Describe rain gauges with special regard to Tipping bucket rain gauge (slide 01 – 5-11)

Agrohydrology

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Describe rain gauges with special regard to Tipping bucket rain gauge (slide 01 – 5-11)

A rain gauge (осадкомер) is an instrument used by meteorologists and hydrologists to gather and measure the amount of liquid precipitation over a set period of time. There are a lot of different types: optic al, acoustic, weighing, Tipping bucket and classical.

Tipping bucket consist of funnel / tube narrow at the down part --- at the top, for collecting precipitation and sending a signal after water falls into tipping bucket.

Disadvantages: is not as accurate as the standard rain gauge because the rainfall may stop before the lever has tipped.

The advantage: is that the character of the rain (light, medium, or heavy) may be easily obtained. Rainfall character is decided by the total amount of rain that has fallen in a set period (usually 1 hour) and by counting the number of 'clicks' in a 10-minute period the observer can decide the character of the rain.

2. Characterize and describe evapotranspiration pans (slide 01 – 36-38 + инет )

Pan evaporation is a measurement that combines or integrates the effects of several climate elements: temperature, humidity, rain fall, drought dispersion, solar radiation, and wind. Evaporation is greatest on hot, windy, dry, sunny days; and is greatly reduced when clouds block the sun and when air is cool, calm, and humid.[1] Pan evaporation measurements enable farmers and ranchers to understand how much water their crops will need

An evaporation pan is used to hold water during observations for the determination of the quantity of evaporation at a given location. Such pans are of varying sizes and shapes, the most commonly used being circular or square.[3] The best known of the pans are the "Class A" evaporation pan and the "Sunken Colorado Pan".[4] In Europe, India and South Africa, a Symon's Pan (or sometimes Symon's Tank) is used. Often the evaporation pans are automated with water level sensors and a small weather station is located nearby.

“class A“ evaporation pan, namely 54 mm/0.5 m (10 inches) in height and 1206 mm / 1.95 m (47.5 inches) in diameter. The evaporation pan is installed on the wooden support, which is set and levelled on the ground in a grassy location, away from bushes, trees and other obstacles which obstruct a natural air flow around the pan, thus representing open water in an open area. Daily the result of evaporation and precipitation is measured within the still well, by means of a high quality evaporation micrometer with a measuring range of 100 mm and an accuracy of 0.02 mm. This accuracy can be obtained because the still well prevents rippling of the water surface.

The sunken Colorado pan is square, 1 m (3 ft) on a side and 0.5 m (18 in.) deep and made of unpainted galvanized iron. As the name suggests, it is buried in the ground to within about 5 cm (2 in.) of its rim. Evaporation from a Sunken Colorado Pan can be compared with a Class A pan using conversion constants. The pan coefficient, on an annual basis, is about 0.8.[7]

The principle of the evaporation pan measurement is the following: r the pan is installed in the field r the pan is filled with a known quantity of water (the surface area of the pan is known and the water depth is measured) r the water is allowed to evaporate during a certain period of time (usually 24 hours). For example, each morning at 7 o’clock a measurement is taken. The rainfall, if any, is measured simultaneously r after 24 hours, the remaining quantity of water (i.e. water depth) is measured r the amount of evaporation per time unit (the difference between the two measured water depths) is calculated; this is the pan evaporation: E pan (in mm/24 hours) r the E pan is multiplied by a pan coefficient, K pan, to obtain the ETo.

Specify the important definitions: Crop factor (KC)

The crop type, variety and development stage should be considered when assessing the evapotranspiration from crops grown in large, well-managed fields. Differences in resistance to transpiration, crop height, crop roughness, reflection, ground cover and crop rooting characteristics result in different ET levels in different types of crops underidentical environmental conditions.

Crop coefficient is the ratio between crop ET and potential ET. Crop coefficient depends on soil cover, soil moisture and crop height. At early stage crop covers only a fraction of soil and covers as it matures. The rate at which plants grow and cover the soil depends on the crop. In the early stage main component of ET is evaporation, while under fully covered condition transpiration is the main component. ET of a crop depends on plant height and leaf area index. Crop coefficients vary with relative humidity and wind velocity. For most of the crops, Kc value increases from a low value at the time of emergence to a maximum value during the period when the crop reaches flowering and

declines as the crop approaches maturity.