MHSE 18: Soil Water

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

2. Chapter 8: Water Balance of Soils

3. Balance equationBased on the law of conservation • “…matter can neither be created nor destroyed but can only change from one state or location to another.” • Accumulation = inflow – outflow +generation – consumption Application into the soil Allows us the estimation of • Evapotranspiration • Groundwater recharge • other soil water components

4. Definition of a systemBoundaries and time resolution Term of interest: ΔW of a given body of soil during a specified time period Duration between measurements or model outputs  determination of soil water fluxes ΔW(t)

5. From previous lecture:Soil water depletion method

6. Definition of a systemCategories ΔW(t) Steady-state no change in storage within the system vs. Transient-state  storage within the system Open system Material is transferred across the system boundary vs. Closed system  no transfer

7. Deriving the balance equation Pg I ET Pn Rin Rout ΔW(t) Ddown Dup Change in storage = Gains – Losses

8. Soil water balance equation Pg - I ± R - ET ± D = ΔW Pg = gross precipitation (also irrigation) I = evaporation of water intercepted during precipitation from living or dead plant surfaces R = net surface runoff/runon ET includes E= evaporation from bare soil (soil evaporation) T = transpiration D = net drainage (deep percolation or capillary rise) ΔW = change in soil water storage

9. Interception of precipitation • The stopping, interrupting, or temporary holding of precipitation in any form by • mulch • forest floor • a vegetative canopy • vegetation residue or • any other physical barrier • and lost as evaporation

10. Canopy interception Source: Hewlett, 1982 Ic= Pg – (Pt + Ps) Terms to know: Interception storage S Crown interception loss IC Throughfall Pt Stemflow PS (Total interception loss It)

11. Factors controlling interception • Vegetative factors • Leaf area index • Height of plants • Crown structure • Meteorological factors • Number and spacing of rain events • Rain intensity • Wind speed

12. Leaf area index (LAI)

13. Seasonal variation of LAI for agricultural plants Source: DVWK, 1996 Source: Kutilek & Nielsen, 1994

14. Measurement of interceptionForest sites Source: Benecke & van der Ploeg, 1976

15. Measurement of interceptionForest sites Pg Ic ΔS Source: Benecke & van der Ploeg, 1976 Pt Ps

16. Modeling of interception I = Interception [mm] P = Precipitation [mm] LAI = Leaf area index [-] a = empirical coefficient [cm] b = soil cover (between 0 and 1) [-]

17. Relation between LAI, precipitation & interception: agricultural plants Source: DVWK, 1996

18. Modeling of soil water flow and soil water componentsWhy do we need it? Measurements… ... aims at understanding of the mechanisms governing the behaviour of the soil. ... are time and money consuming. ... allow often no long-dated prognosis. ... are essential for development and application of numerical models.

19. What Is a Simulation Model? ... a simpler representation of the real world. It can reproduce some but not all of the characteristics of the system. Produce output in response to an input.

20. Classification of soil hydrologic models

21. Flow chart BROOK90 PREC = Precipitation EVAP = Evapotranspiration SEEP = Groundwater outflow FLOW = surface runoff INTS = Snow on vegetative canopy INTR = Rain on vegetative canopy SNOW = Snowpack on ground SWATI = Soil water storage GWAT = Groundwater storage Source: Federer 2003

22. Coupled heat and mass transfermodel for soil-plant-atmosphere system Jansson & Karlberg 2004 Biotic part: Carbon- & Nitrogen fluxes Royal Institute of Technology, Dept. of Land and Water Resources Engineering, Stockholm (Sweden) http://amov.ce.kth.se/coup Abioticpart: Water & Energyfluxes (Svensson et al. 2008) Modelling soil water, C and N turnover in a Poplar plantation 22/10 Santiago de Compostela, May 2011

23. ModelingInput

24. ModelingOutput • Components of the Soil Water Balance • Transpiration, Interception, Evaporation • Drainage (= groundwater recharge) • (Surface runoff) • Water content and pressure head as a function of time and space •  Quality of modeling results depends on • the quality of input data and on the • underlying theory

25. Study results • Measured and simulated soil water components dependend on • climatic conditions, • groundwater depth, • soil properties and • land use

26. Available water capacity (AWC) of the root zone & water consumption • Loess • Silty clay • Loamy sand • Sand

27. Influence of soil, groundwater and landuse on water consumption

28. Influence of landuse and soil on groundwater recharge

29. Measured components of soil water balance of spruce & beech [mm a-1] Source: Benecke & van der Ploeg, 1976

30. Prediction of soil water content and matrix potential at different tree species Model application BROOK90 Rotherdbach (Ore mountains) ---- Spruce (Fichte) ---- Beech (Buche)

31. Take home message Soils without groundwater influence: The ET increases under the same soil and climate condition dependent on landuse as follows: arable crops < grassland < forest The groundwater recharge increases in the following order: forest < grassland < arable crops

32. Take home message • Soils with groundwater influence: • At low groundwater depth, the ET is mainly controlled by the atmospheric conditions. • At deeper groundwater levels, the ET is mainly controlled by the soil hydraulic properties and by the kind of land use. • Groundwater influenced sites serve as a region of water depletion. A groundwater recharge only starts to occur at groundwater level > 100 cm below surface.

33. Simulation of water storage in the field • Homework • Read chapter 4 from textbook „An Introduction to Applied Soil Hydrology“, K. Bohne. • Try to get into the spreadsheet model „SIMULATION.XLS (Section 4.3.1) • Please download simulation.zip that contains: • Bohne_Chapter_4.pdf • SIMULATION.XLS • http://boku.forst.tu-dresden.de/index.php?educational_material_soil_water_ws_13_14_eng