Precipitation

The precipitation (P) is, besides the discharge (R) and the evaporation (E), a further number in the basic hydrological equation.

According to the common equation of the water balance P = R + E for a longer period of time, the equation P = R + E + (S - U) applies for shorter periods of time, considering changes in storage. A considerable part of the precipitation evaporates as steam.

Forms of Precipitation

Condensation causes water drops of a size from 0.005 - 0.01 mm. For the causing of precipitation at first bigger drops must be formed in a cloud. There is precipitation in form of rain, drizzle, fog and dew. Snow, hail and soft hail are hardly relevant in the tropics and subtropics. The different types of precipitation are distinguished by their diameter of drops and the consequential falling speed.

Forms of Precipitation
Form of Precipitation Diameter of Drop in mm
Drizzle 0.08 - 0.6
Little drops 0.6 - 1.4
Big drops 1.4 - 6.0

The falling speed of the drops is between 2.7 and 8.0 m/s.

Purpose of Precipitation Measurement

In order to establish water balances it is essential to measure the precipitation, since it is one of three components of the water balance. The knowledge of precipitation depth and distribution of precipitation are important to be able to make flood forecasts based on precipitation events. Further areas of application are irrigation, drainage, power generation by water power, erosion protection and salination of soil.
Isohytes for monthly and yearly precipitation can be provided by measuring precipitation at different places. Based on these maps, the precipitation for other places can be interpolated.

Measurement of Precipitation

The precipitation is measured with rain gauges according to Hellmann that have a standardized WMO collection area of 200 cm². It is measured 1.0 or 1.5 m above the ground (FAO and WMO guideline) and read out once per day. Wind/rain must not be interfered by adjacent buildings and trees. The minimum distance between the rain gauge recorder and buildings or trees is equal to their height.
The rain gauge recorder according to Hellmann has a standard funnel of 200 cm² (WMO guideline). The collected precipitation flows over a pipe into a container that is equipped with a siphon. Every time the precipitation reaches 10 mm, the self-activated mechanism tips the collected precipitation into a container. A continuously working recorder writes down the precipitation amount.
The recording rain gauge according to Hellmann records the precipitation continuously on graph paper for a duration of up to one month, and it is therefore suitable for remote areas where a daily reading is not possible. A further advantage of the continuous record is the possibility to determine the precipitation intensity (mm/min) on the basis of the rise of the curve.

Units

mm/min; mm/h; mm/month; mm/a;
l/(s x ha);

Analysis

Isohytes for monthly and annual precipitation
Monthly and annual precipitation
Interval of recurrence of precipitation / depth of precipitation and duration
Precipitation hydrograph, cumulative polygon of precipitation