Perhaps the most obvious sign of water deer activity in an area is tufts of white/cream/buff hair, hollow and measuring 40-55 mm (about 1.5-2 in.) in length, on the ground during the winter. Based on Arnold Cooke and Lynne Farrell's observations at Woodwalton Fen, these tufts start appearing during October as the bucks establish their territories in preparation for the rut, with most found in December. This correlates with my observations in Buckinghamshire, where fur tufts seem most apparent during December and January.
Meticulous studies at Woodwalton Fen and Monks Wood - both in Cambridgeshire - by Cooke and Farrell are the source of much that we know of water deer field signs today. The cleaves of the hoofs are approximately similar in length, with straight inner edges. The slot marks left behind are intermediate in size between muntjac and roe; typically 4-5 cm (1.6-2 in.) long by 3-4 cm (1.2-1.6 in.) wide, larger in soft mud or snow.
During their studies at Woodwalton, Cooke and Farrell observed that muntjac tended to walk across tracks between cover; they also had a stride that placed the slots from the right feet exactly between those on the left. Water deer, by contrast, preferred to walk the length of rides and had the slots from the right slightly behind the centre of those on the left. The registration of the tracks (i.e., the degree to which the rear foot steps into the slot left by the corresponding forefoot) while walking is “near perfect” in water deer according to Rowland Brown, Michael Lawrence and Joyce Pope in their 1989 book Animals: Tracks, Trails and Signs. The stride length extends 30-40 cm (12-16 in.), they will use regular paths some 20-25 cm (8-10 in.) wide and, where the path tunnels through reeds or other lanky vegetation, about 50 cm (just over 1.5 ft.) tall.
Droppings are very similar to those of muntjac. Indeed, when discussing the difference, Arnold Cooke told me “I'm never totally confident in distinguishing their dung”. Nonetheless, the pellets produced by water deer are typically slightly longer than those of muntjac, about 10-15 mm long by 5-10 mm (about 0.5 x 0.3 in.) wide, and more cylindrical in shape. One end is pointed and the other slightly rounded, and they're dark brown or black in colour.
Unlike muntjac, which tend to leave droppings in latrines, those of water deer tend to be delivered in distinct defaecations ranging from a handful of pellets to more than one hundred. Indeed, during the rut, droppings are deposited by males in small scrapes to signify territory ownership. Bucks will scrape and defaecate throughout the day and night, resulting in each scrape containing only a few pellets as they try to maximise their reach. We know little about the defaecation behaviour of does. Anecdotally, during one camera trapping session in Buckinghamshire, I observed an adult female frequently using two spots a metre or so apart in approximately the same area of rush bed frequently to deposit pellets. Over 18 urination/defaecation events by this doe that were captured by the camera in the winter of 2020/2021, 13 (72%) were in one (or both) of these two spots.
Water deer spend a significant amount of their time lying up, resting and ruminating (see: Activity). This produces an oval-shaped area of flattened vegetation called a bed or couch. In a sample of couches at one fenland site, Arnold Cooke observed that the average size was 60cm by 40 cm (24 x 16 in.), which corresponds with those I have found in Buckinghamshire. Similarly, Cooke notes that the couches often contained droppings, which I have also observed. I know of no data on whether couches are re-used, either by the same deer or different indviduals.
Finally, while feeding, deer bite their lower incisors against a tough gum pad that replaces the upper incisors present in other mammals. This results in ragged stalks, or leaves with sections missing, which allows their feeding activity to be separated from those of tidier feeders, such as the lagomorphs (rabbits and hares) that bite tooth on tooth. Where the deer take bramble, Arnold Cooke notes that they tend to take the tips of the leaves, leaving behind half a leaf with its rib system exposed. Nonetheless, it is difficult, if not impossible, to distinguish the vegetation damage inflicted by water deer from that caused by Reeves' muntjac, particularly given that the two feed in the same way, often share the same habitat, and are of similar size.