EVAPORATION

INTRODUCTION A process whereby liquid water is converted to water vapour and removed from the vapour surface. Evaporation consists of 2 components i.e. The energy component and the aerodynamic component The energy component is responsible conversion of liquid water to water vapour

INTRODUCTION The aerodynamic component is responsible for vapour removal from the evaporating surface Energy used for evaporation is the net all wave radiation and to a lesser extend ambient temperature Evaporation take place from water surfaces, soil surfaces, plant surfaces and animal surfaces.

Factors affecting evaporation Meteorological factors Radiation, relative humidity, wind speed, air temperature Radiation and temperature provide the energy required to break water bonds to form vapour RH and wind speed constitute the aerodynamic component (vapour removal from the surface)

Factors affecting evaporation Evaporation proceeds when there is a vapour pressure gradient between the evaporating surface and the ambient Wind speed is required for the creation of the vapour pressure gradient Low ambient RH increases the vapour pressure gradient and vice versa

Factors affecting evaporation The higher the vapour pressure gradient, the faster evaporation will take place Crop factors Crop type, variety, development stage, resistance to transpiration (stomatal resistance), crop roughness, ground cover, root charactreristics

Factors affecting evaporation These factors are summarised in the crop coefficient, Kc Management / Environmental factors Soil fertility, soil salinity, mulching, hard soil horizons, pests and diseases, plant density, water availability

Factors affecting evaporation These may limit crop development and reduce ET. Evaporation can be measured in depth or energy terms per unit time The most common unit is mm/day or MJ/m2/day

Conversion factors for evaporation Depth Volume per unit area Energy per unit area mm/day mm/ha/day l/s/ha MJ/m2/day 1mm/day 1 10 0.116 2.45 1m3/ha/day 0.1 0.012 0.245 1l/s/ha 8.640 86.40 21.17 1MJ/m2/day 0.408 4.082 0.047

Conversion factors for evaporation Example On a summer day , net solar radiation received at a dam was 18 MJ/m2/day. If 80 % of the energy was used to vapourise the water, what is the evaporation rate in mm/day? Answer 0.8*18*0.408 = 5.88 mm/day

Terms Reference evapotranspiration, ETo Is ET rate from a reference surface, not short of water A reference surface is a hypothetical grass closely representing an extensive surface of 8 – 15 cm tall, green grass cover of uniform height, actively growing & completely shading the ground Solely a function of meteorological factors i.e. The evaporative demand of the atmosphere

Terms Crop evapotranspiration under standard conditions, ETc is the evaporative demand from crops grown under optimal soil water, excellent management and environmental conditions and achieve full production under the given climatic conditions

Terms Crop evapotranspiration under non standard conditions, ETc adj Is ET from crops grown under management and environmental conditions that differ from standard conditions – calculated by a water stress coefficient, Ks and or by adjusting Kc

Measurement of evaporation Can be measured from soil, plant, water surfaces Measurement can be direct or in direct Some of these methods are: evaporation pan, lysimetry, energy budget, eddy covariance, whole plant evaporation chambers, evaporimeters / atmometers, sap flow gauges, thermal dissipation probe, soil water balance

Measurement of evaporation

Estimation of ETo Usually done using empirical relationships from long term studies Most methods are temperature and radiation based Correlations were established (and are still being established) between ETo and meteorological parameter(s) using long term data

Estimation of ETo Empirical relationships employ simpler models with less input variables and can be broadly applied Not limited to data availability Best suited for humid conditions where the aerodynamic term is relatively small Empirical equations are however less likely accurate in arid and semi arid conditions (aerodynamic component is significant)

Estimation of ETo Local calibration is required for satisfactory results Examples of evaporation equations are: Blaney Criddle Jensen - Haise Hargreaves Priestly & Taylor Senami – Hargreaves Thornthwaite Penman FAO Makkink

Estimation of ETo Blaney Criddle method p is the ratio of actual daily daytime hours to annual mean daily day time hours Tmean is the mean air temperature (oC)

Estimation of ETo FAO Makkink method c is the FAO-24 adjustment factor for mean RH and day time wind speed (c is usually assumed to be 1) Rs is the solar / short wave radiation (MJ/m2/day) ∆ is the slope of the vapour pressure curve (kPa/oC) γ is the psychrometric constant (kPa/oC)

Estimation of ETo Jensen – Haise method Rs is the incoming short wave radiation (W/m2) Ta is the average air temperature at 2m (oC)

Estimation of ETo Senami – Hargreaves method Tmean is the average daily air temperature (oC) Tmax and Tmin are the daily maximum and minimum temperatures respectively (oC) Ra is the extraterrestrial radiation (mm/day)

Estimation of ETo Thornthwaite method ETo = montly reference evapotranspiration (cm) T is the mean monthly temperature (oC) I is a heat index for a given area which is the sum of 12 monthly index values, i

Estimation of ETo i is derived from mean monthly temperatures using: a is an empirically derived exponent which is a function of I

Estimation of ETo The FAO Penman – Monteith method Is the standard method for measuring ETo Was developed based on correlation between energy conservation and the aerodynamics of an area (combination method) The equation is a close, simple representation of the physical and physiological factors governing ETo

Estimation of ETo The method requires all meteorological data required for evaporation to take place Missing data for a site must be estimated before using the method The method is mainly used for calibration of other methods

Estimation of ETo Rn is the net radiation at the surface (MJ/m2/day), G is the soil heat flux (MJ/m2/day), T is the average air temperature (oC), U2 is wind speed measured at 2m height (m/s), (ea-ed) is the vapour pressure deficit (Kpa)

Estimation of ETo ∆ is the slope of the vapour pressure curve (Kpa/oC), γ is the psychrometric constant (kPa/oC) and 900 is a conversion factor