GEOLOGICAL SURVEY OF TFIE TERRITORIES. 259 



Water is the thing demanded, the only element needed except the 

 application of human energy, to render this broad area productive; 

 Nature has supplied all the other elements, ready for use. But how 

 much water is necessary '? From the best records at hand we learn that 

 the region around Washington City receives twenty-one inches rain-fall 

 during the spring and summer months ; that around New York, 23; the 

 vicinity of Cincinnati, 25; Missouri, 26; Michigan, IS; and the region 

 around Leavenworth, 20. But George P. Marsh,* following the state- 

 ment of Boussingault, tells ns that seventeen and one-third inches suffice 

 for the sandy soil and dry climate of Egypt, counting one hundred and 

 fifty days as the length of the irrigating season. As there is no rain 

 there, this entire amount must he furnished by irrigation. 



As shown by experience in Jumna, in India, a discharge of one cubic 

 foot of water per second is sufficient to irrigate two hundred and eighteen 

 acres — one hundred and fifty days here, also, being assumed as the irri- 

 gating season.f This is equivalent to about sixteen and one-third inches, 

 to which mnst be added one-fifth, (the rain supplying one-sixth of the 

 whole amount,) which gives a total of about nineteen and one-half 

 inches. This varies but little from the estimate given by Marsh, and 

 falls between the amounts as given for Michigan and Leavenworth. 

 Assuming the amount given by Marsh to be a sufficient supply for the 

 growing season on the Plains, we arrive at the third inquiry. 



3. What portion of this is furnished by the rain-fall during this part 

 of the year? We have but little data upon this point, yet sufficient is 

 known to enable us to make an approximate estimate. The meteor- 

 ological records kept at Fort Laramie, Santa Fe, and Fort Lyon (as given 

 by Blodget, Foster, and Elliot) show the average rain-fall for the spring 

 and summer, taken together, as follows : 



Fort Laramie, 14.39 inches ; Santa Fe, 11.73 ; Fort Lyon, 6.36. An 

 average of these three places is 10.82 inches, which, deducted from 

 17.34, the amount of water necessary, leaves 6.52 inches to be supplied 

 by irrigation. But I think this estimate of the rain-fall on the Plains is 

 too large, and that a further examination of the more recent observa- 

 tions and records will reduce it. I jutlge that seven inches, or, to make 

 the remainder a round number, 7.34 inches, will be much nearer the 

 correct figure. Nor do I arbitrarily assume this average, but from 

 incomplete data, not necessary to be repeated here. If too small, the 

 calculations based upon it are certainly safe ; if too large, the error will 

 be much. less than it would be if we assume the larger amount, and it 

 must be very near the lowest possible average, which, for the dryest 

 sections, is seldom placed less than five , inches. This, then, will leave 

 10.34, or, in round numbers, ten inches to be supplied by irrigation. 



4. Now, an answer to the fourth question — what is the volume of 

 water brought down by the streams during the spring and summer 1! — 

 w T ould enable us to tell at once the area that can be rendered tillable by 

 the aid of irrigation. But, unfortunately, at this point we are- without 

 reliable statistics. Few or no streams have been accurately measured 

 with this object in view, and but few have even been roughly estimated. 

 While in Italy and India this has been a matter of careful study ; the 

 necessity for irrigating any of our lauds having been but recently felt, 

 it has not been attended to. But the rapid influx of population into 

 the great plains and mountain regions of the West is causing the im- 

 portance of attention not only to this, but to all that bears upon irri- 

 gation, to be felt more and more each year. 



*In "Man and Nature." 



t Smith's Italian Irrigation, vol. I, p. 378. 



