Remove the ends from the rhubarb and cut into thin slices. Add the water and rhubarb to a small pan and place over a medium heat.
Once the mixture comes to a simmer, reduce the heat and simmer for approximately 15 minutes, partially covered, until the rhubarb is soft and the water has taken on some of its color. Try not to stir. If the liquid reduces too quickly, you can add a little more but it should only just cover the rhubarb.
As the rhubarb is cooking, line a fine strainer with cheesecloth (a double layer is good) and place over a measuring jug or bowl. Once the rhubarb is very soft, strain the mixture through the cheescloth and let it drain into the jug/bowl below. Leave it to drain through a minute or two but don't press down on the rhubarb (as this can make the mixture cloudy).
Once drained, discard the rhubarb (or you can use it in something else) and measure out the liquid then return it to the cleaned out pan. You should have around ½ cup (120ml), in which case add ½ cup sugar. If it's less, then use the equivalent volume of sugar.
Return the mixture to the heat and warm gently, just enough to dissolve the sugar, then remove from the heat and allow to cool slightly before storing in a clean bottle or jar. Store in a cool place away from daylight if using soon, or else in the fridge for slightly longer storage.