Lecture 11 Evapotranspiration (4) ET Measurements Lysimeters Evaporation Pans Pan size effects

Lysimeter Allow determination of water balance of a block of soil Maintain wet soil to measure PET Allow soil moisture to fluctuate to measure AET   http://lawr.ucdavis.edu/classes/ hyd143/hyd143week2.html

Weighing Lysimeters Use a balance to assess changes in storage Small lysimeters can be removed and weighed Measure both PET and AET http://www.kfa-juelich.de/icg/icg5/irapic/einlasse.jpg

Draining Lysimeters Soil block is isolated but must be undisturbed and in the same surroundings   Can’t measure change in storage, only inputs and outputs http://www.l-sadwrn.dircon.co.uk/ lysim.html

Evaporation Pan (US Class A) standard of the National Weather Service (NWS) http://www.fao.org/docrep/S2022E/s2022e07.htm

Sunken Colorado Pan http://www.fao.org/docrep/S2022E/s2022e07.htm

Evaporation Pans Solve a simplified water-balance equation for a given period (t): E = W – (V2 – V1) E = Evaporation W = Precipitation during t V1 = Volume at beginning of t V2 = Volume at end of t  

Estimating the Effect of Pan Size Empirical equations to account for energy exchange through the side the pan, in order to estimate free water evaporation: + when Tspan > Ta - when Tspan < Ta   Efw = Evaporation from free-water surface Epan = Pan evaporation P = Atmospheric pressure (mb) vpan = Avg. wind speed 15 cm above pan (km/day) Tspan = Water-surface temperature (C) Ta = Air temperature (C) pan = Proportion of energy lost to evaporation Kohler et al. (1955)

Evaporation Pans Free-water evaporation Expose a cylindrical (or rectangular) pan of water to the atmosphere Provide estimates of potential evapotranspiration (PET) – can be more accurate than formulae Must be related to vegetation canopies using other methods (coefficients etc.) Precipitation is measured in an adjacent rain gauge Pan volume is measured by recording water height (which is maintained by adding or removing measured amounts of water) Pans can be placed on land in clearings or on floating platforms on open water and usually surrounded by fences

Plot Studies Water balance approach applied to small undisturbed plots of land Analogous to small lake for simple evaporation Must select suitable sites where inputs/outputs can be measured Spatial uncertainty increases with size of plot Artificial rain (sprinklers) can be used to assess PET Soil moisture assessed by spot sampling and measurement by: Gravimetric methods Soil block moisture Neutron probe Time domain reflectometry

Catchment Studies Water balance of entire drainage basin Even greater spatial uncertainties   Variable precipitation, soil water capacity Ground water seepage   Better for long term estimates On a yearly basis, change in storage is approx. zero