If you are dehydrated you will typically see a change of stools. Seeing hard pebbles and dry stools is a logical consequence of the body rationing water. Recently, we got a new smart scale ($30 on Amazon) that comes with a smart phone app and does multiple measures, including hydration. I have been running below the desired range, so I am busy hydrating with Gerolsteiner Mineral Water (from Traders Joe) etc.
Conceptually, the availability of water should impact the microbiome. So tonight I started to search out studies to confirm my speculation.
There were five out of the 108 types of intestinal bacterium at the genus level that showed significant differences within and between groups due to water supplementation, and 17 out of 564 species identified by the homology search of OTU representative sequences in the DDBJ 16S database showed the same behavior… Of these, a slight correlation was seen in the intervention group between changes in blood pressure and changes in the Psudoflavonifractor capillosus bacterial count (R = 0.42) Kineothrix sp. (R = 0.36), Feacalibacterium prausnitzii (R = 0.38), and Ruminococcaceae (R = 0.34) showed weak correlations between changes in body temperature and changes in bacterial count
Some related studies that implies impact on the microbiome but failed to do microbiome analysis with the study
- Mild dehydration affects mood in healthy young women, 2011
- Mild dehydration impairs cognitive performance and mood of men. 
- Influence of progressive fluid restriction on mood and physiological markers of dehydration in women. 
- The effect of the consumption of water on the memory and attention of children. 
Dehydration is often reported as a contributor to various conditions
- Myalgic Encephalomyelitis/Chronic Fatigue Syndrome Diagnosis and Management in Young People: A Primer 
Hydration is an often overlooked aspect of improving the microbiome. It has impacts on blood pressure, body temperature, mood and cognitive ability. While we have few studies on it’s microbiome impact, we can likely assume safely that improved hydration will improve the microbiome. It should not be left to a “I drink enough water, I do not feel dehydrated”, but with actual measurements. Improving it is actually a slow process — my goal is to shift from my current level below, to at least 55%.