8 



Effect of soil mulches on soil evaporation. 



The effect of dry soil mulches of different thickness applied on the soil 

 surface after irrigation are shown in the diagrams given below. The first 

 one (Fig 46) is for the California soil whose daily rate of evaporation is 

 given above. The second one (Fig. 47) is for a Wenatchee orchard sandy 

 loani soil typical of the orchard lands of the north central portion of the 

 State of Washington. These diagrams show the effectiveness of different 

 depths of mulches. A 9 inch mulch practically stopped all evaporation. 



These experiments represent theoretical conditions which can not be 

 obtained in the field. It is, however, possible to approach these conditions 

 by producing a mulch by cultivation. Conditions more nearly like field 

 conditions were followed in another experiment made in California on a 

 sandy loam soil. In this experiment 8 inches of water was applied to the 

 soil surface in a furrow 4 inches deep. The water was applied during the 

 first two days and at the end of the third day some of the tanks were culti- 

 vated 6 inches deep, the others receiving no cultivation. The evaporation 

 loss during the first three days was 10.5 per cent, or .84 inch, during the 

 next three days it was 1.2 per cent, from the cultivated tanks and 3.6 per 

 cent, from uncultivated tanks, showing that the loss from cultivated soil 

 was one-third that from uncultivated eoil. 



Effect of depth of furrows on soil evaporation. 



The experiments made on California orchard soil imitated as much as 

 possible actual field practice. A depth of 5 *4 inches of water was applied 

 to different pots, on the surface and in furrows 3, 6, and 9 inches deep. 

 The irrigation lasted two days and the soil was cultivated at the end of the 

 third day. The evaporation losses for the first three days and for the next 

 seven days following cultivation were as shown in the accompanying dia- 

 gram. The loss for furrows 12 inches deep was % of the loss obtained 

 with surface irrigation. 



4. Percolation of Water Applied to the Soil. 



To obtain maximum benefits from irrigation water the water applied 

 should be uniformly distributed throughout the soil down to the lower end 

 of the roots and any water which passes beyond the capillary reach of the 

 lower ends of roots is wasted. It flows underground either into some drain- 

 age channel or may cause considerable damage by the waterlogging and 

 the rise of alkali in the lands below. In actual practice 't is impossible to 

 obtain perfect distribution of water and have no waste, but by careful 

 irrigation it is possible to minimize the loss by percolation. 



Effect of texture of soil and subsoil on percolation. 



When water is applied in irrigation, the soil directly in contact with the 

 water is saturated, a portion of this water moves downwards and wets the 

 soil beneath furnishing capillary water or free moisture to the roots below. 

 If sufficient water has been added, the excess water passes into some drain- 

 age channels or down to the level of standing water. This excess water is 

 wasted. This loss by percolation is usually greater in open soils well 

 drained through which gravity water percolates readily and it is very easy 

 for a careless and wasteful irrigator to use a great deal more water than 

 necessary. Some of the waste can not be eliminated because when a soil 

 is irrigated by furrows the upper end of the furrows where the water is 

 turned in receives an excess of water. In some southern California orch- 

 ards with careful irrigation, it has been found that the water at the upper 



