Actually, the main “cotton growing area” in early Texas was the Brazos River Valley from Waco South to the Gulf. East Texas was not particularly suited for Cotton farming. It is red clay hills or sandy loam with Pine forests. The Brazos River Valley is Blackland “Bottomland” and makes for great cotton farming. You don’t need 50” of rain to farm cotton.
Heavy rains at the wrong time can wipe out the crop.
I live east of Houston in Chambers County. We get 50” average annual rainfall and were never a huge Cotton producing area. A little further to the east is Beaumont which averages over 60” of rain and can easily get 70”+ in a “wet” year. That area was never a major cotton producer either. (But its great country for rice farming.)
I wasn’t trying to draw precise lines for agricultural purposes. For example, Katy used to be rice fields. My point was that rainfall patterns change dramatically as you move through Houston, and you don’t have to go far west of Houston before not a lot grows easily without irrigation (actually a lot of cotton is grown in arid parts of Texas today because of irrigation). In any event, most of Texas has a fairly harsh climate, certainly compared to central Mexico, which, when you add in Apaches, Commanches, and Karankawas, accounts in large part for the relative lack of enthusiasm by Spaniards/Mexicans for settlement north of the Rio Grande in the 17th, 18th, and early 19th centuries.