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Use of double mass curve analysis | Test for the consistency of records (Double mass curve)

Use of double mass curve analysis

The double mass curve is used to check the consistency of many kinds of hydrological data by comparing data for a single station with that of pattern composed of the data from the several other station in the area. The double mass curve can be used to adjust inconsistent precipitation data.

The theory of double mass curve is based upon the fact that a plot of the two cumulative quantities during the same period exhibit a straight line so long as the proportionality between the two remains unchanged and the slope of the line represent the proportionality. This method can be smooth at a time series and suppress random elements in the series. In recent 30 yrs, Chinese scholars analyzed the effects of soil and water conservation measures and land use/cover changes on runoff and sediments using double mass curve method and have achieved the good results. In this study, double-mass curves of precipitation vs sediments are plotted for the two contrastive periods to estimates change in regression slopes to quantify the overall efficiency of the soil conservation measures before and after transition years. 


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Test for the consistency of records (Double mass curve)

If the conditions relevant to the recording of a rain gauge station have undergone a significant changes during the periods of record, inconsistency would arise in rainfall data of that station. This inconsistency would be felt from the time significant changes took place. Some of the common causes for the inconsistency of records are:

  1. Shifting of the rain gauge station to a new location.
  2. The neighborhood of the station undergoing a marked change.
  3. Change in the ecosystem due to calamities such as forest fires, landslides and 
  4. Occurrences of observation errors from a certain dates.
The checking for the inconsistency of the records is done by the double mass curve techniques. This techniques is based upon the principal that when each recorded data comes from the same parent population, they are consistent.

A group of five to ten base stations in the neighborhood of the problems station x is selected. The data of the annual (or monthly or seasonal mean ) rainfall of the station x and also the average rainfall of the group of the base station covering a long period is arranged in the reverse chronological order ( i.e. the latest record as the first entry and the oldest record as the last entry in the list ). The accumulated precipitation of the station X (i.e. Σ Px ) and the accumulated values of the average of the groups of the base stations (i.e. Σ Pav for various consecutive time periods. A decided break in the slope of the resulting plot indicates a change in precipitation regime of the station X. The precipitation values at station X beyond the period of change is corrected by using the relation.

Fig. Double mass curve



Snow gauges

Like rain gauges, snow gauges receptacles to catch precipitation as it falls in specified sampling area. Here a large cylindrical receiver 203mm in diameter is used to collect the snow as it falls. The height of the cylinder depends upon the snow storage needed at a spot as a consequences of accessibility etc. and may range from the 60cm to several meters. The receiver is mounted on a tower to keep the rim of the gauge above the anticipated maximum depth of accumulated snow in the area. The top of the cylinder is usually a funnel like fulcrum of the cone with side slope not less than 1H:6V, to maximize the deposit of ice on the exterior of the gauge. Also a windshield is provided at the top. Melting agent or heating system are sometimes provided in the remote snow gauges to reduce the size of the containers. The snow collected in the cylinder is brought to the warm room and the snow melted by adding a pre-measured quantity of hot water. Through weighting or by volume measurements, the water equivalent of snow is ascertained and recorded.


Snow tubes 

Water equivalent of the accumulated snow is measured by the means of snow tubes which are essentially a set of telescopic metal tubes. While a tube size of 40mm diameter is in normal use, higher sizes up to 90mm diameter are also in use. The main tube is provided with cutter edge for easy penetration as well as to enable extracting of core sample . Additional lengths of the tube can be attached to the main tubes depending upon the depth of snow. 

 To extract the sample, a tube is driven into the snow deposits till it reaches the bottom of the deposits and then twisted and turned to cut a core. The core is extracted carefully and studied for its physical properties and then melted to obtain water equivalent of the snow core. Obviously, a large numbers of samples are needed to obtain the representative values for the large area deposits. Usually, the sampling is done along an established route with specified location called snow course


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Location: Kathmandu 44600, Nepal

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