This attached 2013 paper (Review) should give you a comprehensive review on this topic. Paper title: Root traits contributing to plant productivity under drought
In the article, it mentions: "Root traits associated with maintaining plant productivity under drought include small fine root diameters, long specific root length ,and considerable root length density, especially at depths in soil with available water." They discussed the details of these "parameters" across the paper (see the Figure 1 for outline).