Estimating pump power in U. S. customary units:

HP = (Q x H)/(1714 x eff)

HP = horsepower (1 HP = 746 watts)
Q = flow, in gallons per minute
H = head or pressure, in pounds per square inch
eff= decimal pump efficiency, varies with pump (example: 0.50, not 50%)

Specified flow: Q = 48 gpm
Specified head: H = 35 psi
eff = 0.50, average for small single stage centrifugal around best eff. point

HP = (48 x 35)/(1714 x 0.50) = 1.96 HP, use next larger size (2 HP)

Note: efficiency can be much higher for multistage and larger single stage pumps. Efficiency can also be much lower if a given pump is operated much different than most efficient flow and pressure. It is wise to look at a few pump curves (graph of pump pressure versus flow, often includes efficiency too).

For example, from looking at one pump in the Goulds 3656 series, the "1x2-7" model with "F" impeller: with 2 HP motor it will deliver 48 gpm at 39 psi. Efficiency is about 54%. Allowing some pipe friction, this might be about right to deliver 35 psi at the sprinklers.

Your actual elevation differences (if any), pipe sizes and lengths will influence the pressure delivered to the sprinklers. You can write back with details of the layout if you want.

Tank drawdown and refill rates:

Output flow 48 gpm for total of 8 hours. Storage volume 4,000 gallons.

pumped volume = 48 gpm x 60 min/hr x 8 hr = 23,040 gallons

Depending on requirements at start of next day, either 23,000 or 19,000 gallons has to be pumped in to complete the 8 hour run.

IF the sprinklers are run in short blocks of time over 24 hours AND IF the tank can end the day empty:

minimum refill flow = (19,000 gal)/(24 hrs x 60 min/hr) = 13.2 gpm

At the other extreme, assuming a continuous 8 hour output, starting with a full tank and ending empty, AND refill begins at start of output:

minimum refill flow = (19,000 gal)/(8 hrs x 60 min/hr) = 39.6 gpm

(Tank would refill in less than 2 additional hours.)

So, the irrigation schedule may depend a lot on how fast water can be delivered into the tank.

to: Irri_guy
Thanks for your message.Have to irrigate a socer field that is with Bahia grass on(Paspalum Notatum)the field have 335 feets long and 223 wide.The main line and the secondary line is a pipe of 1 1/2" and six sprinklers that will be located across the field;spaced at 36 feet, one line of six sprinkler will be irrigating and another line will be waiting for the time when is changed the area of irrigation.The auxiliary pump will be located at the outlet of the tank at the same level that will be the sprinkler ad will not have head.-In how many hours can irrigate the field with of 1" of water? Will have enough water refilling the tank at 10 GPM input?and what will be the size of the pump?Thank you very much

Emilio,

I do drip irrigation in vineyards, so I am a little vague on grass and sprinkler work. Give me a little time, and maybe the turf experts can jump in here.

I think you are planning six sprinklers in a "lateral" across the 223 foot direction. You are planning 9 or 10 laterals, spaced 36 feet, along the 335 foot direction. Is this correct?

Each sprinkler is 8 gpm at 35 psi, and six sprinklers times 8 gpm = 48 gpm per lateral. Is this correct?

If those are correct, I think you will have a problem with the pipe size. If the "main line" runs down the middle of the field there would be two branches to each lateral, with three sprinklers and 24 gpm to each side. This would be okay. If the main line is on one side and all six sprinklers connect in one 1 1/2 inch pipe, there probably would be too much friction and pressure loss. The flow from the first sprinklers would be bigger than to the last sprinklers.

The main line in 1 1/2 inch would also be too small and have too much friction and pressure loss. I would probably use 2 1/2 inch pipe or maybe 3 inch for the main line. This is if 48 gpm is the flow rate.

There is a bigger problem if the refill rate is 10 gpm. To put 1 inch of water on the field is a lot of water: (1 inch = 1/12 foot)

335 ft x 223 ft x 1/12 ft = 6,225 cubic feet, x 7.48 gallon per cu ft = 46,572 gallons.

Starting with a full storage tank, it will take three days to bring in that much water. You would have to wait 6 hours and 40 minutes to refill every 4,000 gallons. This would make it hard to use the field to play on. Is there any way to refill faster? 40 gpm would be good.

just a thought, not regarding the irrigation even.
Bahia grass here thrives on very little water, even when it is nearly 100 degrees with no rain.
I do not know the needs of bahia in california, but it would seem 1inch of rain at any one time would be alot of overkill.
I would think 1/2 would be more than sufficient at any one time... and certainly 1/2inch per day would definatly keep even a high traffic area like a soccer field moist and green.

dp

I was hoping somebody who knows about grass would come by.

The original poster is in Honduras, I think CA is Central America. There is no city of Honduras in California.

On his member page, Emelio_ch describes the climate as very hot, dry Dec to May, wet June to Dec. Not California!

To: Irri_guy

Thanks for your message,also hope some expert on grasses will help me.Yes I live in the city of Choluteca in the south of the Republic of Honduras in Central America

honduras I would assume gets alot of rain.
it rains 55-65inches a year here, and despite the fact I live in a swamp, the soil is very sandy.
Bahia here is used by the road companies to seed embankments and near bridges, which is mostly clay fill dirt.
Even when bermuda grass is browning bahia is still happy... in fact it grows best in June-August normally requiring mowing every 6days at the least (unirrigated).

but I'm no grassing expert... I grow ferns for a living and do a lil irrigation for residential yards.
(if you grow ferns, you might as well know about irrigation, hehe)

dp