Rainfall Runoff Relationship

How to derive Hydrograph from discharge from rainfall measurement. Sharing an example from lab base study. The derivation of relationships between the rainfall over a catchment area and the resulting flow in a river is a fundamental problem for the hydrologist. To facilitate comparisons it is usual to express values for rainfall and river discharge in similar terms. The amount of precipitation (rain, snow, etc.) falling on a catchment area is normally expressed in millimetres (mm) depth, but may be converted into a total volume of water, cubic metres (m3) falling on the catchment. Alternatively, the river discharge (flow rate), measured in cubic metres per second (m3/s) for a comparable time period may be converted into total volume (m3) and expressed as an equivalent depth of water (mm) over the catchment area. The discharge, often termed runoff for the defined period of time, is then easily compared with rainfall depths over the same time period. Estimating runoff or discharge from rainfall measurements is very much dependent on the timescale being considered. For short durations (hours) the complex interrelationship between rainfall and runoff is not easily defined, but as the time period lengthens, the connection becomes simpler until, on an annual basis, a straight-line correlation may be obtained. Note: Credits to Pn Zainab Mohamed Yusof co-teaching the same subject..