PROBLEMS FROM LAND OCCUPANCY 171 



data are too meager to show the long-term trends in water 

 levels. 



Most of the detailed records of water-level changes in wells 

 cover only the last decade or two, and records begun as early 

 as 1920 are considered long records. So far as known, there 

 are fewer than 50 wells where data have been collected for 

 half a century or longer. Very few records have been analyzed 

 in sufficient detail to deduce the quantities of water repre- 

 sented in the changes of storage indicated by the water-level 

 fluctuations. For most areas not enough is known of the hy- 

 drologic characteristics to permit such analysis. 



The causes of water-level fluctuations are many. In order 

 to discriminate the effects of changes in land use, it is neces- 

 sary to know the effects of these other factors. One very sig- 

 nificant factor is pumping. As shown in the preceding chap- 

 ter, lowering of water levels is characteristic of wells in areas 

 of ground-water development and is the physical expression 

 of the process of diverting water to wells and from its path 

 of natural movement. Wherever water levels are affected by 

 pumping, any effects of changed land use are likely to be 

 hidden by these more pronounced fluctuations. Most of the 

 long records of water-level fluctuations have been obtained 

 from wells in areas of pumping. 



Climatic factors are also responsible for significant changes 

 in ground-water storage. Water levels fluctuate in daily, 

 seasonal, and climatic patterns. In many wells water levels 

 decline during dry years and rise during years when precipita- 

 tion is above normal. Several hydrologic studies suggest there 

 may have been trends in climate during the past century that 

 would tend to reduce the replenishment to ground-water 

 reservoirs and thus cause some decline of water levels in wells. 

 In the absence of century-long records of ground-water levels, 

 it is necessary to rely on records of other phases of the hydro- 

 logic cycle, from which changes in ground-water storage may 

 be inferred on the basis of known interrelationships of water 

 in all phases of the cycle. 



