WATER SUPPLY ON THE GREAT PLAINS. 



in 



the Yellowstone and Missouri valleys. Such cases 

 do not, however, controvert the general truth of the 

 proposition that the rainfall of a drainage area or 

 basin is the source of the supply of its water. It will 

 be in favor of brevity if we use the word basin as 

 the full equivalent of drainage area. 



The full significance of the use of the word basin 

 should, however, not be lost sight of, and in some 

 cases it is. It means all the land enclosed by the 

 highest rim that bounds on every side the valley in 

 which runs the stream whose basin is under consider- 

 ation, AND all the subsidiary valleys whose surplus 

 waters naturally find their way into it. Thus the 

 basin of the Arkansas has its western water shed 

 among the rugged ridges from north of Pike's Peak 

 to south of the Raton mountains, and its northern 

 water shed is a sinuous line from near Palmer lake 

 across the plains south of the Smoky Hill river and 

 between the Neosho and Marmaton to enclose the 

 head waters of Spring river, and so by the flanks of 

 the Ozarks to the confluence of the Arkansas with 

 the Father of Waters, while the southern divide of 

 this great basin would be a similar line from eastern 

 New Mexico across the Texan Panhandle north of 

 the Red river, and across Oklahoma and the In- 

 dian Territory to the mouth of the Arkansas. This 

 includes many smaller basins, some of which as be- 

 longing to rivers of the plains, as the Big Sandy, the 

 Medicine, the Neosho, and others, derive no drop of 

 water from the mountains; and others, as the basins 

 of the two Candians, the Purgatoire (locally Picket- 

 wire}, the Fountain, and the Cimarron, have their 

 highest springs in the high valleys on the flanks of 

 the mountains. The Kaw river basin has no mount- 

 ain supply. Its subsidiary basins, of the Republican, 

 Smoky Hill, Solomon and Saline, have their supplies 

 all from precipitation east of the 104th meridian the 

 midregion of the Great Plains. So, too, the Loup 

 rivers and the Running Water of Nebraska. 



Think, then, of any great basin, as the Arkansas, 

 or any small basin, as that of the Solomon river, and 

 the truth remains that the rainfall of the basin is the 

 source of the water supply of the basin. 



The precipitation rain, snow or hail on any 

 given area can readily be followed to its immediate 

 disposition. Suppose the quantity for a year at a 

 given spot as shown by a rain gauge was twenty 

 inches. 



WHAT BECOMES OF THE RAINFALL. 



What became of it? It was disposed of imme- 

 diately in three ways: (1) Some of it soaked into 

 the earth. This may be called the seepage. (2) Some 

 of it ran off to lower levels and to the natural drain- 

 age channels. (3) Some of it was taken back into the 

 air by evaporation from the spot v .iere it fell. The 

 relative proportions of these vary vith every down- 



fall on the same area and in the same downfall on 

 different areas. On the same area with an inch of 

 rain falling within an hour, the runoff would be large 

 and the seepage comparatively small. The same 

 rainfall spread gently through twenty-four hours 

 would give a large seepage and small runoff. Evap- 

 oration would vary with the direction of the wind, 

 and the moisture or siccity of the atmosphere after 

 the rainfall. On different areas seepage would vary 

 with the porosity of the soil, and thus indirectly 

 would affect the quantity of r moff and evaporation. 

 There are probably no areas where accurate or 

 even approximate measurements of these three items 

 of the immediate disposition of rainfall have been 

 made. But when we go beyond immediate results, 

 the question of seepage may be eliminated from the 

 problem. The water that soaks into the ground is 

 eventually returned to the surface to be evaporated 

 on exposed surfaces, or through the pores of plants ; 

 or it works downward, and all in excess of the rock 

 saturation below becomes runoff, issuing from springs 

 or otherwise. Thus, then, the ultimate destination of 

 all rainfall is either evaporation or runoff, as the sat- 

 uration of buried strata is practically the same 

 always. If, then, numerous continuous observations 

 give us the total rainfall of any basin, and if careful 

 measurement can be made of the outflow of water at 

 the mouth of the main drainage channel, then the 

 measurement of that outflow would give us the total 

 final runoff of the region, and the difference of that 

 from the totality of rainfall would be the totality of 

 evaporation. Suppose the Kaw river outflow at Kan- 

 sas City were carefully measured for a year, that 

 runoff would be the excess 0} rainfall over evapora- 

 tion in the whole basin extending into Colorado and 

 Nebraska and having an area of not less than 50,000 

 square miles. But if at Kansas City there are con- 

 siderable depths of sand and gravel under the bed of 

 the Kaw and under the bottom land adjacent, which 

 would convey water as underflow to the underflow of 

 the Missouri valley, then measurement is impossible, 

 and the relation of runoff to evaporation and rainfall 

 cannot be ascertained. There are valleys where the na- 

 ture of the rocky bottom of the river bed and the val- 

 ley itself enables the outflowing runoff to be measured 

 with approximate accuracy. Such measurements 

 have resulted in the statement of proportions of run- 

 off to evaporation given in No. 6 above. 



ON THE GREAT PLAINS. 



The application of facts observed elsewhere is not 

 easy to the region of the Great Plains. And in con- 

 nection with the subject of irrigation and the ques- 

 tion of how much underground water is recoverable 

 for artificial use, the statement as to the relation of 

 the totality of outflowing runoff to the rainfall of the ba- 



