172 CONSERVATION OF GROUND WATER 



The long-term precipitation records from about 125 towns 

 have been used in studies of rainfall-runoff relationships prior 

 to 1935. 13 These studies conclude that there was a severe 

 drought of wide extent between 1830 and 1850, followed by 

 a general upward trend over most of the United States until 

 about 1885; toward the end of that period the precipitation 

 over much of the United States was greater than at any other 

 time in the century. There followed a general decrease in 

 precipitation, with widespread drought conditions toward 

 the end of the century, except in the Pacific Northwest where 

 precipitation reached an 80-year maximum about 1900. Be- 

 ginning in 1915, the trend over the nation was upward, with 

 maxima between 1910 and 1915 in the Rocky Mountains and 

 Great Plains, and as late as 1925 in Texas. In each of these 

 regions these maxima exceeded those of the 1880's. The de- 

 clining trend which culminated in the drought of the 1930's 

 began as early as 1910 in some areas, and as late as 1925 in 

 others. The minimum of the 1930's was generally the lowest 

 of record along both the Atlantic and Pacific Coasts but was 

 not so low as the minimum of 1895 to 1905 in most of the 

 interior. Since 1940 the trend has been generally upward ex- 

 cept in the Southwest. 



Regional temperatures also have an important bearing on 

 ground-water replenishment, because as temperature in- 

 creases the opportunity for evaporation and transpiration 

 increases and less water is left for downward percolation. 

 Records of temperature are generally about as long and as 

 numerous as those of precipitation, and several studies of long- 

 term trends have been published. 14 In 70 widely distributed lo- 

 calities included in Hoyt's study, an increase in temperature 

 has been quite general, and all but one of these localities 

 recorded an increase since 1910. Kincer shows that tempera- 



13 Hoyt, W. G., et at., Rainfall and Runoff in the U.S., U.S. Geol. Survey 

 Water-Supply Paper 772, pp. 20-110, 1936. 



i* Kincer, J. B., Is Our Climate Changing? A Study of Long-term Precipita- 

 tion Trends, Monthly Weather Rev., vol. 61, pp. 251-260, 1933; Our Changing 

 Climate, Trans. Am. Geophys. Union, vol. 27, pp. 342-347, 1946. 



Hoyt, W. G., ibid. 



