We currently live on a property which has soil with a 5-10 percent clay content and gently sloping ground. I am not looking for specifics but what do you think Sepp's general strategy for earthworks would be in a situation like this? Would he still try to make ponds and seal with pigs. I know this is a pretty non-specific question but general strategies would be great! Thanks for what you are doing with Holzer AgroEcology!
Do you know what layers are in the soil? It is quite common for there to be a clay layer at some point in the soil horizon, making use of this layer is the most ecological and economical method for water retention. Do you have a well log? If so you can check this for the various layers that are in your soil without having to do any digging. This will also let you know about deep layers that you would have to go through great extents to find with an excavator.
Test models are really the only things that provide the answers to earthworks. Terraces are usually a sure bet to improve water retention regardless of soil type. Keep in mind that the specifics for terraces vary with the content of the parent material, but in my experience this seems to be a technique than can be adapted to a wide variety of geological and climatic conditions. It's tough to say more without more specifics, those are the first thoughts that come to mind.