122 HORTICULTURE, FORESTRY, FLORICULTURE 



done. If, on the other hand, more than its capacity is supplied trou- 

 ble will surely follow. 



A proper appreciation of seepage conditions often involves a 

 knowledge of the ability of water to transmit pressure between two 

 points, due to differences of elevation. This is often manifested in 

 irrigated sections by ground water springing up through the innum- 

 erable soil pores. Such a condition indicates a porous substratum 

 filled with water exerting a pressure. When this substratum is too 

 deep to cut into by a drain, outlet wells connecting with the drains 

 at an ordinary depth afford the best solution. 



Another physical property affecting the movement of ground 

 water and of great importance is that called capillary attraction. 

 This phenomenon was named from the inherent ability of liquids to 

 rise in small hair-like or capillary tubes above their free surfaces. 

 Water rises higher in the tubes and also in soil particles than any of 

 the other liquids. This property is most essential to plant life and 

 crop production, but uncontrolled in alkaline soils, it hastens the 

 accumulations of alkali at the surface. In soils varying from 

 coarse sand to very fine clay, water will rise by this attraction from 2 

 to 6 feet. When the surface is allowed to become hard and baked, 

 as when abandoned or allowed to stand idle, a surface tension is 

 formed which increases this attraction and the consequent evapora- 

 tion. It is the evaporation of large quantities of water containing 

 alkaline salts in solution that does the damage. The evaporated 

 water passes off nearly free, leaving the alkali in the surface soil. 

 Experiments have shown that of the water applied in ordinary irri- 

 gation, without cultivation, from 30 to 50 per cent is evaporated 

 from the surface. If, in addition to this, seepage water from higher 

 lands or canals containing much alkali is within reach of capillary 

 attraction, and the surface poorly cared for, thus increasing evapora- 

 tion, alkali accumulations surely follow. A minimum depth of 4 

 feet for drains, with cultivated or shade crops, has been the practice 

 of some of the drainage engineers though deep drainage from 5 to 

 6 feet is preferred. 



Plans for Drainage. The soils of the arid regions have not been 

 subject to percolating water since their formation, as the humid soils 

 have been, and consequently are without the drainage pores of the 

 latter. If they have any water pores at all, they are those of the 

 water doing the damage. The water applied to the higher lands af- 

 fects the lower lands, where it flows out or comes near the surface, 

 whether in light or heavy soil. In moving laterally through the 

 body of the soil the water will flow along the lines of greatest slope 

 and will not drain well into tile lines parallel to its movement. The 

 drainage water finds its way through some special underground pas- 

 sage, or stratum, or some modification of these, rather than uni- 

 formly through the soil. The method of intercepting the water as 

 it enters the field or soil, either by lateral seepage or some lower pas- 

 sage, is highly essential to success. However, intercepting drains are 

 not all that are necessary. In nearly every instance outlet lines 



