144 GEOLOGICAL SURVEY OF NEW JERSEY. 



If we have accurate measurements of the run-off of a stream 

 during a period of years, and contemporaneous measurements of 

 the run-off upon the catchment, the difference between these 

 two may usually be depended upon to represent the total evap- 

 oration, including the demands of vegetation. The mean tem- 

 perature of the atmosphere should be recorded at the same time. 

 Such observations have a scientific value far above the mere 

 •determination of the run-off for industrial purposes. The rela- 

 tion between rain-fall, run-off and evaporation is one which needs 

 to be established because of its bearing upon agriculture, drain- 

 age, forestry, hygiene and other fields of science and economics. 

 The engineer who gauges a stream should always endeavor to 

 have contemporaneous measurements of rain-fall and tempera- 

 ture. 



If we have such measurements as above indicated for a 

 deforested catchment, and also for a forested catchment under 

 similar climatic conditions, it should be possible to determine 

 whether forests have any very appreciable effect upon evapora- 

 tion. It may be insisted that it is difficult and almost impossible 

 to make such measurements with accuracy, but it should be 

 possible to make gauging with an error not exceeding five per 

 cent., and if the rain-fall can be observed at a number of stations 

 upon the catchment, this also may be closely approximated to, 

 and certainly the result should be sufficiently accurate to show 

 any effect which has economic importance. We have collected, 

 in the following table, the results of such measurements for 

 thirty-four catchments, largely in the eastern United States, 

 with some in the west and a few in England. We have also 

 given the authority for these figures and data, which will enable 

 the reader to understand the care with which the compilation 

 has been made. We do not claim accuracy for these data, but 

 we will show, as we proceed with the analysis, that there is 

 sufficient consistency in the results to warrant such conclusions 

 as we have attempted to draw from them. 



