BUREAU OF SOILS. 507 



the surface is relatively low the wells are artesian. The chief pur- 

 poses of the investifjjation were, first, to ascertain the source of the 

 ground water, and, second, to ascertain its relation to the soil. The 

 observations coupled with the office research render it clear that the 

 ground-water sui)idy is derived partly from locally absorbed rainfall, 

 but largely from the greater rainfall of the mountains absorbed by 

 the westerly outcrops of the strata and carried in accordance with 

 artesian princi|)les tnence eastward, the water rising under its hydro- 

 static head from one porous stratum to another in such manner as 

 to approach the surface. The entire mass of the formations and 

 deposits are thus kej)t saturated, and the water of saturation may be 

 likened to a reservoir with a surface inclining eastward about 10 feet 

 per mile and flowing through the strata at a rate determined by their 

 perviousness, the surface of the reservoir coinciding with the land 

 surface in the permanent stream valleys and in certain basins con- 

 taining permanent ponds. Over perhaps a quarter or a third of the 

 area this ground-water reservoir approaches the surface to within the 

 limits of cajMllarity, and here it supplies the requisite moisture for 

 plant growth, and except in restricted areas it is within reach of 

 flowing or pump wells. This subsurface supply is especially impor- 

 tant in connection with dust mulching, or "dry farming," since this 

 device for checking evaporation permits the moisture to accumulate 

 against the season of growing crops. On the whole the investiga- 

 tions show that the ground water of the plains is an important 

 resource, without which this vast area would be far less j)roductive 

 and habitable than it is. 



The field work was extended into Uncompahgre Valle}' in Colorado, 

 and into a part of California Valley, in order to ascertain the quantity 

 and movement of ground water in intermountain valleys in its rela- 

 tion to the soil. In both valleys it was found that the ground-water 

 supply, collected largely from adjacent mountain slopes during 

 storms, exists in sufficient quantity to yield artesian flow in places, 

 and to approach the surface to witliin reach of capillarity elsewhere. 

 The researches render it clear that throughout both the plains and 

 the mountain regions natural subirrigation through tlie movement 

 of ground water, while hitherto unrecognized, is an im|)ortant factor 

 in determining tlie constitution and productivity of soils. 



The field work extended also into the Southern States, where the 

 destructive effect of soil erosion is great. In addition to records and 

 photograj)hs illustrating the destruction, attention was given to pn)- 

 tective devices, and tlie data were incorporated in a paper on "Soil 

 Erosion," transmitted durhig the year for publication. 



Attention was given also to the role of water in affecting the con- 

 stitution and |)roductivity of soils and the cjuantity of water required 

 for tlie best results in production. In this connection a coefficient 

 for the agricultural duty of water, expressed in terms of production, 

 was worked out. The results of this work also have been transmitted 

 for publication. 



