Evapotranspiration. Rabi H. Mohtar ABE 325

Transcription

1 Evapotranspiration Rabi H. Mohtar ABE 325 Introduction What is it? Factors affecting it? Why we need to estimate it? Latent heat of vaporization: Liquid gas o Energy needed o Cooling process Saturation vapor pressure Factors needed for evaporation to occur: 1. Energy source to supply latent heat of vaporization, and 2. Concentration gradient; air mixing and movement. Fick s Law J j D j c j x where J j = moles of j particles /m 2 per sec. D j = diffusion coefficient

2 Evaporation from bare soil Saturated or near saturated stage 90% of maximum possible (climate controlled) Dry stage > 3 days after saturation (soil controlled, not climate) Transpiration Pressure gradient Water path tubes Plant controlled process (stomata) Transpiration Ratio (TR) water transpired TR dry matter produced TR values: 900 for alfalfa 500 for wheat 250 for sorghum Consumptive use = water used (transpired) + stored in plant tissue. ET Estimation Methods: Methods Climatic Information Air T RH/vap press Lat Elev Rad Wind Penman: x x x x x Jensen-Haise x x x SCS Blaney Criddle x x Thornwaite x

3 Potential evapotranspiration E evaporation when no restriction on the rate from p the surfaces. It is a climatic factor. Under wet conditions. E actual evaporation = E p * crop coefficient E tr reference crop evapotranspiration o E pan kp where kp = pan coefficient Soil water depletion: Water balance o inflow outflow ( 1 2) i Si I D SM i 1 Et t t where E actual ET mm / day n I SM t rz D 1 2 t n rz change in water content time increment in days number of soil layers in the root zone, moisture content at beginning & Si end of period t in mm thickness of layer in mm infiltration in mm drainage in mm

4 Saturated vapor pressure chart Actual vapor pressure: RH ed es* 100 ( e e ) e e vapor pressure deficit s d s s Tave Td where T d = dew point temp, and T ave = average temp for period of interest

5 Solar radiation R (1 ) R R where R n s b b albedo outgoing radiation R s = solar radiation received on a horizontal plane at the earth s surface R n = net radiation Can be measured directly or approximated (estimated) from number of sunshine hours + latitude. Wind speed

6 Evaporation from open water (E) E C e e U 25 ( s d)(1 ) 10 Where C = 11 for small lakes 15 for shallow ponds U 25 = wind speed at 25 ft. elevation Crop E T E T =kce tr or E T = kc E tp Penman s Method Penman (1948) first combined factors to account for a supply of energy and a mechanism to remove the water vapor from the immediate vicinity of the evaporating surface. You should recognize these two factors as the essential ingredients for evaporation. Penman derived an equation for a well watered grass reference crop:

7 E tp ( R G) 6.43( u )( e e ) n 2 s d where E tp = potential evapotranspiration in mm/day R n = net radiation in MJ m -2 d -1 G = heat flux density to the ground in MJ m -2 d -1 λ = latent heat of vaporization computed in MJ/kg by: λ = * 10-3 T u 2 = wind speed measured 2 m above the ground in m s -1 Δ = slope of the saturation vapor pressuretemperature curve, kpa C -1 = psychrometric constant, kpa C -1 e s -e d = vapor pressure deficit determined by e e s s ( T ) ( T ) ( e e ) vapor pressure deficit max min e s d 2 s T kpa The slope of the saturation vapor pressuretemperature curve, Δ, can be computed knowing the mean temperature as follows: Δ = [ T ] d 8am

8 where Δ is in kpa/ C, and T is the mean temperature in C. To calculate the psychrometric constant, you must first calculate P, the atmospheric pressure, which Doorenbos and Pruitt (1977) suggested could be calculated by: P = H where P is in kpa and H is the elevation above sea level in meters. Using P, λ, and c p, the specific heat of water at constant pressure [ kj/(kg C)], the psychrometric constant (in kpa/ C) can be calculated from: c p P Cp specific heat of water at constant pressure ( kj / kg C) T 1 T 1 G 4.2 i i t Where T is mean air temp in Celsius for time periods i+1 and i-1; Δt time in days between midpoints of time periods i+1 and i-1.

9 Example: Estimate the E tr for August 2, 1992 from a field close to Hoytville, Ohio, using Penman s method. Radiation data, wind speed and pan evaporation were measured at the site. The other weather data are taken from the Hoytville daily summaries published by NOAA and summarized below. Given: T avg (Aug 1) = 15.0 C ELEV = m Lat = 41 ⁰N ELEV = m Lat = 41 ⁰N Solution: Step 1: Calculate G Step 2: Calculate λ

10 λ = ( * 10-3 * (18.6) = MJ/kg Step 3: Calculate P Step 4: Calculate γ, remembering c p = kj/(kg C). γ = Step 5: Calculate Δ, Δ = Step 6: Determine R n from Rs as shown in the section on solar radiation. From these calculations, є =.1795, R bo = 6.363, R so = 28.9, R b = 4.25, and R n = Step 7: Determine (e s e d ) shown in the vapor pressure deficit section. The calculated vapor pressure deficit = kpa. Step 8: Substitute into Penman s Equation =1.54 Answer: The potential ET for a site near Hoytville for August 2, 1992, was 1.54 mm/d. The measured pan evaporation was 1.12 mm/d, and multiplying by the Etr/Epan coefficient of 0.92 results in a predicted potential ET of 1.03 mm/d.

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12 TABLE Evapotranspiration Coefficients for Irrigated Crops. (From Blaney and Criddle, 1950). Length of Growing Evapotranspiration Coefficient K Crop Season or Western Southeastern Period States States Alfalfa Between Frosts a b Corn 4 months Cotton 7 months Grains, Small 3 months Grain Sorghums 4 to 5 months 0.70 Orchard, Citrus 7 months Pasture, Grass Between Frosts Potatoes 3 ½ Months Rice 3 to 5 Months Sugar Beets 6 Months Truck Crops, Small 3 Months a lower values for coastal areas; the higher values for areas with an arid climate. b lower values for coastal areas and entire state of Florida; higher values for remainder of region. Evapotranspiration ET ktp 100 k = crop coefficient (Table 10-1) T = mean monthly temperature ( F) p = monthly percent of daytime hrs of yr. ET = inches of ET during the month

13 Design Depth (DD) of irrigation (0.5)( AMC) DD WAE (0.5)(AMC) Irrigation interval ET AMC = Available moisture capacity ET = in/day WAE = water application efficiency