Water Science and the Environment

Transcription

1 Water Science and the Environment HWRS 201 Dr. Zreda Mr. Ghasemian Fall 2015

2 Surface Evaporation: Overview Evaporation is a process that transfers energy from the Earth s surface to the atmosphere. Some evaporation will always occur as long as the air is not saturated (RH = 100%, or M V = M SV ) with water vapor. Solar radiation heats the land surface (or the surface of a body of water like an ocean, sea, lake or river). Heat from the surface is transferred to water in the upper layer of the soil. That causes the water to evaporate thereby transferring heat from the surface to the water. The result is that the surface is cooled. The heat in the water vapor is released later in the atmosphere when the vapor condenses into water droplets.

3 Balances of flows The net flow into most reservoirs in a year is zero! Reservoir Annual inflow Amount Annual Outflow Amount Oceans Return flow 1 Evaporation 426 Surface runoff 40 PPT 385 Total 426 Total 426 Land surface PPT 111 ET 71 Surface runoff 40 Total Atmosphere ET 71 Land PPT 111 Ocean evaporation 426 Ocean PPT 385 Total 497 Total 496

4 Earth s energy balance

5 Latent Heat A change of state (or phase) change is a change between solid, gas, and liquid. Latent heat is the energy involved in changing the state of a substance. Ice to vapor: absorb energy, cool environment (melt, evaporation, sublimation) Vapor to ice: release energy, heat environment (freeze, condense, deposition) 20

6 Sensible Heat vs Latent Heat Sensible heat is what we feel from different temperatures; energy needed to raise T (or released to decrease T) Latent heat is the energy needed to change state (ice to water, water to vapor) 21

7 Potential evaporation Potential evaporation (PE) is the amount of evaporation that would occur if a sufficient water source was available. Actual evaporation is the net result of atmospheric demand for moisture from a surface (PE) and the ability of the surface to supply moisture. Surface and air temperatures, insolation,and wind all affect this. A dryland is a place where annual potential evaporation exceeds annual precipitation. 22

8 Pan evaporation

9 Water path through a plant

10 Measuring evapotranspiration (ET) It s not too hard to put an upper bound on the rate of evaporation. As we ve discussed, the basic ideas are the same as measuring evaporation from the surface of a body of water. Potential evaporation can be measured with an evaporation pan. That s the amount of water that would be evaporated from the land surface if the amount of water in soil was unlimited. But measuring actual evaporation is a harder because the amount of water in soil is limited. And measuring transpiration through plants is even harder. We measure ET at scales ranging from individual leaves to regions

11 Regional scale: The water-balance equation Remember the steady-state equation is, (P + G in ) (Q + ET + G out ) = 0 The total amount of water leaving the region is the runoff, RO = Q + G out Usually the groundwater terms are negligible (G in = 0 and G i << Q). Hence another way to think of runoff is RO = P - ET

12 The water-balance equation: estimating ET With the simplifications on the previous page, we can express the water balance more simply as evapo-transpiration = precipitation runoff or ET = P RO The importance of this is that regional precipitation and runoff are relatively easy to measure, but evapo-transpiration is not.

13 Measuring transpiration: Methods Measurement of liquid water loss Measurement of vapor flow in the atmosphere Remote sensing All these methods have a specific scale of measurement, so the measurements must frequently be scaled up or scaled down depending on the application.

14 Measurement of liquid water loss All these methods depend on a basic water balance model that is averaged over a given amount of time, t. ET net loss (mm ) from soil surface. P net precipitation input (mm ) net amount of water entering or leaving at the surface (runoff) P V Leakage of water out. leakage and runoff are small, approximately equal to the change in storage. V ET

15 Measuring ET in small plots The most straightforward way to measure evapotranspiration from a single plant, or a few, is to use a lysimeter. These are specialized growing tanks that can be set in fairly realistic field conditions. This is especially true of agricultural fields where engineered tanks aren t too hard to set up. Their biggest advantage is that they measure ET directly. The plant(s) is allowed to grow in the tank and the amount of water used for irrigation is carefully measured, as is the overall weight of the system.

16 Measuring ET in small plots Evapotranspiration from one, or a few, plants can be pretty accurately measured in a lysimeter. The disadvantages include the relatively small scale of the experiment: it s still necessary to extrapolate up to a whole field. It s not easy to get a representative soil or vegetation sample of a natural system in the tank. Also, it s difficult to evaluate natural ecosystems, especially in hilly terrain. And the equipment is expensive.

17 ET from soil moisture depletion The way it works. Hydrogen atoms are the right size to interact with neutrons. A neutron probe generates fast neutrons that can bounce into hydrogen atoms in water molecules. The hydrogen atoms slow down the neutrons. The probe counts the number of slow neutrons that come back to it. The ratio of fast neutrons sent out to slow neutrons returned lets the probe count the number of hydrogen atoms encountered and thus the number of water molecules in the soil. Neutron probe By sending out pulses of neutrons at 2 different times, scientists can calculate the difference between the number of water molecules detected with the 2 pulses. That difference is proportional to the evaporation rate.

18 ET from soil moisture depletion ET is estimated as the change in amount over a period of measurement. One issue is inserting the probe without disturbing the plant canopy or soil Instruments that can make these kinds of measurements accurately also include Time-domain reflectometers Capacitance sounders Neutron probe

19 COSMOS: COsmic-ray Soil Moisture Observing System The idea is basically the same as a neutron probe, except Cosmic rays are the source The data comes from a circle (an area) of about 500m radius.

20 COSMOS: Calibration and Validation

21 Meteorological estimates: Eddy covariance The average wind near the land surface is parallel to the ground. However, turbulent eddies in the air can cause local upward and downward motions of the air. Eddy covariance towers measure the difference between the water flux going up from the surface (w 2 below) and going down (w 1 ). Then ET = w 2 w 1. Eddy covariance tower

22 Meteorological estimates: Eddy covariance Eddy covariance towers measure the difference between the water flux going up from the surface (w 2 below) and going down (w 1 ). Then ET = w 2 w 1. These fluxes depend on the local vertical velocity of the air, it s specific humidity, and the air s density. Measuring these things simultaneously is a major challenge, but good towers can yield estimates of ET that are accurate up to 5-10%. The measurements are primarily local so they must be scaled up for regional analyses. Eddy covariance tower

23 Larger scale measurements: Lidar Lidar works like a radar that uses light instead of radio waves. Water vapor lidars send out beams of light that bounce off water molecules in the atmosphere. Kind of like a neutron probe (but on a much larger scale and in the atmosphere instead of the soil), the lidar counts how many photons return from which directions and distances. The return gives a picture of the amount of water vapor in the atmosphere.

24 Larger scale measurements: Lidar Lidar uses reflected light to estimate the quantity of water vapor in a volume of atmosphere.. Water vapor lidars send out beams of light that bounce off water molecules in the atmosphere. Kind of like a neutron probe (but on a much larger scale and in the atmosphere instead of the soil), the lidar counts how many photons return from which directions and distances. The return gives a picture of the amount of water vapor in the atmosphere.

25 Water path through a plant

26 Measuring transpiration in an individual plant One way to estimate transpiration is to take a cutting from a plant and measure the short-term loss in a pipette as water streams through the cutting. This is obviously not completely realistic since no roots are involved and it s not possible to do this with a large plant like a tree. For that it s necessary to extrapolate the loss of water from one branch to a whole tree and that has many uncertainties. Also it s hard to reproduce the actual environmental conditions a plant undergoes in a lab.

27 Measuring transpiration in an individual plant One way to measure transpiration in the field is to put a bag around a branch (or a whole plant) and measure the flow of gases, including water vapor, in and out of the plant. This is somewhat more realistic, but it disturbs the thermal (heat) balance of the plant. The bags keeps air from blowing over the branch. Also, the scale is still limited to a single branch (or plant at most).