EVAPOTRANSPIRACIN REAL Y POTENCIAL

UNIVERSIDAD NACIONAL PEDRO RUIZ GALLO

CURSO:

HIDRAULICA APLIACADA

TEMA:

EVAPOTRANSPIRACIN REAL Y POTENCIAL

DOCENTE:

MSc. JOS ARBUL RAMOS.

ALUMNOS:

AREBALO MONTENEGRO HERNAN CARLOS CALLACA JONATHAN QUEVEDO GARCA NIKKA

INDEXINDICE.. (1)EVAPOTRANSPIRACIN REAL Y POTENCIAL.(2)INTRODUCCIN..(2)DEFINICIN(2)DEFINICIN DE TERMINOS.(2)FACTORES QUE AFECTAN LA EVAPOTRASPIRACIN.(3)APLICACIN DE LA EVAPOTRASPIRACIN..(3)METODOS PARA EL CLCULO DE ETo...(3) MTODO DE PENMAN(4)

MTODO DE THORNTHWAITE(5)

MTODO DE BLANEY-CLIDDLE(6)

MTODO DE TURC(7)

MTODO DE CHRISTIANSEN.(7)

MTODO DE HARGREAVES(8)

ULTIMOS METODOS(9)

EVAPOTRANSPIRATION REAL AND POTENTIAL

1.- INTRODUCTION

The efficient use of water resources in agro- ecosystems of the world has become increasingly important due to the rapid decrease of water resources , industrial development and population growth , drought conditions and degradation of soil quality surface water in many regions. In many cases , evapotranspiration ( ET ), which is the sum of transpiration through the stomata of the plant and soil evaporation , plants and the surface of open water, may be the most important component of the hydrological cycle . Improved for accurate quantification of ET in a field techniques, watershed and regional level are needed to improve the efficient use of water resources and sustainable productivity in the agricultural ecosystem and protect the environment and water quality . A significant proportion of total precipitation that falls on the surface of the land is returned to the atmosphere by ET . As a global average, 60 % of the annual precipitation falling on the land is returned to the atmosphere by ET .

(http://watercenter.unl.edu/downloads/ResearchInBrief/IrmakSuatET.pdf)

2. DEFINITION

Evaporation is the primary process of the transfer of water in the hydrological cycle. The water is transformed into steam and transported to the .The atmosphere evaporation more transpiration of a plant surface with unlimited supply of water is known as the potential evaporation or potential evapotranspiration (PE ) and is the maximum rate due to weather conditions. This PE is the maximum value of the actual evaporation (): PE = when the water supply is unlimited.

actual evaporation () is the amount of water evaporating a normal day.

(http://www.civil.utah.edu/~mizukami/coursework/cveen7920/ETMeasurement.pdf)

3. DEFINITION OF TERMS

Evaporation

Evaporation is the process by which liquid water is converted into steam and is removed from the surface. Water evaporates from a variety of surfaces such as lakes, rivers , pavements , soils and wet vegetation .

Perspiration

Is the vaporization of liquid water contained in plant tissues and the removal of vapor into the atmosphere .

Evapotranspiration ( ET

Combining two separate processes by which water is lost by a part of the soil surface by evaporation and on the other side of harvest is known as transpiration (ET ) .Evaporation and transpiration occur simultaneously and there is no easy way to distinguish between the two processes .

(http://www.engr.scu.edu/~emaurer/classes/ceng140_watres/handouts/FAO_56_Evapotranspiration.pdf)

reference crop evapotranspiration (ETo )

It is denoted as ETo . The reference surface is a crop of grass hypothetical reference with specific characteristics. The use of other denominations as potential ET is strongly discouraged due to ambiguities in their definitions.The only factors affecting ETo are climatic parameters . Consequently, ETo is a climatic parameter and can be calculated from meteorological data . ETo expresses the evaporating power of the atmosphere at a specific place and time of year and does not take into account the characteristics of crops and soil factors . The method of FAO Penman - Monteith is recommended as the only method for determining ETo .

The crop evapotranspiration under standard conditions (ETc )

It is the crop evapotranspiration free, well-fertilized disease , grown in large fields , under optimum soil moisture, and achieving full production in climatic conditions given .

Crop evapotranspiration can be calculated from climatic data and integrating factors directly crop resistance , albedo and air resistance in the Penman- Monteith approach . As there is still a considerable lack of information for different crops , -Monteith Penman method is used to estimate the standard reference crop to determine evapotranspiration rates , ie ETo . Experimentally determined ratios of ETo , called coefficients ETc / ETo , called crop coefficient ( Kc ) are used to refer ETc = Kc ETo .

EVAPOTRANSPIRATION CROP CONDITIONS UNDER NO STANDARD ( ETC AJ)

Is evapotranspiration crop management and environmental conditions which differ from the standard conditions. When the cultivation of crops in the fields , evapotranspiration of true culture may deviate from ETc due to non-optimal , such as the presence of pests and diseases , soil salinity , low soil fertility , water shortages or flooding conditions. This can result in poor plant growth, low plant density and can reduce evapotranspiration rate below ETc. (http://www.kimberly.uidaho.edu/water/fao56/fao56.pdf)

4.- FACTORS AFFECTING EVAPOTRANSPIRATION

Meteorological parametersThe main meteorological parameters are radiation , air temperature , humidity and wind speed .

crops FactorsThe type of crop , variety and stage of development should be considered when assessing the evapotranspiration of crops in large fields and well managed

Management and environmental conditionsFactors such as soil salinity, low soil fertility , limited application of fertilizers , the presence of hard or impenetrable horizons of soil , lack of control of diseases and pests and poor soil management can limit crop development and reduce evapotranspiration .

(http://www.engr.scu.edu/~emaurer/classes/ceng140_watres/handouts/FAO_56_Evapotranspiration.pdf)

5. APPLICATIONS EVAPOTRANSPIRATION

Determine the area that can be irrigated with a given volume of water available. Develop conceptual crop irrigation schedules . estimate the volumes of water that are necessary to assist the crops , if rain is insufficient. Determine in large areas or basins , water volumes required drain . To select the most suitable crops in rainfed areas . Calculate the additional irrigation depth necessary to prevent problems of salinization of soils.

(https://www.idwr.idaho.gov/GeographicInfo/Landsat/PDFs/mapping_et_using_metric.pdf)

6. - METHODS FOR CALCULATING THE ETo

A large number of evapotranspiration formulas developed from 1942-2005 to calculate water use crops that begin with the development of the formula Blaney - Criddle and ending with the Penman - Monteith equation that became the American Society of Civil Engineers ( ASCE) standardized reference ET equation .

(http://www.journalofserviceclimatology.org/articles/2011/Sammis.A.11-working2A.pdf)A.-PENMAN METHOD

THE PENMAN-MONTEITH EQUATION

Various derivations of the Penman equation included a bulk surface resistance term (Penman, 1953; Covey, 1959; Rijtema, 1965; and Monteith, 1965). The resulting equation is now called the Penman-Monteith equation, which may be expressed for daily values as

[8]

where is air density in kg m-3, is specific heat of dry air [~1.013 x 10-3 MJ kg-1 C-1], is mean saturated vapor pressure in kPa computed as the mean at the daily minimum and maximum air temperature in C, is the bulk surface aerodynamic resistance for water vapor in s m-1, is the mean daily ambient vapor pressure in kPa, and is the canopy surface resistance in sm-1. The Penman-Monteith equation represents the evaporating surface as a single big leaf (Raupach and Finnigan, 1988) with two parameters one of which is determined by the atmospheric physics () influenced only slightly by the crop canopy architecture while the other one () depends on the biological behavior of the crop canopy surface and is related to both crop specific parameters (light attenuation, leaf stomatal resistances, etc.) and environmental parameters (irradiance, vapor pressure deficit, etc.). The water vapor aerodynamic resistance can be estimated following (Allen et al, 1989; and Jensen et al., 1990) as

[9]

where is the wind speed measurement height in m, is the momentum roughness length in m, is the relative humidity measurement height in m, and is the vapor roughness length in m. The crop canopy aerodynamic parameters are estimated as follows

[10, 11, & 12]

FAO-56 PENMAN-MONTEITH EQUATIONthey derived the FAO-56 Penman-Monteith equation using the fixed bulk surface resistance (70 s m-1) and the vapor aerodynamic resistance simplified to an inverse function of wind speed ( as

[13]Twhere is the hypothetical reference crop evapotranspiration rate in mm d-1, T is mean air temperature in C, and U2 is wind speed in m s-1 at 2 m above the ground [and RH or dew point and air temperature are assumed to be measured at 2 m above the ground, also].

http://www.hydrol-earth-syst-sci.net/17/1331/2013/hess-17-1331-2013.pdfhttp://www.cprl.ars.usda.gov/pdfs/pm%20colo%20bar%202004%20corrected%209apr04.pdf

B. METHOD THORNTHWAITE

The Thornthwaite method was developed from data of precipitation and runoff in several drainage basins. The result is basically an empirical relationship between potential evapotranspiration and mean air temperature . Despite the obvious simplicity and inherent limitations of the method , does surprisingly well. It is not necessarily the most accurate method , nor has the strong theoretical base. Rather, these distinctions probably belong to one of the methods of steam flow and heat balance . Among the most obvious shortcomings of the empirical relationship of Thornthwaite is the inherent assumption that there is a high correlation between average temperature and some of the other relevant documents such as radiation , humidity , and wind parameters.The empirical formula of Thornthwaite can be used for any location where the daily maximum and minimum temperatures are recorded . It is this simple universal applicability rather than any pretense of exceptional precision , which has led to widespread use of this method.

(http://pluto2000.com/onlinethornthwaitereference.pdf)

FORMULA THORNTHWAITE

The method of estimating potential evapotranspiration from climatological data developed by Thornthwaite (1948 ) was derived from water budget for natural and controlled in moist northeastern United States experiments basins. It is derived from the following general equation for estimating potential evapotranspiration.

where: = unadjusted potential evapotranspiration , in centimeters , of a month of 30 days;T = mean monthly air temperature , in degrees Celsius ;I = heat index ; and = cubic function R

To estimate potential evapotranspiration by this method , the average monthly temperature at the site and the latitude of the place must be known.

RESOLUTION PROCESS

STEP -1 : calculate the monthly heat index . Thornthwaite (1948 ) gives a table of monthly values of heat indexes for the monthly average temperature. The sum of the 12 monthly values gives the heat index (see table ) . Otherwise you can solve the monthly heat index "i" from the monthly temperature ( C ) by the following formula

i=

-2 STEP : The next step is to calculate the heat index (or index annual heat ) , I summing the 12 values of i .

-3 STEP : The next step is to determine the adjusted monthly values of potential evapotranspiration ; calculating by the general formula:

Where:

-4 STEP : Finally you can make the correction for N of days in the month and hours of sunshine

-5 STEP : You can also adjust the monthly potential evapotranspiration values not adjusted for possible sunshine, in units of 30 days of 12 hours each.

(http://pubs.usgs.gov/wsp/1839m/report.pdf)

C. METHOD BLANEY - CLIDDLE

When considering real time, historical and future needs of each method for estimating the only alternative available for consumptive use data operation is a method of temperature. The Blaney -Criddle method was originally developed to calculate ET on a monthly basis , but can be modified to estimate daily ET values with the average daily temperature .

(http://www.nws.noaa.gov/oh/hrl/nwsrfs/users_manual/part2/_pdf/24consuse_et.pdf)

The method of Blaney - Criddle is simple, using the measurement data in a single temperature. It should be noted , however, that this method is not very precise ; provides a rough estimate u " order of magnitude" only. This method and Thornthwaite to be among the poorest temperature methods are considered. In windy areas , dry and sunny , the reference ET underestimated. In the still , humid , cloudy areas , reference is overestimated ET (http://www.sjrwmd.com/technicalreports/pdfs/SP/SJ2001-SP8.pdf)

FORMULA BLANEY - CLIDDLE

The formula Blaney - Criddle originally published included a parameter relative humidity But due to lack of data relative humidity throughout the western United States , a simplified formula excluding the humidity parameter was published in 1950 , where he was consumptive use monthly or seasonal water ( Et ) of a culture in inches :

Utilisation of water monthly consumption (F ) is the average monthly temperature (T ) in degrees Fahrenheit by the monthly percentage of daylight hours ( p ) divided by 100. The crop coefficient kc is an empirical seasonal factor relating the seasonal water use plant for specific crop water use factor of seasonal consumption total generated in experimental conditions where kc can be calculated from measured F and Et . The formula can be applied on a monthly basis by calculating F for each month and monthly for kc reduction , which depends on the rate of development of crop growth. Consequently, the formula Blaney - Criddle applies to both seasonal and monthly consumption calculations using water.

In 1970, the Soil Conservation Service of the USDA ( USDA SCS 1970 ) extended the term of the air temperature to account for the different rates of development in different climates :

Where:

coefficient kc is a monthly growth stage of the crop and climate kt is a coefficient related to the average monthly air temperature (t ) :

Where:

kt = 0.0173t - 0,314 , with a minimum value of 0.300 .

The temperature is again in Fahrenheit and Et in inches. Crop development and monthly coefficients stage of crop growth are different in different climatic conditions and the rate of development is related to the seasonal progression as measured by the growing degree days that have occurred since planting ( Sammis et al. 1985 ) .

http://www.journalofserviceclimatology.org/articles/2011/Sammis.A.11-working2A.pdf

D. METHOD TURC

An empirical method was developed to estimate evapotranspiration Turc regionally. Two equations were developed ; one for annual use and the other for calculating the evapotranspiration by a period of ten days . These equations require measurements of temperature, precipitation , radiation and an estimate of crop yields . FORMULA TURC

ANNUAL EQUATION

Where: E and P are evaporation and precipitation , respectively , in units of mm . L (t) is a function of temperature

Where: t = mean annual air temperature in degrees Chttp://www.usask.ca/hydrology/papers/Martin_Gray_.pdfMONTHLY EQUATION :

Turks developed an equation for calculating monthly evapotranspiration potential as a function of air temperature , humidity and sunlight .The method of Turks has two equations , both to be used depend on the relative humidity (RH ) air .

Where:

K is a constant equal to 0.40 for 30 and 31 months and 0.37 for the month of February.ETo : is the reference evapotranspiration (cal * cm - 2 * d - 1 ) ; Tm is the mean daily air temperature ( c ) ; Rs is the solar radiation incoming shortwave (mm * d - 1 ) and RH is the relative humidity (%).

http://www.ewra.net/ew/pdf/EW_2008_21-22_02.pdf

E.-METHOD E. CHRISTIANSEN

At present, a large number of countries, are not lysimeter for measuring evapotranspiration due to the high costs of implementation and operation they imply . In these cases the estimation of PET should be performed by empirical methodologies that a large number of authors have proposed from about 60 years ago and today are being used . From the first definition , the concept of evapotranspiration has always been linked to a mathematical expression comprising different weather variables involved in the process. Methods for estimating PET can be classified according to the climatological information required to use their respective equation.

CHRISTIANSEN MODEL FOR ESTIMATING THE POTENTIAL EVAPOTRANSPIRATION .

This is an empirical formula which takes into account solar radiation, wind speed, relative humidity, temperature, insolation and elevation for estimating potential evapotranspiration in mm / month. Christiansen's equation has the following form:

where: PET(C), potential evapotranspiraton (in mm/day)

extraterrestrial radiation is taken at the top of the atmosphere and expressed in cal / cmda.

is the temperature coefficient and is calculated using the following equation:

,

default option ; is the average monthly air temperature expressed in C

wind coefficient is calculated according to the following expression:

,

default option ; is the monthly average wind speed at 2.0 m above the ground, expressed in km / h. For the wind speed 2.0 m based on measurements at different heights, Hellmann power law according to which the wind speed calculated W_Z a desired height z is used, is expressed as Equation.

Where z is the distance from the ground in meters at which you want to calculate the wind speed, h is the height of the measurement of wind speed (in meters), W_hes wind speed measured at the height h is an exponent function of surface roughness that is often taken as 1/7 to open land (Banuelos Angeles, Serrano, & Mucio, 2008)

Coefficient RH is defined Equation

,

default option ; Where H is the monthly average relative humidity (in decimal)

The coefficient of sunshine depends on the insolation (S) and is defined by the equation:

,

default option

is the coefficient of elevation and is defined by the following expression:

default option ; Where e is the average elevation of the study area in msnm.

http://www.javeriana.edu.co/biblos/tesis/ingenieria/tesis369.pdf

http://wwwsst.ums.edu.my/data/file/Yu2o19X039xB.pdf

F. METHOD HARGREAVES

Hargreaves, using data from grass lysimeter evapotranspiration accuracy over a period of eight years , seen through regressions that 94 % of the variance in measured ET can be explained by the average temperature and radiation global solar , Rs . As a result , in 1975 , published an equation for predicting ETo based only on these two parameters

Where Rs is the water evaporation unit , in mm day - 1 , and T in C .

https://www.repository.utl.pt/bitstream/10400.5/4250/1/REP-J.L.Teixeira-InTech-Hargreaves_and_other_reduced_set_methods_for_calculating_evapotranspiration.pdf

FORMULA HARGREAVES

Finally conditions for plants and requires measurements of temperature and solar radiation , is as follows :

Where: Extraterrestrial Radiation (mm / day)

The mean air temperature by the method of Hargreaves is calculated by averaging and , is calculated from information about the site (latitude ) and the day of the year .

Hargreaves model is a simple model which requires only two climate parameters , temperature and incident radiation.

http://www.wseas.us/e-library/transactions/environment/2009/31-286.pdf

G.-LAST METHODS

METHOD OF RADIATIONThis method considers the radiation reaches the earth as the greatest contribution or influence factor for evapotranspiration . FAO recommends :

Where::

Radiacion in the equivalent evaporation (mm / day) Sol bright royal measured in hours W = temperature dependent factor and altitude weightage . C = Adjustment factor graphically made in W

http://www.wseas.us/e-library/transactions/environment/2009/31-286.pdf

FORMULA MAKKINK

This is another simplified formula combining the original Penman equation ; does not take into account the aerodynamic component and replaced the net balance of solar radiation with incoming shortwave radiation ( Rs) . The equation is

Where::

http://www.ewra.net/ew/pdf/EW_2008_21-22_02.pdf

pg. 4