Seepage, evapotranspiration, and recharge. Aka the considerations for keeping your pond full.

2. ESTIMATES OF WATER REQUIREMENTS

ftp://ftp.fao.org/fi.....705e02.htm

 

2.1 Water losses by seepage

Water that is lost vertically through the bottom of the pond, horizontally through the dikes by infiltration, and through the drainage system of the pond is called seepage water.

If the dikes of your pond are well built and well maintained and if the drainage system is watertight, the amount of seepage water lost horizontally will be very small. You will need to calculate only the vertical seepage.

Water seepage will be greater from a new pond when it is filled for the first time. The soil structure of the pond will still be good and water will be lost.

After the pond has been filled with water for some time, the water tends to break down the soil structure and the soil pores become sealed by organic matter that collects on the pond bottom. As a result, the soil permeability and losses by seepage will decrease.

 

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This is among the best articles I have found on getting a new pond to hold water. Above is an excerpt.