Question

The areal characteristics of a rain storm are represented by a-

• A. DAD curve
• B. Hyetograph
• C. Mass curve
• D. Double mass curve

Answer 1

The correct option is A DAD curve

(i )Depth area duration (DAD) relationship

• Depth of rainfall at a rain gauge station is called point rainfall.

A depth area duration curve expresses graphically the relation between progressively decreasing average depth of rainfall and progressively increasing area from the centre of the storm outward to its edge for a given duration of rainfall.

• For a rainfall of a given duration, the average depth decreases with the area is an exponential function given by

$\overline{P}=P_0{e}^{-KA"}$

$\overline{P}={\text{Average depth in catchment over area A (km}}^2)$

$P_0=\text{Highest amount of rainfall in catchment at the storm curve.}$

$k,n=\text{Constant.}$

(ii) Double mass curve:

• Inconsistency of record is corrected by using double mass curve technique.

• Thus after correction, the previous record becomes consistent with present day environmental and land use condition.

• $\text{Double mass curve plots between accumulated precipitation of the station (i.e.}\sum P_x\text{) on y axis v/s the accumulated values of the average of the group of base stations (i.e.}\sum P_{\text{avg}})$

• Some of the common cause for inconsistency of the records are

• Shifting of rain gauge station to a new location.

• Neighbourhood of the station undergoing a marked change.

• Replacement of old instrument by new one.

• Change in the ecosystem due to climate such as forest fires, landslides etc.

• Occurrence of observational error from a certain data.