How does a difficulty increase affect a miner's income?
Your profit relates to the amount of hashing power you contribute to the network. Since your mining power is constant, your share of the total hashing power decreases relatively when the network's hashing power increases. I.e. we can deduce the following formula:

newProfit = currentProfit * currentDiff/newDiff.

At a currentProfit of 1BTC/d and a 30% increase in difficulty, you get: 

(1BTC/d)*100/(100+30)= (1BTC/d)/1.3 = 0.76923077 BTC/d

i.e. your profit decreases by ~23%.