710 EXPERIMENT STATION llECORD. 



called " accumulation," and the following equaticm is given to represent the 

 circulation of water in wet years following dry years : Rainfall — run-ofC=loss 

 of water by evai)oration+accuiuulation. 



Years of drought are usually years of increased temperature and therefore of 

 Increased evaporation, and the conclusion is drawn that the difference between 

 rainfall and run-oft" does not represent the real amount of evaporation, but that 

 some of the water evaporated is drawn from the subterranean waters. There- 

 fore for dry years the equation for circulation of water would be: Rainfall — 

 run-off+consumption=loss of water by evaporation, " consumption " being 

 the term used to denote the amount of water taken from the subterranean 

 supply during years of drought. This consumption of subterranean waters in 

 years of drought is proved by lowering of wells and drying up of marshes 

 during such years. This is due largerly to transpiration by forest growth 

 and other forms of vegetation. 



This is confirmed by study of conditions in the upper basin of the Dnieper in 

 the wooded belt of Russia. 



The hydrologic influence of forests under such conditions in years of drought 

 is sometimes entirely injurious, for they become the chief agents of evaporation, 

 although in cold and rainy years they may be useful, since they accumulate more 

 water than they exi)end. On the other hand, the peaty soils of the marshes 

 absorb moisture drained from higher ground and instead of giving it off to the 

 rivers evaporate it into the atmosphere. Then they gradually encroach on the 

 bed of streams and in the end turn them into marshes. 



Summarizing his studies, the author therefore concludes that during dry 

 years, often two in succession, there is apparent a considerable consumption 

 of the reserve moisture held in the soil of the basin and expended by evapora- 

 tion. This expenditure is replaced by the rains of the following year. This 

 consumption and accumulation is often as much as the run-off. The real 

 amount of evaporation is more than the difference between rainfall and run-off 

 in years of drought, but evaporation is not as much as the difference between 

 rainfall and run-off in wet years. The run-off is relatively large in dry years 

 and relatively small in wet years, because of the slowness of the movement 

 of subterranean moisture. 



Forests and marshes of the basin may be considered the most serious agents 

 of evaporation in dry years and their influence extends into the years following 

 dry years. It appears that one should estimate the influence of the forests 

 upon the river flow in the zone given, and generally in central Russia, as 

 almost nil under normal meteorological conditions. This influence is almost 

 negative in dry years in the south and central zones, but it may be that it 

 becomes positive in the north and central zones in wet years if the forests are 

 really capable of accumulating more moisture in times of slight evaporation 

 than they expend in their periods of active growth. 



The part played by marshes in feeding rivers is negative in dry years, insig- 

 nificant in normal years, and not as useful in wet years as one might expect. 

 The draining of marshes lessens useless evaporation and helps to increase 

 flow of rivers in summer. 



It must be admitted that marshes are not the most serious factors in the 

 feeding of rivers by subterranean waters, but rather the sandy soils, which 

 are perfect carriers of subterranean water, not alone to the rivers but also to the 

 marshes, which have great need of them in dry years. 



The role of forests, marshes, and other sorts of vegetable coverings consists 

 chiefly in evaporating the water of the basin and acts indirectly on the forma- 

 tion of atmosi)heric precipitation. 



