Hi Drew,
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.