Rainfall is the most important single factor in determining whether runoff will or will not occur for a given set of environmental conditions. It determines runoff amount and frequency. There are two measurements of rainfall amount that are commonly collected for hydrological purposes: Daily and (runoff) Event rainfall.
Daily Rainfall is probably the most ubiquitously measured meteorological variable. It is the rain that falls awing a 24 how period starting in the morning of one day (commonly 06:00, 07:00 or 08:00 furs) until measurement is made at the same time the following day. Event Rainfall by contrast is the rainfall occurring awing an unspecified time period usually, but not always less than 24 hours, that can be seen to be responsible for subsequent runoff. The collection and use of each has advantages and disadvantages.
In the case of daily rainfall, data are usually available from many stations, even in countries with only the most basic meteorological network. The equipment to measure daily rainfall is relatively cheap, simple to install, read and maintain. All projects should easily achieve adequate instrumentation. In most cases, many years of historical data will be available for analysis from a variety of sources, in addition to that obtained from meteorological offices: these sources include various government departments, water resource and construction projects, state and private farms, schools and interested individuals. Often basic analyses will have been performed on the data (average monthly and annual totals, spatial distribution, etc.). For the analysis of runoff relations, however, daily rainfall can have one serious drawback. It is the lump sum rainfall awing a 24 how period and in some climatic environments may greatly exaggerate the amount of rainfall thought to be responsible for runoff, but despite this drawback, it is the most commonly used climatic variable in runoff studies.
Event rainfall, obtained from the careful examination of the records of an automatically recording rain gauge, can provide a precise and accurate evaluation of the rainfall responsible for runoff and it is often to be preferred for rainfall/runoff analyses. However, recording rain gauges are not usually in widespread use except at important synoptic stations (especially in developing countries). They are expensive to buy, can be difficult to maintain and staff must have a higher level of expertise to operate them. The analysis of data is more complex and time-consuming.
