Irrigation

[vision25]

I am thinking of digging a 1 rai pond for irrigation purposes.

Assumming that 1 inch of water on one acre gives 100,000 litres and the rainfall in Sa Kaew is 55 inches per year then a 1 rai pond will collect 2,200,000 litres of water a year.

Is this correct?

Will evaporation be a problem, how much water will I loose?

Apparently most people here don't line their ponds which is obviously cheaper than plastic or concrete. However, if the pond was larger, say 5 rai would lining be neccessary.

How much will this cost, I have been told that to have a pond of 1 rai dug will be free as the dug out earth will be sold to someone else. Is this true?

Vision25

[chownah]

A container of water will lose water to evaportion at about the same rate as a pond will lose water to evaporation.

Some ponds will benefit from sealing and some will suffer from sealing. Where I live the water table is never more than two metres below the ground level so when I dug a pond I dug it deep enough so that even in the driest of times the bottom of the pond will be below the water table so I will always have water. If I lined my pond it would inhibit the inflow of water from the ground and the pond would not get recharged after pumping. In other places the water table is too low for this type of construction and ponds constantly lose water to the ground...these ponds benefit from being sealed.

I don't know the costs for sealing ponds.

Chownah

P.S. Your calculation of 2,200,000 litres per year seems to be correct.

Chownah

Edited November 14, 2006 by chownah

[RamdomChances]

I'd suggest you look carefully at the placment of the pond, if you just reliy on rainfall, then you would only expect to get 55 inches of fill in your pond, roughly a meter and a half ?. You need to look at geting a catchment area. Mines about 4-500 cu meters (4-500,000 liters) and basicaly can more or less fill in one down pour ( it often overflows ).

As for sealing I'd agree with what Chownah said, depends where you are. I'm not sure on the viability of using it for irrigation if it's above the water table. If you do want to use it for that I'd suggest trying to go as deep as you can. Even if you lose about 1cm in evaporation a day, thats approx 1 meter for 3 months, without pumping anything out of it. A deeper pond will suffer from less evaporation(percentage wise) as it has a lower surface area. A 5 rai 3.5 meter deep pond will hold about 28,000 cu maters when full

[Maizefarmer]

A pond in the North East of Thailand with a surface area of 1rai will loose approx 8780litres of water per day (24hrs) – from evaporation alone!

These figures are based on average climatic conditions measured over a 10 year period, year on year, from 1991 – 2001.

Over the same period of time the average drop in pond depth from evaporation alone, was measured for a 1 rai pond (i.e. 1600 metre squared) to be just over 0.5 cm per day. This figure takes into consideration rainfall over the same surface area but not runoff into the pond from surrounding areas.

Seepage through the base/bottom – impossible to estimate as every area has its own rate of drainage. Take a look at other dams around you and build an average of those losses into your calculations.

Should you line your pond?

In anything other than clay like earth, then to generalise, yes I would line the pond, but in any event do your calculations – work out how much water you need to store and what is it going to cost you to extract the extra earth to make up for the above difference (plus seepage through the base) versus the cost of the lining.

You will only have it dug and taken away for free if it is decent soil that can be used elsewhere.

If the earth is not going to be taken away then use the earth extraced to build up the side of the dam wall - will save the cost of having to dig deeper to get the depth you want.

Edited November 14, 2006 by Maizefarmer