CONSISTENCY OF RAINFALL RECORD
Precipitation puts water in the watershed. Precipitation
measurements help determine water availability for
evaporation and streamflow, and the risk of forest fires,
landslides, and soil erosion. Precipitation is liquid (rain)
or frozen (sleet, hail, graupel, snow) water or a
combination of both falling from the sky.
Precipitation is measured as depth of water that would
accumulate on a horizontal surface. Typical recorded
values for precipitation are the daily, monthly, and
annual totals (depth of water in millimeters), as well as
storm total, maximum intensity (millimeters per minute
or per hour), and duration (hours).
Precipitation is measured at a point using a manual or
automatic recording gauge and over an area using
meteorological radar (e.g., Doppler) or satellite images.
Measurement of rainfall with a gauge is less prone to
error than the measurement of solid precipitation
because it is less susceptible to the influence of wind on
“catch” by gauges. Some gauges are suitable only for
measuring rainfall, and others are used only for solid
precipitation.
Very few can measure both forms reliably. Precipitation
is usually not uniform in spatial distribution, intensity, or
duration within a storm.
Wind flow interacting with watershed topography also
affects the distribution of precipitation. The standard
technique for measuring rainfall is a plastic or metal
cylinder with a sharp edge and funnel-like cover to
minimize evaporation.
Many studies require long term rainfall data; therefore,
a test must be conducted to check homogeneity or self
consistency of the rainfall record. This is necessary
because over a period of time, it may happen that there
be some obstructions (trees, buildings) may have
emerged after the installation of gage or its location
might have changed, or observational procedure might
have changed.
The inconsistency of rainfall record can be checked by
graphical or statistical methods including double mass
curve, the von Neumann ratio test, cumulative deviation,
run test.
Double mass curve method is one of the most common
and widely accepted methods for checking the
consistency of rainfall record.
DOUBLE-MASS ANALYSIS
This method is based on the assumption that the mean
accumulated precipitation for a large group of stations is
not significantly affected by a change or changes in
individual stations. If we plot the mean accumulated
precipitation for several stations against the
accumulated precipitation of the record for the station
that needs to be adjusted, any change in slope will
indicate a “break” in the station record.
