MHSE 18: Soil Water

1. MHSE 18: Soil Water Dr. Stefan Julich Georg Richter

2. Atmosphere Plant Soil Chapter 7: Soil-Plant-Water Relations

3. Ch. 7 Learning Objectives • How does the water move from the soil through the plant to the atmosphere? • How to measure this water movement? • Which influence have soil and weather conditions on water uptake and transpiration? • Which influence has the soil water availability on crop yield?

4. Soil-Plant-Atmosphere Continuum • …is the pathway for water moving from soil through plants to the atmosphere. • various terms to characterize the state of water among scientific areas: • concept of water potential

5. Concept of water potential ψ • is needed to explain direction and rate of water movement • Flow of water occours from high to low potential • Useful hints: • Potential to move • Units are pressure (kPa or MPa) • Water potential of pure water = 0 (at room temperature / pressure)

6. Out side are ψw -95 MPa Leaf (air space) Ψw -7.0 MPa Soil-Plant-Atmosphere Continuum Leaf (cell walls) ψw  -0.8 MPa Stem Ψw -0.6 MPa Root ψw -0.5 MPa WATER FLOW Soil ψw -0.1 MPa

7. Flow of electronic charge • I… electric current • ΔV… electric potential difference • R… resistance Electrical analogy: Ohm`s law • Flow rate of water • q… volume flux of water • Δψ… water potential difference • R… resistance

8. Water movement in the soil(see previous lectures) • …by bulk flow driven by pressure gradient • (soil water gradient) • physical properties of soil set plant available water  Soils are able to store and conduct water • sets first resistance!!!

9. Water uptake by rootsStructure of Roots Root caps with mucilage

10. Water uptake by rootsRoot hairs ensure intimate contact with soil particles • Root hairs • Increase surface area • Improve water & mineral uptake

11. Water uptake by rootsMathematical modeling of water uptake by roots • Microscopic scale or single root model • Macroscopic scale or root system model

12. Water transport through the plantXylem Longitudinal section through xylem & phloem vessels

13. Water transport through the plant vesselsRate of sap flow (JS) – Darcy´s Law Remember previous lecture!!! Analogy to saturated flow in soils

14. Water movement from the leaf to the atmosphere

15. Water movement from the leaf to the atmosphereTranspiration as an Diffusion process

16. Water movement from the leaf to the atmosphere Leaf stomatal resistance • Stomatal regulation of water flow to control plant water balance

17. Transpiration flux Moving air Boundary layer resistance Still air Stomatal aperture

18. Evaporative demand of the atmosphere  VPD • Humidity • Radiation (PAR) • Temperature • Wind speed • Soil water supply • plant available water • (soil temperature) Environmental control of transpiration

19. Environmental control of transpiration • Potential transpiration Tpot: • the transpiration depends only on the atmospheric conditions (no soil water shortage), unstressed transpiration. • Actual transpiration Tact: • the loss of water from plant tissues to the atmosphere under the prevailing atmospheric and soil hydrologic conditions. • Tpot Tact

20. Control of transpirationAtmospheric demand

21. Control of transpirationAtmospheric demand

22. Control on transpirationSoil-water suction under different atmospheric conditions Tactual/ Tpotential Denmead & Shaw 1962

23. Control on transpirationSoil-water content under different atmospheric conditions

24. Continous monitoring of the SPAC

25. Control on transpirationGradual lowering of the water potential

26. Determination of (evapo)transpiration • Soil water balance method • Soil water depletion • Lysimeter • Micrometeorological methods • Bowen ratio • Eddy correlation • Plant physiological methods • Sap flow measurement • Calculation using weathering data • Penman-Monteith-Equation

27. Soil water balance method ET… Evapotranspiration P… Precipitation D… Drainage R… Runoff A… Change in storage

28. Soil water depletion method

29. Lysimeter method Source: Manning 1997

30. Micrometeorological methods

31. H eating wires Heated sensor Copper D T Constantan Reference S a p w o o d Heartwood sensor Differential thermocouple Sap flow Sap flow measurement Heat dissipation method (Granier 1985)

32. Sap flow measurement Trunk Heat Balance method (Čermák & Kučera 2004)

33. Calculation of evapotranspiration λ = latent heat of vaporization [J g-1], Rn = net radiation flux at the canopy surface [J m-2 d-1] G = soil heat flux [J m-2 d-1], ρ is the atmospheric density [g cm-3] cp = specific heat of humid air [J g-1 °C-1] (ea-ed) = vapour pressure deficit [kPa] rc = crop canopy resistance [s m-1] ra = aerodynamic resistance [s m-1] s = slope of the vapor pressure curve [kPa °C-1] γ is the psychrometric constant [kPa °C-1].

34. Take home message • The movement of water through the SPAC involves different transport mechanisms : • In the soil and in the xylem: bulk flow driven by hydraulic gradients. • In the leaf and to the atmosphere: diffusion and convection flow. • In all these situations flow rate is proportional to water potential gradient and inverse proportional to the transfer resistance • transfer resistance between atmosphere and leaf is the largest

35. Plant productivity and soil water Stomata, transpiration & photosynthesis

36. Wheat Water consumption (T) and dry matter yield (P) from Ehlers 1996

37. Water use efficiency (WUE)

38. Take home messages • Crop yield depends on atmospheric factors, nutrient and soil water availability. • Under the same climate conditions crop yield highly depends on soil water supply. • Crops with high productivity and low water use are particulary advantageous in area where water is scare