• Probe consists of boron trifuoride (BF3) as a detector.
• Drawbacks of neutron moisture meter are that it is expensive and moisture content from shallow top layers cannot be estimated.
• Fast neutrons are also slowed down by other sources of hydrogen (present in organic matter), chlorine, boron and iron overestimating the soil moisture content.
• Pressure membrane and pressure plate apparatus is generally used to estimate field capacity, permanent wilting point and moisture content at different pressures.
• Time domain reflectometry (TDR) is a relatively new highly accurate and automatable method for measurement of soil water content and electrical conductivity.
• Water content is inferred from the dielectric permittivity or dielectric constant of the medium, whereas electrical conductivity is inferred from TDR signal attenuation.
• The first application of TDR to soil water measurements was reported by Topp et al. (1980).
• The main advantages of TDR over other soil water content measurement methods are
1) Superior accuracy to within 1 or 2% volumetric water content.
2) Calibration requirements are minimal – in many cases soilspecific calibration is not needed.
3) Lack of radiation hazard associated with neutron probe or gamma – attenuation techniques.
4) TDR has excellent spatial and temporal resolution.
5) Measurements are simple to obtain, and the method is capable of providing continuous measurements through automation and multiplexing.
• The energy status of water and amount of water in the soil are related with the soil moisture characteristic curve.
• As the energy status of water decreases (moving towards more negative values) soil water content also decreases.
• As soil moisture content decreases, more energy has to be applied to extract moisture from the soil.
• The relation between suction (externally applied force) and water content of the soil are represented graphically by a curve which is known as soil moisture characteristic curve.
• The shape of the clay soil curve is almost a straight line with bends on ends while it is L shaped in case of sandy soil.
• The moisture content at a given suction is greater in desorption than in sorption and this phenomenon is known as hysteresis.
• Generally, desorption curve of soil moisture is determined in laboratory.
• Water is needed mainly to meet the demands of evaporation (E), transpiration (T) and metabolic needs of the plants, all together known as consumptive use (CU).
• Since water used in the metabolic activities of the plant is negligible, being only less than 1 % of the quantity of water passing through the plant, ET is practically considered as equal to consumptive use (CU).
• Water requirement = CU + application losses + water needed for special operations.
• Water requirement = Irrigation requirement + effective rainfall + soil profile contribution.
WR = IR + ER + S
• Evapotranspiration and crop growth are directly related in several crops.
• Relationship between dry matter of crop and ET is linear.
• Relationship between ET and crop yield is linear in case of Cereals.
• Relationship between ET and crop yield is quadratic in case of Pulses.
• Reference evapotranspiration can be defined as the rate of evapotranspiration of an extended surface of an 8 to 15 cm tall green grass cover, actively growing completely shading the ground and not short of water.
• Modified Blaney-Criddle method is simple, easy to calculate and require only temperature data of the region.
• Modified Penman method is complicated, more reliable with a possible error of 10 per cent only.
• Possible error for pan evaporation is 15%
Radiation method = 20%
Modified Blaney – Criddle method = 25%
• Modified Blaney-Criddle Method:
ETo = C [P (0.46 T + 8)]
• Radiation method:
ETo = C (W.Rs)
Rs = (0.25 + 0.50 n/N) Ra
• Pan evaporation method:
ETo = Kp.Epan
• Pan evaporation method is simple, fairly reliable and inexpensive and can be adopted under Indian conditions.
• Modified Penman method
ETo = c [ W.Rn + (1 – W). f(U) (ea – ed)]
• Crop coefficient is the ratio between evapotranspiration of crop (ETc) and potential evapotranspiration (ETo).
• Crop coefficient depends on soil cover, soil moisture and crop height.
ET (crop) = Kc × ETo
• ET (crop) is also known as maximum evapotranspiration (ETmax).
• 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 crop emergence to a maximum value during the period when the crop reaches flowering, and declines as the crop approaches maturity.
• Actual evapotranspiration depends on both climate and soil moisture.
• Maximum evapotranspiration depends on climate only as full soil moisture supply is assumed.
• Actual evapotranspiration equals maximum evapotranspiration when available water to the crop is adequate.
• Actual evapotranspiration is measured from field experiments by estimating soil moisture loss during crop growth period.
• Actual evapotranspiration (ETa) is measured accurately with Lysimeters.
• A lysimeter is a container of soil that facilitates the measurements of gains and losses of soil water by weight during crop growth.
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