Irrigation Efficiency Flashcards
1
Q
Explain the importance or relevance of agricultural water management (linked to the issue of water availability, water scarcity and food production, climate change); Explain briefly what was covered about environmental effects of irrigation
A
- Agriculture and mainly irrigation uses 70% of the world’s water supply. Irrigation increases yields of crops by 100-400%. Inefficient irrigation systems cause large volumes of water losses, which is important to minimise especially as the global fresh water supply is depleting, even more so as the climate warms (demand > supply). Food (crops) is essential to sustaining life (and so is water). With efficient agricultural water management, application can be more efficient and systems can be upgraded to reduce losses and thereby preserve water sources while still growing the necessary crops. Also management can prevent contamination of streams and reservoirs.
- Environmental effects of irrigation: NO2, ammonia emissions, nitrate, drying of water bodies, soil can become saline, decreased water flows downstream, increased evaporation and increased gw table in the irrigated area
2
Q
Explain water potential and water potential head and the different components (+units)
A
- Water potential = water is driven by potential, it flows from high to low potential (from soil to leaves)
- Components =
> Gravitational
> Air pressure potential, Pair in soil - Preference air
> Osmotic (relevant when there is a diffusion barrier e.g. cell membranes)
> Soil matric (in unsaturated soil, effect of capillarity and adsorptive forces)
> Hydrostatic pressure potential (saturated soil, below the water table) - The water potential is the potential energy of the water expressed in pressure units (Pa) (energy per volume)
- The water head is the same as water potential but expressed in head units (m), measured using tensiometer
> Water head *pwg = water potential
3
Q
Explain the water retention characteristic and how it is measured
A
- Water retention curve can be drawn by plotting soil water content against pore size distribution which also can display differences across soil type
> As pressure head gets more negative (higher absolute value), soils lose water
> Sandy soil has higher K in saturated conditions but loses water easily so K decreases quickly - Moisture retention characteristics depend on pore size distribution of soil as it is related to particle size distribution and water is stored within pores by adhesion and cohesion
- Determination of moisture retention characteristic on undisturbed soil samples with known bulk volume in the laboratory by letting saturated soil samples subsequently getting in equilibrium with increasing negative pressure heads (suction tables, low and high pressure chambers, equilibrium with vapor pressure)
> Measure the weight of soil sample after hydrostatic equilibrium then convert weight to water retention curve using a model - Sand box method and pressure cells
FIGURES
4
Q
Give and explain the Buckingham-Darcy equation and discuss the variation of the hydraulic conductivity with soil water content
A
- Jw = -K(dH/z)
> Where H = z + h
> Total head = elevation head + pressure head
> h is negative in unsaturated conditions (matric
potential head), positive in saturated conditions
(hydrostatic pressure head), 0 at interface
> Jw = q = Q/A - K of unsaturated soil is less than K of saturated soil (Ks)
5
Q
Explain the concept of field capacity and confront it with graphs that show redistribution of water in a sand and silt loam soil
Interpretation of measured or simulated soil water dynamics during infiltration
A
- Field capacity = situation when rapid drainage has stopped (usually after 2 days of applying water)\
- FC: quantity of water remaining in soil a few days after being wetted and after drainage has ceased.
> Sand has lower FC (~0.1) while silt has higher FC (0.35)
FIGURE - See above graph. Sand: front moves faster in sand as it has bigger pores. The same amount of water has been applied but in the sand, it has spread over a larger depth
- In wetted part, silt loam is saturated but sand is not -> silt loam retains water better
- Soil consists of solid mineral grains with organic matter = soil matrix and pores where water is stored (smaller soil grains = smaller pores)
> Water enters pores, surface tension holds it in matrix -> crops need to overcome suction to extract water from soil which is harder for smaller pores with strong suction
> Soil is mixture of small and large pores, water is first extracted from larger pores then soil gets drier and suction force increases
> If soil is too wet, water will drain out of root zone -> can be beneficial as it drains out salts but we also need air in pores as roots function in an aerobic way + plant respiration
6
Q
Define (or compare) irrigation efficiency and crop water productivity
A
- Irrigation efficiency = (useful water for crop production)/(total water used by irrigation)
- Crop water productivity = (crop yield)/(unit of water) - can be kg of harvest, nutritional value, economic value (more crop per drop)
- Crop water productivity is not a real efficiency, it optimises the use of water for food
7
Q
Define (or compare) classical and net irrigation efficiency
A
- Goal of irrigation: have enough water in the root zone to ensure crop receives water requirement, ETc (while keeping salt conc in soil low)
- Classical irrigation = beneficial/ diverted. Focuses on technical infrastructure, useful output of the irrigation system = water stored in root zone and the leaching requirement. Split into 2 parts: conveyance (how much water reaches border of field/ vol of water diverted from source) and application efficiency (beneficial water use/ volume delivered to border). Classical is important for design of good irrigation systems at field and system level
- Net irrigation = uses perspective of water resources/reusability - some fraction of losses (outflow from field, GW recharge) can be useful to downstream users so it is not actually lost
