a. Soil Moisture. Subsurface (6 inches) moisture of sand on the backshore (about 4.5 to 

 5.0 feet MSL) varies widely and depends on rainfall and on inundations by surges. The range 

 of moisture content, from data collected since 1970, is from about 22 percent to no less 

 than 2 percent, but is usually above 5 percent even after moderate droughts. Field capacity 

 of the sand, i.e., water retained by the sand under well-drained conditions, without the 

 effects of evapotranspiration, is about 3 to 4 percent; laboratory-determined saturation 

 extract is 21.5 percent. Thus, the usual backshore soil moisture varies from greater than 

 field capacity to saturation. Moisture remains high because the water table is usually within 

 2 feet of the surface except during prolonged droughts when it is deeper, and after heavy 

 rains when it is at or above the surface. Only when the water table recedes does complete 

 drainage of soil pores occur. Soil moisture at the beach surface varies more widely, from 

 0.1 percent to saturation. In a south Padre hurricane pass, of lower elevation and hence 

 closer to the water table, soil moisture was even higher. 



On elevated surfaces, drainage is less restricted, and both surface and subsurface moisture 

 contents are lower than on the beach. Near-saturation occurs only briefly, during and 

 immediately after heavy rains. The typical subsurface moisture content range of an elevated 

 dune is between field capacity and 10 percent. 



Moisture values taken at monthly intervals in 1973 from 15 stations along an artificial 

 dune line on Padre Island are shown in Table 17. "Beach toe" and "bay toe" samples were 



Table 17. Percent soil moisture at the surface and at 6 inches from three locations on artificial dunes. 



North Padre 



South Padre 



1973 



Beach toe 



Center of dune 



Bay 



toe 



1973 



Beach toe 



Center of dune 



Bay toe 









6 



| 6 







6 





| 6 



| 6 



| 6 





(inches) 



(inches) 



(inches) 





(inches) 



(inches) 



(inches) 



9 Jan. 



12.2 



19.3 



7.5 



9.4 



11.2 



18.0 



7 Jan. 



2.7 



7.2 



2.7 



3.5 



6.0 



11.2 



19 Feb. 



6.5 



10.6 



4.9 



6.9 



6.5 



10.9 



17 Feb. 



11.7 



8.6 



11.5 



8.7 



11.5 



7.8 



15 Mar. 



0.2 



4.4 



0.1 



4.5 



0.4 



7.4 



29 Mar. 



0.1 



2.7 



0.1 



2.0 



0.1 



3.1 



14 Apr. 



0.5 



4.2 



0.3 



2.4 



0.2 



3.8 



29 Apr. 



0.2 



2.5 



0.2 



0.8 



0.1 



1.6 



13 May 



0.2 



5.9 



0.1 



5.3 



0.3 



5.2 



31 May 



0.2 



2.2 



0.2 



2.0 



0.2 



1.9 



14 June 



1.8 



7.7 



0.2 



4.7 



11.1 



14.9 



23 June 



8.3 



13.1 



6.8 



8.4 



8.7 



12.9 



16 July 



0.2 



3.9 



0.2 



3.0 



0.2 



4.9 



28 July 



0.1 



2.5 



0.1 



2.0 



0.1 



2.8 



19 Aug. 



1.0 



8.4 



0.1 



5.8 



5.6 



12.6 



Aug. 2 





----- 



....... 





....... 





18 Sept. 



8.1 



17.9 



3.2 



5.5 



20.9 



24.8 



12 Sept. 



0.2 



5.1 



0.1 



2.7 



0.2 



7.5 



20 Oct. 



10.0 



22.9 



0.1 



3.7 



18.9 



26.6 



26 Oct. 



1.6 



5.3 



0.2 



2.9 



1.6 



12.5 



18 Nov. 



0.3 



9.4 



0.2 



3.2 



7.8 



23.3 



21 Nov. 



0.4 



4.6 



0.4 



2.7 



0.5 



8.9 



17 Dec. 



0.2 



7.7 



0.2 



3.2 



1.7 



14.8 



14 Dec. 



0.2 



4.0 



0.2 



1.9 



0.2 



6.5 



1. Monthly collections. 



2. No samples taken. 



Each is the mean of five samples from fixed stations. 



77 



