Tools can also measure the water flow and suspended sediment concentration from catchments and river basins. Gauging stations are installed along a stream or river channel and devices measure the suspended sediment load of the water at set time intervals. Sediment yield at the measurement point is expressed as the mass of sediment per area of the catchment for a given time period (for example, tkm-1 yr-1). This method can be used in combination with gross estimates of erosion rates in the catchment to calculate the ratio delivered at the basin outlet to gross erosion within the basin – known as the sediment delivery ratio. The limitations of using this tool in isolation are: i) The sediment measured at the outlet is only a fraction of that actually eroded in the catchment, ii) sediment on hillslopes and along river courses causes a time lag between erosion in the catchment and sediment measurement and iii) the sediment transported in the river is derived from sources other than hillslope erosion (FAO, 2019)