Water Management-3 Point wise notes for competitive exam

Water Management

• 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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