A NATIONAL PLAN FOB AMERICAN FORESTRY 



359 



the Rio Grande and Pecos Rivers, all the main drainages in Texas 

 (fig. 6). The principal streams are the Sabine, Trinity, Brazos, and 

 Colorado Rivers. The extreme variations in climate, soils, and 

 vegetation throughout this extensive area have had a correspondingly 

 profound effect on land utilization as well as on watershed con- 

 ditions. 



CLIMATE IN RELATION TO WATERSHED PROBLEMS 



Precipitation on these drainages comes almost entirely in the 

 form of rainfall the rains varying tremendously in different portions 

 of the basin. In the humid region of eastern Texas and western Louisi- 

 ana the rainfall averages 50 to 60 inches annually as compared with 



MAJOR INFLUENCE 

 MODERATE INFLUENCE 

 [H SLIGHT INFLUENCE 



FIGURE 6 West Gulf drainages, Arkansas Red River drainages, upper Rio Grande Basin and lower 



Mississippi River Basin. 



about 15 inches in the arid region of western Texas. In the latter 

 region evaporation rates are high and this tends to decrease further 

 the supply of moisture available for plant growth. However, a 

 tendency to torrential rainfalls in some portions of the basin makes 

 annual averages unreliable indexes of true rainfall conditions, as in 

 central and western Texas where a single heavy rain may account 

 for as much as two thirds of the average annual precipitation. 

 In eastern Texas and in the Gulf region intense rains are also common 

 but they seldom reach cloudburst proportions. The maximum rain 

 recorded in the Trinity River watershed is 5^ inches an hour and over 

 10 inches in 24 consecutive hours. 



The fact that these torrential rains are most frequent during the 

 winter when pasture and range are least protected and cultivated 



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