Hilliers Manual of Trees and Shrubs rarely gives more than a passing ref to cultivation but for The walnut he reccommends that it be not planted in sites subject to late frosts.

You posit the cause as lack of water....All trees need to be watered in their first year so if this has been neglected it could be a cause.

Trust it prospers next year.
Jardiniere

Wow! And they grow to a great size. :o(

This could be a spider mite infestation. Take a sheet of white paper and while holding it underneath a few of the affected leaves, tap the leaves. If you see tiny specks moving around, that would be a firm indication of mites. For control and or eradication, consult with a tree specialist before taking this matter into your own hands.

Two years running with the same problem sounds serious. I think Graterford is absolutely right about asking a specialist. We've never had this problem with any of ours - in fact we have a problem getting rid of unwanted seedlings. Good luck.
p.s. They say that no other plant will survive or thrive if planted close to walnut tree.

Did you know they also keep mosquitos away?

Here in the USA Black Walnut Trees have been decimated by a fungal plague called the " Thousand Canker Disease." The vector of this disease is the Black Walnut Twig Beetle.
It started on the west coast and has traveled to the east coast by rail and truck transport of black walnut wood products. In some areas there is a complete quarantine of infected trees. Case in point-after the most recent east coast hurricane a black walnut tree came down upon a Pennsylvania residents house. The resident could get no local tree service to take the tree down off his roof, reason, this tree was infected and the area in which he lives has a quarantine in effect. Under that rule no infected tree can be removed from the property, no mulching or burning of remains allowed and any tools, clothing and vehicles that the tree service might use would have to be disinfected totally. The result was that the resident got the tree off of his roof himself but the cut pieces remain on his property. Though your problem is not this one, it could be the beginnings of some other contagious disease which makes it a best practice to obtain an identification and solution of problem first by an expert.

I'm not sure the mosquitos up here know about that, Cammomile!

Thank you all for your comments.

Could it be Walnut anthracnose / Walnut Leaf Blotch ?