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MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



Any disturbance or alteration of the components 

 of this region greatly influence the water balance 

 of the entire system. 



Interception 



Redistribution and loss of water through inter- 

 ception by the canopy is often overlooked. Col- 

 lins (10) reviewed works in which 20 percent 

 of the total rainfall was lost through evaporation 

 of intercepted precipitation in forest vegetation 

 in Wisconsin. Interception losses in conifer for- 

 ests were found to be twice those of hardwood 

 forests of equal canopy density. The conifers 

 intercepted 40 percent compared with 20 percent 

 for the hardwoods on an annual basis. Intercep- 

 tion by crop plants was found to range from 6.9 

 to 35.8 percent of the total precipitation, depend- 

 ing upon density of the vegetative cover. 



In a detailed study of interception by grassland 

 vegetation, Clark (9) found density of Nebraska 

 vegetation to be the major factor. Due to the 

 smaller amount of surface exposed, low-grow- 

 ing, mat-forming species intercepted less mois- 

 ture than did species of greater height. Stratifica- 

 tion of vegetation, as in a true prairie complex, 

 increased interception loss. The water intercepted 

 by the prairie vegetation ranged from 84 per- 

 cent of a 0.32-cm. rain in a 30-minute period to 

 51 percent of a 5-cm. rain in a 1-hour period. As 

 would be expected, wind and other evaporation 

 factors had a large influence. Duration and rate 

 of precipitation was also found to be important, 

 but the type and amount of vegetation was of 

 most importance. 



A spreading, semidesert shrub can collect a 

 sizeable amount of water and channel it down 

 branches and stems, thus improving moisture 

 conditions in its own root zone (11). Redistribu- 

 tion by this means is an important survival mech- 

 anism for the shrub, but it increases the aridity 

 of the areas between shrubs. Stemflow is also 

 important in crop plants, such as corn (Zea mays 

 L.), alfalfa (Medicago saliva L.), and soybeans, 

 (Glycine max [L.] Merrill). 



Removal of all or part of the vegetative cover, 

 or change in species composition in response to 

 grazing will surely influence interception and 

 consequently various other aspects of the water 

 regime. Cooper ' ( / / ) reviewed work done in the 

 California chaparral for which equations for 



quantities of water involved are being developed. 

 This information would be useful for other vege- 

 tation types, particularly if the effects of grazing 

 were included. 



Infiltration And Runoff 



Consider first the soil factors which influence 

 rate of infiltration. Duley (16) reported that a 

 rapid reduction in rate of water intake by bare 

 soils was accompanied by formation of a thin, 

 compact layer at the soil surface. This layer was 

 the result of a severe structural disturbance 

 brought about by the action of raindrops. Al- 

 though the rate of formation and degree of seal- 

 ing depended upon soil type and structure, it 

 was formed in all cases. He found that a thin 

 covering of straw kept the intake rate constant 

 over a 5-hour period. The straw was then re- 

 moved while the water application was continued. 

 The intake rate rapidly decreased to near zero. 

 Removal of the 7.6-mm. compact layer of soil 

 and protection of the newly exposed surface with 

 burlap resulted in an intake rate near that of the 

 originally protected surface. Duley (17) reported 

 that a surface cover of either grass or crop resi- 

 due increased the intake rate over the initial rate 

 for bare soil. The intake rate 90 minutes after 

 start of precipitation was 8.4 and 8.9 cm./hr. for 

 native vegetation 46 cm. tall and for cultivated 

 soil covered with 5,604 kg. straw/ha., respec- 

 tively. At the same time, intake rates were only 

 1.7 and 1.2 cm./hr. for native grass dipped to 

 the soil surface with all debris removed, and for 

 bare cultivated soil, respectively. 



Dee et al. (15) found a high positive correla- 

 tion between water intake and the amount and 

 species of standing vegetation, litter, and vegeta- 

 tion plus litter on Pullman silty clay loam on the 

 High Plains of Texas. Infiltration rates varied 

 under different plant communities. Over a 2-hour 

 period, soil under blue grama (Bouteloua gra- 

 cilis [H.B.K.jLag. ex Steud) absorbed 21.3 cm. 

 of water, compared with 14.2 cm. for windmill 

 grass (C Moris verticilata Nutt.), 9.7 cm. for an- 

 nual weeds, and 5.3 cm. for buffalograss (Buchloe 

 dactyloides [Nutt] Engelm.). A comprehensive 

 study on the Wasatch Front Range in northern 

 Utah (33) showed that plant and litter cover 

 accounted for 73 percent of the variance in the 

 amount of water retained during a 30-minute 



