• Irrigation efficiency at the field level can be increased by selecting suitable method of irrigation, adequate land preparation and engaging an efficient irrigator.
• At project level, irrigation efficiency can be increased by proper conveyance and disitribution system.
• Irrigation efficiency is the ratio usually expressed as per cent of the volume of irrigation water transpired by plants, plus that evaporated from the soil, plus that necessary to regulate the salt concentration in the soil solution and that used by the plant in building plant tissue to the total volume of water diverted, stored or pumped for irrigation.
Ei=Wt + Ws – Re / Wi X 100
Ei = irrigation efficiency
Wt = volume of irrigation water per unit area of land transpired by plants, evaporated from the soil during the crop period (including field preparation and nursery)
Ws = volume of irrigation water per unit area of land to regulate the salt content of soil solution
Re = Effective rainfall
Wi = Volume of water per unit area of land that is stored in a reservoir or diverted for irrigation
• Efficiency of irrigation projects in India is as low as 20 to 40 per cent.
• Major project can irrigate more than 10,000 ha.
• Medium project can irrigate an area between 2,000 to 10,000 ha.
• Minor irrigation project is an irrigation project with a capacity to irrigate less than 2,000 ha.
• Conveyance efficiency indicates the efficiency with which water is conveyed from source of supply to the field. Conveyance efficiency estimates conveyance losses.
Ec = Wf / Ws X 100
Ec = water conveyance efficiency (per cent)
Wf = water delivered at the field
Ws = water delivered at the source
• Water application efficiency is the measure of efficiency with which water delivered to the field is stored in the root zone.
Water application efficiency = Water stored in the root zone / Water needed in the root zone × 100
• Water storage efficiency is expressed as the percentage of water needed in the root zone prior to irrigation to that stored in the root zone during irrigation.
Water storage efficiency = Water stored in the root zone / Water needed in the root zone × 100
• Water distribution efficiency is defined as the percentage of difference from the unity of the ratio between the average numerical deviation from the average depth stored during the irrigation.
Water distribution efficiency = [1-Y / d] X 100
d = average depth of penetration along the run during irrigation
y = average numerical deviation from d
• Water productivity (WP) is used to define the relationship between crop produced and the amount of water involved in crop production, expressed as crop production per unit volume of water.
WP = Grain or seed yield/water applied to the field
• Water use efficiency is defined as the yield of marketable crop produced per unit of water used in evapotranspiration.
WUE = Y/ET
WUE = water-use efficiency (kg/ha mm of water)
Y = marketable yield (kg /ha)
ET = evapotranspiration (mm)
• Yield is more influenced by crop management practices.
• ET is mainly dependent on climate and soil moisture.
• Fertilizers and other cultural practices usually increase WUE, because they relatively increase crop yield more than crop water use.
• Water-use efficiency is highest in finger millet (13.4 kg/ha mm) followed by
wheat (12.6 kg/ha-mm),
groundnut (9.2 kg/ha-mm),
sorghum (9.0 kg/ha-mm),
Maize = pearl millet (8.0 kg/ha-mm) and lowest in rice (3.0 kg/ha-mm).
• Factors affecting water use efficiency are
1) Nature of the plant
2) Climatic conditions
3) Soil moisture content
5) Plant population
• Under adequate irrigation, application of fertilizers increases yields considerably, with small increase in ET and improves WUE.
• Highest yield and WUE is possible only through optimum levels of soil moisture regime, plant population and fertilization.
• Irrigation period is the number of days that can be allowed for applying one irrigation to a given area during the peak consumption use period of the crop that is irrigated.
Irrigation period = Net irrigation requirement/peak use rate
• Duty of water is the quantity of water required for irrigation to bring a crop to maturity.
• Base period is the period of irrigation which crop requires for full maturity.
• Delta is the total depth of water required by a crop during the entire period the crop is in the field.
• Duty is the area irrigated by one cusec discharge of water during the crop period. It is equal to twice the base divided by delta.
• Duty of water is the total volume of irrigation water required for a particular type of crop to mature. It includes consumptive use, evaporation and seepage from ditches and canals, and the water eventually returned to streams by percolation and surface runoff.
Duty of water = 8.64 × Base period/Delta
• The main soluble constituents of irrigation water are calcium, magnesium, sodium as cations and chloride, sulphate, bicarbonate as anions.
• Other ions present in minute quantities in irrigation water are boron, selenium, molybdenum and fluorine which are harmful to animals fed on plants grown on excess concentration of these ions.
• Quality of irrigation water is judged by three parameters
1) Total salt concentration
2) Sodium adsorption ratio
3) Bicarbonate and boron content
• Salt content of irrigation water is measured as electrical conductivity (EC).
• Water containing total dissolved salts to the extent of more than 1.5 dS/m has been classified as saline.
• Saline waters are those which have sodium chloride as the predominant salt.
• Brackish water contains more of salts other than sodium chloride.
• Brackish water is one that is contaminated with acids, bases, salts or organic matter, whereas saline water contains mainly dissolved salts.
• In addition to EC, to determine the quality of irrigation water, sodium adsorption ratio (SAR), residual sodium carbonate (RSC) and boron content are also used to find suitability of irrigation water.
• Residual Sodium Carbonate (RSC) is an indicator of the tendency to precipitate Ca as CaCO3 in irrigation water.
RSC (me/l) = (CO3– + HCO3–) – (Ca2+ + Mg2+)
• Irrigation water is considered safe if its RSC is < 1.25 and unsafe at RSC > 2.5.
• Irrigation water which contains more than 3 ppm boron is harmful to crops, especially on light soils.
• Water quality of most of the Indian rivers is good with EC values < 0.7 dS/m except in Krishna (1.4), Hagari (1.6) and Tungabhadra (1.7) rivers.
• Water quality in tanks and lakes is good.
• Quality of ground water is influenced by soil characteristics, water table and rainfall of the region.
• Water quality in semi arid and arid regions is generally poor with high salt content.
• Crop growth in soils irrigated with poor quality water decreases due to increased osmotic stress and poor physical condition of highly dispersed sodic soils.
• Salinity delays flowering and reduces flower number.
• Irrigation with saline water from fruiting to maturity has less influence on yield’.
• Sodium and chloride of irrigation water is retained in stems in melons.
• Reclamation of alkali soil is more difficult than saline soil because alkali soils have very low permeability.
• Permeameter is a device for measuring the permeability of soils or other materials.
• Exchangeable sodium has to be replaced by calcium and the replaced sodium has to be leached down to lower layers.
• Replacement of excessive exchangeable sodium can be done by any soluble source of calcium and magnesium salts such as calcium chloride, magnesium chloride or gypsum.
• Calcium and magnesium chlorides are too expensive.
• However, gypsum by virtue of its low solubility and cost is quite suitable.
• In calcareous alkali soils, some acid forming reagents can also bring about reclamation by solubilising calcium carbonate thus creating acidity.
• Acidifying materials are
2) Sulphuric acid
3) Iron sulphate
4) Aluminum sulphate
5) Lime sulphur
• In the above materials, sulphur acts slowly because it has to be oxidized by microbial activity before it could act as an ameliorating agent.
• The quantity of amendment necessary for reclamation of any area depends on the quantity of exchangeable sodium present on the clay surface.
• To replace each milliequivalent of sodium by calcium in an area of 1 hectare for a soil depth of 30 cm requires 4.1 tonnes of gypsum.
• Barley, sugarbeet, mustard, cotton, turnips, beetroot, datepalm, coconut etc. are salt tolerant crops.
• Sorghum, pearl millet, finger millet, rice, castor etc. are semi tolerant to salinity.
• Application of FYM or incorporation of green manure crops helps in reducing the adverse effects of irrigation with poor quality water.
• Under salinity conditions, planting seeds on the side of the ridge helps in better germination than those planted on the top of the ridge.
• Heavy water (D2O) is the water in which hydrogen has been replaced by deuterium (hydrogen isotope of mass 2).
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