Soil Physics 2010 Outline Announcements Measuring evaporation

Soil Physics 2010 Announcements Vote your preferences on doodle for review sessions next week Last homework is posted, due Wednesday, April 28th Quiz? YES!

Soil Physics 2010 Quiz, question 1: What factor(s) might limit the actual rate of evaporation from a pan of water (like, for example, the evaporation pan that John Stinn showed)? Big ones: Temperature Solar irradiance Wind speed Vapor pressure deficit Obnoxious ones ☺ : Is there water in the pan? Any solutes in the water? Is the pan covered? What color is the pan? Heater in the pan? Local / secondary ones: Local topography Humidity of the fetch Atmospheric pressure Heat conduction through pan Available energy Maintaining vapor pressure gradient

Soil Physics 2010 Quiz, question 2: Three almost-identical metal boxes, colored black both inside and outside, are sitting in the sun in the Arizona desert. For your convenience, the sun doesn’t move for several days: it is always noon! Each box contains 1 kg of H 2 O at 0 °C, but there are some important differences: Box A is tightly sealed, and the H 2 O is in the form of ice. Box B is tightly sealed, and the H 2 O is in the form of liquid water Box C has no top, and the H 2 O is in the form of liquid water Sketch the expected average H 2 O temperature of each box. 0time 0 Temperature, °C

Soil Physics 2010 Quiz, question 2: explanation Box A is tightly sealed, and the H 2 O is in the form of ice. Temperature will remain near 0 °C until all the ice has melted; after that it will behave like B. Box B is tightly sealed, and the H 2 O is in the form of liquid water. The water will get hotter until the energy coming in equals the energy going out. In other words, it will approach an equilibrium temperature or plateau. Box C has no top, and the H 2 O is in the form of liquid water. Because the top is open, water can evaporate and cool the water, so the plateau will be at a lower temperature. Eventually, once all the water has evaporated, the box will get hotter (but the water won’t, because it’s all gone). 0time 0 Temperature, °C

Soil Physics 2010 Main limitations on actual evaporation from soil time e, mm/day Stage I Stage II Stage III Stage I: evaporation from the surface Energy available at surface Vapor pressure deficit in air near the surface Transport of vapor away from the surface Stage II: evaporation from a retreating drying front Flow of liquid water to the drying front Stage III: evaporation from a stationary drying front Diffusion of water vapor from the drying front to air above the soil surface, which equals… Flow of liquid water to the drying front

Soil Physics 2010 Measuring evapotranspiration Meteorology magic: Eddy Covariance time e, mm/day Stage I Stage II Stage III Measure air mass fluxes in 3 orthogonal directions, many times per second. Simultaneously measure concentration(s) of gas(es) of interest, e.g. H 2 O Make lots of assumptions and do some fancy math

Soil Physics 2010 Measuring Evaporation (from the soil)? Stage I: evaporation from the surface Actual ≈ potential ≈ pan Stage II: evaporation from a retreating drying front ? Stage III: evaporation from a stationary drying front ? time e, mm/day Stage I Stage II Stage III 2 approaches to Stages II & III: Conservation of mass Conservation of energy

Soil Physics 2010 Conservation of Mass Input - Output = Change in Storage Precipitation Irrigation Condensation (dew) Water table rise Lateral flow Run-on Evaporation Transpiration Interception Abstraction Water table fall Lateral flow Runoff  over soil profile Storage in plants Too many variables Errors are too big E = ET – T Precipitation 

Soil Physics 2010 Conservation of Mass: Lysimeter Input - Output = Change in Storage Precipitation Irrigation Condensation (dew) Evaporation Transpiration Interception Abstraction Runoff Deep drainage  over soil profile Storage in plants

Soil Physics 2010 Lysimeters

Soil Physics 2010 Lysimeters

Soil Physics 2010 Conservation of Mass: Lysimeter

Soil Physics 2010 Lysimeter summary Input - Output = Change in Storage Precipitation Irrigation Condensation (dew)  over soil profile Storage in plants Really good for ET For Evaporation only: need 2 lysimeters? Cropped Bare soil Evaporation Transpiration Interception Abstraction Runoff Deep drainage

2 nd approach: Conservation of Energy R N = A + LE + S + M A (heating the air) LE (latent heat) R N (net radiation) ET M M (other) S S (heating the soil) Other: mainly photosynthesis Soil Physics 2010

S How to measure S? S S (heating the soil) Soil Physics 2010 How much does the soil warm up (or cool off)? Need to know temperature and heat capacity… … or the sensible heat flux in and out

Heitman’s soil E method LE LE (evaporation from the soil) Soil Physics 2010 Key concept #1:  = 0.01 is small relative to measurement error, but LE for  = 0.01 is big Key concept #2: LE in the soil is about E, not ET S S (heating the soil)