user: Question: What was the surface in 1981?
Columns: ['Outcome', 'Date', 'Championship', 'Surface', 'Partner', 'Opponents in the final', 'Score in the final']
Types: ['text', 'real', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Winner', 1976.0, 'Madrid, Spain', 'Clay', 'Eduardo Mandarino', 'John Andrews Colin Dibley', '7–6, 4–6, 8–6']
SQL:

assistant: SELECT "Surface" FROM "Table" WHERE "Date" = 1981.0;