Sorry, yesterday was a busy day.
Yes, I calculated the water requirements on an Evapotranspiration Rate for August for that location (Houston). day18, what you need and what you get could be two different things. All areas of the country cannot produce the same amount of water from a well, only a well driller in your area will know for sure.
For instance, Dave Davidson, one of the moderators here can only get (I believe) 1 gpm or 1/10 gpm from his well. So, even for his house he has to pump continually into an above ground storage tank just to have enough water for the house. No way could he use a well to fill the pond. Here where I am at in Northern Indiana, we have a very good aquafier here only 100 feet down. If I care to pay for the electricity, I can pump 2,000 gpm if I dig a big enough well.
I just went over last months utility bill. Our electric costs (Kwh rate, delivery charge, taxes, etc) cost us now $0.239/Kwh. The next county over, that electric rate is $0.209/Kwh. (different utility company)
As for what pump size, that is a big "that depends" question. Even if you wanted to pump 25 gpm, all well pumps are not the same. You have high efficiency pumps and not high efficiency pumps. In other words you could possibly pump the same amount of water with a 2hp pump or a 5 hp pump.
Chris: Here's what the numbers are that I calculated:
If all water loss is from Evapotranspiration, then this is applicable for August (for an average year in Houston).
325,851 (gallons in an Ac/ft of water) ÷ 12 (inches) = 27,154 (gallons per inch) x 4 (inches per month Evapotranspiration Rate loss) = 108,617 (gallons per month per surface acre loss) x 8 (surface acres) = 868,936 ÷ 30 (average days per month) = 28,964 (gallons per day) ÷ 24 (hours in a day) = 1,206 (gallons per hour) ÷ 60 (minutes) = 20.1 gpm if run 24/7.
Data acquired from http://hgciatx.com/water-management/
I have heard that you cannot get more than 100 gpm from a 4" well casing. You will have to talk to your local well driller to get the generalities for a well in your area, and even at that they cannot guarantee how many GPM your well will produce, they can only give you a very educated guess before the actual well is drilled.
To help increase the life of the well, I would recommend putting a double screen on it, AND use stainless steel screen vs. plastic if possible. A double screen will give you twice the amount of screen to pull water through, so that gives you twice the surface area to resist plugging up. A stainless screen can be made with more open area to allow more water to flow through it, so that also increases the efficiency of the well. YES, the stainless screen costs more, but when you look at what the cost of the well is, and what it will cost to replace the screen if it gets plugged, I believe that the stainless screen is cheap in comparison.