Evaporation data are available from two stations near Padre Island. Beeville, about 60 

 miles nortbwest of Corpus Christi, had an annual average evaporation of 59.11 inches. 

 Precipitation during the same period averaged 29.48 inches; wind movement was 51,898 

 miles per year. Weslaco, occupying a similar position relative to Port Isabel as Beeville to 

 Corpus Christi, in 22 years had an average annual evaporation rate of 56.07 inches, and an 

 average annual precipitation rate of 23.80 inches; wind movement was 29,020 miles 

 annually for a 19-year period (Bloodgood, Patterson, and Smith, 1954). Evaporation on the 

 island is less than the mainland due to higher humidity and lower temperatures, although 

 precipitation rates are equal. Nonetheless, there is deficiency in available water. 



6. Soil. 



The coastal topography of the mainland adjacent Padre Island is relatively flat with soils 

 developed from Pleistocene and Recent unconsolidated clastic sediments. The soils of Padre 

 Island developed on Recent marine and eolian sands. Sand particle size from project samples 

 (Table 5) is dominantly fine to very fine sand, and agrees with other investigators (Hayes, 

 1964). Garner (1967) states, "The well sorted sands consist of quartz, chert, heavy minerals, 

 and volcanic and plutonic rock fragments . . . Sands on the northern part of the island 

 contain plutonic and volcanic rock fragments; on the southern part of the island they 

 contain relatively more volcanic rock fragments and relatively fewer plutonic rock 

 fragments." The soil is highly variable in salt content with varying amounts of shell and 

 organic matter. The highest percentage of organic matter from beach sands was 0.1 

 (Table 6). Shell fragments ranged from a trace to 14.9 percent. 



7. Vegetation. 



The vegetational history of the island is incomplete, but the relative species composition 

 is different today r from the pristine condition. The adjacent mainland vegetation is a mixture 

 of live oak (Quercus virginiana), sweet bay (Magnolia virginiana), and tall prairie grasses such 

 as big bluestem (Andropogon Gerardi), tanglehead (Heteropogon contortus), switchgrass 

 (Panicum virgatum), Indian grass (Sorghastrum avenaceum), and four-flowered trichloris 

 (Trichloris pluriflora), and associated broadleaf plants. With one exception, none of these 

 prairie grasses occur today on either Padre Island or Mustang Island, although all are well 

 adapted to sandy, moist sites, similar to those of the mid-island prairie. All are excellent 

 forage species, and are termed decreasers since they do not persist under heavy grazing 

 pressure. One species (big bluestem) has recently (1973) appeared at scattered locations 

 along the roadside on northern Padre, apparently as waifs; the 30 to 40 individual plants are 

 thriving and spreading. 



Padre Island's woody vegetation consists mainly of live oak. Sizable mottes (dense groves 

 surrounded by grassland) exist on the north end of the island and several small mottes occur 

 farther south. On the national seashore, several trees survive in an area once covered by a 

 large motte (Derek Hambly, PAIS Naturalist, personal communication, 1970). Stumps of 

 this motte are occasionally uncovered during hurricanes. Within the remaining mottes on the 



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