It is the study of the processes of precipitation (rainfall, mostly) and evaporation of water from the earth?s surface. Precipitation will be the basic source of water that then either evaporates, or else moves to the streams and rivers, or else seeps into the ground and merges into groundwater. Understanding these processes is vital for the management of surface water and groundwater resources.
Precipitation (and its Measurement)
?Precipitation? is the condensation of water vapour from the atmosphere, and its falling to the earth?s surface. This mostly occurs as rainfall, but also snowfall and hail are forms of precipitation.
Annual rainfall over India varies considerably ? see Map showing ?normal? annual rainfall in centimeters as published by the IMD.
Rainfall is measured basically by collection of rain in a container at the earth?s surface, with the volume of water caught measured at the same time each morning. This is the ?standard? raingauge and there are many thousands of these devices used in India, many with records going back over 100 years. The IMD maintains the master records of these daily-read raingauges, but many Agencies actually manage the network and provide data to the IMD.
For understanding floods and problems with heavy rainfall, the rate at which rain is falling is important, and so there are also automatic raingauges to record rainfall intensity. These are generally of ?tipping bucket? type whereby rain entering the funnel at the top of the gauge drops into a balanced bucket that tips over when a specific volume of water has passed into it, and the tip of the bucket is recorded.
In additional to measurement of rainfall, there is a need to measure snow. This is normally done not as it falls, but as accumulation on the ground. Instruments are used to measure the depth of snow (bouncing a radar beam off the top of the snow) as well as the weight of the snowpack as the density of the snow varies a lot through the year ? initially the snow is light but it gets denser as the weight of snow above compresses the pack, and as it starts to melt it gets even more dense. The weight of snowpack is usually measured using snow pillows ? with the pressure of fluid within the pillow indicating the weight of snow resting on it.
Evaporation and its Measurement
The rate of water lost through evaporation from plants, water surfaces and the ground is a large part of the amount of precipitation falling. This loss of water is important to know accurately to understand the amount of water likely to become flow in rivers, or to seep into the groundwater.
There are a number of devices used to measure evaporation ? the most common being an evaporation pan which is topped up each day after recording the depth of water in the pan. The measured loss to evaporation (after adjusting for rainfall) is an approximate measure of the evaporation rate.
A more reliable method of measuring evaporation has been proven to be using measurement of the energy balance at the earth?s surface and using this to determine the energy available for evaporation ? and thus the potential evaporation rate (potential because if there is not enough water, not all available energy can be used for evaporation). This method uses measurements of temperature, humidity, incoming solar radiation and wind speed and these are measured within a Hydrometeorological climate station ? or using an automatic weather station.
A full climate station
An automatic weather station
Crop Water Needs
When rainfall is insufficient to meet a crop?s water requirements, irrigation water can be provided to avoid water stress in the crop and to get better yields. The water requirements are calculated using knowledge of the expected rainfall, and the potential evaporation rates in the area. The amount of water needed from both these sources is calculated based on corrections to the calculated potential evaporation to allow for changes to the energy balance due to the crops presence (giving an estimate of the ?reference crop evaporation?) and then using crop factors to allow for the characteristics of the crop and its stage of growth. There is an authoritative and practical guide to such calculations provided by the FAO (see website: http://www.fao.org/docrep/x0490e/x0490e00.htm )