Science_blog: evapotranspiration

Showing posts with label evapotranspiration. Show all posts

Saturday, 6 May 2023

Some important facts for crop modelling

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

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

Saturday, 18 March 2023

Adaptations and mitigation: Questions and answers part 6

Q: Please describe relationships between climate extreme and climate change, status of climate change in your country.

Relationships between climate extreme and climate change:

1) A warmer atmosphere can hold more water, fueling more intense rain and snow events. But at the other end of the spectrum, the warming climate can amplify conditions conducive to drought—like heatwaves, evapotranspiration and reduced soil moisture. The combination of these two extremes in one location can increase disasters like flooding and landslides.

2) A growing body of evidence shows strong connections between climate change and extreme events, and impacts once thought of as a distant future threat are already occurring and widespread.

3)The Character and severity of impacts from climate extremes depend not only on the extremes themselves but also on exposure and vulnerability.

4) Changing climate leads to changes in the frequency, intensity, spatial extent, duration,and timing of extreme weather and climate events, and can result in unprecedented extreme weather and climate events.

Status of climate change in India:

a) Extreme Heat: In India, an increase in the linear trend of about 0.4 0C in the surface air temperature has been observed in the past century. A warming trend is visible along the west coast, central India, interior peninsula and the North-Eastern India,but some cooling trends are also visible in the North-West India and parts of south India.

b) Changing Rainfall Patterns: A trend of about 10 to 12% (of the normal) increase in monsoon rains were reported along the west coast, northern Andhra Pradesh and north-western India during the last century. A decreasing trend of about 6 to 8% is observed over the last 100 years over eastern Madhya Pradesh, North-Eastern India and some parts of Gujarat and Kerala (NAPCC,2008).

c) Droughts: Droughts have major consequences. In 1987 and 2002-2003, droughts affected more than half of India’s crop area and led to a huge fall in crop production.

d) Groundwater: it is difficult to predict future ground water levels, falling water tables can be expected to reduce further on account of increasing demand for water from a growing population and agricultural production.

e) Glacier Melt: The available data on snowfall in Himalayan ranges show a recession in some parts of the Himalayan ranges. The river systems of the Brahmaputra, the Ganges and the Indus draws water directly from melting of the Himalayas.

f) Sea level rise: Due to sea level rise, the fresh water sources near the coastal areas will suffer from salt intrusion. Kolkata and Mumbai, both densely populated cities, are particularly vulnerable to the impacts of sea-level rise, tropical cyclones, and river flooding.

g) Agriculture and food security: Food production in India is sensitive to climate change like variations in temperature and monsoon rainfall. Rise in temperature has a direct impact on the Rabi crop and every 1 degree celcius rise will reduce wheat production by 4 to 5 Million Tons. Every small change in temperature and rainfall has significant effect on the quality and quantity of fruits, vegetables, tea, coffee, basmati rice and aromatic and medicinal plants. It is predicted that a loss of 10 to 40% in production may occur by 2100 due to climate change (NAPCC).

h) Water Security: Urbanization, population growth, economic development, and increasing demand for water from agriculture and industry are likely to aggravate the situation further.

i) Energy Security: Climate-related impacts on water resources can undermine the two dominant forms of power generation in India - hydropower and thermal power generation - both of which depend on adequate water supplies to function effectively.

j) Health: Effluent emissions to water bodies and salination of rivers through sea level rise may increase the incidence of water borne diseases. Deaths due to heat wave are reported from several parts of the country from time to time, particularly during the summer. It is observed that changes in climatic patterns may alter the distribution of vector species (Malaria, Kala-azar, Filaria, Chikungunia, Encephalitis ) and increase its spread in new areas. An increase in temperature and relative humidity may enlarge the transmission windows.