David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Modeling of Environmental Systems• The next portion of this course will examine the balance / flows / cycling of three quantities that are present in ecosystems:– Energy– Water–Nutrients• We will look at each of these at two scales:– Global– Ecosystem• Before we can build models of these phenomena, we need to have some background on the functioning of these systems with respect to these quantitiesDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Water Budget Equations• Assuming we can bound a terrestrial ecosystem, and further assume that it is in a steady state over the period we wish to study it, we can come up with the following water budget:dVdt= 0 = p + si + gi - so – go - etwhere: p is precipitationsi is surface water inflowgi is groundwater inflowso is surface water outflowgo is groundwater outflow(all quantities averaged over the period)• Recalling groundwater’s long residence time, we can take the difference between groundwater terms to be negligible, and since water flows from land to ocean, we will remove the si termDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Water Budget Equations• This leaves us with the following equation:dVdt= 0 = p -so -etp = so + etorHornberger et al. 1998. Elements of Physical Hydrology. The Johns Hopkins University Press, Baltimore and London.David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Water Flow Processes Up Close• Taking a closer look, we can identify a few more processes:Soil Water DrainageRunoffGroundwaterStemflow UptakeThroughfallPrecipitation Transpiration Evaporationp = so + et• We will focus on how the uptake and trans-piration processes work• Soil water is drawn upthrough the plant root systems, moves through the plant, and is expelled from the plant leaves as transpiration into the atmosphere. How and why does this happen?David Tenenbaum – GEOG 110 – UNC-CH Fall 2005The Concept of Water Potential• The reason is because there is a constant gradient of water potential (from soil to root to xylem to leaf to atmosphere), and this potential gradient is constantly dragging the water up and out• Water potential is the free energy of water, measured in units of pressure (the commonly used SI unit is Megapascales [Mpa])• Pure water in liquid form at 20oC and 1 atmospheric pressure (e.g. distilled water sitting in an unenclosed basin at sea level) has zero water potential• In terrestrial ecosystems, the water in the stocks is reduced in concentration when compared with pure liquid water, thus water potentials are negativeDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Water Potential vs. Relative Humidity•An unsaturated atmosphere has a large water potentialSalisbury, FB and C. Ross. 1969. Plant Physiology. Wadsworth, Belmont, CA.David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Soil Water Potential• Although the gradient that will draw water from the soil to the atmosphere (via plants) is usually present, there are counteracting forces along the way as well• The water in soil is held to the soil particles as thin film of water, against the pull of gravity– The thicker the film, the less tightly the water at the outer rim is held and the more easily it can be taken by plants• This attractive force between water and surfaces is called matric potential• The total amount of water a soil can hold per unit soil volume is a function of the surface area of all the particles within that volume and the amount of air spacepresent between these particles, and varies by soil typeDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Soil Water Potential• Soil matric potentials that are less negative than approximately -0.03 Mpa are too weak to retain water against gravity– Soil water content at this point is called the field capacity•As soils dry, and these films become thinner, the remaining water is held more tightly and the matric potential becomes more negative• Many plants cannot extract water from soils with matric potentials that are more negative than approximately –1.5 Mpa– This condition is called the permanent wilting point• Soils are a mixture of particles of various sizes, so a soil’s aggregate performance is a function of the mixtureDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Soil Water PotentialDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Water Movement through the Soil-Plant-Atmosphere Continuum• Soil water that can be freed from the soil can proceed to the atmosphere in two ways:• Evaporation - Water in the soil evaporates directly into the atmosphere. Evaporation only affects the thin surface layer of soils, as the resistance to liquid water movement in soils is high• Transpiration - Plants provide an ideal conduit for the movement of water between soils and the atmosphere. Roots grow deep into the soil and can tap into water reserves far from the surface, providing a pathway between the deeper soil and the atmosphereDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Water Movement through the Soil-Plant-Atmosphere Continuum• The movement of water from the soil through plant and into the atmosphere is controlled by stomata, tiny holes on the back of leaves•The atmosphere is usually drierthan the air inside the stomata, thus there exists a water potential gradient (the potential in the outside air is more negative) causing the water move from the stomata into the atmosphereDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Water Movement through the Soil-Plant-Atmosphere Continuum• The negative water potential in the atmosphere is transferred to a continuous column of liquid water that begins in the root and ends in the leaf• The tissue that the water passes through is called xylem, which provides an uninterrupted pathway for water movement– The tension is conducted out through the roots, and through contact between roots and soil, to the water adhering to soil particles• This water column must be continuous. Any air gaps in the system will relieve the tension and stop the movement of water. Root surfaces must be in direct contact with the soil water filmDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Water Movement through the Soil-Plant-Atmosphere Continuum• The actual rate of flow of water up through the plant, and thus from the soil to the atmosphere is a function of the differences in water potential between these two ends of the gradient and the resistance to the flow•