feeder to 

 iiTigate the lower parts of die dechvity : a third 

 channel receives the water and distributes it 

 lower do^\^l. until the last pours it into the river. 

 This is called catch-work, because the water is 

 caught from one channel to another. This me- 

 thod is only applicable -where there is a consid- 

 erable fall of water and a gentle declivity to- 

 \vards the river. But it must be borne in mind 

 that the water is deteriorated for the purpose of 

 irrigation, when it has passed over the land, and 

 that it is not advantageous to let it flow over a 

 great extent when a fresh suppl.y can be ob- 

 tained : but where only a small portion of water 

 can be commanded, tliat must be made the most 

 of; and it wiU irrigate three or four portions of 

 land in succession, without there being any very 

 marked difference in the effect : beyond this it 

 rapidly lo.ses its fertilizing qualities. This is not 

 owing to the water having deposited the ferti- 

 lizing substances which it held in solution, or 

 which were diffused through it, but it is owing 

 to its having taken up some which are detri- 

 mental to vegetation, and being saturated with 

 them : at least this is the most probable opinion 

 when all circumstances are taken into the ac- 

 count. 



The general principle of irrigation may be 



described as the suppK^ing of ever}- portion of 



the surface vith an abundance of water, and 



taking it off again rapidly. In many situations 



' the great difficulty in irrigation arises from the 



! want of a supply of water ; but even then a par- 



I tial irrigation maj' be effected, which, ahhoagh 



j not perfect, will have its advantages. A small 



, rilf; which is often quite dry m summer, may 



; stilf, by judicious management, be made to im- 



