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 2005The Global Water Cycle• The circulation of water is the largest movement of matter on the surface of the Earth• In addition to water being a huge mass movement, it also acts as the medium by which a significant amount of energy is moved from place to place: – The circulation of water is closely related to the distribution of heat energy on the Earth’s surface: Through evaporation and precipitation, water transfers much of the heat in the tropics to the polar regions• The annual availability of water on land is the single most important factor that determines the growth of plants at a given locationDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Units for Measuring Water• Water can be measured using a wide variety of units, and each is more or less convenient under different circumstances:1. Volume – Used most often when working with large amounts of water (i.e. km3of water in the global water balance)2. Weight –Again, usually for large balances: Conveniently, 1 liter of water = 1 kilogram of weight = 1000 cm3at standard temperature and pressure3. Depth equivalent –Here, volumes of water are assumed to be spread out uniformly over some area(e.g. mm, cm, or m of precipitation over an area)David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Measuring Water in the Atmosphere• A significant proportion of the water on Earth is in a gaseous form in the atmosphere, and naturally we need to approach measuring water in a gaseous form slightly differently• To measure the quantity of gas within a given control volume, we need to consider its contribution to the total pressure, as one of several gases present• If a container contains n well mixed gases, g1, g2, g3…gn, the total gaseous pressure exerted on the inner wall of the container e can be expressed as the following: e = e(g1)+e(g2)+e(g3)+…+e(gn), where e(g1) would the pressure if only g1 was presentDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Measuring Water in the Atmosphere• When we refer to the partial pressure contributed by gaseous water in the atmosphere, we call it vapor pressure• At a given total pressure and temperature, there is a maximum amount of vapor that a given unit of atmosphere can contain, and this is referred to as saturated vapor pressure (e0)•The difference between the actual vapor pressure (ea) and the saturated vapor pressure is referred to as the vapor pressure deficit (VPD = e0-ea)• We more commonly calculate the ratio of actual to saturated vapor pressure to express the amount of water in the atmosphere, called relative humidity (RH = ea/ e0 * 100%)David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Saturation Vapor PressureHornberger et al. 1998. Elements of Physical Hydrology. The Johns Hopkins University Press, Baltimore and London.David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Mechanisms of Water MovementHornberger et al. 1998. Elements of Physical Hydrology. The Johns Hopkins University Press, Baltimore and London.David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Mechanisms of Water Movement• Precipitation – Removes water from the atmosphere as either liquid water (rain) or as a solid (snow), potentially falling on the land surface or in the ocean• Snowmelt runoff – In colder climes, snow can accumulate and then melt at some time later• Infiltration and surface runoff – Some of the water which reaches the surface as a liquid will enter the soil. The remainder will move along the surface• Evaporation and transpiration – Liquid water can return to the atmosphere as vapor from the land surface or oceans, or via plants’ transpiration• Groundwater runoff – Remaining subsurface water will eventually reach the oceanDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Magnitude of Flows in the Water CycleHornberger et al. 1998. Elements of Physical Hydrology. The Johns Hopkins University Press, Baltimore and London.David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Magnitude of Flows and StoresAtmosphere (13,000)Ocean (1,350,000,000)• The oceans contain 97% of the water on the planet(1.35 billion km3of water)425,000Evaporation• The atmosphere contains comparatively little water, ~ 13,000 km3Stores in km3Flows in km3/yearDavid Tenenbaum – GEOG 110 – UNC-CH Fall 2005Magnitude of Flows and StoresAtmosphere (13,000)Ice(33,000,000)Soil(122,000)Ocean (1,350,000,000)Ground water(15,300,000)40,000425,000Evaporation385,000Precipitation40,000111,000PrecipitationLandSurface• The transfers from oceans to land (via the atmosphere) and vice-versa (via ground water etc.) are equalTrans.71,000Evapotranspiration• Evapotranspiration returns 71,000 km3of water from the land surface to the atmosphere each yearStores in km3Flows in km3/year•Loops?David Tenenbaum – GEOG 110 – UNC-CH Fall 2005Residence Times• A useful way of comparing the magnitude of hydrologic stores to the flows that add and remove water to them is through the notion of residence time, which describes on average how long a molecule of water will spend in a particular storeTr= Volume / Net flow rateHornberger et al. 1998. Elements of Physical Hydrology. The Johns Hopkins University Press, Baltimore and London.David Tenenbaum – GEOG 110 – UNC-CH Fall 2005From the Global Scale to Regional• While considering the water cycle globally gives us the big picture, it obscures some important regional differences in the magnitude of processes:– Evaporation from the oceans ranges from 4 mm/day in the tropics to less than 1 mm/day in the high latitudes. Evaporationfrom the oceans is greater than precipitation in the tropics, thus providing water to the atmosphere– On land precipitation exceeds evapotranspiration in tropical rainforests, e.g. more than 50% of rainfall becomes runoff in the Amazon. In desert regions, precipitation is essentially equal to rainfall, thus there is no runoff – Nearly all rainfall into the oceans originates from the oceans; much of the rainfall in