ABSTRACT
Precipitation is the main input of water within a catchment water balance. Its measurement is fraught with difficulties and any small errors will be magnified enormously at the catchment scale. It is also highly variable in time and space. Despite these difficulties, precipitation is one of the most regularly measured hydrological variables, and good rainfall records are available for many regions in the world. In this chapter we describe different types of precipitation (e.g. rain, drizzle, snow, hail) and the fundamental process of precipitation formation. We introduce different mechanisms of precipitation (cyclonic, frontal, orographic, convective) and the factors that determine the spatial distribution of precipitation. We introduce methods for measuring precipitation directly (e.g. rain and snow gauges), and also methods for estimating it indirectly (e.g. ground-based radar and satellites). We discuss errors in precipitation measurement and approaches for moving from estimation of precipitation at a point to characterising precipitation over a wider area. We highlight the importance of understanding and analysing rainfall intensity, and finally we briefly consider the water quality aspects of precipitation.
