Water use efficiency (WUE) is the ability of vegetation carbon-uptake at the expense of water transpired or total water supply. The traditional WUE metrics like the rain-use-efficiency (RUE) are likely subject to certain biases since they essentially account for surface and subsurface runoff as a water supply to vegetation systems. Here, we develop a new catchment-scale WUE based on the coupled water and carbon balance concept, and apply it to more than 400 near-natural catchments across conterminous US. We are able to analytically identify the major controlling factors then validate using the real observations. We will further examine the spatio-temporal variations of WUE in both growing- and non-growing seasons across gradients of climate conditions, vegetation types and landscape properties.