THE IKKIGATION AGE. 



83 



DRAINAGE PROBLEMS OF IRRIGATION.* 



BY C. G. ELLIOTT. 



The drainage Of irrigated lands seems like a prop- 

 osition involving a contradiction of terms. Yet its 

 necessity is a fact in many of the older-irrigated areas of 

 the West. Had the necessity of this been suggested 

 to the early promoters of irrigation improvements, they 

 would have pointed to the heated sands and barren 

 wastes as furnishing a sufficiently conclusive answer and 

 dismissed the matter with scarcely a passing thought. 

 However, such a necessity exists and is acknowledged 

 to-day. By the side of productive fields are seen areas of 

 marsh and bog similar in appearance to those found in 

 the well watered parts of the rainfall belt. Thousands 

 of acres which were once the pride of the farmer, afford- 

 ing him abundant crops and ample remuneration for his 

 labor, are now waste, by reason of the water-logged con- 

 dition of the land. Villages and towns are also victims 

 of this unforeseen condition, and find the necessity of 

 drainage forced upon them as a- sanitary measure. 



The causes which have produced these undesirable 

 conditions mav be briefly given. In the improvement of 

 arid lands it is necessary to lead the water required 

 for irrigation in ditches which are constructed for long 

 distances through soils often exceedingly open by reason 

 of the large per cent of gra.vel and sand which they 

 contain. As a- result, much loss by leakage occurs, 

 especially during the early history of every canal. In 

 many localities this is never effectively checked, but is 

 looked upon as an unavoidable loss which must be pro- 

 vided for. In some of the older ditches upon which 

 measurements have been made, this loss is found to be 

 from 30 to 50 per cent of the total volume of water 

 taken from the stream. It is continual during the sea- 

 son when water is supplied to the land. In addition to 

 the supply from this source may be mentioned the reck- 

 less and prodigal use of water which frequently pre- 

 vails in early irrigation practice. The soil at first is 

 dry and deep and apparently will contain an unlimited 

 supply of water, but unless some underflow furnishes 

 a ivlief, the ground fills up and the water appears on 

 the surface at lower Bevels, or at points where there is 

 some underground b&rrier which obstructs its course. 

 Where the surface is generally level, as in some of the 

 districts in southern California, the filling up is more 

 uniform and manifests itself in a more general satura- 

 tion of the soil, with its accompanying results. This 

 is commonly known as "seepage" and is a condition 

 becoming more familiar to irrigators each succeeding 

 year. 



The saturation of the soil is not the only injury 

 done by the seepage. The soils in nearly ever.y locality 

 in the arid regions contain large quantities of sodium 

 chloride, sodium sulphate and sodium carbonate, and 

 other salts which are derived from the rocks from which 

 the soils originate, and are distributed through them, 

 forming an important part of their fertility under 

 normal conditions. When, however, any portion of 

 the soil contains an excess of water, these substances are 

 dissolved from the solid portions and are held in solu- 

 tion. The water, when brought to the surface by either 

 gravity or capillary attraction and evaporated, leaves 

 the salts concentrated at or near the surface. The 

 continuation of this process results in the accumulation 

 of alkali, as it is commonly called, in such strength as 



*Read before the Engineering Section of the American 

 Association for the Advancement of Science. Dec. 31, 1902. 



to destroy all vegetation except such as is indigenous 

 to alkali soils. While these salts are valuable when 

 properly distributed in the soil, more than one-tenth 

 of one per cent has been found to be injurious to the 

 larger part of cultivated plants. The dryness of the 

 atmosphere produces an excessive evaporation whenever 

 moisture is brought to the surface, resulting in a rapid 

 concentration of the salts, especially during the most 

 heated season in the arid climates. 



This is briefly the situation in some of the irrigated 

 districts of this country, and, it may be added, that a 

 counterpart may be found in foreign irrigated districts 

 which has attracted the attention of investigators for 

 some years. The engineering problems connected with 

 the treatment of this growing evil, which is curtailing 

 the profits of the irrigation farmer, are somewhat differ- 

 ent from those to be dealt with in humid lands where 

 drainage is successfully practiced. The rise of soil 

 water must be controlled or limited. to a certain horizon 

 below the surface. Owing to the finely divided structure 

 of this soil and its chemical composition, its capillary 

 power is such that this horizon should be distant not 

 less than 5 feet from the surface, and for many kinds 

 of plants a greater depth is preferable. Otherwise the 

 water will continually pass to the surface where, being 

 evaporated, it will occasion the results before described. 

 The source of the water is the supply contained in the 

 irrigation canals, which always occupy a higher level 

 than the land irrigated by them. It places a constant 

 head upon the soil water, in many cases similar to the 

 force which produces artesian flow. In other cases, 

 water under this head follows courses in the lower soil 

 until arrested by some less permeable barrier, it appears 

 at the surface. 



The remedy, in the light of present investigation, is 

 to cut off the supply rather than to remove the water 

 after it has appeared, or, in the case of large and level 

 tracts, such as are found in southern California, to so 

 arrange the drains as to arrest the supply from beneath 

 rather than remove it after it reaches the surface. In 

 land which requires reclamation, water flows upward 

 instead of downward, either directly' or after it has passed 

 through an inclined stratum of soil, and is then arrested 

 and forced to the. surface. In either case, it is clear that 

 if a drain can be located in such a way as to intercept 

 this flow before it appears too near the surface, an im- 

 portant part of the problem has been solved. It is not 

 necessary to intercept and remove all of the excess of 

 water, since the soil will care for a part of it without 

 detriment to its surface production. In the fruit belt 

 of Fresno, Cal., it has been estimated that in order to 

 control the water line of the soil properly, 2% cubic 

 feet of water per second per square mile should be 

 removed. Each location, however, demands a special 

 investigation to determine this quantity on account of 

 the varying conditions of surface, soil, and physical 

 structure of the land through which the supply ditch 

 passes. 



The proper location of drains is, perhaps, the key 

 to the situation and furnishes a field for practical in- 

 vestigation of a peculiar nature, since it is not neces- 

 sarily the number or kind of drains which will accomp- 

 lish the work, but their intelligent and skillful location, 

 both as to surface locality and depth. Various elements 

 enter into this phase of the work which ca.a not be 

 mentioned at this time. It is sufficient to say that pres- 

 ent drainage practice in irrigated land, though of limited 



