🕑 Reading time: 1 minute
Analysis of Rainfall DataBefore using rainfall records at a station for any analysis, the data has to be checked for
Estimation of Missing Rainfall DataRequired to find the missing annual precipitation at station X (not included in the m neighbouring stations), given the annual precipitation values at m neighbouring raingauge stations . Let the normal annual precipitation at the m+1 stations be If the normal precipitations at the different stations are within 10% of the normal annual precipitation at station X, then If the normal precipitation at any of the m stations vary by more than 10% from the normal annual precipitation at station X, then is estimated by the normal ratio method as
Test for Consistency of RecordInconsistency would arise in the rainfall data at a raingauge station if conditions relevant to recording of rainfall at the station undergoes significant change during the period of record. It would be felt from the time a significant change took place. Some common causes for inconsistency of record
- Shifting of the raingauge to a new location
- Neighbourhood of the station is undergoing significant changes
- Change in the ecosystem due to natural calamities such as forest fires, landslides etc
- Errors in observation from a certain date
- Let X be the station where inconsistency in rainfall records is observed.
- Select a group of about 5 to 10 base stations in the neighbourhood of station X.
- Data of annual or monthly mean rainfall of station X as well as the average rainfall of the group of base stations over a long time period is arranged in reverse chronological order ie. the latest record is the first entry and the oldest record is the last entry in the list.
- Accumulated precipitation at station X and the accumulated values of the average precipitation of the group of base stations are computed from the latest records.
- A plot of vs for various consecutive time periods is prepared.
- A break in the slope of this plot indicates a change in the precipitation regime of station X.
- Precipitation values at X beyond the period of change of regime is corrected as
Figure: Double mass curveThus older records are brought to the new regime at X.
- The more homogeneous the base station records are, the more accurate will be the corrected values at station X.
- A change in slope is taken as significant only if it persists for more than 5 years.
- The double mass curve technique is also useful in checking arithmetical errors in transferring rainfall data from one record to another.
Presentation of Rainfall Data
Mass Curve of Rainfall
- It is a plot of accumulated precipitation against time, plotted in chronological order.
- Records of float type, weighing bucket type etc raingauges are of this form.
- It gives information on duration and magnitude of a storm.Intensity at various time intervals in a storm = slope of the curve.
- It can be prepared for non-recording raingauges also if the approximate start and end of a storm are known.
Figure: Mass curve of rainfall
- It is a plot of rainfall intensity against time interval.
- Time interval depends on the purpose (urban drainage problems – small durations, flood flow computations from large catchments~ 6h).
- It can be derived from the mass curve of rainfall.
- It is represented as a bar chart.
- It is useful in developing design storms to predict extreme floods.
- Area under a hyetograph = total precipitation received in that time period.
Figure: Hyetograph of a storm