Gravitational water, defined as the water that is held at a potential greater than -1/3 bar and freely drain by gravity.
Capillary water, held by force of attraction between soil particles and water molecules and retained in the between the water potential -1/3 to -31 bars.
Hygroscopic water, water that is held by the soil particles at a suction of more than -31 bars.
Available water, water retained between field capacity (-1/3 bar) to permanent wilting point (-15 bars)
Soil Water Measurement :
Techniques for measuring the soil moisture can be grouped into the following two categories:
Direct measurement: gravimetric or thermo-gravimetric procedure
Indirect measurement : The indirect measurement category includes the following:
Radiological method – Neutron scattering, gamma attenuation technique (e.g., using neutron moisture meter)
Electromagnetic method – Time domain reflectrometry (TDR), TDR FM, Diviner
Tensiometry method – using tensiometer
Psychrometer method
Different types of radioactive sources produce the following types of radiation:
Neutrons
Gamma rays (Photons)
Alpha particles
Beta particles
Water content of soil is inferred from the dielectric permittivity of the medium. Electrical conductivity is inferred from TDR signal attenuation.
Psychrometer Method approach measures the vapor pressure of the water in equilibrium with the soil and hence measures the total soil-water potential.
Indirect Estimation of Soil Moisture Characteristic:
Statistical approaches, which either relate parameters in specific hydraulic models with texture and other soil properties using regression analysis
Physico-empirical approaches, which transform the PSD curve into a soil water characteristic curve by relating soil particle size to a corresponding pore diameter in a soil capillary model.
Factors Affecting Hysteresis The factors influencing hysteresis are the following:
Pore-size geometry
Entrapped air
Moisture absorption (capillarity) and release (drainage) behavior of the soil
Swelling and shrinkage of soil
Field methods for determination of saturated hydraulic in the absence of watertable or far above the water-table are as follows:
Bore-hole constant water level method
Permeameter method
Field Measurement of Ksat
(a) Auger-hole method (for unconfined water-table condition)
(b) Well-pumping method (under both confined and unconfined condition)
Measurement of Unsaturated Hydraulic Conductivity:
(a) Instantaneous profile method: The instantaneous profile method for determining unsaturated hydraulic conductivity and diffusivity is based on Darcian analysis of transient soil water content
(b) Internal drainage method
(c) Saturating unsaturated soil core or column
The growth cycle of wheat has mainly the following divisions: germination, seedling establishment and leaf production, tillering and head differentiation, stem and head growth, head emergence and flowering, and grain filling and maturity.
The phases were germination and emergence (0–8 days), tillering (8–43 days), stem elongation (33–65 days), heading (55–75 days), flowering (61–81 days), and grain formation and ripening (65–115 days) for wheat.
Winter wheat has growing period of 120 to 160 days
Crown root initiation is most critical stage for irrigation
The minimum daily temp for growth is 50 C
The mean daily temp for optimum growth and tillering is in the range of 150-200C
Rice grown about 40% of the irrigated area in india
The optimum temp is required for potato is 18-200 C
Water requirement of major crops Crop Water requirement (cm) Crop Water requirement (cm) Wheat (winter) 30–40, Wheat (spring) 40–55, Corn 50–70, Rice 110–160 Cotton 70–100, Maize 40–60 (cm)
Many methods of scheduling irrigations are used by the farmers. These methods include the following: Crop observation, “Feeling” the soil, Application of irrigation weekly to bring the water to a set amount (say, 5–7 cm for rice), Rotation basis (provision of irrigation scheme or pump owner), Pan evaporation observation, Check book method/irrigation calendar.
Duty is the depth of water required by a crop during its growing period for successfully grown up
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