If you assume duck to mean "irrelevant decorative elements" there are a few things that strike me as likely: (1) the "squares" only roughly indicate location of the target area; (2) squares represent volume, which is incongruent when overlaid on 2d geography; (3) shading of mountains/geographic features doesn't add detail.
Also problematic: comparison of h20 volume applied to crop types is difficult to discern using grids. Quick, which crop color uses more water??
If you want to compare volume to crop type then you should use a plain column chart, where it is easy to distinguish relative heights. This is analogous to why bar/column charts are preferable to pie charts - humans are better at comparing heights/lengths vs area/volume. If, on the other hand, the point of the graph is to compare crop type irrigation by area, you would want to display a column graph with regions next to each other, but grouped by crop type.
As far as a superbly produced duck, my guess is that the Applied Irrigation Water is a duck, but a superb duck, at least when compared to the monstrous duck on the preceding page of the Visual Display of Quantitative Information 2nd Ed.