Some folks argue that a wider spacing of plants will use less water, since there are less plants to draw it up. Others argue that tighter spacing will use less water, since plants will shade one another, create a moist microclimate, and reduce water lost to cooling the plants.
I'm sure which strategy to use depends on climate, rainfall, latitude, soil type and water storage, type of plant(s) in the system, irrigation if any, management goals, and other factors.
How should we go about determining which way to go in a given case? Anyone know of studies, etc. that are relevant?
The following link may have some information of interest. Certainly corn in the midwest US has received a ton of research attention. When I was growing up, it was relatively easy to walk between corn rows during pheasant season....I'm thinking those rows in the early 70's were ~36" apart or wider. Narrower rows slowly are becoming more popular, but as you can see from the data within the link, it's not such a clear story on yield. I bring this link up because, even though it focuses on yield and not on water retention or needs, much if not most midwestern corn is grown without irrigation. The same would probably be said of soybeans, but I don't know if the analysis on plant density has been done as thoroughly, although I suspect it has. Also not directly addressed in the link is till versus no-till influence on the parameters evaluated. And it seems pretty clear that much of this will depend of what kind of plants are being used to evaluate plant numbers, spacing, ground cover, etc. with regard to water needs and usage.
On water use: "The potential of narrow rows to increase yields by improving water uptake is less clear. Barbieri et al. (2012) found that narrow rows increased water uptake during the early stages of crop growth, likely due to deeper and more uniform distribution of roots in the soil profile, but this advantage diminished as the season progressed. Total seasonal crop evapotranspiration ultimately did not differ between row spacings. Conversely, Sharratt and McWilliams (2005) found that narrow-row corn did have greater total soil water extraction in 1 year of a 2-year study......The effect of corn row spacing on water use likely depends on moisture availability patterns during the growing season. In cases where drought stress persists during the growing season, increased water extraction early may reduce water that is available later in the season. Increased early water uptake may have the added effect of creating greater demand for water later in the season due to improved early crop growth. If water is not limited later in the season, the greater early uptake may be advantageous for the crop. However, research does not indicate any broad advantage to narrow-row corn under drought stress conditions."