Crop Water Demand Flashcards
Explain what the reference evapotranspiration (ETo) is, and/or the difference between ETo, ETc, and ETc,adjusted
- ETo = reference evapotranspiration = evapotranspiration rate from well- watered grass on a reference surface. The only factors affecting ETo are climatic parameters -> FAO PM method is only way to calculate ETo
- ETc = crop water requirement for maximum production = crop evapotranspiration under standard conditions and optimal soil conditions (well-fertilized, disease free) and achieving full production. Ground cover, canopy properties and aerodynamics resistance of the crop are different from grass. ETc = Kc*ETo Kc is the crop coefficient (changes throughout season)
- ETc,adjusted = ETc but including environmental and management factors = ETc * Ks where Ks is the stress factor (Ks < 1 when stressed)
- Soil evap = non productive consumptive use and crop transpiration = productive consumptive water use
- ETo dependent on weather parameters, but not on crop characteristics (reference crop) and not on management and environmental factors (standard non limiting conditions)
- ETc dependent on weather parameters and on crop characteristics but not on management and environmental factors (standard non limiting conditions)
- ETcadj dependent on weather parameters, crop characteristics and environmental factors
Explain how the Kc factor varies over the 4 growth stages, and why the curve has this shape (explain the factors that determine the magnitude of the Kc factor in the different growth stages)
- Kc depends on crop type, growth stage, climatic conditions and wetting frequency.
- Kc = Kcb + Ke = basal crop coefficient (transpired by crop) + soil evaporation coefficient
- Initial: lots of bare soil = lots of evaporation, high Ke (and low Kcb)
- Development: crop starts to grow and leaves appear, you have more ET but ground gets covered so less evaporation, increasing Kcb
- Mid stage: soil is fully shaded, Kcb is dominant and Ke is very small
- Late stage: transpiration is low and soil still covered, decreasing Kcb but Ke remains constant
FIGURE - Goal: higher Kcb as that is more productive use rather than Ke which is lost to atmosphere
Explain the 3 ways to calculate ETo (FAO-PM, Hargreaves-Samani, evaporation pan); you don’t need to be able to give the equations, but you need to be able to explain the variables and their units in the equations). And you need to know how to choose between FAO-PM and Hargreaves-Samani (depends on the data you have)
- FAO-PM: most accurate, requires a complete weather station of good quality, need data on temperature, humidity, wind speed, solar radiation - can estimate some data if needed
> Need: ETo, the slope of the vapor pressure curve (kPa/°C), the net radiation at the crop surface (MJ/m²/day), the soil heat flux density (MJ/m²/day), the psychrometric constant (kPa/°C), the mean daily air temperature (°C), the wind speed at 2 meters above ground level (m/s), the saturation vapor pressure (kPa), the actual vapor pressure (kPa) - Hargreaves-Samani: good when you know Tmax and Tmin accurately
> Need: ETo, Tmin, Tmax, Tmean, extraterrestrial radiation
Low data requirement but less accurate - Practical method using evaporation pan you estimate ETo
> ETo = Kp*Epan where Kp is the pan coefficient, depends on environment surrounding the pan as well as the pan size and shape
Needs precise installation and calibration
Explain strategies to reduce soil evaporation (see MOOC module 2: video crop evapotranspiration)
Irrigate at night, use mulch to reduce soil evaporation, supply water directly to the rootzone of the crop using drip irrigation
Explain the concepts of soil water content at field capacity and at permanent wilting point, the critical water content, the (water content at the) management allowed deficit, the total available water (TAW) and/or the readily available water (RAW)
- FC = max water that can be held in soil (excess will drain quickly), often defined as water content after 2 days of draining a very wet soil (note: we want to fill root zone up to FC but also need to apply excess to maintain low salinity)
- PWP = plant can no longer extract water from soil and it will die. The remaining water is held in too small pores and roots cannot overcome high soil water retention
- Critical water content = point where plants begin to experience stress, depends on the crop
- MAD = when should the farmer water the crop, can be higher, equal to or lower than CWC
- TAW = maximum possible water storage, TAW = FC - WP
- RAW = easily extractable water, RAW = FC - CW = p*TAW (RAW is a fraction of TAW, p is adjusted based on crop and demand and root depth)
Discuss which soil and plant sensors could be used for irrigation scheduling (covered in MOOC companion text but also during excursion). What are advantages and disadvantages of the different options?
- Soil water sensors: monitor water content in root zone. Cons = expensive and hard to find optimal depth and number of sensors needed
> Tensiometer: measures soil water tension through a porous cup
> Measure electrical conductivity
> TDR probes are accurate but $$
> Weig soil sample, dry at 105 C and weigh again. Time consuming, delay in results - Plant sensors:
> Measure leaf water potential using pressure chamber to provide information on soil moisture content. Choose leaf that is shaded by other leaves, put tin foil and ziplock over it so it comes into equilibrium with plant/stem then put leaf in chamber and squeeze until water comes out.
> High pressure indicates low soil moisture
> Cons: leaf water potential changes throughout the day
> Measure diameter of trees to determine moisture - harder to do, different for each crop - Good to monitor water stress to achieve optimal plant growth and yield
Explain how irrigation scheduling could be done by simulating the soil water balance in real time with weather data
Collecting real-time weather data (temperature, rainfall, and solar radiation) -> integrate into a soil water balance model. By considering factors like crop water requirements, evapotranspiration rates, and soil moisture levels monitored through sensors, the model accurately predicts the water needs of the crops. This information enables farmers to schedule irrigation at the appropriate time and in the right amounts, ensuring efficient water use, optimal crop growth, and minimizing the risk of over- or under-irrigation
Explain why the critical water content is crop-dependent and also varies with the ETc
- CWC occurs when crop water demand (ETc) cannot be met by water supply, therefore it is dependent on what the ETC is. Supply depends on the soil water content and root density (denser root system may be able to take up enough water).
- The drier the soil, the lower the K, the slower water flows to roots
Explain the soil water balance with a drawing and by naming and explaining all inputs of water into and outputs out of the root zone
FIGURE
- Capillary rise: shallow groundwater enters root zone
> Capillary rise is due to adhesion (water attracted to solids) and cohesion (mutual attraction between water molecules)
Give and explain the equation that relates relative yield to relative ET, and illustrate it with a graph
- Ya/Ym = 1 - Ky(1-ETCadj/ETC)
- (ETc-ETc,adj)/ETC is the relative evapotranspiration deficit - is either 0 or positive
- Slope differs for different crops
- Bigger Ky = for same relative evaporation deficit, bigger yield decrease = more sensitive to water stress
