Incorporation of Vegetation Processes in Hydrological Models

1. Incorporation of Vegetation Processes in Hydrological Models Sandra Dashora Susannah Hughes Dejan Marković Bruce Mitchell Dennis Rotheray Simon Yhdego Contents: 1.Introduction – Hydrology and vegetation 2.How the CHASM model includes vegetation processes 3.How the SWAP model includes vegetation processes 4.Relative strengths & weaknesses of the two models 5.Conclusions

2. Hydrology The perpetual, global, donor-recipient cycle of moisture exchange THE WATER CYCLE Water gathers in reservoirs (oceans, seas, rivers, lakes, groundwater, icecaps) Evaporation from these, and transpiration from biomass translates into precipitation. Some of this evaporates straight back into the atmosphere, while the rest trickles down to other recipients to re-enter the cycle elsewhere.

3. Soil Hydrology and ModelsHydrology at a local scale – the Unsaturated Soil Zone • The local application of the global water cycle looks more closely at factors of local geology and soil porosity as well as infiltration, storage, redistribution, drainage, evaporation and transpiration. • At this scale, moisture is either intercepted (captured by vegetation) before it reaches the unsaturated zone or it moves upwards, sideways or downwards within the zone: • upwards = evaporation, transpiration • sideways = watercourses, groundwater flow • downwards = drainage : water will infiltrate through the ‘unsaturated zone’ (soil, where the voids between soil particles may be filled with air and/or water) towards the water table and ultimately percolate through the ‘saturated’ zone (bedrock, where voids are completed filled with water). • Amongst the most significant influences on soil hydrology, and one of the hardest to model, is the absence or presence (and type) of vegetation cover • Under normal conditions, most vegetation only affects the upper unsaturated zone • It is possible to model how changes in one aspect of the hydrological cycle (e.g. increased rainfall) will affect another. • The presence of vegetation in the unsaturated zone is a major complication in these computer simulations. • Each mechanism in both the soil and the vegetation must be identified and added into the model.

4. Key variables for Soil HydrologySoil Properties, Vegetation Mechanisms and Other Variables SOIL PROPERTIES HYDRAULIC CONDUCTIVITY - the soil’s ability to transmit water POROSITY - the density of pores (voids) within the soil PERMEABILITY - how connected these pores are INFILTRATION RATES - competing forces of gravity and capillary action (a combination of surface tension & absorption) MECHANICAL RESISTANCE: soils of high bulk density (e.g. clayey or stony soils can affect vegetation growth) AERATION: the oxygen content of the soil can fall affecting vegetation (e.g. compaction of the soil, waterlogging) VEGETATION MECHANISMS EVAPO-TRANSPIRATION – depleting soil moisture by vegetative uptake; water is brought to the surface by osmosis in roots and also by transpiration of plants. The denser the vegetation the higher the transpiration rates. Shallow soils are directly affected by evaporation whereas deeper layers are more influenced by gravity and capillary action. ROUGHENING OF GROUND – roots and stems increase the roughness of the surface and therefore the permeability of the soil INCREASED RESISTANCE TO EROSION – roots bind soil particles at the ground surface, reducing their susceptibility to erosion CANOPY COVER – the shade of the canopy prevents desiccation of the soil which otherwise reduces rainfall’s infiltration capacity INTERCEPTION – foliage intercepts rainfall causing evaporative and absorptive losses that alters the amount of rainfall available to infiltrate the soil. Leaf area index (LAI) is a useful measure towards these mechanisms. There are three main types: CANOPY INTERCEPTION: some water sitting in the canopy / on leaves will be evaporated or used in transpiration before it can filter down to the ground (either from rain or fog) STEMFLOW: rain is intercepted by stems and branches and then flows down trunks into soil THROUGHFLOW: rain falls onto leaves and subsequently drips to the ground OTHER VARIABLES AFFECTING SOIL HYDROLOGY DEPTH OF SOIL – generally very deep in the humid tropics and relatively shallow in temperate region TYPE OF VEGETATION – tree roots go deeper into the soil and can access water at various levels creating greater transpirative effects. Therefore trees are more resistant to summer droughts as opposed to shallow rooted shrubs .

5. What is the CHASM model?CHASM – Combined Hydrology and Stability Model • CHASM is an integrated slope hydrology/slope stability software package that aids the assessment of slope stability conditions • More specifically, CHASM allows: • direct input of storm • hydrographs (hourly intensities) • or design rainfall • incorporation of vegetation • cover effects for both slope • hydrology and slope stability • The specification of multiple • soil strata and associated • properties

6. How CHASM works • The slope is divided into a series of rectangular columns, each subdivided into regular cells • The model simulates detention storage, infiltration, evapo-transpiration, and unsaturated and saturated flow regimes • The hydrology scheme facilitates representation of slope plan curvature • The use of a graphical user-interface (GUI) enables a more seamless and user-friendly operation • Results are then visualised through charted and tabular outputs to aid decision making processes

7. CHASM - Vegetation Modelling • In CHASM, the Interception model depends on the type of vegetation, and many types can be specified using the customisable input parameters. For example: • Chasm can capture the effect of dense stands of tall grass being flattened by intense rainfall to form a semi-permeable ‘thatch’ barrier • Properties of the top (upper) soil strata, including its permeability. • Strength parameters of roots, including tensile strength, root area, cohesion and shear strength. • Interception by trees is modelled by a free throughfall coefficient and canopy storage capacity to reflect the structure of the canopy, and a stemflow-partitioning coefficient and trunk storage capacity. • Evapotranspiration and root water uptake are modelled in CHASM using the Penman–Monteith equation. • The interaction between roots and soil (reinforcement and surcharge) can be quantified using a simple perpendicular root model. • Rainfall inputs can be set using amount of precipitation and duration of storm events

8. Uses of CHASM model • Assessing the role of surface cover on slope stability • Establishing the controls on stability afforded by differing slope plan curvatures • Through the full dynamic linkages of the model relating design rainfall to factor of safety • Probabilistic analyses • Implementation through a GIS environment to provide wide area stability assessment for prioritization of remedial works • A practical application of CHASM can be found at MoSSaiC: http://www.mossaic.org/projectsand http://www.mossaic.org/landslides

9. What is the SWAP model?SWAP – Soil, Water, Atmosphere, Plant • Soil – Water – Atmosphere – Plant finite difference model • Simulates transport of WATER through SATURATED and UNSATURATED soils • Uses soil hydraulic functions q, h and K from Richard’ Equation to simulate vertical movement of water as well as : • SOLUTES (e.g. pesticides, salt through root uptake) and • HEAT (soil temperature affects decomposition rate of solutes) • Operates at field scales and can incorporate whole growing seasons. • Works on several levels within the ground: • TOP LAYER (atmosphere & vegetation) • BASE LAYER (soil and groundwater interactions)

10. How SWAP works Integration of the four main inputs to the system (Soil, Water, Atmosphere and Plants) in the computer model, using a 1 dimensional approach. Each vertical cell can be individually set to reflect local parameters e.g. variations in the transport of water between different soil or rock types.

11. SWAP - Vegetation Modeling • SOIL FACTORS • The Soil column is divided into finite number of layers to reflect its heterogeneity: e.g. peat, or cracks in clay which allow macropore flow, can be simulated. • SWAP can accommodate: hysteresis (difference between soil drying out and getting wet) in retention function; spatial variability of soil hydraulic functions; and preferential flow in water repellent soils • VEGETATION FACTORS • Vegetation processes involved in SWAP include: Rate of phenological development (timing of bud burst etc and hence length of growing season); interception of global radiation; CO2 assimilation; biomass accumulation of leaves/stems/storage/root extension (latter affected by any stress in root zone e.g. salinity) • Carbohydrates are used by vegetation as both energy to maintain live biomass (maintenance respiration) and in creating structural matter (growth respiration) • Root uptake of solutes such as pesticides and salt transport affects crop growth through processes of convection, diffusion, dispersion, adsorption and decomposition • Potential gross photosynthesis calculated from absorbed radiation and leaf area index can decrease through stress if salinity levels increase. Also uses factors such as albedo, crop height and minimum resistance • Up to 3 crops a year can be input into SWAP • WATER FACTORS • SWAP can use variable irrigation /drainage influences, drainage is considered as lateral discharge • Up to 5 orders of surface water channels can be used (ephemeral streams to wide rivers) with varying stream bed geometry such as bed level / width / slope / spacing • Calculates infiltration rates (summer) or discharge rates (winter)

12. Uses of SWAP model • Calculation of potential evapo-transpiration rates of wet canopies, dry canopies and wet bare soils to give potential transpiration and potential evaporation values • Incorporation of WOFOST (World Food Studies) which is a crop growth model to study the effect of climate change on crop production e.g. in identifying crop model parameters in Thailand (Srinuandee, 2004). • Comparison of the effects of setting a single water-table level across a whole catchment versus using variable ground water levels • Creation of a dataset for validation studies of thermal infrared (TIR) measurements (difficult to measure in the field). SWAP was used to as part of a CUPID model to calculate values across a range of canopy types and meteorological conditions. • Assessing the most efficient way to achieve high crop yields by setting canal drainage levels on agricultural land in the Netherlands (comparing whether one single water table across a large area is more advantageous than variable water table levels to reflect topography, Bierkens, 1999)

13. Advantages of the models Both CHASM and SWAP are similar in many respects but each have certain advantages which are listed below CHASM Uses generally accepted hydrological equations for modelling, e.g. Darcy’s Law for infiltration during rainfall; Richards’ equation for vertical flow Incorporates parameters for direct effects of vegetation on slope stability from both groundcover and canopy, e.g. interception, stemflow and throughfall, evapotranspiration, hydraulic conductivity, surcharge and root cohesion Also incorporates the indirect effects of vegetation such as soil suction, permeability and additional absorptive properties Specification in the software of multiple soil strata and associated properties in a 3D setting makes it more robust. In addition, by using an intuitive GUI, CHASM successfully minimises demands on parameter inputs, thereby minimising the risk of user errors SWAP Can simulate the transport of both water, solutes (salts/pesticides) and heat through the unsaturated and saturated zones. Can accurately handle rapid soil water movement in dry soils during infiltration with great speed (40-70 year periods calculated in a few minutes) Can incorporate snow and frost values in cold regions It can estimate evapotranspiration by two step processes by: - calculating potential evapotranspiration using minimum value canopy resistance and actual air resistance; - calculating actual evapotranspiration using reduction in root water uptake due to stress e.g. Salinity Values for slopes with runoff can be calculated by using runon and runoff options for a sequence of soil profiles SWAP can calculate comparative interceptions rates of varying types of vegetation, from agricultural crops to forests and as such is useful to a range of disciplines.

14. Limitations of the models And likewise, with regards to limitations, which are listed below: SWAP Very slow processing of CPU if very low saturated hydraulic conductivity (k) values are used The model doesn’t take into account the unknown effects on max. evaporation rates in top few cm of soils The model predictions are affected by splashing rain, dry crust formation, root extension and variations in cultivation practices SWAP is developed for calculations with daily meteorological input data In SWAP there is no simulation of regional groundwater hydrology; no interaction and between crop growth and nutrient availability; no non-equilibrium sorption of pesticides no simulation of metabolites Rain duration and evaporation rates are not taken into account in rainfall interception calculations • CHASM • Soils exhibiting strong anisotropy in hydraulic conductivity or those dominated by macropore flow are not capable of being modelled by CHASM. • The scheme has limitations in the context of the description of the failure mechanisms; The stress strain and progressive failure mechanism is not incorporated into the software. • The real kinematics of slope failure is 3D: CHASM being 2D grossly simplifies the problem. Further, dividing 2D areas into columns also simplifies the model. • There are also limitations that relate to process representation and numerical implementation • (It should be noted that the limitations outlined above are not considered restrictive!)

15. How do the models compare? • Given the advantages and disadvantages outlined over the last two slides it is clear that whilst both models set out to incorporate the effects of vegetation into their calculations, they are designed to fulfill different needs. • CHASM is designed for and is intrinsically better at calculations involving the stability of slopes and how vegetation can positively or adversely affect this. • SWAP is primarily of benefit to crop calculations and the agro-forestry industry by comparing different scenarios to local hydrological conditions to achieve maximum benefit to yields. Its wide ranging input parameters means it is of equal use to studies of crop yields in the humid tropics as temperate applications to minimise loss of crops as water table levels vary. • Since both models clearly have their own unique functions and methods of dealing with effects of vegetation, they can’t be seen to be “better than” or “worse than” each other. Choice of software would be dependant on specific requirements of the research/task at hand.

16. Conclusions • The impacts of the soil hydrology on computer models of aspects of the global water cycle can be achieved in several different ways. • The unsaturated, soil zone is one of the most complicated to include in models and here we have attempted to compare 2 models which have endeavored to do this, although for different end reasons. Both incorporate the vegetation processes at work in this zone and, being dynamic, biological systems, these processes are often difficult to predict and break down. Depending on the output information required (agricultural or engineering focus’) either 1D or 2D models can be used to gain a better understanding of the impacts on the soil zone. • CHASM and SWAP are just 2 such models exploring this area – other research has been undertaken via different methodologies and would be a good starting point for further research into the advantages and disadvantages of incorporating vegetation processes into computer modelling techniques. E.g. SWIMv1/SWIMv2 available from Scientific Software Group website (http://www.ssg-software.com/) • A biohydrology conference planned in 2009 would be a very useful comparison point for exploring these and other models and their practical application within the hydrological world: http://www.ih.savba.sk/biohydrology2009/

17. References & Acknowledgements • M.F.P. Bierkens et al. 1999. Comparison of two modes of surface water control using a soil water model and surface elevation data. Geoderma, 89, 149 – 175 • R.S. Clemente et al. Testing and comparison of three unsaturated soil water flow models. 1994. Agricultural Water Management, 25, 135 – 152 • T.J. Kelleners et al. 1999. Spatially variable soil hydraulic properties for simulation of field-scale solute transport in the unsaturated zone. Geoderma, 92, 19 - 215 • Freeze & Cherry. Groundwater. 1979. Prentice & Hall, New Jersey • M. Price. Introducing groundwater. 1996. Stanley Thornes Ltd, Cheltenham • E. Menashe. 2004. Trees, soils, geology and soil stability. Greenbelt consulting, www.greenbeltconsulting.com • P. Crow. 2005. The influence of soils and species on tree root depth. Forestry Research Information Note • SWAP website: http://www.swap.alterra.nl • V. Arora. 2002. Modeling vegetation as a dynamic component in soil-vegetation-atmosphere transfer schemes and hydrological models. Reviews of Geophysics, 40 (2), 1006 • L. Jia et al. 2002. Modeling of TIR radiative transfer in the soil-vegetation-atmosphere system: sensitivity to soil water content and LAI and simulation of complex scenes. Geoscience & Remote Sensing Symposium, IEEE International. 1, 39-41 • P. Srinuandee et al. SWAP crop model Parameter Identification using SPOT Vegetation in Suphanburi, Thailand. Asian Institute of Technology, Pathumthani. Thailand.2004. www.GISdevelopment.net • T. Davie. Fundamentals of Hydrology 2002 Routledge, London • MG Anderson and KS Richards (eds.). Slope stability : geotechnical engineering and geomorphology. c1987 Wiley, Chichester • in particular: D.R. Greenway. Vegetation and slope stability. Chapter 6 • CHASM Help File (CHASM Version 4) • CHASM Website: http://www.chasm.info • A.B. Fourie. Predicting rainfall-induced slope instability. Proc. Instn Civ. Engrs Geotech. Engng, 1996, 119, Oct., 211±218 • P.L. Wilkinson, G. M.G. Anderson, D.M. Lloyd and J. Renaud. Landslide hazard and bioengineering: towards providing improved decision support through integrated numerical model development. Environmental Modelling & Software 17 (2002) 333–344 • A.J.C. Collinson and M.G. Anderson. Using A Combined Slope Hydrology/Stability Model To Identify Suitable Conditions For Landslide Prevention By Vegetation In The Humid Tropics. Earth Surface Processes And Landforms, Vol. 21,737-747 (1996) • M.G. Anderson. CHASM Demo (for version 3.4) – http://www.mossaic.org/publications/downloads/ChasmDemo.ppt • J. Greenwood. Response of Slope Stability to Vegetation changes due to Climate Change. Presentation, 26 October 2005 1 2 3 4 SWAP Model:http://www.swap.alterra.nl CHASM Model: http://www.chasm.info Hydrology: Slide 3 - Scots Pine image:http://www.treeforall.org.uk/GetDigging/OurNativeTrees Image 1: http://theonlinephotographer.typepad.com/the_online_photographer/2007/06/index.html and http://www.mercuryfilms.ca/ML_credits.html Image 2, 3: http://www.hyd.gov.hk/contractwebsites/cpr/Progress%20Photos/HY200002_photos/photo0403.htm?item=4&projid=1 Image 4: http://seis.natsci.csulb.edu/bperry/Mass%20Wasting/creepSierrasS.jpg