research paper - mulch and water

When my son (8th grade) was choosing his science fair project this year, I suggested he do an experiment on the effect of mulch on water retention in soil. His teacher accepted the proposal. We have the experiment design pretty well in hand, but he also has to write a research paper on the topic. I have a lot of gardening books, including conventional, organic, permaculture, biodynamic, and probably a few other approaches. Most of them assert that mulch reduces irrigation requirements, a few discuss the mechanisms, but none that I can find refer to any research on the topic.
If anyone has sources we can look at on this topic, please send them my way. This can include any experimental data, academic research, or expert advice, but more specific than generic recommendations.
Help is much appreciated.

The USDA Natural Resources Conservation Services (NRCS) should have information. Google "mulch water conservation usda" and a bunch of stuff comes up. Also your local Ag Extension should have information particular to your area.